The Guile Reference Manual
**************************

This manual documents Guile version 2.0.5.

   Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2009,
2010, 2011, 2012 Free Software Foundation.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
copy of the license is included in the section entitled "GNU Free
Documentation License."


Table of Contents
*****************

The Guile Reference Manual
Preface
  Contributors to this Manual
  The Guile License
1 Introduction
  1.1 Guile and Scheme
  1.2 Combining with C Code
  1.3 Guile and the GNU Project
  1.4 Interactive Programming
  1.5 Supporting Multiple Languages
  1.6 Obtaining and Installing Guile
  1.7 Organisation of this Manual
  1.8 Typographical Conventions
2 Hello Guile!
  2.1 Running Guile Interactively
  2.2 Running Guile Scripts
  2.3 Linking Guile into Programs
  2.4 Writing Guile Extensions
  2.5 Using the Guile Module System
    2.5.1 Using Modules
    2.5.2 Writing new Modules
    2.5.3 Putting Extensions into Modules
  2.6 Reporting Bugs
3 Hello Scheme!
  3.1 Data Types, Values and Variables
    3.1.1 Latent Typing
    3.1.2 Values and Variables
    3.1.3 Defining and Setting Variables
  3.2 The Representation and Use of Procedures
    3.2.1 Procedures as Values
    3.2.2 Simple Procedure Invocation
    3.2.3 Creating and Using a New Procedure
    3.2.4 Lambda Alternatives
  3.3 Expressions and Evaluation
    3.3.1 Evaluating Expressions and Executing Programs
      3.3.1.1 Evaluating Literal Data
      3.3.1.2 Evaluating a Variable Reference
      3.3.1.3 Evaluating a Procedure Invocation Expression
      3.3.1.4 Evaluating Special Syntactic Expressions
    3.3.2 Tail calls
    3.3.3 Using the Guile REPL
    3.3.4 Summary of Common Syntax
  3.4 The Concept of Closure
    3.4.1 Names, Locations, Values and Environments
    3.4.2 Local Variables and Environments
    3.4.3 Environment Chaining
    3.4.4 Lexical Scope
      3.4.4.1 An Example of Non-Lexical Scoping
    3.4.5 Closure
    3.4.6 Example 1: A Serial Number Generator
    3.4.7 Example 2: A Shared Persistent Variable
    3.4.8 Example 3: The Callback Closure Problem
    3.4.9 Example 4: Object Orientation
  3.5 Further Reading
4 Programming in Scheme
  4.1 Guile's Implementation of Scheme
  4.2 Invoking Guile
    4.2.1 Command-line Options
    4.2.2 Environment Variables
  4.3 Guile Scripting
    4.3.1 The Top of a Script File
    4.3.2 The Meta Switch
    4.3.3 Command Line Handling
    4.3.4 Scripting Examples
  4.4 Using Guile Interactively
    4.4.1 The Init File, `~/.guile'
    4.4.2 Readline
    4.4.3 Value History
    4.4.4 REPL Commands
      4.4.4.1 Help Commands
      4.4.4.2 Module Commands
      4.4.4.3 Language Commands
      4.4.4.4 Compile Commands
      4.4.4.5 Profile Commands
      4.4.4.6 Debug Commands
      4.4.4.7 Inspect Commands
      4.4.4.8 System Commands
    4.4.5 Error Handling
    4.4.6 Interactive Debugging
  4.5 Using Guile in Emacs
  4.6 Using Guile Tools
  4.7 Installing Site Packages
5 Programming in C
  5.1 Parallel Installations
  5.2 Linking Programs With Guile
    5.2.1 Guile Initialization Functions
    5.2.2 A Sample Guile Main Program
  5.3 Linking Guile with Libraries
    5.3.1 A Sample Guile Extension
  5.4 General concepts for using libguile
    5.4.1 Dynamic Types
    5.4.2 Garbage Collection
    5.4.3 Control Flow
    5.4.4 Asynchronous Signals
    5.4.5 Multi-Threading
  5.5 Defining New Types (Smobs)
    5.5.1 Describing a New Type
    5.5.2 Creating Smob Instances
    5.5.3 Type checking
    5.5.4 Garbage Collecting Smobs
    5.5.5 Garbage Collecting Simple Smobs
    5.5.6 Remembering During Operations
    5.5.7 Double Smobs
    5.5.8 The Complete Example
  5.6 Function Snarfing
  5.7 An Overview of Guile Programming
    5.7.1 How One Might Extend Dia Using Guile
      5.7.1.1 Deciding Why You Want to Add Guile
      5.7.1.2 Four Steps Required to Add Guile
      5.7.1.3 How to Represent Dia Data in Scheme
      5.7.1.4 Writing Guile Primitives for Dia
      5.7.1.5 Providing a Hook for the Evaluation of Scheme Code
      5.7.1.6 Top-level Structure of Guile-enabled Dia
      5.7.1.7 Going Further with Dia and Guile
    5.7.2 Why Scheme is More Hackable Than C
    5.7.3 Example: Using Guile for an Application Testbed
    5.7.4 A Choice of Programming Options
      5.7.4.1 What Functionality is Already Available?
      5.7.4.2 Functional and Performance Constraints
      5.7.4.3 Your Preferred Programming Style
      5.7.4.4 What Controls Program Execution?
    5.7.5 How About Application Users?
  5.8 Autoconf Support
    5.8.1 Autoconf Background
    5.8.2 Autoconf Macros
    5.8.3 Using Autoconf Macros
6 API Reference
  6.1 Overview of the Guile API
  6.2 Deprecation
  6.3 The SCM Type
  6.4 Initializing Guile
  6.5 Snarfing Macros
  6.6 Simple Generic Data Types
    6.6.1 Booleans
    6.6.2 Numerical data types
      6.6.2.1 Scheme's Numerical "Tower"
      6.6.2.2 Integers
      6.6.2.3 Real and Rational Numbers
      6.6.2.4 Complex Numbers
      6.6.2.5 Exact and Inexact Numbers
      6.6.2.6 Read Syntax for Numerical Data
      6.6.2.7 Operations on Integer Values
      6.6.2.8 Comparison Predicates
      6.6.2.9 Converting Numbers To and From Strings
      6.6.2.10 Complex Number Operations
      6.6.2.11 Arithmetic Functions
      6.6.2.12 Scientific Functions
      6.6.2.13 Bitwise Operations
      6.6.2.14 Random Number Generation
    6.6.3 Characters
    6.6.4 Character Sets
      6.6.4.1 Character Set Predicates/Comparison
      6.6.4.2 Iterating Over Character Sets
      6.6.4.3 Creating Character Sets
      6.6.4.4 Querying Character Sets
      6.6.4.5 Character-Set Algebra
      6.6.4.6 Standard Character Sets
    6.6.5 Strings
      6.6.5.1 String Read Syntax
      6.6.5.2 String Predicates
      6.6.5.3 String Constructors
      6.6.5.4 List/String conversion
      6.6.5.5 String Selection
      6.6.5.6 String Modification
      6.6.5.7 String Comparison
      6.6.5.8 String Searching
      6.6.5.9 Alphabetic Case Mapping
      6.6.5.10 Reversing and Appending Strings
      6.6.5.11 Mapping, Folding, and Unfolding
      6.6.5.12 Miscellaneous String Operations
      6.6.5.13 Conversion to/from C
      6.6.5.14 String Internals
    6.6.6 Bytevectors
      6.6.6.1 Endianness
      6.6.6.2 Manipulating Bytevectors
      6.6.6.3 Interpreting Bytevector Contents as Integers
      6.6.6.4 Converting Bytevectors to/from Integer Lists
      6.6.6.5 Interpreting Bytevector Contents as Floating Point Numbers
      6.6.6.6 Interpreting Bytevector Contents as Unicode Strings
      6.6.6.7 Accessing Bytevectors with the Generalized Vector API
      6.6.6.8 Accessing Bytevectors with the SRFI-4 API
    6.6.7 Symbols
      6.6.7.1 Symbols as Discrete Data
      6.6.7.2 Symbols as Lookup Keys
      6.6.7.3 Symbols as Denoting Variables
      6.6.7.4 Operations Related to Symbols
      6.6.7.5 Function Slots and Property Lists
      6.6.7.6 Extended Read Syntax for Symbols
      6.6.7.7 Uninterned Symbols
    6.6.8 Keywords
      6.6.8.1 Why Use Keywords?
      6.6.8.2 Coding With Keywords
      6.6.8.3 Keyword Read Syntax
      6.6.8.4 Keyword Procedures
    6.6.9 "Functionality-Centric" Data Types
  6.7 Compound Data Types
    6.7.1 Pairs
    6.7.2 Lists
      6.7.2.1 List Read Syntax
      6.7.2.2 List Predicates
      6.7.2.3 List Constructors
      6.7.2.4 List Selection
      6.7.2.5 Append and Reverse
      6.7.2.6 List Modification
      6.7.2.7 List Searching
      6.7.2.8 List Mapping
    6.7.3 Vectors
      6.7.3.1 Read Syntax for Vectors
      6.7.3.2 Dynamic Vector Creation and Validation
      6.7.3.3 Accessing and Modifying Vector Contents
      6.7.3.4 Vector Accessing from C
      6.7.3.5 Uniform Numeric Vectors
    6.7.4 Bit Vectors
    6.7.5 Generalized Vectors
    6.7.6 Arrays
      6.7.6.1 Array Syntax
      6.7.6.2 Array Procedures
      6.7.6.3 Shared Arrays
      6.7.6.4 Accessing Arrays from C
    6.7.7 VLists
    6.7.8 Records
    6.7.9 Structures
      6.7.9.1 Vtables
      6.7.9.2 Structure Basics
      6.7.9.3 Vtable Contents
      6.7.9.4 Vtable Vtables
    6.7.10 Dictionary Types
    6.7.11 Association Lists
      6.7.11.1 Alist Key Equality
      6.7.11.2 Adding or Setting Alist Entries
      6.7.11.3 Retrieving Alist Entries
      6.7.11.4 Removing Alist Entries
      6.7.11.5 Sloppy Alist Functions
      6.7.11.6 Alist Example
    6.7.12 VList-Based Hash Lists or "VHashes"
    6.7.13 Hash Tables
      6.7.13.1 Hash Table Examples
      6.7.13.2 Hash Table Reference
  6.8 Smobs
  6.9 Procedures
    6.9.1 Lambda: Basic Procedure Creation
    6.9.2 Primitive Procedures
    6.9.3 Compiled Procedures
    6.9.4 Optional Arguments
      6.9.4.1 lambda* and define*.
      6.9.4.2 (ice-9 optargs)
    6.9.5 Case-lambda
    6.9.6 Higher-Order Functions
    6.9.7 Procedure Properties and Meta-information
    6.9.8 Procedures with Setters
    6.9.9 Inlinable Procedures
  6.10 Macros
    6.10.1 Defining Macros
    6.10.2 Syntax-rules Macros
      6.10.2.1 Patterns
      6.10.2.2 Hygiene
      6.10.2.3 Shorthands
      6.10.2.4 Further Information
    6.10.3 Support for the `syntax-case' System
      6.10.3.1 Why `syntax-case'?
    6.10.4 Syntax Transformer Helpers
    6.10.5 Lisp-style Macro Definitions
    6.10.6 Identifier Macros
    6.10.7 Syntax Parameters
    6.10.8 Eval-when
    6.10.9 Internal Macros
  6.11 General Utility Functions
    6.11.1 Equality
    6.11.2 Object Properties
    6.11.3 Sorting
    6.11.4 Copying Deep Structures
    6.11.5 General String Conversion
    6.11.6 Hooks
      6.11.6.1 Hook Usage by Example
      6.11.6.2 Hook Reference
      6.11.6.3 Handling Scheme-level hooks from C code
      6.11.6.4 Hooks For C Code.
      6.11.6.5 Hooks for Garbage Collection
      6.11.6.6 Hooks into the Guile REPL
  6.12 Definitions and Variable Bindings
    6.12.1 Top Level Variable Definitions
    6.12.2 Local Variable Bindings
    6.12.3 Internal definitions
    6.12.4 Querying variable bindings
  6.13 Controlling the Flow of Program Execution
    6.13.1 Sequencing and Splicing
    6.13.2 Simple Conditional Evaluation
    6.13.3 Conditional Evaluation of a Sequence of Expressions
    6.13.4 Iteration mechanisms
    6.13.5 Prompts
      6.13.5.1 Prompt Primitives
      6.13.5.2 Shift, Reset, and All That
    6.13.6 Continuations
    6.13.7 Returning and Accepting Multiple Values
    6.13.8 Exceptions
      6.13.8.1 Exception Terminology
      6.13.8.2 Catching Exceptions
      6.13.8.3 Throw Handlers
      6.13.8.4 Throwing Exceptions
      6.13.8.5 How Guile Implements Exceptions
    6.13.9 Procedures for Signaling Errors
    6.13.10 Dynamic Wind
    6.13.11 How to Handle Errors
      6.13.11.1 C Support
      6.13.11.2 Signalling Type Errors
    6.13.12 Continuation Barriers
  6.14 Input and Output
    6.14.1 Ports
    6.14.2 Reading
    6.14.3 Writing
    6.14.4 Closing
    6.14.5 Random Access
    6.14.6 Line Oriented and Delimited Text
    6.14.7 Block reading and writing
    6.14.8 Default Ports for Input, Output and Errors
    6.14.9 Types of Port
      6.14.9.1 File Ports
      6.14.9.2 String Ports
      6.14.9.3 Soft Ports
      6.14.9.4 Void Ports
    6.14.10 R6RS I/O Ports
      6.14.10.1 File Names
      6.14.10.2 File Options
      6.14.10.3 Buffer Modes
      6.14.10.4 Transcoders
      6.14.10.5 The End-of-File Object
      6.14.10.6 Port Manipulation
      6.14.10.7 Input Ports
      6.14.10.8 Binary Input
      6.14.10.9 Textual Input
      6.14.10.10 Output Ports
      6.14.10.11 Binary Output
      6.14.10.12 Textual Output
    6.14.11 Using and Extending Ports in C
      6.14.11.1 C Port Interface
      6.14.11.2 Port Implementation
  6.15 Regular Expressions
    6.15.1 Regexp Functions
    6.15.2 Match Structures
    6.15.3 Backslash Escapes
  6.16 LALR(1) Parsing
  6.17 Reading and Evaluating Scheme Code
    6.17.1 Scheme Syntax: Standard and Guile Extensions
      6.17.1.1 Expression Syntax
      6.17.1.2 Comments
      6.17.1.3 Block Comments
      6.17.1.4 Case Sensitivity
      6.17.1.5 Keyword Syntax
      6.17.1.6 Reader Extensions
    6.17.2 Reading Scheme Code
    6.17.3 Writing Scheme Values
    6.17.4 Procedures for On the Fly Evaluation
    6.17.5 Compiling Scheme Code
    6.17.6 Loading Scheme Code from File
    6.17.7 Load Paths
    6.17.8 Character Encoding of Source Files
    6.17.9 Delayed Evaluation
    6.17.10 Local Evaluation
    6.17.11 Local Inclusion
  6.18 Memory Management and Garbage Collection
    6.18.1 Function related to Garbage Collection
    6.18.2 Memory Blocks
      6.18.2.1 Upgrading from scm_must_malloc et al.
    6.18.3 Weak References
      6.18.3.1 Weak hash tables
      6.18.3.2 Weak vectors
    6.18.4 Guardians
  6.19 Modules
    6.19.1 General Information about Modules
    6.19.2 Using Guile Modules
    6.19.3 Creating Guile Modules
    6.19.4 Modules and the File System
    6.19.5 R6RS Version References
    6.19.6 R6RS Libraries
    6.19.7 Variables
    6.19.8 Module System Reflection
    6.19.9 Accessing Modules from C
    6.19.10 Included Guile Modules
    6.19.11 provide and require
    6.19.12 Environments
  6.20 Foreign Function Interface
    6.20.1 Foreign Libraries
    6.20.2 Foreign Functions
    6.20.3 C Extensions
    6.20.4 Modules and Extensions
    6.20.5 Foreign Pointers
      6.20.5.1 Foreign Types
      6.20.5.2 Foreign Variables
      6.20.5.3 Void Pointers and Byte Access
      6.20.5.4 Foreign Structs
    6.20.6 Dynamic FFI
  6.21 Threads, Mutexes, Asyncs and Dynamic Roots
    6.21.1 Arbiters
    6.21.2 Asyncs
      6.21.2.1 System asyncs
      6.21.2.2 User asyncs
    6.21.3 Threads
    6.21.4 Mutexes and Condition Variables
    6.21.5 Blocking in Guile Mode
    6.21.6 Critical Sections
    6.21.7 Fluids and Dynamic States
    6.21.8 Parameters
    6.21.9 Futures
    6.21.10 Parallel forms
  6.22 Configuration, Features and Runtime Options
    6.22.1 Configuration, Build and Installation
    6.22.2 Feature Tracking
      6.22.2.1 Feature Manipulation
      6.22.2.2 Common Feature Symbols
    6.22.3 Runtime Options
      6.22.3.1 Examples of option use
  6.23 Support for Other Languages
    6.23.1 Using Other Languages
    6.23.2 Emacs Lisp
      6.23.2.1 Nil
      6.23.2.2 Equality
      6.23.2.3 Dynamic Binding
      6.23.2.4 Other Elisp Features
    6.23.3 ECMAScript
  6.24 Support for Internationalization
    6.24.1 Internationalization with Guile
    6.24.2 Text Collation
    6.24.3 Character Case Mapping
    6.24.4 Number Input and Output
    6.24.5 Accessing Locale Information
    6.24.6 Gettext Support
  6.25 Debugging Infrastructure
    6.25.1 Evaluation and the Scheme Stack
      6.25.1.1 Stack Capture
      6.25.1.2 Stacks
      6.25.1.3 Frames
    6.25.2 Source Properties
    6.25.3 Programmatic Error Handling
      6.25.3.1 Catching Exceptions
      6.25.3.2 Capturing the full error stack
      6.25.3.3 Pre-Unwind Debugging
      6.25.3.4 Debug options
    6.25.4 Traps
      6.25.4.1 VM Hooks
      6.25.4.2 Trap Interface
      6.25.4.3 Low-Level Traps
      6.25.4.4 Tracing Traps
      6.25.4.5 Trap States
      6.25.4.6 High-Level Traps
  6.26 Code Coverage Reports
7 Guile Modules
  7.1 SLIB
    7.1.1 SLIB installation
    7.1.2 JACAL
  7.2 POSIX System Calls and Networking
    7.2.1 POSIX Interface Conventions
    7.2.2 Ports and File Descriptors
    7.2.3 File System
    7.2.4 User Information
    7.2.5 Time
    7.2.6 Runtime Environment
    7.2.7 Processes
    7.2.8 Signals
    7.2.9 Terminals and Ptys
    7.2.10 Pipes
    7.2.11 Networking
      7.2.11.1 Network Address Conversion
      7.2.11.2 Network Databases
      7.2.11.3 Network Socket Address
      7.2.11.4 Network Sockets and Communication
      7.2.11.5 Network Socket Examples
    7.2.12 System Identification
    7.2.13 Locales
    7.2.14 Encryption
  7.3 HTTP, the Web, and All That
    7.3.1 Types and the Web
    7.3.2 Universal Resource Identifiers
    7.3.3 The Hyper-Text Transfer Protocol
    7.3.4 HTTP Headers
      7.3.4.1 HTTP Header Types
      7.3.4.2 General Headers
      7.3.4.3 Entity Headers
      7.3.4.4 Request Headers
      7.3.4.5 Response Headers
    7.3.5 HTTP Requests
      7.3.5.1 An Important Note on Character Sets
      7.3.5.2 Request API
    7.3.6 HTTP Responses
    7.3.7 Web Client
    7.3.8 Web Server
    7.3.9 Web Examples
      7.3.9.1 Hello, World!
      7.3.9.2 Inspecting the Request
      7.3.9.3 Higher-Level Interfaces
      7.3.9.4 Conclusion
  7.4 The (ice-9 getopt-long) Module
    7.4.1 A Short getopt-long Example
    7.4.2 How to Write an Option Specification
    7.4.3 Expected Command Line Format
    7.4.4 Reference Documentation for `getopt-long'
    7.4.5 Reference Documentation for `option-ref'
  7.5 SRFI Support Modules
    7.5.1 About SRFI Usage
    7.5.2 SRFI-0 - cond-expand
    7.5.3 SRFI-1 - List library
      7.5.3.1 Constructors
      7.5.3.2 Predicates
      7.5.3.3 Selectors
      7.5.3.4 Length, Append, Concatenate, etc.
      7.5.3.5 Fold, Unfold & Map
      7.5.3.6 Filtering and Partitioning
      7.5.3.7 Searching
      7.5.3.8 Deleting
      7.5.3.9 Association Lists
      7.5.3.10 Set Operations on Lists
    7.5.4 SRFI-2 - and-let*
    7.5.5 SRFI-4 - Homogeneous numeric vector datatypes
      7.5.5.1 SRFI-4 - Overview
      7.5.5.2 SRFI-4 - API
      7.5.5.3 SRFI-4 - Generic operations
      7.5.5.4 SRFI-4 - Relation to bytevectors
      7.5.5.5 SRFI-4 - Guile extensions
    7.5.6 SRFI-6 - Basic String Ports
    7.5.7 SRFI-8 - receive
    7.5.8 SRFI-9 - define-record-type
      Non-toplevel Record Definitions
      Custom Printers
    7.5.9 SRFI-10 - Hash-Comma Reader Extension
    7.5.10 SRFI-11 - let-values
    7.5.11 SRFI-13 - String Library
    7.5.12 SRFI-14 - Character-set Library
    7.5.13 SRFI-16 - case-lambda
    7.5.14 SRFI-17 - Generalized set!
    7.5.15 SRFI-18 - Multithreading support
      7.5.15.1 SRFI-18 Threads
      7.5.15.2 SRFI-18 Mutexes
      7.5.15.3 SRFI-18 Condition variables
      7.5.15.4 SRFI-18 Time
      7.5.15.5 SRFI-18 Exceptions
    7.5.16 SRFI-19 - Time/Date Library
      7.5.16.1 SRFI-19 Introduction
      7.5.16.2 SRFI-19 Time
      7.5.16.3 SRFI-19 Date
      7.5.16.4 SRFI-19 Time/Date conversions
      7.5.16.5 SRFI-19 Date to string
      7.5.16.6 SRFI-19 String to date
    7.5.17 SRFI-23 - Error Reporting
    7.5.18 SRFI-26 - specializing parameters
    7.5.19 SRFI-27 - Sources of Random Bits
      7.5.19.1 The Default Random Source
      7.5.19.2 Random Sources
      7.5.19.3 Obtaining random number generator procedures
    7.5.20 SRFI-30 - Nested Multi-line Comments
    7.5.21 SRFI-31 - A special form `rec' for recursive evaluation
    7.5.22 SRFI-34 - Exception handling for programs
    7.5.23 SRFI-35 - Conditions
    7.5.24 SRFI-37 - args-fold
    7.5.25 SRFI-38 - External Representation for Data With Shared Structure
    7.5.26 SRFI-39 - Parameters
    7.5.27 SRFI-42 - Eager Comprehensions
    7.5.28 SRFI-45 - Primitives for Expressing Iterative Lazy Algorithms
    7.5.29 SRFI-55 - Requiring Features
    7.5.30 SRFI-60 - Integers as Bits
    7.5.31 SRFI-61 - A more general `cond' clause
    7.5.32 SRFI-67 - Compare procedures
    7.5.33 SRFI-69 - Basic hash tables
      7.5.33.1 Creating hash tables
      7.5.33.2 Accessing table items
      7.5.33.3 Table properties
      7.5.33.4 Hash table algorithms
    7.5.34 SRFI-88 Keyword Objects
    7.5.35 SRFI-98 Accessing environment variables.
  7.6 R6RS Support
    7.6.1 Incompatibilities with the R6RS
    7.6.2 R6RS Standard Libraries
      7.6.2.1 Library Usage
      7.6.2.2 rnrs base
      7.6.2.3 rnrs unicode
      7.6.2.4 rnrs bytevectors
      7.6.2.5 rnrs lists
      7.6.2.6 rnrs sorting
      7.6.2.7 rnrs control
      7.6.2.8 R6RS Records
      7.6.2.9 rnrs records syntactic
      7.6.2.10 rnrs records procedural
      7.6.2.11 rnrs records inspection
      7.6.2.12 rnrs exceptions
      7.6.2.13 rnrs conditions
      7.6.2.14 I/O Conditions
      7.6.2.15 rnrs io ports
      7.6.2.16 rnrs io simple
      7.6.2.17 rnrs files
      7.6.2.18 rnrs programs
      7.6.2.19 rnrs arithmetic fixnums
      7.6.2.20 rnrs arithmetic flonums
      7.6.2.21 rnrs arithmetic bitwise
      7.6.2.22 rnrs syntax-case
      7.6.2.23 rnrs hashtables
      7.6.2.24 rnrs enums
      7.6.2.25 rnrs
      7.6.2.26 rnrs eval
      7.6.2.27 rnrs mutable-pairs
      7.6.2.28 rnrs mutable-strings
      7.6.2.29 rnrs r5rs
  7.7 Pattern Matching
  7.8 Readline Support
    7.8.1 Loading Readline Support
    7.8.2 Readline Options
    7.8.3 Readline Functions
      7.8.3.1 Readline Port
      7.8.3.2 Completion
  7.9 Pretty Printing
  7.10 Formatted Output
  7.11 File Tree Walk
  7.12 Queues
  7.13 Streams
  7.14 Buffered Input
  7.15 Expect
  7.16 `sxml-match': Pattern Matching of SXML
    Syntax
    Matching XML Elements
    Ellipses in Patterns
    Ellipses in Quasiquote'd Output
    Matching Nodesets
    Matching the "Rest" of a Nodeset
    Matching the Unmatched Attributes
    Default Values in Attribute Patterns
    Guards in Patterns
    Catamorphisms
    Named-Catamorphisms
    `sxml-match-let' and `sxml-match-let*'
  7.17 The Scheme shell (scsh)
8 Standard Library
  8.1 (statprof)
    8.1.1 Overview
    8.1.2 Implementation notes
    8.1.3 Usage
  8.2 (sxml apply-templates)
    8.2.1 Overview
    8.2.2 Usage
  8.3 (sxml fold)
    8.3.1 Overview
    8.3.2 Usage
  8.4 (sxml simple)
    8.4.1 Overview
    8.4.2 Usage
  8.5 (sxml ssax)
    8.5.1 Overview
    8.5.2 Usage
  8.6 (sxml ssax input-parse)
    8.6.1 Overview
    8.6.2 Usage
  8.7 (sxml transform)
    8.7.1 Overview
    8.7.2 Usage
  8.8 (sxml xpath)
    8.8.1 Overview
    8.8.2 Usage
  8.9 (texinfo)
    8.9.1 Overview
    8.9.2 Usage
  8.10 (texinfo docbook)
    8.10.1 Overview
    8.10.2 Usage
  8.11 (texinfo html)
    8.11.1 Overview
    8.11.2 Usage
  8.12 (texinfo indexing)
    8.12.1 Overview
    8.12.2 Usage
  8.13 (texinfo string-utils)
    8.13.1 Overview
    8.13.2 Usage
  8.14 (texinfo plain-text)
    8.14.1 Overview
    8.14.2 Usage
  8.15 (texinfo serialize)
    8.15.1 Overview
    8.15.2 Usage
  8.16 (texinfo reflection)
    8.16.1 Overview
    8.16.2 Usage
9 GOOPS
  9.1 Copyright Notice
  9.2 Class Definition
  9.3 Instance Creation and Slot Access
  9.4 Slot Options
  9.5 Illustrating Slot Description
  9.6 Methods and Generic Functions
    9.6.1 Accessors
    9.6.2 Extending Primitives
    9.6.3 Merging Generics
    9.6.4 Next-method
    9.6.5 Generic Function and Method Examples
    9.6.6 Handling Invocation Errors
  9.7 Inheritance
    9.7.1 Class Precedence List
    9.7.2 Sorting Methods
  9.8 Introspection
    9.8.1 Classes
    9.8.2 Instances
    9.8.3 Slots
    9.8.4 Generic Functions
    9.8.5 Accessing Slots
  9.9 Error Handling
  9.10 GOOPS Object Miscellany
  9.11 The Metaobject Protocol
    9.11.1 Metaobjects and the Metaobject Protocol
    9.11.2 Metaclasses
    9.11.3 MOP Specification
    9.11.4 Instance Creation Protocol
    9.11.5 Class Definition Protocol
    9.11.6 Customizing Class Definition
    9.11.7 Method Definition
    9.11.8 Method Definition Internals
    9.11.9 Generic Function Internals
    9.11.10 Generic Function Invocation
  9.12 Redefining a Class
    9.12.1 Default Class Redefinition Behaviour
    9.12.2 Customizing Class Redefinition
  9.13 Changing the Class of an Instance
10 Guile Implementation
  10.1 A Brief History of Guile
    10.1.1 The Emacs Thesis
    10.1.2 Early Days
    10.1.3 A Scheme of Many Maintainers
    10.1.4 A Timeline of Selected Guile Releases
    10.1.5 Status, or: Your Help Needed
  10.2 Data Representation
    10.2.1 A Simple Representation
    10.2.2 Faster Integers
    10.2.3 Cheaper Pairs
    10.2.4 Conservative Garbage Collection
    10.2.5 The SCM Type in Guile
      10.2.5.1 Relationship between `SCM' and `scm_t_bits'
      10.2.5.2 Immediate objects
      10.2.5.3 Non-immediate objects
      10.2.5.4 Allocating Cells
      10.2.5.5 Heap Cell Type Information
      10.2.5.6 Accessing Cell Entries
  10.3 A Virtual Machine for Guile
    10.3.1 Why a VM?
    10.3.2 VM Concepts
    10.3.3 Stack Layout
    10.3.4 Variables and the VM
    10.3.5 Compiled Procedures are VM Programs
    10.3.6 Instruction Set
      10.3.6.1 Lexical Environment Instructions
      10.3.6.2 Top-Level Environment Instructions
      10.3.6.3 Procedure Call and Return Instructions
      10.3.6.4 Function Prologue Instructions
      10.3.6.5 Trampoline Instructions
      10.3.6.6 Branch Instructions
      10.3.6.7 Data Constructor Instructions
      10.3.6.8 Loading Instructions
      10.3.6.9 Dynamic Environment Instructions
      10.3.6.10 Miscellaneous Instructions
      10.3.6.11 Inlined Scheme Instructions
      10.3.6.12 Inlined Mathematical Instructions
      10.3.6.13 Inlined Bytevector Instructions
  10.4 Compiling to the Virtual Machine
    10.4.1 Compiler Tower
    10.4.2 The Scheme Compiler
    10.4.3 Tree-IL
    10.4.4 GLIL
    10.4.5 Assembly
    10.4.6 Bytecode and Objcode
    10.4.7 Writing New High-Level Languages
    10.4.8 Extending the Compiler
Appendix A GNU Free Documentation License
Concept Index
Procedure Index
Variable Index
Type Index
R5RS Index


Preface
*******

This manual describes how to use Guile, GNU's Ubiquitous Intelligent
Language for Extensions.  It relates particularly to Guile version
2.0.5.

Contributors to this Manual
===========================

Like Guile itself, the Guile reference manual is a living entity, cared
for by many people over a long period of time.  As such, it is hard to
identify individuals of whom to say "yes, this person, she wrote the
manual."

   Still, among the many contributions, some caretakers stand out.
First among them is Neil Jerram, who has been working on this document
for ten years now.  Neil's attention both to detail and to the big
picture have made a real difference in the understanding of a
generation of Guile hackers.

   Next we should note Marius Vollmer's effect on this document.  Marius
maintained Guile during a period in which Guile's API was
clarified--put to the fire, so to speak--and he had the good sense to
effect the same change on the manual.

   Martin Grabmueller made substantial contributions throughout the
manual in preparation for the Guile 1.6 release, including filling out
a lot of the documentation of Scheme data types, control mechanisms and
procedures.  In addition, he wrote the documentation for Guile's SRFI
modules and modules associated with the Guile REPL.

   Ludovic Courtès and Andy Wingo, the Guile maintainers at the time of
this writing (late 2010), have also made their dent in the manual,
writing documentation for new modules and subsystems in Guile 2.0.  They
are also responsible for ensuring that the existing text retains its
relevance as Guile evolves.  *Note Reporting Bugs::, for more
information on reporting problems in this manual.

   The content for the first versions of this manual incorporated and
was inspired by documents from Aubrey Jaffer, author of the SCM system
on which Guile was based, and from Tom Lord, Guile's first maintainer.
Although most of this text has been rewritten, all of it was important,
and some of the structure remains.

   The manual for the first versions of Guile were largely written,
edited, and compiled by Mark Galassi and Jim Blandy.  In particular,
Jim wrote the original tutorial on Guile's data representation and the
C API for accessing Guile objects.

   Significant portions were also contributed by Thien-Thi Nguyen, Kevin
Ryde, Mikael Djurfeldt, Christian Lynbech, Julian Graham, Gary Houston,
Tim Pierce, and a few dozen more.  You, reader, are most welcome to join
their esteemed ranks.  Visit Guile's web site at
`http://www.gnu.org/software/guile/' to find out how to get involved.

The Guile License
=================

Guile is Free Software.  Guile is copyrighted, not public domain, and
there are restrictions on its distribution or redistribution, but these
restrictions are designed to permit everything a cooperating person
would want to do.

   * The Guile library (libguile) and supporting files are published
     under the terms of the GNU Lesser General Public License version 3
     or later.  See the files `COPYING.LESSER' and `COPYING'.

   * The Guile readline module is published under the terms of the GNU
     General Public License version 3 or later.  See the file `COPYING'.

   * The manual you're now reading is published under the terms of the
     GNU Free Documentation License (*note GNU Free Documentation
     License::).

   C code linking to the Guile library is subject to terms of that
library.  Basically such code may be published on any terms, provided
users can re-link against a new or modified version of Guile.

   C code linking to the Guile readline module is subject to the terms
of that module.  Basically such code must be published on Free terms.

   Scheme level code written to be run by Guile (but not derived from
Guile itself) is not restricted in any way, and may be published on any
terms.  We encourage authors to publish on Free terms.

   You must be aware there is no warranty whatsoever for Guile.  This is
described in full in the licenses.

1 Introduction
**************

Guile is an implementation of the Scheme programming language.  Scheme
(`http://schemers.org/') is an elegant and conceptually simple dialect
of Lisp, originated by Guy Steele and Gerald Sussman, and since evolved
by the series of reports known as RnRS (the Revised^n Reports on
Scheme).

   Unlike, for example, Python or Perl, Scheme has no benevolent
dictator.  There are many Scheme implementations, with different
characteristics and with communities and academic activities around
them, and the language develops as a result of the interplay between
these.  Guile's particular characteristics are that

   * it is easy to combine with other code written in C

   * it has a historical and continuing connection with the GNU Project

   * it emphasizes interactive and incremental programming

   * it actually supports several languages, not just Scheme.

The next few sections explain what we mean by these points.  The
sections after that cover how you can obtain and install Guile, and the
typographical conventions that we use in this manual.

1.1 Guile and Scheme
====================

Guile implements Scheme as described in the Revised^5 Report on the
Algorithmic Language Scheme (usually known as R5RS), providing clean
and general data and control structures.  Guile goes beyond the rather
austere language presented in R5RS, extending it with a module system,
full access to POSIX system calls, networking support, multiple threads,
dynamic linking, a foreign function call interface, powerful string
processing, and many other features needed for programming in the real
world.

   The Scheme community has recently agreed and published R6RS, the
latest installment in the RnRS series.  R6RS significantly expands the
core Scheme language, and standardises many non-core functions that
implementations--including Guile--have previously done in different
ways.  Guile has been updated to incorporate some of the features of
R6RS, and to adjust some existing features to conform to the R6RS
specification, but it is by no means a complete R6RS implementation.
*Note R6RS Support::.

   Between R5RS and R6RS, the SRFI process (`http://srfi.schemers.org/')
standardised interfaces for many practical needs, such as multithreaded
programming and multidimensional arrays.  Guile supports many SRFIs, as
documented in detail in *note SRFI Support::.

   In summary, so far as relationship to the Scheme standards is
concerned, Guile is an R5RS implementation with many extensions, some
of which conform to SRFIs or to the relevant parts of R6RS.

1.2 Combining with C Code
=========================

Like a shell, Guile can run interactively--reading expressions from the
user, evaluating them, and displaying the results--or as a script
interpreter, reading and executing Scheme code from a file.  Guile also
provides an object library, "libguile", that allows other applications
to easily incorporate a complete Scheme interpreter.  An application
can then use Guile as an extension language, a clean and powerful
configuration language, or as multi-purpose "glue", connecting
primitives provided by the application.  It is easy to call Scheme code
from C code and vice versa, giving the application designer full
control of how and when to invoke the interpreter.  Applications can
add new functions, data types, control structures, and even syntax to
Guile, creating a domain-specific language tailored to the task at
hand, but based on a robust language design.

   This kind of combination is helped by four aspects of Guile's design
and history.  First is that Guile has always been targeted as an
extension language.  Hence its C API has always been of great
importance, and has been developed accordingly.  Second and third are
rather technical points--that Guile uses conservative garbage
collection, and that it implements the Scheme concept of continuations
by copying and reinstating the C stack--but whose practical consequence
is that most existing C code can be glued into Guile as is, without
needing modifications to cope with strange Scheme execution flows.
Last is the module system, which helps extensions to coexist without
stepping on each others' toes.

   Guile's module system allows one to break up a large program into
manageable sections with well-defined interfaces between them.  Modules
may contain a mixture of interpreted and compiled code; Guile can use
either static or dynamic linking to incorporate compiled code.  Modules
also encourage developers to package up useful collections of routines
for general distribution; as of this writing, one can find Emacs
interfaces, database access routines, compilers, GUI toolkit
interfaces, and HTTP client functions, among others.

1.3 Guile and the GNU Project
=============================

Guile was conceived by the GNU Project following the fantastic success
of Emacs Lisp as an extension language within Emacs.  Just as Emacs
Lisp allowed complete and unanticipated applications to be written
within the Emacs environment, the idea was that Guile should do the
same for other GNU Project applications.  This remains true today.

   The idea of extensibility is closely related to the GNU project's
primary goal, that of promoting software freedom.  Software freedom
means that people receiving a software package can modify or enhance it
to their own desires, including in ways that may not have occurred at
all to the software's original developers.  For programs written in a
compiled language like C, this freedom covers modifying and rebuilding
the C code; but if the program also provides an extension language,
that is usually a much friendlier and lower-barrier-of-entry way for
the user to start making their own changes.

   Guile is now used by GNU project applications such as AutoGen,
Lilypond, Denemo, Mailutils, TeXmacs and Gnucash, and we hope that
there will be many more in future.

1.4 Interactive Programming
===========================

Non-free software has no interest in its users being able to see how it
works.  They are supposed to just accept it, or to report problems and
hope that the source code owners will choose to work on them.

   Free software aims to work reliably just as much as non-free
software does, but it should also empower its users by making its
workings available.  This is useful for many reasons, including
education, auditing and enhancements, as well as for debugging problems.

   The ideal free software system achieves this by making it easy for
interested users to see the source code for a feature that they are
using, and to follow through that source code step-by-step, as it runs.
In Emacs, good examples of this are the source code hyperlinks in the
help system, and `edebug'.  Then, for bonus points and maximising the
ability for the user to experiment quickly with code changes, the
system should allow parts of the source code to be modified and
reloaded into the running program, to take immediate effect.

   Guile is designed for this kind of interactive programming, and this
distinguishes it from many Scheme implementations that instead
prioritise running a fixed Scheme program as fast as possible--because
there are tradeoffs between performance and the ability to modify parts
of an already running program.  There are faster Schemes than Guile,
but Guile is a GNU project and so prioritises the GNU vision of
programming freedom and experimentation.

1.5 Supporting Multiple Languages
=================================

Since the 2.0 release, Guile's architecture supports compiling any
language to its core virtual machine bytecode, and Scheme is just one
of the supported languages.  Other supported languages are Emacs Lisp,
ECMAScript (commonly known as Javascript) and Brainfuck, and work is
under discussion for Lua, Ruby and Python.

   This means that users can program applications which use Guile in
the language of their choice, rather than having the tastes of the
application's author imposed on them.

1.6 Obtaining and Installing Guile
==================================

Guile can be obtained from the main GNU archive site
`ftp://ftp.gnu.org' or any of its mirrors.  The file will be named
guile-VERSION.tar.gz.  The current version is 2.0.5, so the file you
should grab is:

   `ftp://ftp.gnu.org/gnu/guile/guile-2.0.5.tar.gz'

   To unbundle Guile use the instruction

     zcat guile-2.0.5.tar.gz | tar xvf -

which will create a directory called `guile-2.0.5' with all the
sources.  You can look at the file `INSTALL' for detailed instructions
on how to build and install Guile, but you should be able to just do

     cd guile-2.0.5
     ./configure
     make
     make install

   This will install the Guile executable `guile', the Guile library
`libguile' and various associated header files and support libraries. It
will also install the Guile reference manual.

   Since this manual frequently refers to the Scheme "standard", also
known as R5RS, or the "Revised^5 Report on the Algorithmic Language
Scheme", we have included the report in the Guile distribution; see
*note Introduction: (r5rs)Top.  This will also be installed in your
info directory.

1.7 Organisation of this Manual
===============================

The rest of this manual is organised into the following chapters.

*Chapter 2: Hello Guile!*
     A whirlwind tour shows how Guile can be used interactively and as
     a script interpreter, how to link Guile into your own applications,
     and how to write modules of interpreted and compiled code for use
     with Guile.  Everything introduced here is documented again and in
     full by the later parts of the manual.

*Chapter 3: Hello Scheme!*
     For readers new to Scheme, this chapter provides an introduction
     to the basic ideas of the Scheme language.  This material would
     apply to any Scheme implementation and so does not make reference
     to anything Guile-specific.

*Chapter 4: Programming in Scheme*
     Provides an overview of programming in Scheme with Guile.  It
     covers how to invoke the `guile' program from the command-line and
     how to write scripts in Scheme.  It also introduces the extensions
     that Guile offers beyond standard Scheme.

*Chapter 5: Programming in C*
     Provides an overview of how to use Guile in a C program.  It
     discusses the fundamental concepts that you need to understand to
     access the features of Guile, such as dynamic types and the garbage
     collector.  It explains in a tutorial like manner how to define new
     data types and functions for the use by Scheme programs.

*Chapter 6: Guile API Reference*
     This part of the manual documents the Guile API in
     functionality-based groups with the Scheme and C interfaces
     presented side by side.

*Chapter 7: Guile Modules*
     Describes some important modules, distributed as part of the Guile
     distribution, that extend the functionality provided by the Guile
     Scheme core.

*Chapter 8: GOOPS*
     Describes GOOPS, an object oriented extension to Guile that
     provides classes, multiple inheritance and generic functions.


1.8 Typographical Conventions
=============================

We use some conventions in this manual.

   * For some procedures, notably type predicates, we use "iff" to mean
     "if and only if".  The construct is usually something like: `Return
     VAL iff CONDITION', where VAL is usually "#t" or "non-#f".  This
     typically means that VAL is returned if CONDITION holds, and that
     `#f' is returned otherwise.  To clarify: VAL will *only* be
     returned when CONDITION is true.  

   * In examples and procedure descriptions and all other places where
     the evaluation of Scheme expression is shown, we use some notation
     for denoting the output and evaluation results of expressions.

     The symbol `=>' is used to tell which value is returned by an
     evaluation:

          (+ 1 2)
          => 3

     Some procedures produce some output besides returning a value.
     This is denoted by the symbol `-|'.

          (begin (display 1) (newline) 'hooray)
          -| 1
          => hooray

     As you can see, this code prints `1' (denoted by `-|'), and
     returns `hooray' (denoted by `=>').


2 Hello Guile!
**************

This chapter presents a quick tour of all the ways that Guile can be
used.  There are additional examples in the `examples/' directory in
the Guile source distribution.  It also explains how best to report any
problems that you find.

   The following examples assume that Guile has been installed in
`/usr/local/'.

2.1 Running Guile Interactively
===============================

In its simplest form, Guile acts as an interactive interpreter for the
Scheme programming language, reading and evaluating Scheme expressions
the user enters from the terminal.  Here is a sample interaction between
Guile and a user; the user's input appears after the `$' and
`scheme@(guile-user)>' prompts:

     $ guile
     scheme@(guile-user)> (+ 1 2 3)                ; add some numbers
     $1 = 6
     scheme@(guile-user)> (define (factorial n)    ; define a function
                            (if (zero? n) 1 (* n (factorial (- n 1)))))
     scheme@(guile-user)> (factorial 20)
     $2 = 2432902008176640000
     scheme@(guile-user)> (getpwnam "root")        ; look in /etc/passwd
     $3 = #("root" "x" 0 0 "root" "/root" "/bin/bash")
     scheme@(guile-user)> C-d
     $

2.2 Running Guile Scripts
=========================

Like AWK, Perl, or any shell, Guile can interpret script files.  A Guile
script is simply a file of Scheme code with some extra information at
the beginning which tells the operating system how to invoke Guile, and
then tells Guile how to handle the Scheme code.

   Here is a trivial Guile script.  *Note Guile Scripting::, for more
details.

     #!/usr/local/bin/guile -s
     !#
     (display "Hello, world!")
     (newline)

2.3 Linking Guile into Programs
===============================

The Guile interpreter is available as an object library, to be linked
into applications using Scheme as a configuration or extension language.

   Here is `simple-guile.c', source code for a program that will
produce a complete Guile interpreter.  In addition to all usual
functions provided by Guile, it will also offer the function
`my-hostname'.

     #include <stdlib.h>
     #include <libguile.h>

     static SCM
     my_hostname (void)
     {
       char *s = getenv ("HOSTNAME");
       if (s == NULL)
         return SCM_BOOL_F;
       else
         return scm_from_locale_string (s);
     }

     static void
     inner_main (void *data, int argc, char **argv)
     {
       scm_c_define_gsubr ("my-hostname", 0, 0, 0, my_hostname);
       scm_shell (argc, argv);
     }

     int
     main (int argc, char **argv)
     {
       scm_boot_guile (argc, argv, inner_main, 0);
       return 0; /* never reached */
     }

   When Guile is correctly installed on your system, the above program
can be compiled and linked like this:

     $ gcc -o simple-guile simple-guile.c \
         `pkg-config --cflags --libs guile-2.0`

   When it is run, it behaves just like the `guile' program except that
you can also call the new `my-hostname' function.

     $ ./simple-guile
     scheme@(guile-user)> (+ 1 2 3)
     $1 = 6
     scheme@(guile-user)> (my-hostname)
     "burns"

2.4 Writing Guile Extensions
============================

You can link Guile into your program and make Scheme available to the
users of your program.  You can also link your library into Guile and
make its functionality available to all users of Guile.

   A library that is linked into Guile is called an "extension", but it
really just is an ordinary object library.

   The following example shows how to write a simple extension for Guile
that makes the `j0' function available to Scheme code.

     #include <math.h>
     #include <libguile.h>

     SCM
     j0_wrapper (SCM x)
     {
       return scm_make_real (j0 (scm_num2dbl (x, "j0")));
     }

     void
     init_bessel ()
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
     }

   This C source file needs to be compiled into a shared library.  Here
is how to do it on GNU/Linux:

     gcc `pkg-config --cflags guile-2.0` \
       -shared -o libguile-bessel.so -fPIC bessel.c

   For creating shared libraries portably, we recommend the use of GNU
Libtool (*note Introduction: (libtool)Top.).

   A shared library can be loaded into a running Guile process with the
function `load-extension'.  The `j0' is then immediately available:

     $ guile
     scheme@(guile-user)> (load-extension "./libguile-bessel" "init_bessel")
     scheme@(guile-user)> (j0 2)
     $1 = 0.223890779141236

   For more on how to install your extension, *note Installing Site
Packages::.

2.5 Using the Guile Module System
=================================

Guile has support for dividing a program into "modules".  By using
modules, you can group related code together and manage the composition
of complete programs from largely independent parts.

   For more details on the module system beyond this introductory
material, *Note Modules::.

2.5.1 Using Modules
-------------------

Guile comes with a lot of useful modules, for example for string
processing or command line parsing.  Additionally, there exist many
Guile modules written by other Guile hackers, but which have to be
installed manually.

   Here is a sample interactive session that shows how to use the
`(ice-9 popen)' module which provides the means for communicating with
other processes over pipes together with the `(ice-9 rdelim)' module
that provides the function `read-line'.

     $ guile
     scheme@(guile-user)> (use-modules (ice-9 popen))
     scheme@(guile-user)> (use-modules (ice-9 rdelim))
     scheme@(guile-user)> (define p (open-input-pipe "ls -l"))
     scheme@(guile-user)> (read-line p)
     $1 = "total 30"
     scheme@(guile-user)> (read-line p)
     $2 = "drwxr-sr-x    2 mgrabmue mgrabmue     1024 Mar 29 19:57 CVS"

2.5.2 Writing new Modules
-------------------------

You can create new modules using the syntactic form `define-module'.
All definitions following this form until the next `define-module' are
placed into the new module.

   One module is usually placed into one file, and that file is
installed in a location where Guile can automatically find it.  The
following session shows a simple example.

     $ cat /usr/local/share/guile/site/foo/bar.scm

     (define-module (foo bar)
       #:export (frob))

     (define (frob x) (* 2 x))

     $ guile
     scheme@(guile-user)> (use-modules (foo bar))
     scheme@(guile-user)> (frob 12)
     $1 = 24

   For more on how to install your module, *note Installing Site
Packages::.

2.5.3 Putting Extensions into Modules
-------------------------------------

In addition to Scheme code you can also put things that are defined in
C into a module.

   You do this by writing a small Scheme file that defines the module
and call `load-extension' directly in the body of the module.

     $ cat /usr/local/share/guile/site/math/bessel.scm

     (define-module (math bessel)
       #:export (j0))

     (load-extension "libguile-bessel" "init_bessel")

     $ file /usr/local/lib/guile/2.0/extensions/libguile-bessel.so
     ... ELF 32-bit LSB shared object ...
     $ guile
     scheme@(guile-user)> (use-modules (math bessel))
     scheme@(guile-user)> (j0 2)
     $1 = 0.223890779141236

   *Note Modules and Extensions::, for more information.

2.6 Reporting Bugs
==================

Any problems with the installation should be reported to
<bug-guile@gnu.org>.

   If you find a bug in Guile, please report it to the Guile
developers, so they can fix it.  They may also be able to suggest
workarounds when it is not possible for you to apply the bug-fix or
install a new version of Guile yourself.

   Before sending in bug reports, please check with the following list
that you really have found a bug.

   * Whenever documentation and actual behavior differ, you have
     certainly found a bug, either in the documentation or in the
     program.

   * When Guile crashes, it is a bug.

   * When Guile hangs or takes forever to complete a task, it is a bug.

   * When calculations produce wrong results, it is a bug.

   * When Guile signals an error for valid Scheme programs, it is a bug.

   * When Guile does not signal an error for invalid Scheme programs,
     it may be a bug, unless this is explicitly documented.

   * When some part of the documentation is not clear and does not make
     sense to you even after re-reading the section, it is a bug.

   Before reporting the bug, check whether any programs you have loaded
into Guile, including your `.guile' file, set any variables that may
affect the functioning of Guile.  Also, see whether the problem happens
in a freshly started Guile without loading your `.guile' file (start
Guile with the `-q' switch to prevent loading the init file).  If the
problem does _not_ occur then, you must report the precise contents of
any programs that you must load into Guile in order to cause the
problem to occur.

   When you write a bug report, please make sure to include as much of
the information described below in the report.  If you can't figure out
some of the items, it is not a problem, but the more information we
get, the more likely we can diagnose and fix the bug.

   * The version number of Guile. You can get this information from
     invoking `guile --version' at your shell, or calling `(version)'
     from within Guile.

   * Your machine type, as determined by the `config.guess' shell
     script. If you have a Guile checkout, this file is located in
     `build-aux'; otherwise you can fetch the latest version from
     `http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD'.

          $ build-aux/config.guess
          x86_64-unknown-linux-gnu

   * If you installed Guile from a binary package, the version of that
     package. On systems that use RPM, use `rpm -qa | grep guile'. On
     systems that use DPKG, `dpkg -l | grep guile'.

   * If you built Guile yourself, the build configuration that you used:

          $ ./config.status --config
          '--enable-error-on-warning' '--disable-deprecated'...

   * A complete description of how to reproduce the bug.

     If you have a Scheme program that produces the bug, please include
     it in the bug report. If your program is too big to include.
     please try to reduce your code to a minimal test case.

     If you can reproduce your problem at the REPL, that is best. Give a
     transcript of the expressions you typed at the REPL.

   * A description of the incorrect behavior.  For example, "The Guile
     process gets a fatal signal," or, "The resulting output is as
     follows, which I think is wrong."

     If the manifestation of the bug is a Guile error message, it is
     important to report the precise text of the error message, and a
     backtrace showing how the Scheme program arrived at the error.
     This can be done using the `,backtrace' command in Guile's
     debugger.

   If your bug causes Guile to crash, additional information from a
low-level debugger such as GDB might be helpful. If you have built Guile
yourself, you can run Guile under GDB via the
`meta/gdb-uninstalled-guile' script. Instead of invoking Guile as
usual, invoke the wrapper script, type `run' to start the process, then
`backtrace' when the crash comes. Include that backtrace in your report.

3 Hello Scheme!
***************

In this chapter, we introduce the basic concepts that underpin the
elegance and power of the Scheme language.

   Readers who already possess a background knowledge of Scheme may
happily skip this chapter.  For the reader who is new to the language,
however, the following discussions on data, procedures, expressions and
closure are designed to provide a minimum level of Scheme understanding
that is more or less assumed by the chapters that follow.

   The style of this introductory material aims about halfway between
the terse precision of R5RS and the discursiveness of existing Scheme
tutorials.  For pointers to useful Scheme resources on the web, please
see *note Further Reading::.

3.1 Data Types, Values and Variables
====================================

This section discusses the representation of data types and values, what
it means for Scheme to be a "latently typed" language, and the role of
variables.  We conclude by introducing the Scheme syntaxes for defining
a new variable, and for changing the value of an existing variable.

3.1.1 Latent Typing
-------------------

The term "latent typing" is used to describe a computer language, such
as Scheme, for which you cannot, _in general_, simply look at a
program's source code and determine what type of data will be
associated with a particular variable, or with the result of a
particular expression.

   Sometimes, of course, you _can_ tell from the code what the type of
an expression will be.  If you have a line in your program that sets the
variable `x' to the numeric value 1, you can be certain that,
immediately after that line has executed (and in the absence of multiple
threads), `x' has the numeric value 1.  Or if you write a procedure
that is designed to concatenate two strings, it is likely that the rest
of your application will always invoke this procedure with two string
parameters, and quite probable that the procedure would go wrong in some
way if it was ever invoked with parameters that were not both strings.

   Nevertheless, the point is that there is nothing in Scheme which
requires the procedure parameters always to be strings, or `x' always
to hold a numeric value, and there is no way of declaring in your
program that such constraints should always be obeyed.  In the same
vein, there is no way to declare the expected type of a procedure's
return value.

   Instead, the types of variables and expressions are only known - in
general - at run time.  If you _need_ to check at some point that a
value has the expected type, Scheme provides run time procedures that
you can invoke to do so.  But equally, it can be perfectly valid for two
separate invocations of the same procedure to specify arguments with
different types, and to return values with different types.

   The next subsection explains what this means in practice, for the
ways that Scheme programs use data types, values and variables.

3.1.2 Values and Variables
--------------------------

Scheme provides many data types that you can use to represent your data.
Primitive types include characters, strings, numbers and procedures.
Compound types, which allow a group of primitive and compound values to
be stored together, include lists, pairs, vectors and multi-dimensional
arrays.  In addition, Guile allows applications to define their own data
types, with the same status as the built-in standard Scheme types.

   As a Scheme program runs, values of all types pop in and out of
existence.  Sometimes values are stored in variables, but more commonly
they pass seamlessly from being the result of one computation to being
one of the parameters for the next.

   Consider an example.  A string value is created because the
interpreter reads in a literal string from your program's source code.
Then a numeric value is created as the result of calculating the length
of the string.  A second numeric value is created by doubling the
calculated length.  Finally the program creates a list with two
elements - the doubled length and the original string itself - and
stores this list in a program variable.

   All of the values involved here - in fact, all values in Scheme -
carry their type with them.  In other words, every value "knows," at
runtime, what kind of value it is.  A number, a string, a list,
whatever.

   A variable, on the other hand, has no fixed type.  A variable - `x',
say - is simply the name of a location - a box - in which you can store
any kind of Scheme value.  So the same variable in a program may hold a
number at one moment, a list of procedures the next, and later a pair
of strings.  The "type" of a variable - insofar as the idea is
meaningful at all - is simply the type of whatever value the variable
happens to be storing at a particular moment.

3.1.3 Defining and Setting Variables
------------------------------------

To define a new variable, you use Scheme's `define' syntax like this:

     (define VARIABLE-NAME VALUE)

   This makes a new variable called VARIABLE-NAME and stores VALUE in
it as the variable's initial value.  For example:

     ;; Make a variable `x' with initial numeric value 1.
     (define x 1)

     ;; Make a variable `organization' with an initial string value.
     (define organization "Free Software Foundation")

   (In Scheme, a semicolon marks the beginning of a comment that
continues until the end of the line.  So the lines beginning `;;' are
comments.)

   Changing the value of an already existing variable is very similar,
except that `define' is replaced by the Scheme syntax `set!', like this:

     (set! VARIABLE-NAME NEW-VALUE)

   Remember that variables do not have fixed types, so NEW-VALUE may
have a completely different type from whatever was previously stored in
the location named by VARIABLE-NAME.  Both of the following examples
are therefore correct.

     ;; Change the value of `x' to 5.
     (set! x 5)

     ;; Change the value of `organization' to the FSF's street number.
     (set! organization 545)

   In these examples, VALUE and NEW-VALUE are literal numeric or string
values.  In general, however, VALUE and NEW-VALUE can be any Scheme
expression.  Even though we have not yet covered the forms that Scheme
expressions can take (*note About Expressions::), you can probably
guess what the following `set!' example does...

     (set! x (+ x 1))

   (Note: this is not a complete description of `define' and `set!',
because we need to introduce some other aspects of Scheme before the
missing pieces can be filled in.  If, however, you are already familiar
with the structure of Scheme, you may like to read about those missing
pieces immediately by jumping ahead to the following references.

   * *note Lambda Alternatives::, to read about an alternative form of
     the `define' syntax that can be used when defining new procedures.

   * *note Procedures with Setters::, to read about an alternative form
     of the `set!' syntax that helps with changing a single value in
     the depths of a compound data structure.)

   * *Note Internal Definitions::, to read about using `define' other
     than at top level in a Scheme program, including a discussion of
     when it works to use `define' rather than `set!' to change the
     value of an existing variable.

3.2 The Representation and Use of Procedures
============================================

This section introduces the basics of using and creating Scheme
procedures.  It discusses the representation of procedures as just
another kind of Scheme value, and shows how procedure invocation
expressions are constructed.  We then explain how `lambda' is used to
create new procedures, and conclude by presenting the various shorthand
forms of `define' that can be used instead of writing an explicit
`lambda' expression.

3.2.1 Procedures as Values
--------------------------

One of the great simplifications of Scheme is that a procedure is just
another type of value, and that procedure values can be passed around
and stored in variables in exactly the same way as, for example, strings
and lists.  When we talk about a built-in standard Scheme procedure such
as `open-input-file', what we actually mean is that there is a
pre-defined top level variable called `open-input-file', whose value is
a procedure that implements what R5RS says that `open-input-file'
should do.

   Note that this is quite different from many dialects of Lisp --
including Emacs Lisp -- in which a program can use the same name with
two quite separate meanings: one meaning identifies a Lisp function,
while the other meaning identifies a Lisp variable, whose value need
have nothing to do with the function that is associated with the first
meaning.  In these dialects, functions and variables are said to live in
different "namespaces".

   In Scheme, on the other hand, all names belong to a single unified
namespace, and the variables that these names identify can hold any kind
of Scheme value, including procedure values.

   One consequence of the "procedures as values" idea is that, if you
don't happen to like the standard name for a Scheme procedure, you can
change it.

   For example, `call-with-current-continuation' is a very important
standard Scheme procedure, but it also has a very long name!  So, many
programmers use the following definition to assign the same procedure
value to the more convenient name `call/cc'.

     (define call/cc call-with-current-continuation)

   Let's understand exactly how this works.  The definition creates a
new variable `call/cc', and then sets its value to the value of the
variable `call-with-current-continuation'; the latter value is a
procedure that implements the behaviour that R5RS specifies under the
name "call-with-current-continuation".  So `call/cc' ends up holding
this value as well.

   Now that `call/cc' holds the required procedure value, you could
choose to use `call-with-current-continuation' for a completely
different purpose, or just change its value so that you will get an
error if you accidentally use `call-with-current-continuation' as a
procedure in your program rather than `call/cc'.  For example:

     (set! call-with-current-continuation "Not a procedure any more!")

   Or you could just leave `call-with-current-continuation' as it was.
It's perfectly fine for more than one variable to hold the same
procedure value.

3.2.2 Simple Procedure Invocation
---------------------------------

A procedure invocation in Scheme is written like this:

     (PROCEDURE [ARG1 [ARG2 ...]])

   In this expression, PROCEDURE can be any Scheme expression whose
value is a procedure.  Most commonly, however, PROCEDURE is simply the
name of a variable whose value is a procedure.

   For example, `string-append' is a standard Scheme procedure whose
behaviour is to concatenate together all the arguments, which are
expected to be strings, that it is given.  So the expression

     (string-append "/home" "/" "andrew")

is a procedure invocation whose result is the string value
`"/home/andrew"'.

   Similarly, `string-length' is a standard Scheme procedure that
returns the length of a single string argument, so

     (string-length "abc")

is a procedure invocation whose result is the numeric value 3.

   Each of the parameters in a procedure invocation can itself be any
Scheme expression.  Since a procedure invocation is itself a type of
expression, we can put these two examples together to get

     (string-length (string-append "/home" "/" "andrew"))

-- a procedure invocation whose result is the numeric value 12.

   (You may be wondering what happens if the two examples are combined
the other way round.  If we do this, we can make a procedure invocation
expression that is _syntactically_ correct:

     (string-append "/home" (string-length "abc"))

but when this expression is executed, it will cause an error, because
the result of `(string-length "abc")' is a numeric value, and
`string-append' is not designed to accept a numeric value as one of its
arguments.)

3.2.3 Creating and Using a New Procedure
----------------------------------------

Scheme has lots of standard procedures, and Guile provides all of these
via predefined top level variables.  All of these standard procedures
are documented in the later chapters of this reference manual.

   Before very long, though, you will want to create new procedures that
encapsulate aspects of your own applications' functionality.  To do
this, you can use the famous `lambda' syntax.

   For example, the value of the following Scheme expression

     (lambda (name address) EXPRESSION ...)

is a newly created procedure that takes two arguments: `name' and
`address'.  The behaviour of the new procedure is determined by the
sequence of EXPRESSIONs in the "body" of the procedure definition.
(Typically, these EXPRESSIONs would use the arguments in some way, or
else there wouldn't be any point in giving them to the procedure.)
When invoked, the new procedure returns a value that is the value of
the last EXPRESSION in the procedure body.

   To make things more concrete, let's suppose that the two arguments
are both strings, and that the purpose of this procedure is to form a
combined string that includes these arguments.  Then the full lambda
expression might look like this:

     (lambda (name address)
       (string-append "Name=" name ":Address=" address))

   We noted in the previous subsection that the PROCEDURE part of a
procedure invocation expression can be any Scheme expression whose value
is a procedure.  But that's exactly what a lambda expression is!  So we
can use a lambda expression directly in a procedure invocation, like
this:

     ((lambda (name address)
        (string-append "Name=" name ":Address=" address))
      "FSF"
      "Cambridge")

This is a valid procedure invocation expression, and its result is the
string:

     "Name=FSF:Address=Cambridge"

   It is more common, though, to store the procedure value in a
variable --

     (define make-combined-string
       (lambda (name address)
         (string-append "Name=" name ":Address=" address)))

-- and then to use the variable name in the procedure invocation:

     (make-combined-string "FSF" "Cambridge")

Which has exactly the same result.

   It's important to note that procedures created using `lambda' have
exactly the same status as the standard built in Scheme procedures, and
can be invoked, passed around, and stored in variables in exactly the
same ways.

3.2.4 Lambda Alternatives
-------------------------

Since it is so common in Scheme programs to want to create a procedure
and then store it in a variable, there is an alternative form of the
`define' syntax that allows you to do just that.

   A `define' expression of the form

     (define (NAME [ARG1 [ARG2 ...]])
       EXPRESSION ...)

is exactly equivalent to the longer form

     (define NAME
       (lambda ([ARG1 [ARG2 ...]])
         EXPRESSION ...))

   So, for example, the definition of `make-combined-string' in the
previous subsection could equally be written:

     (define (make-combined-string name address)
       (string-append "Name=" name ":Address=" address))

   This kind of procedure definition creates a procedure that requires
exactly the expected number of arguments.  There are two further forms
of the `lambda' expression, which create a procedure that can accept a
variable number of arguments:

     (lambda (ARG1 ... . ARGS) EXPRESSION ...)

     (lambda ARGS EXPRESSION ...)

The corresponding forms of the alternative `define' syntax are:

     (define (NAME ARG1 ... . ARGS) EXPRESSION ...)

     (define (NAME . ARGS) EXPRESSION ...)

For details on how these forms work, see *Note Lambda::.

   (It could be argued that the alternative `define' forms are rather
confusing, especially for newcomers to the Scheme language, as they hide
both the role of `lambda' and the fact that procedures are values that
are stored in variables in the some way as any other kind of value.  On
the other hand, they are very convenient, and they are also a good
example of another of Scheme's powerful features: the ability to specify
arbitrary syntactic transformations at run time, which can be applied to
subsequently read input.)

3.3 Expressions and Evaluation
==============================

So far, we have met expressions that _do_ things, such as the `define'
expressions that create and initialize new variables, and we have also
talked about expressions that have _values_, for example the value of
the procedure invocation expression:

     (string-append "/home" "/" "andrew")

but we haven't yet been precise about what causes an expression like
this procedure invocation to be reduced to its "value", or how the
processing of such expressions relates to the execution of a Scheme
program as a whole.

   This section clarifies what we mean by an expression's value, by
introducing the idea of "evaluation".  It discusses the side effects
that evaluation can have, explains how each of the various types of
Scheme expression is evaluated, and describes the behaviour and use of
the Guile REPL as a mechanism for exploring evaluation.  The section
concludes with a very brief summary of Scheme's common syntactic
expressions.

3.3.1 Evaluating Expressions and Executing Programs
---------------------------------------------------

In Scheme, the process of executing an expression is known as
"evaluation".  Evaluation has two kinds of result:

   * the "value" of the evaluated expression

   * the "side effects" of the evaluation, which consist of any effects
     of evaluating the expression that are not represented by the value.

   Of the expressions that we have met so far, `define' and `set!'
expressions have side effects -- the creation or modification of a
variable -- but no value; `lambda' expressions have values -- the newly
constructed procedures -- but no side effects; and procedure invocation
expressions, in general, have either values, or side effects, or both.

   It is tempting to try to define more intuitively what we mean by
"value" and "side effects", and what the difference between them is.
In general, though, this is extremely difficult.  It is also
unnecessary; instead, we can quite happily define the behaviour of a
Scheme program by specifying how Scheme executes a program as a whole,
and then by describing the value and side effects of evaluation for each
type of expression individually.

So, some(1) definitions...

   * A Scheme program consists of a sequence of expressions.

   * A Scheme interpreter executes the program by evaluating these
     expressions in order, one by one.

   * An expression can be

        * a piece of literal data, such as a number `2.3' or a string
          `"Hello world!"'

        * a variable name

        * a procedure invocation expression

        * one of Scheme's special syntactic expressions.

The following subsections describe how each of these types of expression
is evaluated.

   ---------- Footnotes ----------

   (1) These definitions are approximate.  For the whole and detailed
truth, see *note R5RS syntax: (r5rs)Formal syntax and semantics.

3.3.1.1 Evaluating Literal Data
...............................

When a literal data expression is evaluated, the value of the expression
is simply the value that the expression describes.  The evaluation of a
literal data expression has no side effects.

So, for example,

   * the value of the expression `"abc"' is the string value `"abc"'

   * the value of the expression `3+4i' is the complex number 3 + 4i

   * the value of the expression `#(1 2 3)' is a three-element vector
     containing the numeric values 1, 2 and 3.

   For any data type which can be expressed literally like this, the
syntax of the literal data expression for that data type -- in other
words, what you need to write in your code to indicate a literal value
of that type -- is known as the data type's "read syntax".  This manual
specifies the read syntax for each such data type in the section that
describes that data type.

   Some data types do not have a read syntax.  Procedures, for example,
cannot be expressed as literal data; they must be created using a
`lambda' expression (*note Creating a Procedure::) or implicitly using
the shorthand form of `define' (*note Lambda Alternatives::).

3.3.1.2 Evaluating a Variable Reference
.......................................

When an expression that consists simply of a variable name is evaluated,
the value of the expression is the value of the named variable.  The
evaluation of a variable reference expression has no side effects.

   So, after

     (define key "Paul Evans")

the value of the expression `key' is the string value `"Paul Evans"'.
If KEY is then modified by

     (set! key 3.74)

the value of the expression `key' is the numeric value 3.74.

   If there is no variable with the specified name, evaluation of the
variable reference expression signals an error.

3.3.1.3 Evaluating a Procedure Invocation Expression
....................................................

This is where evaluation starts getting interesting!  As already noted,
a procedure invocation expression has the form

     (PROCEDURE [ARG1 [ARG2 ...]])

where PROCEDURE must be an expression whose value, when evaluated, is a
procedure.

   The evaluation of a procedure invocation expression like this
proceeds by

   * evaluating individually the expressions PROCEDURE, ARG1, ARG2, and
     so on

   * calling the procedure that is the value of the PROCEDURE
     expression with the list of values obtained from the evaluations of
     ARG1, ARG2 etc. as its parameters.

   For a procedure defined in Scheme, "calling the procedure with the
list of values as its parameters" means binding the values to the
procedure's formal parameters and then evaluating the sequence of
expressions that make up the body of the procedure definition.  The
value of the procedure invocation expression is the value of the last
evaluated expression in the procedure body.  The side effects of calling
the procedure are the combination of the side effects of the sequence of
evaluations of expressions in the procedure body.

   For a built-in procedure, the value and side-effects of calling the
procedure are best described by that procedure's documentation.

   Note that the complete side effects of evaluating a procedure
invocation expression consist not only of the side effects of the
procedure call, but also of any side effects of the preceding
evaluation of the expressions PROCEDURE, ARG1, ARG2, and so on.

   To illustrate this, let's look again at the procedure invocation
expression:

     (string-length (string-append "/home" "/" "andrew"))

   In the outermost expression, PROCEDURE is `string-length' and ARG1
is `(string-append "/home" "/" "andrew")'.

   * Evaluation of `string-length', which is a variable, gives a
     procedure value that implements the expected behaviour for
     "string-length".

   * Evaluation of `(string-append "/home" "/" "andrew")', which is
     another procedure invocation expression, means evaluating each of

        * `string-append', which gives a procedure value that
          implements the expected behaviour for "string-append"

        * `"/home"', which gives the string value `"/home"'

        * `"/"', which gives the string value `"/"'

        * `"andrew"', which gives the string value `"andrew"'

     and then invoking the procedure value with this list of string
     values as its arguments.  The resulting value is a single string
     value that is the concatenation of all the arguments, namely
     `"/home/andrew"'.

   In the evaluation of the outermost expression, the interpreter can
now invoke the procedure value obtained from PROCEDURE with the value
obtained from ARG1 as its arguments.  The resulting value is a numeric
value that is the length of the argument string, which is 12.

3.3.1.4 Evaluating Special Syntactic Expressions
................................................

When a procedure invocation expression is evaluated, the procedure and
_all_ the argument expressions must be evaluated before the procedure
can be invoked.  Special syntactic expressions are special because they
are able to manipulate their arguments in an unevaluated form, and can
choose whether to evaluate any or all of the argument expressions.

   Why is this needed?  Consider a program fragment that asks the user
whether or not to delete a file, and then deletes the file if the user
answers yes.

     (if (string=? (read-answer "Should I delete this file?")
                   "yes")
         (delete-file file))

   If the outermost `(if ...)' expression here was a procedure
invocation expression, the expression `(delete-file file)', whose side
effect is to actually delete a file, would already have been evaluated
before the `if' procedure even got invoked!  Clearly this is no use --
the whole point of an `if' expression is that the "consequent"
expression is only evaluated if the condition of the `if' expression is
"true".

   Therefore `if' must be special syntax, not a procedure.  Other
special syntaxes that we have already met are `define', `set!' and
`lambda'.  `define' and `set!' are syntax because they need to know the
variable _name_ that is given as the first argument in a `define' or
`set!' expression, not that variable's value.  `lambda' is syntax
because it does not immediately evaluate the expressions that define
the procedure body; instead it creates a procedure object that
incorporates these expressions so that they can be evaluated in the
future, when that procedure is invoked.

   The rules for evaluating each special syntactic expression are
specified individually for each special syntax.  For a summary of
standard special syntax, see *Note Syntax Summary::.

3.3.2 Tail calls
----------------

Scheme is "properly tail recursive", meaning that tail calls or
recursions from certain contexts do not consume stack space or other
resources and can therefore be used on arbitrarily large data or for an
arbitrarily long calculation.  Consider for example,

     (define (foo n)
       (display n)
       (newline)
       (foo (1+ n)))

     (foo 1)
     -|
     1
     2
     3
     ...

   `foo' prints numbers infinitely, starting from the given N.  It's
implemented by printing N then recursing to itself to print N+1 and so
on.  This recursion is a tail call, it's the last thing done, and in
Scheme such tail calls can be made without limit.

   Or consider a case where a value is returned, a version of the SRFI-1
`last' function (*note SRFI-1 Selectors::) returning the last element
of a list,

     (define (my-last lst)
       (if (null? (cdr lst))
           (car lst)
           (my-last (cdr lst))))

     (my-last '(1 2 3)) => 3

   If the list has more than one element, `my-last' applies itself to
the `cdr'.  This recursion is a tail call, there's no code after it,
and the return value is the return value from that call.  In Scheme
this can be used on an arbitrarily long list argument.


   A proper tail call is only available from certain contexts, namely
the following special form positions,

   * `and' -- last expression

   * `begin' -- last expression

   * `case' -- last expression in each clause

   * `cond' -- last expression in each clause, and the call to a `=>'
     procedure is a tail call

   * `do' -- last result expression

   * `if' -- "true" and "false" leg expressions

   * `lambda' -- last expression in body

   * `let', `let*', `letrec', `let-syntax', `letrec-syntax' -- last
     expression in body

   * `or' -- last expression

The following core functions make tail calls,

   * `apply' -- tail call to given procedure

   * `call-with-current-continuation' -- tail call to the procedure
     receiving the new continuation

   * `call-with-values' -- tail call to the values-receiving procedure

   * `eval' -- tail call to evaluate the form

   * `string-any', `string-every' -- tail call to predicate on the last
     character (if that point is reached)


   The above are just core functions and special forms.  Tail calls in
other modules are described with the relevant documentation, for
example SRFI-1 `any' and `every' (*note SRFI-1 Searching::).

   It will be noted there are a lot of places which could potentially be
tail calls, for instance the last call in a `for-each', but only those
explicitly described are guaranteed.

3.3.3 Using the Guile REPL
--------------------------

If you start Guile without specifying a particular program for it to
execute, Guile enters its standard Read Evaluate Print Loop -- or
"REPL" for short.  In this mode, Guile repeatedly reads in the next
Scheme expression that the user types, evaluates it, and prints the
resulting value.

   The REPL is a useful mechanism for exploring the evaluation behaviour
described in the previous subsection.  If you type `string-append', for
example, the REPL replies `#<primitive-procedure string-append>',
illustrating the relationship between the variable `string-append' and
the procedure value stored in that variable.

   In this manual, the notation => is used to mean "evaluates to".
Wherever you see an example of the form

     EXPRESSION
     =>
     RESULT

feel free to try it out yourself by typing EXPRESSION into the REPL and
checking that it gives the expected RESULT.

3.3.4 Summary of Common Syntax
------------------------------

This subsection lists the most commonly used Scheme syntactic
expressions, simply so that you will recognize common special syntax
when you see it.  For a full description of each of these syntaxes,
follow the appropriate reference.

   `lambda' (*note Lambda::) is used to construct procedure objects.

   `define' (*note Top Level::) is used to create a new variable and
set its initial value.

   `set!' (*note Top Level::) is used to modify an existing variable's
value.

   `let', `let*' and `letrec' (*note Local Bindings::) create an inner
lexical environment for the evaluation of a sequence of expressions, in
which a specified set of local variables is bound to the values of a
corresponding set of expressions.  For an introduction to environments,
see *Note About Closure::.

   `begin' (*note begin::) executes a sequence of expressions in order
and returns the value of the last expression.  Note that this is not the
same as a procedure which returns its last argument, because the
evaluation of a procedure invocation expression does not guarantee to
evaluate the arguments in order.

   `if' and `cond' (*note Conditionals::) provide conditional
evaluation of argument expressions depending on whether one or more
conditions evaluate to "true" or "false".

   `case' (*note Conditionals::) provides conditional evaluation of
argument expressions depending on whether a variable has one of a
specified group of values.

   `and' (*note and or::) executes a sequence of expressions in order
until either there are no expressions left, or one of them evaluates to
"false".

   `or' (*note and or::) executes a sequence of expressions in order
until either there are no expressions left, or one of them evaluates to
"true".

3.4 The Concept of Closure
==========================

The concept of "closure" is the idea that a lambda expression
"captures" the variable bindings that are in lexical scope at the point
where the lambda expression occurs.  The procedure created by the
lambda expression can refer to and mutate the captured bindings, and the
values of those bindings persist between procedure calls.

   This section explains and explores the various parts of this idea in
more detail.

3.4.1 Names, Locations, Values and Environments
-----------------------------------------------

We said earlier that a variable name in a Scheme program is associated
with a location in which any kind of Scheme value may be stored.
(Incidentally, the term "vcell" is often used in Lisp and Scheme
circles as an alternative to "location".)  Thus part of what we mean
when we talk about "creating a variable" is in fact establishing an
association between a name, or identifier, that is used by the Scheme
program code, and the variable location to which that name refers.
Although the value that is stored in that location may change, the
location to which a given name refers is always the same.

   We can illustrate this by breaking down the operation of the
`define' syntax into three parts: `define'

   * creates a new location

   * establishes an association between that location and the name
     specified as the first argument of the `define' expression

   * stores in that location the value obtained by evaluating the second
     argument of the `define' expression.

   A collection of associations between names and locations is called an
"environment".  When you create a top level variable in a program using
`define', the name-location association for that variable is added to
the "top level" environment.  The "top level" environment also includes
name-location associations for all the procedures that are supplied by
standard Scheme.

   It is also possible to create environments other than the top level
one, and to create variable bindings, or name-location associations, in
those environments.  This ability is a key ingredient in the concept of
closure; the next subsection shows how it is done.

3.4.2 Local Variables and Environments
--------------------------------------

We have seen how to create top level variables using the `define'
syntax (*note Definition::).  It is often useful to create variables
that are more limited in their scope, typically as part of a procedure
body.  In Scheme, this is done using the `let' syntax, or one of its
modified forms `let*' and `letrec'.  These syntaxes are described in
full later in the manual (*note Local Bindings::).  Here our purpose is
to illustrate their use just enough that we can see how local variables
work.

   For example, the following code uses a local variable `s' to
simplify the computation of the area of a triangle given the lengths of
its three sides.

     (define a 5.3)
     (define b 4.7)
     (define c 2.8)

     (define area
       (let ((s (/ (+ a b c) 2)))
         (sqrt (* s (- s a) (- s b) (- s c)))))

   The effect of the `let' expression is to create a new environment
and, within this environment, an association between the name `s' and a
new location whose initial value is obtained by evaluating `(/ (+ a b
c) 2)'.  The expressions in the body of the `let', namely `(sqrt (* s
(- s a) (- s b) (- s c)))', are then evaluated in the context of the
new environment, and the value of the last expression evaluated becomes
the value of the whole `let' expression, and therefore the value of the
variable `area'.

3.4.3 Environment Chaining
--------------------------

In the example of the previous subsection, we glossed over an important
point.  The body of the `let' expression in that example refers not
only to the local variable `s', but also to the top level variables
`a', `b', `c' and `sqrt'.  (`sqrt' is the standard Scheme procedure for
calculating a square root.)  If the body of the `let' expression is
evaluated in the context of the _local_ `let' environment, how does the
evaluation get at the values of these top level variables?

   The answer is that the local environment created by a `let'
expression automatically has a reference to its containing environment
-- in this case the top level environment -- and that the Scheme
interpreter automatically looks for a variable binding in the containing
environment if it doesn't find one in the local environment.  More
generally, every environment except for the top level one has a
reference to its containing environment, and the interpreter keeps
searching back up the chain of environments -- from most local to top
level -- until it either finds a variable binding for the required
identifier or exhausts the chain.

   This description also determines what happens when there is more than
one variable binding with the same name.  Suppose, continuing the
example of the previous subsection, that there was also a pre-existing
top level variable `s' created by the expression:

     (define s "Some beans, my lord!")

   Then both the top level environment and the local `let' environment
would contain bindings for the name `s'.  When evaluating code within
the `let' body, the interpreter looks first in the local `let'
environment, and so finds the binding for `s' created by the `let'
syntax.  Even though this environment has a reference to the top level
environment, which also has a binding for `s', the interpreter doesn't
get as far as looking there.  When evaluating code outside the `let'
body, the interpreter looks up variable names in the top level
environment, so the name `s' refers to the top level variable.

   Within the `let' body, the binding for `s' in the local environment
is said to "shadow" the binding for `s' in the top level environment.

3.4.4 Lexical Scope
-------------------

The rules that we have just been describing are the details of how
Scheme implements "lexical scoping".  This subsection takes a brief
diversion to explain what lexical scope means in general and to present
an example of non-lexical scoping.

   "Lexical scope" in general is the idea that

   * an identifier at a particular place in a program always refers to
     the same variable location -- where "always" means "every time
     that the containing expression is executed", and that

   * the variable location to which it refers can be determined by
     static examination of the source code context in which that
     identifier appears, without having to consider the flow of
     execution through the program as a whole.

   In practice, lexical scoping is the norm for most programming
languages, and probably corresponds to what you would intuitively
consider to be "normal".  You may even be wondering how the situation
could possibly -- and usefully -- be otherwise.  To demonstrate that
another kind of scoping is possible, therefore, and to compare it
against lexical scoping, the following subsection presents an example
of non-lexical scoping and examines in detail how its behavior differs
from the corresponding lexically scoped code.

3.4.4.1 An Example of Non-Lexical Scoping
.........................................

To demonstrate that non-lexical scoping does exist and can be useful, we
present the following example from Emacs Lisp, which is a "dynamically
scoped" language.

     (defvar currency-abbreviation "USD")

     (defun currency-string (units hundredths)
       (concat currency-abbreviation
               (number-to-string units)
               "."
               (number-to-string hundredths)))

     (defun french-currency-string (units hundredths)
       (let ((currency-abbreviation "FRF"))
         (currency-string units hundredths)))

   The question to focus on here is: what does the identifier
`currency-abbreviation' refer to in the `currency-string' function?
The answer, in Emacs Lisp, is that all variable bindings go onto a
single stack, and that `currency-abbreviation' refers to the topmost
binding from that stack which has the name "currency-abbreviation".
The binding that is created by the `defvar' form, to the value `"USD"',
is only relevant if none of the code that calls `currency-string'
rebinds the name "currency-abbreviation" in the meanwhile.

   The second function `french-currency-string' works precisely by
taking advantage of this behaviour.  It creates a new binding for the
name "currency-abbreviation" which overrides the one established by the
`defvar' form.

     ;; Note!  This is Emacs Lisp evaluation, not Scheme!
     (french-currency-string 33 44)
     =>
     "FRF33.44"

   Now let's look at the corresponding, _lexically scoped_ Scheme code:

     (define currency-abbreviation "USD")

     (define (currency-string units hundredths)
       (string-append currency-abbreviation
                      (number->string units)
                      "."
                      (number->string hundredths)))

     (define (french-currency-string units hundredths)
       (let ((currency-abbreviation "FRF"))
         (currency-string units hundredths)))

   According to the rules of lexical scoping, the
`currency-abbreviation' in `currency-string' refers to the variable
location in the innermost environment at that point in the code which
has a binding for `currency-abbreviation', which is the variable
location in the top level environment created by the preceding `(define
currency-abbreviation ...)' expression.

   In Scheme, therefore, the `french-currency-string' procedure does
not work as intended.  The variable binding that it creates for
"currency-abbreviation" is purely local to the code that forms the body
of the `let' expression.  Since this code doesn't directly use the name
"currency-abbreviation" at all, the binding is pointless.

     (french-currency-string 33 44)
     =>
     "USD33.44"

   This begs the question of how the Emacs Lisp behaviour can be
implemented in Scheme.  In general, this is a design question whose
answer depends upon the problem that is being addressed.  In this case,
the best answer may be that `currency-string' should be redesigned so
that it can take an optional third argument.  This third argument, if
supplied, is interpreted as a currency abbreviation that overrides the
default.

   It is possible to change `french-currency-string' so that it mostly
works without changing `currency-string', but the fix is inelegant, and
susceptible to interrupts that could leave the `currency-abbreviation'
variable in the wrong state:

     (define (french-currency-string units hundredths)
       (set! currency-abbreviation "FRF")
       (let ((result (currency-string units hundredths)))
         (set! currency-abbreviation "USD")
         result))

   The key point here is that the code does not create any local binding
for the identifier `currency-abbreviation', so all occurrences of this
identifier refer to the top level variable.

3.4.5 Closure
-------------

Consider a `let' expression that doesn't contain any `lambda's:

     (let ((s (/ (+ a b c) 2)))
       (sqrt (* s (- s a) (- s b) (- s c))))

When the Scheme interpreter evaluates this, it

   * creates a new environment with a reference to the environment that
     was current when it encountered the `let'

   * creates a variable binding for `s' in the new environment, with
     value given by `(/ (+ a b c) 2)'

   * evaluates the expression in the body of the `let' in the context of
     the new local environment, and remembers the value `V'

   * forgets the local environment

   * continues evaluating the expression that contained the `let', using
     the value `V' as the value of the `let' expression, in the context
     of the containing environment.

   After the `let' expression has been evaluated, the local environment
that was created is simply forgotten, and there is no longer any way to
access the binding that was created in this environment.  If the same
code is evaluated again, it will follow the same steps again, creating
a second new local environment that has no connection with the first,
and then forgetting this one as well.

   If the `let' body contains a `lambda' expression, however, the local
environment is _not_ forgotten.  Instead, it becomes associated with
the procedure that is created by the `lambda' expression, and is
reinstated every time that that procedure is called.  In detail, this
works as follows.

   * When the Scheme interpreter evaluates a `lambda' expression, to
     create a procedure object, it stores the current environment as
     part of the procedure definition.

   * Then, whenever that procedure is called, the interpreter
     reinstates the environment that is stored in the procedure
     definition and evaluates the procedure body within the context of
     that environment.

   The result is that the procedure body is always evaluated in the
context of the environment that was current when the procedure was
created.

   This is what is meant by "closure".  The next few subsections
present examples that explore the usefulness of this concept.

3.4.6 Example 1: A Serial Number Generator
------------------------------------------

This example uses closure to create a procedure with a variable binding
that is private to the procedure, like a local variable, but whose value
persists between procedure calls.

     (define (make-serial-number-generator)
       (let ((current-serial-number 0))
         (lambda ()
           (set! current-serial-number (+ current-serial-number 1))
           current-serial-number)))

     (define entry-sn-generator (make-serial-number-generator))

     (entry-sn-generator)
     =>
     1

     (entry-sn-generator)
     =>
     2

   When `make-serial-number-generator' is called, it creates a local
environment with a binding for `current-serial-number' whose initial
value is 0, then, within this environment, creates a procedure.  The
local environment is stored within the created procedure object and so
persists for the lifetime of the created procedure.

   Every time the created procedure is invoked, it increments the value
of the `current-serial-number' binding in the captured environment and
then returns the current value.

   Note that `make-serial-number-generator' can be called again to
create a second serial number generator that is independent of the
first.  Every new invocation of `make-serial-number-generator' creates
a new local `let' environment and returns a new procedure object with
an association to this environment.

3.4.7 Example 2: A Shared Persistent Variable
---------------------------------------------

This example uses closure to create two procedures, `get-balance' and
`deposit', that both refer to the same captured local environment so
that they can both access the `balance' variable binding inside that
environment.  The value of this variable binding persists between calls
to either procedure.

   Note that the captured `balance' variable binding is private to
these two procedures: it is not directly accessible to any other code.
It can only be accessed indirectly via `get-balance' or `deposit', as
illustrated by the `withdraw' procedure.

     (define get-balance #f)
     (define deposit #f)

     (let ((balance 0))
       (set! get-balance
             (lambda ()
               balance))
       (set! deposit
             (lambda (amount)
               (set! balance (+ balance amount))
               balance)))

     (define (withdraw amount)
       (deposit (- amount)))

     (get-balance)
     =>
     0

     (deposit 50)
     =>
     50

     (withdraw 75)
     =>
     -25

   An important detail here is that the `get-balance' and `deposit'
variables must be set up by `define'ing them at top level and then
`set!'ing their values inside the `let' body.  Using `define' within
the `let' body would not work: this would create variable bindings
within the local `let' environment that would not be accessible at top
level.

3.4.8 Example 3: The Callback Closure Problem
---------------------------------------------

A frequently used programming model for library code is to allow an
application to register a callback function for the library to call when
some particular event occurs.  It is often useful for the application to
make several such registrations using the same callback function, for
example if several similar library events can be handled using the same
application code, but the need then arises to distinguish the callback
function calls that are associated with one callback registration from
those that are associated with different callback registrations.

   In languages without the ability to create functions dynamically,
this problem is usually solved by passing a `user_data' parameter on the
registration call, and including the value of this parameter as one of
the parameters on the callback function.  Here is an example of
declarations using this solution in C:

     typedef void (event_handler_t) (int event_type,
                                     void *user_data);

     void register_callback (int event_type,
                             event_handler_t *handler,
                             void *user_data);

   In Scheme, closure can be used to achieve the same functionality
without requiring the library code to store a `user-data' for each
callback registration.

     ;; In the library:

     (define (register-callback event-type handler-proc)
       ...)

     ;; In the application:

     (define (make-handler event-type user-data)
       (lambda ()
         ...
         <code referencing event-type and user-data>
         ...))

     (register-callback event-type
                        (make-handler event-type ...))

   As far as the library is concerned, `handler-proc' is a procedure
with no arguments, and all the library has to do is call it when the
appropriate event occurs.  From the application's point of view, though,
the handler procedure has used closure to capture an environment that
includes all the context that the handler code needs -- `event-type'
and `user-data' -- to handle the event correctly.

3.4.9 Example 4: Object Orientation
-----------------------------------

Closure is the capture of an environment, containing persistent variable
bindings, within the definition of a procedure or a set of related
procedures.  This is rather similar to the idea in some object oriented
languages of encapsulating a set of related data variables inside an
"object", together with a set of "methods" that operate on the
encapsulated data.  The following example shows how closure can be used
to emulate the ideas of objects, methods and encapsulation in Scheme.

     (define (make-account)
       (let ((balance 0))
         (define (get-balance)
           balance)
         (define (deposit amount)
           (set! balance (+ balance amount))
           balance)
         (define (withdraw amount)
           (deposit (- amount)))

         (lambda args
           (apply
             (case (car args)
               ((get-balance) get-balance)
               ((deposit) deposit)
               ((withdraw) withdraw)
               (else (error "Invalid method!")))
             (cdr args)))))

   Each call to `make-account' creates and returns a new procedure,
created by the expression in the example code that begins "(lambda
args".

     (define my-account (make-account))

     my-account
     =>
     #<procedure args>

   This procedure acts as an account object with methods `get-balance',
`deposit' and `withdraw'.  To apply one of the methods to the account,
you call the procedure with a symbol indicating the required method as
the first parameter, followed by any other parameters that are required
by that method.

     (my-account 'get-balance)
     =>
     0

     (my-account 'withdraw 5)
     =>
     -5

     (my-account 'deposit 396)
     =>
     391

     (my-account 'get-balance)
     =>
     391

   Note how, in this example, both the current balance and the helper
procedures `get-balance', `deposit' and `withdraw', used to implement
the guts of the account object's methods, are all stored in variable
bindings within the private local environment captured by the `lambda'
expression that creates the account object procedure.

3.5 Further Reading
===================

   * The website `http://www.schemers.org/' is a good starting point for
     all things Scheme.

   * Dorai Sitaram's online Scheme tutorial, "Teach Yourself Scheme in
     Fixnum Days", at
     `http://www.ccs.neu.edu/home/dorai/t-y-scheme/t-y-scheme.html'.
     Includes a nice explanation of continuations.

   * The complete text of "Structure and Interpretation of Computer
     Programs", the classic introduction to computer science and Scheme
     by Hal Abelson, Jerry Sussman and Julie Sussman, is now available
     online at `http://mitpress.mit.edu/sicp/sicp.html'.  This site
     also provides teaching materials related to the book, and all the
     source code used in the book, in a form suitable for loading and
     running.

4 Programming in Scheme
***********************

Guile's core language is Scheme, and a lot can be achieved simply by
using Guile to write and run Scheme programs -- as opposed to having to
dive into C code.  In this part of the manual, we explain how to use
Guile in this mode, and describe the tools that Guile provides to help
you with script writing, debugging, and packaging your programs for
distribution.

   For detailed reference information on the variables, functions, and
so on that make up Guile's application programming interface (API), see
*note API Reference::.

4.1 Guile's Implementation of Scheme
====================================

Guile's core language is Scheme, which is specified and described in the
series of reports known as "RnRS".  "RnRS" is shorthand for the
"Revised^n Report on the Algorithmic Language Scheme".  Guile complies
fully with R5RS (*note Introduction: (r5rs)Top.), and implements some
aspects of R6RS.

   Guile also has many extensions that go beyond these reports.  Some of
the areas where Guile extends R5RS are:

   * Guile's interactive documentation system

   * Guile's support for POSIX-compliant network programming

   * GOOPS - Guile's framework for object oriented programming.

4.2 Invoking Guile
==================

Many features of Guile depend on and can be changed by information that
the user provides either before or when Guile is started.  Below is a
description of what information to provide and how to provide it.

4.2.1 Command-line Options
--------------------------

Here we describe Guile's command-line processing in detail.  Guile
processes its arguments from left to right, recognizing the switches
described below.  For examples, see *note Scripting Examples::.

`SCRIPT ARG...'
`-s SCRIPT ARG...'
     By default, Guile will read a file named on the command line as a
     script.  Any command-line arguments ARG... following SCRIPT become
     the script's arguments; the `command-line' function returns a list
     of strings of the form `(SCRIPT ARG...)'.

     It is possible to name a file using a leading hyphen, for example,
     `-myfile.scm'.  In this case, the file name must be preceded by
     `-s' to tell Guile that a (script) file is being named.

     Scripts are read and evaluated as Scheme source code just as the
     `load' function would.  After loading SCRIPT, Guile exits.

`-c EXPR ARG...'
     Evaluate EXPR as Scheme code, and then exit.  Any command-line
     arguments ARG... following EXPR become command-line arguments; the
     `command-line' function returns a list of strings of the form
     `(GUILE ARG...)', where GUILE is the path of the Guile executable.

`-- ARG...'
     Run interactively, prompting the user for expressions and
     evaluating them.  Any command-line arguments ARG... following the
     `--' become command-line arguments for the interactive session; the
     `command-line' function returns a list of strings of the form
     `(GUILE ARG...)', where GUILE is the path of the Guile executable.

`-L DIRECTORY'
     Add DIRECTORY to the front of Guile's module load path.  The given
     directories are searched in the order given on the command line and
     before any directories in the `GUILE_LOAD_PATH' environment
     variable.  Paths added here are _not_ in effect during execution of
     the user's `.guile' file.

`-x EXTENSION'
     Add EXTENSION to the front of Guile's load extension list (*note
     `%load-extensions': Load Paths.).  The specified extensions are
     tried in the order given on the command line, and before the
     default load extensions.  Extensions added here are _not_ in
     effect during execution of the user's `.guile' file.

`-l FILE'
     Load Scheme source code from FILE, and continue processing the
     command line.

`-e FUNCTION'
     Make FUNCTION the "entry point" of the script.  After loading the
     script file (with `-s') or evaluating the expression (with `-c'),
     apply FUNCTION to a list containing the program name and the
     command-line arguments--the list provided by the `command-line'
     function.

     A `-e' switch can appear anywhere in the argument list, but Guile
     always invokes the FUNCTION as the _last_ action it performs.
     This is weird, but because of the way script invocation works under
     POSIX, the `-s' option must always come last in the list.

     The FUNCTION is most often a simple symbol that names a function
     that is defined in the script.  It can also be of the form `(@
     MODULE-NAME SYMBOL)', and in that case, the symbol is looked up in
     the module named MODULE-NAME.

     For compatibility with some versions of Guile 1.4, you can also
     use the form `(symbol ...)' (that is, a list of only symbols that
     doesn't start with `@'), which is equivalent to `(@ (symbol ...)
     main)', or `(symbol ...)  symbol' (that is, a list of only symbols
     followed by a symbol), which is equivalent to `(@ (symbol ...)
     symbol)'.  We recommend to use the equivalent forms directly since
     they correspond to the `(@ ...)'  read syntax that can be used in
     normal code.  See *note Using Guile Modules:: and *note Scripting
     Examples::.

`-ds'
     Treat a final `-s' option as if it occurred at this point in the
     command line; load the script here.

     This switch is necessary because, although the POSIX script
     invocation mechanism effectively requires the `-s' option to
     appear last, the programmer may well want to run the script before
     other actions requested on the command line.  For examples, see
     *note Scripting Examples::.

`\'
     Read more command-line arguments, starting from the second line of
     the script file.  *Note The Meta Switch::.

`--use-srfi=LIST'
     The option `--use-srfi' expects a comma-separated list of numbers,
     each representing a SRFI module to be loaded into the interpreter
     before evaluating a script file or starting the REPL.
     Additionally, the feature identifier for the loaded SRFIs is
     recognized by the procedure `cond-expand' when this option is used.

     Here is an example that loads the modules SRFI-8 ('receive') and
     SRFI-13 ('string library') before the GUILE interpreter is started:

          guile --use-srfi=8,13

`--debug'
     Start with the debugging virtual machine (VM) engine.  Using the
     debugging VM will enable support for VM hooks, which are needed for
     tracing, breakpoints, and accurate call counts when profiling.  The
     debugging VM is slower than the regular VM, though, by about ten
     percent.  *Note VM Hooks::, for more information.

     By default, the debugging VM engine is only used when entering an
     interactive session.  When executing a script with `-s' or `-c',
     the normal, faster VM is used by default.

`--no-debug'
     Do not use the debugging VM engine, even when entering an
     interactive session.

     Note that, despite the name, Guile running with `--no-debug'
     _does_ support the usual debugging facilities, such as printing a
     detailed backtrace upon error.  The only difference with `--debug'
     is lack of support for VM hooks and the facilities that build upon
     it (see above).

`-q'
     Do not load the initialization file, `.guile'.  This option only
     has an effect when running interactively; running scripts does not
     load the `.guile' file.  *Note Init File::.

`--listen[=P]'
     While this program runs, listen on a local port or a path for REPL
     clients.  If P starts with a number, it is assumed to be a local
     port on which to listen.  If it starts with a forward slash, it is
     assumed to be a path to a UNIX domain socket on which to listen.

     If P is not given, the default is local port 37146.  If you look
     at it upside down, it almost spells "Guile".  If you have netcat
     installed, you should be able to `nc localhost 37146' and get a
     Guile prompt.  Alternately you can fire up Emacs and connect to the
     process; see *note Using Guile in Emacs:: for more details.

     Note that opening a port allows anyone who can connect to that
     port--in the TCP case, any local user--to do anything Guile can
     do, as the user that the Guile process is running as.  Do not use
     `--listen' on multi-user machines.  Of course, if you do not pass
     `--listen' to Guile, no port will be opened.

     That said, `--listen' is great for interactive debugging and
     development.

`--auto-compile'
     Compile source files automatically (default behavior).

`--fresh-auto-compile'
     Treat the auto-compilation cache as invalid, forcing recompilation.

`--no-auto-compile'
     Disable automatic source file compilation.

`-h, --help'
     Display help on invoking Guile, and then exit.

`-v, --version'
     Display the current version of Guile, and then exit.


4.2.2 Environment Variables
---------------------------

The "environment" is a feature of the operating system; it consists of
a collection of variables with names and values.  Each variable is
called an "environment variable" (or, sometimes, a "shell variable");
environment variable names are case-sensitive, and it is conventional
to use upper-case letters only.  The values are all text strings, even
those that are written as numerals.  (Note that here we are referring
to names and values that are defined in the operating system shell from
which Guile is invoked.  This is not the same as a Scheme environment
that is defined within a running instance of Guile.  For a description
of Scheme environments, *note About Environments::.)

   How to set environment variables before starting Guile depends on the
operating system and, especially, the shell that you are using.  For
example, here is how to tell Guile to provide detailed warning messages
about deprecated features by setting `GUILE_WARN_DEPRECATED' using Bash:

     $ export GUILE_WARN_DEPRECATED="detailed"
     $ guile

Or, detailed warnings can be turned on for a single invocation using:

     $ env GUILE_WARN_DEPRECATED="detailed" guile

   If you wish to retrieve or change the value of the shell environment
variables that affect the run-time behavior of Guile from within a
running instance of Guile, see *note Runtime Environment::.

   Here are the environment variables that affect the run-time behavior
of Guile:

`GUILE_AUTO_COMPILE'
     This is a flag that can be used to tell Guile whether or not to
     compile Scheme source files automatically.  Starting with Guile
     2.0, Scheme source files will be compiled automatically, by
     default.

     If a compiled (`.go') file corresponding to a `.scm' file is not
     found or is not newer than the `.scm' file, the `.scm' file will
     be compiled on the fly, and the resulting `.go' file stored away.
     An advisory note will be printed on the console.

     Compiled files will be stored in the directory
     `$XDG_CACHE_HOME/guile/ccache', where `XDG_CACHE_HOME' defaults to
     the directory `$HOME/.cache'.  This directory will be created if
     it does not already exist.

     Note that this mechanism depends on the timestamp of the `.go' file
     being newer than that of the `.scm' file; if the `.scm' or `.go'
     files are moved after installation, care should be taken to
     preserve their original timestamps.

     Set `GUILE_AUTO_COMPILE' to zero (0), to prevent Scheme files from
     being compiled automatically.  Set this variable to "fresh" to tell
     Guile to compile Scheme files whether they are newer than the
     compiled files or not.

     *Note Compilation::.

`GUILE_HISTORY'
     This variable names the file that holds the Guile REPL command
     history.  You can specify a different history file by setting this
     environment variable.  By default, the history file is
     `$HOME/.guile_history'.

`GUILE_LOAD_COMPILED_PATH'
     This variable may be used to augment the path that is searched for
     compiled Scheme files (`.go' files) when loading.  Its value should
     be a colon-separated list of directories, which will be prefixed
     to the value of the default search path stored in
     `%load-compiled-path'.

     Here is an example using the Bash shell that adds the current
     directory, `.', and the relative directory `../my-library' to
     `%load-compiled-path':

          $ export GUILE_LOAD_COMPILED_PATH=".:../my-library"
          $ guile -c '(display %load-compiled-path) (newline)'
          (. ../my-library /usr/local/lib/guile/2.0/ccache)

`GUILE_LOAD_PATH'
     This variable may be used to augment the path that is searched for
     Scheme files when loading.  Its value should be a colon-separated
     list of directories, which will be prefixed to the value of the
     default search path stored in `%load-path'.

     Here is an example using the Bash shell that adds the current
     directory and the parent of the current directory to `%load-path':

          $ env GUILE_LOAD_PATH=".:.." \
          guile -c '(display %load-path) (newline)'
          (. .. /usr/local/share/guile/2.0 \
          /usr/local/share/guile/site/2.0 \
          /usr/local/share/guile/site /usr/local/share/guile)

     (Note: The line breaks, above, are for documentation purposes
     only, and not required in the actual example.)

`GUILE_WARN_DEPRECATED'
     As Guile evolves, some features will be eliminated or replaced by
     newer features.  To help users migrate their code as this
     evolution occurs, Guile will issue warning messages about code
     that uses features that have been marked for eventual elimination.
     `GUILE_WARN_DEPRECATED' can be set to "no" to tell Guile not to
     display these warning messages, or set to "detailed" to tell Guile
     to display more lengthy messages describing the warning.  *Note
     Deprecation::.

`HOME'
     Guile uses the environment variable `HOME', the name of your home
     directory, to locate various files, such as `.guile' or
     `.guile_history'.

`LTDL_LIBRARY_PATH'
     Guile now adds its install prefix to the `LTDL_LIBRARY_PATH'.

     Users may now install Guile in non-standard directories and run
     `/path/to/bin/guile', without having also to set
     `LTDL_LIBRARY_PATH' to include `/path/to/lib'.


4.3 Guile Scripting
===================

Like AWK, Perl, or any shell, Guile can interpret script files.  A Guile
script is simply a file of Scheme code with some extra information at
the beginning which tells the operating system how to invoke Guile, and
then tells Guile how to handle the Scheme code.

4.3.1 The Top of a Script File
------------------------------

The first line of a Guile script must tell the operating system to use
Guile to evaluate the script, and then tell Guile how to go about doing
that.  Here is the simplest case:

   * The first two characters of the file must be `#!'.

     The operating system interprets this to mean that the rest of the
     line is the name of an executable that can interpret the script.
     Guile, however, interprets these characters as the beginning of a
     multi-line comment, terminated by the characters `!#' on a line by
     themselves.  (This is an extension to the syntax described in
     R5RS, added to support shell scripts.)

   * Immediately after those two characters must come the full pathname
     to the Guile interpreter.  On most systems, this would be
     `/usr/local/bin/guile'.

   * Then must come a space, followed by a command-line argument to
     pass to Guile; this should be `-s'.  This switch tells Guile to
     run a script, instead of soliciting the user for input from the
     terminal.  There are more elaborate things one can do here; see
     *note The Meta Switch::.

   * Follow this with a newline.

   * The second line of the script should contain only the characters
     `!#' -- just like the top of the file, but reversed.  The
     operating system never reads this far, but Guile treats this as
     the end of the comment begun on the first line by the `#!'
     characters.

   * If this source code file is not ASCII or ISO-8859-1 encoded, a
     coding declaration such as `coding: utf-8' should appear in a
     comment somewhere in the first five lines of the file: see *note
     Character Encoding of Source Files::.

   * The rest of the file should be a Scheme program.


   Guile reads the program, evaluating expressions in the order that
they appear.  Upon reaching the end of the file, Guile exits.

4.3.2 The Meta Switch
---------------------

Guile's command-line switches allow the programmer to describe
reasonably complicated actions in scripts.  Unfortunately, the POSIX
script invocation mechanism only allows one argument to appear on the
`#!' line after the path to the Guile executable, and imposes arbitrary
limits on that argument's length.  Suppose you wrote a script starting
like this:
     #!/usr/local/bin/guile -e main -s
     !#
     (define (main args)
       (map (lambda (arg) (display arg) (display " "))
            (cdr args))
       (newline))
   The intended meaning is clear: load the file, and then call `main'
on the command-line arguments.  However, the system will treat
everything after the Guile path as a single argument -- the string `"-e
main -s"' -- which is not what we want.

   As a workaround, the meta switch `\' allows the Guile programmer to
specify an arbitrary number of options without patching the kernel.  If
the first argument to Guile is `\', Guile will open the script file
whose name follows the `\', parse arguments starting from the file's
second line (according to rules described below), and substitute them
for the `\' switch.

   Working in concert with the meta switch, Guile treats the characters
`#!' as the beginning of a comment which extends through the next line
containing only the characters `!#'.  This sort of comment may appear
anywhere in a Guile program, but it is most useful at the top of a
file, meshing magically with the POSIX script invocation mechanism.

   Thus, consider a script named `/u/jimb/ekko' which starts like this:
     #!/usr/local/bin/guile \
     -e main -s
     !#
     (define (main args)
             (map (lambda (arg) (display arg) (display " "))
                  (cdr args))
             (newline))

   Suppose a user invokes this script as follows:
     $ /u/jimb/ekko a b c

   Here's what happens:
   * the operating system recognizes the `#!' token at the top of the
     file, and rewrites the command line to:
          /usr/local/bin/guile \ /u/jimb/ekko a b c
     This is the usual behavior, prescribed by POSIX.

   * When Guile sees the first two arguments, `\ /u/jimb/ekko', it opens
     `/u/jimb/ekko', parses the three arguments `-e', `main', and `-s'
     from it, and substitutes them for the `\' switch.  Thus, Guile's
     command line now reads:
          /usr/local/bin/guile -e main -s /u/jimb/ekko a b c

   * Guile then processes these switches: it loads `/u/jimb/ekko' as a
     file of Scheme code (treating the first three lines as a comment),
     and then performs the application `(main "/u/jimb/ekko" "a" "b"
     "c")'.


   When Guile sees the meta switch `\', it parses command-line argument
from the script file according to the following rules:
   * Each space character terminates an argument.  This means that two
     spaces in a row introduce an argument `""'.

   * The tab character is not permitted (unless you quote it with the
     backslash character, as described below), to avoid confusion.

   * The newline character terminates the sequence of arguments, and
     will also terminate a final non-empty argument.  (However, a
     newline following a space will not introduce a final empty-string
     argument; it only terminates the argument list.)

   * The backslash character is the escape character.  It escapes
     backslash, space, tab, and newline.  The ANSI C escape sequences
     like `\n' and `\t' are also supported.  These produce argument
     constituents; the two-character combination `\n' doesn't act like
     a terminating newline.  The escape sequence `\NNN' for exactly
     three octal digits reads as the character whose ASCII code is NNN.
     As above, characters produced this way are argument constituents.
     Backslash followed by other characters is not allowed.


4.3.3 Command Line Handling
---------------------------

The ability to accept and handle command line arguments is very
important when writing Guile scripts to solve particular problems, such
as extracting information from text files or interfacing with existing
command line applications.  This chapter describes how Guile makes
command line arguments available to a Guile script, and the utilities
that Guile provides to help with the processing of command line
arguments.

   When a Guile script is invoked, Guile makes the command line
arguments accessible via the procedure `command-line', which returns the
arguments as a list of strings.

   For example, if the script

     #! /usr/local/bin/guile -s
     !#
     (write (command-line))
     (newline)

is saved in a file `cmdline-test.scm' and invoked using the command
line `./cmdline-test.scm bar.txt -o foo -frumple grob', the output is

     ("./cmdline-test.scm" "bar.txt" "-o" "foo" "-frumple" "grob")

   If the script invocation includes a `-e' option, specifying a
procedure to call after loading the script, Guile will call that
procedure with `(command-line)' as its argument.  So a script that uses
`-e' doesn't need to refer explicitly to `command-line' in its code.
For example, the script above would have identical behaviour if it was
written instead like this:

     #! /usr/local/bin/guile \
     -e main -s
     !#
     (define (main args)
       (write args)
       (newline))

   (Note the use of the meta switch `\' so that the script invocation
can include more than one Guile option: *Note The Meta Switch::.)

   These scripts use the `#!' POSIX convention so that they can be
executed using their own file names directly, as in the example command
line `./cmdline-test.scm bar.txt -o foo -frumple grob'.  But they can
also be executed by typing out the implied Guile command line in full,
as in:

     $ guile -s ./cmdline-test.scm bar.txt -o foo -frumple grob

or

     $ guile -e main -s ./cmdline-test2.scm bar.txt -o foo -frumple grob

   Even when a script is invoked using this longer form, the arguments
that the script receives are the same as if it had been invoked using
the short form.  Guile ensures that the `(command-line)' or `-e'
arguments are independent of how the script is invoked, by stripping off
the arguments that Guile itself processes.

   A script is free to parse and handle its command line arguments in
any way that it chooses.  Where the set of possible options and
arguments is complex, however, it can get tricky to extract all the
options, check the validity of given arguments, and so on.  This task
can be greatly simplified by taking advantage of the module `(ice-9
getopt-long)', which is distributed with Guile, *Note getopt-long::.

4.3.4 Scripting Examples
------------------------

To start with, here are some examples of invoking Guile directly:

`guile -- a b c'
     Run Guile interactively; `(command-line)' will return
     `("/usr/local/bin/guile" "a" "b" "c")'.

`guile -s /u/jimb/ex2 a b c'
     Load the file `/u/jimb/ex2'; `(command-line)' will return
     `("/u/jimb/ex2" "a" "b" "c")'.

`guile -c '(write %load-path) (newline)''
     Write the value of the variable `%load-path', print a newline, and
     exit.

`guile -e main -s /u/jimb/ex4 foo'
     Load the file `/u/jimb/ex4', and then call the function `main',
     passing it the list `("/u/jimb/ex4" "foo")'.

`guile -l first -ds -l last -s script'
     Load the files `first', `script', and `last', in that order.  The
     `-ds' switch says when to process the `-s' switch.  For a more
     motivated example, see the scripts below.


   Here is a very simple Guile script:
     #!/usr/local/bin/guile -s
     !#
     (display "Hello, world!")
     (newline)
   The first line marks the file as a Guile script.  When the user
invokes it, the system runs `/usr/local/bin/guile' to interpret the
script, passing `-s', the script's filename, and any arguments given to
the script as command-line arguments.  When Guile sees `-s SCRIPT', it
loads SCRIPT.  Thus, running this program produces the output:
     Hello, world!

   Here is a script which prints the factorial of its argument:
     #!/usr/local/bin/guile -s
     !#
     (define (fact n)
       (if (zero? n) 1
         (* n (fact (- n 1)))))

     (display (fact (string->number (cadr (command-line)))))
     (newline)
   In action:
     $ ./fact 5
     120
     $

   However, suppose we want to use the definition of `fact' in this
file from another script.  We can't simply `load' the script file, and
then use `fact''s definition, because the script will try to compute
and display a factorial when we load it.  To avoid this problem, we
might write the script this way:

     #!/usr/local/bin/guile \
     -e main -s
     !#
     (define (fact n)
       (if (zero? n) 1
         (* n (fact (- n 1)))))

     (define (main args)
       (display (fact (string->number (cadr args))))
       (newline))
   This version packages the actions the script should perform in a
function, `main'.  This allows us to load the file purely for its
definitions, without any extraneous computation taking place.  Then we
used the meta switch `\' and the entry point switch `-e' to tell Guile
to call `main' after loading the script.
     $ ./fact 50
     30414093201713378043612608166064768844377641568960512000000000000

   Suppose that we now want to write a script which computes the
`choose' function: given a set of M distinct objects, `(choose N M)' is
the number of distinct subsets containing N objects each.  It's easy to
write `choose' given `fact', so we might write the script this way:
     #!/usr/local/bin/guile \
     -l fact -e main -s
     !#
     (define (choose n m)
       (/ (fact m) (* (fact (- m n)) (fact n))))

     (define (main args)
       (let ((n (string->number (cadr args)))
             (m (string->number (caddr args))))
         (display (choose n m))
         (newline)))

   The command-line arguments here tell Guile to first load the file
`fact', and then run the script, with `main' as the entry point.  In
other words, the `choose' script can use definitions made in the `fact'
script.  Here are some sample runs:
     $ ./choose 0 4
     1
     $ ./choose 1 4
     4
     $ ./choose 2 4
     6
     $ ./choose 3 4
     4
     $ ./choose 4 4
     1
     $ ./choose 50 100
     100891344545564193334812497256

4.4 Using Guile Interactively
=============================

When you start up Guile by typing just `guile', without a `-c' argument
or the name of a script to execute, you get an interactive interpreter
where you can enter Scheme expressions, and Guile will evaluate them
and print the results for you.  Here are some simple examples.

     scheme@(guile-user)> (+ 3 4 5)
     $1 = 12
     scheme@(guile-user)> (display "Hello world!\n")
     Hello world!
     scheme@(guile-user)> (values 'a 'b)
     $2 = a
     $3 = b

This mode of use is called a "REPL", which is short for
"Read-Eval-Print Loop", because the Guile interpreter first reads the
expression that you have typed, then evaluates it, and then prints the
result.

   The prompt shows you what language and module you are in. In this
case, the current language is `scheme', and the current module is
`(guile-user)'. *Note Other Languages::, for more information on Guile's
support for languages other than Scheme.

4.4.1 The Init File, `~/.guile'
-------------------------------

When run interactively, Guile will load a local initialization file from
`~/.guile'.  This file should contain Scheme expressions for evaluation.

   This facility lets the user customize their interactive Guile
environment, pulling in extra modules or parameterizing the REPL
implementation.

   To run Guile without loading the init file, use the `-q'
command-line option.

4.4.2 Readline
--------------

To make it easier for you to repeat and vary previously entered
expressions, or to edit the expression that you're typing in, Guile can
use the GNU Readline library.  This is not enabled by default because
of licensing reasons, but all you need to activate Readline is the
following pair of lines.

     scheme@(guile-user)> (use-modules (ice-9 readline))
     scheme@(guile-user)> (activate-readline)

   It's a good idea to put these two lines (without the
`scheme@(guile-user)>' prompts) in your `.guile' file.  *Note Init
File::, for more on `.guile'.

4.4.3 Value History
-------------------

Just as Readline helps you to reuse a previous input line, "value
history" allows you to use the _result_ of a previous evaluation in a
new expression.  When value history is enabled, each evaluation result
is automatically assigned to the next in the sequence of variables
`$1', `$2', ....  You can then use these variables in subsequent
expressions.

     scheme@(guile-user)> (iota 10)
     $1 = (0 1 2 3 4 5 6 7 8 9)
     scheme@(guile-user)> (apply * (cdr $1))
     $2 = 362880
     scheme@(guile-user)> (sqrt $2)
     $3 = 602.3952191045344
     scheme@(guile-user)> (cons $2 $1)
     $4 = (362880 0 1 2 3 4 5 6 7 8 9)

   Value history is enabled by default, because Guile's REPL imports the
`(ice-9 history)' module. Value history may be turned off or on within
the repl, using the options interface:

     scheme@(guile-user)> ,option value-history #f
     scheme@(guile-user)> 'foo
     foo
     scheme@(guile-user)> ,option value-history #t
     scheme@(guile-user)> 'bar
     $5 = bar

   Note that previously recorded values are still accessible, even if
value history is off. In rare cases, these references to past
computations can cause Guile to use too much memory. One may clear
these values, possibly enabling garbage collection, via the
`clear-value-history!' procedure, described below.

   The programmatic interface to value history is in a module:

     (use-modules (ice-9 history))

 -- Scheme Procedure: value-history-enabled?
     Return true iff value history is enabled.

 -- Scheme Procedure: enable-value-history!
     Turn on value history, if it was off.

 -- Scheme Procedure: disable-value-history!
     Turn off value history, if it was on.

 -- Scheme Procedure: clear-value-history!
     Clear the value history. If the stored values are not captured by
     some other data structure or closure, they may then be reclaimed
     by the garbage collector.

4.4.4 REPL Commands
-------------------

The REPL exists to read expressions, evaluate them, and then print their
results. But sometimes one wants to tell the REPL to evaluate an
expression in a different way, or to do something else altogether. A
user can affect the way the REPL works with a "REPL command".

   The previous section had an example of a command, in the form of
`,option'.

     scheme@(guile-user)> ,option value-history #t

Commands are distinguished from expressions by their initial comma
(`,'). Since a comma cannot begin an expression in most languages, it
is an effective indicator to the REPL that the following text forms a
command, not an expression.

   REPL commands are convenient because they are always there. Even if
the current module doesn't have a binding for `pretty-print', one can
always `,pretty-print'.

   The following sections document the various commands, grouped
together by functionality. Many of the commands have abbreviations; see
the online help (`,help') for more information.

4.4.4.1 Help Commands
.....................

When Guile starts interactively, it notifies the user that help can be
had by typing `,help'. Indeed, `help' is a command, and a particularly
useful one, as it allows the user to discover the rest of the commands.

 -- REPL Command: help [`all' | group | `[-c]' command]
     Show help.

     With one argument, tries to look up the argument as a group name,
     giving help on that group if successful. Otherwise tries to look
     up the argument as a command, giving help on the command.

     If there is a command whose name is also a group name, use the `-c
     COMMAND' form to give help on the command instead of the group.

     Without any argument, a list of help commands and command groups
     are displayed.

 -- REPL Command: show [topic]
     Gives information about Guile.

     With one argument, tries to show a particular piece of information;
     currently supported topics are `warranty' (or `w'), `copying' (or
     `c'), and `version' (or `v').

     Without any argument, a list of topics is displayed.

 -- REPL Command: apropos regexp
     Find bindings/modules/packages.

 -- REPL Command: describe obj
     Show description/documentation.

4.4.4.2 Module Commands
.......................

 -- REPL Command: module [module]
     Change modules / Show current module.

 -- REPL Command: import [module ...]
     Import modules / List those imported.

 -- REPL Command: load file
     Load a file in the current module.

 -- REPL Command: reload [module]
     Reload the given module, or the current module if none was given.

 -- REPL Command: binding
     List current bindings.

 -- REPL Command: in module expression
 -- REPL Command: in module command [args ...]
     Evaluate an expression, or alternatively, execute another
     meta-command in the context of a module.  For example, `,in (foo
     bar) ,binding' will show the bindings in the module `(foo bar)'.

4.4.4.3 Language Commands
.........................

 -- REPL Command: language language
     Change languages.

4.4.4.4 Compile Commands
........................

 -- REPL Command: compile exp
     Generate compiled code.

 -- REPL Command: compile-file file
     Compile a file.

 -- REPL Command: expand exp
     Expand any macros in a form.

 -- REPL Command: optimize exp
     Run the optimizer on a piece of code and print the result.

 -- REPL Command: disassemble exp
     Disassemble a compiled procedure.

 -- REPL Command: disassemble-file file
     Disassemble a file.

4.4.4.5 Profile Commands
........................

 -- REPL Command: time exp
     Time execution.

 -- REPL Command: profile exp
     Profile execution.

 -- REPL Command: trace exp
     Trace execution.

4.4.4.6 Debug Commands
......................

These debugging commands are only available within a recursive REPL;
they do not work at the top level.

 -- REPL Command: backtrace [count] [#:width w] [#:full? f]
     Print a backtrace.

     Print a backtrace of all stack frames, or innermost COUNT frames.
     If COUNT is negative, the last COUNT frames will be shown.

 -- REPL Command: up [count]
     Select a calling stack frame.

     Select and print stack frames that called this one.  An argument
     says how many frames up to go.

 -- REPL Command: down [count]
     Select a called stack frame.

     Select and print stack frames called by this one.  An argument
     says how many frames down to go.

 -- REPL Command: frame [idx]
     Show a frame.

     Show the selected frame.  With an argument, select a frame by
     index, then show it.

 -- REPL Command: procedure
     Print the procedure for the selected frame.

 -- REPL Command: locals
     Show local variables.

     Show locally-bound variables in the selected frame.

 -- REPL Command: error-message
 -- REPL Command: error
     Show error message.

     Display the message associated with the error that started the
     current debugging REPL.

 -- REPL Command: registers
     Show the VM registers associated with the current frame.

     *Note Stack Layout::, for more information on VM stack frames.

 -- REPL Command: width [cols]
     Sets the number of display columns in the output of `,backtrace'
     and `,locals' to COLS.  If COLS is not given, the width of the
     terminal is used.

   The next 3 commands work at any REPL.

 -- REPL Command: break proc
     Set a breakpoint at PROC.

 -- REPL Command: break-at-source file line
     Set a breakpoint at the given source location.

 -- REPL Command: tracepoint proc
     Set a tracepoint on the given procedure. This will cause all calls
     to the procedure to print out a tracing message. *Note Tracing
     Traps::, for more information.

   The rest of the commands in this subsection all apply only when the
stack is "continuable" -- in other words when it makes sense for the
program that the stack comes from to continue running.  Usually this
means that the program stopped because of a trap or a breakpoint.

 -- REPL Command: step
     Tell the debugged program to step to the next source location.

 -- REPL Command: next
     Tell the debugged program to step to the next source location in
     the same frame.  (See *note Traps:: for the details of how this
     works.)

 -- REPL Command: finish
     Tell the program being debugged to continue running until the
     completion of the current stack frame, and at that time to print
     the result and reenter the REPL.

4.4.4.7 Inspect Commands
........................

 -- REPL Command: inspect EXP
     Inspect the result(s) of evaluating EXP.

 -- REPL Command: pretty-print EXP
     Pretty-print the result(s) of evaluating EXP.

4.4.4.8 System Commands
.......................

 -- REPL Command: gc
     Garbage collection.

 -- REPL Command: statistics
     Display statistics.

 -- REPL Command: option [key value]
     List/show/set options.

 -- REPL Command: quit
     Quit this session.

   Current REPL options include:

`compile-options'
     The options used when compiling expressions entered at the REPL.
     *Note Compilation::, for more on compilation options.

`interp'
     Whether to interpret or compile expressions given at the REPL, if
     such a choice is available.  Off by default (indicating
     compilation).

`prompt'
     A customized REPL prompt.  `#f' by default, indicating the default
     prompt.

`value-history'
     Whether value history is on or not.  *Note Value History::.

`on-error'
     What to do when an error happens.  By default, `debug', meaning to
     enter the debugger.  Other values include `backtrace', to show a
     backtrace without entering the debugger, or `report', to simply
     show a short error printout.

   Default values for REPL options may be set using
`repl-default-option-set!' from `(system repl common)':

 -- Scheme Procedure: repl-set-default-option! key value
     Set the default value of a REPL option.  This function is
     particularly useful in a user's init file.  *Note Init File::.

4.4.5 Error Handling
--------------------

When code being evaluated from the REPL hits an error, Guile enters a
new prompt, allowing you to inspect the context of the error.

     scheme@(guile-user)> (map string-append '("a" "b") '("c" #\d))
     ERROR: In procedure string-append:
     ERROR: Wrong type (expecting string): #\d
     Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
     scheme@(guile-user) [1]>

   The new prompt runs inside the old one, in the dynamic context of the
error.  It is a recursive REPL, augmented with a reified representation
of the stack, ready for debugging.

   `,backtrace' (abbreviated `,bt') displays the Scheme call stack at
the point where the error occurred:

     scheme@(guile-user) [1]> ,bt
                1 (map #<procedure string-append _> ("a" "b") ("c" #\d))
                0 (string-append "b" #\d)

   In the above example, the backtrace doesn't have much source
information, as `map' and `string-append' are both primitives. But in
the general case, the space on the left of the backtrace indicates the
line and column in which a given procedure calls another.

   You can exit a recursive REPL in the same way that you exit any REPL:
via `(quit)', `,quit' (abbreviated `,q'), or `C-d', among other options.

4.4.6 Interactive Debugging
---------------------------

A recursive debugging REPL exposes a number of other meta-commands that
inspect the state of the computation at the time of the error. These
commands allow you to

   * display the Scheme call stack at the point where the error
     occurred;

   * move up and down the call stack, to see in detail the expression
     being evaluated, or the procedure being applied, in each "frame";
     and

   * examine the values of variables and expressions in the context of
     each frame.

*Note Debug Commands::, for documentation of the individual commands.
This section aims to give more of a walkthrough of a typical debugging
session.

   First, we're going to need a good error. Let's try to macroexpand the
expression `(unquote foo)', outside of a `quasiquote' form, and see how
the macroexpander reports this error.

     scheme@(guile-user)> (macroexpand '(unquote foo))
     ERROR: In procedure macroexpand:
     ERROR: unquote: expression not valid outside of quasiquote in (unquote foo)
     Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
     scheme@(guile-user) [1]>

   The `backtrace' command, which can also be invoked as `bt', displays
the call stack (aka backtrace) at the point where the debugger was
entered:

     scheme@(guile-user) [1]> ,bt
     In ice-9/psyntax.scm:
       1130:21  3 (chi-top (unquote foo) () ((top)) e (eval) (hygiene #))
       1071:30  2 (syntax-type (unquote foo) () ((top)) #f #f (# #) #f)
       1368:28  1 (chi-macro #<procedure de9360 at ice-9/psyntax.scm...> ...)
     In unknown file:
                0 (scm-error syntax-error macroexpand "~a: ~a in ~a" # #f)

   A call stack consists of a sequence of stack "frames", with each
frame describing one procedure which is waiting to do something with the
values returned by another. Here we see that there are four frames on
the stack.

   Note that `macroexpand' is not on the stack - it must have made a
tail call to `chi-top', as indeed we would find if we searched
`ice-9/psyntax.scm' for its definition.

   When you enter the debugger, the innermost frame is selected, which
means that the commands for getting information about the "current"
frame, or for evaluating expressions in the context of the current
frame, will do so by default with respect to the innermost frame.  To
select a different frame, so that these operations will apply to it
instead, use the `up', `down' and `frame' commands like this:

     scheme@(guile-user) [1]> ,up
     In ice-9/psyntax.scm:
       1368:28  1 (chi-macro #<procedure de9360 at ice-9/psyntax.scm...> ...)
     scheme@(guile-user) [1]> ,frame 3
     In ice-9/psyntax.scm:
       1130:21  3 (chi-top (unquote foo) () ((top)) e (eval) (hygiene #))
     scheme@(guile-user) [1]> ,down
     In ice-9/psyntax.scm:
       1071:30  2 (syntax-type (unquote foo) () ((top)) #f #f (# #) #f)

   Perhaps we're interested in what's going on in frame 2, so we take a
look at its local variables:

     scheme@(guile-user) [1]> ,locals
       Local variables:
       $1 = e = (unquote foo)
       $2 = r = ()
       $3 = w = ((top))
       $4 = s = #f
       $5 = rib = #f
       $6 = mod = (hygiene guile-user)
       $7 = for-car? = #f
       $8 = first = unquote
       $9 = ftype = macro
       $10 = fval = #<procedure de9360 at ice-9/psyntax.scm:2817:2 (x)>
       $11 = fe = unquote
       $12 = fw = ((top))
       $13 = fs = #f
       $14 = fmod = (hygiene guile-user)

   All of the values are accessible by their value-history names (`$N'):

     scheme@(guile-user) [1]> $10
     $15 = #<procedure de9360 at ice-9/psyntax.scm:2817:2 (x)>

   We can even invoke the procedure at the REPL directly:

     scheme@(guile-user) [1]> ($10 'not-going-to-work)
     ERROR: In procedure macroexpand:
     ERROR: source expression failed to match any pattern in not-going-to-work
     Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.

   Well at this point we've caused an error within an error. Let's just
quit back to the top level:

     scheme@(guile-user) [2]> ,q
     scheme@(guile-user) [1]> ,q
     scheme@(guile-user)>

   Finally, as a word to the wise: hackers close their REPL prompts with
`C-d'.

4.5 Using Guile in Emacs
========================

Any text editor can edit Scheme, but some are better than others.  Emacs
is the best, of course, and not just because it is a fine text editor.
Emacs has good support for Scheme out of the box, with sensible
indentation rules, parenthesis-matching, syntax highlighting, and even a
set of keybindings for structural editing, allowing navigation,
cut-and-paste, and transposition operations that work on balanced
S-expressions.

   As good as it is, though, two things will vastly improve your
experience with Emacs and Guile.

   The first is Taylor Campbell's Paredit
(http://www.emacswiki.org/emacs/ParEdit).  You should not code in any
dialect of Lisp without Paredit.  (They say that unopinionated writing
is boring--hence this tone--but it's the truth, regardless.)  Paredit
is the bee's knees.

   The second is José Antonio Ortega Ruiz's Geiser
(http://www.nongnu.org/geiser/).  Geiser complements Emacs'
`scheme-mode' with tight integration to running Guile processes via a
`comint-mode' REPL buffer.

   Of course there are keybindings to switch to the REPL, and a good
REPL environment, but Geiser goes beyond that, providing:

   * Form evaluation in the context of the current file's module.

   * Macro expansion.

   * File/module loading and/or compilation.

   * Namespace-aware identifier completion (including local bindings,
     names visible in the current module, and module names).

   * Autodoc: the echo area shows information about the signature of the
     procedure/macro around point automatically.

   * Jump to definition of identifier at point.

   * Access to documentation (including docstrings when the
     implementation provides it).

   * Listings of identifiers exported by a given module.

   * Listings of callers/callees of procedures.

   * Rudimentary support for debugging and error navigation.

   * Support for multiple, simultaneous REPLs.

   See Geiser's web page at `http://www.nongnu.org/geiser/', for more
information.

4.6 Using Guile Tools
=====================

Guile also comes with a growing number of command-line utilities: a
compiler, a disassembler, some module inspectors, and in the future, a
system to install Guile packages from the internet.  These tools may be
invoked using the `guild' program.

     $ guild compile -o foo.go foo.scm
     wrote `foo.go'

   This program used to be called `guile-tools' up to Guile version
2.0.1, and for backward compatibility it still may be called as such.
However we changed the name to `guild', not only because it is
pleasantly shorter and easier to read, but also because this tool will
serve to bind Guile wizards together, by allowing hackers to share code
with each other using a CPAN-like system.

   *Note Compilation::, for more on `guild compile'.

   A complete list of guild scripts can be had by invoking `guild
list', or simply `guild'.

4.7 Installing Site Packages
============================

At some point, you will probably want to share your code with other
people.  To do so effectively, it is important to follow a set of common
conventions, to make it easy for the user to install and use your
package.

   The first thing to do is to install your Scheme files where Guile can
find them.  When Guile goes to find a Scheme file, it will search a
"load path" to find the file: first in Guile's own path, then in paths
for "site packages".  A site package is any Scheme code that is
installed and not part of Guile itself.  *Note Load Paths::, for more
on load paths.

   There are several site paths, for historical reasons, but the one
that should generally be used can be obtained by invoking the
`%site-dir' procedure.  *Note Build Config::.  If Guile
2.0 is installed on your system in `/usr/', then `(%site-dir)' will be
`/usr/share/guile/site/2.0'.  Scheme files should be installed there.

   If you do not install compiled `.go' files, Guile will compile your
modules and programs when they are first used, and cache them in the
user's home directory.  *Note Compilation::, for more on
auto-compilation.  However, it is better to compile the files before
they are installed, and to just copy the files to a place that Guile can
find them.

   As with Scheme files, Guile searches a path to find compiled `.go'
files, the `%load-compiled-path'.  By default, this path has two
entries: a path for Guile's files, and a path for site packages.  You
should install your `.go' files into the latter.  Currently there is no
procedure to get at this path, which is probably a bug.  As in the
previous example, if Guile 2.0 is installed on your system in `/usr/',
then the place to put compiled files for site packages will be
`/usr/lib/guile/2.0/site-ccache'.

   Note that a `.go' file will only be loaded in preference to a `.scm'
file if it is newer.  For that reason, you should install your Scheme
files first, and your compiled files second.  `Load Paths', for more on
the loading process.

   Finally, although this section is only about Scheme, sometimes you
need to install C extensions too.  Shared libraries should be installed
in the "extensions dir".  This value can be had from the build config
(*note Build Config::).  Again, if Guile 2.0 is installed on your
system in `/usr/', then the extensions dir will be
`/usr/lib/guile/2.0/extensions'.

5 Programming in C
******************

This part of the manual explains the general concepts that you need to
understand when interfacing to Guile from C.  You will learn about how
the latent typing of Scheme is embedded into the static typing of C, how
the garbage collection of Guile is made available to C code, and how
continuations influence the control flow in a C program.

   This knowledge should make it straightforward to add new functions to
Guile that can be called from Scheme.  Adding new data types is also
possible and is done by defining "smobs".

   The *note Programming Overview:: section of this part contains
general musings and guidelines about programming with Guile.  It
explores different ways to design a program around Guile, or how to
embed Guile into existing programs.

   For a pedagogical yet detailed explanation of how the data
representation of Guile is implemented, *Note Data Representation::.
You don't need to know the details given there to use Guile from C, but
they are useful when you want to modify Guile itself or when you are
just curious about how it is all done.

   For detailed reference information on the variables, functions etc.
that make up Guile's application programming interface (API), *Note API
Reference::.

5.1 Parallel Installations
==========================

Guile provides strong API and ABI stability guarantees during stable
series, so that if a user writes a program against Guile version 2.0.3,
it will be compatible with some future version 2.0.7.  We say in this
case that 2.0 is the "effective version", composed of the major and
minor versions, in this case 2 and 0.

   Users may install multiple effective versions of Guile, with each
version's headers, libraries, and Scheme files under their own
directories.  This provides the necessary stability guarantee for users,
while also allowing Guile developers to evolve the language and its
implementation.

   However, parallel installability does have a down-side, in that users
need to know which version of Guile to ask for, when they build against
Guile.  Guile solves this problem by installing a file to be read by the
`pkg-config' utility, a tool to query installed packages by name.
Guile encodes the version into its pkg-config name, so that users can
ask for `guile-2.0' or `guile-2.2', as appropriate.

   For effective version 2.0, for example, you would invoke `pkg-config
--cflags --libs guile-2.0' to get the compilation and linking flags
necessary to link to version 2.0 of Guile.  You would typically run
`pkg-config' during the configuration phase of your program and use the
obtained information in the Makefile.

   Guile's `pkg-config' file, `guile-2.0.pc', defines additional useful
variables:

`sitedir'
     The default directory where Guile looks for Scheme source and
     compiled files (*note %site-dir: Installing Site Packages.).  Run
     `pkg-config guile-2.0 --variable=sitedir' to see its value.  *Note
     GUILE_SITE_DIR: Autoconf Macros, for more on how to use it from
     Autoconf.

`extensiondir'
     The default directory where Guile looks for extensions--i.e.,
     shared libraries providing additional features (*note Modules and
     Extensions::).  Run `pkg-config guile-2.0 --variable=extensiondir'
     to see its value.

See the `pkg-config' man page, for more information, or its web site,
`http://pkg-config.freedesktop.org/'.  *Note Autoconf Support::, for
more on checking for Guile from within a `configure.ac' file.

5.2 Linking Programs With Guile
===============================

This section covers the mechanics of linking your program with Guile on
a typical POSIX system.

   The header file `<libguile.h>' provides declarations for all of
Guile's functions and constants.  You should `#include' it at the head
of any C source file that uses identifiers described in this manual.
Once you've compiled your source files, you need to link them against
the Guile object code library, `libguile'.

   As noted in the previous section, `<libguile.h>' is not in the
default search path for headers.  The following command lines give
respectively the C compilation and link flags needed to build programs
using Guile 2.0:

     pkg-config guile-2.0 --cflags
     pkg-config guile-2.0 --libs

5.2.1 Guile Initialization Functions
------------------------------------

To initialize Guile, you can use one of several functions.  The first,
`scm_with_guile', is the most portable way to initialize Guile.  It
will initialize Guile when necessary and then call a function that you
can specify.  Multiple threads can call `scm_with_guile' concurrently
and it can also be called more than once in a given thread.  The global
state of Guile will survive from one call of `scm_with_guile' to the
next.  Your function is called from within `scm_with_guile' since the
garbage collector of Guile needs to know where the stack of each thread
is.

   A second function, `scm_init_guile', initializes Guile for the
current thread.  When it returns, you can use the Guile API in the
current thread.  This function employs some non-portable magic to learn
about stack bounds and might thus not be available on all platforms.

   One common way to use Guile is to write a set of C functions which
perform some useful task, make them callable from Scheme, and then link
the program with Guile.  This yields a Scheme interpreter just like
`guile', but augmented with extra functions for some specific
application -- a special-purpose scripting language.

   In this situation, the application should probably process its
command-line arguments in the same manner as the stock Guile
interpreter.  To make that straightforward, Guile provides the
`scm_boot_guile' and `scm_shell' function.

   For more about these functions, see *note Initialization::.

5.2.2 A Sample Guile Main Program
---------------------------------

Here is `simple-guile.c', source code for a `main' and an `inner_main'
function that will produce a complete Guile interpreter.

     /* simple-guile.c --- how to start up the Guile
        interpreter from C code.  */

     /* Get declarations for all the scm_ functions.  */
     #include <libguile.h>

     static void
     inner_main (void *closure, int argc, char **argv)
     {
       /* module initializations would go here */
       scm_shell (argc, argv);
     }

     int
     main (int argc, char **argv)
     {
       scm_boot_guile (argc, argv, inner_main, 0);
       return 0; /* never reached */
     }

   The `main' function calls `scm_boot_guile' to initialize Guile,
passing it `inner_main'.  Once `scm_boot_guile' is ready, it invokes
`inner_main', which calls `scm_shell' to process the command-line
arguments in the usual way.

   Here is a Makefile which you can use to compile the above program.
It uses `pkg-config' to learn about the necessary compiler and linker
flags.
     # Use GCC, if you have it installed.
     CC=gcc

     # Tell the C compiler where to find <libguile.h>
     CFLAGS=`pkg-config --cflags guile-2.0`

     # Tell the linker what libraries to use and where to find them.
     LIBS=`pkg-config --libs guile-2.0`

     simple-guile: simple-guile.o
             ${CC} simple-guile.o ${LIBS} -o simple-guile

     simple-guile.o: simple-guile.c
             ${CC} -c ${CFLAGS} simple-guile.c

   If you are using the GNU Autoconf package to make your application
more portable, Autoconf will settle many of the details in the Makefile
above automatically, making it much simpler and more portable; we
recommend using Autoconf with Guile.  Here is a `configure.ac' file for
`simple-guile' that uses the standard `PKG_CHECK_MODULES' macro to
check for Guile.  Autoconf will process this file into a `configure'
script.  We recommend invoking Autoconf via the `autoreconf' utility.

     AC_INIT(simple-guile.c)

     # Find a C compiler.
     AC_PROG_CC

     # Check for Guile
     PKG_CHECK_MODULES([GUILE], [guile-2.0])

     # Generate a Makefile, based on the results.
     AC_OUTPUT(Makefile)

   Run `autoreconf -vif' to generate `configure'.

   Here is a `Makefile.in' template, from which the `configure' script
produces a Makefile customized for the host system:
     # The configure script fills in these values.
     CC=@CC@
     CFLAGS=@GUILE_CFLAGS@
     LIBS=@GUILE_LIBS@

     simple-guile: simple-guile.o
             ${CC} simple-guile.o ${LIBS} -o simple-guile
     simple-guile.o: simple-guile.c
             ${CC} -c ${CFLAGS} simple-guile.c

   The developer should use Autoconf to generate the `configure' script
from the `configure.ac' template, and distribute `configure' with the
application.  Here's how a user might go about building the application:

     $ ls
     Makefile.in     configure*      configure.ac    simple-guile.c
     $ ./configure
     checking for gcc... ccache gcc
     checking whether the C compiler works... yes
     checking for C compiler default output file name... a.out
     checking for suffix of executables...
     checking whether we are cross compiling... no
     checking for suffix of object files... o
     checking whether we are using the GNU C compiler... yes
     checking whether ccache gcc accepts -g... yes
     checking for ccache gcc option to accept ISO C89... none needed
     checking for pkg-config... /usr/bin/pkg-config
     checking pkg-config is at least version 0.9.0... yes
     checking for GUILE... yes
     configure: creating ./config.status
     config.status: creating Makefile
     $ make
     [...]
     $ ./simple-guile
     guile> (+ 1 2 3)
     6
     guile> (getpwnam "jimb")
     #("jimb" "83Z7d75W2tyJQ" 4008 10 "Jim Blandy" "/u/jimb"
       "/usr/local/bin/bash")
     guile> (exit)
     $

5.3 Linking Guile with Libraries
================================

The previous section has briefly explained how to write programs that
make use of an embedded Guile interpreter.  But sometimes, all you want
to do is make new primitive procedures and data types available to the
Scheme programmer.  Writing a new version of `guile' is inconvenient in
this case and it would in fact make the life of the users of your new
features needlessly hard.

   For example, suppose that there is a program `guile-db' that is a
version of Guile with additional features for accessing a database.
People who want to write Scheme programs that use these features would
have to use `guile-db' instead of the usual `guile' program.  Now
suppose that there is also a program `guile-gtk' that extends Guile
with access to the popular Gtk+ toolkit for graphical user interfaces.
People who want to write GUIs in Scheme would have to use `guile-gtk'.
Now, what happens when you want to write a Scheme application that uses
a GUI to let the user access a database?  You would have to write a
_third_ program that incorporates both the database stuff and the GUI
stuff.  This might not be easy (because `guile-gtk' might be a quite
obscure program, say) and taking this example further makes it easy to
see that this approach can not work in practice.

   It would have been much better if both the database features and the
GUI feature had been provided as libraries that can just be linked with
`guile'.  Guile makes it easy to do just this, and we encourage you to
make your extensions to Guile available as libraries whenever possible.

   You write the new primitive procedures and data types in the normal
fashion, and link them into a shared library instead of into a
stand-alone program.  The shared library can then be loaded dynamically
by Guile.

5.3.1 A Sample Guile Extension
------------------------------

This section explains how to make the Bessel functions of the C library
available to Scheme.  First we need to write the appropriate glue code
to convert the arguments and return values of the functions from Scheme
to C and back.  Additionally, we need a function that will add them to
the set of Guile primitives.  Because this is just an example, we will
only implement this for the `j0' function.

   Consider the following file `bessel.c'.

     #include <math.h>
     #include <libguile.h>

     SCM
     j0_wrapper (SCM x)
     {
       return scm_from_double (j0 (scm_to_double (x)));
     }

     void
     init_bessel ()
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
     }

   This C source file needs to be compiled into a shared library.  Here
is how to do it on GNU/Linux:

     gcc `pkg-config --cflags guile-2.0` \
       -shared -o libguile-bessel.so -fPIC bessel.c

   For creating shared libraries portably, we recommend the use of GNU
Libtool (*note Introduction: (libtool)Top.).

   A shared library can be loaded into a running Guile process with the
function `load-extension'.  In addition to the name of the library to
load, this function also expects the name of a function from that
library that will be called to initialize it.  For our example, we are
going to call the function `init_bessel' which will make `j0_wrapper'
available to Scheme programs with the name `j0'.  Note that we do not
specify a filename extension such as `.so' when invoking
`load-extension'.  The right extension for the host platform will be
provided automatically.

     (load-extension "libguile-bessel" "init_bessel")
     (j0 2)
     => 0.223890779141236

   For this to work, `load-extension' must be able to find
`libguile-bessel', of course.  It will look in the places that are
usual for your operating system, and it will additionally look into the
directories listed in the `LTDL_LIBRARY_PATH' environment variable.

   To see how these Guile extensions via shared libraries relate to the
module system, *Note Putting Extensions into Modules::.

5.4 General concepts for using libguile
=======================================

When you want to embed the Guile Scheme interpreter into your program or
library, you need to link it against the `libguile' library (*note
Linking Programs With Guile::).  Once you have done this, your C code
has access to a number of data types and functions that can be used to
invoke the interpreter, or make new functions that you have written in
C available to be called from Scheme code, among other things.

   Scheme is different from C in a number of significant ways, and Guile
tries to make the advantages of Scheme available to C as well.  Thus, in
addition to a Scheme interpreter, libguile also offers dynamic types,
garbage collection, continuations, arithmetic on arbitrary sized
numbers, and other things.

   The two fundamental concepts are dynamic types and garbage
collection.  You need to understand how libguile offers them to C
programs in order to use the rest of libguile.  Also, the more general
control flow of Scheme caused by continuations needs to be dealt with.

   Running asynchronous signal handlers and multi-threading is known to
C code already, but there are of course a few additional rules when
using them together with libguile.

5.4.1 Dynamic Types
-------------------

Scheme is a dynamically-typed language; this means that the system
cannot, in general, determine the type of a given expression at compile
time.  Types only become apparent at run time.  Variables do not have
fixed types; a variable may hold a pair at one point, an integer at the
next, and a thousand-element vector later.  Instead, values, not
variables, have fixed types.

   In order to implement standard Scheme functions like `pair?' and
`string?' and provide garbage collection, the representation of every
value must contain enough information to accurately determine its type
at run time.  Often, Scheme systems also use this information to
determine whether a program has attempted to apply an operation to an
inappropriately typed value (such as taking the `car' of a string).

   Because variables, pairs, and vectors may hold values of any type,
Scheme implementations use a uniform representation for values -- a
single type large enough to hold either a complete value or a pointer
to a complete value, along with the necessary typing information.

   In Guile, this uniform representation of all Scheme values is the C
type `SCM'.  This is an opaque type and its size is typically equivalent
to that of a pointer to `void'.  Thus, `SCM' values can be passed
around efficiently and they take up reasonably little storage on their
own.

   The most important rule is: You never access a `SCM' value directly;
you only pass it to functions or macros defined in libguile.

   As an obvious example, although a `SCM' variable can contain
integers, you can of course not compute the sum of two `SCM' values by
adding them with the C `+' operator.  You must use the libguile
function `scm_sum'.

   Less obvious and therefore more important to keep in mind is that you
also cannot directly test `SCM' values for trueness.  In Scheme, the
value `#f' is considered false and of course a `SCM' variable can
represent that value.  But there is no guarantee that the `SCM'
representation of `#f' looks false to C code as well.  You need to use
`scm_is_true' or `scm_is_false' to test a `SCM' value for trueness or
falseness, respectively.

   You also can not directly compare two `SCM' values to find out
whether they are identical (that is, whether they are `eq?' in Scheme
terms).  You need to use `scm_is_eq' for this.

   The one exception is that you can directly assign a `SCM' value to a
`SCM' variable by using the C `=' operator.

   The following (contrived) example shows how to do it right.  It
implements a function of two arguments (A and FLAG) that returns A+1 if
FLAG is true, else it returns A unchanged.

     SCM
     my_incrementing_function (SCM a, SCM flag)
     {
       SCM result;

       if (scm_is_true (flag))
         result = scm_sum (a, scm_from_int (1));
       else
         result = a;

       return result;
     }

   Often, you need to convert between `SCM' values and appropriate C
values.  For example, we needed to convert the integer `1' to its `SCM'
representation in order to add it to A.  Libguile provides many
function to do these conversions, both from C to `SCM' and from `SCM'
to C.

   The conversion functions follow a common naming pattern: those that
make a `SCM' value from a C value have names of the form `scm_from_TYPE
(...)' and those that convert a `SCM' value to a C value use the form
`scm_to_TYPE (...)'.

   However, it is best to avoid converting values when you can.  When
you must combine C values and `SCM' values in a computation, it is
often better to convert the C values to `SCM' values and do the
computation by using libguile functions than to the other way around
(converting `SCM' to C and doing the computation some other way).

   As a simple example, consider this version of
`my_incrementing_function' from above:

     SCM
     my_other_incrementing_function (SCM a, SCM flag)
     {
       int result;

       if (scm_is_true (flag))
         result = scm_to_int (a) + 1;
       else
         result = scm_to_int (a);

       return scm_from_int (result);
     }

   This version is much less general than the original one: it will only
work for values A that can fit into a `int'.  The original function
will work for all values that Guile can represent and that `scm_sum'
can understand, including integers bigger than `long long', floating
point numbers, complex numbers, and new numerical types that have been
added to Guile by third-party libraries.

   Also, computing with `SCM' is not necessarily inefficient.  Small
integers will be encoded directly in the `SCM' value, for example, and
do not need any additional memory on the heap.  See *note Data
Representation:: to find out the details.

   Some special `SCM' values are available to C code without needing to
convert them from C values:

Scheme value   C representation
#f             SCM_BOOL_F
#t             SCM_BOOL_T
()             SCM_EOL

   In addition to `SCM', Guile also defines the related type
`scm_t_bits'.  This is an unsigned integral type of sufficient size to
hold all information that is directly contained in a `SCM' value.  The
`scm_t_bits' type is used internally by Guile to do all the bit
twiddling explained in *note Data Representation::, but you will
encounter it occasionally in low-level user code as well.

5.4.2 Garbage Collection
------------------------

As explained above, the `SCM' type can represent all Scheme values.
Some values fit entirely into a `SCM' value (such as small integers),
but other values require additional storage in the heap (such as
strings and vectors).  This additional storage is managed automatically
by Guile.  You don't need to explicitly deallocate it when a `SCM'
value is no longer used.

   Two things must be guaranteed so that Guile is able to manage the
storage automatically: it must know about all blocks of memory that have
ever been allocated for Scheme values, and it must know about all Scheme
values that are still being used.  Given this knowledge, Guile can
periodically free all blocks that have been allocated but are not used
by any active Scheme values.  This activity is called "garbage
collection".

   It is easy for Guile to remember all blocks of memory that it has
allocated for use by Scheme values, but you need to help it with finding
all Scheme values that are in use by C code.

   You do this when writing a SMOB mark function, for example (*note
Garbage Collecting Smobs::).  By calling this function, the garbage
collector learns about all references that your SMOB has to other `SCM'
values.

   Other references to `SCM' objects, such as global variables of type
`SCM' or other random data structures in the heap that contain fields
of type `SCM', can be made visible to the garbage collector by calling
the functions `scm_gc_protect' or `scm_permanent_object'.  You normally
use these functions for long lived objects such as a hash table that is
stored in a global variable.  For temporary references in local
variables or function arguments, using these functions would be too
expensive.

   These references are handled differently: Local variables (and
function arguments) of type `SCM' are automatically visible to the
garbage collector.  This works because the collector scans the stack for
potential references to `SCM' objects and considers all referenced
objects to be alive.  The scanning considers each and every word of the
stack, regardless of what it is actually used for, and then decides
whether it could possibly be a reference to a `SCM' object.  Thus, the
scanning is guaranteed to find all actual references, but it might also
find words that only accidentally look like references.  These `false
positives' might keep `SCM' objects alive that would otherwise be
considered dead.  While this might waste memory, keeping an object
around longer than it strictly needs to is harmless.  This is why this
technique is called "conservative garbage collection".  In practice,
the wasted memory seems to be no problem.

   The stack of every thread is scanned in this way and the registers of
the CPU and all other memory locations where local variables or function
parameters might show up are included in this scan as well.

   The consequence of the conservative scanning is that you can just
declare local variables and function parameters of type `SCM' and be
sure that the garbage collector will not free the corresponding objects.

   However, a local variable or function parameter is only protected as
long as it is really on the stack (or in some register).  As an
optimization, the C compiler might reuse its location for some other
value and the `SCM' object would no longer be protected.  Normally,
this leads to exactly the right behavior: the compiler will only
overwrite a reference when it is no longer needed and thus the object
becomes unprotected precisely when the reference disappears, just as
wanted.

   There are situations, however, where a `SCM' object needs to be
around longer than its reference from a local variable or function
parameter.  This happens, for example, when you retrieve some pointer
from a smob and work with that pointer directly.  The reference to the
`SCM' smob object might be dead after the pointer has been retrieved,
but the pointer itself (and the memory pointed to) is still in use and
thus the smob object must be protected.  The compiler does not know
about this connection and might overwrite the `SCM' reference too early.

   To get around this problem, you can use `scm_remember_upto_here_1'
and its cousins.  It will keep the compiler from overwriting the
reference.  For a typical example of its use, see *note Remembering
During Operations::.

5.4.3 Control Flow
------------------

Scheme has a more general view of program flow than C, both locally and
non-locally.

   Controlling the local flow of control involves things like gotos,
loops, calling functions and returning from them.  Non-local control
flow refers to situations where the program jumps across one or more
levels of function activations without using the normal call or return
operations.

   The primitive means of C for local control flow is the `goto'
statement, together with `if'.  Loops done with `for', `while' or `do'
could in principle be rewritten with just `goto' and `if'.  In Scheme,
the primitive means for local control flow is the _function call_
(together with `if').  Thus, the repetition of some computation in a
loop is ultimately implemented by a function that calls itself, that
is, by recursion.

   This approach is theoretically very powerful since it is easier to
reason formally about recursion than about gotos.  In C, using
recursion exclusively would not be practical, though, since it would eat
up the stack very quickly.  In Scheme, however, it is practical:
function calls that appear in a "tail position" do not use any
additional stack space (*note Tail Calls::).

   A function call is in a tail position when it is the last thing the
calling function does.  The value returned by the called function is
immediately returned from the calling function.  In the following
example, the call to `bar-1' is in a tail position, while the call to
`bar-2' is not.  (The call to `1-' in `foo-2' is in a tail position,
though.)

     (define (foo-1 x)
       (bar-1 (1- x)))

     (define (foo-2 x)
       (1- (bar-2 x)))

   Thus, when you take care to recurse only in tail positions, the
recursion will only use constant stack space and will be as good as a
loop constructed from gotos.

   Scheme offers a few syntactic abstractions (`do' and "named" `let')
that make writing loops slightly easier.

   But only Scheme functions can call other functions in a tail
position: C functions can not.  This matters when you have, say, two
functions that call each other recursively to form a common loop.  The
following (unrealistic) example shows how one might go about
determining whether a non-negative integer N is even or odd.

     (define (my-even? n)
       (cond ((zero? n) #t)
             (else (my-odd? (1- n)))))

     (define (my-odd? n)
       (cond ((zero? n) #f)
             (else (my-even? (1- n)))))

   Because the calls to `my-even?' and `my-odd?' are in tail positions,
these two procedures can be applied to arbitrary large integers without
overflowing the stack.  (They will still take a lot of time, of course.)

   However, when one or both of the two procedures would be rewritten in
C, it could no longer call its companion in a tail position (since C
does not have this concept).  You might need to take this consideration
into account when deciding which parts of your program to write in
Scheme and which in C.

   In addition to calling functions and returning from them, a Scheme
program can also exit non-locally from a function so that the control
flow returns directly to an outer level.  This means that some functions
might not return at all.

   Even more, it is not only possible to jump to some outer level of
control, a Scheme program can also jump back into the middle of a
function that has already exited.  This might cause some functions to
return more than once.

   In general, these non-local jumps are done by invoking
"continuations" that have previously been captured using
`call-with-current-continuation'.  Guile also offers a slightly
restricted set of functions, `catch' and `throw', that can only be used
for non-local exits.  This restriction makes them more efficient.
Error reporting (with the function `error') is implemented by invoking
`throw', for example.  The functions `catch' and `throw' belong to the
topic of "exceptions".

   Since Scheme functions can call C functions and vice versa, C code
can experience the more general control flow of Scheme as well.  It is
possible that a C function will not return at all, or will return more
than once.  While C does offer `setjmp' and `longjmp' for non-local
exits, it is still an unusual thing for C code.  In contrast, non-local
exits are very common in Scheme, mostly to report errors.

   You need to be prepared for the non-local jumps in the control flow
whenever you use a function from `libguile': it is best to assume that
any `libguile' function might signal an error or run a pending signal
handler (which in turn can do arbitrary things).

   It is often necessary to take cleanup actions when the control
leaves a function non-locally.  Also, when the control returns
non-locally, some setup actions might be called for.  For example, the
Scheme function `with-output-to-port' needs to modify the global state
so that `current-output-port' returns the port passed to
`with-output-to-port'.  The global output port needs to be reset to its
previous value when `with-output-to-port' returns normally or when it
is exited non-locally.  Likewise, the port needs to be set again when
control enters non-locally.

   Scheme code can use the `dynamic-wind' function to arrange for the
setting and resetting of the global state.  C code can use the
corresponding `scm_internal_dynamic_wind' function, or a
`scm_dynwind_begin'/`scm_dynwind_end' pair together with suitable
'dynwind actions' (*note Dynamic Wind::).

   Instead of coping with non-local control flow, you can also prevent
it by erecting a _continuation barrier_, *Note Continuation Barriers::.
The function `scm_c_with_continuation_barrier', for example, is
guaranteed to return exactly once.

5.4.4 Asynchronous Signals
--------------------------

You can not call libguile functions from handlers for POSIX signals, but
you can register Scheme handlers for POSIX signals such as `SIGINT'.
These handlers do not run during the actual signal delivery.  Instead,
they are run when the program (more precisely, the thread that the
handler has been registered for) reaches the next _safe point_.

   The libguile functions themselves have many such safe points.
Consequently, you must be prepared for arbitrary actions anytime you
call a libguile function.  For example, even `scm_cons' can contain a
safe point and when a signal handler is pending for your thread,
calling `scm_cons' will run this handler and anything might happen,
including a non-local exit although `scm_cons' would not ordinarily do
such a thing on its own.

   If you do not want to allow the running of asynchronous signal
handlers, you can block them temporarily with
`scm_dynwind_block_asyncs', for example.  See *Note System asyncs::.

   Since signal handling in Guile relies on safe points, you need to
make sure that your functions do offer enough of them.  Normally,
calling libguile functions in the normal course of action is all that
is needed.  But when a thread might spent a long time in a code section
that calls no libguile function, it is good to include explicit safe
points.  This can allow the user to interrupt your code with <C-c>, for
example.

   You can do this with the macro `SCM_TICK'.  This macro is
syntactically a statement.  That is, you could use it like this:

     while (1)
       {
         SCM_TICK;
         do_some_work ();
       }

   Frequent execution of a safe point is even more important in multi
threaded programs, *Note Multi-Threading::.

5.4.5 Multi-Threading
---------------------

Guile can be used in multi-threaded programs just as well as in
single-threaded ones.

   Each thread that wants to use functions from libguile must put itself
into _guile mode_ and must then follow a few rules.  If it doesn't want
to honor these rules in certain situations, a thread can temporarily
leave guile mode (but can no longer use libguile functions during that
time, of course).

   Threads enter guile mode by calling `scm_with_guile',
`scm_boot_guile', or `scm_init_guile'.  As explained in the reference
documentation for these functions, Guile will then learn about the
stack bounds of the thread and can protect the `SCM' values that are
stored in local variables.  When a thread puts itself into guile mode
for the first time, it gets a Scheme representation and is listed by
`all-threads', for example.

   Threads in guile mode can block (e.g., do blocking I/O) without
causing any problems(1); temporarily leaving guile mode with
`scm_without_guile' before blocking slightly improves GC performance,
though.  For some common blocking operations, Guile provides
convenience functions.  For example, if you want to lock a pthread
mutex while in guile mode, you might want to use
`scm_pthread_mutex_lock' which is just like `pthread_mutex_lock' except
that it leaves guile mode while blocking.

   All libguile functions are (intended to be) robust in the face of
multiple threads using them concurrently.  This means that there is no
risk of the internal data structures of libguile becoming corrupted in
such a way that the process crashes.

   A program might still produce nonsensical results, though.  Taking
hashtables as an example, Guile guarantees that you can use them from
multiple threads concurrently and a hashtable will always remain a valid
hashtable and Guile will not crash when you access it.  It does not
guarantee, however, that inserting into it concurrently from two threads
will give useful results: only one insertion might actually happen, none
might happen, or the table might in general be modified in a totally
arbitrary manner.  (It will still be a valid hashtable, but not the one
that you might have expected.)  Guile might also signal an error when it
detects a harmful race condition.

   Thus, you need to put in additional synchronizations when multiple
threads want to use a single hashtable, or any other mutable Scheme
object.

   When writing C code for use with libguile, you should try to make it
robust as well.  An example that converts a list into a vector will help
to illustrate.  Here is a correct version:

     SCM
     my_list_to_vector (SCM list)
     {
       SCM vector = scm_make_vector (scm_length (list), SCM_UNDEFINED);
       size_t len, i;

       len = SCM_SIMPLE_VECTOR_LENGTH (vector);
       i = 0;
       while (i < len && scm_is_pair (list))
         {
           SCM_SIMPLE_VECTOR_SET (vector, i, SCM_CAR (list));
           list = SCM_CDR (list);
           i++;
         }

       return vector;
     }

   The first thing to note is that storing into a `SCM' location
concurrently from multiple threads is guaranteed to be robust: you don't
know which value wins but it will in any case be a valid `SCM' value.

   But there is no guarantee that the list referenced by LIST is not
modified in another thread while the loop iterates over it.  Thus, while
copying its elements into the vector, the list might get longer or
shorter.  For this reason, the loop must check both that it doesn't
overrun the vector (`SCM_SIMPLE_VECTOR_SET' does no range-checking) and
that it doesn't overrun the list (`SCM_CAR' and `SCM_CDR' likewise do
no type checking).

   It is safe to use `SCM_CAR' and `SCM_CDR' on the local variable LIST
once it is known that the variable contains a pair.  The contents of
the pair might change spontaneously, but it will always stay a valid
pair (and a local variable will of course not spontaneously point to a
different Scheme object).

   Likewise, a simple vector such as the one returned by
`scm_make_vector' is guaranteed to always stay the same length so that
it is safe to only use SCM_SIMPLE_VECTOR_LENGTH once and store the
result.  (In the example, VECTOR is safe anyway since it is a fresh
object that no other thread can possibly know about until it is
returned from `my_list_to_vector'.)

   Of course the behavior of `my_list_to_vector' is suboptimal when
LIST does indeed get asynchronously lengthened or shortened in another
thread.  But it is robust: it will always return a valid vector.  That
vector might be shorter than expected, or its last elements might be
unspecified, but it is a valid vector and if a program wants to rule
out these cases, it must avoid modifying the list asynchronously.

   Here is another version that is also correct:

     SCM
     my_pedantic_list_to_vector (SCM list)
     {
       SCM vector = scm_make_vector (scm_length (list), SCM_UNDEFINED);
       size_t len, i;

       len = SCM_SIMPLE_VECTOR_LENGTH (vector);
       i = 0;
       while (i < len)
         {
           SCM_SIMPLE_VECTOR_SET (vector, i, scm_car (list));
           list = scm_cdr (list);
           i++;
         }

       return vector;
     }

   This version uses the type-checking and thread-robust functions
`scm_car' and `scm_cdr' instead of the faster, but less robust macros
`SCM_CAR' and `SCM_CDR'.  When the list is shortened (that is, when
LIST holds a non-pair), `scm_car' will throw an error.  This might be
preferable to just returning a half-initialized vector.

   The API for accessing vectors and arrays of various kinds from C
takes a slightly different approach to thread-robustness.  In order to
get at the raw memory that stores the elements of an array, you need to
_reserve_ that array as long as you need the raw memory.  During the
time an array is reserved, its elements can still spontaneously change
their values, but the memory itself and other things like the size of
the array are guaranteed to stay fixed.  Any operation that would
change these parameters of an array that is currently reserved will
signal an error.  In order to avoid these errors, a program should of
course put suitable synchronization mechanisms in place.  As you can
see, Guile itself is again only concerned about robustness, not about
correctness: without proper synchronization, your program will likely
not be correct, but the worst consequence is an error message.

   Real thread-safeness often requires that a critical section of code
is executed in a certain restricted manner.  A common requirement is
that the code section is not entered a second time when it is already
being executed.  Locking a mutex while in that section ensures that no
other thread will start executing it, blocking asyncs ensures that no
asynchronous code enters the section again from the current thread, and
the error checking of Guile mutexes guarantees that an error is
signalled when the current thread accidentally reenters the critical
section via recursive function calls.

   Guile provides two mechanisms to support critical sections as
outlined above.  You can either use the macros
`SCM_CRITICAL_SECTION_START' and `SCM_CRITICAL_SECTION_END' for very
simple sections; or use a dynwind context together with a call to
`scm_dynwind_critical_section'.

   The macros only work reliably for critical sections that are
guaranteed to not cause a non-local exit.  They also do not detect an
accidental reentry by the current thread.  Thus, you should probably
only use them to delimit critical sections that do not contain calls to
libguile functions or to other external functions that might do
complicated things.

   The function `scm_dynwind_critical_section', on the other hand, will
correctly deal with non-local exits because it requires a dynwind
context.  Also, by using a separate mutex for each critical section, it
can detect accidental reentries.

   ---------- Footnotes ----------

   (1) In Guile 1.8, a thread blocking in guile mode would prevent
garbage collection to occur.  Thus, threads had to leave guile mode
whenever they could block.  This is no longer needed with Guile 2.0.

5.5 Defining New Types (Smobs)
==============================

"Smobs" are Guile's mechanism for adding new primitive types to the
system.  The term "smob" was coined by Aubrey Jaffer, who says it comes
from "small object", referring to the fact that they are quite limited
in size: they can hold just one pointer to a larger memory block plus
16 extra bits.

   To define a new smob type, the programmer provides Guile with some
essential information about the type -- how to print it, how to garbage
collect it, and so on -- and Guile allocates a fresh type tag for it.
The programmer can then use `scm_c_define_gsubr' to make a set of C
functions visible to Scheme code that create and operate on these
objects.

   (You can find a complete version of the example code used in this
section in the Guile distribution, in `doc/example-smob'.  That
directory includes a makefile and a suitable `main' function, so you
can build a complete interactive Guile shell, extended with the
datatypes described here.)

5.5.1 Describing a New Type
---------------------------

To define a new type, the programmer must write four functions to
manage instances of the type:

`mark'
     Guile will apply this function to each instance of the new type it
     encounters during garbage collection.  This function is
     responsible for telling the collector about any other `SCM' values
     that the object has stored.  The default smob mark function does
     nothing.  *Note Garbage Collecting Smobs::, for more details.

`free'
     Guile will apply this function to each instance of the new type
     that is to be deallocated.  The function should release all
     resources held by the object.  This is analogous to the Java
     finalization method- it is invoked at an unspecified time (when
     garbage collection occurs) after the object is dead.  The default
     free function frees the smob data (if the size of the struct
     passed to `scm_make_smob_type' is non-zero) using `scm_gc_free'.
     *Note Garbage Collecting Smobs::, for more details.

     This function operates while the heap is in an inconsistent state
     and must therefore be careful.  *Note Smobs::, for details about
     what this function is allowed to do.

`print'
     Guile will apply this function to each instance of the new type to
     print the value, as for `display' or `write'.  The default print
     function prints `#<NAME ADDRESS>' where `NAME' is the first
     argument passed to `scm_make_smob_type'.

`equalp'
     If Scheme code asks the `equal?' function to compare two instances
     of the same smob type, Guile calls this function.  It should return
     `SCM_BOOL_T' if A and B should be considered `equal?', or
     `SCM_BOOL_F' otherwise.  If `equalp' is `NULL', `equal?' will
     assume that two instances of this type are never `equal?' unless
     they are `eq?'.


   To actually register the new smob type, call `scm_make_smob_type'.
It returns a value of type `scm_t_bits' which identifies the new smob
type.

   The four special functions described above are registered by calling
one of `scm_set_smob_mark', `scm_set_smob_free', `scm_set_smob_print',
or `scm_set_smob_equalp', as appropriate.  Each function is intended to
be used at most once per type, and the call should be placed
immediately following the call to `scm_make_smob_type'.

   There can only be at most 256 different smob types in the system.
Instead of registering a huge number of smob types (for example, one
for each relevant C struct in your application), it is sometimes better
to register just one and implement a second layer of type dispatching
on top of it.  This second layer might use the 16 extra bits to extend
its type, for example.

   Here is how one might declare and register a new type representing
eight-bit gray-scale images:

     #include <libguile.h>

     struct image {
       int width, height;
       char *pixels;

       /* The name of this image */
       SCM name;

       /* A function to call when this image is
          modified, e.g., to update the screen,
          or SCM_BOOL_F if no action necessary */
       SCM update_func;
     };

     static scm_t_bits image_tag;

     void
     init_image_type (void)
     {
       image_tag = scm_make_smob_type ("image", sizeof (struct image));
       scm_set_smob_mark (image_tag, mark_image);
       scm_set_smob_free (image_tag, free_image);
       scm_set_smob_print (image_tag, print_image);
     }

5.5.2 Creating Smob Instances
-----------------------------

Normally, smobs can have one _immediate_ word of data.  This word
stores either a pointer to an additional memory block that holds the
real data, or it might hold the data itself when it fits.  The word is
large enough for a `SCM' value, a pointer to `void', or an integer that
fits into a `size_t' or `ssize_t'.

   You can also create smobs that have two or three immediate words, and
when these words suffice to store all data, it is more efficient to use
these super-sized smobs instead of using a normal smob plus a memory
block.  *Note Double Smobs::, for their discussion.

   Guile provides functions for managing memory which are often helpful
when implementing smobs.  *Note Memory Blocks::.

   To retrieve the immediate word of a smob, you use the macro
`SCM_SMOB_DATA'.  It can be set with `SCM_SET_SMOB_DATA'.  The 16 extra
bits can be accessed with `SCM_SMOB_FLAGS' and `SCM_SET_SMOB_FLAGS'.

   The two macros `SCM_SMOB_DATA' and `SCM_SET_SMOB_DATA' treat the
immediate word as if it were of type `scm_t_bits', which is an unsigned
integer type large enough to hold a pointer to `void'.  Thus you can
use these macros to store arbitrary pointers in the smob word.

   When you want to store a `SCM' value directly in the immediate word
of a smob, you should use the macros `SCM_SMOB_OBJECT' and
`SCM_SET_SMOB_OBJECT' to access it.

   Creating a smob instance can be tricky when it consists of multiple
steps that allocate resources and might fail.  It is recommended that
you go about creating a smob in the following way:

   * Allocate the memory block for holding the data with
     `scm_gc_malloc'.

   * Initialize it to a valid state without calling any functions that
     might cause a non-local exits.  For example, initialize pointers
     to NULL.  Also, do not store `SCM' values in it that must be
     protected.  Initialize these fields with `SCM_BOOL_F'.

     A valid state is one that can be safely acted upon by the _mark_
     and _free_ functions of your smob type.

   * Create the smob using `SCM_NEWSMOB', passing it the initialized
     memory block.  (This step will always succeed.)

   * Complete the initialization of the memory block by, for example,
     allocating additional resources and making it point to them.

   This procedure ensures that the smob is in a valid state as soon as
it exists, that all resources that are allocated for the smob are
properly associated with it so that they can be properly freed, and
that no `SCM' values that need to be protected are stored in it while
the smob does not yet completely exist and thus can not protect them.

   Continuing the example from above, if the global variable
`image_tag' contains a tag returned by `scm_make_smob_type', here is
how we could construct a smob whose immediate word contains a pointer
to a freshly allocated `struct image':

     SCM
     make_image (SCM name, SCM s_width, SCM s_height)
     {
       SCM smob;
       struct image *image;
       int width = scm_to_int (s_width);
       int height = scm_to_int (s_height);

       /* Step 1: Allocate the memory block.
        */
       image = (struct image *)
          scm_gc_malloc (sizeof (struct image), "image");

       /* Step 2: Initialize it with straight code.
        */
       image->width = width;
       image->height = height;
       image->pixels = NULL;
       image->name = SCM_BOOL_F;
       image->update_func = SCM_BOOL_F;

       /* Step 3: Create the smob.
        */
       SCM_NEWSMOB (smob, image_tag, image);

       /* Step 4: Finish the initialization.
        */
       image->name = name;
       image->pixels =
          scm_gc_malloc (width * height, "image pixels");

       return smob;
     }

   Let us look at what might happen when `make_image' is called.

   The conversions of S_WIDTH and S_HEIGHT to `int's might fail and
signal an error, thus causing a non-local exit.  This is not a problem
since no resources have been allocated yet that would have to be freed.

   The allocation of IMAGE in step 1 might fail, but this is likewise
no problem.

   Step 2 can not exit non-locally.  At the end of it, the IMAGE struct
is in a valid state for the `mark_image' and `free_image' functions
(see below).

   Step 3 can not exit non-locally either.  This is guaranteed by Guile.
After it, SMOB contains a valid smob that is properly initialized and
protected, and in turn can properly protect the Scheme values in its
IMAGE struct.

   But before the smob is completely created, `SCM_NEWSMOB' might cause
the garbage collector to run.  During this garbage collection, the
`SCM' values in the IMAGE struct would be invisible to Guile.  It only
gets to know about them via the `mark_image' function, but that
function can not yet do its job since the smob has not been created
yet.  Thus, it is important to not store `SCM' values in the IMAGE
struct until after the smob has been created.

   Step 4, finally, might fail and cause a non-local exit.  In that
case, the complete creation of the smob has not been successful, but it
does nevertheless exist in a valid state.  It will eventually be freed
by the garbage collector, and all the resources that have been allocated
for it will be correctly freed by `free_image'.

5.5.3 Type checking
-------------------

Functions that operate on smobs should check that the passed `SCM'
value indeed is a suitable smob before accessing its data.  They can do
this with `scm_assert_smob_type'.

   For example, here is a simple function that operates on an image
smob, and checks the type of its argument.

     SCM
     clear_image (SCM image_smob)
     {
       int area;
       struct image *image;

       scm_assert_smob_type (image_tag, image_smob);

       image = (struct image *) SCM_SMOB_DATA (image_smob);
       area = image->width * image->height;
       memset (image->pixels, 0, area);

       /* Invoke the image's update function.
        */
       if (scm_is_true (image->update_func))
         scm_call_0 (image->update_func);

       scm_remember_upto_here_1 (image_smob);

       return SCM_UNSPECIFIED;
     }

   See *note Remembering During Operations:: for an explanation of the
call to `scm_remember_upto_here_1'.

5.5.4 Garbage Collecting Smobs
------------------------------

Once a smob has been released to the tender mercies of the Scheme
system, it must be prepared to survive garbage collection.  Guile calls
the _mark_ and _free_ functions of the smob to manage this.

   As described in more detail elsewhere (*note Conservative GC::),
every object in the Scheme system has a "mark bit", which the garbage
collector uses to tell live objects from dead ones.  When collection
starts, every object's mark bit is clear.  The collector traces pointers
through the heap, starting from objects known to be live, and sets the
mark bit on each object it encounters.  When it can find no more
unmarked objects, the collector walks all objects, live and dead, frees
those whose mark bits are still clear, and clears the mark bit on the
others.

   The two main portions of the collection are called the "mark phase",
during which the collector marks live objects, and the "sweep phase",
during which the collector frees all unmarked objects.

   The mark bit of a smob lives in a special memory region.  When the
collector encounters a smob, it sets the smob's mark bit, and uses the
smob's type tag to find the appropriate _mark_ function for that smob.
It then calls this _mark_ function, passing it the smob as its only
argument.

   The _mark_ function is responsible for marking any other Scheme
objects the smob refers to.  If it does not do so, the objects' mark
bits will still be clear when the collector begins to sweep, and the
collector will free them.  If this occurs, it will probably break, or at
least confuse, any code operating on the smob; the smob's `SCM' values
will have become dangling references.

   To mark an arbitrary Scheme object, the _mark_ function calls
`scm_gc_mark'.

   Thus, here is how we might write `mark_image':

     SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_mark (image->name);
       scm_gc_mark (image->update_func);

       return SCM_BOOL_F;
     }

   Note that, even though the image's `update_func' could be an
arbitrarily complex structure (representing a procedure and any values
enclosed in its environment), `scm_gc_mark' will recurse as necessary
to mark all its components.  Because `scm_gc_mark' sets an object's
mark bit before it recurses, it is not confused by circular structures.

   As an optimization, the collector will mark whatever value is
returned by the _mark_ function; this helps limit depth of recursion
during the mark phase.  Thus, the code above should really be written
as:
     SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_mark (image->name);
       return image->update_func;
     }

   Finally, when the collector encounters an unmarked smob during the
sweep phase, it uses the smob's tag to find the appropriate _free_
function for the smob.  It then calls that function, passing it the smob
as its only argument.

   The _free_ function must release any resources used by the smob.
However, it must not free objects managed by the collector; the
collector will take care of them.  For historical reasons, the return
type of the _free_ function should be `size_t', an unsigned integral
type; the _free_ function should always return zero.

   Here is how we might write the `free_image' function for the image
smob type:
     size_t
     free_image (SCM image_smob)
     {
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_free (image->pixels,
                    image->width * image->height,
                    "image pixels");
       scm_gc_free (image, sizeof (struct image), "image");

       return 0;
     }

   During the sweep phase, the garbage collector will clear the mark
bits on all live objects.  The code which implements a smob need not do
this itself.

   There is no way for smob code to be notified when collection is
complete.

   It is usually a good idea to minimize the amount of processing done
during garbage collection; keep the _mark_ and _free_ functions very
simple.  Since collections occur at unpredictable times, it is easy for
any unusual activity to interfere with normal code.

5.5.5 Garbage Collecting Simple Smobs
-------------------------------------

It is often useful to define very simple smob types -- smobs which have
no data to mark, other than the cell itself, or smobs whose immediate
data word is simply an ordinary Scheme object, to be marked recursively.
Guile provides some functions to handle these common cases; you can use
this function as your smob type's _mark_ function, if your smob's
structure is simple enough.

   If the smob refers to no other Scheme objects, then no action is
necessary; the garbage collector has already marked the smob cell
itself.  In that case, you can use zero as your mark function.

   If the smob refers to exactly one other Scheme object via its first
immediate word, you can use `scm_markcdr' as its mark function.  Its
definition is simply:

     SCM
     scm_markcdr (SCM obj)
     {
       return SCM_SMOB_OBJECT (obj);
     }

5.5.6 Remembering During Operations
-----------------------------------

It's important that a smob is visible to the garbage collector whenever
its contents are being accessed.  Otherwise it could be freed while
code is still using it.

   For example, consider a procedure to convert image data to a list of
pixel values.

     SCM
     image_to_list (SCM image_smob)
     {
       struct image *image;
       SCM lst;
       int i;

       scm_assert_smob_type (image_tag, image_smob);

       image = (struct image *) SCM_SMOB_DATA (image_smob);
       lst = SCM_EOL;
       for (i = image->width * image->height - 1; i >= 0; i--)
         lst = scm_cons (scm_from_char (image->pixels[i]), lst);

       scm_remember_upto_here_1 (image_smob);
       return lst;
     }

   In the loop, only the `image' pointer is used and the C compiler has
no reason to keep the `image_smob' value anywhere.  If `scm_cons'
results in a garbage collection, `image_smob' might not be on the stack
or anywhere else and could be freed, leaving the loop accessing freed
data.  The use of `scm_remember_upto_here_1' prevents this, by creating
a reference to `image_smob' after all data accesses.

   There's no need to do the same for `lst', since that's the return
value and the compiler will certainly keep it in a register or
somewhere throughout the routine.

   The `clear_image' example previously shown (*note Type checking::)
also used `scm_remember_upto_here_1' for this reason.

   It's only in quite rare circumstances that a missing
`scm_remember_upto_here_1' will bite, but when it happens the
consequences are serious.  Fortunately the rule is simple: whenever
calling a Guile library function or doing something that might, ensure
that the `SCM' of a smob is referenced past all accesses to its
insides.  Do this by adding an `scm_remember_upto_here_1' if there are
no other references.

   In a multi-threaded program, the rule is the same.  As far as a given
thread is concerned, a garbage collection still only occurs within a
Guile library function, not at an arbitrary time.  (Guile waits for all
threads to reach one of its library functions, and holds them there
while the collector runs.)

5.5.7 Double Smobs
------------------

Smobs are called smob because they are small: they normally have only
room for one `void*' or `SCM' value plus 16 bits.  The reason for this
is that smobs are directly implemented by using the low-level, two-word
cells of Guile that are also used to implement pairs, for example.
(*note Data Representation:: for the details.)  One word of the
two-word cells is used for `SCM_SMOB_DATA' (or `SCM_SMOB_OBJECT'), the
other contains the 16-bit type tag and the 16 extra bits.

   In addition to the fundamental two-word cells, Guile also has
four-word cells, which are appropriately called "double cells".  You
can use them for "double smobs" and get two more immediate words of
type `scm_t_bits'.

   A double smob is created with `SCM_NEWSMOB2' or `SCM_NEWSMOB3'
instead of `SCM_NEWSMOB'.  Its immediate words can be retrieved as
`scm_t_bits' with `SCM_SMOB_DATA_2' and `SCM_SMOB_DATA_3' in addition to
`SCM_SMOB_DATA'.  Unsurprisingly, the words can be set to `scm_t_bits'
values with `SCM_SET_SMOB_DATA_2' and `SCM_SET_SMOB_DATA_3'.

   Of course there are also `SCM_SMOB_OBJECT_2', `SCM_SMOB_OBJECT_3',
`SCM_SET_SMOB_OBJECT_2', and `SCM_SET_SMOB_OBJECT_3'.

5.5.8 The Complete Example
--------------------------

Here is the complete text of the implementation of the image datatype,
as presented in the sections above.  We also provide a definition for
the smob's _print_ function, and make some objects and functions
static, to clarify exactly what the surrounding code is using.

   As mentioned above, you can find this code in the Guile
distribution, in `doc/example-smob'.  That directory includes a
makefile and a suitable `main' function, so you can build a complete
interactive Guile shell, extended with the datatypes described here.)

     /* file "image-type.c" */

     #include <stdlib.h>
     #include <libguile.h>

     static scm_t_bits image_tag;

     struct image {
       int width, height;
       char *pixels;

       /* The name of this image */
       SCM name;

       /* A function to call when this image is
          modified, e.g., to update the screen,
          or SCM_BOOL_F if no action necessary */
       SCM update_func;
     };

     static SCM
     make_image (SCM name, SCM s_width, SCM s_height)
     {
       SCM smob;
       struct image *image;
       int width = scm_to_int (s_width);
       int height = scm_to_int (s_height);

       /* Step 1: Allocate the memory block.
        */
       image = (struct image *)
          scm_gc_malloc (sizeof (struct image), "image");

       /* Step 2: Initialize it with straight code.
        */
       image->width = width;
       image->height = height;
       image->pixels = NULL;
       image->name = SCM_BOOL_F;
       image->update_func = SCM_BOOL_F;

       /* Step 3: Create the smob.
        */
       SCM_NEWSMOB (smob, image_tag, image);

       /* Step 4: Finish the initialization.
        */
       image->name = name;
       image->pixels =
          scm_gc_malloc (width * height, "image pixels");

       return smob;
     }

     SCM
     clear_image (SCM image_smob)
     {
       int area;
       struct image *image;

       scm_assert_smob_type (image_tag, image_smob);

       image = (struct image *) SCM_SMOB_DATA (image_smob);
       area = image->width * image->height;
       memset (image->pixels, 0, area);

       /* Invoke the image's update function.
        */
       if (scm_is_true (image->update_func))
         scm_call_0 (image->update_func);

       scm_remember_upto_here_1 (image_smob);

       return SCM_UNSPECIFIED;
     }

     static SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_mark (image->name);
       return image->update_func;
     }

     static size_t
     free_image (SCM image_smob)
     {
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_gc_free (image->pixels,
                    image->width * image->height,
                    "image pixels");
       scm_gc_free (image, sizeof (struct image), "image");

       return 0;
     }

     static int
     print_image (SCM image_smob, SCM port, scm_print_state *pstate)
     {
       struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

       scm_puts ("#<image ", port);
       scm_display (image->name, port);
       scm_puts (">", port);

       /* non-zero means success */
       return 1;
     }

     void
     init_image_type (void)
     {
       image_tag = scm_make_smob_type ("image", sizeof (struct image));
       scm_set_smob_mark (image_tag, mark_image);
       scm_set_smob_free (image_tag, free_image);
       scm_set_smob_print (image_tag, print_image);

       scm_c_define_gsubr ("clear-image", 1, 0, 0, clear_image);
       scm_c_define_gsubr ("make-image", 3, 0, 0, make_image);
     }

   Here is a sample build and interaction with the code from the
`example-smob' directory, on the author's machine:

     zwingli:example-smob$ make CC=gcc
     gcc `pkg-config --cflags guile-2.0` -c image-type.c -o image-type.o
     gcc `pkg-config --cflags guile-2.0` -c myguile.c -o myguile.o
     gcc image-type.o myguile.o `pkg-config --libs guile-2.0` -o myguile
     zwingli:example-smob$ ./myguile
     guile> make-image
     #<primitive-procedure make-image>
     guile> (define i (make-image "Whistler's Mother" 100 100))
     guile> i
     #<image Whistler's Mother>
     guile> (clear-image i)
     guile> (clear-image 4)
     ERROR: In procedure clear-image in expression (clear-image 4):
     ERROR: Wrong type (expecting image): 4
     ABORT: (wrong-type-arg)

     Type "(backtrace)" to get more information.
     guile>

5.6 Function Snarfing
=====================

When writing C code for use with Guile, you typically define a set of C
functions, and then make some of them visible to the Scheme world by
calling `scm_c_define_gsubr' or related functions.  If you have many
functions to publish, it can sometimes be annoying to keep the list of
calls to `scm_c_define_gsubr' in sync with the list of function
definitions.

   Guile provides the `guile-snarf' program to manage this problem.
Using this tool, you can keep all the information needed to define the
function alongside the function definition itself; `guile-snarf' will
extract this information from your source code, and automatically
generate a file of calls to `scm_c_define_gsubr' which you can
`#include' into an initialization function.

   The snarfing mechanism works for many kind of initialization actions,
not just for collecting calls to `scm_c_define_gsubr'.  For a full list
of what can be done, *Note Snarfing Macros::.

   The `guile-snarf' program is invoked like this:

     guile-snarf [-o OUTFILE] [CPP-ARGS ...]

   This command will extract initialization actions to OUTFILE.  When
no OUTFILE has been specified or when OUTFILE is `-', standard output
will be used.  The C preprocessor is called with CPP-ARGS (which
usually include an input file) and the output is filtered to extract
the initialization actions.

   If there are errors during processing, OUTFILE is deleted and the
program exits with non-zero status.

   During snarfing, the pre-processor macro `SCM_MAGIC_SNARFER' is
defined.  You could use this to avoid including snarfer output files
that don't yet exist by writing code like this:

     #ifndef SCM_MAGIC_SNARFER
     #include "foo.x"
     #endif

   Here is how you might define the Scheme function `clear-image',
implemented by the C function `clear_image':

     #include <libguile.h>

     SCM_DEFINE (clear_image, "clear-image", 1, 0, 0,
                 (SCM image_smob),
                 "Clear the image.")
     {
       /* C code to clear the image in `image_smob'... */
     }

     void
     init_image_type ()
     {
     #include "image-type.x"
     }

   The `SCM_DEFINE' declaration says that the C function `clear_image'
implements a Scheme function called `clear-image', which takes one
required argument (of type `SCM' and named `image_smob'), no optional
arguments, and no rest argument.  The string `"Clear the image."'
provides a short help text for the function, it is called a "docstring".

   `SCM_DEFINE' macro also defines a static array of characters
initialized to the Scheme name of the function.  In this case,
`s_clear_image' is set to the C string, "clear-image".  You might want
to use this symbol when generating error messages.

   Assuming the text above lives in a file named `image-type.c', you
will need to execute the following command to prepare this file for
compilation:

     guile-snarf -o image-type.x image-type.c

   This scans `image-type.c' for `SCM_DEFINE' declarations, and writes
to `image-type.x' the output:

     scm_c_define_gsubr ("clear-image", 1, 0, 0, (SCM (*)() ) clear_image);

   When compiled normally, `SCM_DEFINE' is a macro which expands to the
function header for `clear_image'.

   Note that the output file name matches the `#include' from the input
file.  Also, you still need to provide all the same information you
would if you were using `scm_c_define_gsubr' yourself, but you can
place the information near the function definition itself, so it is
less likely to become incorrect or out-of-date.

   If you have many files that `guile-snarf' must process, you should
consider using a fragment like the following in your Makefile:

     snarfcppopts = $(DEFS) $(INCLUDES) $(CPPFLAGS) $(CFLAGS)
     .SUFFIXES: .x
     .c.x:
     	guile-snarf -o $@ $< $(snarfcppopts)

   This tells make to run `guile-snarf' to produce each needed `.x'
file from the corresponding `.c' file.

   The program `guile-snarf' passes its command-line arguments directly
to the C preprocessor, which it uses to extract the information it
needs from the source code. this means you can pass normal compilation
flags to `guile-snarf' to define preprocessor symbols, add header file
directories, and so on.

5.7 An Overview of Guile Programming
====================================

Guile is designed as an extension language interpreter that is
straightforward to integrate with applications written in C (and C++).
The big win here for the application developer is that Guile
integration, as the Guile web page says, "lowers your project's
hacktivation energy."  Lowering the hacktivation energy means that you,
as the application developer, _and your users_, reap the benefits that
flow from being able to extend the application in a high level
extension language rather than in plain old C.

   In abstract terms, it's difficult to explain what this really means
and what the integration process involves, so instead let's begin by
jumping straight into an example of how you might integrate Guile into
an existing program, and what you could expect to gain by so doing.
With that example under our belts, we'll then return to a more general
analysis of the arguments involved and the range of programming options
available.

5.7.1 How One Might Extend Dia Using Guile
------------------------------------------

Dia is a free software program for drawing schematic diagrams like flow
charts and floor plans (`http://www.gnome.org/projects/dia/').  This
section conducts the thought experiment of adding Guile to Dia.  In so
doing, it aims to illustrate several of the steps and considerations
involved in adding Guile to applications in general.

5.7.1.1 Deciding Why You Want to Add Guile
..........................................

First off, you should understand why you want to add Guile to Dia at
all, and that means forming a picture of what Dia does and how it does
it.  So, what are the constituents of the Dia application?

   * Most importantly, the "application domain objects" -- in other
     words, the concepts that differentiate Dia from another
     application such as a word processor or spreadsheet: shapes,
     templates, connectors, pages, plus the properties of all these
     things.

   * The code that manages the graphical face of the application,
     including the layout and display of the objects above.

   * The code that handles input events, which indicate that the
     application user is wanting to do something.

(In other words, a textbook example of the "model - view - controller"
paradigm.)

   Next question: how will Dia benefit once the Guile integration is
complete?  Several (positive!) answers are possible here, and the choice
is obviously up to the application developers.  Still, one answer is
that the main benefit will be the ability to manipulate Dia's
application domain objects from Scheme.

   Suppose that Dia made a set of procedures available in Scheme,
representing the most basic operations on objects such as shapes,
connectors, and so on.  Using Scheme, the application user could then
write code that builds upon these basic operations to create more
complex procedures.  For example, given basic procedures to enumerate
the objects on a page, to determine whether an object is a square, and
to change the fill pattern of a single shape, the user can write a
Scheme procedure to change the fill pattern of all squares on the
current page:

     (define (change-squares'-fill-pattern new-pattern)
       (for-each-shape current-page
         (lambda (shape)
           (if (square? shape)
               (change-fill-pattern shape new-pattern)))))

5.7.1.2 Four Steps Required to Add Guile
........................................

Assuming this objective, four steps are needed to achieve it.

   First, you need a way of representing your application-specific
objects -- such as `shape' in the previous example -- when they are
passed into the Scheme world.  Unless your objects are so simple that
they map naturally into builtin Scheme data types like numbers and
strings, you will probably want to use Guile's "SMOB" interface to
create a new Scheme data type for your objects.

   Second, you need to write code for the basic operations like
`for-each-shape' and `square?' such that they access and manipulate
your existing data structures correctly, and then make these operations
available as "primitives" on the Scheme level.

   Third, you need to provide some mechanism within the Dia application
that a user can hook into to cause arbitrary Scheme code to be
evaluated.

   Finally, you need to restructure your top-level application C code a
little so that it initializes the Guile interpreter correctly and
declares your "SMOBs" and "primitives" to the Scheme world.

   The following subsections expand on these four points in turn.

5.7.1.3 How to Represent Dia Data in Scheme
...........................................

For all but the most trivial applications, you will probably want to
allow some representation of your domain objects to exist on the Scheme
level.  This is where the idea of SMOBs comes in, and with it issues of
lifetime management and garbage collection.

   To get more concrete about this, let's look again at the example we
gave earlier of how application users can use Guile to build
higher-level functions from the primitives that Dia itself provides.

     (define (change-squares'-fill-pattern new-pattern)
       (for-each-shape current-page
         (lambda (shape)
           (if (square? shape)
               (change-fill-pattern shape new-pattern)))))

   Consider what is stored here in the variable `shape'.  For each
shape on the current page, the `for-each-shape' primitive calls
`(lambda (shape) ...)' with an argument representing that shape.
Question is: how is that argument represented on the Scheme level?  The
issues are as follows.

   * Whatever the representation, it has to be decodable again by the C
     code for the `square?' and `change-fill-pattern' primitives.  In
     other words, a primitive like `square?' has somehow to be able to
     turn the value that it receives back into something that points to
     the underlying C structure describing a shape.

   * The representation must also cope with Scheme code holding on to
     the value for later use.  What happens if the Scheme code stores
     `shape' in a global variable, but then that shape is deleted (in a
     way that the Scheme code is not aware of), and later on some other
     Scheme code uses that global variable again in a call to, say,
     `square?'?

   * The lifetime and memory allocation of objects that exist _only_ in
     the Scheme world is managed automatically by Guile's garbage
     collector using one simple rule: when there are no remaining
     references to an object, the object is considered dead and so its
     memory is freed.  But for objects that exist in both C and Scheme,
     the picture is more complicated; in the case of Dia, where the
     `shape' argument passes transiently in and out of the Scheme
     world, it would be quite wrong the *delete* the underlying C shape
     just because the Scheme code has finished evaluation.  How do we
     avoid this happening?

   One resolution of these issues is for the Scheme-level
representation of a shape to be a new, Scheme-specific C structure
wrapped up as a SMOB.  The SMOB is what is passed into and out of
Scheme code, and the Scheme-specific C structure inside the SMOB points
to Dia's underlying C structure so that the code for primitives like
`square?' can get at it.

   To cope with an underlying shape being deleted while Scheme code is
still holding onto a Scheme shape value, the underlying C structure
should have a new field that points to the Scheme-specific SMOB.  When a
shape is deleted, the relevant code chains through to the
Scheme-specific structure and sets its pointer back to the underlying
structure to NULL.  Thus the SMOB value for the shape continues to
exist, but any primitive code that tries to use it will detect that the
underlying shape has been deleted because the underlying structure
pointer is NULL.

   So, to summarize the steps involved in this resolution of the problem
(and assuming that the underlying C structure for a shape is `struct
dia_shape'):

   * Define a new Scheme-specific structure that _points_ to the
     underlying C structure:

          struct dia_guile_shape
          {
            struct dia_shape * c_shape;   /* NULL => deleted */
          }

   * Add a field to `struct dia_shape' that points to its `struct
     dia_guile_shape' if it has one --

          struct dia_shape
          {
            ...
            struct dia_guile_shape * guile_shape;
          }

     -- so that C code can set `guile_shape->c_shape' to NULL when the
     underlying shape is deleted.

   * Wrap `struct dia_guile_shape' as a SMOB type.

   * Whenever you need to represent a C shape onto the Scheme level,
     create a SMOB instance for it, and pass that.

   * In primitive code that receives a shape SMOB instance, check the
     `c_shape' field when decoding it, to find out whether the
     underlying C shape is still there.

   As far as memory management is concerned, the SMOB values and their
Scheme-specific structures are under the control of the garbage
collector, whereas the underlying C structures are explicitly managed in
exactly the same way that Dia managed them before we thought of adding
Guile.

   When the garbage collector decides to free a shape SMOB value, it
calls the "SMOB free" function that was specified when defining the
shape SMOB type.  To maintain the correctness of the `guile_shape' field
in the underlying C structure, this function should chain through to the
underlying C structure (if it still exists) and set its `guile_shape'
field to NULL.

   For full documentation on defining and using SMOB types, see *note
Defining New Types (Smobs)::.

5.7.1.4 Writing Guile Primitives for Dia
........................................

Once the details of object representation are decided, writing the
primitive function code that you need is usually straightforward.

   A primitive is simply a C function whose arguments and return value
are all of type `SCM', and whose body does whatever you want it to do.
As an example, here is a possible implementation of the `square?'
primitive:

     static SCM square_p (SCM shape)
     {
       struct dia_guile_shape * guile_shape;

       /* Check that arg is really a shape SMOB. */
       scm_assert_smob_type (shape_tag, shape);

       /* Access Scheme-specific shape structure. */
       guile_shape = SCM_SMOB_DATA (shape);

       /* Find out if underlying shape exists and is a
          square; return answer as a Scheme boolean. */
       return scm_from_bool (guile_shape->c_shape &&
                             (guile_shape->c_shape->type == DIA_SQUARE));
     }

   Notice how easy it is to chain through from the `SCM shape'
parameter that `square_p' receives -- which is a SMOB -- to the
Scheme-specific structure inside the SMOB, and thence to the underlying
C structure for the shape.

   In this code, `scm_assert_smob_type', `SCM_SMOB_DATA', and
`scm_from_bool' are from the standard Guile API.  We assume that
`shape_tag' was given to us when we made the shape SMOB type, using
`scm_make_smob_type'.  The call to `scm_assert_smob_type' ensures that
SHAPE is indeed a shape.  This is needed to guard against Scheme code
using the `square?' procedure incorrectly, as in `(square? "hello")';
Scheme's latent typing means that usage errors like this must be caught
at run time.

   Having written the C code for your primitives, you need to make them
available as Scheme procedures by calling the `scm_c_define_gsubr'
function.  `scm_c_define_gsubr' (*note Primitive Procedures::) takes
arguments that specify the Scheme-level name for the primitive and how
many required, optional and rest arguments it can accept.  The
`square?' primitive always requires exactly one argument, so the call
to make it available in Scheme reads like this:

     scm_c_define_gsubr ("square?", 1, 0, 0, square_p);

   For where to put this call, see the subsection after next on the
structure of Guile-enabled code (*note Dia Structure::).

5.7.1.5 Providing a Hook for the Evaluation of Scheme Code
..........................................................

To make the Guile integration useful, you have to design some kind of
hook into your application that application users can use to cause their
Scheme code to be evaluated.

   Technically, this is straightforward; you just have to decide on a
mechanism that is appropriate for your application.  Think of Emacs, for
example: when you type `<ESC> :', you get a prompt where you can type
in any Elisp code, which Emacs will then evaluate.  Or, again like
Emacs, you could provide a mechanism (such as an init file) to allow
Scheme code to be associated with a particular key sequence, and
evaluate the code when that key sequence is entered.

   In either case, once you have the Scheme code that you want to
evaluate, as a null terminated string, you can tell Guile to evaluate
it by calling the `scm_c_eval_string' function.

5.7.1.6 Top-level Structure of Guile-enabled Dia
................................................

Let's assume that the pre-Guile Dia code looks structurally like this:

   * `main ()'

        * do lots of initialization and setup stuff

        * enter Gtk main loop

   When you add Guile to a program, one (rather technical) requirement
is that Guile's garbage collector needs to know where the bottom of the
C stack is.  The easiest way to ensure this is to use `scm_boot_guile'
like this:

   * `main ()'

        * do lots of initialization and setup stuff

        * `scm_boot_guile (argc, argv, inner_main, NULL)'

   * `inner_main ()'

        * define all SMOB types

        * export primitives to Scheme using `scm_c_define_gsubr'

        * enter Gtk main loop

   In other words, you move the guts of what was previously in your
`main' function into a new function called `inner_main', and then add a
`scm_boot_guile' call, with `inner_main' as a parameter, to the end of
`main'.

   Assuming that you are using SMOBs and have written primitive code as
described in the preceding subsections, you also need to insert calls to
declare your new SMOBs and export the primitives to Scheme.  These
declarations must happen _inside_ the dynamic scope of the
`scm_boot_guile' call, but also _before_ any code is run that could
possibly use them -- the beginning of `inner_main' is an ideal place
for this.

5.7.1.7 Going Further with Dia and Guile
........................................

The steps described so far implement an initial Guile integration that
already gives a lot of additional power to Dia application users.  But
there are further steps that you could take, and it's interesting to
consider a few of these.

   In general, you could progressively move more of Dia's source code
from C into Scheme.  This might make the code more maintainable and
extensible, and it could open the door to new programming paradigms that
are tricky to effect in C but straightforward in Scheme.

   A specific example of this is that you could use the guile-gtk
package, which provides Scheme-level procedures for most of the Gtk+
library, to move the code that lays out and displays Dia objects from C
to Scheme.

   As you follow this path, it naturally becomes less useful to
maintain a distinction between Dia's original non-Guile-related source
code, and its later code implementing SMOBs and primitives for the
Scheme world.

   For example, suppose that the original source code had a
`dia_change_fill_pattern' function:

     void dia_change_fill_pattern (struct dia_shape * shape,
                                   struct dia_pattern * pattern)
     {
       /* real pattern change work */
     }

   During initial Guile integration, you add a `change_fill_pattern'
primitive for Scheme purposes, which accesses the underlying structures
from its SMOB values and uses `dia_change_fill_pattern' to do the real
work:

     SCM change_fill_pattern (SCM shape, SCM pattern)
     {
       struct dia_shape * d_shape;
       struct dia_pattern * d_pattern;

       ...

       dia_change_fill_pattern (d_shape, d_pattern);

       return SCM_UNSPECIFIED;
     }

   At this point, it makes sense to keep `dia_change_fill_pattern' and
`change_fill_pattern' separate, because `dia_change_fill_pattern' can
also be called without going through Scheme at all, say because the
user clicks a button which causes a C-registered Gtk+ callback to be
called.

   But, if the code for creating buttons and registering their
callbacks is moved into Scheme (using guile-gtk), it may become true
that `dia_change_fill_pattern' can no longer be called other than
through Scheme.  In which case, it makes sense to abolish it and move
its contents directly into `change_fill_pattern', like this:

     SCM change_fill_pattern (SCM shape, SCM pattern)
     {
       struct dia_shape * d_shape;
       struct dia_pattern * d_pattern;

       ...

       /* real pattern change work */

       return SCM_UNSPECIFIED;
     }

   So further Guile integration progressively _reduces_ the amount of
functional C code that you have to maintain over the long term.

   A similar argument applies to data representation.  In the
discussion of SMOBs earlier, issues arose because of the different
memory management and lifetime models that normally apply to data
structures in C and in Scheme.  However, with further Guile
integration, you can resolve this issue in a more radical way by
allowing all your data structures to be under the control of the
garbage collector, and kept alive by references from the Scheme world.
Instead of maintaining an array or linked list of shapes in C, you
would instead maintain a list in Scheme.

   Rather like the coalescing of `dia_change_fill_pattern' and
`change_fill_pattern', the practical upshot of such a change is that
you would no longer have to keep the `dia_shape' and `dia_guile_shape'
structures separate, and so wouldn't need to worry about the pointers
between them.  Instead, you could change the SMOB definition to wrap
the `dia_shape' structure directly, and send `dia_guile_shape' off to
the scrap yard.  Cut out the middle man!

   Finally, we come to the holy grail of Guile's free software /
extension language approach.  Once you have a Scheme representation for
interesting Dia data types like shapes, and a handy bunch of primitives
for manipulating them, it suddenly becomes clear that you have a bundle
of functionality that could have far-ranging use beyond Dia itself.  In
other words, the data types and primitives could now become a library,
and Dia becomes just one of the many possible applications using that
library -- albeit, at this early stage, a rather important one!

   In this model, Guile becomes just the glue that binds everything
together.  Imagine an application that usefully combined functionality
from Dia, Gnumeric and GnuCash -- it's tricky right now, because no
such application yet exists; but it'll happen some day ...

5.7.2 Why Scheme is More Hackable Than C
----------------------------------------

Underlying Guile's value proposition is the assumption that programming
in a high level language, specifically Guile's implementation of Scheme,
is necessarily better in some way than programming in C.  What do we
mean by this claim, and how can we be so sure?

   One class of advantages applies not only to Scheme, but more
generally to any interpretable, high level, scripting language, such as
Emacs Lisp, Python, Ruby, or TeX's macro language.  Common features of
all such languages, when compared to C, are that:

   * They lend themselves to rapid and experimental development cycles,
     owing usually to a combination of their interpretability and the
     integrated development environment in which they are used.

   * They free developers from some of the low level bookkeeping tasks
     associated with C programming, notably memory management.

   * They provide high level features such as container objects and
     exception handling that make common programming tasks easier.

   In the case of Scheme, particular features that make programming
easier -- and more fun! -- are its powerful mechanisms for abstracting
parts of programs (closures -- *note About Closure::) and for iteration
(*note while do::).

   The evidence in support of this argument is empirical: the huge
amount of code that has been written in extension languages for
applications that support this mechanism.  Most notable are extensions
written in Emacs Lisp for GNU Emacs, in TeX's macro language for TeX,
and in Script-Fu for the Gimp, but there is increasingly now a
significant code eco-system for Guile-based applications as well, such
as Lilypond and GnuCash.  It is close to inconceivable that similar
amounts of functionality could have been added to these applications
just by writing new code in their base implementation languages.

5.7.3 Example: Using Guile for an Application Testbed
-----------------------------------------------------

As an example of what this means in practice, imagine writing a testbed
for an application that is tested by submitting various requests (via a
C interface) and validating the output received.  Suppose further that
the application keeps an idea of its current state, and that the
"correct" output for a given request may depend on the current
application state.  A complete "white box"(1) test plan for this
application would aim to submit all possible requests in each
distinguishable state, and validate the output for all request/state
combinations.

   To write all this test code in C would be very tedious.  Suppose
instead that the testbed code adds a single new C function, to submit an
arbitrary request and return the response, and then uses Guile to export
this function as a Scheme procedure.  The rest of the testbed can then
be written in Scheme, and so benefits from all the advantages of
programming in Scheme that were described in the previous section.

   (In this particular example, there is an additional benefit of
writing most of the testbed in Scheme.  A common problem for white box
testing is that mistakes and mistaken assumptions in the application
under test can easily be reproduced in the testbed code.  It is more
difficult to copy mistakes like this when the testbed is written in a
different language from the application.)

   ---------- Footnotes ----------

   (1) A "white box" test plan is one that incorporates knowledge of
the internal design of the application under test.

5.7.4 A Choice of Programming Options
-------------------------------------

The preceding arguments and example point to a model of Guile
programming that is applicable in many cases.  According to this model,
Guile programming involves a balance between C and Scheme programming,
with the aim being to extract the greatest possible Scheme level benefit
from the least amount of C level work.

   The C level work required in this model usually consists of packaging
and exporting functions and application objects such that they can be
seen and manipulated on the Scheme level.  To help with this, Guile's C
language interface includes utility features that aim to make this kind
of integration very easy for the application developer.  These features
are documented later in this part of the manual: see REFFIXME.

   This model, though, is really just one of a range of possible
programming options.  If all of the functionality that you need is
available from Scheme, you could choose instead to write your whole
application in Scheme (or one of the other high level languages that
Guile supports through translation), and simply use Guile as an
interpreter for Scheme.  (In the future, we hope that Guile will also be
able to compile Scheme code, so lessening the performance gap between C
and Scheme code.)  Or, at the other end of the C-Scheme scale, you
could write the majority of your application in C, and only call out to
Guile occasionally for specific actions such as reading a configuration
file or executing a user-specified extension.  The choices boil down to
two basic questions:

   * Which parts of the application do you write in C, and which in
     Scheme (or another high level translated language)?

   * How do you design the interface between the C and Scheme parts of
     your application?

   These are of course design questions, and the right design for any
given application will always depend upon the particular requirements
that you are trying to meet.  In the context of Guile, however, there
are some generally applicable considerations that can help you when
designing your answers.

5.7.4.1 What Functionality is Already Available?
................................................

Suppose, for the sake of argument, that you would prefer to write your
whole application in Scheme.  Then the API available to you consists of:

   * standard Scheme

   * plus the extensions to standard Scheme provided by Guile in its
     core distribution

   * plus any additional functionality that you or others have packaged
     so that it can be loaded as a Guile Scheme module.

   A module in the last category can either be a pure Scheme module --
in other words a collection of utility procedures coded in Scheme -- or
a module that provides a Scheme interface to an extension library coded
in C -- in other words a nice package where someone else has done the
work of wrapping up some useful C code for you.  The set of available
modules is growing quickly and already includes such useful examples as
`(gtk gtk)', which makes Gtk+ drawing functions available in Scheme,
and `(database postgres)', which provides SQL access to a Postgres
database.

   Given the growing collection of pre-existing modules, it is quite
feasible that your application could be implemented by combining a
selection of these modules together with new application code written in
Scheme.

   If this approach is not enough, because the functionality that your
application needs is not already available in this form, and it is
impossible to write the new functionality in Scheme, you will need to
write some C code.  If the required function is already available in C
(e.g. in a library), all you need is a little glue to connect it to the
world of Guile.  If not, you need both to write the basic code and to
plumb it into Guile.

   In either case, two general considerations are important.  Firstly,
what is the interface by which the functionality is presented to the
Scheme world?  Does the interface consist only of function calls (for
example, a simple drawing interface), or does it need to include
"objects" of some kind that can be passed between C and Scheme and
manipulated by both worlds.  Secondly, how does the lifetime and memory
management of objects in the C code relate to the garbage collection
governed approach of Scheme objects?  In the case where the basic C
code is not already written, most of the difficulties of memory
management can be avoided by using Guile's C interface features from
the start.

   For the full documentation on writing C code for Guile and connecting
existing C code to the Guile world, see REFFIXME.

5.7.4.2 Functional and Performance Constraints
..............................................

5.7.4.3 Your Preferred Programming Style
........................................

5.7.4.4 What Controls Program Execution?
........................................

5.7.5 How About Application Users?
----------------------------------

So far we have considered what Guile programming means for an
application developer.  But what if you are instead _using_ an existing
Guile-based application, and want to know what your options are for
programming and extending this application?

   The answer to this question varies from one application to another,
because the options available depend inevitably on whether the
application developer has provided any hooks for you to hang your own
code on and, if there are such hooks, what they allow you to do.(1)
For example...

   * If the application permits you to load and execute any Guile code,
     the world is your oyster.  You can extend the application in any
     way that you choose.

   * A more cautious application might allow you to load and execute
     Guile code, but only in a "safe" environment, where the interface
     available is restricted by the application from the standard Guile
     API.

   * Or a really fearful application might not provide a hook to really
     execute user code at all, but just use Scheme syntax as a
     convenient way for users to specify application data or
     configuration options.

   In the last two cases, what you can do is, by definition, restricted
by the application, and you should refer to the application's own
manual to find out your options.

   The most well known example of the first case is Emacs, with its
extension language Emacs Lisp: as well as being a text editor, Emacs
supports the loading and execution of arbitrary Emacs Lisp code.  The
result of such openness has been dramatic: Emacs now benefits from
user-contributed Emacs Lisp libraries that extend the basic editing
function to do everything from reading news to psychoanalysis and
playing adventure games.  The only limitation is that extensions are
restricted to the functionality provided by Emacs's built-in set of
primitive operations.  For example, you can interact and display data by
manipulating the contents of an Emacs buffer, but you can't pop-up and
draw a window with a layout that is totally different to the Emacs
standard.

   This situation with a Guile application that supports the loading of
arbitrary user code is similar, except perhaps even more so, because
Guile also supports the loading of extension libraries written in C.
This last point enables user code to add new primitive operations to
Guile, and so to bypass the limitation present in Emacs Lisp.

   At this point, the distinction between an application developer and
an application user becomes rather blurred.  Instead of seeing yourself
as a user extending an application, you could equally well say that you
are developing a new application of your own using some of the primitive
functionality provided by the original application.  As such, all the
discussions of the preceding sections of this chapter are relevant to
how you can proceed with developing your extension.

   ---------- Footnotes ----------

   (1) Of course, in the world of free software, you always have the
freedom to modify the application's source code to your own
requirements.  Here we are concerned with the extension options that the
application has provided for without your needing to modify its source
code.

5.8 Autoconf Support
====================

Autoconf, a part of the GNU build system, makes it easy for users to
build your package.  This section documents Guile's Autoconf support.

5.8.1 Autoconf Background
-------------------------

As explained in the `GNU Autoconf Manual', any package needs
configuration at build-time (*note Introduction: (autoconf)Top.).  If
your package uses Guile (or uses a package that in turn uses Guile),
you probably need to know what specific Guile features are available
and details about them.

   The way to do this is to write feature tests and arrange for their
execution by the `configure' script, typically by adding the tests to
`configure.ac', and running `autoconf' to create `configure'.  Users of
your package then run `configure' in the normal way.

   Macros are a way to make common feature tests easy to express.
Autoconf provides a wide range of macros (*note Existing Tests:
(autoconf)Existing Tests.), and Guile installation provides
Guile-specific tests in the areas of: program detection, compilation
flags reporting, and Scheme module checks.

5.8.2 Autoconf Macros
---------------------

As mentioned earlier in this chapter, Guile supports parallel
installation, and uses `pkg-config' to let the user choose which
version of Guile they are interested in.  `pkg-config' has its own set
of Autoconf macros that are probably installed on most every
development system.  The most useful of these macros is
`PKG_CHECK_MODULES'.

     PKG_CHECK_MODULES([GUILE], [guile-2.0])

   This example looks for Guile and sets the `GUILE_CFLAGS' and
`GUILE_LIBS' variables accordingly, or prints an error and exits if
Guile was not found.

   Guile comes with additional Autoconf macros providing more
information, installed as `PREFIX/share/aclocal/guile.m4'.  Their names
all begin with `GUILE_'.

 -- Autoconf Macro: GUILE_PROGS
     This macro looks for programs `guile', `guile-config' and
     `guile-tools', and sets variables GUILE, GUILE_CONFIG and
     GUILE_TOOLS, to their paths, respectively.  If either of the first
     two is not found, signal error.

     The variables are marked for substitution, as by `AC_SUBST'.


 -- Autoconf Macro: GUILE_FLAGS
     This macro runs the `guile-config' script, installed with Guile, to
     find out where Guile's header files and libraries are installed.
     It sets four variables, GUILE_CFLAGS, GUILE_LDFLAGS, GUILE_LIBS,
     and GUILE_LTLIBS.

     GUILE_CFLAGS: flags to pass to a C or C++ compiler to build code
     that uses Guile header files.  This is almost always just one or
     more `-I' flags.

     GUILE_LDFLAGS: flags to pass to the compiler to link a program
     against Guile.  This includes `-lguile' for the Guile library
     itself, any libraries that Guile itself requires (like
     -lqthreads), and so on.  It may also include one or more `-L' flag
     to tell the compiler where to find the libraries.  But it does not
     include flags that influence the program's runtime search path for
     libraries, and will therefore lead to a program that fails to
     start, unless all necessary libraries are installed in a standard
     location such as `/usr/lib'.

     GUILE_LIBS and GUILE_LTLIBS: flags to pass to the compiler or to
     libtool, respectively, to link a program against Guile.  It
     includes flags that augment the program's runtime search path for
     libraries, so that shared libraries will be found at the location
     where they were during linking, even in non-standard locations.
     GUILE_LIBS is to be used when linking the program directly with
     the compiler, whereas GUILE_LTLIBS is to be used when linking the
     program is done through libtool.

     The variables are marked for substitution, as by `AC_SUBST'.


 -- Autoconf Macro: GUILE_SITE_DIR
     This looks for Guile's "site" directory, usually something like
     PREFIX/share/guile/site, and sets var GUILE_SITE to the path.
     Note that the var name is different from the macro name.

     The variable is marked for substitution, as by `AC_SUBST'.


 -- Autoconf Macro: GUILE_CHECK_RETVAL var check
     VAR is a shell variable name to be set to the return value.  CHECK
     is a Guile Scheme expression, evaluated with "$GUILE -c", and
     returning either 0 or non-#f to indicate the check passed.
     Non-0 number or #f indicates failure.     Avoid using the
     character "#" since that confuses autoconf.


 -- Autoconf Macro: GUILE_MODULE_CHECK var module featuretest
          description
     VAR is a shell variable name to be set to "yes" or "no".  MODULE
     is a list of symbols, like: (ice-9 common-list).  FEATURETEST is
     an expression acceptable to GUILE_CHECK, q.v.  DESCRIPTION is a
     present-tense verb phrase (passed to AC_MSG_CHECKING).


 -- Autoconf Macro: GUILE_MODULE_AVAILABLE var module
     VAR is a shell variable name to be set to "yes" or "no".  MODULE
     is a list of symbols, like: (ice-9 common-list).


 -- Autoconf Macro: GUILE_MODULE_REQUIRED symlist
     SYMLIST is a list of symbols, WITHOUT surrounding parens, like:
     ice-9 common-list.


 -- Autoconf Macro: GUILE_MODULE_EXPORTS var module modvar
     VAR is a shell variable to be set to "yes" or "no".  MODULE is a
     list of symbols, like: (ice-9 common-list).  MODVAR is the Guile
     Scheme variable to check.


 -- Autoconf Macro: GUILE_MODULE_REQUIRED_EXPORT module modvar
     MODULE is a list of symbols, like: (ice-9 common-list).  MODVAR is
     the Guile Scheme variable to check.


5.8.3 Using Autoconf Macros
---------------------------

Using the autoconf macros is straightforward: Add the macro "calls"
(actually instantiations) to `configure.ac', run `aclocal', and finally,
run `autoconf'.  If your system doesn't have guile.m4 installed, place
the desired macro definitions (`AC_DEFUN' forms) in `acinclude.m4', and
`aclocal' will do the right thing.

   Some of the macros can be used inside normal shell constructs: `if
foo ; then GUILE_BAZ ; fi', but this is not guaranteed.  It's probably
a good idea to instantiate macros at top-level.

   We now include two examples, one simple and one complicated.

   The first example is for a package that uses libguile, and thus
needs to know how to compile and link against it.  So we use
`PKG_CHECK_MODULES' to set the vars `GUILE_CFLAGS' and `GUILE_LIBS',
which are automatically substituted in the Makefile.

     In configure.ac:

       PKG_CHECK_MODULES([GUILE], [guile-2.0])

     In Makefile.in:

       GUILE_CFLAGS  = @GUILE_CFLAGS@
       GUILE_LIBS = @GUILE_LIBS@

       myprog.o: myprog.c
               $(CC) -o $ $(GUILE_CFLAGS) $<
       myprog: myprog.o
               $(CC) -o $ $< $(GUILE_LIBS)

   The second example is for a package of Guile Scheme modules that
uses an external program and other Guile Scheme modules (some might
call this a "pure scheme" package).  So we use the `GUILE_SITE_DIR'
macro, a regular `AC_PATH_PROG' macro, and the `GUILE_MODULE_AVAILABLE'
macro.

     In configure.ac:

       GUILE_SITE_DIR

       probably_wont_work=""

       # pgtype pgtable
       GUILE_MODULE_AVAILABLE(have_guile_pg, (database postgres))
       test $have_guile_pg = no &&
           probably_wont_work="(my pgtype) (my pgtable) $probably_wont_work"

       # gpgutils
       AC_PATH_PROG(GNUPG,gpg)
       test x"$GNUPG" = x &&
           probably_wont_work="(my gpgutils) $probably_wont_work"

       if test ! "$probably_wont_work" = "" ; then
           p="         ***"
           echo
           echo "$p"
           echo "$p NOTE:"
           echo "$p The following modules probably won't work:"
           echo "$p   $probably_wont_work"
           echo "$p They can be installed anyway, and will work if their"
           echo "$p dependencies are installed later.  Please see README."
           echo "$p"
           echo
       fi

     In Makefile.in:

       instdir = @GUILE_SITE@/my

       install:
             $(INSTALL) my/*.scm $(instdir)

6 API Reference
***************

Guile provides an application programming interface ("API") to
developers in two core languages: Scheme and C.  This part of the manual
contains reference documentation for all of the functionality that is
available through both Scheme and C interfaces.

6.1 Overview of the Guile API
=============================

Guile's application programming interface ("API") makes functionality
available that an application developer can use in either C or Scheme
programming.  The interface consists of "elements" that may be macros,
functions or variables in C, and procedures, variables, syntax or other
types of object in Scheme.

   Many elements are available to both Scheme and C, in a form that is
appropriate.  For example, the `assq' Scheme procedure is also
available as `scm_assq' to C code.  These elements are documented only
once, addressing both the Scheme and C aspects of them.

   The Scheme name of an element is related to its C name in a regular
way.  Also, a C function takes its parameters in a systematic way.

   Normally, the name of a C function can be derived given its Scheme
name, using some simple textual transformations:

   * Replace `-' (hyphen) with `_' (underscore).

   * Replace `?' (question mark) with `_p'.

   * Replace `!' (exclamation point) with `_x'.

   * Replace internal `->' with `_to_'.

   * Replace `<=' (less than or equal) with `_leq'.

   * Replace `>=' (greater than or equal) with `_geq'.

   * Replace `<' (less than) with `_less'.

   * Replace `>' (greater than) with `_gr'.

   * Prefix with `scm_'.


   A C function always takes a fixed number of arguments of type `SCM',
even when the corresponding Scheme function takes a variable number.

   For some Scheme functions, some last arguments are optional; the
corresponding C function must always be invoked with all optional
arguments specified.  To get the effect as if an argument has not been
specified, pass `SCM_UNDEFINED' as its value.  You can not do this for
an argument in the middle; when one argument is `SCM_UNDEFINED' all the
ones following it must be `SCM_UNDEFINED' as well.

   Some Scheme functions take an arbitrary number of _rest_ arguments;
the corresponding C function must be invoked with a list of all these
arguments.  This list is always the last argument of the C function.

   These two variants can also be combined.

   The type of the return value of a C function that corresponds to a
Scheme function is always `SCM'.  In the descriptions below, types are
therefore often omitted but for the return value and for the arguments.

6.2 Deprecation
===============

From time to time functions and other features of Guile become obsolete.
Guile's "deprecation" is a mechanism that can help you cope with this.

   When you use a feature that is deprecated, you will likely get a
warning message at run-time. Also, if you have a new enough toolchain,
using a deprecated function from `libguile' will cause a link-time
warning.

   The primary source for information about just what interfaces are
deprecated in a given release is the file `NEWS'.  That file also
documents what you should use instead of the obsoleted things.

   The file `README' contains instructions on how to control the
inclusion or removal of the deprecated features from the public API of
Guile, and how to control the deprecation warning messages.

   The idea behind this mechanism is that normally all deprecated
interfaces are available, but you get feedback when compiling and
running code that uses them, so that you can migrate to the newer APIs
at your leisure.

6.3 The SCM Type
================

Guile represents all Scheme values with the single C type `SCM'.  For
an introduction to this topic, *Note Dynamic Types::.

 -- C Type: SCM
     `SCM' is the user level abstract C type that is used to represent
     all of Guile's Scheme objects, no matter what the Scheme object
     type is.  No C operation except assignment is guaranteed to work
     with variables of type `SCM', so you should only use macros and
     functions to work with `SCM' values.  Values are converted between
     C data types and the `SCM' type with utility functions and macros.
   
 -- C Type: scm_t_bits
     `scm_t_bits' is an unsigned integral data type that is guaranteed
     to be large enough to hold all information that is required to
     represent any Scheme object.  While this data type is mostly used
     to implement Guile's internals, the use of this type is also
     necessary to write certain kinds of extensions to Guile.

 -- C Type: scm_t_signed_bits
     This is a signed integral type of the same size as `scm_t_bits'.

 -- C Macro: scm_t_bits SCM_UNPACK (SCM X)
     Transforms the `SCM' value X into its representation as an
     integral type.  Only after applying `SCM_UNPACK' it is possible to
     access the bits and contents of the `SCM' value.

 -- C Macro: SCM SCM_PACK (scm_t_bits X)
     Takes a valid integral representation of a Scheme object and
     transforms it into its representation as a `SCM' value.

6.4 Initializing Guile
======================

Each thread that wants to use functions from the Guile API needs to put
itself into guile mode with either `scm_with_guile' or
`scm_init_guile'.  The global state of Guile is initialized
automatically when the first thread enters guile mode.

   When a thread wants to block outside of a Guile API function, it
should leave guile mode temporarily with `scm_without_guile', *Note
Blocking::.

   Threads that are created by `call-with-new-thread' or
`scm_spawn_thread' start out in guile mode so you don't need to
initialize them.

 -- C Function: void * scm_with_guile (void *(*func)(void *), void
          *data)
     Call FUNC, passing it DATA and return what FUNC returns.  While
     FUNC is running, the current thread is in guile mode and can thus
     use the Guile API.

     When `scm_with_guile' is called from guile mode, the thread remains
     in guile mode when `scm_with_guile' returns.

     Otherwise, it puts the current thread into guile mode and, if
     needed, gives it a Scheme representation that is contained in the
     list returned by `all-threads', for example.  This Scheme
     representation is not removed when `scm_with_guile' returns so
     that a given thread is always represented by the same Scheme value
     during its lifetime, if at all.

     When this is the first thread that enters guile mode, the global
     state of Guile is initialized before calling `func'.

     The function FUNC is called via `scm_with_continuation_barrier';
     thus, `scm_with_guile' returns exactly once.

     When `scm_with_guile' returns, the thread is no longer in guile
     mode (except when `scm_with_guile' was called from guile mode, see
     above).  Thus, only `func' can store `SCM' variables on the stack
     and be sure that they are protected from the garbage collector.
     See `scm_init_guile' for another approach at initializing Guile
     that does not have this restriction.

     It is OK to call `scm_with_guile' while a thread has temporarily
     left guile mode via `scm_without_guile'.  It will then simply
     temporarily enter guile mode again.

 -- C Function: void scm_init_guile ()
     Arrange things so that all of the code in the current thread
     executes as if from within a call to `scm_with_guile'.  That is,
     all functions called by the current thread can assume that `SCM'
     values on their stack frames are protected from the garbage
     collector (except when the thread has explicitly left guile mode,
     of course).

     When `scm_init_guile' is called from a thread that already has been
     in guile mode once, nothing happens.  This behavior matters when
     you call `scm_init_guile' while the thread has only temporarily
     left guile mode: in that case the thread will not be in guile mode
     after `scm_init_guile' returns.  Thus, you should not use
     `scm_init_guile' in such a scenario.

     When a uncaught throw happens in a thread that has been put into
     guile mode via `scm_init_guile', a short message is printed to the
     current error port and the thread is exited via `scm_pthread_exit
     (NULL)'.  No restrictions are placed on continuations.

     The function `scm_init_guile' might not be available on all
     platforms since it requires some stack-bounds-finding magic that
     might not have been ported to all platforms that Guile runs on.
     Thus, if you can, it is better to use `scm_with_guile' or its
     variation `scm_boot_guile' instead of this function.

 -- C Function: void scm_boot_guile (int ARGC, char **ARGV, void
          (*MAIN_FUNC) (void *DATA, int ARGC, char **ARGV), void *DATA)
     Enter guile mode as with `scm_with_guile' and call MAIN_FUNC,
     passing it DATA, ARGC, and ARGV as indicated.  When MAIN_FUNC
     returns, `scm_boot_guile' calls `exit (0)'; `scm_boot_guile' never
     returns.  If you want some other exit value, have MAIN_FUNC call
     `exit' itself.  If you don't want to exit at all, use
     `scm_with_guile' instead of `scm_boot_guile'.

     The function `scm_boot_guile' arranges for the Scheme
     `command-line' function to return the strings given by ARGC and
     ARGV.  If MAIN_FUNC modifies ARGC or ARGV, it should call
     `scm_set_program_arguments' with the final list, so Scheme code
     will know which arguments have been processed (*note Runtime
     Environment::).

 -- C Function: void scm_shell (int ARGC, char **ARGV)
     Process command-line arguments in the manner of the `guile'
     executable.  This includes loading the normal Guile initialization
     files, interacting with the user or running any scripts or
     expressions specified by `-s' or `-e' options, and then exiting.
     *Note Invoking Guile::, for more details.

     Since this function does not return, you must do all
     application-specific initialization before calling this function.

6.5 Snarfing Macros
===================

The following macros do two different things: when compiled normally,
they expand in one way; when processed during snarfing, they cause the
`guile-snarf' program to pick up some initialization code, *Note
Function Snarfing::.

   The descriptions below use the term `normally' to refer to the case
when the code is compiled normally, and `while snarfing' when the code
is processed by `guile-snarf'.

 -- C Macro: SCM_SNARF_INIT (code)
     Normally, `SCM_SNARF_INIT' expands to nothing; while snarfing, it
     causes CODE to be included in the initialization action file,
     followed by a semicolon.

     This is the fundamental macro for snarfing initialization actions.
     The more specialized macros below use it internally.

 -- C Macro: SCM_DEFINE (c_name, scheme_name, req, opt, var, arglist,
          docstring)
     Normally, this macro expands into

          static const char s_C_NAME[] = SCHEME_NAME;
          SCM
          C_NAME ARGLIST

     While snarfing, it causes

          scm_c_define_gsubr (s_C_NAME, REQ, OPT, VAR,
                              C_NAME);

     to be added to the initialization actions.  Thus, you can use it to
     declare a C function named C_NAME that will be made available to
     Scheme with the name SCHEME_NAME.

     Note that the ARGLIST argument must have parentheses around it.

 -- C Macro: SCM_SYMBOL (c_name, scheme_name)
 -- C Macro: SCM_GLOBAL_SYMBOL (c_name, scheme_name)
     Normally, these macros expand into

          static SCM C_NAME

     or

          SCM C_NAME

     respectively.  While snarfing, they both expand into the
     initialization code

          C_NAME = scm_permanent_object (scm_from_locale_symbol (SCHEME_NAME));

     Thus, you can use them declare a static or global variable of type
     `SCM' that will be initialized to the symbol named SCHEME_NAME.

 -- C Macro: SCM_KEYWORD (c_name, scheme_name)
 -- C Macro: SCM_GLOBAL_KEYWORD (c_name, scheme_name)
     Normally, these macros expand into

          static SCM C_NAME

     or

          SCM C_NAME

     respectively.  While snarfing, they both expand into the
     initialization code

          C_NAME = scm_permanent_object (scm_c_make_keyword (SCHEME_NAME));

     Thus, you can use them declare a static or global variable of type
     `SCM' that will be initialized to the keyword named SCHEME_NAME.

 -- C Macro: SCM_VARIABLE (c_name, scheme_name)
 -- C Macro: SCM_GLOBAL_VARIABLE (c_name, scheme_name)
     These macros are equivalent to `SCM_VARIABLE_INIT' and
     `SCM_GLOBAL_VARIABLE_INIT', respectively, with a VALUE of
     `SCM_BOOL_F'.

 -- C Macro: SCM_VARIABLE_INIT (c_name, scheme_name, value)
 -- C Macro: SCM_GLOBAL_VARIABLE_INIT (c_name, scheme_name, value)
     Normally, these macros expand into

          static SCM C_NAME

     or

          SCM C_NAME

     respectively.  While snarfing, they both expand into the
     initialization code

          C_NAME = scm_permanent_object (scm_c_define (SCHEME_NAME, VALUE));

     Thus, you can use them declare a static or global C variable of
     type `SCM' that will be initialized to the object representing the
     Scheme variable named SCHEME_NAME in the current module.  The
     variable will be defined when it doesn't already exist.  It is
     always set to VALUE.

6.6 Simple Generic Data Types
=============================

This chapter describes those of Guile's simple data types which are
primarily used for their role as items of generic data.  By "simple" we
mean data types that are not primarily used as containers to hold other
data -- i.e. pairs, lists, vectors and so on.  For the documentation of
such "compound" data types, see *note Compound Data Types::.

6.6.1 Booleans
--------------

The two boolean values are `#t' for true and `#f' for false.

   Boolean values are returned by predicate procedures, such as the
general equality predicates `eq?', `eqv?' and `equal?' (*note
Equality::) and numerical and string comparison operators like
`string=?' (*note String Comparison::) and `<=' (*note Comparison::).

     (<= 3 8)
     => #t

     (<= 3 -3)
     => #f

     (equal? "house" "houses")
     => #f

     (eq? #f #f)
     =>
     #t

   In test condition contexts like `if' and `cond' (*note
Conditionals::), where a group of subexpressions will be evaluated only
if a CONDITION expression evaluates to "true", "true" means any value
at all except `#f'.

     (if #t "yes" "no")
     => "yes"

     (if 0 "yes" "no")
     => "yes"

     (if #f "yes" "no")
     => "no"

   A result of this asymmetry is that typical Scheme source code more
often uses `#f' explicitly than `#t': `#f' is necessary to represent an
`if' or `cond' false value, whereas `#t' is not necessary to represent
an `if' or `cond' true value.

   It is important to note that `#f' is *not* equivalent to any other
Scheme value.  In particular, `#f' is not the same as the number 0
(like in C and C++), and not the same as the "empty list" (like in some
Lisp dialects).

   In C, the two Scheme boolean values are available as the two
constants `SCM_BOOL_T' for `#t' and `SCM_BOOL_F' for `#f'.  Care must
be taken with the false value `SCM_BOOL_F': it is not false when used
in C conditionals.  In order to test for it, use `scm_is_false' or
`scm_is_true'.

 -- Scheme Procedure: not x
 -- C Function: scm_not (x)
     Return `#t' if X is `#f', else return `#f'.

 -- Scheme Procedure: boolean? obj
 -- C Function: scm_boolean_p (obj)
     Return `#t' if OBJ is either `#t' or `#f', else return `#f'.

 -- C Macro: SCM SCM_BOOL_T
     The `SCM' representation of the Scheme object `#t'.

 -- C Macro: SCM SCM_BOOL_F
     The `SCM' representation of the Scheme object `#f'.

 -- C Function: int scm_is_true (SCM obj)
     Return `0' if OBJ is `#f', else return `1'.

 -- C Function: int scm_is_false (SCM obj)
     Return `1' if OBJ is `#f', else return `0'.

 -- C Function: int scm_is_bool (SCM obj)
     Return `1' if OBJ is either `#t' or `#f', else return `0'.

 -- C Function: SCM scm_from_bool (int val)
     Return `#f' if VAL is `0', else return `#t'.

 -- C Function: int scm_to_bool (SCM val)
     Return `1' if VAL is `SCM_BOOL_T', return `0' when VAL is
     `SCM_BOOL_F', else signal a `wrong type' error.

     You should probably use `scm_is_true' instead of this function
     when you just want to test a `SCM' value for trueness.

6.6.2 Numerical data types
--------------------------

Guile supports a rich "tower" of numerical types -- integer, rational,
real and complex -- and provides an extensive set of mathematical and
scientific functions for operating on numerical data.  This section of
the manual documents those types and functions.

   You may also find it illuminating to read R5RS's presentation of
numbers in Scheme, which is particularly clear and accessible: see
*note Numbers: (r5rs)Numbers.

6.6.2.1 Scheme's Numerical "Tower"
..................................

Scheme's numerical "tower" consists of the following categories of
numbers:

"integers"
     Whole numbers, positive or negative; e.g. -5, 0, 18.

"rationals"
     The set of numbers that can be expressed as P/Q where P and Q are
     integers; e.g. 9/16 works, but pi (an irrational number) doesn't.
     These include integers (N/1).

"real numbers"
     The set of numbers that describes all possible positions along a
     one-dimensional line. This includes rationals as well as irrational
     numbers.

"complex numbers"
     The set of numbers that describes all possible positions in a two
     dimensional space. This includes real as well as imaginary numbers
     (A+Bi, where A is the "real part", B is the "imaginary part", and
     i is the square root of -1.)

   It is called a tower because each category "sits on" the one that
follows it, in the sense that every integer is also a rational, every
rational is also real, and every real number is also a complex number
(but with zero imaginary part).

   In addition to the classification into integers, rationals, reals and
complex numbers, Scheme also distinguishes between whether a number is
represented exactly or not.  For example, the result of 2*sin(pi/4) is
exactly 2^(1/2), but Guile can represent neither pi/4 nor 2^(1/2)
exactly.  Instead, it stores an inexact approximation, using the C type
`double'.

   Guile can represent exact rationals of any magnitude, inexact
rationals that fit into a C `double', and inexact complex numbers with
`double' real and imaginary parts.

   The `number?' predicate may be applied to any Scheme value to
discover whether the value is any of the supported numerical types.

 -- Scheme Procedure: number? obj
 -- C Function: scm_number_p (obj)
     Return `#t' if OBJ is any kind of number, else `#f'.

   For example:

     (number? 3)
     => #t

     (number? "hello there!")
     => #f

     (define pi 3.141592654)
     (number? pi)
     => #t

 -- C Function: int scm_is_number (SCM obj)
     This is equivalent to `scm_is_true (scm_number_p (obj))'.

   The next few subsections document each of Guile's numerical data
types in detail.

6.6.2.2 Integers
................

Integers are whole numbers, that is numbers with no fractional part,
such as 2, 83, and -3789.

   Integers in Guile can be arbitrarily big, as shown by the following
example.

     (define (factorial n)
       (let loop ((n n) (product 1))
         (if (= n 0)
             product
             (loop (- n 1) (* product n)))))

     (factorial 3)
     => 6

     (factorial 20)
     => 2432902008176640000

     (- (factorial 45))
     => -119622220865480194561963161495657715064383733760000000000

   Readers whose background is in programming languages where integers
are limited by the need to fit into just 4 or 8 bytes of memory may find
this surprising, or suspect that Guile's representation of integers is
inefficient.  In fact, Guile achieves a near optimal balance of
convenience and efficiency by using the host computer's native
representation of integers where possible, and a more general
representation where the required number does not fit in the native
form.  Conversion between these two representations is automatic and
completely invisible to the Scheme level programmer.

   C has a host of different integer types, and Guile offers a host of
functions to convert between them and the `SCM' representation.  For
example, a C `int' can be handled with `scm_to_int' and `scm_from_int'.
Guile also defines a few C integer types of its own, to help with
differences between systems.

   C integer types that are not covered can be handled with the generic
`scm_to_signed_integer' and `scm_from_signed_integer' for signed types,
or with `scm_to_unsigned_integer' and `scm_from_unsigned_integer' for
unsigned types.

   Scheme integers can be exact and inexact.  For example, a number
written as `3.0' with an explicit decimal-point is inexact, but it is
also an integer.  The functions `integer?' and `scm_is_integer' report
true for such a number, but the functions `scm_is_signed_integer' and
`scm_is_unsigned_integer' only allow exact integers and thus report
false.  Likewise, the conversion functions like `scm_to_signed_integer'
only accept exact integers.

   The motivation for this behavior is that the inexactness of a number
should not be lost silently.  If you want to allow inexact integers,
you can explicitly insert a call to `inexact->exact' or to its C
equivalent `scm_inexact_to_exact'.  (Only inexact integers will be
converted by this call into exact integers; inexact non-integers will
become exact fractions.)

 -- Scheme Procedure: integer? x
 -- C Function: scm_integer_p (x)
     Return `#t' if X is an exact or inexact integer number, else `#f'.

          (integer? 487)
          => #t

          (integer? 3.0)
          => #t

          (integer? -3.4)
          => #f

          (integer? +inf.0)
          => #t

 -- C Function: int scm_is_integer (SCM x)
     This is equivalent to `scm_is_true (scm_integer_p (x))'.

 -- C Type: scm_t_int8
 -- C Type: scm_t_uint8
 -- C Type: scm_t_int16
 -- C Type: scm_t_uint16
 -- C Type: scm_t_int32
 -- C Type: scm_t_uint32
 -- C Type: scm_t_int64
 -- C Type: scm_t_uint64
 -- C Type: scm_t_intmax
 -- C Type: scm_t_uintmax
     The C types are equivalent to the corresponding ISO C types but are
     defined on all platforms, with the exception of `scm_t_int64' and
     `scm_t_uint64', which are only defined when a 64-bit type is
     available.  For example, `scm_t_int8' is equivalent to `int8_t'.

     You can regard these definitions as a stop-gap measure until all
     platforms provide these types.  If you know that all the platforms
     that you are interested in already provide these types, it is
     better to use them directly instead of the types provided by Guile.

 -- C Function: int scm_is_signed_integer (SCM x, scm_t_intmax min,
          scm_t_intmax max)
 -- C Function: int scm_is_unsigned_integer (SCM x, scm_t_uintmax min,
          scm_t_uintmax max)
     Return `1' when X represents an exact integer that is between MIN
     and MAX, inclusive.

     These functions can be used to check whether a `SCM' value will
     fit into a given range, such as the range of a given C integer
     type.  If you just want to convert a `SCM' value to a given C
     integer type, use one of the conversion functions directly.

 -- C Function: scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax
          min, scm_t_intmax max)
 -- C Function: scm_t_uintmax scm_to_unsigned_integer (SCM x,
          scm_t_uintmax min, scm_t_uintmax max)
     When X represents an exact integer that is between MIN and MAX
     inclusive, return that integer.  Else signal an error, either a
     `wrong-type' error when X is not an exact integer, or an
     `out-of-range' error when it doesn't fit the given range.

 -- C Function: SCM scm_from_signed_integer (scm_t_intmax x)
 -- C Function: SCM scm_from_unsigned_integer (scm_t_uintmax x)
     Return the `SCM' value that represents the integer X.  This
     function will always succeed and will always return an exact
     number.

 -- C Function: char scm_to_char (SCM x)
 -- C Function: signed char scm_to_schar (SCM x)
 -- C Function: unsigned char scm_to_uchar (SCM x)
 -- C Function: short scm_to_short (SCM x)
 -- C Function: unsigned short scm_to_ushort (SCM x)
 -- C Function: int scm_to_int (SCM x)
 -- C Function: unsigned int scm_to_uint (SCM x)
 -- C Function: long scm_to_long (SCM x)
 -- C Function: unsigned long scm_to_ulong (SCM x)
 -- C Function: long long scm_to_long_long (SCM x)
 -- C Function: unsigned long long scm_to_ulong_long (SCM x)
 -- C Function: size_t scm_to_size_t (SCM x)
 -- C Function: ssize_t scm_to_ssize_t (SCM x)
 -- C Function: scm_t_int8 scm_to_int8 (SCM x)
 -- C Function: scm_t_uint8 scm_to_uint8 (SCM x)
 -- C Function: scm_t_int16 scm_to_int16 (SCM x)
 -- C Function: scm_t_uint16 scm_to_uint16 (SCM x)
 -- C Function: scm_t_int32 scm_to_int32 (SCM x)
 -- C Function: scm_t_uint32 scm_to_uint32 (SCM x)
 -- C Function: scm_t_int64 scm_to_int64 (SCM x)
 -- C Function: scm_t_uint64 scm_to_uint64 (SCM x)
 -- C Function: scm_t_intmax scm_to_intmax (SCM x)
 -- C Function: scm_t_uintmax scm_to_uintmax (SCM x)
     When X represents an exact integer that fits into the indicated C
     type, return that integer.  Else signal an error, either a
     `wrong-type' error when X is not an exact integer, or an
     `out-of-range' error when it doesn't fit the given range.

     The functions `scm_to_long_long', `scm_to_ulong_long',
     `scm_to_int64', and `scm_to_uint64' are only available when the
     corresponding types are.

 -- C Function: SCM scm_from_char (char x)
 -- C Function: SCM scm_from_schar (signed char x)
 -- C Function: SCM scm_from_uchar (unsigned char x)
 -- C Function: SCM scm_from_short (short x)
 -- C Function: SCM scm_from_ushort (unsigned short x)
 -- C Function: SCM scm_from_int (int x)
 -- C Function: SCM scm_from_uint (unsigned int x)
 -- C Function: SCM scm_from_long (long x)
 -- C Function: SCM scm_from_ulong (unsigned long x)
 -- C Function: SCM scm_from_long_long (long long x)
 -- C Function: SCM scm_from_ulong_long (unsigned long long x)
 -- C Function: SCM scm_from_size_t (size_t x)
 -- C Function: SCM scm_from_ssize_t (ssize_t x)
 -- C Function: SCM scm_from_int8 (scm_t_int8 x)
 -- C Function: SCM scm_from_uint8 (scm_t_uint8 x)
 -- C Function: SCM scm_from_int16 (scm_t_int16 x)
 -- C Function: SCM scm_from_uint16 (scm_t_uint16 x)
 -- C Function: SCM scm_from_int32 (scm_t_int32 x)
 -- C Function: SCM scm_from_uint32 (scm_t_uint32 x)
 -- C Function: SCM scm_from_int64 (scm_t_int64 x)
 -- C Function: SCM scm_from_uint64 (scm_t_uint64 x)
 -- C Function: SCM scm_from_intmax (scm_t_intmax x)
 -- C Function: SCM scm_from_uintmax (scm_t_uintmax x)
     Return the `SCM' value that represents the integer X.  These
     functions will always succeed and will always return an exact
     number.

 -- C Function: void scm_to_mpz (SCM val, mpz_t rop)
     Assign VAL to the multiple precision integer ROP.  VAL must be an
     exact integer, otherwise an error will be signalled.  ROP must
     have been initialized with `mpz_init' before this function is
     called.  When ROP is no longer needed the occupied space must be
     freed with `mpz_clear'.  *Note Initializing Integers:
     (gmp)Initializing Integers, for details.

 -- C Function: SCM scm_from_mpz (mpz_t val)
     Return the `SCM' value that represents VAL.

6.6.2.3 Real and Rational Numbers
.................................

Mathematically, the real numbers are the set of numbers that describe
all possible points along a continuous, infinite, one-dimensional line.
The rational numbers are the set of all numbers that can be written as
fractions P/Q, where P and Q are integers.  All rational numbers are
also real, but there are real numbers that are not rational, for
example the square root of 2, and pi.

   Guile can represent both exact and inexact rational numbers, but it
cannot represent precise finite irrational numbers.  Exact rationals are
represented by storing the numerator and denominator as two exact
integers.  Inexact rationals are stored as floating point numbers using
the C type `double'.

   Exact rationals are written as a fraction of integers.  There must be
no whitespace around the slash:

     1/2
     -22/7

   Even though the actual encoding of inexact rationals is in binary, it
may be helpful to think of it as a decimal number with a limited number
of significant figures and a decimal point somewhere, since this
corresponds to the standard notation for non-whole numbers.  For
example:

     0.34
     -0.00000142857931198
     -5648394822220000000000.0
     4.0

   The limited precision of Guile's encoding means that any finite
"real" number in Guile can be written in a rational form, by
multiplying and then dividing by sufficient powers of 10 (or in fact,
2).  For example, `-0.00000142857931198' is the same as -142857931198
divided by 100000000000000000.  In Guile's current incarnation,
therefore, the `rational?' and `real?' predicates are equivalent for
finite numbers.

   Dividing by an exact zero leads to a error message, as one might
expect.  However, dividing by an inexact zero does not produce an error.
Instead, the result of the division is either plus or minus infinity,
depending on the sign of the divided number and the sign of the zero
divisor (some platforms support signed zeroes `-0.0' and `+0.0'; `0.0'
is the same as `+0.0').

   Dividing zero by an inexact zero yields a NaN (`not a number')
value, although they are actually considered numbers by Scheme.
Attempts to compare a NaN value with any number (including itself)
using `=', `<', `>', `<=' or `>=' always returns `#f'.  Although a NaN
value is not `=' to itself, it is both `eqv?' and `equal?' to itself
and other NaN values.  However, the preferred way to test for them is
by using `nan?'.

   The real NaN values and infinities are written `+nan.0', `+inf.0'
and `-inf.0'.  This syntax is also recognized by `read' as an extension
to the usual Scheme syntax.  These special values are considered by
Scheme to be inexact real numbers but not rational.  Note that non-real
complex numbers may also contain infinities or NaN values in their real
or imaginary parts.  To test a real number to see if it is infinite, a
NaN value, or neither, use `inf?', `nan?', or `finite?', respectively.
Every real number in Scheme belongs to precisely one of those three
classes.

   On platforms that follow IEEE 754 for their floating point
arithmetic, the `+inf.0', `-inf.0', and `+nan.0' values are implemented
using the corresponding IEEE 754 values.  They behave in arithmetic
operations like IEEE 754 describes it, i.e., `(= +nan.0 +nan.0)' =>
`#f'.

 -- Scheme Procedure: real? obj
 -- C Function: scm_real_p (obj)
     Return `#t' if OBJ is a real number, else `#f'.  Note that the
     sets of integer and rational values form subsets of the set of
     real numbers, so the predicate will also be fulfilled if OBJ is an
     integer number or a rational number.

 -- Scheme Procedure: rational? x
 -- C Function: scm_rational_p (x)
     Return `#t' if X is a rational number, `#f' otherwise.  Note that
     the set of integer values forms a subset of the set of rational
     numbers, i.e. the predicate will also be fulfilled if X is an
     integer number.

 -- Scheme Procedure: rationalize x eps
 -- C Function: scm_rationalize (x, eps)
     Returns the _simplest_ rational number differing from X by no more
     than EPS.

     As required by R5RS, `rationalize' only returns an exact result
     when both its arguments are exact.  Thus, you might need to use
     `inexact->exact' on the arguments.

          (rationalize (inexact->exact 1.2) 1/100)
          => 6/5


 -- Scheme Procedure: inf? x
 -- C Function: scm_inf_p (x)
     Return `#t' if the real number X is `+inf.0' or `-inf.0'.
     Otherwise return `#f'.

 -- Scheme Procedure: nan? x
 -- C Function: scm_nan_p (x)
     Return `#t' if the real number X is `+nan.0', or `#f' otherwise.

 -- Scheme Procedure: finite? x
 -- C Function: scm_finite_p (x)
     Return `#t' if the real number X is neither infinite nor a NaN,
     `#f' otherwise.

 -- Scheme Procedure: nan
 -- C Function: scm_nan ()
     Return `+nan.0', a NaN value.

 -- Scheme Procedure: inf
 -- C Function: scm_inf ()
     Return `+inf.0', positive infinity.

 -- Scheme Procedure: numerator x
 -- C Function: scm_numerator (x)
     Return the numerator of the rational number X.

 -- Scheme Procedure: denominator x
 -- C Function: scm_denominator (x)
     Return the denominator of the rational number X.

 -- C Function: int scm_is_real (SCM val)
 -- C Function: int scm_is_rational (SCM val)
     Equivalent to `scm_is_true (scm_real_p (val))' and `scm_is_true
     (scm_rational_p (val))', respectively.

 -- C Function: double scm_to_double (SCM val)
     Returns the number closest to VAL that is representable as a
     `double'.  Returns infinity for a VAL that is too large in
     magnitude.  The argument VAL must be a real number.

 -- C Function: SCM scm_from_double (double val)
     Return the `SCM' value that represents VAL.  The returned value is
     inexact according to the predicate `inexact?', but it will be
     exactly equal to VAL.

6.6.2.4 Complex Numbers
.......................

Complex numbers are the set of numbers that describe all possible points
in a two-dimensional space.  The two coordinates of a particular point
in this space are known as the "real" and "imaginary" parts of the
complex number that describes that point.

   In Guile, complex numbers are written in rectangular form as the sum
of their real and imaginary parts, using the symbol `i' to indicate the
imaginary part.

     3+4i
     =>
     3.0+4.0i

     (* 3-8i 2.3+0.3i)
     =>
     9.3-17.5i

Polar form can also be used, with an `@' between magnitude and angle,

     1@3.141592 => -1.0      (approx)
     -1@1.57079 => 0.0-1.0i  (approx)

   Guile represents a complex number as a pair of inexact reals, so the
real and imaginary parts of a complex number have the same properties of
inexactness and limited precision as single inexact real numbers.

   Note that each part of a complex number may contain any inexact real
value, including the special values `+nan.0', `+inf.0' and `-inf.0', as
well as either of the signed zeroes `0.0' or `-0.0'.

 -- Scheme Procedure: complex? z
 -- C Function: scm_complex_p (z)
     Return `#t' if X is a complex number, `#f' otherwise.  Note that
     the sets of real, rational and integer values form subsets of the
     set of complex numbers, i.e. the predicate will also be fulfilled
     if X is a real, rational or integer number.

 -- C Function: int scm_is_complex (SCM val)
     Equivalent to `scm_is_true (scm_complex_p (val))'.

6.6.2.5 Exact and Inexact Numbers
.................................

R5RS requires that, with few exceptions, a calculation involving inexact
numbers always produces an inexact result.  To meet this requirement,
Guile distinguishes between an exact integer value such as `5' and the
corresponding inexact integer value which, to the limited precision
available, has no fractional part, and is printed as `5.0'.  Guile will
only convert the latter value to the former when forced to do so by an
invocation of the `inexact->exact' procedure.

   The only exception to the above requirement is when the values of the
inexact numbers do not affect the result.  For example `(expt n 0)' is
`1' for any value of `n', therefore `(expt 5.0 0)' is permitted to
return an exact `1'.

 -- Scheme Procedure: exact? z
 -- C Function: scm_exact_p (z)
     Return `#t' if the number Z is exact, `#f' otherwise.

          (exact? 2)
          => #t

          (exact? 0.5)
          => #f

          (exact? (/ 2))
          => #t


 -- C Function: int scm_is_exact (SCM z)
     Return a `1' if the number Z is exact, and `0' otherwise.  This is
     equivalent to `scm_is_true (scm_exact_p (z))'.

     An alternate approch to testing the exactness of a number is to
     use `scm_is_signed_integer' or `scm_is_unsigned_integer'.

 -- Scheme Procedure: inexact? z
 -- C Function: scm_inexact_p (z)
     Return `#t' if the number Z is inexact, `#f' else.

 -- C Function: int scm_is_inexact (SCM z)
     Return a `1' if the number Z is inexact, and `0' otherwise.  This
     is equivalent to `scm_is_true (scm_inexact_p (z))'.

 -- Scheme Procedure: inexact->exact z
 -- C Function: scm_inexact_to_exact (z)
     Return an exact number that is numerically closest to Z, when
     there is one.  For inexact rationals, Guile returns the exact
     rational that is numerically equal to the inexact rational.
     Inexact complex numbers with a non-zero imaginary part can not be
     made exact.

          (inexact->exact 0.5)
          => 1/2

     The following happens because 12/10 is not exactly representable
     as a `double' (on most platforms).  However, when reading a decimal
     number that has been marked exact with the "#e" prefix, Guile is
     able to represent it correctly.

          (inexact->exact 1.2)
          => 5404319552844595/4503599627370496

          #e1.2
          => 6/5


 -- Scheme Procedure: exact->inexact z
 -- C Function: scm_exact_to_inexact (z)
     Convert the number Z to its inexact representation.

6.6.2.6 Read Syntax for Numerical Data
......................................

The read syntax for integers is a string of digits, optionally preceded
by a minus or plus character, a code indicating the base in which the
integer is encoded, and a code indicating whether the number is exact
or inexact.  The supported base codes are:

`#b'
`#B'
     the integer is written in binary (base 2)

`#o'
`#O'
     the integer is written in octal (base 8)

`#d'
`#D'
     the integer is written in decimal (base 10)

`#x'
`#X'
     the integer is written in hexadecimal (base 16)

   If the base code is omitted, the integer is assumed to be decimal.
The following examples show how these base codes are used.

     -13
     => -13

     #d-13
     => -13

     #x-13
     => -19

     #b+1101
     => 13

     #o377
     => 255

   The codes for indicating exactness (which can, incidentally, be
applied to all numerical values) are:

`#e'
`#E'
     the number is exact

`#i'
`#I'
     the number is inexact.

   If the exactness indicator is omitted, the number is exact unless it
contains a radix point.  Since Guile can not represent exact complex
numbers, an error is signalled when asking for them.

     (exact? 1.2)
     => #f

     (exact? #e1.2)
     => #t

     (exact? #e+1i)
     ERROR: Wrong type argument

   Guile also understands the syntax `+inf.0' and `-inf.0' for plus and
minus infinity, respectively.  The value must be written exactly as
shown, that is, they always must have a sign and exactly one zero digit
after the decimal point.  It also understands `+nan.0' and `-nan.0' for
the special `not-a-number' value.  The sign is ignored for
`not-a-number' and the value is always printed as `+nan.0'.

6.6.2.7 Operations on Integer Values
....................................

 -- Scheme Procedure: odd? n
 -- C Function: scm_odd_p (n)
     Return `#t' if N is an odd number, `#f' otherwise.

 -- Scheme Procedure: even? n
 -- C Function: scm_even_p (n)
     Return `#t' if N is an even number, `#f' otherwise.

 -- Scheme Procedure: quotient n d
 -- Scheme Procedure: remainder n d
 -- C Function: scm_quotient (n, d)
 -- C Function: scm_remainder (n, d)
     Return the quotient or remainder from N divided by D.  The
     quotient is rounded towards zero, and the remainder will have the
     same sign as N.  In all cases quotient and remainder satisfy N =
     Q*D + R.

          (remainder 13 4) => 1
          (remainder -13 4) => -1

     See also `truncate-quotient', `truncate-remainder' and related
     operations in *note Arithmetic::.

 -- Scheme Procedure: modulo n d
 -- C Function: scm_modulo (n, d)
     Return the remainder from N divided by D, with the same sign as D.

          (modulo 13 4) => 1
          (modulo -13 4) => 3
          (modulo 13 -4) => -3
          (modulo -13 -4) => -1

     See also `floor-quotient', `floor-remainder' and related
     operations in *note Arithmetic::.

 -- Scheme Procedure: gcd x...
 -- C Function: scm_gcd (x, y)
     Return the greatest common divisor of all arguments.  If called
     without arguments, 0 is returned.

     The C function `scm_gcd' always takes two arguments, while the
     Scheme function can take an arbitrary number.

 -- Scheme Procedure: lcm x...
 -- C Function: scm_lcm (x, y)
     Return the least common multiple of the arguments.  If called
     without arguments, 1 is returned.

     The C function `scm_lcm' always takes two arguments, while the
     Scheme function can take an arbitrary number.

 -- Scheme Procedure: modulo-expt n k m
 -- C Function: scm_modulo_expt (n, k, m)
     Return N raised to the integer exponent K, modulo M.

          (modulo-expt 2 3 5)
             => 3

 -- Scheme Procedure:  exact-integer-sqrt K
 -- C Function: void scm_exact_integer_sqrt (SCM K, SCM *S, SCM *R)
     Return two exact non-negative integers S and R such that K = S^2 +
     R and S^2 <= K < (S + 1)^2.  An error is raised if K is not an
     exact non-negative integer.

          (exact-integer-sqrt 10) => 3 and 1

6.6.2.8 Comparison Predicates
.............................

The C comparison functions below always takes two arguments, while the
Scheme functions can take an arbitrary number.  Also keep in mind that
the C functions return one of the Scheme boolean values `SCM_BOOL_T' or
`SCM_BOOL_F' which are both true as far as C is concerned.  Thus,
always write `scm_is_true (scm_num_eq_p (x, y))' when testing the two
Scheme numbers `x' and `y' for equality, for example.

 -- Scheme Procedure: =
 -- C Function: scm_num_eq_p (x, y)
     Return `#t' if all parameters are numerically equal.

 -- Scheme Procedure: <
 -- C Function: scm_less_p (x, y)
     Return `#t' if the list of parameters is monotonically increasing.

 -- Scheme Procedure: >
 -- C Function: scm_gr_p (x, y)
     Return `#t' if the list of parameters is monotonically decreasing.

 -- Scheme Procedure: <=
 -- C Function: scm_leq_p (x, y)
     Return `#t' if the list of parameters is monotonically
     non-decreasing.

 -- Scheme Procedure: >=
 -- C Function: scm_geq_p (x, y)
     Return `#t' if the list of parameters is monotonically
     non-increasing.

 -- Scheme Procedure: zero? z
 -- C Function: scm_zero_p (z)
     Return `#t' if Z is an exact or inexact number equal to zero.

 -- Scheme Procedure: positive? x
 -- C Function: scm_positive_p (x)
     Return `#t' if X is an exact or inexact number greater than zero.

 -- Scheme Procedure: negative? x
 -- C Function: scm_negative_p (x)
     Return `#t' if X is an exact or inexact number less than zero.

6.6.2.9 Converting Numbers To and From Strings
..............................................

The following procedures read and write numbers according to their
external representation as defined by R5RS (*note R5RS Lexical
Structure: (r5rs)Lexical structure.).  *Note the `(ice-9 i18n)' module:
Number Input and Output, for locale-dependent number parsing.

 -- Scheme Procedure: number->string n [radix]
 -- C Function: scm_number_to_string (n, radix)
     Return a string holding the external representation of the number
     N in the given RADIX.  If N is inexact, a radix of 10 will be used.

 -- Scheme Procedure: string->number string [radix]
 -- C Function: scm_string_to_number (string, radix)
     Return a number of the maximally precise representation expressed
     by the given STRING. RADIX must be an exact integer, either 2, 8,
     10, or 16. If supplied, RADIX is a default radix that may be
     overridden by an explicit radix prefix in STRING (e.g. "#o177").
     If RADIX is not supplied, then the default radix is 10. If string
     is not a syntactically valid notation for a number, then
     `string->number' returns `#f'.

 -- C Function: SCM scm_c_locale_stringn_to_number (const char *string,
          size_t len, unsigned radix)
     As per `string->number' above, but taking a C string, as pointer
     and length.  The string characters should be in the current locale
     encoding (`locale' in the name refers only to that, there's no
     locale-dependent parsing).

6.6.2.10 Complex Number Operations
..................................

 -- Scheme Procedure: make-rectangular real_part imaginary_part
 -- C Function: scm_make_rectangular (real_part, imaginary_part)
     Return a complex number constructed of the given REAL-PART and
     IMAGINARY-PART parts.

 -- Scheme Procedure: make-polar mag ang
 -- C Function: scm_make_polar (mag, ang)
     Return the complex number MAG * e^(i * ANG).

 -- Scheme Procedure: real-part z
 -- C Function: scm_real_part (z)
     Return the real part of the number Z.

 -- Scheme Procedure: imag-part z
 -- C Function: scm_imag_part (z)
     Return the imaginary part of the number Z.

 -- Scheme Procedure: magnitude z
 -- C Function: scm_magnitude (z)
     Return the magnitude of the number Z. This is the same as `abs'
     for real arguments, but also allows complex numbers.

 -- Scheme Procedure: angle z
 -- C Function: scm_angle (z)
     Return the angle of the complex number Z.

 -- C Function: SCM scm_c_make_rectangular (double re, double im)
 -- C Function: SCM scm_c_make_polar (double x, double y)
     Like `scm_make_rectangular' or `scm_make_polar', respectively, but
     these functions take `double's as their arguments.

 -- C Function: double scm_c_real_part (z)
 -- C Function: double scm_c_imag_part (z)
     Returns the real or imaginary part of Z as a `double'.

 -- C Function: double scm_c_magnitude (z)
 -- C Function: double scm_c_angle (z)
     Returns the magnitude or angle of Z as a `double'.

6.6.2.11 Arithmetic Functions
.............................

The C arithmetic functions below always takes two arguments, while the
Scheme functions can take an arbitrary number.  When you need to invoke
them with just one argument, for example to compute the equivalent of
`(- x)', pass `SCM_UNDEFINED' as the second one: `scm_difference (x,
SCM_UNDEFINED)'.

 -- Scheme Procedure: + z1 ...
 -- C Function: scm_sum (z1, z2)
     Return the sum of all parameter values.  Return 0 if called
     without any parameters.

 -- Scheme Procedure: - z1 z2 ...
 -- C Function: scm_difference (z1, z2)
     If called with one argument Z1, -Z1 is returned. Otherwise the sum
     of all but the first argument are subtracted from the first
     argument.

 -- Scheme Procedure: * z1 ...
 -- C Function: scm_product (z1, z2)
     Return the product of all arguments.  If called without arguments,
     1 is returned.

 -- Scheme Procedure: / z1 z2 ...
 -- C Function: scm_divide (z1, z2)
     Divide the first argument by the product of the remaining
     arguments.  If called with one argument Z1, 1/Z1 is returned.

 -- Scheme Procedure: 1+ z
 -- C Function: scm_oneplus (z)
     Return Z + 1.

 -- Scheme Procedure: 1- z
 -- C function: scm_oneminus (z)
     Return Z - 1.

 -- Scheme Procedure: abs x
 -- C Function: scm_abs (x)
     Return the absolute value of X.

     X must be a number with zero imaginary part.  To calculate the
     magnitude of a complex number, use `magnitude' instead.

 -- Scheme Procedure: max x1 x2 ...
 -- C Function: scm_max (x1, x2)
     Return the maximum of all parameter values.

 -- Scheme Procedure: min x1 x2 ...
 -- C Function: scm_min (x1, x2)
     Return the minimum of all parameter values.

 -- Scheme Procedure: truncate x
 -- C Function: scm_truncate_number (x)
     Round the inexact number X towards zero.

 -- Scheme Procedure: round x
 -- C Function: scm_round_number (x)
     Round the inexact number X to the nearest integer.  When exactly
     halfway between two integers, round to the even one.

 -- Scheme Procedure: floor x
 -- C Function: scm_floor (x)
     Round the number X towards minus infinity.

 -- Scheme Procedure: ceiling x
 -- C Function: scm_ceiling (x)
     Round the number X towards infinity.

 -- C Function: double scm_c_truncate (double x)
 -- C Function: double scm_c_round (double x)
     Like `scm_truncate_number' or `scm_round_number', respectively,
     but these functions take and return `double' values.

 -- Scheme Procedure:  euclidean/ X Y
 -- Scheme Procedure:  euclidean-quotient X Y
 -- Scheme Procedure:  euclidean-remainder X Y
 -- C Function: void scm_euclidean_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_euclidean_quotient (SCM X, SCM Y)
 -- C Function: SCM scm_euclidean_remainder (SCM X, SCM Y)
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `euclidean-quotient' returns the
     integer Q and `euclidean-remainder' returns the real number R such
     that X = Q*Y + R and 0 <= R < |Y|.  `euclidean/' returns both Q and
     R, and is more efficient than computing each separately.  Note
     that when Y > 0, `euclidean-quotient' returns floor(X/Y),
     otherwise it returns ceiling(X/Y).

     Note that these operators are equivalent to the R6RS operators
     `div', `mod', and `div-and-mod'.

          (euclidean-quotient 123 10) => 12
          (euclidean-remainder 123 10) => 3
          (euclidean/ 123 10) => 12 and 3
          (euclidean/ 123 -10) => -12 and 3
          (euclidean/ -123 10) => -13 and 7
          (euclidean/ -123 -10) => 13 and 7
          (euclidean/ -123.2 -63.5) => 2.0 and 3.8
          (euclidean/ 16/3 -10/7) => -3 and 22/21

 -- Scheme Procedure:  floor/ X Y
 -- Scheme Procedure:  floor-quotient X Y
 -- Scheme Procedure:  floor-remainder X Y
 -- C Function: void scm_floor_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_floor_quotient (X, Y)
 -- C Function: SCM scm_floor_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `floor-quotient' returns the integer
     Q and `floor-remainder' returns the real number R such that Q =
     floor(X/Y) and X = Q*Y + R.  `floor/' returns both Q and R, and is
     more efficient than computing each separately.  Note that R, if
     non-zero, will have the same sign as Y.

     When X and Y are integers, `floor-remainder' is equivalent to the
     R5RS integer-only operator `modulo'.

          (floor-quotient 123 10) => 12
          (floor-remainder 123 10) => 3
          (floor/ 123 10) => 12 and 3
          (floor/ 123 -10) => -13 and -7
          (floor/ -123 10) => -13 and 7
          (floor/ -123 -10) => 12 and -3
          (floor/ -123.2 -63.5) => 1.0 and -59.7
          (floor/ 16/3 -10/7) => -4 and -8/21

 -- Scheme Procedure:  ceiling/ X Y
 -- Scheme Procedure:  ceiling-quotient X Y
 -- Scheme Procedure:  ceiling-remainder X Y
 -- C Function: void scm_ceiling_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_ceiling_quotient (X, Y)
 -- C Function: SCM scm_ceiling_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `ceiling-quotient' returns the
     integer Q and `ceiling-remainder' returns the real number R such
     that Q = ceiling(X/Y) and X = Q*Y + R.  `ceiling/' returns both Q
     and R, and is more efficient than computing each separately.  Note
     that R, if non-zero, will have the opposite sign of Y.

          (ceiling-quotient 123 10) => 13
          (ceiling-remainder 123 10) => -7
          (ceiling/ 123 10) => 13 and -7
          (ceiling/ 123 -10) => -12 and 3
          (ceiling/ -123 10) => -12 and -3
          (ceiling/ -123 -10) => 13 and 7
          (ceiling/ -123.2 -63.5) => 2.0 and 3.8
          (ceiling/ 16/3 -10/7) => -3 and 22/21

 -- Scheme Procedure:  truncate/ X Y
 -- Scheme Procedure:  truncate-quotient X Y
 -- Scheme Procedure:  truncate-remainder X Y
 -- C Function: void scm_truncate_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_truncate_quotient (X, Y)
 -- C Function: SCM scm_truncate_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `truncate-quotient' returns the
     integer Q and `truncate-remainder' returns the real number R such
     that Q is X/Y rounded toward zero, and X = Q*Y + R.  `truncate/'
     returns both Q and R, and is more efficient than computing each
     separately.  Note that R, if non-zero, will have the same sign as
     X.

     When X and Y are integers, these operators are equivalent to the
     R5RS integer-only operators `quotient' and `remainder'.

          (truncate-quotient 123 10) => 12
          (truncate-remainder 123 10) => 3
          (truncate/ 123 10) => 12 and 3
          (truncate/ 123 -10) => -12 and 3
          (truncate/ -123 10) => -12 and -3
          (truncate/ -123 -10) => 12 and -3
          (truncate/ -123.2 -63.5) => 1.0 and -59.7
          (truncate/ 16/3 -10/7) => -3 and 22/21

 -- Scheme Procedure:  centered/ X Y
 -- Scheme Procedure:  centered-quotient X Y
 -- Scheme Procedure:  centered-remainder X Y
 -- C Function: void scm_centered_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_centered_quotient (SCM X, SCM Y)
 -- C Function: SCM scm_centered_remainder (SCM X, SCM Y)
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `centered-quotient' returns the
     integer Q and `centered-remainder' returns the real number R such
     that X = Q*Y + R and -|Y/2| <= R < |Y/2|.  `centered/' returns
     both Q and R, and is more efficient than computing each separately.

     Note that `centered-quotient' returns X/Y rounded to the nearest
     integer.  When X/Y lies exactly half-way between two integers, the
     tie is broken according to the sign of Y.  If Y > 0, ties are
     rounded toward positive infinity, otherwise they are rounded
     toward negative infinity.  This is a consequence of the
     requirement that -|Y/2| <= R < |Y/2|.

     Note that these operators are equivalent to the R6RS operators
     `div0', `mod0', and `div0-and-mod0'.

          (centered-quotient 123 10) => 12
          (centered-remainder 123 10) => 3
          (centered/ 123 10) => 12 and 3
          (centered/ 123 -10) => -12 and 3
          (centered/ -123 10) => -12 and -3
          (centered/ -123 -10) => 12 and -3
          (centered/ 125 10) => 13 and -5
          (centered/ 127 10) => 13 and -3
          (centered/ 135 10) => 14 and -5
          (centered/ -123.2 -63.5) => 2.0 and 3.8
          (centered/ 16/3 -10/7) => -4 and -8/21

 -- Scheme Procedure:  round/ X Y
 -- Scheme Procedure:  round-quotient X Y
 -- Scheme Procedure:  round-remainder X Y
 -- C Function: void scm_round_divide (SCM X, SCM Y, SCM *Q, SCM *R)
 -- C Function: SCM scm_round_quotient (X, Y)
 -- C Function: SCM scm_round_remainder (X, Y)
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `round-quotient' returns the integer
     Q and `round-remainder' returns the real number R such that X =
     Q*Y + R and Q is X/Y rounded to the nearest integer, with ties
     going to the nearest even integer.  `round/' returns both Q and R,
     and is more efficient than computing each separately.

     Note that `round/' and `centered/' are almost equivalent, but
     their behavior differs when X/Y lies exactly half-way between two
     integers.  In this case, `round/' chooses the nearest even
     integer, whereas `centered/' chooses in such a way to satisfy the
     constraint -|Y/2| <= R < |Y/2|, which is stronger than the
     corresponding constraint for `round/', -|Y/2| <= R <= |Y/2|.  In
     particular, when X and Y are integers, the number of possible
     remainders returned by `centered/' is |Y|, whereas the number of
     possible remainders returned by `round/' is |Y|+1 when Y is even.

          (round-quotient 123 10) => 12
          (round-remainder 123 10) => 3
          (round/ 123 10) => 12 and 3
          (round/ 123 -10) => -12 and 3
          (round/ -123 10) => -12 and -3
          (round/ -123 -10) => 12 and -3
          (round/ 125 10) => 12 and 5
          (round/ 127 10) => 13 and -3
          (round/ 135 10) => 14 and -5
          (round/ -123.2 -63.5) => 2.0 and 3.8
          (round/ 16/3 -10/7) => -4 and -8/21

6.6.2.12 Scientific Functions
.............................

The following procedures accept any kind of number as arguments,
including complex numbers.

 -- Scheme Procedure: sqrt z
     Return the square root of Z.  Of the two possible roots (positive
     and negative), the one with a positive real part is returned, or
     if that's zero then a positive imaginary part.  Thus,

          (sqrt 9.0)       => 3.0
          (sqrt -9.0)      => 0.0+3.0i
          (sqrt 1.0+1.0i)  => 1.09868411346781+0.455089860562227i
          (sqrt -1.0-1.0i) => 0.455089860562227-1.09868411346781i

 -- Scheme Procedure: expt z1 z2
     Return Z1 raised to the power of Z2.

 -- Scheme Procedure: sin z
     Return the sine of Z.

 -- Scheme Procedure: cos z
     Return the cosine of Z.

 -- Scheme Procedure: tan z
     Return the tangent of Z.

 -- Scheme Procedure: asin z
     Return the arcsine of Z.

 -- Scheme Procedure: acos z
     Return the arccosine of Z.

 -- Scheme Procedure: atan z
 -- Scheme Procedure: atan y x
     Return the arctangent of Z, or of Y/X.

 -- Scheme Procedure: exp z
     Return e to the power of Z, where e is the base of natural
     logarithms (2.71828...).

 -- Scheme Procedure: log z
     Return the natural logarithm of Z.

 -- Scheme Procedure: log10 z
     Return the base 10 logarithm of Z.

 -- Scheme Procedure: sinh z
     Return the hyperbolic sine of Z.

 -- Scheme Procedure: cosh z
     Return the hyperbolic cosine of Z.

 -- Scheme Procedure: tanh z
     Return the hyperbolic tangent of Z.

 -- Scheme Procedure: asinh z
     Return the hyperbolic arcsine of Z.

 -- Scheme Procedure: acosh z
     Return the hyperbolic arccosine of Z.

 -- Scheme Procedure: atanh z
     Return the hyperbolic arctangent of Z.

6.6.2.13 Bitwise Operations
...........................

For the following bitwise functions, negative numbers are treated as
infinite precision twos-complements.  For instance -6 is bits
...111010, with infinitely many ones on the left.  It can be seen that
adding 6 (binary 110) to such a bit pattern gives all zeros.

 -- Scheme Procedure: logand n1 n2 ...
 -- C Function: scm_logand (n1, n2)
     Return the bitwise AND of the integer arguments.

          (logand) => -1
          (logand 7) => 7
          (logand #b111 #b011 #b001) => 1

 -- Scheme Procedure: logior n1 n2 ...
 -- C Function: scm_logior (n1, n2)
     Return the bitwise OR of the integer arguments.

          (logior) => 0
          (logior 7) => 7
          (logior #b000 #b001 #b011) => 3

 -- Scheme Procedure: logxor n1 n2 ...
 -- C Function: scm_loxor (n1, n2)
     Return the bitwise XOR of the integer arguments.  A bit is set in
     the result if it is set in an odd number of arguments.

          (logxor) => 0
          (logxor 7) => 7
          (logxor #b000 #b001 #b011) => 2
          (logxor #b000 #b001 #b011 #b011) => 1

 -- Scheme Procedure: lognot n
 -- C Function: scm_lognot (n)
     Return the integer which is the ones-complement of the integer
     argument, ie. each 0 bit is changed to 1 and each 1 bit to 0.

          (number->string (lognot #b10000000) 2)
             => "-10000001"
          (number->string (lognot #b0) 2)
             => "-1"

 -- Scheme Procedure: logtest j k
 -- C Function: scm_logtest (j, k)
     Test whether J and K have any 1 bits in common.  This is
     equivalent to `(not (zero? (logand j k)))', but without actually
     calculating the `logand', just testing for non-zero.

          (logtest #b0100 #b1011) => #f
          (logtest #b0100 #b0111) => #t

 -- Scheme Procedure: logbit? index j
 -- C Function: scm_logbit_p (index, j)
     Test whether bit number INDEX in J is set.  INDEX starts from 0
     for the least significant bit.

          (logbit? 0 #b1101) => #t
          (logbit? 1 #b1101) => #f
          (logbit? 2 #b1101) => #t
          (logbit? 3 #b1101) => #t
          (logbit? 4 #b1101) => #f

 -- Scheme Procedure: ash n cnt
 -- C Function: scm_ash (n, cnt)
     Return N shifted left by CNT bits, or shifted right if CNT is
     negative.  This is an "arithmetic" shift.

     This is effectively a multiplication by 2^CNT, and when CNT is
     negative it's a division, rounded towards negative infinity.
     (Note that this is not the same rounding as `quotient' does.)

     With N viewed as an infinite precision twos complement, `ash'
     means a left shift introducing zero bits, or a right shift
     dropping bits.

          (number->string (ash #b1 3) 2)     => "1000"
          (number->string (ash #b1010 -1) 2) => "101"

          ;; -23 is bits ...11101001, -6 is bits ...111010
          (ash -23 -2) => -6

 -- Scheme Procedure: logcount n
 -- C Function: scm_logcount (n)
     Return the number of bits in integer N.  If N is positive, the
     1-bits in its binary representation are counted.  If negative, the
     0-bits in its two's-complement binary representation are counted.
     If zero, 0 is returned.

          (logcount #b10101010)
             => 4
          (logcount 0)
             => 0
          (logcount -2)
             => 1

 -- Scheme Procedure: integer-length n
 -- C Function: scm_integer_length (n)
     Return the number of bits necessary to represent N.

     For positive N this is how many bits to the most significant one
     bit.  For negative N it's how many bits to the most significant
     zero bit in twos complement form.

          (integer-length #b10101010) => 8
          (integer-length #b1111)     => 4
          (integer-length 0)          => 0
          (integer-length -1)         => 0
          (integer-length -256)       => 8
          (integer-length -257)       => 9

 -- Scheme Procedure: integer-expt n k
 -- C Function: scm_integer_expt (n, k)
     Return N raised to the power K.  K must be an exact integer, N can
     be any number.

     Negative K is supported, and results in 1/n^abs(k) in the usual
     way.  N^0 is 1, as usual, and that includes 0^0 is 1.

          (integer-expt 2 5)   => 32
          (integer-expt -3 3)  => -27
          (integer-expt 5 -3)  => 1/125
          (integer-expt 0 0)   => 1

 -- Scheme Procedure: bit-extract n start end
 -- C Function: scm_bit_extract (n, start, end)
     Return the integer composed of the START (inclusive) through END
     (exclusive) bits of N.  The STARTth bit becomes the 0-th bit in
     the result.

          (number->string (bit-extract #b1101101010 0 4) 2)
             => "1010"
          (number->string (bit-extract #b1101101010 4 9) 2)
             => "10110"

6.6.2.14 Random Number Generation
.................................

Pseudo-random numbers are generated from a random state object, which
can be created with `seed->random-state' or `datum->random-state'.  An
external representation (i.e. one which can written with `write' and
read with `read') of a random state object can be obtained via
`random-state->datum'.  The STATE parameter to the various functions
below is optional, it defaults to the state object in the
`*random-state*' variable.

 -- Scheme Procedure: copy-random-state [state]
 -- C Function: scm_copy_random_state (state)
     Return a copy of the random state STATE.

 -- Scheme Procedure: random n [state]
 -- C Function: scm_random (n, state)
     Return a number in [0, N).

     Accepts a positive integer or real n and returns a number of the
     same type between zero (inclusive) and N (exclusive). The values
     returned have a uniform distribution.

 -- Scheme Procedure: random:exp [state]
 -- C Function: scm_random_exp (state)
     Return an inexact real in an exponential distribution with mean 1.
     For an exponential distribution with mean U use `(* U
     (random:exp))'.

 -- Scheme Procedure: random:hollow-sphere! vect [state]
 -- C Function: scm_random_hollow_sphere_x (vect, state)
     Fills VECT with inexact real random numbers the sum of whose
     squares is equal to 1.0.  Thinking of VECT as coordinates in space
     of dimension N = `(vector-length VECT)', the coordinates are
     uniformly distributed over the surface of the unit n-sphere.

 -- Scheme Procedure: random:normal [state]
 -- C Function: scm_random_normal (state)
     Return an inexact real in a normal distribution.  The distribution
     used has mean 0 and standard deviation 1.  For a normal
     distribution with mean M and standard deviation D use `(+ M (* D
     (random:normal)))'.

 -- Scheme Procedure: random:normal-vector! vect [state]
 -- C Function: scm_random_normal_vector_x (vect, state)
     Fills VECT with inexact real random numbers that are independent
     and standard normally distributed (i.e., with mean 0 and variance
     1).

 -- Scheme Procedure: random:solid-sphere! vect [state]
 -- C Function: scm_random_solid_sphere_x (vect, state)
     Fills VECT with inexact real random numbers the sum of whose
     squares is less than 1.0.  Thinking of VECT as coordinates in
     space of dimension N = `(vector-length VECT)', the coordinates are
     uniformly distributed within the unit N-sphere.

 -- Scheme Procedure: random:uniform [state]
 -- C Function: scm_random_uniform (state)
     Return a uniformly distributed inexact real random number in [0,1).

 -- Scheme Procedure: seed->random-state seed
 -- C Function: scm_seed_to_random_state (seed)
     Return a new random state using SEED.

 -- Scheme Procedure: datum->random-state datum
 -- C Function: scm_datum_to_random_state (datum)
     Return a new random state from DATUM, which should have been
     obtained by `random-state->datum'.

 -- Scheme Procedure: random-state->datum state
 -- C Function: scm_random_state_to_datum (state)
     Return a datum representation of STATE that may be written out and
     read back with the Scheme reader.

 -- Scheme Procedure: random-state-from-platform
 -- C Function: scm_random_state_from_platform ()
     Construct a new random state seeded from a platform-specific
     source of entropy, appropriate for use in non-security-critical
     applications.  Currently `/dev/urandom' is tried first, or else
     the seed is based on the time, date, process ID, an address from a
     freshly allocated heap cell, an address from the local stack
     frame, and a high-resolution timer if available.

 -- Variable: *random-state*
     The global random state used by the above functions when the STATE
     parameter is not given.

   Note that the initial value of `*random-state*' is the same every
time Guile starts up.  Therefore, if you don't pass a STATE parameter
to the above procedures, and you don't set `*random-state*' to
`(seed->random-state your-seed)', where `your-seed' is something that
_isn't_ the same every time, you'll get the same sequence of "random"
numbers on every run.

   For example, unless the relevant source code has changed, `(map
random (cdr (iota 30)))', if the first use of random numbers since
Guile started up, will always give:

     (map random (cdr (iota 19)))
     =>
     (0 1 1 2 2 2 1 2 6 7 10 0 5 3 12 5 5 12)

   To seed the random state in a sensible way for non-security-critical
applications, do this during initialization of your program:

     (set! *random-state* (random-state-from-platform))

6.6.3 Characters
----------------

In Scheme, there is a data type to describe a single character.

   Defining what exactly a character _is_ can be more complicated than
it seems.  Guile follows the advice of R6RS and uses The Unicode
Standard to help define what a character is.  So, for Guile, a
character is anything in the Unicode Character Database.

   The Unicode Character Database is basically a table of characters
indexed using integers called 'code points'.  Valid code points are in
the ranges 0 to `#xD7FF' inclusive or `#xE000' to `#x10FFFF' inclusive,
which is about 1.1 million code points.

   Any code point that has been assigned to a character or that has
otherwise been given a meaning by Unicode is called a 'designated code
point'.  Most of the designated code points, about 200,000 of them,
indicate characters, accents or other combining marks that modify other
characters, symbols, whitespace, and control characters.  Some are not
characters but indicators that suggest how to format or display
neighboring characters.

   If a code point is not a designated code point - if it has not been
assigned to a character by The Unicode Standard - it is a 'reserved
code point', meaning that they are reserved for future use.  Most of
the code points, about 800,000, are 'reserved code points'.

   By convention, a Unicode code point is written as "U+XXXX" where
"XXXX" is a hexadecimal number.  Please note that this convenient
notation is not valid code.  Guile does not interpret "U+XXXX" as a
character.

   In Scheme, a character literal is written as `#\NAME' where NAME is
the name of the character that you want.  Printable characters have
their usual single character name; for example, `#\a' is a lower case
`a'.

   Some of the code points are 'combining characters' that are not meant
to be printed by themselves but are instead meant to modify the
appearance of the previous character.  For combining characters, an
alternate form of the character literal is `#\' followed by U+25CC (a
small, dotted circle), followed by the combining character.  This
allows the combining character to be drawn on the circle, not on the
backslash of `#\'.

   Many of the non-printing characters, such as whitespace characters
and control characters, also have names.

   The most commonly used non-printing characters have long character
names, described in the table below.

Character Name  Codepoint
`#\nul'         U+0000
`#\alarm'       u+0007
`#\backspace'   U+0008
`#\tab'         U+0009
`#\linefeed'    U+000A
`#\newline'     U+000A
`#\vtab'        U+000B
`#\page'        U+000C
`#\return'      U+000D
`#\esc'         U+001B
`#\space'       U+0020
`#\delete'      U+007F

   There are also short names for all of the "C0 control characters"
(those with code points below 32).  The following table lists the short
name for each character.

0 = `#\nul'        1 = `#\soh'        2 = `#\stx'        3 = `#\etx'
4 = `#\eot'        5 = `#\enq'        6 = `#\ack'        7 = `#\bel'
8 = `#\bs'         9 = `#\ht'         10 = `#\lf'        11 = `#\vt'
12 = `#\ff'        13 = `#\cr'        14 = `#\so'        15 = `#\si'
16 = `#\dle'       17 = `#\dc1'       18 = `#\dc2'       19 = `#\dc3'
20 = `#\dc4'       21 = `#\nak'       22 = `#\syn'       23 = `#\etb'
24 = `#\can'       25 = `#\em'        26 = `#\sub'       27 = `#\esc'
28 = `#\fs'        29 = `#\gs'        30 = `#\rs'        31 = `#\us'
32 = `#\sp'                                              

   The short name for the "delete" character (code point U+007F) is
`#\del'.

   There are also a few alternative names left over for compatibility
with previous versions of Guile.

Alternate       Standard
`#\nl'          `#\newline'
`#\np'          `#\page'
`#\null'        `#\nul'

   Characters may also be written using their code point values.  They
can be written with as an octal number, such as `#\10' for `#\bs' or
`#\177' for `#\del'.

   If one prefers hex to octal, there is an additional syntax for
character escapes: `#\xHHHH' - the letter 'x' followed by a hexadecimal
number of one to eight digits.

 -- Scheme Procedure: char? x
 -- C Function: scm_char_p (x)
     Return `#t' iff X is a character, else `#f'.

   Fundamentally, the character comparison operations below are numeric
comparisons of the character's code points.

 -- Scheme Procedure: char=? x y
     Return `#t' iff code point of X is equal to the code point of Y,
     else `#f'.

 -- Scheme Procedure: char<? x y
     Return `#t' iff the code point of X is less than the code point of
     Y, else `#f'.

 -- Scheme Procedure: char<=? x y
     Return `#t' iff the code point of X is less than or equal to the
     code point of Y, else `#f'.

 -- Scheme Procedure: char>? x y
     Return `#t' iff the code point of X is greater than the code point
     of Y, else `#f'.

 -- Scheme Procedure: char>=? x y
     Return `#t' iff the code point of X is greater than or equal to
     the code point of Y, else `#f'.

   Case-insensitive character comparisons use _Unicode case folding_.
In case folding comparisons, if a character is lowercase and has an
uppercase form that can be expressed as a single character, it is
converted to uppercase before comparison.  All other characters undergo
no conversion before the comparison occurs.  This includes the German
sharp S (Eszett) which is not uppercased before conversion because its
uppercase form has two characters.  Unicode case folding is language
independent: it uses rules that are generally true, but, it cannot
cover all cases for all languages.

 -- Scheme Procedure: char-ci=? x y
     Return `#t' iff the case-folded code point of X is the same as the
     case-folded code point of Y, else `#f'.

 -- Scheme Procedure: char-ci<? x y
     Return `#t' iff the case-folded code point of X is less than the
     case-folded code point of Y, else `#f'.

 -- Scheme Procedure: char-ci<=? x y
     Return `#t' iff the case-folded code point of X is less than or
     equal to the case-folded code point of Y, else `#f'.

 -- Scheme Procedure: char-ci>? x y
     Return `#t' iff the case-folded code point of X is greater than
     the case-folded code point of Y, else `#f'.

 -- Scheme Procedure: char-ci>=? x y
     Return `#t' iff the case-folded code point of X is greater than or
     equal to the case-folded code point of Y, else `#f'.

 -- Scheme Procedure: char-alphabetic? chr
 -- C Function: scm_char_alphabetic_p (chr)
     Return `#t' iff CHR is alphabetic, else `#f'.

 -- Scheme Procedure: char-numeric? chr
 -- C Function: scm_char_numeric_p (chr)
     Return `#t' iff CHR is numeric, else `#f'.

 -- Scheme Procedure: char-whitespace? chr
 -- C Function: scm_char_whitespace_p (chr)
     Return `#t' iff CHR is whitespace, else `#f'.

 -- Scheme Procedure: char-upper-case? chr
 -- C Function: scm_char_upper_case_p (chr)
     Return `#t' iff CHR is uppercase, else `#f'.

 -- Scheme Procedure: char-lower-case? chr
 -- C Function: scm_char_lower_case_p (chr)
     Return `#t' iff CHR is lowercase, else `#f'.

 -- Scheme Procedure: char-is-both? chr
 -- C Function: scm_char_is_both_p (chr)
     Return `#t' iff CHR is either uppercase or lowercase, else `#f'.

 -- Scheme Procedure: char-general-category chr
 -- C Function: scm_char_general_category (chr)
     Return a symbol giving the two-letter name of the Unicode general
     category assigned to CHR or `#f' if no named category is assigned.
     The following table provides a list of category names along with
     their meanings.

     Lu      Uppercase letter            Pf      Final quote punctuation
     Ll      Lowercase letter            Po      Other punctuation
     Lt      Titlecase letter            Sm      Math symbol
     Lm      Modifier letter             Sc      Currency symbol
     Lo      Other letter                Sk      Modifier symbol
     Mn      Non-spacing mark            So      Other symbol
     Mc      Combining spacing mark      Zs      Space separator
     Me      Enclosing mark              Zl      Line separator
     Nd      Decimal digit number        Zp      Paragraph separator
     Nl      Letter number               Cc      Control
     No      Other number                Cf      Format
     Pc      Connector punctuation       Cs      Surrogate
     Pd      Dash punctuation            Co      Private use
     Ps      Open punctuation            Cn      Unassigned
     Pe      Close punctuation                   
     Pi      Initial quote punctuation           

 -- Scheme Procedure: char->integer chr
 -- C Function: scm_char_to_integer (chr)
     Return the code point of CHR.

 -- Scheme Procedure: integer->char n
 -- C Function: scm_integer_to_char (n)
     Return the character that has code point N.  The integer N must be
     a valid code point.  Valid code points are in the ranges 0 to
     `#xD7FF' inclusive or `#xE000' to `#x10FFFF' inclusive.

 -- Scheme Procedure: char-upcase chr
 -- C Function: scm_char_upcase (chr)
     Return the uppercase character version of CHR.

 -- Scheme Procedure: char-downcase chr
 -- C Function: scm_char_downcase (chr)
     Return the lowercase character version of CHR.

 -- Scheme Procedure: char-titlecase chr
 -- C Function: scm_char_titlecase (chr)
     Return the titlecase character version of CHR if one exists;
     otherwise return the uppercase version.

     For most characters these will be the same, but the Unicode
     Standard includes certain digraph compatibility characters, such
     as `U+01F3' "dz", for which the uppercase and titlecase characters
     are different (`U+01F1' "DZ" and `U+01F2' "Dz" in this case,
     respectively).

 -- C Function: scm_t_wchar scm_c_upcase (scm_t_wchar C)
 -- C Function: scm_t_wchar scm_c_downcase (scm_t_wchar C)
 -- C Function: scm_t_wchar scm_c_titlecase (scm_t_wchar C)
     These C functions take an integer representation of a Unicode
     codepoint and return the codepoint corresponding to its uppercase,
     lowercase, and titlecase forms respectively.  The type
     `scm_t_wchar' is a signed, 32-bit integer.

6.6.4 Character Sets
--------------------

The features described in this section correspond directly to SRFI-14.

   The data type "charset" implements sets of characters (*note
Characters::).  Because the internal representation of character sets
is not visible to the user, a lot of procedures for handling them are
provided.

   Character sets can be created, extended, tested for the membership
of a characters and be compared to other character sets.

6.6.4.1 Character Set Predicates/Comparison
...........................................

Use these procedures for testing whether an object is a character set,
or whether several character sets are equal or subsets of each other.
`char-set-hash' can be used for calculating a hash value, maybe for
usage in fast lookup procedures.

 -- Scheme Procedure: char-set? obj
 -- C Function: scm_char_set_p (obj)
     Return `#t' if OBJ is a character set, `#f' otherwise.

 -- Scheme Procedure: char-set= . char_sets
 -- C Function: scm_char_set_eq (char_sets)
     Return `#t' if all given character sets are equal.

 -- Scheme Procedure: char-set<= . char_sets
 -- C Function: scm_char_set_leq (char_sets)
     Return `#t' if every character set CSi is a subset of character
     set CSi+1.

 -- Scheme Procedure: char-set-hash cs [bound]
 -- C Function: scm_char_set_hash (cs, bound)
     Compute a hash value for the character set CS.  If BOUND is given
     and non-zero, it restricts the returned value to the range 0 ...
     BOUND - 1.

6.6.4.2 Iterating Over Character Sets
.....................................

Character set cursors are a means for iterating over the members of a
character sets.  After creating a character set cursor with
`char-set-cursor', a cursor can be dereferenced with `char-set-ref',
advanced to the next member with `char-set-cursor-next'.  Whether a
cursor has passed past the last element of the set can be checked with
`end-of-char-set?'.

   Additionally, mapping and (un-)folding procedures for character sets
are provided.

 -- Scheme Procedure: char-set-cursor cs
 -- C Function: scm_char_set_cursor (cs)
     Return a cursor into the character set CS.

 -- Scheme Procedure: char-set-ref cs cursor
 -- C Function: scm_char_set_ref (cs, cursor)
     Return the character at the current cursor position CURSOR in the
     character set CS.  It is an error to pass a cursor for which
     `end-of-char-set?' returns true.

 -- Scheme Procedure: char-set-cursor-next cs cursor
 -- C Function: scm_char_set_cursor_next (cs, cursor)
     Advance the character set cursor CURSOR to the next character in
     the character set CS.  It is an error if the cursor given
     satisfies `end-of-char-set?'.

 -- Scheme Procedure: end-of-char-set? cursor
 -- C Function: scm_end_of_char_set_p (cursor)
     Return `#t' if CURSOR has reached the end of a character set, `#f'
     otherwise.

 -- Scheme Procedure: char-set-fold kons knil cs
 -- C Function: scm_char_set_fold (kons, knil, cs)
     Fold the procedure KONS over the character set CS, initializing it
     with KNIL.

 -- Scheme Procedure: char-set-unfold p f g seed [base_cs]
 -- C Function: scm_char_set_unfold (p, f, g, seed, base_cs)
     This is a fundamental constructor for character sets.
        * G is used to generate a series of "seed" values from the
          initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of the seed values.

        * F maps each seed value to a character. These characters are
          added to the base character set BASE_CS to form the result;
          BASE_CS defaults to the empty set.

 -- Scheme Procedure: char-set-unfold! p f g seed base_cs
 -- C Function: scm_char_set_unfold_x (p, f, g, seed, base_cs)
     This is a fundamental constructor for character sets.
        * G is used to generate a series of "seed" values from the
          initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of the seed values.

        * F maps each seed value to a character. These characters are
          added to the base character set BASE_CS to form the result;
          BASE_CS defaults to the empty set.

 -- Scheme Procedure: char-set-for-each proc cs
 -- C Function: scm_char_set_for_each (proc, cs)
     Apply PROC to every character in the character set CS.  The return
     value is not specified.

 -- Scheme Procedure: char-set-map proc cs
 -- C Function: scm_char_set_map (proc, cs)
     Map the procedure PROC over every character in CS.  PROC must be a
     character -> character procedure.

6.6.4.3 Creating Character Sets
...............................

New character sets are produced with these procedures.

 -- Scheme Procedure: char-set-copy cs
 -- C Function: scm_char_set_copy (cs)
     Return a newly allocated character set containing all characters
     in CS.

 -- Scheme Procedure: char-set . rest
 -- C Function: scm_char_set (rest)
     Return a character set containing all given characters.

 -- Scheme Procedure: list->char-set list [base_cs]
 -- C Function: scm_list_to_char_set (list, base_cs)
     Convert the character list LIST to a character set.  If the
     character set BASE_CS is given, the character in this set are also
     included in the result.

 -- Scheme Procedure: list->char-set! list base_cs
 -- C Function: scm_list_to_char_set_x (list, base_cs)
     Convert the character list LIST to a character set.  The
     characters are added to BASE_CS and BASE_CS is returned.

 -- Scheme Procedure: string->char-set str [base_cs]
 -- C Function: scm_string_to_char_set (str, base_cs)
     Convert the string STR to a character set.  If the character set
     BASE_CS is given, the characters in this set are also included in
     the result.

 -- Scheme Procedure: string->char-set! str base_cs
 -- C Function: scm_string_to_char_set_x (str, base_cs)
     Convert the string STR to a character set.  The characters from
     the string are added to BASE_CS, and BASE_CS is returned.

 -- Scheme Procedure: char-set-filter pred cs [base_cs]
 -- C Function: scm_char_set_filter (pred, cs, base_cs)
     Return a character set containing every character from CS so that
     it satisfies PRED.  If provided, the characters from BASE_CS are
     added to the result.

 -- Scheme Procedure: char-set-filter! pred cs base_cs
 -- C Function: scm_char_set_filter_x (pred, cs, base_cs)
     Return a character set containing every character from CS so that
     it satisfies PRED.  The characters are added to BASE_CS and
     BASE_CS is returned.

 -- Scheme Procedure: ucs-range->char-set lower upper [error [base_cs]]
 -- C Function: scm_ucs_range_to_char_set (lower, upper, error, base_cs)
     Return a character set containing all characters whose character
     codes lie in the half-open range [LOWER,UPPER).

     If ERROR is a true value, an error is signalled if the specified
     range contains characters which are not contained in the
     implemented character range.  If ERROR is `#f', these characters
     are silently left out of the resulting character set.

     The characters in BASE_CS are added to the result, if given.

 -- Scheme Procedure: ucs-range->char-set! lower upper error base_cs
 -- C Function: scm_ucs_range_to_char_set_x (lower, upper, error,
          base_cs)
     Return a character set containing all characters whose character
     codes lie in the half-open range [LOWER,UPPER).

     If ERROR is a true value, an error is signalled if the specified
     range contains characters which are not contained in the
     implemented character range.  If ERROR is `#f', these characters
     are silently left out of the resulting character set.

     The characters are added to BASE_CS and BASE_CS is returned.

 -- Scheme Procedure: ->char-set x
 -- C Function: scm_to_char_set (x)
     Coerces x into a char-set. X may be a string, character or
     char-set. A string is converted to the set of its constituent
     characters; a character is converted to a singleton set; a
     char-set is returned as-is.

6.6.4.4 Querying Character Sets
...............................

Access the elements and other information of a character set with these
procedures.

 -- Scheme Procedure: %char-set-dump cs
     Returns an association list containing debugging information for
     CS. The association list has the following entries.
    `char-set'
          The char-set itself

    `len'
          The number of groups of contiguous code points the char-set
          contains

    `ranges'
          A list of lists where each sublist is a range of code points
          and their associated characters
     The return value of this function cannot be relied upon to be
     consistent between versions of Guile and should not be used in
     code.

 -- Scheme Procedure: char-set-size cs
 -- C Function: scm_char_set_size (cs)
     Return the number of elements in character set CS.

 -- Scheme Procedure: char-set-count pred cs
 -- C Function: scm_char_set_count (pred, cs)
     Return the number of the elements int the character set CS which
     satisfy the predicate PRED.

 -- Scheme Procedure: char-set->list cs
 -- C Function: scm_char_set_to_list (cs)
     Return a list containing the elements of the character set CS.

 -- Scheme Procedure: char-set->string cs
 -- C Function: scm_char_set_to_string (cs)
     Return a string containing the elements of the character set CS.
     The order in which the characters are placed in the string is not
     defined.

 -- Scheme Procedure: char-set-contains? cs ch
 -- C Function: scm_char_set_contains_p (cs, ch)
     Return `#t' iff the character CH is contained in the character set
     CS.

 -- Scheme Procedure: char-set-every pred cs
 -- C Function: scm_char_set_every (pred, cs)
     Return a true value if every character in the character set CS
     satisfies the predicate PRED.

 -- Scheme Procedure: char-set-any pred cs
 -- C Function: scm_char_set_any (pred, cs)
     Return a true value if any character in the character set CS
     satisfies the predicate PRED.

6.6.4.5 Character-Set Algebra
.............................

Character sets can be manipulated with the common set algebra operation,
such as union, complement, intersection etc.  All of these procedures
provide side-effecting variants, which modify their character set
argument(s).

 -- Scheme Procedure: char-set-adjoin cs . rest
 -- C Function: scm_char_set_adjoin (cs, rest)
     Add all character arguments to the first argument, which must be a
     character set.

 -- Scheme Procedure: char-set-delete cs . rest
 -- C Function: scm_char_set_delete (cs, rest)
     Delete all character arguments from the first argument, which must
     be a character set.

 -- Scheme Procedure: char-set-adjoin! cs . rest
 -- C Function: scm_char_set_adjoin_x (cs, rest)
     Add all character arguments to the first argument, which must be a
     character set.

 -- Scheme Procedure: char-set-delete! cs . rest
 -- C Function: scm_char_set_delete_x (cs, rest)
     Delete all character arguments from the first argument, which must
     be a character set.

 -- Scheme Procedure: char-set-complement cs
 -- C Function: scm_char_set_complement (cs)
     Return the complement of the character set CS.

   Note that the complement of a character set is likely to contain many
reserved code points (code points that are not associated with
characters).  It may be helpful to modify the output of
`char-set-complement' by computing its intersection with the set of
designated code points, `char-set:designated'.

 -- Scheme Procedure: char-set-union . rest
 -- C Function: scm_char_set_union (rest)
     Return the union of all argument character sets.

 -- Scheme Procedure: char-set-intersection . rest
 -- C Function: scm_char_set_intersection (rest)
     Return the intersection of all argument character sets.

 -- Scheme Procedure: char-set-difference cs1 . rest
 -- C Function: scm_char_set_difference (cs1, rest)
     Return the difference of all argument character sets.

 -- Scheme Procedure: char-set-xor . rest
 -- C Function: scm_char_set_xor (rest)
     Return the exclusive-or of all argument character sets.

 -- Scheme Procedure: char-set-diff+intersection cs1 . rest
 -- C Function: scm_char_set_diff_plus_intersection (cs1, rest)
     Return the difference and the intersection of all argument
     character sets.

 -- Scheme Procedure: char-set-complement! cs
 -- C Function: scm_char_set_complement_x (cs)
     Return the complement of the character set CS.

 -- Scheme Procedure: char-set-union! cs1 . rest
 -- C Function: scm_char_set_union_x (cs1, rest)
     Return the union of all argument character sets.

 -- Scheme Procedure: char-set-intersection! cs1 . rest
 -- C Function: scm_char_set_intersection_x (cs1, rest)
     Return the intersection of all argument character sets.

 -- Scheme Procedure: char-set-difference! cs1 . rest
 -- C Function: scm_char_set_difference_x (cs1, rest)
     Return the difference of all argument character sets.

 -- Scheme Procedure: char-set-xor! cs1 . rest
 -- C Function: scm_char_set_xor_x (cs1, rest)
     Return the exclusive-or of all argument character sets.

 -- Scheme Procedure: char-set-diff+intersection! cs1 cs2 . rest
 -- C Function: scm_char_set_diff_plus_intersection_x (cs1, cs2, rest)
     Return the difference and the intersection of all argument
     character sets.

6.6.4.6 Standard Character Sets
...............................

In order to make the use of the character set data type and procedures
useful, several predefined character set variables exist.

   These character sets are locale independent and are not recomputed
upon a `setlocale' call.  They contain characters from the whole range
of Unicode code points. For instance, `char-set:letter' contains about
94,000 characters.

 -- Scheme Variable: char-set:lower-case
 -- C Variable: scm_char_set_lower_case
     All lower-case characters.

 -- Scheme Variable: char-set:upper-case
 -- C Variable: scm_char_set_upper_case
     All upper-case characters.

 -- Scheme Variable: char-set:title-case
 -- C Variable: scm_char_set_title_case
     All single characters that function as if they were an upper-case
     letter followed by a lower-case letter.

 -- Scheme Variable: char-set:letter
 -- C Variable: scm_char_set_letter
     All letters.  This includes `char-set:lower-case',
     `char-set:upper-case', `char-set:title-case', and many letters
     that have no case at all.  For example, Chinese and Japanese
     characters typically have no concept of case.

 -- Scheme Variable: char-set:digit
 -- C Variable: scm_char_set_digit
     All digits.

 -- Scheme Variable: char-set:letter+digit
 -- C Variable: scm_char_set_letter_and_digit
     The union of `char-set:letter' and `char-set:digit'.

 -- Scheme Variable: char-set:graphic
 -- C Variable: scm_char_set_graphic
     All characters which would put ink on the paper.

 -- Scheme Variable: char-set:printing
 -- C Variable: scm_char_set_printing
     The union of `char-set:graphic' and `char-set:whitespace'.

 -- Scheme Variable: char-set:whitespace
 -- C Variable: scm_char_set_whitespace
     All whitespace characters.

 -- Scheme Variable: char-set:blank
 -- C Variable: scm_char_set_blank
     All horizontal whitespace characters, which notably includes
     `#\space' and `#\tab'.

 -- Scheme Variable: char-set:iso-control
 -- C Variable: scm_char_set_iso_control
     The ISO control characters are the C0 control characters (U+0000 to
     U+001F), delete (U+007F), and the C1 control characters (U+0080 to
     U+009F).

 -- Scheme Variable: char-set:punctuation
 -- C Variable: scm_char_set_punctuation
     All punctuation characters, such as the characters
     `!"#%&'()*,-./:;?@[\\]_{}'

 -- Scheme Variable: char-set:symbol
 -- C Variable: scm_char_set_symbol
     All symbol characters, such as the characters `$+<=>^`|~'.

 -- Scheme Variable: char-set:hex-digit
 -- C Variable: scm_char_set_hex_digit
     The hexadecimal digits `0123456789abcdefABCDEF'.

 -- Scheme Variable: char-set:ascii
 -- C Variable: scm_char_set_ascii
     All ASCII characters.

 -- Scheme Variable: char-set:empty
 -- C Variable: scm_char_set_empty
     The empty character set.

 -- Scheme Variable: char-set:designated
 -- C Variable: scm_char_set_designated
     This character set contains all designated code points.  This
     includes all the code points to which Unicode has assigned a
     character or other meaning.

 -- Scheme Variable: char-set:full
 -- C Variable: scm_char_set_full
     This character set contains all possible code points.  This
     includes both designated and reserved code points.

6.6.5 Strings
-------------

Strings are fixed-length sequences of characters.  They can be created
by calling constructor procedures, but they can also literally get
entered at the REPL or in Scheme source files.

   Strings always carry the information about how many characters they
are composed of with them, so there is no special end-of-string
character, like in C.  That means that Scheme strings can contain any
character, even the `#\nul' character `\0'.

   To use strings efficiently, you need to know a bit about how Guile
implements them.  In Guile, a string consists of two parts, a head and
the actual memory where the characters are stored.  When a string (or a
substring of it) is copied, only a new head gets created, the memory is
usually not copied.  The two heads start out pointing to the same
memory.

   When one of these two strings is modified, as with `string-set!',
their common memory does get copied so that each string has its own
memory and modifying one does not accidentally modify the other as well.
Thus, Guile's strings are `copy on write'; the actual copying of their
memory is delayed until one string is written to.

   This implementation makes functions like `substring' very efficient
in the common case that no modifications are done to the involved
strings.

   If you do know that your strings are getting modified right away, you
can use `substring/copy' instead of `substring'.  This function
performs the copy immediately at the time of creation.  This is more
efficient, especially in a multi-threaded program.  Also,
`substring/copy' can avoid the problem that a short substring holds on
to the memory of a very large original string that could otherwise be
recycled.

   If you want to avoid the copy altogether, so that modifications of
one string show up in the other, you can use `substring/shared'.  The
strings created by this procedure are called "mutation sharing
substrings" since the substring and the original string share
modifications to each other.

   If you want to prevent modifications, use `substring/read-only'.

   Guile provides all procedures of SRFI-13 and a few more.

6.6.5.1 String Read Syntax
..........................

The read syntax for strings is an arbitrarily long sequence of
characters enclosed in double quotes (").

   Backslash is an escape character and can be used to insert the
following special characters.  \" and \\ are R5RS standard, the next
seven are R6RS standard -- notice they follow C syntax -- and the
remaining four are Guile extensions.

\\
     Backslash character.

\"
     Double quote character (an unescaped " is otherwise the end of the
     string).

\a
     Bell character (ASCII 7).

\f
     Formfeed character (ASCII 12).

\n
     Newline character (ASCII 10).

\r
     Carriage return character (ASCII 13).

\t
     Tab character (ASCII 9).

\v
     Vertical tab character (ASCII 11).

\b
     Backspace character (ASCII 8).

\0
     NUL character (ASCII 0).

\ followed by newline (ASCII 10)
     Nothing.  This way if \ is the last character in a line, the
     string will continue with the first character from the next line,
     without a line break.

     If the `hungry-eol-escapes' reader option is enabled, which is not
     the case by default, leading whitespace on the next line is
     discarded.

          "foo\
            bar"
          => "foo  bar"
          (read-enable 'hungry-eol-escapes)
          "foo\
            bar"
          => "foobar"

\xHH
     Character code given by two hexadecimal digits.  For example \x7f
     for an ASCII DEL (127).

\uHHHH
     Character code given by four hexadecimal digits.  For example
     \u0100 for a capital A with macron (U+0100).

\UHHHHHH
     Character code given by six hexadecimal digits.  For example
     \U010402.

The following are examples of string literals:

     "foo"
     "bar plonk"
     "Hello World"
     "\"Hi\", he said."

   The three escape sequences `\xHH', `\uHHHH' and `\UHHHHHH' were
chosen to not break compatibility with code written for previous
versions of Guile.  The R6RS specification suggests a different,
incompatible syntax for hex escapes: `\xHHHH;' - a character code
followed by one to eight hexadecimal digits terminated with a
semicolon.  If this escape format is desired instead, it can be enabled
with the reader option `r6rs-hex-escapes'.

     (read-enable 'r6rs-hex-escapes)

   For more on reader options, *Note Scheme Read::.

6.6.5.2 String Predicates
.........................

The following procedures can be used to check whether a given string
fulfills some specified property.

 -- Scheme Procedure: string? obj
 -- C Function: scm_string_p (obj)
     Return `#t' if OBJ is a string, else `#f'.

 -- C Function: int scm_is_string (SCM obj)
     Returns `1' if OBJ is a string, `0' otherwise.

 -- Scheme Procedure: string-null? str
 -- C Function: scm_string_null_p (str)
     Return `#t' if STR's length is zero, and `#f' otherwise.
          (string-null? "")  => #t
          y                    => "foo"
          (string-null? y)     => #f

 -- Scheme Procedure: string-any char_pred s [start [end]]
 -- C Function: scm_string_any (char_pred, s, start, end)
     Check if CHAR_PRED is true for any character in string S.

     CHAR_PRED can be a character to check for any equal to that, or a
     character set (*note Character Sets::) to check for any in that
     set, or a predicate procedure to call.

     For a procedure, calls `(CHAR_PRED c)' are made successively on
     the characters from START to END.  If CHAR_PRED returns true (ie.
     non-`#f'), `string-any' stops and that return value is the return
     from `string-any'.  The call on the last character (ie. at END-1),
     if that point is reached, is a tail call.

     If there are no characters in S (ie. START equals END) then the
     return is `#f'.

 -- Scheme Procedure: string-every char_pred s [start [end]]
 -- C Function: scm_string_every (char_pred, s, start, end)
     Check if CHAR_PRED is true for every character in string S.

     CHAR_PRED can be a character to check for every character equal to
     that, or a character set (*note Character Sets::) to check for
     every character being in that set, or a predicate procedure to
     call.

     For a procedure, calls `(CHAR_PRED c)' are made successively on
     the characters from START to END.  If CHAR_PRED returns `#f',
     `string-every' stops and returns `#f'.  The call on the last
     character (ie. at END-1), if that point is reached, is a tail call
     and the return from that call is the return from `string-every'.

     If there are no characters in S (ie. START equals END) then the
     return is `#t'.

6.6.5.3 String Constructors
...........................

The string constructor procedures create new string objects, possibly
initializing them with some specified character data.  See also *Note
String Selection::, for ways to create strings from existing strings.

 -- Scheme Procedure: string char...
     Return a newly allocated string made from the given character
     arguments.

          (string #\x #\y #\z) => "xyz"
          (string)             => ""

 -- Scheme Procedure: list->string lst
 -- C Function: scm_string (lst)
     Return a newly allocated string made from a list of characters.

          (list->string '(#\a #\b #\c)) => "abc"

 -- Scheme Procedure: reverse-list->string lst
 -- C Function: scm_reverse_list_to_string (lst)
     Return a newly allocated string made from a list of characters, in
     reverse order.

          (reverse-list->string '(#\a #\B #\c)) => "cBa"

 -- Scheme Procedure: make-string k [chr]
 -- C Function: scm_make_string (k, chr)
     Return a newly allocated string of length K.  If CHR is given,
     then all elements of the string are initialized to CHR, otherwise
     the contents of the STRING are unspecified.

 -- C Function: SCM scm_c_make_string (size_t len, SCM chr)
     Like `scm_make_string', but expects the length as a `size_t'.

 -- Scheme Procedure: string-tabulate proc len
 -- C Function: scm_string_tabulate (proc, len)
     PROC is an integer->char procedure.  Construct a string of size
     LEN by applying PROC to each index to produce the corresponding
     string element.  The order in which PROC is applied to the indices
     is not specified.

 -- Scheme Procedure: string-join ls [delimiter [grammar]]
 -- C Function: scm_string_join (ls, delimiter, grammar)
     Append the string in the string list LS, using the string DELIM as
     a delimiter between the elements of LS.  GRAMMAR is a symbol which
     specifies how the delimiter is placed between the strings, and
     defaults to the symbol `infix'.

    `infix'
          Insert the separator between list elements.  An empty string
          will produce an empty list.

    `string-infix'
          Like `infix', but will raise an error if given the empty list.

    `suffix'
          Insert the separator after every list element.

    `prefix'
          Insert the separator before each list element.

6.6.5.4 List/String conversion
..............................

When processing strings, it is often convenient to first convert them
into a list representation by using the procedure `string->list', work
with the resulting list, and then convert it back into a string.  These
procedures are useful for similar tasks.

 -- Scheme Procedure: string->list str [start [end]]
 -- C Function: scm_substring_to_list (str, start, end)
 -- C Function: scm_string_to_list (str)
     Convert the string STR into a list of characters.

 -- Scheme Procedure: string-split str chr
 -- C Function: scm_string_split (str, chr)
     Split the string STR into a list of substrings delimited by
     appearances of the character CHR.  Note that an empty substring
     between separator characters will result in an empty string in the
     result list.

          (string-split "root:x:0:0:root:/root:/bin/bash" #\:)
          =>
          ("root" "x" "0" "0" "root" "/root" "/bin/bash")

          (string-split "::" #\:)
          =>
          ("" "" "")

          (string-split "" #\:)
          =>
          ("")

6.6.5.5 String Selection
........................

Portions of strings can be extracted by these procedures.  `string-ref'
delivers individual characters whereas `substring' can be used to
extract substrings from longer strings.

 -- Scheme Procedure: string-length string
 -- C Function: scm_string_length (string)
     Return the number of characters in STRING.

 -- C Function: size_t scm_c_string_length (SCM str)
     Return the number of characters in STR as a `size_t'.

 -- Scheme Procedure: string-ref str k
 -- C Function: scm_string_ref (str, k)
     Return character K of STR using zero-origin indexing. K must be a
     valid index of STR.

 -- C Function: SCM scm_c_string_ref (SCM str, size_t k)
     Return character K of STR using zero-origin indexing. K must be a
     valid index of STR.

 -- Scheme Procedure: string-copy str [start [end]]
 -- C Function: scm_substring_copy (str, start, end)
 -- C Function: scm_string_copy (str)
     Return a copy of the given string STR.

     The returned string shares storage with STR initially, but it is
     copied as soon as one of the two strings is modified.

 -- Scheme Procedure: substring str start [end]
 -- C Function: scm_substring (str, start, end)
     Return a new string formed from the characters of STR beginning
     with index START (inclusive) and ending with index END (exclusive).
     STR must be a string, START and END must be exact integers
     satisfying:

     0 <= START <= END <= `(string-length STR)'.

     The returned string shares storage with STR initially, but it is
     copied as soon as one of the two strings is modified.

 -- Scheme Procedure: substring/shared str start [end]
 -- C Function: scm_substring_shared (str, start, end)
     Like `substring', but the strings continue to share their storage
     even if they are modified.  Thus, modifications to STR show up in
     the new string, and vice versa.

 -- Scheme Procedure: substring/copy str start [end]
 -- C Function: scm_substring_copy (str, start, end)
     Like `substring', but the storage for the new string is copied
     immediately.

 -- Scheme Procedure: substring/read-only str start [end]
 -- C Function: scm_substring_read_only (str, start, end)
     Like `substring', but the resulting string can not be modified.

 -- C Function: SCM scm_c_substring (SCM str, size_t start, size_t end)
 -- C Function: SCM scm_c_substring_shared (SCM str, size_t start,
          size_t end)
 -- C Function: SCM scm_c_substring_copy (SCM str, size_t start, size_t
          end)
 -- C Function: SCM scm_c_substring_read_only (SCM str, size_t start,
          size_t end)
     Like `scm_substring', etc. but the bounds are given as a `size_t'.

 -- Scheme Procedure: string-take s n
 -- C Function: scm_string_take (s, n)
     Return the N first characters of S.

 -- Scheme Procedure: string-drop s n
 -- C Function: scm_string_drop (s, n)
     Return all but the first N characters of S.

 -- Scheme Procedure: string-take-right s n
 -- C Function: scm_string_take_right (s, n)
     Return the N last characters of S.

 -- Scheme Procedure: string-drop-right s n
 -- C Function: scm_string_drop_right (s, n)
     Return all but the last N characters of S.

 -- Scheme Procedure: string-pad s len [chr [start [end]]]
 -- Scheme Procedure: string-pad-right s len [chr [start [end]]]
 -- C Function: scm_string_pad (s, len, chr, start, end)
 -- C Function: scm_string_pad_right (s, len, chr, start, end)
     Take characters START to END from the string S and either pad with
     CHAR or truncate them to give LEN characters.

     `string-pad' pads or truncates on the left, so for example

          (string-pad "x" 3)     => "  x"
          (string-pad "abcde" 3) => "cde"

     `string-pad-right' pads or truncates on the right, so for example

          (string-pad-right "x" 3)     => "x  "
          (string-pad-right "abcde" 3) => "abc"

 -- Scheme Procedure: string-trim s [char_pred [start [end]]]
 -- Scheme Procedure: string-trim-right s [char_pred [start [end]]]
 -- Scheme Procedure: string-trim-both s [char_pred [start [end]]]
 -- C Function: scm_string_trim (s, char_pred, start, end)
 -- C Function: scm_string_trim_right (s, char_pred, start, end)
 -- C Function: scm_string_trim_both (s, char_pred, start, end)
     Trim occurrences of CHAR_PRED from the ends of S.

     `string-trim' trims CHAR_PRED characters from the left (start) of
     the string, `string-trim-right' trims them from the right (end) of
     the string, `string-trim-both' trims from both ends.

     CHAR_PRED can be a character, a character set, or a predicate
     procedure to call on each character.  If CHAR_PRED is not given
     the default is whitespace as per `char-set:whitespace' (*note
     Standard Character Sets::).

          (string-trim " x ")              => "x "
          (string-trim-right "banana" #\a) => "banan"
          (string-trim-both ".,xy:;" char-set:punctuation)
                            => "xy"
          (string-trim-both "xyzzy" (lambda (c)
                                       (or (eqv? c #\x)
                                           (eqv? c #\y))))
                            => "zz"

6.6.5.6 String Modification
...........................

These procedures are for modifying strings in-place.  This means that
the result of the operation is not a new string; instead, the original
string's memory representation is modified.

 -- Scheme Procedure: string-set! str k chr
 -- C Function: scm_string_set_x (str, k, chr)
     Store CHR in element K of STR and return an unspecified value. K
     must be a valid index of STR.

 -- C Function: void scm_c_string_set_x (SCM str, size_t k, SCM chr)
     Like `scm_string_set_x', but the index is given as a `size_t'.

 -- Scheme Procedure: string-fill! str chr [start [end]]
 -- C Function: scm_substring_fill_x (str, chr, start, end)
 -- C Function: scm_string_fill_x (str, chr)
     Stores CHR in every element of the given STR and returns an
     unspecified value.

 -- Scheme Procedure: substring-fill! str start end fill
 -- C Function: scm_substring_fill_x (str, start, end, fill)
     Change every character in STR between START and END to FILL.

          (define y (string-copy "abcdefg"))
          (substring-fill! y 1 3 #\r)
          y
          => "arrdefg"

 -- Scheme Procedure: substring-move! str1 start1 end1 str2 start2
 -- C Function: scm_substring_move_x (str1, start1, end1, str2, start2)
     Copy the substring of STR1 bounded by START1 and END1 into STR2
     beginning at position START2.  STR1 and STR2 can be the same
     string.

 -- Scheme Procedure: string-copy! target tstart s [start [end]]
 -- C Function: scm_string_copy_x (target, tstart, s, start, end)
     Copy the sequence of characters from index range [START, END) in
     string S to string TARGET, beginning at index TSTART.  The
     characters are copied left-to-right or right-to-left as needed -
     the copy is guaranteed to work, even if TARGET and S are the same
     string.  It is an error if the copy operation runs off the end of
     the target string.

6.6.5.7 String Comparison
.........................

The procedures in this section are similar to the character ordering
predicates (*note Characters::), but are defined on character sequences.

   The first set is specified in R5RS and has names that end in `?'.
The second set is specified in SRFI-13 and the names have not ending
`?'.

   The predicates ending in `-ci' ignore the character case when
comparing strings.  For now, case-insensitive comparison is done using
the R5RS rules, where every lower-case character that has a single
character upper-case form is converted to uppercase before comparison.
See *Note the `(ice-9 i18n)' module: Text Collation, for
locale-dependent string comparison.

 -- Scheme Procedure: string=? [s1 [s2 . rest]]
 -- C Function: scm_i_string_equal_p (s1, s2, rest)
     Lexicographic equality predicate; return `#t' if the two strings
     are the same length and contain the same characters in the same
     positions, otherwise return `#f'.

     The procedure `string-ci=?' treats upper and lower case letters as
     though they were the same character, but `string=?' treats upper
     and lower case as distinct characters.

 -- Scheme Procedure: string<? [s1 [s2 . rest]]
 -- C Function: scm_i_string_less_p (s1, s2, rest)
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically less than S2.

 -- Scheme Procedure: string<=? [s1 [s2 . rest]]
 -- C Function: scm_i_string_leq_p (s1, s2, rest)
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically less than or equal to S2.

 -- Scheme Procedure: string>? [s1 [s2 . rest]]
 -- C Function: scm_i_string_gr_p (s1, s2, rest)
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically greater than S2.

 -- Scheme Procedure: string>=? [s1 [s2 . rest]]
 -- C Function: scm_i_string_geq_p (s1, s2, rest)
     Lexicographic ordering predicate; return `#t' if S1 is
     lexicographically greater than or equal to S2.

 -- Scheme Procedure: string-ci=? [s1 [s2 . rest]]
 -- C Function: scm_i_string_ci_equal_p (s1, s2, rest)
     Case-insensitive string equality predicate; return `#t' if the two
     strings are the same length and their component characters match
     (ignoring case) at each position; otherwise return `#f'.

 -- Scheme Procedure: string-ci<? [s1 [s2 . rest]]
 -- C Function: scm_i_string_ci_less_p (s1, s2, rest)
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically less than S2 regardless of case.

 -- Scheme Procedure: string-ci<=? [s1 [s2 . rest]]
 -- C Function: scm_i_string_ci_leq_p (s1, s2, rest)
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically less than or equal to S2 regardless of
     case.

 -- Scheme Procedure: string-ci>? [s1 [s2 . rest]]
 -- C Function: scm_i_string_ci_gr_p (s1, s2, rest)
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically greater than S2 regardless of case.

 -- Scheme Procedure: string-ci>=? [s1 [s2 . rest]]
 -- C Function: scm_i_string_ci_geq_p (s1, s2, rest)
     Case insensitive lexicographic ordering predicate; return `#t' if
     S1 is lexicographically greater than or equal to S2 regardless of
     case.

 -- Scheme Procedure: string-compare s1 s2 proc_lt proc_eq proc_gt
          [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt,
          start1, end1, start2, end2)
     Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending
     upon whether S1 is less than, equal to, or greater than S2.  The
     mismatch index is the largest index I such that for every 0 <= J <
     I, S1[J] = S2[J] - that is, I is the first position that does not
     match.

 -- Scheme Procedure: string-compare-ci s1 s2 proc_lt proc_eq proc_gt
          [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_compare_ci (s1, s2, proc_lt, proc_eq,
          proc_gt, start1, end1, start2, end2)
     Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending
     upon whether S1 is less than, equal to, or greater than S2.  The
     mismatch index is the largest index I such that for every 0 <= J <
     I, S1[J] = S2[J] - that is, I is the first position where the
     lowercased letters do not match.


 -- Scheme Procedure: string= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_eq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are not equal, a true value otherwise.

 -- Scheme Procedure: string<> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_neq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are equal, a true value otherwise.

 -- Scheme Procedure: string< s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_lt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater or equal to S2, a true value
     otherwise.

 -- Scheme Procedure: string> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_gt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less or equal to S2, a true value otherwise.

 -- Scheme Procedure: string<= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_le (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater to S2, a true value otherwise.

 -- Scheme Procedure: string>= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ge (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less to S2, a true value otherwise.

 -- Scheme Procedure: string-ci= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_eq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are not equal, a true value otherwise.
     The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci<> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_neq (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 and S2 are equal, a true value otherwise.  The
     character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci< s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_lt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater or equal to S2, a true value
     otherwise.  The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci> s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_gt (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less or equal to S2, a true value otherwise.
     The character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci<= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_le (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is greater to S2, a true value otherwise.  The
     character comparison is done case-insensitively.

 -- Scheme Procedure: string-ci>= s1 s2 [start1 [end1 [start2 [end2]]]]
 -- C Function: scm_string_ci_ge (s1, s2, start1, end1, start2, end2)
     Return `#f' if S1 is less to S2, a true value otherwise.  The
     character comparison is done case-insensitively.

 -- Scheme Procedure: string-hash s [bound [start [end]]]
 -- C Function: scm_substring_hash (s, bound, start, end)
     Compute a hash value for S.  The optional argument BOUND is a
     non-negative exact integer specifying the range of the hash
     function. A positive value restricts the return value to the range
     [0,bound).

 -- Scheme Procedure: string-hash-ci s [bound [start [end]]]
 -- C Function: scm_substring_hash_ci (s, bound, start, end)
     Compute a hash value for S.  The optional argument BOUND is a
     non-negative exact integer specifying the range of the hash
     function. A positive value restricts the return value to the range
     [0,bound).

   Because the same visual appearance of an abstract Unicode character
can be obtained via multiple sequences of Unicode characters, even the
case-insensitive string comparison functions described above may return
`#f' when presented with strings containing different representations
of the same character.  For example, the Unicode character "LATIN SMALL
LETTER S WITH DOT BELOW AND DOT ABOVE" can be represented with a single
character (U+1E69) or by the character "LATIN SMALL LETTER S" (U+0073)
followed by the combining marks "COMBINING DOT BELOW" (U+0323) and
"COMBINING DOT ABOVE" (U+0307).

   For this reason, it is often desirable to ensure that the strings to
be compared are using a mutually consistent representation for every
character.  The Unicode standard defines two methods of normalizing the
contents of strings: Decomposition, which breaks composite characters
into a set of constituent characters with an ordering defined by the
Unicode Standard; and composition, which performs the converse.

   There are two decomposition operations.  "Canonical decomposition"
produces character sequences that share the same visual appearance as
the original characters, while "compatibility decomposition" produces
ones whose visual appearances may differ from the originals but which
represent the same abstract character.

   These operations are encapsulated in the following set of
normalization forms:

"NFD"
     Characters are decomposed to their canonical forms.

"NFKD"
     Characters are decomposed to their compatibility forms.

"NFC"
     Characters are decomposed to their canonical forms, then composed.

"NFKC"
     Characters are decomposed to their compatibility forms, then
     composed.


   The functions below put their arguments into one of the forms
described above.

 -- Scheme Procedure: string-normalize-nfd s
 -- C Function: scm_string_normalize_nfd (s)
     Return the `NFD' normalized form of S.

 -- Scheme Procedure: string-normalize-nfkd s
 -- C Function: scm_string_normalize_nfkd (s)
     Return the `NFKD' normalized form of S.

 -- Scheme Procedure: string-normalize-nfc s
 -- C Function: scm_string_normalize_nfc (s)
     Return the `NFC' normalized form of S.

 -- Scheme Procedure: string-normalize-nfkc s
 -- C Function: scm_string_normalize_nfkc (s)
     Return the `NFKC' normalized form of S.

6.6.5.8 String Searching
........................

 -- Scheme Procedure: string-index s char_pred [start [end]]
 -- C Function: scm_string_index (s, char_pred, start, end)
     Search through the string S from left to right, returning the
     index of the first occurrence of a character which

        * equals CHAR_PRED, if it is character,

        * satisfies the predicate CHAR_PRED, if it is a procedure,

        * is in the set CHAR_PRED, if it is a character set.

     Return `#f' if no match is found.

 -- Scheme Procedure: string-rindex s char_pred [start [end]]
 -- C Function: scm_string_rindex (s, char_pred, start, end)
     Search through the string S from right to left, returning the
     index of the last occurrence of a character which

        * equals CHAR_PRED, if it is character,

        * satisfies the predicate CHAR_PRED, if it is a procedure,

        * is in the set if CHAR_PRED is a character set.

     Return `#f' if no match is found.

 -- Scheme Procedure: string-prefix-length s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_prefix_length (s1, s2, start1, end1, start2,
          end2)
     Return the length of the longest common prefix of the two strings.

 -- Scheme Procedure: string-prefix-length-ci s1 s2 [start1 [end1
          [start2 [end2]]]]
 -- C Function: scm_string_prefix_length_ci (s1, s2, start1, end1,
          start2, end2)
     Return the length of the longest common prefix of the two strings,
     ignoring character case.

 -- Scheme Procedure: string-suffix-length s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_suffix_length (s1, s2, start1, end1, start2,
          end2)
     Return the length of the longest common suffix of the two strings.

 -- Scheme Procedure: string-suffix-length-ci s1 s2 [start1 [end1
          [start2 [end2]]]]
 -- C Function: scm_string_suffix_length_ci (s1, s2, start1, end1,
          start2, end2)
     Return the length of the longest common suffix of the two strings,
     ignoring character case.

 -- Scheme Procedure: string-prefix? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_prefix_p (s1, s2, start1, end1, start2, end2)
     Is S1 a prefix of S2?

 -- Scheme Procedure: string-prefix-ci? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_prefix_ci_p (s1, s2, start1, end1, start2,
          end2)
     Is S1 a prefix of S2, ignoring character case?

 -- Scheme Procedure: string-suffix? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_suffix_p (s1, s2, start1, end1, start2, end2)
     Is S1 a suffix of S2?

 -- Scheme Procedure: string-suffix-ci? s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_suffix_ci_p (s1, s2, start1, end1, start2,
          end2)
     Is S1 a suffix of S2, ignoring character case?

 -- Scheme Procedure: string-index-right s char_pred [start [end]]
 -- C Function: scm_string_index_right (s, char_pred, start, end)
     Search through the string S from right to left, returning the
     index of the last occurrence of a character which

        * equals CHAR_PRED, if it is character,

        * satisfies the predicate CHAR_PRED, if it is a procedure,

        * is in the set if CHAR_PRED is a character set.

     Return `#f' if no match is found.

 -- Scheme Procedure: string-skip s char_pred [start [end]]
 -- C Function: scm_string_skip (s, char_pred, start, end)
     Search through the string S from left to right, returning the
     index of the first occurrence of a character which

        * does not equal CHAR_PRED, if it is character,

        * does not satisfy the predicate CHAR_PRED, if it is a
          procedure,

        * is not in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-skip-right s char_pred [start [end]]
 -- C Function: scm_string_skip_right (s, char_pred, start, end)
     Search through the string S from right to left, returning the
     index of the last occurrence of a character which

        * does not equal CHAR_PRED, if it is character,

        * does not satisfy the predicate CHAR_PRED, if it is a
          procedure,

        * is not in the set if CHAR_PRED is a character set.

 -- Scheme Procedure: string-count s char_pred [start [end]]
 -- C Function: scm_string_count (s, char_pred, start, end)
     Return the count of the number of characters in the string S which

        * equals CHAR_PRED, if it is character,

        * satisfies the predicate CHAR_PRED, if it is a procedure.

        * is in the set CHAR_PRED, if it is a character set.

 -- Scheme Procedure: string-contains s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_contains (s1, s2, start1, end1, start2, end2)
     Does string S1 contain string S2?  Return the index in S1 where S2
     occurs as a substring, or false.  The optional start/end indices
     restrict the operation to the indicated substrings.

 -- Scheme Procedure: string-contains-ci s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_contains_ci (s1, s2, start1, end1, start2,
          end2)
     Does string S1 contain string S2?  Return the index in S1 where S2
     occurs as a substring, or false.  The optional start/end indices
     restrict the operation to the indicated substrings.  Character
     comparison is done case-insensitively.

6.6.5.9 Alphabetic Case Mapping
...............................

These are procedures for mapping strings to their upper- or lower-case
equivalents, respectively, or for capitalizing strings.

   They use the basic case mapping rules for Unicode characters.  No
special language or context rules are considered.  The resulting strings
are guaranteed to be the same length as the input strings.

   *Note the `(ice-9 i18n)' module: Character Case Mapping, for
locale-dependent case conversions.

 -- Scheme Procedure: string-upcase str [start [end]]
 -- C Function: scm_substring_upcase (str, start, end)
 -- C Function: scm_string_upcase (str)
     Upcase every character in `str'.

 -- Scheme Procedure: string-upcase! str [start [end]]
 -- C Function: scm_substring_upcase_x (str, start, end)
 -- C Function: scm_string_upcase_x (str)
     Destructively upcase every character in `str'.

          (string-upcase! y)
          => "ARRDEFG"
          y
          => "ARRDEFG"

 -- Scheme Procedure: string-downcase str [start [end]]
 -- C Function: scm_substring_downcase (str, start, end)
 -- C Function: scm_string_downcase (str)
     Downcase every character in STR.

 -- Scheme Procedure: string-downcase! str [start [end]]
 -- C Function: scm_substring_downcase_x (str, start, end)
 -- C Function: scm_string_downcase_x (str)
     Destructively downcase every character in STR.

          y
          => "ARRDEFG"
          (string-downcase! y)
          => "arrdefg"
          y
          => "arrdefg"

 -- Scheme Procedure: string-capitalize str
 -- C Function: scm_string_capitalize (str)
     Return a freshly allocated string with the characters in STR,
     where the first character of every word is capitalized.

 -- Scheme Procedure: string-capitalize! str
 -- C Function: scm_string_capitalize_x (str)
     Upcase the first character of every word in STR destructively and
     return STR.

          y                      => "hello world"
          (string-capitalize! y) => "Hello World"
          y                      => "Hello World"

 -- Scheme Procedure: string-titlecase str [start [end]]
 -- C Function: scm_string_titlecase (str, start, end)
     Titlecase every first character in a word in STR.

 -- Scheme Procedure: string-titlecase! str [start [end]]
 -- C Function: scm_string_titlecase_x (str, start, end)
     Destructively titlecase every first character in a word in STR.

6.6.5.10 Reversing and Appending Strings
........................................

 -- Scheme Procedure: string-reverse str [start [end]]
 -- C Function: scm_string_reverse (str, start, end)
     Reverse the string STR.  The optional arguments START and END
     delimit the region of STR to operate on.

 -- Scheme Procedure: string-reverse! str [start [end]]
 -- C Function: scm_string_reverse_x (str, start, end)
     Reverse the string STR in-place.  The optional arguments START and
     END delimit the region of STR to operate on.  The return value is
     unspecified.

 -- Scheme Procedure: string-append . args
 -- C Function: scm_string_append (args)
     Return a newly allocated string whose characters form the
     concatenation of the given strings, ARGS.

          (let ((h "hello "))
            (string-append h "world"))
          => "hello world"

 -- Scheme Procedure: string-append/shared . rest
 -- C Function: scm_string_append_shared (rest)
     Like `string-append', but the result may share memory with the
     argument strings.

 -- Scheme Procedure: string-concatenate ls
 -- C Function: scm_string_concatenate (ls)
     Append the elements of LS (which must be strings) together into a
     single string.  Guaranteed to return a freshly allocated string.

 -- Scheme Procedure: string-concatenate-reverse ls [final_string [end]]
 -- C Function: scm_string_concatenate_reverse (ls, final_string, end)
     Without optional arguments, this procedure is equivalent to

          (string-concatenate (reverse ls))

     If the optional argument FINAL_STRING is specified, it is consed
     onto the beginning to LS before performing the list-reverse and
     string-concatenate operations.  If END is given, only the
     characters of FINAL_STRING up to index END are used.

     Guaranteed to return a freshly allocated string.

 -- Scheme Procedure: string-concatenate/shared ls
 -- C Function: scm_string_concatenate_shared (ls)
     Like `string-concatenate', but the result may share memory with
     the strings in the list LS.

 -- Scheme Procedure: string-concatenate-reverse/shared ls
          [final_string [end]]
 -- C Function: scm_string_concatenate_reverse_shared (ls,
          final_string, end)
     Like `string-concatenate-reverse', but the result may share memory
     with the strings in the LS arguments.

6.6.5.11 Mapping, Folding, and Unfolding
........................................

 -- Scheme Procedure: string-map proc s [start [end]]
 -- C Function: scm_string_map (proc, s, start, end)
     PROC is a char->char procedure, it is mapped over S.  The order in
     which the procedure is applied to the string elements is not
     specified.

 -- Scheme Procedure: string-map! proc s [start [end]]
 -- C Function: scm_string_map_x (proc, s, start, end)
     PROC is a char->char procedure, it is mapped over S.  The order in
     which the procedure is applied to the string elements is not
     specified.  The string S is modified in-place, the return value is
     not specified.

 -- Scheme Procedure: string-for-each proc s [start [end]]
 -- C Function: scm_string_for_each (proc, s, start, end)
     PROC is mapped over S in left-to-right order.  The return value is
     not specified.

 -- Scheme Procedure: string-for-each-index proc s [start [end]]
 -- C Function: scm_string_for_each_index (proc, s, start, end)
     Call `(PROC i)' for each index i in S, from left to right.

     For example, to change characters to alternately upper and lower
     case,

          (define str (string-copy "studly"))
          (string-for-each-index
              (lambda (i)
                (string-set! str i
                  ((if (even? i) char-upcase char-downcase)
                   (string-ref str i))))
              str)
          str => "StUdLy"

 -- Scheme Procedure: string-fold kons knil s [start [end]]
 -- C Function: scm_string_fold (kons, knil, s, start, end)
     Fold KONS over the characters of S, with KNIL as the terminating
     element, from left to right.  KONS must expect two arguments: The
     actual character and the last result of KONS' application.

 -- Scheme Procedure: string-fold-right kons knil s [start [end]]
 -- C Function: scm_string_fold_right (kons, knil, s, start, end)
     Fold KONS over the characters of S, with KNIL as the terminating
     element, from right to left.  KONS must expect two arguments: The
     actual character and the last result of KONS' application.

 -- Scheme Procedure: string-unfold p f g seed [base [make_final]]
 -- C Function: scm_string_unfold (p, f, g, seed, base, make_final)
        * G is used to generate a series of _seed_ values from the
          initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of these seed values.

        * F maps each seed value to the corresponding character in the
          result string.  These chars are assembled into the string in
          a left-to-right order.

        * BASE is the optional initial/leftmost portion of the
          constructed string; it default to the empty string.

        * MAKE_FINAL is applied to the terminal seed value (on which P
          returns true) to produce the final/rightmost portion of the
          constructed string.  The default is nothing extra.

 -- Scheme Procedure: string-unfold-right p f g seed [base [make_final]]
 -- C Function: scm_string_unfold_right (p, f, g, seed, base,
          make_final)
        * G is used to generate a series of _seed_ values from the
          initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...

        * P tells us when to stop - when it returns true when applied
          to one of these seed values.

        * F maps each seed value to the corresponding character in the
          result string.  These chars are assembled into the string in
          a right-to-left order.

        * BASE is the optional initial/rightmost portion of the
          constructed string; it default to the empty string.

        * MAKE_FINAL is applied to the terminal seed value (on which P
          returns true) to produce the final/leftmost portion of the
          constructed string.  It defaults to `(lambda (x) )'.

6.6.5.12 Miscellaneous String Operations
........................................

 -- Scheme Procedure: xsubstring s from [to [start [end]]]
 -- C Function: scm_xsubstring (s, from, to, start, end)
     This is the _extended substring_ procedure that implements
     replicated copying of a substring of some string.

     S is a string, START and END are optional arguments that demarcate
     a substring of S, defaulting to 0 and the length of S.  Replicate
     this substring up and down index space, in both the positive and
     negative directions.  `xsubstring' returns the substring of this
     string beginning at index FROM, and ending at TO, which defaults
     to FROM + (END - START).

 -- Scheme Procedure: string-xcopy! target tstart s sfrom [sto [start
          [end]]]
 -- C Function: scm_string_xcopy_x (target, tstart, s, sfrom, sto,
          start, end)
     Exactly the same as `xsubstring', but the extracted text is
     written into the string TARGET starting at index TSTART.  The
     operation is not defined if `(eq?  TARGET S)' or these arguments
     share storage - you cannot copy a string on top of itself.

 -- Scheme Procedure: string-replace s1 s2 [start1 [end1 [start2
          [end2]]]]
 -- C Function: scm_string_replace (s1, s2, start1, end1, start2, end2)
     Return the string S1, but with the characters START1 ... END1
     replaced by the characters START2 ... END2 from S2.

 -- Scheme Procedure: string-tokenize s [token_set [start [end]]]
 -- C Function: scm_string_tokenize (s, token_set, start, end)
     Split the string S into a list of substrings, where each substring
     is a maximal non-empty contiguous sequence of characters from the
     character set TOKEN_SET, which defaults to `char-set:graphic'.  If
     START or END indices are provided, they restrict `string-tokenize'
     to operating on the indicated substring of S.

 -- Scheme Procedure: string-filter char_pred s [start [end]]
 -- C Function: scm_string_filter (char_pred, s, start, end)
     Filter the string S, retaining only those characters which satisfy
     CHAR_PRED.

     If CHAR_PRED is a procedure, it is applied to each character as a
     predicate, if it is a character, it is tested for equality and if
     it is a character set, it is tested for membership.

 -- Scheme Procedure: string-delete char_pred s [start [end]]
 -- C Function: scm_string_delete (char_pred, s, start, end)
     Delete characters satisfying CHAR_PRED from S.

     If CHAR_PRED is a procedure, it is applied to each character as a
     predicate, if it is a character, it is tested for equality and if
     it is a character set, it is tested for membership.

6.6.5.13 Conversion to/from C
.............................

When creating a Scheme string from a C string or when converting a
Scheme string to a C string, the concept of character encoding becomes
important.

   In C, a string is just a sequence of bytes, and the character
encoding describes the relation between these bytes and the actual
characters that make up the string.  For Scheme strings, character
encoding is not an issue (most of the time), since in Scheme you never
get to see the bytes, only the characters.

   Converting to C and converting from C each have their own challenges.

   When converting from C to Scheme, it is important that the sequence
of bytes in the C string be valid with respect to its encoding.  ASCII
strings, for example, can't have any bytes greater than 127.  An ASCII
byte greater than 127 is considered _ill-formed_ and cannot be
converted into a Scheme character.

   Problems can occur in the reverse operation as well.  Not all
character encodings can hold all possible Scheme characters.  Some
encodings, like ASCII for example, can only describe a small subset of
all possible characters.  So, when converting to C, one must first
decide what to do with Scheme characters that can't be represented in
the C string.

   Converting a Scheme string to a C string will often allocate fresh
memory to hold the result.  You must take care that this memory is
properly freed eventually.  In many cases, this can be achieved by
using `scm_dynwind_free' inside an appropriate dynwind context, *Note
Dynamic Wind::.

 -- C Function: SCM scm_from_locale_string (const char *str)
 -- C Function: SCM scm_from_locale_stringn (const char *str, size_t
          len)
     Creates a new Scheme string that has the same contents as STR when
     interpreted in the character encoding of the current locale.

     For `scm_from_locale_string', STR must be null-terminated.

     For `scm_from_locale_stringn', LEN specifies the length of STR in
     bytes, and STR does not need to be null-terminated.  If LEN is
     `(size_t)-1', then STR does need to be null-terminated and the
     real length will be found with `strlen'.

     If the C string is ill-formed, an error will be raised.

     Note that these functions should _not_ be used to convert C string
     constants, because there is no guarantee that the current locale
     will match that of the source code.  To convert C string
     constants, use `scm_from_latin1_string', `scm_from_utf8_string' or
     `scm_from_utf32_string'.

 -- C Function: SCM scm_take_locale_string (char *str)
 -- C Function: SCM scm_take_locale_stringn (char *str, size_t len)
     Like `scm_from_locale_string' and `scm_from_locale_stringn',
     respectively, but also frees STR with `free' eventually.  Thus,
     you can use this function when you would free STR anyway
     immediately after creating the Scheme string.  In certain cases,
     Guile can then use STR directly as its internal representation.

 -- C Function: char * scm_to_locale_string (SCM str)
 -- C Function: char * scm_to_locale_stringn (SCM str, size_t *lenp)
     Returns a C string with the same contents as STR in the character
     encoding of the current locale.  The C string must be freed with
     `free' eventually, maybe by using `scm_dynwind_free', *Note
     Dynamic Wind::.

     For `scm_to_locale_string', the returned string is null-terminated
     and an error is signalled when STR contains `#\nul' characters.

     For `scm_to_locale_stringn' and LENP not `NULL', STR might contain
     `#\nul' characters and the length of the returned string in bytes
     is stored in `*LENP'.  The returned string will not be
     null-terminated in this case.  If LENP is `NULL',
     `scm_to_locale_stringn' behaves like `scm_to_locale_string'.

     If a character in STR cannot be represented in the character
     encoding of the current locale, the default port conversion
     strategy is used.  *Note Ports::, for more on conversion
     strategies.

     If the conversion strategy is `error', an error will be raised.  If
     it is `substitute', a replacement character, such as a question
     mark, will be inserted in its place.  If it is `escape', a hex
     escape will be inserted in its place.

 -- C Function: size_t scm_to_locale_stringbuf (SCM str, char *buf,
          size_t max_len)
     Puts STR as a C string in the current locale encoding into the
     memory pointed to by BUF.  The buffer at BUF has room for MAX_LEN
     bytes and `scm_to_local_stringbuf' will never store more than
     that.  No terminating `'\0'' will be stored.

     The return value of `scm_to_locale_stringbuf' is the number of
     bytes that are needed for all of STR, regardless of whether BUF
     was large enough to hold them.  Thus, when the return value is
     larger than MAX_LEN, only MAX_LEN bytes have been stored and you
     probably need to try again with a larger buffer.

   For most situations, string conversion should occur using the current
locale, such as with the functions above.  But there may be cases where
one wants to convert strings from a character encoding other than the
locale's character encoding.  For these cases, the lower-level functions
`scm_to_stringn' and `scm_from_stringn' are provided.  These functions
should seldom be necessary if one is properly using locales.

 -- C Type: scm_t_string_failed_conversion_handler
     This is an enumerated type that can take one of three values:
     `SCM_FAILED_CONVERSION_ERROR',
     `SCM_FAILED_CONVERSION_QUESTION_MARK', and
     `SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE'.  They are used to indicate
     a strategy for handling characters that cannot be converted to or
     from a given character encoding.  `SCM_FAILED_CONVERSION_ERROR'
     indicates that a conversion should throw an error if some
     characters cannot be converted.
     `SCM_FAILED_CONVERSION_QUESTION_MARK' indicates that a conversion
     should replace unconvertable characters with the question mark
     character.  And, `SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE' requests
     that a conversion should replace an unconvertable character with
     an escape sequence.

     While all three strategies apply when converting Scheme strings to
     C, only `SCM_FAILED_CONVERSION_ERROR' and
     `SCM_FAILED_CONVERSION_QUESTION_MARK' can be used when converting C
     strings to Scheme.

 -- C Function: char *scm_to_stringn (SCM str, size_t *lenp, const char
          *encoding, scm_t_string_failed_conversion_handler handler)
     This function returns a newly allocated C string from the Guile
     string STR.  The length of the returned string in bytes will be
     returned in LENP.  The character encoding of the C string is
     passed as the ASCII, null-terminated C string ENCODING.  The
     HANDLER parameter gives a strategy for dealing with characters
     that cannot be converted into ENCODING.

     If LENP is `NULL', this function will return a null-terminated C
     string.  It will throw an error if the string contains a null
     character.

 -- C Function: SCM scm_from_stringn (const char *str, size_t len,
          const char *encoding, scm_t_string_failed_conversion_handler
          handler)
     This function returns a scheme string from the C string STR.  The
     length in bytes of the C string is input as LEN.  The encoding of
     the C string is passed as the ASCII, null-terminated C string
     `encoding'.  The HANDLER parameters suggests a strategy for
     dealing with unconvertable characters.

   The following conversion functions are provided as a convenience for
the most commonly used encodings.

 -- C Function: SCM scm_from_latin1_string (const char *str)
 -- C Function: SCM scm_from_utf8_string (const char *str)
 -- C Function: SCM scm_from_utf32_string (const scm_t_wchar *str)
     Return a scheme string from the null-terminated C string STR,
     which is ISO-8859-1-, UTF-8-, or UTF-32-encoded.  These functions
     should be used to convert hard-coded C string constants into
     Scheme strings.

 -- C Function: SCM scm_from_latin1_stringn (const char *str, size_t
          len)
 -- C Function: SCM scm_from_utf8_stringn (const char *str, size_t len)
 -- C Function: SCM scm_from_utf32_stringn (const scm_t_wchar *str,
          size_t len)
     Return a scheme string from C string STR, which is ISO-8859-1-,
     UTF-8-, or UTF-32-encoded, of length LEN.  LEN is the number of
     bytes pointed to by STR for `scm_from_latin1_stringn' and
     `scm_from_utf8_stringn'; it is the number of elements (code points)
     in STR in the case of `scm_from_utf32_stringn'.

 -- C function: char *scm_to_latin1_stringn (SCM str, size_t *lenp)
 -- C function: char *scm_to_utf8_stringn (SCM str, size_t *lenp)
 -- C function: scm_t_wchar *scm_to_utf32_stringn (SCM str, size_t
          *lenp)
     Return a newly allocated, ISO-8859-1-, UTF-8-, or UTF-32-encoded C
     string from Scheme string STR.  An error is thrown when STR cannot
     be converted to the specified encoding.  If LENP is `NULL', the
     returned C string will be null terminated, and an error will be
     thrown if the C string would otherwise contain null characters.
     If LENP is not `NULL', the string is not null terminated, and the
     length of the returned string is returned in LENP.  The length
     returned is the number of bytes for `scm_to_latin1_stringn' and
     `scm_to_utf8_stringn'; it is the number of elements (code points)
     for `scm_to_utf32_stringn'.

6.6.5.14 String Internals
.........................

Guile stores each string in memory as a contiguous array of Unicode code
points along with an associated set of attributes.  If all of the code
points of a string have an integer range between 0 and 255 inclusive,
the code point array is stored as one byte per code point: it is stored
as an ISO-8859-1 (aka Latin-1) string.  If any of the code points of the
string has an integer value greater that 255, the code point array is
stored as four bytes per code point: it is stored as a UTF-32 string.

   Conversion between the one-byte-per-code-point and
four-bytes-per-code-point representations happens automatically as
necessary.

   No API is provided to set the internal representation of strings;
however, there are pair of procedures available to query it.  These are
debugging procedures.  Using them in production code is discouraged,
since the details of Guile's internal representation of strings may
change from release to release.

 -- Scheme Procedure: string-bytes-per-char str
 -- C Function: scm_string_bytes_per_char (str)
     Return the number of bytes used to encode a Unicode code point in
     string STR.  The result is one or four.

 -- Scheme Procedure: %string-dump str
 -- C Function: scm_sys_string_dump (str)
     Returns an association list containing debugging information for
     STR. The association list has the following entries.
    `string'
          The string itself.

    `start'
          The start index of the string into its stringbuf

    `length'
          The length of the string

    `shared'
          If this string is a substring, it returns its parent string.
          Otherwise, it returns `#f'

    `read-only'
          `#t' if the string is read-only

    `stringbuf-chars'
          A new string containing this string's stringbuf's characters

    `stringbuf-length'
          The number of characters in this stringbuf

    `stringbuf-shared'
          `#t' if this stringbuf is shared

    `stringbuf-wide'
          `#t' if this stringbuf's characters are stored in a 32-bit
          buffer, or `#f' if they are stored in an 8-bit buffer

6.6.6 Bytevectors
-----------------

A "bytevector" is a raw bit string.  The `(rnrs bytevectors)' module
provides the programming interface specified by the Revised^6 Report on
the Algorithmic Language Scheme (R6RS) (http://www.r6rs.org/).  It
contains procedures to manipulate bytevectors and interpret their
contents in a number of ways: bytevector contents can be accessed as
signed or unsigned integer of various sizes and endianness, as IEEE-754
floating point numbers, or as strings.  It is a useful tool to encode
and decode binary data.

   The R6RS (Section 4.3.4) specifies an external representation for
bytevectors, whereby the octets (integers in the range 0-255) contained
in the bytevector are represented as a list prefixed by `#vu8':

     #vu8(1 53 204)

   denotes a 3-byte bytevector containing the octets 1, 53, and 204.
Like string literals, booleans, etc., bytevectors are "self-quoting",
i.e., they do not need to be quoted:

     #vu8(1 53 204)
     => #vu8(1 53 204)

   Bytevectors can be used with the binary input/output primitives of
the R6RS (*note R6RS I/O Ports::).

6.6.6.1 Endianness
..................

Some of the following procedures take an ENDIANNESS parameter.  The
"endianness" is defined as the order of bytes in multi-byte numbers:
numbers encoded in "big endian" have their most significant bytes
written first, whereas numbers encoded in "little endian" have their
least significant bytes first(1).

   Little-endian is the native endianness of the IA32 architecture and
its derivatives, while big-endian is native to SPARC and PowerPC, among
others. The `native-endianness' procedure returns the native endianness
of the machine it runs on.

 -- Scheme Procedure: native-endianness
 -- C Function: scm_native_endianness ()
     Return a value denoting the native endianness of the host machine.

 -- Scheme Macro: endianness symbol
     Return an object denoting the endianness specified by SYMBOL.  If
     SYMBOL is neither `big' nor `little' then an error is raised at
     expand-time.

 -- C Variable: scm_endianness_big
 -- C Variable: scm_endianness_little
     The objects denoting big- and little-endianness, respectively.

   ---------- Footnotes ----------

   (1) Big-endian and little-endian are the most common "endiannesses",
but others do exist. For instance, the GNU MP library allows "word
order" to be specified independently of "byte order" (*note Integer
Import and Export: (gmp)Integer Import and Export.).

6.6.6.2 Manipulating Bytevectors
................................

Bytevectors can be created, copied, and analyzed with the following
procedures and C functions.

 -- Scheme Procedure: make-bytevector len [fill]
 -- C Function: scm_make_bytevector (len, fill)
 -- C Function: scm_c_make_bytevector (size_t len)
     Return a new bytevector of LEN bytes.  Optionally, if FILL is
     given, fill it with FILL; FILL must be in the range [-128,255].

 -- Scheme Procedure: bytevector? obj
 -- C Function: scm_bytevector_p (obj)
     Return true if OBJ is a bytevector.

 -- C Function: int scm_is_bytevector (SCM obj)
     Equivalent to `scm_is_true (scm_bytevector_p (obj))'.

 -- Scheme Procedure: bytevector-length bv
 -- C Function: scm_bytevector_length (bv)
     Return the length in bytes of bytevector BV.

 -- C Function: size_t scm_c_bytevector_length (SCM bv)
     Likewise, return the length in bytes of bytevector BV.

 -- Scheme Procedure: bytevector=? bv1 bv2
 -- C Function: scm_bytevector_eq_p (bv1, bv2)
     Return is BV1 equals to BV2--i.e., if they have the same length
     and contents.

 -- Scheme Procedure: bytevector-fill! bv fill
 -- C Function: scm_bytevector_fill_x (bv, fill)
     Fill bytevector BV with FILL, a byte.

 -- Scheme Procedure: bytevector-copy! source source-start target
          target-start len
 -- C Function: scm_bytevector_copy_x (source, source_start, target,
          target_start, len)
     Copy LEN bytes from SOURCE into TARGET, starting reading from
     SOURCE-START (a positive index within SOURCE) and start writing at
     TARGET-START.  It is permitted for the SOURCE and TARGET regions
     to overlap.

 -- Scheme Procedure: bytevector-copy bv
 -- C Function: scm_bytevector_copy (bv)
     Return a newly allocated copy of BV.

 -- C Function: scm_t_uint8 scm_c_bytevector_ref (SCM bv, size_t index)
     Return the byte at INDEX in bytevector BV.

 -- C Function: void scm_c_bytevector_set_x (SCM bv, size_t index,
          scm_t_uint8 value)
     Set the byte at INDEX in BV to VALUE.

   Low-level C macros are available.  They do not perform any
type-checking; as such they should be used with care.

 -- C Macro: size_t SCM_BYTEVECTOR_LENGTH (bv)
     Return the length in bytes of bytevector BV.

 -- C Macro: signed char * SCM_BYTEVECTOR_CONTENTS (bv)
     Return a pointer to the contents of bytevector BV.

6.6.6.3 Interpreting Bytevector Contents as Integers
....................................................

The contents of a bytevector can be interpreted as a sequence of
integers of any given size, sign, and endianness.

     (let ((bv (make-bytevector 4)))
       (bytevector-u8-set! bv 0 #x12)
       (bytevector-u8-set! bv 1 #x34)
       (bytevector-u8-set! bv 2 #x56)
       (bytevector-u8-set! bv 3 #x78)

       (map (lambda (number)
              (number->string number 16))
            (list (bytevector-u8-ref bv 0)
                  (bytevector-u16-ref bv 0 (endianness big))
                  (bytevector-u32-ref bv 0 (endianness little)))))

     => ("12" "1234" "78563412")

   The most generic procedures to interpret bytevector contents as
integers are described below.

 -- Scheme Procedure: bytevector-uint-ref bv index endianness size
 -- C Function: scm_bytevector_uint_ref (bv, index, endianness, size)
     Return the SIZE-byte long unsigned integer at index INDEX in BV,
     decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector-sint-ref bv index endianness size
 -- C Function: scm_bytevector_sint_ref (bv, index, endianness, size)
     Return the SIZE-byte long signed integer at index INDEX in BV,
     decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector-uint-set! bv index value endianness
          size
 -- C Function: scm_bytevector_uint_set_x (bv, index, value,
          endianness, size)
     Set the SIZE-byte long unsigned integer at INDEX to VALUE, encoded
     according to ENDIANNESS.

 -- Scheme Procedure: bytevector-sint-set! bv index value endianness
          size
 -- C Function: scm_bytevector_sint_set_x (bv, index, value,
          endianness, size)
     Set the SIZE-byte long signed integer at INDEX to VALUE, encoded
     according to ENDIANNESS.

   The following procedures are similar to the ones above, but
specialized to a given integer size:

 -- Scheme Procedure: bytevector-u8-ref bv index
 -- Scheme Procedure: bytevector-s8-ref bv index
 -- Scheme Procedure: bytevector-u16-ref bv index endianness
 -- Scheme Procedure: bytevector-s16-ref bv index endianness
 -- Scheme Procedure: bytevector-u32-ref bv index endianness
 -- Scheme Procedure: bytevector-s32-ref bv index endianness
 -- Scheme Procedure: bytevector-u64-ref bv index endianness
 -- Scheme Procedure: bytevector-s64-ref bv index endianness
 -- C Function: scm_bytevector_u8_ref (bv, index)
 -- C Function: scm_bytevector_s8_ref (bv, index)
 -- C Function: scm_bytevector_u16_ref (bv, index, endianness)
 -- C Function: scm_bytevector_s16_ref (bv, index, endianness)
 -- C Function: scm_bytevector_u32_ref (bv, index, endianness)
 -- C Function: scm_bytevector_s32_ref (bv, index, endianness)
 -- C Function: scm_bytevector_u64_ref (bv, index, endianness)
 -- C Function: scm_bytevector_s64_ref (bv, index, endianness)
     Return the unsigned N-bit (signed) integer (where N is 8, 16, 32
     or 64) from BV at INDEX, decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector-u8-set! bv index value
 -- Scheme Procedure: bytevector-s8-set! bv index value
 -- Scheme Procedure: bytevector-u16-set! bv index value endianness
 -- Scheme Procedure: bytevector-s16-set! bv index value endianness
 -- Scheme Procedure: bytevector-u32-set! bv index value endianness
 -- Scheme Procedure: bytevector-s32-set! bv index value endianness
 -- Scheme Procedure: bytevector-u64-set! bv index value endianness
 -- Scheme Procedure: bytevector-s64-set! bv index value endianness
 -- C Function: scm_bytevector_u8_set_x (bv, index, value)
 -- C Function: scm_bytevector_s8_set_x (bv, index, value)
 -- C Function: scm_bytevector_u16_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_s16_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_u32_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_s32_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_u64_set_x (bv, index, value, endianness)
 -- C Function: scm_bytevector_s64_set_x (bv, index, value, endianness)
     Store VALUE as an N-bit (signed) integer (where N is 8, 16, 32 or
     64) in BV at INDEX, encoded according to ENDIANNESS.

   Finally, a variant specialized for the host's endianness is available
for each of these functions (with the exception of the `u8' accessors,
for obvious reasons):

 -- Scheme Procedure: bytevector-u16-native-ref bv index
 -- Scheme Procedure: bytevector-s16-native-ref bv index
 -- Scheme Procedure: bytevector-u32-native-ref bv index
 -- Scheme Procedure: bytevector-s32-native-ref bv index
 -- Scheme Procedure: bytevector-u64-native-ref bv index
 -- Scheme Procedure: bytevector-s64-native-ref bv index
 -- C Function: scm_bytevector_u16_native_ref (bv, index)
 -- C Function: scm_bytevector_s16_native_ref (bv, index)
 -- C Function: scm_bytevector_u32_native_ref (bv, index)
 -- C Function: scm_bytevector_s32_native_ref (bv, index)
 -- C Function: scm_bytevector_u64_native_ref (bv, index)
 -- C Function: scm_bytevector_s64_native_ref (bv, index)
     Return the unsigned N-bit (signed) integer (where N is 8, 16, 32
     or 64) from BV at INDEX, decoded according to the host's native
     endianness.

 -- Scheme Procedure: bytevector-u16-native-set! bv index value
 -- Scheme Procedure: bytevector-s16-native-set! bv index value
 -- Scheme Procedure: bytevector-u32-native-set! bv index value
 -- Scheme Procedure: bytevector-s32-native-set! bv index value
 -- Scheme Procedure: bytevector-u64-native-set! bv index value
 -- Scheme Procedure: bytevector-s64-native-set! bv index value
 -- C Function: scm_bytevector_u16_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_s16_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_u32_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_s32_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_u64_native_set_x (bv, index, value)
 -- C Function: scm_bytevector_s64_native_set_x (bv, index, value)
     Store VALUE as an N-bit (signed) integer (where N is 8, 16, 32 or
     64) in BV at INDEX, encoded according to the host's native
     endianness.

6.6.6.4 Converting Bytevectors to/from Integer Lists
....................................................

Bytevector contents can readily be converted to/from lists of signed or
unsigned integers:

     (bytevector->sint-list (u8-list->bytevector (make-list 4 255))
                            (endianness little) 2)
     => (-1 -1)

 -- Scheme Procedure: bytevector->u8-list bv
 -- C Function: scm_bytevector_to_u8_list (bv)
     Return a newly allocated list of unsigned 8-bit integers from the
     contents of BV.

 -- Scheme Procedure: u8-list->bytevector lst
 -- C Function: scm_u8_list_to_bytevector (lst)
     Return a newly allocated bytevector consisting of the unsigned
     8-bit integers listed in LST.

 -- Scheme Procedure: bytevector->uint-list bv endianness size
 -- C Function: scm_bytevector_to_uint_list (bv, endianness, size)
     Return a list of unsigned integers of SIZE bytes representing the
     contents of BV, decoded according to ENDIANNESS.

 -- Scheme Procedure: bytevector->sint-list bv endianness size
 -- C Function: scm_bytevector_to_sint_list (bv, endianness, size)
     Return a list of signed integers of SIZE bytes representing the
     contents of BV, decoded according to ENDIANNESS.

 -- Scheme Procedure: uint-list->bytevector lst endianness size
 -- C Function: scm_uint_list_to_bytevector (lst, endianness, size)
     Return a new bytevector containing the unsigned integers listed in
     LST and encoded on SIZE bytes according to ENDIANNESS.

 -- Scheme Procedure: sint-list->bytevector lst endianness size
 -- C Function: scm_sint_list_to_bytevector (lst, endianness, size)
     Return a new bytevector containing the signed integers listed in
     LST and encoded on SIZE bytes according to ENDIANNESS.

6.6.6.5 Interpreting Bytevector Contents as Floating Point Numbers
..................................................................

Bytevector contents can also be accessed as IEEE-754 single- or
double-precision floating point numbers (respectively 32 and 64-bit
long) using the procedures described here.

 -- Scheme Procedure: bytevector-ieee-single-ref bv index endianness
 -- Scheme Procedure: bytevector-ieee-double-ref bv index endianness
 -- C Function: scm_bytevector_ieee_single_ref (bv, index, endianness)
 -- C Function: scm_bytevector_ieee_double_ref (bv, index, endianness)
     Return the IEEE-754 single-precision floating point number from BV
     at INDEX according to ENDIANNESS.

 -- Scheme Procedure: bytevector-ieee-single-set! bv index value
          endianness
 -- Scheme Procedure: bytevector-ieee-double-set! bv index value
          endianness
 -- C Function: scm_bytevector_ieee_single_set_x (bv, index, value,
          endianness)
 -- C Function: scm_bytevector_ieee_double_set_x (bv, index, value,
          endianness)
     Store real number VALUE in BV at INDEX according to ENDIANNESS.

   Specialized procedures are also available:

 -- Scheme Procedure: bytevector-ieee-single-native-ref bv index
 -- Scheme Procedure: bytevector-ieee-double-native-ref bv index
 -- C Function: scm_bytevector_ieee_single_native_ref (bv, index)
 -- C Function: scm_bytevector_ieee_double_native_ref (bv, index)
     Return the IEEE-754 single-precision floating point number from BV
     at INDEX according to the host's native endianness.

 -- Scheme Procedure: bytevector-ieee-single-native-set! bv index value
 -- Scheme Procedure: bytevector-ieee-double-native-set! bv index value
 -- C Function: scm_bytevector_ieee_single_native_set_x (bv, index,
          value)
 -- C Function: scm_bytevector_ieee_double_native_set_x (bv, index,
          value)
     Store real number VALUE in BV at INDEX according to the host's
     native endianness.

6.6.6.6 Interpreting Bytevector Contents as Unicode Strings
...........................................................

Bytevector contents can also be interpreted as Unicode strings encoded
in one of the most commonly available encoding formats.

     (utf8->string (u8-list->bytevector '(99 97 102 101)))
     => "cafe"

     (string->utf8 "café") ;; SMALL LATIN LETTER E WITH ACUTE ACCENT
     => #vu8(99 97 102 195 169)

 -- Scheme Procedure: string->utf8 str
 -- Scheme Procedure: string->utf16 str [endianness]
 -- Scheme Procedure: string->utf32 str [endianness]
 -- C Function: scm_string_to_utf8 (str)
 -- C Function: scm_string_to_utf16 (str, endianness)
 -- C Function: scm_string_to_utf32 (str, endianness)
     Return a newly allocated bytevector that contains the UTF-8,
     UTF-16, or UTF-32 (aka. UCS-4) encoding of STR.  For UTF-16 and
     UTF-32, ENDIANNESS should be the symbol `big' or `little'; when
     omitted, it defaults to big endian.

 -- Scheme Procedure: utf8->string utf
 -- Scheme Procedure: utf16->string utf [endianness]
 -- Scheme Procedure: utf32->string utf [endianness]
 -- C Function: scm_utf8_to_string (utf)
 -- C Function: scm_utf16_to_string (utf, endianness)
 -- C Function: scm_utf32_to_string (utf, endianness)
     Return a newly allocated string that contains from the UTF-8-,
     UTF-16-, or UTF-32-decoded contents of bytevector UTF.  For UTF-16
     and UTF-32, ENDIANNESS should be the symbol `big' or `little';
     when omitted, it defaults to big endian.

6.6.6.7 Accessing Bytevectors with the Generalized Vector API
.............................................................

As an extension to the R6RS, Guile allows bytevectors to be manipulated
with the "generalized vector" procedures (*note Generalized Vectors::).
This also allows bytevectors to be accessed using the generic "array"
procedures (*note Array Procedures::).  When using these APIs, bytes
are accessed one at a time as 8-bit unsigned integers:

     (define bv #vu8(0 1 2 3))

     (generalized-vector? bv)
     => #t

     (generalized-vector-ref bv 2)
     => 2

     (generalized-vector-set! bv 2 77)
     (array-ref bv 2)
     => 77

     (array-type bv)
     => vu8

6.6.6.8 Accessing Bytevectors with the SRFI-4 API
.................................................

Bytevectors may also be accessed with the SRFI-4 API. *Note SRFI-4 and
Bytevectors::, for more information.

6.6.7 Symbols
-------------

Symbols in Scheme are widely used in three ways: as items of discrete
data, as lookup keys for alists and hash tables, and to denote variable
references.

   A "symbol" is similar to a string in that it is defined by a
sequence of characters.  The sequence of characters is known as the
symbol's "name".  In the usual case -- that is, where the symbol's name
doesn't include any characters that could be confused with other
elements of Scheme syntax -- a symbol is written in a Scheme program by
writing the sequence of characters that make up the name, _without_ any
quotation marks or other special syntax.  For example, the symbol whose
name is "multiply-by-2" is written, simply:

     multiply-by-2

   Notice how this differs from a _string_ with contents
"multiply-by-2", which is written with double quotation marks, like
this:

     "multiply-by-2"

   Looking beyond how they are written, symbols are different from
strings in two important respects.

   The first important difference is uniqueness.  If the same-looking
string is read twice from two different places in a program, the result
is two _different_ string objects whose contents just happen to be the
same.  If, on the other hand, the same-looking symbol is read twice
from two different places in a program, the result is the _same_ symbol
object both times.

   Given two read symbols, you can use `eq?' to test whether they are
the same (that is, have the same name).  `eq?' is the most efficient
comparison operator in Scheme, and comparing two symbols like this is
as fast as comparing, for example, two numbers.  Given two strings, on
the other hand, you must use `equal?' or `string=?', which are much
slower comparison operators, to determine whether the strings have the
same contents.

     (define sym1 (quote hello))
     (define sym2 (quote hello))
     (eq? sym1 sym2) => #t

     (define str1 "hello")
     (define str2 "hello")
     (eq? str1 str2) => #f
     (equal? str1 str2) => #t

   The second important difference is that symbols, unlike strings, are
not self-evaluating.  This is why we need the `(quote ...)'s in the
example above: `(quote hello)' evaluates to the symbol named "hello"
itself, whereas an unquoted `hello' is _read_ as the symbol named
"hello" and evaluated as a variable reference ... about which more
below (*note Symbol Variables::).

6.6.7.1 Symbols as Discrete Data
................................

Numbers and symbols are similar to the extent that they both lend
themselves to `eq?' comparison.  But symbols are more descriptive than
numbers, because a symbol's name can be used directly to describe the
concept for which that symbol stands.

   For example, imagine that you need to represent some colours in a
computer program.  Using numbers, you would have to choose arbitrarily
some mapping between numbers and colours, and then take care to use that
mapping consistently:

     ;; 1=red, 2=green, 3=purple

     (if (eq? (colour-of car) 1)
         ...)

You can make the mapping more explicit and the code more readable by
defining constants:

     (define red 1)
     (define green 2)
     (define purple 3)

     (if (eq? (colour-of car) red)
         ...)

But the simplest and clearest approach is not to use numbers at all, but
symbols whose names specify the colours that they refer to:

     (if (eq? (colour-of car) 'red)
         ...)

   The descriptive advantages of symbols over numbers increase as the
set of concepts that you want to describe grows.  Suppose that a car
object can have other properties as well, such as whether it has or
uses:

   * automatic or manual transmission

   * leaded or unleaded fuel

   * power steering (or not).

Then a car's combined property set could be naturally represented and
manipulated as a list of symbols:

     (properties-of car1)
     =>
     (red manual unleaded power-steering)

     (if (memq 'power-steering (properties-of car1))
         (display "Unfit people can drive this car.\n")
         (display "You'll need strong arms to drive this car!\n"))
     -|
     Unfit people can drive this car.

   Remember, the fundamental property of symbols that we are relying on
here is that an occurrence of `'red' in one part of a program is an
_indistinguishable_ symbol from an occurrence of `'red' in another part
of a program; this means that symbols can usefully be compared using
`eq?'.  At the same time, symbols have naturally descriptive names.
This combination of efficiency and descriptive power makes them ideal
for use as discrete data.

6.6.7.2 Symbols as Lookup Keys
..............................

Given their efficiency and descriptive power, it is natural to use
symbols as the keys in an association list or hash table.

   To illustrate this, consider a more structured representation of the
car properties example from the preceding subsection.  Rather than
mixing all the properties up together in a flat list, we could use an
association list like this:

     (define car1-properties '((colour . red)
                               (transmission . manual)
                               (fuel . unleaded)
                               (steering . power-assisted)))

   Notice how this structure is more explicit and extensible than the
flat list.  For example it makes clear that `manual' refers to the
transmission rather than, say, the windows or the locking of the car.
It also allows further properties to use the same symbols among their
possible values without becoming ambiguous:

     (define car1-properties '((colour . red)
                               (transmission . manual)
                               (fuel . unleaded)
                               (steering . power-assisted)
                               (seat-colour . red)
                               (locking . manual)))

   With a representation like this, it is easy to use the efficient
`assq-XXX' family of procedures (*note Association Lists::) to extract
or change individual pieces of information:

     (assq-ref car1-properties 'fuel) => unleaded
     (assq-ref car1-properties 'transmission) => manual

     (assq-set! car1-properties 'seat-colour 'black)
     =>
     ((colour . red)
      (transmission . manual)
      (fuel . unleaded)
      (steering . power-assisted)
      (seat-colour . black)
      (locking . manual)))

   Hash tables also have keys, and exactly the same arguments apply to
the use of symbols in hash tables as in association lists.  The hash
value that Guile uses to decide where to add a symbol-keyed entry to a
hash table can be obtained by calling the `symbol-hash' procedure:

 -- Scheme Procedure: symbol-hash symbol
 -- C Function: scm_symbol_hash (symbol)
     Return a hash value for SYMBOL.

   See *note Hash Tables:: for information about hash tables in
general, and for why you might choose to use a hash table rather than
an association list.

6.6.7.3 Symbols as Denoting Variables
.....................................

When an unquoted symbol in a Scheme program is evaluated, it is
interpreted as a variable reference, and the result of the evaluation is
the appropriate variable's value.

   For example, when the expression `(string-length "abcd")' is read
and evaluated, the sequence of characters `string-length' is read as
the symbol whose name is "string-length".  This symbol is associated
with a variable whose value is the procedure that implements string
length calculation.  Therefore evaluation of the `string-length' symbol
results in that procedure.

   The details of the connection between an unquoted symbol and the
variable to which it refers are explained elsewhere.  See *note Binding
Constructs::, for how associations between symbols and variables are
created, and *note Modules::, for how those associations are affected by
Guile's module system.

6.6.7.4 Operations Related to Symbols
.....................................

Given any Scheme value, you can determine whether it is a symbol using
the `symbol?' primitive:

 -- Scheme Procedure: symbol? obj
 -- C Function: scm_symbol_p (obj)
     Return `#t' if OBJ is a symbol, otherwise return `#f'.

 -- C Function: int scm_is_symbol (SCM val)
     Equivalent to `scm_is_true (scm_symbol_p (val))'.

   Once you know that you have a symbol, you can obtain its name as a
string by calling `symbol->string'.  Note that Guile differs by default
from R5RS on the details of `symbol->string' as regards
case-sensitivity:

 -- Scheme Procedure: symbol->string s
 -- C Function: scm_symbol_to_string (s)
     Return the name of symbol S as a string.  By default, Guile reads
     symbols case-sensitively, so the string returned will have the
     same case variation as the sequence of characters that caused S to
     be created.

     If Guile is set to read symbols case-insensitively (as specified by
     R5RS), and S comes into being as part of a literal expression
     (*note Literal expressions: (r5rs)Literal expressions.) or by a
     call to the `read' or `string-ci->symbol' procedures, Guile
     converts any alphabetic characters in the symbol's name to lower
     case before creating the symbol object, so the string returned
     here will be in lower case.

     If S was created by `string->symbol', the case of characters in
     the string returned will be the same as that in the string that was
     passed to `string->symbol', regardless of Guile's case-sensitivity
     setting at the time S was created.

     It is an error to apply mutation procedures like `string-set!' to
     strings returned by this procedure.

   Most symbols are created by writing them literally in code.  However
it is also possible to create symbols programmatically using the
following procedures:

 -- Scheme Procedure: symbol char...
     Return a newly allocated symbol made from the given character
     arguments.

          (symbol #\x #\y #\z) => xyz

 -- Scheme Procedure: list->symbol lst
     Return a newly allocated symbol made from a list of characters.

          (list->symbol '(#\a #\b #\c)) => abc

 -- Scheme Procedure: symbol-append . args
     Return a newly allocated symbol whose characters form the
     concatenation of the given symbols, ARGS.

          (let ((h 'hello))
            (symbol-append h 'world))
          => helloworld

 -- Scheme Procedure: string->symbol string
 -- C Function: scm_string_to_symbol (string)
     Return the symbol whose name is STRING.  This procedure can create
     symbols with names containing special characters or letters in the
     non-standard case, but it is usually a bad idea to create such
     symbols because in some implementations of Scheme they cannot be
     read as themselves.

 -- Scheme Procedure: string-ci->symbol str
 -- C Function: scm_string_ci_to_symbol (str)
     Return the symbol whose name is STR.  If Guile is currently
     reading symbols case-insensitively, STR is converted to lowercase
     before the returned symbol is looked up or created.

   The following examples illustrate Guile's detailed behaviour as
regards the case-sensitivity of symbols:

     (read-enable 'case-insensitive)   ; R5RS compliant behaviour

     (symbol->string 'flying-fish)    => "flying-fish"
     (symbol->string 'Martin)         => "martin"
     (symbol->string
        (string->symbol "Malvina"))   => "Malvina"

     (eq? 'mISSISSIppi 'mississippi)  => #t
     (string->symbol "mISSISSIppi")   => mISSISSIppi
     (eq? 'bitBlt (string->symbol "bitBlt")) => #f
     (eq? 'LolliPop
       (string->symbol (symbol->string 'LolliPop))) => #t
     (string=? "K. Harper, M.D."
       (symbol->string
         (string->symbol "K. Harper, M.D."))) => #t

     (read-disable 'case-insensitive)   ; Guile default behaviour

     (symbol->string 'flying-fish)    => "flying-fish"
     (symbol->string 'Martin)         => "Martin"
     (symbol->string
        (string->symbol "Malvina"))   => "Malvina"

     (eq? 'mISSISSIppi 'mississippi)  => #f
     (string->symbol "mISSISSIppi")   => mISSISSIppi
     (eq? 'bitBlt (string->symbol "bitBlt")) => #t
     (eq? 'LolliPop
       (string->symbol (symbol->string 'LolliPop))) => #t
     (string=? "K. Harper, M.D."
       (symbol->string
         (string->symbol "K. Harper, M.D."))) => #t

   From C, there are lower level functions that construct a Scheme
symbol from a C string in the current locale encoding.

   When you want to do more from C, you should convert between symbols
and strings using `scm_symbol_to_string' and `scm_string_to_symbol' and
work with the strings.

 -- C Function: scm_from_latin1_symbol (const char *name)
 -- C Function: scm_from_utf8_symbol (const char *name)
     Construct and return a Scheme symbol whose name is specified by the
     null-terminated C string NAME.  These are appropriate when the C
     string is hard-coded in the source code.

 -- C Function: scm_from_locale_symbol (const char *name)
 -- C Function: scm_from_locale_symboln (const char *name, size_t len)
     Construct and return a Scheme symbol whose name is specified by
     NAME.  For `scm_from_locale_symbol', NAME must be null terminated;
     for `scm_from_locale_symboln' the length of NAME is specified
     explicitly by LEN.

     Note that these functions should _not_ be used when NAME is a C
     string constant, because there is no guarantee that the current
     locale will match that of the source code.  In such cases, use
     `scm_from_latin1_symbol' or `scm_from_utf8_symbol'.

 -- C Function: SCM scm_take_locale_symbol (char *str)
 -- C Function: SCM scm_take_locale_symboln (char *str, size_t len)
     Like `scm_from_locale_symbol' and `scm_from_locale_symboln',
     respectively, but also frees STR with `free' eventually.  Thus,
     you can use this function when you would free STR anyway
     immediately after creating the Scheme string.  In certain cases,
     Guile can then use STR directly as its internal representation.

   The size of a symbol can also be obtained from C:

 -- C Function: size_t scm_c_symbol_length (SCM sym)
     Return the number of characters in SYM.

   Finally, some applications, especially those that generate new Scheme
code dynamically, need to generate symbols for use in the generated
code.  The `gensym' primitive meets this need:

 -- Scheme Procedure: gensym [prefix]
 -- C Function: scm_gensym (prefix)
     Create a new symbol with a name constructed from a prefix and a
     counter value.  The string PREFIX can be specified as an optional
     argument.  Default prefix is ` g'.  The counter is increased by 1
     at each call.  There is no provision for resetting the counter.

   The symbols generated by `gensym' are _likely_ to be unique, since
their names begin with a space and it is only otherwise possible to
generate such symbols if a programmer goes out of their way to do so.
Uniqueness can be guaranteed by instead using uninterned symbols (*note
Symbol Uninterned::), though they can't be usefully written out and
read back in.

6.6.7.5 Function Slots and Property Lists
.........................................

In traditional Lisp dialects, symbols are often understood as having
three kinds of value at once:

   * a "variable" value, which is used when the symbol appears in code
     in a variable reference context

   * a "function" value, which is used when the symbol appears in code
     in a function name position (i.e. as the first element in an
     unquoted list)

   * a "property list" value, which is used when the symbol is given as
     the first argument to Lisp's `put' or `get' functions.

   Although Scheme (as one of its simplifications with respect to Lisp)
does away with the distinction between variable and function namespaces,
Guile currently retains some elements of the traditional structure in
case they turn out to be useful when implementing translators for other
languages, in particular Emacs Lisp.

   Specifically, Guile symbols have two extra slots, one for a symbol's
property list, and one for its "function value."  The following
procedures are provided to access these slots.

 -- Scheme Procedure: symbol-fref symbol
 -- C Function: scm_symbol_fref (symbol)
     Return the contents of SYMBOL's "function slot".

 -- Scheme Procedure: symbol-fset! symbol value
 -- C Function: scm_symbol_fset_x (symbol, value)
     Set the contents of SYMBOL's function slot to VALUE.

 -- Scheme Procedure: symbol-pref symbol
 -- C Function: scm_symbol_pref (symbol)
     Return the "property list" currently associated with SYMBOL.

 -- Scheme Procedure: symbol-pset! symbol value
 -- C Function: scm_symbol_pset_x (symbol, value)
     Set SYMBOL's property list to VALUE.

 -- Scheme Procedure: symbol-property sym prop
     From SYM's property list, return the value for property PROP.  The
     assumption is that SYM's property list is an association list
     whose keys are distinguished from each other using `equal?'; PROP
     should be one of the keys in that list.  If the property list has
     no entry for PROP, `symbol-property' returns `#f'.

 -- Scheme Procedure: set-symbol-property! sym prop val
     In SYM's property list, set the value for property PROP to VAL, or
     add a new entry for PROP, with value VAL, if none already exists.
     For the structure of the property list, see `symbol-property'.

 -- Scheme Procedure: symbol-property-remove! sym prop
     From SYM's property list, remove the entry for property PROP, if
     there is one.  For the structure of the property list, see
     `symbol-property'.

   Support for these extra slots may be removed in a future release,
and it is probably better to avoid using them.  For a more modern and
Schemely approach to properties, see *note Object Properties::.

6.6.7.6 Extended Read Syntax for Symbols
........................................

The read syntax for a symbol is a sequence of letters, digits, and
"extended alphabetic characters", beginning with a character that
cannot begin a number.  In addition, the special cases of `+', `-', and
`...' are read as symbols even though numbers can begin with `+', `-'
or `.'.

   Extended alphabetic characters may be used within identifiers as if
they were letters.  The set of extended alphabetic characters is:

     ! $ % & * + - . / : < = > ? @ ^ _ ~

   In addition to the standard read syntax defined above (which is taken
from R5RS (*note Formal syntax: (r5rs)Formal syntax.)), Guile provides
an extended symbol read syntax that allows the inclusion of unusual
characters such as space characters, newlines and parentheses.  If (for
whatever reason) you need to write a symbol containing characters not
mentioned above, you can do so as follows.

   * Begin the symbol with the characters `#{',

   * write the characters of the symbol and

   * finish the symbol with the characters `}#'.

   Here are a few examples of this form of read syntax.  The first
symbol needs to use extended syntax because it contains a space
character, the second because it contains a line break, and the last
because it looks like a number.

     #{foo bar}#

     #{what
     ever}#

     #{4242}#

   Although Guile provides this extended read syntax for symbols,
widespread usage of it is discouraged because it is not portable and not
very readable.

6.6.7.7 Uninterned Symbols
..........................

What makes symbols useful is that they are automatically kept unique.
There are no two symbols that are distinct objects but have the same
name.  But of course, there is no rule without exception.  In addition
to the normal symbols that have been discussed up to now, you can also
create special "uninterned" symbols that behave slightly differently.

   To understand what is different about them and why they might be
useful, we look at how normal symbols are actually kept unique.

   Whenever Guile wants to find the symbol with a specific name, for
example during `read' or when executing `string->symbol', it first
looks into a table of all existing symbols to find out whether a symbol
with the given name already exists.  When this is the case, Guile just
returns that symbol.  When not, a new symbol with the name is created
and entered into the table so that it can be found later.

   Sometimes you might want to create a symbol that is guaranteed
`fresh', i.e. a symbol that did not exist previously.  You might also
want to somehow guarantee that no one else will ever unintentionally
stumble across your symbol in the future.  These properties of a symbol
are often needed when generating code during macro expansion.  When
introducing new temporary variables, you want to guarantee that they
don't conflict with variables in other people's code.

   The simplest way to arrange for this is to create a new symbol but
not enter it into the global table of all symbols.  That way, no one
will ever get access to your symbol by chance.  Symbols that are not in
the table are called "uninterned".  Of course, symbols that _are_ in
the table are called "interned".

   You create new uninterned symbols with the function `make-symbol'.
You can test whether a symbol is interned or not with
`symbol-interned?'.

   Uninterned symbols break the rule that the name of a symbol uniquely
identifies the symbol object.  Because of this, they can not be written
out and read back in like interned symbols.  Currently, Guile has no
support for reading uninterned symbols.  Note that the function
`gensym' does not return uninterned symbols for this reason.

 -- Scheme Procedure: make-symbol name
 -- C Function: scm_make_symbol (name)
     Return a new uninterned symbol with the name NAME.  The returned
     symbol is guaranteed to be unique and future calls to
     `string->symbol' will not return it.

 -- Scheme Procedure: symbol-interned? symbol
 -- C Function: scm_symbol_interned_p (symbol)
     Return `#t' if SYMBOL is interned, otherwise return `#f'.

   For example:

     (define foo-1 (string->symbol "foo"))
     (define foo-2 (string->symbol "foo"))
     (define foo-3 (make-symbol "foo"))
     (define foo-4 (make-symbol "foo"))

     (eq? foo-1 foo-2)
     => #t
     ; Two interned symbols with the same name are the same object,

     (eq? foo-1 foo-3)
     => #f
     ; but a call to make-symbol with the same name returns a
     ; distinct object.

     (eq? foo-3 foo-4)
     => #f
     ; A call to make-symbol always returns a new object, even for
     ; the same name.

     foo-3
     => #<uninterned-symbol foo 8085290>
     ; Uninterned symbols print differently from interned symbols,

     (symbol? foo-3)
     => #t
     ; but they are still symbols,

     (symbol-interned? foo-3)
     => #f
     ; just not interned.

6.6.8 Keywords
--------------

Keywords are self-evaluating objects with a convenient read syntax that
makes them easy to type.

   Guile's keyword support conforms to R5RS, and adds a (switchable)
read syntax extension to permit keywords to begin with `:' as well as
`#:', or to end with `:'.

6.6.8.1 Why Use Keywords?
.........................

Keywords are useful in contexts where a program or procedure wants to be
able to accept a large number of optional arguments without making its
interface unmanageable.

   To illustrate this, consider a hypothetical `make-window' procedure,
which creates a new window on the screen for drawing into using some
graphical toolkit.  There are many parameters that the caller might
like to specify, but which could also be sensibly defaulted, for
example:

   * color depth - Default: the color depth for the screen

   * background color - Default: white

   * width - Default: 600

   * height - Default: 400

   If `make-window' did not use keywords, the caller would have to pass
in a value for each possible argument, remembering the correct argument
order and using a special value to indicate the default value for that
argument:

     (make-window 'default              ;; Color depth
                  'default              ;; Background color
                  800                   ;; Width
                  100                   ;; Height
                  ...)                  ;; More make-window arguments

   With keywords, on the other hand, defaulted arguments are omitted,
and non-default arguments are clearly tagged by the appropriate
keyword.  As a result, the invocation becomes much clearer:

     (make-window #:width 800 #:height 100)

   On the other hand, for a simpler procedure with few arguments, the
use of keywords would be a hindrance rather than a help.  The primitive
procedure `cons', for example, would not be improved if it had to be
invoked as

     (cons #:car x #:cdr y)

   So the decision whether to use keywords or not is purely pragmatic:
use them if they will clarify the procedure invocation at point of call.

6.6.8.2 Coding With Keywords
............................

If a procedure wants to support keywords, it should take a rest argument
and then use whatever means is convenient to extract keywords and their
corresponding arguments from the contents of that rest argument.

   The following example illustrates the principle: the code for
`make-window' uses a helper procedure called `get-keyword-value' to
extract individual keyword arguments from the rest argument.

     (define (get-keyword-value args keyword default)
       (let ((kv (memq keyword args)))
         (if (and kv (>= (length kv) 2))
             (cadr kv)
             default)))

     (define (make-window . args)
       (let ((depth  (get-keyword-value args #:depth  screen-depth))
             (bg     (get-keyword-value args #:bg     "white"))
             (width  (get-keyword-value args #:width  800))
             (height (get-keyword-value args #:height 100))
             ...)
         ...))

   But you don't need to write `get-keyword-value'.  The `(ice-9
optargs)' module provides a set of powerful macros that you can use to
implement keyword-supporting procedures like this:

     (use-modules (ice-9 optargs))

     (define (make-window . args)
       (let-keywords args #f ((depth  screen-depth)
                              (bg     "white")
                              (width  800)
                              (height 100))
         ...))

Or, even more economically, like this:

     (use-modules (ice-9 optargs))

     (define* (make-window #:key (depth  screen-depth)
                                 (bg     "white")
                                 (width  800)
                                 (height 100))
       ...)

   For further details on `let-keywords', `define*' and other
facilities provided by the `(ice-9 optargs)' module, see *note Optional
Arguments::.

6.6.8.3 Keyword Read Syntax
...........................

Guile, by default, only recognizes a keyword syntax that is compatible
with R5RS.  A token of the form `#:NAME', where `NAME' has the same
syntax as a Scheme symbol (*note Symbol Read Syntax::), is the external
representation of the keyword named `NAME'.  Keyword objects print
using this syntax as well, so values containing keyword objects can be
read back into Guile.  When used in an expression, keywords are
self-quoting objects.

   If the `keyword' read option is set to `'prefix', Guile also
recognizes the alternative read syntax `:NAME'.  Otherwise, tokens of
the form `:NAME' are read as symbols, as required by R5RS.

   If the `keyword' read option is set to `'postfix', Guile recognizes
the SRFI-88 read syntax `NAME:' (*note SRFI-88::).  Otherwise, tokens
of this form are read as symbols.

   To enable and disable the alternative non-R5RS keyword syntax, you
use the `read-set!' procedure documented *note Scheme Read::.  Note that
the `prefix' and `postfix' syntax are mutually exclusive.

     (read-set! keywords 'prefix)

     #:type
     =>
     #:type

     :type
     =>
     #:type

     (read-set! keywords 'postfix)

     type:
     =>
     #:type

     :type
     =>
     :type

     (read-set! keywords #f)

     #:type
     =>
     #:type

     :type
     -|
     ERROR: In expression :type:
     ERROR: Unbound variable: :type
     ABORT: (unbound-variable)

6.6.8.4 Keyword Procedures
..........................

 -- Scheme Procedure: keyword? obj
 -- C Function: scm_keyword_p (obj)
     Return `#t' if the argument OBJ is a keyword, else `#f'.

 -- Scheme Procedure: keyword->symbol keyword
 -- C Function: scm_keyword_to_symbol (keyword)
     Return the symbol with the same name as KEYWORD.

 -- Scheme Procedure: symbol->keyword symbol
 -- C Function: scm_symbol_to_keyword (symbol)
     Return the keyword with the same name as SYMBOL.

 -- C Function: int scm_is_keyword (SCM obj)
     Equivalent to `scm_is_true (scm_keyword_p (OBJ))'.

 -- C Function: SCM scm_from_locale_keyword (const char *name)
 -- C Function: SCM scm_from_locale_keywordn (const char *name, size_t
          len)
     Equivalent to `scm_symbol_to_keyword (scm_from_locale_symbol
     (NAME))' and `scm_symbol_to_keyword (scm_from_locale_symboln
     (NAME, LEN))', respectively.

     Note that these functions should _not_ be used when NAME is a C
     string constant, because there is no guarantee that the current
     locale will match that of the source code.  In such cases, use
     `scm_from_latin1_keyword' or `scm_from_utf8_keyword'.

 -- C Function: SCM scm_from_latin1_keyword (const char *name)
 -- C Function: SCM scm_from_utf8_keyword (const char *name)
     Equivalent to `scm_symbol_to_keyword (scm_from_latin1_symbol
     (NAME))' and `scm_symbol_to_keyword (scm_from_utf8_symbol
     (NAME))', respectively.

6.6.9 "Functionality-Centric" Data Types
----------------------------------------

Procedures and macros are documented in their own sections: see *note
Procedures:: and *note Macros::.

   Variable objects are documented as part of the description of Guile's
module system: see *note Variables::.

   Asyncs, dynamic roots and fluids are described in the section on
scheduling: see *note Scheduling::.

   Hooks are documented in the section on general utility functions: see
*note Hooks::.

   Ports are described in the section on I/O: see *note Input and
Output::.

   Regular expressions are described in their own section: see *note
Regular Expressions::.

6.7 Compound Data Types
=======================

This chapter describes Guile's compound data types.  By "compound" we
mean that the primary purpose of these data types is to act as
containers for other kinds of data (including other compound objects).
For instance, a (non-uniform) vector with length 5 is a container that
can hold five arbitrary Scheme objects.

   The various kinds of container object differ from each other in how
their memory is allocated, how they are indexed, and how particular
values can be looked up within them.

6.7.1 Pairs
-----------

Pairs are used to combine two Scheme objects into one compound object.
Hence the name: A pair stores a pair of objects.

   The data type "pair" is extremely important in Scheme, just like in
any other Lisp dialect.  The reason is that pairs are not only used to
make two values available as one object, but that pairs are used for
constructing lists of values.  Because lists are so important in Scheme,
they are described in a section of their own (*note Lists::).

   Pairs can literally get entered in source code or at the REPL, in the
so-called "dotted list" syntax.  This syntax consists of an opening
parentheses, the first element of the pair, a dot, the second element
and a closing parentheses.  The following example shows how a pair
consisting of the two numbers 1 and 2, and a pair containing the symbols
`foo' and `bar' can be entered.  It is very important to write the
whitespace before and after the dot, because otherwise the Scheme
parser would not be able to figure out where to split the tokens.

     (1 . 2)
     (foo . bar)

   But beware, if you want to try out these examples, you have to
"quote" the expressions.  More information about quotation is available
in the section *note Expression Syntax::.  The correct way to try these
examples is as follows.

     '(1 . 2)
     =>
     (1 . 2)
     '(foo . bar)
     =>
     (foo . bar)

   A new pair is made by calling the procedure `cons' with two
arguments.  Then the argument values are stored into a newly allocated
pair, and the pair is returned.  The name `cons' stands for
"construct".  Use the procedure `pair?' to test whether a given Scheme
object is a pair or not.

 -- Scheme Procedure: cons x y
 -- C Function: scm_cons (x, y)
     Return a newly allocated pair whose car is X and whose cdr is Y.
     The pair is guaranteed to be different (in the sense of `eq?')
     from every previously existing object.

 -- Scheme Procedure: pair? x
 -- C Function: scm_pair_p (x)
     Return `#t' if X is a pair; otherwise return `#f'.

 -- C Function: int scm_is_pair (SCM x)
     Return 1 when X is a pair; otherwise return 0.

   The two parts of a pair are traditionally called "car" and "cdr".
They can be retrieved with procedures of the same name (`car' and
`cdr'), and can be modified with the procedures `set-car!' and
`set-cdr!'.  Since a very common operation in Scheme programs is to
access the car of a car of a pair, or the car of the cdr of a pair,
etc., the procedures called `caar', `cadr' and so on are also
predefined.

 -- Scheme Procedure: car pair
 -- Scheme Procedure: cdr pair
 -- C Function: scm_car (pair)
 -- C Function: scm_cdr (pair)
     Return the car or the cdr of PAIR, respectively.

 -- C Macro: SCM SCM_CAR (SCM pair)
 -- C Macro: SCM SCM_CDR (SCM pair)
     These two macros are the fastest way to access the car or cdr of a
     pair; they can be thought of as compiling into a single memory
     reference.

     These macros do no checking at all.  The argument PAIR must be a
     valid pair.

 -- Scheme Procedure: cddr pair
 -- Scheme Procedure: cdar pair
 -- Scheme Procedure: cadr pair
 -- Scheme Procedure: caar pair
 -- Scheme Procedure: cdddr pair
 -- Scheme Procedure: cddar pair
 -- Scheme Procedure: cdadr pair
 -- Scheme Procedure: cdaar pair
 -- Scheme Procedure: caddr pair
 -- Scheme Procedure: cadar pair
 -- Scheme Procedure: caadr pair
 -- Scheme Procedure: caaar pair
 -- Scheme Procedure: cddddr pair
 -- Scheme Procedure: cdddar pair
 -- Scheme Procedure: cddadr pair
 -- Scheme Procedure: cddaar pair
 -- Scheme Procedure: cdaddr pair
 -- Scheme Procedure: cdadar pair
 -- Scheme Procedure: cdaadr pair
 -- Scheme Procedure: cdaaar pair
 -- Scheme Procedure: cadddr pair
 -- Scheme Procedure: caddar pair
 -- Scheme Procedure: cadadr pair
 -- Scheme Procedure: cadaar pair
 -- Scheme Procedure: caaddr pair
 -- Scheme Procedure: caadar pair
 -- Scheme Procedure: caaadr pair
 -- Scheme Procedure: caaaar pair
 -- C Function: scm_cddr (pair)
 -- C Function: scm_cdar (pair)
 -- C Function: scm_cadr (pair)
 -- C Function: scm_caar (pair)
 -- C Function: scm_cdddr (pair)
 -- C Function: scm_cddar (pair)
 -- C Function: scm_cdadr (pair)
 -- C Function: scm_cdaar (pair)
 -- C Function: scm_caddr (pair)
 -- C Function: scm_cadar (pair)
 -- C Function: scm_caadr (pair)
 -- C Function: scm_caaar (pair)
 -- C Function: scm_cddddr (pair)
 -- C Function: scm_cdddar (pair)
 -- C Function: scm_cddadr (pair)
 -- C Function: scm_cddaar (pair)
 -- C Function: scm_cdaddr (pair)
 -- C Function: scm_cdadar (pair)
 -- C Function: scm_cdaadr (pair)
 -- C Function: scm_cdaaar (pair)
 -- C Function: scm_cadddr (pair)
 -- C Function: scm_caddar (pair)
 -- C Function: scm_cadadr (pair)
 -- C Function: scm_cadaar (pair)
 -- C Function: scm_caaddr (pair)
 -- C Function: scm_caadar (pair)
 -- C Function: scm_caaadr (pair)
 -- C Function: scm_caaaar (pair)
     These procedures are compositions of `car' and `cdr', where for
     example `caddr' could be defined by

          (define caddr (lambda (x) (car (cdr (cdr x)))))

     `cadr', `caddr' and `cadddr' pick out the second, third or fourth
     elements of a list, respectively.  SRFI-1 provides the same under
     the names `second', `third' and `fourth' (*note SRFI-1
     Selectors::).

 -- Scheme Procedure: set-car! pair value
 -- C Function: scm_set_car_x (pair, value)
     Stores VALUE in the car field of PAIR.  The value returned by
     `set-car!' is unspecified.

 -- Scheme Procedure: set-cdr! pair value
 -- C Function: scm_set_cdr_x (pair, value)
     Stores VALUE in the cdr field of PAIR.  The value returned by
     `set-cdr!' is unspecified.

6.7.2 Lists
-----------

A very important data type in Scheme--as well as in all other Lisp
dialects--is the data type "list".(1)

   This is the short definition of what a list is:

   * Either the empty list `()',

   * or a pair which has a list in its cdr.

   ---------- Footnotes ----------

   (1) Strictly speaking, Scheme does not have a real datatype "list".
Lists are made up of "chained pairs", and only exist by definition--a
list is a chain of pairs which looks like a list.

6.7.2.1 List Read Syntax
........................

The syntax for lists is an opening parentheses, then all the elements of
the list (separated by whitespace) and finally a closing
parentheses.(1).

     (1 2 3)            ; a list of the numbers 1, 2 and 3
     ("foo" bar 3.1415) ; a string, a symbol and a real number
     ()                 ; the empty list

   The last example needs a bit more explanation.  A list with no
elements, called the "empty list", is special in some ways.  It is used
for terminating lists by storing it into the cdr of the last pair that
makes up a list.  An example will clear that up:

     (car '(1))
     =>
     1
     (cdr '(1))
     =>
     ()

   This example also shows that lists have to be quoted when written
(*note Expression Syntax::), because they would otherwise be
mistakingly taken as procedure applications (*note Simple Invocation::).

   ---------- Footnotes ----------

   (1) Note that there is no separation character between the list
elements, like a comma or a semicolon.

6.7.2.2 List Predicates
.......................

Often it is useful to test whether a given Scheme object is a list or
not.  List-processing procedures could use this information to test
whether their input is valid, or they could do different things
depending on the datatype of their arguments.

 -- Scheme Procedure: list? x
 -- C Function: scm_list_p (x)
     Return `#t' iff X is a proper list, else `#f'.

   The predicate `null?' is often used in list-processing code to tell
whether a given list has run out of elements.  That is, a loop somehow
deals with the elements of a list until the list satisfies `null?'.
Then, the algorithm terminates.

 -- Scheme Procedure: null? x
 -- C Function: scm_null_p (x)
     Return `#t' iff X is the empty list, else `#f'.

 -- C Function: int scm_is_null (SCM x)
     Return 1 when X is the empty list; otherwise return 0.

6.7.2.3 List Constructors
.........................

This section describes the procedures for constructing new lists.
`list' simply returns a list where the elements are the arguments,
`cons*' is similar, but the last argument is stored in the cdr of the
last pair of the list.

 -- Scheme Procedure: list elem1 ... elemN
 -- C Function: scm_list_1 (elem1)
 -- C Function: scm_list_2 (elem1, elem2)
 -- C Function: scm_list_3 (elem1, elem2, elem3)
 -- C Function: scm_list_4 (elem1, elem2, elem3, elem4)
 -- C Function: scm_list_5 (elem1, elem2, elem3, elem4, elem5)
 -- C Function: scm_list_n (elem1, ..., elemN, SCM_UNDEFINED)
     Return a new list containing elements ELEM1 to ELEMN.

     `scm_list_n' takes a variable number of arguments, terminated by
     the special `SCM_UNDEFINED'.  That final `SCM_UNDEFINED' is not
     included in the list.  None of ELEM1 to ELEMN can themselves be
     `SCM_UNDEFINED', or `scm_list_n' will terminate at that point.

 -- Scheme Procedure: cons* arg1 arg2 ...
     Like `list', but the last arg provides the tail of the constructed
     list, returning `(cons ARG1 (cons ARG2 (cons ... ARGN)))'.
     Requires at least one argument.  If given one argument, that
     argument is returned as result.  This function is called `list*'
     in some other Schemes and in Common LISP.

 -- Scheme Procedure: list-copy lst
 -- C Function: scm_list_copy (lst)
     Return a (newly-created) copy of LST.

 -- Scheme Procedure: make-list n [init]
     Create a list containing of N elements, where each element is
     initialized to INIT.  INIT defaults to the empty list `()' if not
     given.

   Note that `list-copy' only makes a copy of the pairs which make up
the spine of the lists.  The list elements are not copied, which means
that modifying the elements of the new list also modifies the elements
of the old list.  On the other hand, applying procedures like
`set-cdr!' or `delv!' to the new list will not alter the old list.  If
you also need to copy the list elements (making a deep copy), use the
procedure `copy-tree' (*note Copying::).

6.7.2.4 List Selection
......................

These procedures are used to get some information about a list, or to
retrieve one or more elements of a list.

 -- Scheme Procedure: length lst
 -- C Function: scm_length (lst)
     Return the number of elements in list LST.

 -- Scheme Procedure: last-pair lst
 -- C Function: scm_last_pair (lst)
     Return the last pair in LST, signalling an error if LST is
     circular.

 -- Scheme Procedure: list-ref list k
 -- C Function: scm_list_ref (list, k)
     Return the Kth element from LIST.

 -- Scheme Procedure: list-tail lst k
 -- Scheme Procedure: list-cdr-ref lst k
 -- C Function: scm_list_tail (lst, k)
     Return the "tail" of LST beginning with its Kth element.  The
     first element of the list is considered to be element 0.

     `list-tail' and `list-cdr-ref' are identical.  It may help to
     think of `list-cdr-ref' as accessing the Kth cdr of the list, or
     returning the results of cdring K times down LST.

 -- Scheme Procedure: list-head lst k
 -- C Function: scm_list_head (lst, k)
     Copy the first K elements from LST into a new list, and return it.

6.7.2.5 Append and Reverse
..........................

`append' and `append!' are used to concatenate two or more lists in
order to form a new list.  `reverse' and `reverse!' return lists with
the same elements as their arguments, but in reverse order.  The
procedure variants with an `!' directly modify the pairs which form the
list, whereas the other procedures create new pairs.  This is why you
should be careful when using the side-effecting variants.

 -- Scheme Procedure: append lst1 ... lstN
 -- Scheme Procedure: append! lst1 ... lstN
 -- C Function: scm_append (lstlst)
 -- C Function: scm_append_x (lstlst)
     Return a list comprising all the elements of lists LST1 to LSTN.

          (append '(x) '(y))          =>  (x y)
          (append '(a) '(b c d))      =>  (a b c d)
          (append '(a (b)) '((c)))    =>  (a (b) (c))

     The last argument LSTN may actually be any object; an improper
     list results if the last argument is not a proper list.

          (append '(a b) '(c . d))    =>  (a b c . d)
          (append '() 'a)             =>  a

     `append' doesn't modify the given lists, but the return may share
     structure with the final LSTN.  `append!' modifies the given lists
     to form its return.

     For `scm_append' and `scm_append_x', LSTLST is a list of the list
     operands LST1 ... LSTN.  That LSTLST itself is not modified or
     used in the return.

 -- Scheme Procedure: reverse lst
 -- Scheme Procedure: reverse! lst [newtail]
 -- C Function: scm_reverse (lst)
 -- C Function: scm_reverse_x (lst, newtail)
     Return a list comprising the elements of LST, in reverse order.

     `reverse' constructs a new list, `reverse!' modifies LST in
     constructing its return.

     For `reverse!', the optional NEWTAIL is appended to the result.
     NEWTAIL isn't reversed, it simply becomes the list tail.  For
     `scm_reverse_x', the NEWTAIL parameter is mandatory, but can be
     `SCM_EOL' if no further tail is required.

6.7.2.6 List Modification
.........................

The following procedures modify an existing list, either by changing
elements of the list, or by changing the list structure itself.

 -- Scheme Procedure: list-set! list k val
 -- C Function: scm_list_set_x (list, k, val)
     Set the Kth element of LIST to VAL.

 -- Scheme Procedure: list-cdr-set! list k val
 -- C Function: scm_list_cdr_set_x (list, k, val)
     Set the Kth cdr of LIST to VAL.

 -- Scheme Procedure: delq item lst
 -- C Function: scm_delq (item, lst)
     Return a newly-created copy of LST with elements `eq?' to ITEM
     removed.  This procedure mirrors `memq': `delq' compares elements
     of LST against ITEM with `eq?'.

 -- Scheme Procedure: delv item lst
 -- C Function: scm_delv (item, lst)
     Return a newly-created copy of LST with elements `eqv?' to ITEM
     removed.  This procedure mirrors `memv': `delv' compares elements
     of LST against ITEM with `eqv?'.

 -- Scheme Procedure: delete item lst
 -- C Function: scm_delete (item, lst)
     Return a newly-created copy of LST with elements `equal?' to ITEM
     removed.  This procedure mirrors `member': `delete' compares
     elements of LST against ITEM with `equal?'.

     See also SRFI-1 which has an extended `delete' (*note SRFI-1
     Deleting::), and also an `lset-difference' which can delete
     multiple ITEMs in one call (*note SRFI-1 Set Operations::).

 -- Scheme Procedure: delq! item lst
 -- Scheme Procedure: delv! item lst
 -- Scheme Procedure: delete! item lst
 -- C Function: scm_delq_x (item, lst)
 -- C Function: scm_delv_x (item, lst)
 -- C Function: scm_delete_x (item, lst)
     These procedures are destructive versions of `delq', `delv' and
     `delete': they modify the pointers in the existing LST rather than
     creating a new list.  Caveat evaluator: Like other destructive
     list functions, these functions cannot modify the binding of LST,
     and so cannot be used to delete the first element of LST
     destructively.

 -- Scheme Procedure: delq1! item lst
 -- C Function: scm_delq1_x (item, lst)
     Like `delq!', but only deletes the first occurrence of ITEM from
     LST.  Tests for equality using `eq?'.  See also `delv1!' and
     `delete1!'.

 -- Scheme Procedure: delv1! item lst
 -- C Function: scm_delv1_x (item, lst)
     Like `delv!', but only deletes the first occurrence of ITEM from
     LST.  Tests for equality using `eqv?'.  See also `delq1!' and
     `delete1!'.

 -- Scheme Procedure: delete1! item lst
 -- C Function: scm_delete1_x (item, lst)
     Like `delete!', but only deletes the first occurrence of ITEM from
     LST.  Tests for equality using `equal?'.  See also `delq1!' and
     `delv1!'.

 -- Scheme Procedure: filter pred lst
 -- Scheme Procedure: filter! pred lst
     Return a list containing all elements from LST which satisfy the
     predicate PRED.  The elements in the result list have the same
     order as in LST.  The order in which PRED is applied to the list
     elements is not specified.

     `filter' does not change LST, but the result may share a tail with
     it.  `filter!' may modify LST to construct its return.

6.7.2.7 List Searching
......................

The following procedures search lists for particular elements.  They use
different comparison predicates for comparing list elements with the
object to be searched.  When they fail, they return `#f', otherwise
they return the sublist whose car is equal to the search object, where
equality depends on the equality predicate used.

 -- Scheme Procedure: memq x lst
 -- C Function: scm_memq (x, lst)
     Return the first sublist of LST whose car is `eq?' to X where the
     sublists of LST are the non-empty lists returned by `(list-tail
     LST K)' for K less than the length of LST.  If X does not occur in
     LST, then `#f' (not the empty list) is returned.

 -- Scheme Procedure: memv x lst
 -- C Function: scm_memv (x, lst)
     Return the first sublist of LST whose car is `eqv?' to X where the
     sublists of LST are the non-empty lists returned by `(list-tail
     LST K)' for K less than the length of LST.  If X does not occur in
     LST, then `#f' (not the empty list) is returned.

 -- Scheme Procedure: member x lst
 -- C Function: scm_member (x, lst)
     Return the first sublist of LST whose car is `equal?' to X where
     the sublists of LST are the non-empty lists returned by
     `(list-tail LST K)' for K less than the length of LST.  If X does
     not occur in LST, then `#f' (not the empty list) is returned.

     See also SRFI-1 which has an extended `member' function (*note
     SRFI-1 Searching::).

6.7.2.8 List Mapping
....................

List processing is very convenient in Scheme because the process of
iterating over the elements of a list can be highly abstracted.  The
procedures in this section are the most basic iterating procedures for
lists.  They take a procedure and one or more lists as arguments, and
apply the procedure to each element of the list.  They differ in their
return value.

 -- Scheme Procedure: map proc arg1 arg2 ...
 -- Scheme Procedure: map-in-order proc arg1 arg2 ...
 -- C Function: scm_map (proc, arg1, args)
     Apply PROC to each element of the list ARG1 (if only two arguments
     are given), or to the corresponding elements of the argument lists
     (if more than two arguments are given).  The result(s) of the
     procedure applications are saved and returned in a list.  For
     `map', the order of procedure applications is not specified,
     `map-in-order' applies the procedure from left to right to the list
     elements.

 -- Scheme Procedure: for-each proc arg1 arg2 ...
     Like `map', but the procedure is always applied from left to right,
     and the result(s) of the procedure applications are thrown away.
     The return value is not specified.

   See also SRFI-1 which extends these functions to take lists of
unequal lengths (*note SRFI-1 Fold and Map::).

6.7.3 Vectors
-------------

Vectors are sequences of Scheme objects.  Unlike lists, the length of a
vector, once the vector is created, cannot be changed.  The advantage of
vectors over lists is that the time required to access one element of a
vector given its "position" (synonymous with "index"), a zero-origin
number, is constant, whereas lists have an access time linear to the
position of the accessed element in the list.

   Vectors can contain any kind of Scheme object; it is even possible to
have different types of objects in the same vector.  For vectors
containing vectors, you may wish to use arrays, instead.  Note, too,
that vectors are the special case of one dimensional non-uniform arrays
and that most array procedures operate happily on vectors (*note
Arrays::).

6.7.3.1 Read Syntax for Vectors
...............................

Vectors can literally be entered in source code, just like strings,
characters or some of the other data types.  The read syntax for vectors
is as follows: A sharp sign (`#'), followed by an opening parentheses,
all elements of the vector in their respective read syntax, and finally
a closing parentheses.  The following are examples of the read syntax
for vectors; where the first vector only contains numbers and the
second three different object types: a string, a symbol and a number in
hexadecimal notation.

     #(1 2 3)
     #("Hello" foo #xdeadbeef)

   Like lists, vectors have to be quoted:

     '#(a b c) => #(a b c)

6.7.3.2 Dynamic Vector Creation and Validation
..............................................

Instead of creating a vector implicitly by using the read syntax just
described, you can create a vector dynamically by calling one of the
`vector' and `list->vector' primitives with the list of Scheme values
that you want to place into a vector.  The size of the vector thus
created is determined implicitly by the number of arguments given.

 -- Scheme Procedure: vector . l
 -- Scheme Procedure: list->vector l
 -- C Function: scm_vector (l)
     Return a newly allocated vector composed of the given arguments.
     Analogous to `list'.

          (vector 'a 'b 'c) => #(a b c)

   The inverse operation is `vector->list':

 -- Scheme Procedure: vector->list v
 -- C Function: scm_vector_to_list (v)
     Return a newly allocated list composed of the elements of V.

          (vector->list '#(dah dah didah)) =>  (dah dah didah)
          (list->vector '(dididit dah)) =>  #(dididit dah)

   To allocate a vector with an explicitly specified size, use
`make-vector'.  With this primitive you can also specify an initial
value for the vector elements (the same value for all elements, that
is):

 -- Scheme Procedure: make-vector len [fill]
 -- C Function: scm_make_vector (len, fill)
     Return a newly allocated vector of LEN elements.  If a second
     argument is given, then each position is initialized to FILL.
     Otherwise the initial contents of each position is unspecified.

 -- C Function: SCM scm_c_make_vector (size_t k, SCM fill)
     Like `scm_make_vector', but the length is given as a `size_t'.

   To check whether an arbitrary Scheme value _is_ a vector, use the
`vector?' primitive:

 -- Scheme Procedure: vector? obj
 -- C Function: scm_vector_p (obj)
     Return `#t' if OBJ is a vector, otherwise return `#f'.

 -- C Function: int scm_is_vector (SCM obj)
     Return non-zero when OBJ is a vector, otherwise return `zero'.

6.7.3.3 Accessing and Modifying Vector Contents
...............................................

`vector-length' and `vector-ref' return information about a given
vector, respectively its size and the elements that are contained in
the vector.

 -- Scheme Procedure: vector-length vector
 -- C Function: scm_vector_length vector
     Return the number of elements in VECTOR as an exact integer.

 -- C Function: size_t scm_c_vector_length (SCM v)
     Return the number of elements in VECTOR as a `size_t'.

 -- Scheme Procedure: vector-ref vector k
 -- C Function: scm_vector_ref vector k
     Return the contents of position K of VECTOR.  K must be a valid
     index of VECTOR.
          (vector-ref '#(1 1 2 3 5 8 13 21) 5) => 8
          (vector-ref '#(1 1 2 3 5 8 13 21)
              (let ((i (round (* 2 (acos -1)))))
                (if (inexact? i)
                  (inexact->exact i)
                     i))) => 13

 -- C Function: SCM scm_c_vector_ref (SCM v, size_t k)
     Return the contents of position K (a `size_t') of VECTOR.

   A vector created by one of the dynamic vector constructor procedures
(*note Vector Creation::) can be modified using the following
procedures.

   _NOTE:_ According to R5RS, it is an error to use any of these
procedures on a literally read vector, because such vectors should be
considered as constants.  Currently, however, Guile does not detect this
error.

 -- Scheme Procedure: vector-set! vector k obj
 -- C Function: scm_vector_set_x vector k obj
     Store OBJ in position K of VECTOR.  K must be a valid index of
     VECTOR.  The value returned by `vector-set!' is unspecified.
          (let ((vec (vector 0 '(2 2 2 2) "Anna")))
            (vector-set! vec 1 '("Sue" "Sue"))
            vec) =>  #(0 ("Sue" "Sue") "Anna")

 -- C Function: void scm_c_vector_set_x (SCM v, size_t k, SCM obj)
     Store OBJ in position K (a `size_t') of V.

 -- Scheme Procedure: vector-fill! v fill
 -- C Function: scm_vector_fill_x (v, fill)
     Store FILL in every position of VECTOR.  The value returned by
     `vector-fill!' is unspecified.

 -- Scheme Procedure: vector-copy vec
 -- C Function: scm_vector_copy (vec)
     Return a copy of VEC.

 -- Scheme Procedure: vector-move-left! vec1 start1 end1 vec2 start2
 -- C Function: scm_vector_move_left_x (vec1, start1, end1, vec2,
          start2)
     Copy elements from VEC1, positions START1 to END1, to VEC2
     starting at position START2.  START1 and START2 are inclusive
     indices; END1 is exclusive.

     `vector-move-left!' copies elements in leftmost order.  Therefore,
     in the case where VEC1 and VEC2 refer to the same vector,
     `vector-move-left!' is usually appropriate when START1 is greater
     than START2.

 -- Scheme Procedure: vector-move-right! vec1 start1 end1 vec2 start2
 -- C Function: scm_vector_move_right_x (vec1, start1, end1, vec2,
          start2)
     Copy elements from VEC1, positions START1 to END1, to VEC2
     starting at position START2.  START1 and START2 are inclusive
     indices; END1 is exclusive.

     `vector-move-right!' copies elements in rightmost order.
     Therefore, in the case where VEC1 and VEC2 refer to the same
     vector, `vector-move-right!' is usually appropriate when START1 is
     less than START2.

6.7.3.4 Vector Accessing from C
...............................

A vector can be read and modified from C with the functions
`scm_c_vector_ref' and `scm_c_vector_set_x', for example.  In addition
to these functions, there are two more ways to access vectors from C
that might be more efficient in certain situations: you can restrict
yourself to "simple vectors" and then use the very fast _simple vector
macros_; or you can use the very general framework for accessing all
kinds of arrays (*note Accessing Arrays from C::), which is more
verbose, but can deal efficiently with all kinds of vectors (and
arrays).  For vectors, you can use the `scm_vector_elements' and
`scm_vector_writable_elements' functions as shortcuts.

 -- C Function: int scm_is_simple_vector (SCM obj)
     Return non-zero if OBJ is a simple vector, else return zero.  A
     simple vector is a vector that can be used with the `SCM_SIMPLE_*'
     macros below.

     The following functions are guaranteed to return simple vectors:
     `scm_make_vector', `scm_c_make_vector', `scm_vector',
     `scm_list_to_vector'.

 -- C Macro: size_t SCM_SIMPLE_VECTOR_LENGTH (SCM vec)
     Evaluates to the length of the simple vector VEC.  No type
     checking is done.

 -- C Macro: SCM SCM_SIMPLE_VECTOR_REF (SCM vec, size_t idx)
     Evaluates to the element at position IDX in the simple vector VEC.
     No type or range checking is done.

 -- C Macro: void SCM_SIMPLE_VECTOR_SET (SCM vec, size_t idx, SCM val)
     Sets the element at position IDX in the simple vector VEC to VAL.
     No type or range checking is done.

 -- C Function: const SCM * scm_vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Acquire a handle for the vector VEC and return a pointer to the
     elements of it.  This pointer can only be used to read the
     elements of VEC.  When VEC is not a vector, an error is signaled.
     The handle must eventually be released with
     `scm_array_handle_release'.

     The variables pointed to by LENP and INCP are filled with the
     number of elements of the vector and the increment (number of
     elements) between successive elements, respectively.  Successive
     elements of VEC need not be contiguous in their underlying "root
     vector" returned here; hence the increment is not necessarily
     equal to 1 and may well be negative too (*note Shared Arrays::).

     The following example shows the typical way to use this function.
     It creates a list of all elements of VEC (in reverse order).

          scm_t_array_handle handle;
          size_t i, len;
          ssize_t inc;
          const SCM *elt;
          SCM list;

          elt = scm_vector_elements (vec, &handle, &len, &inc);
          list = SCM_EOL;
          for (i = 0; i < len; i++, elt += inc)
            list = scm_cons (*elt, list);
          scm_array_handle_release (&handle);


 -- C Function: SCM * scm_vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_elements' but the pointer can be used to modify
     the vector.

     The following example shows the typical way to use this function.
     It fills a vector with `#t'.

          scm_t_array_handle handle;
          size_t i, len;
          ssize_t inc;
          SCM *elt;

          elt = scm_vector_writable_elements (vec, &handle, &len, &inc);
          for (i = 0; i < len; i++, elt += inc)
            *elt = SCM_BOOL_T;
          scm_array_handle_release (&handle);


6.7.3.5 Uniform Numeric Vectors
...............................

A uniform numeric vector is a vector whose elements are all of a single
numeric type.  Guile offers uniform numeric vectors for signed and
unsigned 8-bit, 16-bit, 32-bit, and 64-bit integers, two sizes of
floating point values, and complex floating-point numbers of these two
sizes. *Note SRFI-4::, for more information.

   For many purposes, bytevectors work just as well as uniform vectors,
and have the advantage that they integrate well with binary input and
output.  *Note Bytevectors::, for more information on bytevectors.

6.7.4 Bit Vectors
-----------------

Bit vectors are zero-origin, one-dimensional arrays of booleans.  They
are displayed as a sequence of `0's and `1's prefixed by `#*', e.g.,

     (make-bitvector 8 #f) =>
     #*00000000

   Bit vectors are also generalized vectors, *Note Generalized
Vectors::, and can thus be used with the array procedures, *Note
Arrays::.  Bit vectors are the special case of one dimensional bit
arrays.

 -- Scheme Procedure: bitvector? obj
 -- C Function: scm_bitvector_p (obj)
     Return `#t' when OBJ is a bitvector, else return `#f'.

 -- C Function: int scm_is_bitvector (SCM obj)
     Return `1' when OBJ is a bitvector, else return `0'.

 -- Scheme Procedure: make-bitvector len [fill]
 -- C Function: scm_make_bitvector (len, fill)
     Create a new bitvector of length LEN and optionally initialize all
     elements to FILL.

 -- C Function: SCM scm_c_make_bitvector (size_t len, SCM fill)
     Like `scm_make_bitvector', but the length is given as a `size_t'.

 -- Scheme Procedure: bitvector . bits
 -- C Function: scm_bitvector (bits)
     Create a new bitvector with the arguments as elements.

 -- Scheme Procedure: bitvector-length vec
 -- C Function: scm_bitvector_length (vec)
     Return the length of the bitvector VEC.

 -- C Function: size_t scm_c_bitvector_length (SCM vec)
     Like `scm_bitvector_length', but the length is returned as a
     `size_t'.

 -- Scheme Procedure: bitvector-ref vec idx
 -- C Function: scm_bitvector_ref (vec, idx)
     Return the element at index IDX of the bitvector VEC.

 -- C Function: SCM scm_c_bitvector_ref (SCM obj, size_t idx)
     Return the element at index IDX of the bitvector VEC.

 -- Scheme Procedure: bitvector-set! vec idx val
 -- C Function: scm_bitvector_set_x (vec, idx, val)
     Set the element at index IDX of the bitvector VEC when VAL is
     true, else clear it.

 -- C Function: SCM scm_c_bitvector_set_x (SCM obj, size_t idx, SCM val)
     Set the element at index IDX of the bitvector VEC when VAL is
     true, else clear it.

 -- Scheme Procedure: bitvector-fill! vec val
 -- C Function: scm_bitvector_fill_x (vec, val)
     Set all elements of the bitvector VEC when VAL is true, else clear
     them.

 -- Scheme Procedure: list->bitvector list
 -- C Function: scm_list_to_bitvector (list)
     Return a new bitvector initialized with the elements of LIST.

 -- Scheme Procedure: bitvector->list vec
 -- C Function: scm_bitvector_to_list (vec)
     Return a new list initialized with the elements of the bitvector
     VEC.

 -- Scheme Procedure: bit-count bool bitvector
 -- C Function: scm_bit_count (bool, bitvector)
     Return a count of how many entries in BITVECTOR are equal to BOOL.
     For example,

          (bit-count #f #*000111000)  => 6

 -- Scheme Procedure: bit-position bool bitvector start
 -- C Function: scm_bit_position (bool, bitvector, start)
     Return the index of the first occurrence of BOOL in BITVECTOR,
     starting from START.  If there is no BOOL entry between START and
     the end of BITVECTOR, then return `#f'.  For example,

          (bit-position #t #*000101 0)  => 3
          (bit-position #f #*0001111 3) => #f

 -- Scheme Procedure: bit-invert! bitvector
 -- C Function: scm_bit_invert_x (bitvector)
     Modify BITVECTOR by replacing each element with its negation.

 -- Scheme Procedure: bit-set*! bitvector uvec bool
 -- C Function: scm_bit_set_star_x (bitvector, uvec, bool)
     Set entries of BITVECTOR to BOOL, with UVEC selecting the entries
     to change.  The return value is unspecified.

     If UVEC is a bit vector, then those entries where it has `#t' are
     the ones in BITVECTOR which are set to BOOL.  UVEC and BITVECTOR
     must be the same length.  When BOOL is `#t' it's like UVEC is
     OR'ed into BITVECTOR.  Or when BOOL is `#f' it can be seen as an
     ANDNOT.

          (define bv #*01000010)
          (bit-set*! bv #*10010001 #t)
          bv
          => #*11010011

     If UVEC is a uniform vector of unsigned long integers, then
     they're indexes into BITVECTOR which are set to BOOL.

          (define bv #*01000010)
          (bit-set*! bv #u(5 2 7) #t)
          bv
          => #*01100111

 -- Scheme Procedure: bit-count* bitvector uvec bool
 -- C Function: scm_bit_count_star (bitvector, uvec, bool)
     Return a count of how many entries in BITVECTOR are equal to BOOL,
     with UVEC selecting the entries to consider.

     UVEC is interpreted in the same way as for `bit-set*!' above.
     Namely, if UVEC is a bit vector then entries which have `#t' there
     are considered in BITVECTOR.  Or if UVEC is a uniform vector of
     unsigned long integers then it's the indexes in BITVECTOR to
     consider.

     For example,

          (bit-count* #*01110111 #*11001101 #t) => 3
          (bit-count* #*01110111 #u(7 0 4) #f)  => 2

 -- C Function: const scm_t_uint32 * scm_bitvector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *offp, size_t *lenp,
          ssize_t *incp)
     Like `scm_vector_elements' (*note Vector Accessing from C::), but
     for bitvectors.  The variable pointed to by OFFP is set to the
     value returned by `scm_array_handle_bit_elements_offset'.  See
     `scm_array_handle_bit_elements' for how to use the returned
     pointer and the offset.

 -- C Function: scm_t_uint32 * scm_bitvector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *offp, size_t *lenp,
          ssize_t *incp)
     Like `scm_bitvector_elements', but the pointer is good for reading
     and writing.

6.7.5 Generalized Vectors
-------------------------

Guile has a number of data types that are generally vector-like:
strings, uniform numeric vectors, bytevectors, bitvectors, and of course
ordinary vectors of arbitrary Scheme values.  These types are disjoint:
a Scheme value belongs to at most one of the five types listed above.

   If you want to gloss over this distinction and want to treat all four
types with common code, you can use the procedures in this section.
They work with the _generalized vector_ type, which is the union of the
five vector-like types.

 -- Scheme Procedure: generalized-vector? obj
 -- C Function: scm_generalized_vector_p (obj)
     Return `#t' if OBJ is a vector, bytevector, string, bitvector, or
     uniform numeric vector.

 -- Scheme Procedure: generalized-vector-length v
 -- C Function: scm_generalized_vector_length (v)
     Return the length of the generalized vector V.

 -- Scheme Procedure: generalized-vector-ref v idx
 -- C Function: scm_generalized_vector_ref (v, idx)
     Return the element at index IDX of the generalized vector V.

 -- Scheme Procedure: generalized-vector-set! v idx val
 -- C Function: scm_generalized_vector_set_x (v, idx, val)
     Set the element at index IDX of the generalized vector V to VAL.

 -- Scheme Procedure: generalized-vector->list v
 -- C Function: scm_generalized_vector_to_list (v)
     Return a new list whose elements are the elements of the
     generalized vector V.

 -- C Function: int scm_is_generalized_vector (SCM obj)
     Return `1' if OBJ is a vector, string, bitvector, or uniform
     numeric vector; else return `0'.

 -- C Function: size_t scm_c_generalized_vector_length (SCM v)
     Return the length of the generalized vector V.

 -- C Function: SCM scm_c_generalized_vector_ref (SCM v, size_t idx)
     Return the element at index IDX of the generalized vector V.

 -- C Function: void scm_c_generalized_vector_set_x (SCM v, size_t idx,
          SCM val)
     Set the element at index IDX of the generalized vector V to VAL.

 -- C Function: void scm_generalized_vector_get_handle (SCM v,
          scm_t_array_handle *handle)
     Like `scm_array_get_handle' but an error is signalled when V is
     not of rank one.  You can use `scm_array_handle_ref' and
     `scm_array_handle_set' to read and write the elements of V, or you
     can use functions like `scm_array_handle_<foo>_elements' to deal
     with specific types of vectors.

6.7.6 Arrays
------------

"Arrays" are a collection of cells organized into an arbitrary number
of dimensions.  Each cell can be accessed in constant time by supplying
an index for each dimension.

   In the current implementation, an array uses a generalized vector for
the actual storage of its elements.  Any kind of generalized vector
will do, so you can have arrays of uniform numeric values, arrays of
characters, arrays of bits, and of course, arrays of arbitrary Scheme
values.  For example, arrays with an underlying `c64vector' might be
nice for digital signal processing, while arrays made from a `u8vector'
might be used to hold gray-scale images.

   The number of dimensions of an array is called its "rank".  Thus, a
matrix is an array of rank 2, while a vector has rank 1.  When
accessing an array element, you have to specify one exact integer for
each dimension.  These integers are called the "indices" of the
element.  An array specifies the allowed range of indices for each
dimension via an inclusive lower and upper bound.  These bounds can
well be negative, but the upper bound must be greater than or equal to
the lower bound minus one.  When all lower bounds of an array are zero,
it is called a "zero-origin" array.

   Arrays can be of rank 0, which could be interpreted as a scalar.
Thus, a zero-rank array can store exactly one object and the list of
indices of this element is the empty list.

   Arrays contain zero elements when one of their dimensions has a zero
length.  These empty arrays maintain information about their shape: a
matrix with zero columns and 3 rows is different from a matrix with 3
columns and zero rows, which again is different from a vector of length
zero.

   Generalized vectors, such as strings, uniform numeric vectors,
bytevectors, bit vectors and ordinary vectors, are the special case of
one dimensional arrays.

6.7.6.1 Array Syntax
....................

An array is displayed as `#' followed by its rank, followed by a tag
that describes the underlying vector, optionally followed by
information about its shape, and finally followed by the cells,
organized into dimensions using parentheses.

   In more words, the array tag is of the form

       #<rank><vectag><@lower><:len><@lower><:len>...

   where `<rank>' is a positive integer in decimal giving the rank of
the array.  It is omitted when the rank is 1 and the array is non-shared
and has zero-origin (see below).  For shared arrays and for a non-zero
origin, the rank is always printed even when it is 1 to distinguish
them from ordinary vectors.

   The `<vectag>' part is the tag for a uniform numeric vector, like
`u8', `s16', etc, `b' for bitvectors, or `a' for strings.  It is empty
for ordinary vectors.

   The `<@lower>' part is a `@' character followed by a signed integer
in decimal giving the lower bound of a dimension.  There is one
`<@lower>' for each dimension.  When all lower bounds are zero, all
`<@lower>' parts are omitted.

   The `<:len>' part is a `:' character followed by an unsigned integer
in decimal giving the length of a dimension.  Like for the lower
bounds, there is one `<:len>' for each dimension, and the `<:len>' part
always follows the `<@lower>' part for a dimension.  Lengths are only
then printed when they can't be deduced from the nested lists of
elements of the array literal, which can happen when at least one
length is zero.

   As a special case, an array of rank 0 is printed as
`#0<vectag>(<scalar>)', where `<scalar>' is the result of printing the
single element of the array.

   Thus,

`#(1 2 3)'
     is an ordinary array of rank 1 with lower bound 0 in dimension 0.
     (I.e., a regular vector.)

`#@2(1 2 3)'
     is an ordinary array of rank 1 with lower bound 2 in dimension 0.

`#2((1 2 3) (4 5 6))'
     is a non-uniform array of rank 2; a 3x3 matrix with index ranges
     0..2 and 0..2.

`#u32(0 1 2)'
     is a uniform u8 array of rank 1.

`#2u32@2@3((1 2) (2 3))'
     is a uniform u8 array of rank 2 with index ranges 2..3 and 3..4.

`#2()'
     is a two-dimensional array with index ranges 0..-1 and 0..-1, i.e.
     both dimensions have length zero.

`#2:0:2()'
     is a two-dimensional array with index ranges 0..-1 and 0..1, i.e.
     the first dimension has length zero, but the second has length 2.

`#0(12)'
     is a rank-zero array with contents 12.


   In addition, bytevectors are also arrays, but use a different syntax
(*note Bytevectors::):

`#vu8(1 2 3)'
     is a 3-byte long bytevector, with contents 1, 2, 3.


6.7.6.2 Array Procedures
........................

When an array is created, the range of each dimension must be
specified, e.g., to create a 2x3 array with a zero-based index:

     (make-array 'ho 2 3) => #2((ho ho ho) (ho ho ho))

   The range of each dimension can also be given explicitly, e.g.,
another way to create the same array:

     (make-array 'ho '(0 1) '(0 2)) => #2((ho ho ho) (ho ho ho))

   The following procedures can be used with arrays (or vectors).  An
argument shown as IDX... means one parameter for each dimension in the
array.  A IDXLIST argument means a list of such values, one for each
dimension.

 -- Scheme Procedure: array? obj
 -- C Function: scm_array_p (obj, unused)
     Return `#t' if the OBJ is an array, and `#f' if not.

     The second argument to scm_array_p is there for historical reasons,
     but it is not used.  You should always pass `SCM_UNDEFINED' as its
     value.

 -- Scheme Procedure: typed-array? obj type
 -- C Function: scm_typed_array_p (obj, type)
     Return `#t' if the OBJ is an array of type TYPE, and `#f' if not.

 -- C Function: int scm_is_array (SCM obj)
     Return `1' if the OBJ is an array and `0' if not.

 -- C Function: int scm_is_typed_array (SCM obj, SCM type)
     Return `0' if the OBJ is an array of type TYPE, and `1' if not.

 -- Scheme Procedure: make-array fill bound ...
 -- C Function: scm_make_array (fill, bounds)
     Equivalent to `(make-typed-array #t FILL BOUND ...)'.

 -- Scheme Procedure: make-typed-array type fill bound ...
 -- C Function: scm_make_typed_array (type, fill, bounds)
     Create and return an array that has as many dimensions as there are
     BOUNDs and (maybe) fill it with FILL.

     The underlying storage vector is created according to TYPE, which
     must be a symbol whose name is the `vectag' of the array as
     explained above, or `#t' for ordinary, non-specialized arrays.

     For example, using the symbol `f64' for TYPE will create an array
     that uses a `f64vector' for storing its elements, and `a' will use
     a string.

     When FILL is not the special _unspecified_ value, the new array is
     filled with FILL.  Otherwise, the initial contents of the array is
     unspecified.  The special _unspecified_ value is stored in the
     variable `*unspecified*' so that for example `(make-typed-array
     'u32 *unspecified* 4)' creates a uninitialized `u32' vector of
     length 4.

     Each BOUND may be a positive non-zero integer N, in which case the
     index for that dimension can range from 0 through N-1; or an
     explicit index range specifier in the form `(LOWER UPPER)', where
     both LOWER and UPPER are integers, possibly less than zero, and
     possibly the same number (however, LOWER cannot be greater than
     UPPER).

 -- Scheme Procedure: list->array dimspec list
     Equivalent to `(list->typed-array #t DIMSPEC LIST)'.

 -- Scheme Procedure: list->typed-array type dimspec list
 -- C Function: scm_list_to_typed_array (type, dimspec, list)
     Return an array of the type indicated by TYPE with elements the
     same as those of LIST.

     The argument DIMSPEC determines the number of dimensions of the
     array and their lower bounds.  When DIMSPEC is an exact integer,
     it gives the number of dimensions directly and all lower bounds are
     zero.  When it is a list of exact integers, then each element is
     the lower index bound of a dimension, and there will be as many
     dimensions as elements in the list.

 -- Scheme Procedure: array-type array
     Return the type of ARRAY.  This is the `vectag' used for printing
     ARRAY (or `#t' for ordinary arrays) and can be used with
     `make-typed-array' to create an array of the same kind as ARRAY.

 -- Scheme Procedure: array-ref array idx ...
     Return the element at `(idx ...)' in ARRAY.

          (define a (make-array 999 '(1 2) '(3 4)))
          (array-ref a 2 4) => 999

 -- Scheme Procedure: array-in-bounds? array idx ...
 -- C Function: scm_array_in_bounds_p (array, idxlist)
     Return `#t' if the given index would be acceptable to `array-ref'.

          (define a (make-array #f '(1 2) '(3 4)))
          (array-in-bounds? a 2 3) => #t
          (array-in-bounds? a 0 0) => #f

 -- Scheme Procedure: array-set! array obj idx ...
 -- C Function: scm_array_set_x (array, obj, idxlist)
     Set the element at `(idx ...)' in ARRAY to OBJ.  The return value
     is unspecified.

          (define a (make-array #f '(0 1) '(0 1)))
          (array-set! a #t 1 1)
          a => #2((#f #f) (#f #t))

 -- Scheme Procedure: array-shape array
 -- Scheme Procedure: array-dimensions array
 -- C Function: scm_array_dimensions (array)
     Return a list of the bounds for each dimension of ARRAY.

     `array-shape' gives `(LOWER UPPER)' for each dimension.
     `array-dimensions' instead returns just UPPER+1 for dimensions
     with a 0 lower bound.  Both are suitable as input to `make-array'.

     For example,

          (define a (make-array 'foo '(-1 3) 5))
          (array-shape a)      => ((-1 3) (0 4))
          (array-dimensions a) => ((-1 3) 5)

 -- Scheme Procedure: array-rank obj
 -- C Function: scm_array_rank (obj)
     Return the rank of ARRAY.

 -- C Function: size_t scm_c_array_rank (SCM array)
     Return the rank of ARRAY as a `size_t'.

 -- Scheme Procedure: array->list array
 -- C Function: scm_array_to_list (array)
     Return a list consisting of all the elements, in order, of ARRAY.

 -- Scheme Procedure: array-copy! src dst
 -- Scheme Procedure: array-copy-in-order! src dst
 -- C Function: scm_array_copy_x (src, dst)
     Copy every element from vector or array SRC to the corresponding
     element of DST.  DST must have the same rank as SRC, and be at
     least as large in each dimension.  The return value is unspecified.

 -- Scheme Procedure: array-fill! array fill
 -- C Function: scm_array_fill_x (array, fill)
     Store FILL in every element of ARRAY.  The value returned is
     unspecified.

 -- Scheme Procedure: array-equal? array1 array2 ...
     Return `#t' if all arguments are arrays with the same shape, the
     same type, and have corresponding elements which are either
     `equal?' or `array-equal?'.  This function differs from `equal?'
     (*note Equality::) in that all arguments must be arrays.

 -- Scheme Procedure: array-map! dst proc src1 ... srcN
 -- Scheme Procedure: array-map-in-order! dst proc src1 ... srcN
 -- C Function: scm_array_map_x (dst, proc, srclist)
     Set each element of the DST array to values obtained from calls to
     PROC.  The value returned is unspecified.

     Each call is `(PROC ELEM1 ... ELEMN)', where each ELEM is from the
     corresponding SRC array, at the DST index.  `array-map-in-order!'
     makes the calls in row-major order, `array-map!' makes them in an
     unspecified order.

     The SRC arrays must have the same number of dimensions as DST, and
     must have a range for each dimension which covers the range in
     DST.  This ensures all DST indices are valid in each SRC.

 -- Scheme Procedure: array-for-each proc src1 ... srcN
 -- C Function: scm_array_for_each (proc, src1, srclist)
     Apply PROC to each tuple of elements of SRC1 ...  SRCN, in
     row-major order.  The value returned is unspecified.

 -- Scheme Procedure: array-index-map! dst proc
 -- C Function: scm_array_index_map_x (dst, proc)
     Set each element of the DST array to values returned by calls to
     PROC.  The value returned is unspecified.

     Each call is `(PROC I1 ... IN)', where I1...IN is the destination
     index, one parameter for each dimension.  The order in which the
     calls are made is unspecified.

     For example, to create a 4x4 matrix representing a cyclic group,

              / 0 1 2 3 \
              | 1 2 3 0 |
              | 2 3 0 1 |
              \ 3 0 1 2 /

          (define a (make-array #f 4 4))
          (array-index-map! a (lambda (i j)
                                (modulo (+ i j) 4)))

 -- Scheme Procedure: uniform-array-read! ra [port_or_fd [start [end]]]
 -- C Function: scm_uniform_array_read_x (ra, port_or_fd, start, end)
     Attempt to read all elements of URA, in lexicographic order, as
     binary objects from PORT-OR-FDES.  If an end of file is
     encountered, the objects up to that point are put into URA
     (starting at the beginning) and the remainder of the array is
     unchanged.

     The optional arguments START and END allow a specified region of a
     vector (or linearized array) to be read, leaving the remainder of
     the vector unchanged.

     `uniform-array-read!' returns the number of objects read.
     PORT-OR-FDES may be omitted, in which case it defaults to the value
     returned by `(current-input-port)'.

 -- Scheme Procedure: uniform-array-write v [port_or_fd [start [end]]]
 -- C Function: scm_uniform_array_write (v, port_or_fd, start, end)
     Writes all elements of URA as binary objects to PORT-OR-FDES.

     The optional arguments START and END allow a specified region of a
     vector (or linearized array) to be written.

     The number of objects actually written is returned.  PORT-OR-FDES
     may be omitted, in which case it defaults to the value returned by
     `(current-output-port)'.

6.7.6.3 Shared Arrays
.....................

 -- Scheme Procedure: make-shared-array oldarray mapfunc bound ...
 -- C Function: scm_make_shared_array (oldarray, mapfunc, boundlist)
     Return a new array which shares the storage of OLDARRAY.  Changes
     made through either affect the same underlying storage.  The
     BOUND... arguments are the shape of the new array, the same as
     `make-array' (*note Array Procedures::).

     MAPFUNC translates coordinates from the new array to the OLDARRAY.
     It's called as `(MAPFUNC newidx1 ...)' with one parameter for each
     dimension of the new array, and should return a list of indices
     for OLDARRAY, one for each dimension of OLDARRAY.

     MAPFUNC must be affine linear, meaning that each OLDARRAY index
     must be formed by adding integer multiples (possibly negative) of
     some or all of NEWIDX1 etc, plus a possible integer offset.  The
     multiples and offset must be the same in each call.


     One good use for a shared array is to restrict the range of some
     dimensions, so as to apply say `array-for-each' or `array-fill!'
     to only part of an array.  The plain `list' function can be used
     for MAPFUNC in this case, making no changes to the index values.
     For example,

          (make-shared-array #2((a b c) (d e f) (g h i)) list 3 2)
          => #2((a b) (d e) (g h))

     The new array can have fewer dimensions than OLDARRAY, for example
     to take a column from an array.

          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i) (list i 2))
                             '(0 2))
          => #1(c f i)

     A diagonal can be taken by using the single new array index for
     both row and column in the old array.  For example,

          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i) (list i i))
                             '(0 2))
          => #1(a e i)

     Dimensions can be increased by for instance considering portions
     of a one dimensional array as rows in a two dimensional array.
     (`array-contents' below can do the opposite, flattening an array.)

          (make-shared-array #1(a b c d e f g h i j k l)
                             (lambda (i j) (list (+ (* i 3) j)))
                             4 3)
          => #2((a b c) (d e f) (g h i) (j k l))

     By negating an index the order that elements appear can be
     reversed.  The following just reverses the column order,

          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i j) (list i (- 2 j)))
                             3 3)
          => #2((c b a) (f e d) (i h g))

     A fixed offset on indexes allows for instance a change from a 0
     based to a 1 based array,

          (define x #2((a b c) (d e f) (g h i)))
          (define y (make-shared-array x
                                       (lambda (i j) (list (1- i) (1- j)))
                                       '(1 3) '(1 3)))
          (array-ref x 0 0) => a
          (array-ref y 1 1) => a

     A multiple on an index allows every Nth element of an array to be
     taken.  The following is every third element,

          (make-shared-array #1(a b c d e f g h i j k l)
                             (lambda (i) (list (* i 3)))
                             4)
          => #1(a d g j)

     The above examples can be combined to make weird and wonderful
     selections from an array, but it's important to note that because
     MAPFUNC must be affine linear, arbitrary permutations are not
     possible.

     In the current implementation, MAPFUNC is not called for every
     access to the new array but only on some sample points to
     establish a base and stride for new array indices in OLDARRAY
     data.  A few sample points are enough because MAPFUNC is linear.

 -- Scheme Procedure: shared-array-increments array
 -- C Function: scm_shared_array_increments (array)
     For each dimension, return the distance between elements in the
     root vector.

 -- Scheme Procedure: shared-array-offset array
 -- C Function: scm_shared_array_offset (array)
     Return the root vector index of the first element in the array.

 -- Scheme Procedure: shared-array-root array
 -- C Function: scm_shared_array_root (array)
     Return the root vector of a shared array.

 -- Scheme Procedure: array-contents array [strict]
 -- C Function: scm_array_contents (array, strict)
     If ARRAY may be "unrolled" into a one dimensional shared array
     without changing their order (last subscript changing fastest),
     then `array-contents' returns that shared array, otherwise it
     returns `#f'.  All arrays made by `make-array' and
     `make-typed-array' may be unrolled, some arrays made by
     `make-shared-array' may not be.

     If the optional argument STRICT is provided, a shared array will
     be returned only if its elements are stored internally contiguous
     in memory.

 -- Scheme Procedure: transpose-array array dim1 ...
 -- C Function: scm_transpose_array (array, dimlist)
     Return an array sharing contents with ARRAY, but with dimensions
     arranged in a different order.  There must be one DIM argument for
     each dimension of ARRAY.  DIM1, DIM2, ... should be integers
     between 0 and the rank of the array to be returned.  Each integer
     in that range must appear at least once in the argument list.

     The values of DIM1, DIM2, ... correspond to dimensions in the
     array to be returned, and their positions in the argument list to
     dimensions of ARRAY.  Several DIMs may have the same value, in
     which case the returned array will have smaller rank than ARRAY.

          (transpose-array '#2((a b) (c d)) 1 0) => #2((a c) (b d))
          (transpose-array '#2((a b) (c d)) 0 0) => #1(a d)
          (transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) =>
                          #2((a 4) (b 5) (c 6))

6.7.6.4 Accessing Arrays from C
...............................

For interworking with external C code, Guile provides an API to allow C
code to access the elements of a Scheme array.  In particular, for
uniform numeric arrays, the API exposes the underlying uniform data as a
C array of numbers of the relevant type.

   While pointers to the elements of an array are in use, the array
itself must be protected so that the pointer remains valid.  Such a
protected array is said to be "reserved".  A reserved array can be read
but modifications to it that would cause the pointer to its elements to
become invalid are prevented.  When you attempt such a modification, an
error is signalled.

   (This is similar to locking the array while it is in use, but without
the danger of a deadlock.  In a multi-threaded program, you will need
additional synchronization to avoid modifying reserved arrays.)

   You must take care to always unreserve an array after reserving it,
even in the presence of non-local exits.  If a non-local exit can
happen between these two calls, you should install a dynwind context
that releases the array when it is left (*note Dynamic Wind::).

   In addition, array reserving and unreserving must be properly
paired.  For instance, when reserving two or more arrays in a certain
order, you need to unreserve them in the opposite order.

   Once you have reserved an array and have retrieved the pointer to its
elements, you must figure out the layout of the elements in memory.
Guile allows slices to be taken out of arrays without actually making a
copy, such as making an alias for the diagonal of a matrix that can be
treated as a vector.  Arrays that result from such an operation are not
stored contiguously in memory and when working with their elements
directly, you need to take this into account.

   The layout of array elements in memory can be defined via a _mapping
function_ that computes a scalar position from a vector of indices.
The scalar position then is the offset of the element with the given
indices from the start of the storage block of the array.

   In Guile, this mapping function is restricted to be "affine": all
mapping functions of Guile arrays can be written as `p = b + c[0]*i[0]
+ c[1]*i[1] + ... + c[n-1]*i[n-1]' where `i[k]' is the kth index and
`n' is the rank of the array.  For example, a matrix of size 3x3 would
have `b == 0', `c[0] == 3' and `c[1] == 1'.  When you transpose this
matrix (with `transpose-array', say), you will get an array whose
mapping function has `b == 0', `c[0] == 1' and `c[1] == 3'.

   The function `scm_array_handle_dims' gives you (indirect) access to
the coefficients `c[k]'.

   Note that there are no functions for accessing the elements of a
character array yet.  Once the string implementation of Guile has been
changed to use Unicode, we will provide them.

 -- C Type: scm_t_array_handle
     This is a structure type that holds all information necessary to
     manage the reservation of arrays as explained above.  Structures
     of this type must be allocated on the stack and must only be
     accessed by the functions listed below.

 -- C Function: void scm_array_get_handle (SCM array,
          scm_t_array_handle *handle)
     Reserve ARRAY, which must be an array, and prepare HANDLE to be
     used with the functions below.  You must eventually call
     `scm_array_handle_release' on HANDLE, and do this in a properly
     nested fashion, as explained above.  The structure pointed to by
     HANDLE does not need to be initialized before calling this
     function.

 -- C Function: void scm_array_handle_release (scm_t_array_handle
          *handle)
     End the array reservation represented by HANDLE.  After a call to
     this function, HANDLE might be used for another reservation.

 -- C Function: size_t scm_array_handle_rank (scm_t_array_handle
          *handle)
     Return the rank of the array represented by HANDLE.

 -- C Type: scm_t_array_dim
     This structure type holds information about the layout of one
     dimension of an array.  It includes the following fields:

    `ssize_t lbnd'
    `ssize_t ubnd'
          The lower and upper bounds (both inclusive) of the
          permissible index range for the given dimension.  Both values
          can be negative, but LBND is always less than or equal to
          UBND.

    `ssize_t inc'
          The distance from one element of this dimension to the next.
          Note, too, that this can be negative.

 -- C Function: const scm_t_array_dim * scm_array_handle_dims
          (scm_t_array_handle *handle)
     Return a pointer to a C vector of information about the dimensions
     of the array represented by HANDLE.  This pointer is valid as long
     as the array remains reserved.  As explained above, the
     `scm_t_array_dim' structures returned by this function can be used
     calculate the position of an element in the storage block of the
     array from its indices.

     This position can then be used as an index into the C array pointer
     returned by the various `scm_array_handle_<foo>_elements'
     functions, or with `scm_array_handle_ref' and
     `scm_array_handle_set'.

     Here is how one can compute the position POS of an element given
     its indices in the vector INDICES:

          ssize_t indices[RANK];
          scm_t_array_dim *dims;
          ssize_t pos;
          size_t i;

          pos = 0;
          for (i = 0; i < RANK; i++)
            {
              if (indices[i] < dims[i].lbnd || indices[i] > dims[i].ubnd)
                out_of_range ();
              pos += (indices[i] - dims[i].lbnd) * dims[i].inc;
            }

 -- C Function: ssize_t scm_array_handle_pos (scm_t_array_handle
          *handle, SCM indices)
     Compute the position corresponding to INDICES, a list of indices.
     The position is computed as described above for
     `scm_array_handle_dims'.  The number of the indices and their
     range is checked and an appropriate error is signalled for invalid
     indices.

 -- C Function: SCM scm_array_handle_ref (scm_t_array_handle *handle,
          ssize_t pos)
     Return the element at position POS in the storage block of the
     array represented by HANDLE.  Any kind of array is acceptable.  No
     range checking is done on POS.

 -- C Function: void scm_array_handle_set (scm_t_array_handle *handle,
          ssize_t pos, SCM val)
     Set the element at position POS in the storage block of the array
     represented by HANDLE to VAL.  Any kind of array is acceptable.
     No range checking is done on POS.  An error is signalled when the
     array can not store VAL.

 -- C Function: const SCM * scm_array_handle_elements
          (scm_t_array_handle *handle)
     Return a pointer to the elements of a ordinary array of general
     Scheme values (i.e., a non-uniform array) for reading.  This
     pointer is valid as long as the array remains reserved.

 -- C Function: SCM * scm_array_handle_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_elements', but the pointer is good for
     reading and writing.

 -- C Function: const void * scm_array_handle_uniform_elements
          (scm_t_array_handle *handle)
     Return a pointer to the elements of a uniform numeric array for
     reading.  This pointer is valid as long as the array remains
     reserved.  The size of each element is given by
     `scm_array_handle_uniform_element_size'.

 -- C Function: void * scm_array_handle_uniform_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_uniform_elements', but the pointer is good
     reading and writing.

 -- C Function: size_t scm_array_handle_uniform_element_size
          (scm_t_array_handle *handle)
     Return the size of one element of the uniform numeric array
     represented by HANDLE.

 -- C Function: const scm_t_uint8 * scm_array_handle_u8_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int8 * scm_array_handle_s8_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_uint16 * scm_array_handle_u16_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int16 * scm_array_handle_s16_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_uint32 * scm_array_handle_u32_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int32 * scm_array_handle_s32_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_uint64 * scm_array_handle_u64_elements
          (scm_t_array_handle *handle)
 -- C Function: const scm_t_int64 * scm_array_handle_s64_elements
          (scm_t_array_handle *handle)
 -- C Function: const float * scm_array_handle_f32_elements
          (scm_t_array_handle *handle)
 -- C Function: const double * scm_array_handle_f64_elements
          (scm_t_array_handle *handle)
 -- C Function: const float * scm_array_handle_c32_elements
          (scm_t_array_handle *handle)
 -- C Function: const double * scm_array_handle_c64_elements
          (scm_t_array_handle *handle)
     Return a pointer to the elements of a uniform numeric array of the
     indicated kind for reading.  This pointer is valid as long as the
     array remains reserved.

     The pointers for `c32' and `c64' uniform numeric arrays point to
     pairs of floating point numbers.  The even index holds the real
     part, the odd index the imaginary part of the complex number.

 -- C Function: scm_t_uint8 * scm_array_handle_u8_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int8 * scm_array_handle_s8_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_uint16 * scm_array_handle_u16_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int16 * scm_array_handle_s16_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_uint32 * scm_array_handle_u32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int32 * scm_array_handle_s32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_uint64 * scm_array_handle_u64_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: scm_t_int64 * scm_array_handle_s64_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: float * scm_array_handle_f32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: double * scm_array_handle_f64_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: float * scm_array_handle_c32_writable_elements
          (scm_t_array_handle *handle)
 -- C Function: double * scm_array_handle_c64_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_<kind>_elements', but the pointer is good
     for reading and writing.

 -- C Function: const scm_t_uint32 * scm_array_handle_bit_elements
          (scm_t_array_handle *handle)
     Return a pointer to the words that store the bits of the
     represented array, which must be a bit array.

     Unlike other arrays, bit arrays have an additional offset that
     must be figured into index calculations.  That offset is returned
     by `scm_array_handle_bit_elements_offset'.

     To find a certain bit you first need to calculate its position as
     explained above for `scm_array_handle_dims' and then add the
     offset.  This gives the absolute position of the bit, which is
     always a non-negative integer.

     Each word of the bit array storage block contains exactly 32 bits,
     with the least significant bit in that word having the lowest
     absolute position number.  The next word contains the next 32 bits.

     Thus, the following code can be used to access a bit whose position
     according to `scm_array_handle_dims' is given in POS:

          SCM bit_array;
          scm_t_array_handle handle;
          scm_t_uint32 *bits;
          ssize_t pos;
          size_t abs_pos;
          size_t word_pos, mask;

          scm_array_get_handle (&bit_array, &handle);
          bits = scm_array_handle_bit_elements (&handle);

          pos = ...
          abs_pos = pos + scm_array_handle_bit_elements_offset (&handle);
          word_pos = abs_pos / 32;
          mask = 1L << (abs_pos % 32);

          if (bits[word_pos] & mask)
            /* bit is set. */

          scm_array_handle_release (&handle);


 -- C Function: scm_t_uint32 * scm_array_handle_bit_writable_elements
          (scm_t_array_handle *handle)
     Like `scm_array_handle_bit_elements' but the pointer is good for
     reading and writing.  You must take care not to modify bits
     outside of the allowed index range of the array, even for
     contiguous arrays.

6.7.7 VLists
------------

The `(ice-9 vlist)' module provides an implementation of the "VList"
data structure designed by Phil Bagwell in 2002.  VLists are immutable
lists, which can contain any Scheme object.  They improve on standard
Scheme linked lists in several areas:

   * Random access has typically constant-time complexity.

   * Computing the length of a VList has time complexity logarithmic in
     the number of elements.

   * VLists use less storage space than standard lists.

   * VList elements are stored in contiguous regions, which improves
     memory locality and leads to more efficient use of hardware caches.

   The idea behind VLists is to store vlist elements in increasingly
large contiguous blocks (implemented as vectors here).  These blocks
are linked to one another using a pointer to the next block and an
offset within that block.  The size of these blocks form a geometric
series with ratio `block-growth-factor' (2 by default).

   The VList structure also serves as the basis for the "VList-based
hash lists" or "vhashes", an immutable dictionary type (*note
VHashes::).

   However, the current implementation in `(ice-9 vlist)' has several
noteworthy shortcomings:

   * It is _not_ thread-safe.  Although operations on vlists are all
     "referentially transparent" (i.e., purely functional), adding
     elements to a vlist with `vlist-cons' mutates part of its internal
     structure, which makes it non-thread-safe.  This could be fixed,
     but it would slow down `vlist-cons'.

   * `vlist-cons' always allocates at least as much memory as `cons'.
     Again, Phil Bagwell describes how to fix it, but that would
     require tuning the garbage collector in a way that may not be
     generally beneficial.

   * `vlist-cons' is a Scheme procedure compiled to bytecode, and it
     does not compete with the straightforward C implementation of
     `cons', and with the fact that the VM has a special `cons'
     instruction.


   We hope to address these in the future.

   The programming interface exported by `(ice-9 vlist)' is defined
below.  Most of it is the same as SRFI-1 with an added `vlist-' prefix
to function names.

 -- Scheme Procedure: vlist? obj
     Return true if OBJ is a VList.

 -- Scheme Variable: vlist-null
     The empty VList.  Note that it's possible to create an empty VList
     not `eq?' to `vlist-null'; thus, callers should always use
     `vlist-null?' when testing whether a VList is empty.

 -- Scheme Procedure: vlist-null? vlist
     Return true if VLIST is empty.

 -- Scheme Procedure: vlist-cons item vlist
     Return a new vlist with ITEM as its head and VLIST as its tail.

 -- Scheme Procedure: vlist-head vlist
     Return the head of VLIST.

 -- Scheme Procedure: vlist-tail vlist
     Return the tail of VLIST.

 -- Scheme Variable: block-growth-factor
     A fluid that defines the growth factor of VList blocks, 2 by
     default.

   The functions below provide the usual set of higher-level list
operations.

 -- Scheme Procedure: vlist-fold proc init vlist
 -- Scheme Procedure: vlist-fold-right proc init vlist
     Fold over VLIST, calling PROC for each element, as for SRFI-1
     `fold' and `fold-right' (*note `fold': SRFI-1.).

 -- Scheme Procedure: vlist-ref vlist index
     Return the element at index INDEX in VLIST.  This is typically a
     constant-time operation.

 -- Scheme Procedure: vlist-length vlist
     Return the length of VLIST.  This is typically logarithmic in the
     number of elements in VLIST.

 -- Scheme Procedure: vlist-reverse vlist
     Return a new VLIST whose content are those of VLIST in reverse
     order.

 -- Scheme Procedure: vlist-map proc vlist
     Map PROC over the elements of VLIST and return a new vlist.

 -- Scheme Procedure: vlist-for-each proc vlist
     Call PROC on each element of VLIST.  The result is unspecified.

 -- Scheme Procedure: vlist-drop vlist count
     Return a new vlist that does not contain the COUNT first elements
     of VLIST.  This is typically a constant-time operation.

 -- Scheme Procedure: vlist-take vlist count
     Return a new vlist that contains only the COUNT first elements of
     VLIST.

 -- Scheme Procedure: vlist-filter pred vlist
     Return a new vlist containing all the elements from VLIST that
     satisfy PRED.

 -- Scheme Procedure: vlist-delete x vlist [equal?]
     Return a new vlist corresponding to VLIST without the elements
     EQUAL? to X.

 -- Scheme Procedure: vlist-unfold p f g seed [tail-gen]
 -- Scheme Procedure: vlist-unfold-right p f g seed [tail]
     Return a new vlist, as for SRFI-1 `unfold' and `unfold-right'
     (*note `unfold': SRFI-1.).

 -- Scheme Procedure: vlist-append vlists ...
     Append the given vlists and return the resulting vlist.

 -- Scheme Procedure: list->vlist lst
     Return a new vlist whose contents correspond to LST.

 -- Scheme Procedure: vlist->list vlist
     Return a new list whose contents match those of VLIST.

6.7.8 Records
-------------

A "record type" is a first class object representing a user-defined
data type.  A "record" is an instance of a record type.

 -- Scheme Procedure: record? obj
     Return `#t' if OBJ is a record of any type and `#f' otherwise.

     Note that `record?' may be true of any Scheme value; there is no
     promise that records are disjoint with other Scheme types.

 -- Scheme Procedure: make-record-type type-name field-names [print]
     Create and return a new "record-type descriptor".

     TYPE-NAME is a string naming the type.  Currently it's only used
     in the printed representation of records, and in diagnostics.
     FIELD-NAMES is a list of symbols naming the fields of a record of
     the type.  Duplicates are not allowed among these symbols.

          (make-record-type "employee" '(name age salary))

     The optional PRINT argument is a function used by `display',
     `write', etc, for printing a record of the new type.  It's called
     as `(PRINT record port)' and should look at RECORD and write to
     PORT.

 -- Scheme Procedure: record-constructor rtd [field-names]
     Return a procedure for constructing new members of the type
     represented by RTD.  The returned procedure accepts exactly as
     many arguments as there are symbols in the given list,
     FIELD-NAMES; these are used, in order, as the initial values of
     those fields in a new record, which is returned by the constructor
     procedure.  The values of any fields not named in that list are
     unspecified.  The FIELD-NAMES argument defaults to the list of
     field names in the call to `make-record-type' that created the
     type represented by RTD; if the FIELD-NAMES argument is provided,
     it is an error if it contains any duplicates or any symbols not in
     the default list.

 -- Scheme Procedure: record-predicate rtd
     Return a procedure for testing membership in the type represented
     by RTD.  The returned procedure accepts exactly one argument and
     returns a true value if the argument is a member of the indicated
     record type; it returns a false value otherwise.

 -- Scheme Procedure: record-accessor rtd field-name
     Return a procedure for reading the value of a particular field of a
     member of the type represented by RTD.  The returned procedure
     accepts exactly one argument which must be a record of the
     appropriate type; it returns the current value of the field named
     by the symbol FIELD-NAME in that record.  The symbol FIELD-NAME
     must be a member of the list of field-names in the call to
     `make-record-type' that created the type represented by RTD.

 -- Scheme Procedure: record-modifier rtd field-name
     Return a procedure for writing the value of a particular field of a
     member of the type represented by RTD.  The returned procedure
     accepts exactly two arguments: first, a record of the appropriate
     type, and second, an arbitrary Scheme value; it modifies the field
     named by the symbol FIELD-NAME in that record to contain the given
     value.  The returned value of the modifier procedure is
     unspecified.  The symbol FIELD-NAME must be a member of the list
     of field-names in the call to `make-record-type' that created the
     type represented by RTD.

 -- Scheme Procedure: record-type-descriptor record
     Return a record-type descriptor representing the type of the given
     record.  That is, for example, if the returned descriptor were
     passed to `record-predicate', the resulting predicate would return
     a true value when passed the given record.  Note that it is not
     necessarily the case that the returned descriptor is the one that
     was passed to `record-constructor' in the call that created the
     constructor procedure that created the given record.

 -- Scheme Procedure: record-type-name rtd
     Return the type-name associated with the type represented by rtd.
     The returned value is `eqv?' to the TYPE-NAME argument given in
     the call to `make-record-type' that created the type represented by
     RTD.

 -- Scheme Procedure: record-type-fields rtd
     Return a list of the symbols naming the fields in members of the
     type represented by RTD.  The returned value is `equal?' to the
     field-names argument given in the call to `make-record-type' that
     created the type represented by RTD.

6.7.9 Structures
----------------

A "structure" is a first class data type which holds Scheme values or C
words in fields numbered 0 upwards.  A "vtable" represents a structure
type, giving field types and permissions, and an optional print
function for `write' etc.

   Structures are lower level than records (*note Records::) but have
some extra features.  The vtable system allows sets of types be
constructed, with class data.  The uninterpreted words can
inter-operate with C code, allowing arbitrary pointers or other values
to be stored along side usual Scheme `SCM' values.

6.7.9.1 Vtables
...............

A vtable is a structure type, specifying its layout, and other
information.  A vtable is actually itself a structure, but there's no
need to worry about that initially (*note Vtable Contents::.)

 -- Scheme Procedure: make-vtable fields [print]
     Create a new vtable.

     FIELDS is a string describing the fields in the structures to be
     created.  Each field is represented by two characters, a type
     letter and a permissions letter, for example `"pw"'.  The types
     are as follows.

        * `p' - a Scheme value.  "p" stands for "protected" meaning
          it's protected against garbage collection.

        * `u' - an arbitrary word of data (an `scm_t_bits').  At the
          Scheme level it's read and written as an unsigned integer.
          "u" stands for "uninterpreted" (it's not treated as a Scheme
          value), or "unprotected" (it's not marked during GC), or
          "unsigned long" (its size), or all of these things.

        * `s' - a self-reference.  Such a field holds the `SCM' value
          of the structure itself (a circular reference).  This can be
          useful in C code where you might have a pointer to the data
          array, and want to get the Scheme `SCM' handle for the
          structure.  In Scheme code it has no use.

     The second letter for each field is a permission code,

        * `w' - writable, the field can be read and written.

        * `r' - read-only, the field can be read but not written.

        * `o' - opaque, the field can be neither read nor written at the
          Scheme level.  This can be used for fields which should only
          be used from C code.

        * `W',`R',`O' - a tail array, with permissions for the array
          fields as per `w',`r',`o'.

     A tail array is further fields at the end of a structure.  The last
     field in the layout string might be for instance `pW' to have a
     tail of writable Scheme-valued fields.  The `pW' field itself
     holds the tail size, and the tail fields come after it.

     Here are some examples.

          (make-vtable "pw")      ;; one writable field
          (make-vtable "prpw")    ;; one read-only and one writable
          (make-vtable "pwuwuw")  ;; one scheme and two uninterpreted

          (make-vtable "prpW")    ;; one fixed then a tail array

     The optional PRINT argument is a function called by `display' and
     `write' (etc) to give a printed representation of a structure
     created from this vtable.  It's called `(PRINT struct port)' and
     should look at STRUCT and write to PORT.  The default print merely
     gives a form like `#<struct ADDR:ADDR>' with a pair of machine
     addresses.

     The following print function for example shows the two fields of
     its structure.

          (make-vtable "prpw"
                       (lambda (struct port)
                         (display "#<" port)
                         (display (struct-ref struct 0) port)
                         (display " and " port)
                         (display (struct-ref struct 1) port)
                         (display ">" port)))

6.7.9.2 Structure Basics
........................

This section describes the basic procedures for working with
structures.  `make-struct' creates a structure, and `struct-ref' and
`struct-set!' access write fields.

 -- Scheme Procedure: make-struct vtable tail-size [init...]
 -- C Function: scm_make_struct (vtable, tail_size, init_list)
     Create a new structure, with layout per the given VTABLE (*note
     Vtables::).

     TAIL-SIZE is the size of the tail array if VTABLE specifies a tail
     array.  TAIL-SIZE should be 0 when VTABLE doesn't specify a tail
     array.

     The optional INIT... arguments are initial values for the fields
     of the structure (and the tail array).  This is the only way to
     put values in read-only fields.  If there are fewer INIT arguments
     than fields then the defaults are `#f' for a Scheme field (type
     `p') or 0 for an uninterpreted field (type `u').

     Type `s' self-reference fields, permission `o' opaque fields, and
     the count field of a tail array are all ignored for the INIT
     arguments, ie. an argument is not consumed by such a field.  An
     `s' is always set to the structure itself, an `o' is always set to
     `#f' or 0 (with the intention that C code will do something to it
     later), and the tail count is always the given TAIL-SIZE.

     For example,

          (define v (make-vtable "prpwpw"))
          (define s (make-struct v 0 123 "abc" 456))
          (struct-ref s 0) => 123
          (struct-ref s 1) => "abc"

          (define v (make-vtable "prpW"))
          (define s (make-struct v 6 "fixed field" 'x 'y))
          (struct-ref s 0) => "fixed field"
          (struct-ref s 1) => 2    ;; tail size
          (struct-ref s 2) => x    ;; tail array ...
          (struct-ref s 3) => y
          (struct-ref s 4) => #f

 -- Scheme Procedure: struct? obj
 -- C Function: scm_struct_p (obj)
     Return `#t' if OBJ is a structure, or `#f' if not.

 -- Scheme Procedure: struct-ref struct n
 -- C Function: scm_struct_ref (struct, n)
     Return the contents of field number N in STRUCT.  The first field
     is number 0.

     An error is thrown if N is out of range, or if the field cannot be
     read because it's `o' opaque.

 -- Scheme Procedure: struct-set! struct n value
 -- C Function: scm_struct_set_x (struct, n, value)
     Set field number N in STRUCT to VALUE.  The first field is number
     0.

     An error is thrown if N is out of range, or if the field cannot be
     written because it's `r' read-only or `o' opaque.

 -- Scheme Procedure: struct-vtable struct
 -- C Function: scm_struct_vtable (struct)
     Return the vtable used by STRUCT.

     This can be used to examine the layout of an unknown structure, see
     *note Vtable Contents::.

6.7.9.3 Vtable Contents
.......................

A vtable is itself a structure, with particular fields that hold
information about the structures to be created.  These include the
fields of those structures, and the print function for them.  The
variables below allow access to those fields.

 -- Scheme Procedure: struct-vtable? obj
 -- C Function: scm_struct_vtable_p (obj)
     Return `#t' if OBJ is a vtable structure.

     Note that because vtables are simply structures with a particular
     layout, `struct-vtable?' can potentially return true on an
     application structure which merely happens to look like a vtable.

 -- Scheme Variable: vtable-index-layout
 -- C Macro: scm_vtable_index_layout
     The field number of the layout specification in a vtable.  The
     layout specification is a symbol like `pwpw' formed from the fields
     string passed to `make-vtable', or created by `make-struct-layout'
     (*note Vtable Vtables::).

          (define v (make-vtable "pwpw" 0))
          (struct-ref v vtable-index-layout) => pwpw

     This field is read-only, since the layout of structures using a
     vtable cannot be changed.

 -- Scheme Variable: vtable-index-vtable
 -- C Macro: scm_vtable_index_vtable
     A self-reference to the vtable, ie. a type `s' field.  This is
     used by C code within Guile and has no use at the Scheme level.

 -- Scheme Variable: vtable-index-printer
 -- C Macro: scm_vtable_index_printer
     The field number of the printer function.  This field contains `#f'
     if the default print function should be used.

          (define (my-print-func struct port)
            ...)
          (define v (make-vtable "pwpw" my-print-func))
          (struct-ref v vtable-index-printer) => my-print-func

     This field is writable, allowing the print function to be changed
     dynamically.

 -- Scheme Procedure: struct-vtable-name vtable
 -- Scheme Procedure: set-struct-vtable-name! vtable name
 -- C Function: scm_struct_vtable_name (vtable)
 -- C Function: scm_set_struct_vtable_name_x (vtable, name)
     Get or set the name of VTABLE.  NAME is a symbol and is used in
     the default print function when printing structures created from
     VTABLE.

          (define v (make-vtable "pw"))
          (set-struct-vtable-name! v 'my-name)

          (define s (make-struct v 0))
          (display s) -| #<my-name b7ab3ae0:b7ab3730>

 -- Scheme Procedure: struct-vtable-tag vtable
 -- C Function: scm_struct_vtable_tag (vtable)
     Return the tag of the given VTABLE.

6.7.9.4 Vtable Vtables
......................

As noted above, a vtable is a structure and that structure is itself
described by a vtable.  Such a "vtable of a vtable" can be created with
`make-vtable-vtable' below.  This can be used to build sets of related
vtables, possibly with extra application fields.

   This second level of vtable can be a little confusing.  The ball
example below is a typical use, adding a "class data" field to the
vtables, from which instance structures are created.  The current
implementation of Guile's own records (*note Records::) does something
similar, a record type descriptor is a vtable with room to hold the
field names of the records to be created from it.

 -- Scheme Procedure: make-vtable-vtable user-fields tail-size [print]
 -- C Function: scm_make_vtable_vtable (user_fields, tail_size,
          print_and_init_list)
     Create a "vtable-vtable" which can be used to create vtables.  This
     vtable-vtable is also a vtable, and is self-describing, meaning its
     vtable is itself.  The following is a simple usage.

          (define vt-vt (make-vtable-vtable "" 0))
          (define vt    (make-struct vt-vt 0
                                     (make-struct-layout "pwpw"))
          (define s     (make-struct vt 0 123 456))

          (struct-ref s 0) => 123

     `make-struct' is used to create a vtable from the vtable-vtable.
     The first initializer is a layout object (field
     `vtable-index-layout'), usually obtained from `make-struct-layout'
     (below).  An optional second initializer is a printer function
     (field `vtable-index-printer'), used as described under
     `make-vtable' (*note Vtables::).


     USER-FIELDS is a layout string giving extra fields to have in the
     vtables.  A vtable starts with some base fields as per *note
     Vtable Contents::, and USER-FIELDS is appended.  The USER-FIELDS
     start at field number `vtable-offset-user' (below), and exist in
     both the vtable-vtable and in the vtables created from it.  Such
     fields provide space for "class data".  For example,

          (define vt-of-vt (make-vtable-vtable "pw" 0))
          (define vt       (make-struct vt-of-vt 0))
          (struct-set! vt vtable-offset-user "my class data")

     TAIL-SIZE is the size of the tail array in the vtable-vtable
     itself, if USER-FIELDS specifies a tail array.  This should be 0
     if nothing extra is required or the format has no tail array.  The
     tail array field such as `pW' holds the tail array size, as usual,
     and is followed by the extra space.

          (define vt-vt (make-vtable-vtable "pW" 20))
          (define my-vt-tail-start (1+ vtable-offset-user))
          (struct-set! vt-vt (+ 3 my-vt-tail-start) "data in tail")

     The optional PRINT argument is used by `display' and `write' (etc)
     to print the vtable-vtable and any vtables created from it.  It's
     called as `(PRINT vtable port)' and should look at VTABLE and
     write to PORT.  The default is the usual structure print function,
     which just gives machine addresses.

 -- Scheme Procedure: make-struct-layout fields
 -- C Function: scm_make_struct_layout (fields)
     Return a structure layout symbol, from a FIELDS string.  FIELDS is
     as described under `make-vtable' (*note Vtables::).  An invalid
     FIELDS string is an error.

          (make-struct-layout "prpW") => prpW
          (make-struct-layout "blah") => ERROR

 -- Scheme Variable: vtable-offset-user
 -- C Macro: scm_vtable_offset_user
     The first field in a vtable which is available for application use.
     Such fields only exist when specified by USER-FIELDS in
     `make-vtable-vtable' above.


   Here's an extended vtable-vtable example, creating classes of
"balls".  Each class has a "colour", which is fixed.  Instances of
those classes are created, and such each such ball has an "owner",
which can be changed.

     (define ball-root (make-vtable-vtable "pr" 0))

     (define (make-ball-type ball-color)
       (make-struct ball-root 0
     	       (make-struct-layout "pw")
                    (lambda (ball port)
                      (format port "#<a ~A ball owned by ~A>"
                              (color ball)
                              (owner ball)))
                    ball-color))
     (define (color ball)
       (struct-ref (struct-vtable ball) vtable-offset-user))
     (define (owner ball)
       (struct-ref ball 0))

     (define red (make-ball-type 'red))
     (define green (make-ball-type 'green))

     (define (make-ball type owner) (make-struct type 0 owner))

     (define ball (make-ball green 'Nisse))
     ball => #<a green ball owned by Nisse>

6.7.10 Dictionary Types
-----------------------

A "dictionary" object is a data structure used to index information in
a user-defined way.  In standard Scheme, the main aggregate data types
are lists and vectors.  Lists are not really indexed at all, and
vectors are indexed only by number (e.g. `(vector-ref foo 5)').  Often
you will find it useful to index your data on some other type; for
example, in a library catalog you might want to look up a book by the
name of its author.  Dictionaries are used to help you organize
information in such a way.

   An "association list" (or "alist" for short) is a list of key-value
pairs.  Each pair represents a single quantity or object; the `car' of
the pair is a key which is used to identify the object, and the `cdr'
is the object's value.

   A "hash table" also permits you to index objects with arbitrary
keys, but in a way that makes looking up any one object extremely fast.
A well-designed hash system makes hash table lookups almost as fast as
conventional array or vector references.

   Alists are popular among Lisp programmers because they use only the
language's primitive operations (lists, "car", "cdr" and the equality
primitives).  No changes to the language core are necessary.
Therefore, with Scheme's built-in list manipulation facilities, it is
very convenient to handle data stored in an association list.  Also,
alists are highly portable and can be easily implemented on even the
most minimal Lisp systems.

   However, alists are inefficient, especially for storing large
quantities of data.  Because we want Guile to be useful for large
software systems as well as small ones, Guile provides a rich set of
tools for using either association lists or hash tables.

6.7.11 Association Lists
------------------------

An association list is a conventional data structure that is often used
to implement simple key-value databases.  It consists of a list of
entries in which each entry is a pair.  The "key" of each entry is the
`car' of the pair and the "value" of each entry is the `cdr'.

     ASSOCIATION LIST ::=  '( (KEY1 . VALUE1)
                              (KEY2 . VALUE2)
                              (KEY3 . VALUE3)
                              ...
                            )

Association lists are also known, for short, as "alists".

   The structure of an association list is just one example of the
infinite number of possible structures that can be built using pairs
and lists.  As such, the keys and values in an association list can be
manipulated using the general list structure procedures `cons', `car',
`cdr', `set-car!', `set-cdr!' and so on.  However, because association
lists are so useful, Guile also provides specific procedures for
manipulating them.

6.7.11.1 Alist Key Equality
...........................

All of Guile's dedicated association list procedures, apart from
`acons', come in three flavours, depending on the level of equality
that is required to decide whether an existing key in the association
list is the same as the key that the procedure call uses to identify the
required entry.

   * Procedures with "assq" in their name use `eq?' to determine key
     equality.

   * Procedures with "assv" in their name use `eqv?' to determine key
     equality.

   * Procedures with "assoc" in their name use `equal?' to determine
     key equality.

   `acons' is an exception because it is used to build association
lists which do not require their entries' keys to be unique.

6.7.11.2 Adding or Setting Alist Entries
........................................

`acons' adds a new entry to an association list and returns the
combined association list.  The combined alist is formed by consing the
new entry onto the head of the alist specified in the `acons' procedure
call.  So the specified alist is not modified, but its contents become
shared with the tail of the combined alist that `acons' returns.

   In the most common usage of `acons', a variable holding the original
association list is updated with the combined alist:

     (set! address-list (acons name address address-list))

   In such cases, it doesn't matter that the old and new values of
`address-list' share some of their contents, since the old value is
usually no longer independently accessible.

   Note that `acons' adds the specified new entry regardless of whether
the alist may already contain entries with keys that are, in some
sense, the same as that of the new entry.  Thus `acons' is ideal for
building alists where there is no concept of key uniqueness.

     (set! task-list (acons 3 "pay gas bill" '()))
     task-list
     =>
     ((3 . "pay gas bill"))

     (set! task-list (acons 3 "tidy bedroom" task-list))
     task-list
     =>
     ((3 . "tidy bedroom") (3 . "pay gas bill"))

   `assq-set!', `assv-set!' and `assoc-set!' are used to add or replace
an entry in an association list where there _is_ a concept of key
uniqueness.  If the specified association list already contains an
entry whose key is the same as that specified in the procedure call,
the existing entry is replaced by the new one.  Otherwise, the new
entry is consed onto the head of the old association list to create the
combined alist.  In all cases, these procedures return the combined
alist.

   `assq-set!' and friends _may_ destructively modify the structure of
the old association list in such a way that an existing variable is
correctly updated without having to `set!' it to the value returned:

     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "16 Bow Street"))

     (assoc-set! address-list "james" "1a London Road")
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

   Or they may not:

     (assoc-set! address-list "bob" "11 Newington Avenue")
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))

     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

   The only safe way to update an association list variable when adding
or replacing an entry like this is to `set!' the variable to the
returned value:

     (set! address-list
           (assoc-set! address-list "bob" "11 Newington Avenue"))
     address-list
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))

   Because of this slight inconvenience, you may find it more
convenient to use hash tables to store dictionary data.  If your
application will not be modifying the contents of an alist very often,
this may not make much difference to you.

   If you need to keep the old value of an association list in a form
independent from the list that results from modification by `acons',
`assq-set!', `assv-set!' or `assoc-set!', use `list-copy' to copy the
old association list before modifying it.

 -- Scheme Procedure: acons key value alist
 -- C Function: scm_acons (key, value, alist)
     Add a new key-value pair to ALIST.  A new pair is created whose
     car is KEY and whose cdr is VALUE, and the pair is consed onto
     ALIST, and the new list is returned.  This function is _not_
     destructive; ALIST is not modified.

 -- Scheme Procedure: assq-set! alist key val
 -- Scheme Procedure: assv-set! alist key value
 -- Scheme Procedure: assoc-set! alist key value
 -- C Function: scm_assq_set_x (alist, key, val)
 -- C Function: scm_assv_set_x (alist, key, val)
 -- C Function: scm_assoc_set_x (alist, key, val)
     Reassociate KEY in ALIST with VALUE: find any existing ALIST entry
     for KEY and associate it with the new VALUE.  If ALIST does not
     contain an entry for KEY, add a new one.  Return the (possibly
     new) alist.

     These functions do not attempt to verify the structure of ALIST,
     and so may cause unusual results if passed an object that is not an
     association list.

6.7.11.3 Retrieving Alist Entries
.................................

`assq', `assv' and `assoc' find the entry in an alist for a given key,
and return the `(KEY . VALUE)' pair.  `assq-ref', `assv-ref' and
`assoc-ref' do a similar lookup, but return just the VALUE.

 -- Scheme Procedure: assq key alist
 -- Scheme Procedure: assv key alist
 -- Scheme Procedure: assoc key alist
 -- C Function: scm_assq (key, alist)
 -- C Function: scm_assv (key, alist)
 -- C Function: scm_assoc (key, alist)
     Return the first entry in ALIST with the given KEY.  The return is
     the pair `(KEY . VALUE)' from ALIST.  If there's no matching entry
     the return is `#f'.

     `assq' compares keys with `eq?', `assv' uses `eqv?' and `assoc'
     uses `equal?'.  See also SRFI-1 which has an extended `assoc'
     (*note SRFI-1 Association Lists::).

 -- Scheme Procedure: assq-ref alist key
 -- Scheme Procedure: assv-ref alist key
 -- Scheme Procedure: assoc-ref alist key
 -- C Function: scm_assq_ref (alist, key)
 -- C Function: scm_assv_ref (alist, key)
 -- C Function: scm_assoc_ref (alist, key)
     Return the value from the first entry in ALIST with the given KEY,
     or `#f' if there's no such entry.

     `assq-ref' compares keys with `eq?', `assv-ref' uses `eqv?' and
     `assoc-ref' uses `equal?'.

     Notice these functions have the KEY argument last, like other
     `-ref' functions, but this is opposite to what `assq' etc above
     use.

     When the return is `#f' it can be either KEY not found, or an
     entry which happens to have value `#f' in the `cdr'.  Use `assq'
     etc above if you need to differentiate these cases.

6.7.11.4 Removing Alist Entries
...............................

To remove the element from an association list whose key matches a
specified key, use `assq-remove!', `assv-remove!' or `assoc-remove!'
(depending, as usual, on the level of equality required between the key
that you specify and the keys in the association list).

   As with `assq-set!' and friends, the specified alist may or may not
be modified destructively, and the only safe way to update a variable
containing the alist is to `set!' it to the value that `assq-remove!'
and friends return.

     address-list
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))

     (set! address-list (assoc-remove! address-list "mary"))
     address-list
     =>
     (("bob" . "11 Newington Avenue") ("james" . "1a London Road"))

   Note that, when `assq/v/oc-remove!' is used to modify an association
list that has been constructed only using the corresponding
`assq/v/oc-set!', there can be at most one matching entry in the alist,
so the question of multiple entries being removed in one go does not
arise.  If `assq/v/oc-remove!' is applied to an association list that
has been constructed using `acons', or an `assq/v/oc-set!' with a
different level of equality, or any mixture of these, it removes only
the first matching entry from the alist, even if the alist might
contain further matching entries.  For example:

     (define address-list '())
     (set! address-list (assq-set! address-list "mary" "11 Elm Street"))
     (set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
     address-list
     =>
     (("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))

     (set! address-list (assoc-remove! address-list "mary"))
     address-list
     =>
     (("mary" . "11 Elm Street"))

   In this example, the two instances of the string "mary" are not the
same when compared using `eq?', so the two `assq-set!' calls add two
distinct entries to `address-list'.  When compared using `equal?', both
"mary"s in `address-list' are the same as the "mary" in the
`assoc-remove!' call, but `assoc-remove!' stops after removing the
first matching entry that it finds, and so one of the "mary" entries is
left in place.

 -- Scheme Procedure: assq-remove! alist key
 -- Scheme Procedure: assv-remove! alist key
 -- Scheme Procedure: assoc-remove! alist key
 -- C Function: scm_assq_remove_x (alist, key)
 -- C Function: scm_assv_remove_x (alist, key)
 -- C Function: scm_assoc_remove_x (alist, key)
     Delete the first entry in ALIST associated with KEY, and return
     the resulting alist.

6.7.11.5 Sloppy Alist Functions
...............................

`sloppy-assq', `sloppy-assv' and `sloppy-assoc' behave like the
corresponding non-`sloppy-' procedures, except that they return `#f'
when the specified association list is not well-formed, where the
non-`sloppy-' versions would signal an error.

   Specifically, there are two conditions for which the non-`sloppy-'
procedures signal an error, which the `sloppy-' procedures handle
instead by returning `#f'.  Firstly, if the specified alist as a whole
is not a proper list:

     (assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
     =>
     ERROR: In procedure assoc in expression (assoc "mary" (quote #)):
     ERROR: Wrong type argument in position 2 (expecting
        association list): ((1 . 2) ("key" . "door") . "open sesame")

     (sloppy-assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
     =>
     #f

Secondly, if one of the entries in the specified alist is not a pair:

     (assoc 2 '((1 . 1) 2 (3 . 9)))
     =>
     ERROR: In procedure assoc in expression (assoc 2 (quote #)):
     ERROR: Wrong type argument in position 2 (expecting
        association list): ((1 . 1) 2 (3 . 9))

     (sloppy-assoc 2 '((1 . 1) 2 (3 . 9)))
     =>
     #f

   Unless you are explicitly working with badly formed association
lists, it is much safer to use the non-`sloppy-' procedures, because
they help to highlight coding and data errors that the `sloppy-'
versions would silently cover up.

 -- Scheme Procedure: sloppy-assq key alist
 -- C Function: scm_sloppy_assq (key, alist)
     Behaves like `assq' but does not do any error checking.
     Recommended only for use in Guile internals.

 -- Scheme Procedure: sloppy-assv key alist
 -- C Function: scm_sloppy_assv (key, alist)
     Behaves like `assv' but does not do any error checking.
     Recommended only for use in Guile internals.

 -- Scheme Procedure: sloppy-assoc key alist
 -- C Function: scm_sloppy_assoc (key, alist)
     Behaves like `assoc' but does not do any error checking.
     Recommended only for use in Guile internals.

6.7.11.6 Alist Example
......................

Here is a longer example of how alists may be used in practice.

     (define capitals '(("New York" . "Albany")
                        ("Oregon"   . "Salem")
                        ("Florida"  . "Miami")))

     ;; What's the capital of Oregon?
     (assoc "Oregon" capitals)       => ("Oregon" . "Salem")
     (assoc-ref capitals "Oregon")   => "Salem"

     ;; We left out South Dakota.
     (set! capitals
           (assoc-set! capitals "South Dakota" "Pierre"))
     capitals
     => (("South Dakota" . "Pierre")
         ("New York" . "Albany")
         ("Oregon" . "Salem")
         ("Florida" . "Miami"))

     ;; And we got Florida wrong.
     (set! capitals
           (assoc-set! capitals "Florida" "Tallahassee"))
     capitals
     => (("South Dakota" . "Pierre")
         ("New York" . "Albany")
         ("Oregon" . "Salem")
         ("Florida" . "Tallahassee"))

     ;; After Oregon secedes, we can remove it.
     (set! capitals
           (assoc-remove! capitals "Oregon"))
     capitals
     => (("South Dakota" . "Pierre")
         ("New York" . "Albany")
         ("Florida" . "Tallahassee"))

6.7.12 VList-Based Hash Lists or "VHashes"
------------------------------------------

The `(ice-9 vlist)' module provides an implementation of "VList-based
hash lists" (*note VLists::).  VList-based hash lists, or "vhashes",
are an immutable dictionary type similar to association lists that maps
"keys" to "values".  However, unlike association lists, accessing a
value given its key is typically a constant-time operation.

   The VHash programming interface of `(ice-9 vlist)' is mostly the
same as that of association lists found in SRFI-1, with procedure names
prefixed by `vhash-' instead of `alist-' (*note SRFI-1 Association
Lists::).

   In addition, vhashes can be manipulated using VList operations:

     (vlist-head (vhash-consq 'a 1 vlist-null))
     => (a . 1)

     (define vh1 (vhash-consq 'b 2 (vhash-consq 'a 1 vlist-null)))
     (define vh2 (vhash-consq 'c 3 (vlist-tail vh1)))

     (vhash-assq 'a vh2)
     => (a . 1)
     (vhash-assq 'b vh2)
     => #f
     (vhash-assq 'c vh2)
     => (c . 3)
     (vlist->list vh2)
     => ((c . 3) (a . 1))

   However, keep in mind that procedures that construct new VLists
(`vlist-map', `vlist-filter', etc.) return raw VLists, not vhashes:

     (define vh (alist->vhash '((a . 1) (b . 2) (c . 3)) hashq))
     (vhash-assq 'a vh)
     => (a . 1)

     (define vl
       ;; This will create a raw vlist.
       (vlist-filter (lambda (key+value) (odd? (cdr key+value))) vh))
     (vhash-assq 'a vl)
     => ERROR: Wrong type argument in position 2

     (vlist->list vl)
     => ((a . 1) (c . 3))

 -- Scheme Procedure: vhash? obj
     Return true if OBJ is a vhash.

 -- Scheme Procedure: vhash-cons key value vhash [hash-proc]
 -- Scheme Procedure: vhash-consq key value vhash
 -- Scheme Procedure: vhash-consv key value vhash
     Return a new hash list based on VHASH where KEY is associated with
     VALUE, using HASH-PROC to compute the hash of KEY.  VHASH must be
     either `vlist-null' or a vhash returned by a previous call to
     `vhash-cons'.  HASH-PROC defaults to `hash' (*note `hash'
     procedure: Hash Table Reference.).  With `vhash-consq', the
     `hashq' hash function is used; with `vhash-consv' the `hashv' hash
     function is used.

     All `vhash-cons' calls made to construct a vhash should use the
     same HASH-PROC.  Failing to do that, the result is undefined.

 -- Scheme Procedure: vhash-assoc key vhash [equal? [hash-proc]]
 -- Scheme Procedure: vhash-assq key vhash
 -- Scheme Procedure: vhash-assv key vhash
     Return the first key/value pair from VHASH whose key is equal to
     KEY according to the EQUAL? equality predicate (which defaults to
     `equal?'), and using HASH-PROC (which defaults to `hash') to
     compute the hash of KEY.  The second form uses `eq?' as the
     equality predicate and `hashq' as the hash function; the last form
     uses `eqv?' and `hashv'.

     Note that it is important to consistently use the same hash
     function for HASH-PROC as was passed to `vhash-cons'.  Failing to
     do that, the result is unpredictable.

 -- Scheme Procedure: vhash-delete key vhash [equal? [hash-proc]]
 -- Scheme Procedure: vhash-delq key vhash
 -- Scheme Procedure: vhash-delv key vhash
     Remove all associations from VHASH with KEY, comparing keys with
     EQUAL? (which defaults to `equal?'), and computing the hash of KEY
     using HASH-PROC (which defaults to `hash').  The second form uses
     `eq?' as the equality predicate and `hashq' as the hash function;
     the last one uses `eqv?' and `hashv'.

     Again the choice of HASH-PROC must be consistent with previous
     calls to `vhash-cons'.

 -- Scheme Procedure: vhash-fold proc init vhash
 -- Scheme Procedure: vhash-fold-right proc init vhash
     Fold over the key/value elements of VHASH in the given direction,
     with each call to PROC having the form `(PROC key value result)',
     where RESULT is the result of the previous call to PROC and INIT
     the value of RESULT for the first call to PROC.

 -- Scheme Procedure: vhash-fold* proc init key vhash [equal? [hash]]
 -- Scheme Procedure: vhash-foldq* proc init key vhash
 -- Scheme Procedure: vhash-foldv* proc init key vhash
     Fold over all the values associated with KEY in VHASH, with each
     call to PROC having the form `(proc value result)', where RESULT
     is the result of the previous call to PROC and INIT the value of
     RESULT for the first call to PROC.

     Keys in VHASH are hashed using HASH are compared using EQUAL?.
     The second form uses `eq?' as the equality predicate and `hashq' as
     the hash function; the third one uses `eqv?' and `hashv'.

     Example:

          (define vh
            (alist->vhash '((a . 1) (a . 2) (z . 0) (a . 3))))

          (vhash-fold* cons '() 'a vh)
          => (3 2 1)

          (vhash-fold* cons '() 'z vh)
          => (0)

 -- Scheme Procedure: alist->vhash alist [hash-proc]
     Return the vhash corresponding to ALIST, an association list, using
     HASH-PROC to compute key hashes.  When omitted, HASH-PROC defaults
     to `hash'.

6.7.13 Hash Tables
------------------

Hash tables are dictionaries which offer similar functionality as
association lists: They provide a mapping from keys to values.  The
difference is that association lists need time linear in the size of
elements when searching for entries, whereas hash tables can normally
search in constant time.  The drawback is that hash tables require a
little bit more memory, and that you can not use the normal list
procedures (*note Lists::) for working with them.

   Guile provides two types of hashtables.  One is an abstract data type
that can only be manipulated with the functions in this section.  The
other type is concrete: it uses a normal vector with alists as
elements.  The advantage of the abstract hash tables is that they will
be automatically resized when they become too full or too empty.

6.7.13.1 Hash Table Examples
............................

For demonstration purposes, this section gives a few usage examples of
some hash table procedures, together with some explanation what they do.

   First we start by creating a new hash table with 31 slots, and
populate it with two key/value pairs.

     (define h (make-hash-table 31))

     ;; This is an opaque object
     h
     =>
     #<hash-table 0/31>

     ;; We can also use a vector of alists.
     (define h (make-vector 7 '()))

     h
     =>
     #(() () () () () () ())

     ;; Inserting into a hash table can be done with hashq-set!
     (hashq-set! h 'foo "bar")
     =>
     "bar"

     (hashq-set! h 'braz "zonk")
     =>
     "zonk"

     ;; Or with hash-create-handle!
     (hashq-create-handle! h 'frob #f)
     =>
     (frob . #f)

     ;; The vector now contains three elements in the alists and the frob
     ;; entry is at index (hashq 'frob).
     h
     =>
     #(((braz . "zonk")) ((foo . "bar")) () () () () ((frob . #f)))

     (hashq 'frob 7)
     =>
     6

   You can get the value for a given key with the procedure
`hashq-ref', but the problem with this procedure is that you cannot
reliably determine whether a key does exists in the table.  The reason
is that the procedure returns `#f' if the key is not in the table, but
it will return the same value if the key is in the table and just
happens to have the value `#f', as you can see in the following
examples.

     (hashq-ref h 'foo)
     =>
     "bar"

     (hashq-ref h 'frob)
     =>
     #f

     (hashq-ref h 'not-there)
     =>
     #f

   Better is to use the procedure `hashq-get-handle', which makes a
distinction between the two cases.  Just like `assq', this procedure
returns a key/value-pair on success, and `#f' if the key is not found.

     (hashq-get-handle h 'foo)
     =>
     (foo . "bar")

     (hashq-get-handle h 'not-there)
     =>
     #f

   There is no procedure for calculating the number of key/value-pairs
in a hash table, but `hash-fold' can be used for doing exactly that.

     (hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
     =>
     3

6.7.13.2 Hash Table Reference
.............................

Like the association list functions, the hash table functions come in
several varieties, according to the equality test used for the keys.
Plain `hash-' functions use `equal?', `hashq-' functions use `eq?',
`hashv-' functions use `eqv?', and the `hashx-' functions use an
application supplied test.

   A single `make-hash-table' creates a hash table suitable for use
with any set of functions, but it's imperative that just one set is
then used consistently, or results will be unpredictable.

   Hash tables are implemented as a vector indexed by a hash value
formed from the key, with an association list of key/value pairs for
each bucket in case distinct keys hash together.  Direct access to the
pairs in those lists is provided by the `-handle-' functions.  The
abstract kind of hash tables hide the vector in an opaque object that
represents the hash table, while for the concrete kind the vector _is_
the hashtable.

   When the number of table entries in an abstract hash table goes above
a threshold, the vector is made larger and the entries are rehashed, to
prevent the bucket lists from becoming too long and slowing down
accesses.  When the number of entries goes below a threshold, the
vector is shrunk to save space.

   A abstract hash table is created with `make-hash-table'.  To create
a vector that is suitable as a hash table, use `(make-vector SIZE
'())', for example.

   For the `hashx-' "extended" routines, an application supplies a HASH
function producing an integer index like `hashq' etc below, and an
ASSOC alist search function like `assq' etc (*note Retrieving Alist
Entries::).  Here's an example of such functions implementing
case-insensitive hashing of string keys,

     (use-modules (srfi srfi-1)
                  (srfi srfi-13))

     (define (my-hash str size)
       (remainder (string-hash-ci str) size))
     (define (my-assoc str alist)
       (find (lambda (pair) (string-ci=? str (car pair))) alist))

     (define my-table (make-hash-table))
     (hashx-set! my-hash my-assoc my-table "foo" 123)

     (hashx-ref my-hash my-assoc my-table "FOO")
     => 123

   In a `hashx-' HASH function the aim is to spread keys across the
vector, so bucket lists don't become long.  But the actual values are
arbitrary as long as they're in the range 0 to SIZE-1.  Helpful
functions for forming a hash value, in addition to `hashq' etc below,
include `symbol-hash' (*note Symbol Keys::), `string-hash' and
`string-hash-ci' (*note String Comparison::), and `char-set-hash'
(*note Character Set Predicates/Comparison::).


 -- Scheme Procedure: make-hash-table [size]
     Create a new abstract hash table object, with an optional minimum
     vector SIZE.

     When SIZE is given, the table vector will still grow and shrink
     automatically, as described above, but with SIZE as a minimum.  If
     an application knows roughly how many entries the table will hold
     then it can use SIZE to avoid rehashing when initial entries are
     added.

 -- Scheme Procedure: hash-table? obj
 -- C Function: scm_hash_table_p (obj)
     Return `#t' if OBJ is a abstract hash table object.

 -- Scheme Procedure: hash-clear! table
 -- C Function: scm_hash_clear_x (table)
     Remove all items from TABLE (without triggering a resize).

 -- Scheme Procedure: hash-ref table key [dflt]
 -- Scheme Procedure: hashq-ref table key [dflt]
 -- Scheme Procedure: hashv-ref table key [dflt]
 -- Scheme Procedure: hashx-ref hash assoc table key [dflt]
 -- C Function: scm_hash_ref (table, key, dflt)
 -- C Function: scm_hashq_ref (table, key, dflt)
 -- C Function: scm_hashv_ref (table, key, dflt)
 -- C Function: scm_hashx_ref (hash, assoc, table, key, dflt)
     Lookup KEY in the given hash TABLE, and return the associated
     value.  If KEY is not found, return DFLT, or `#f' if DFLT is not
     given.

 -- Scheme Procedure: hash-set! table key val
 -- Scheme Procedure: hashq-set! table key val
 -- Scheme Procedure: hashv-set! table key val
 -- Scheme Procedure: hashx-set! hash assoc table key val
 -- C Function: scm_hash_set_x (table, key, val)
 -- C Function: scm_hashq_set_x (table, key, val)
 -- C Function: scm_hashv_set_x (table, key, val)
 -- C Function: scm_hashx_set_x (hash, assoc, table, key, val)
     Associate VAL with KEY in the given hash TABLE.  If KEY is already
     present then it's associated value is changed.  If it's not
     present then a new entry is created.

 -- Scheme Procedure: hash-remove! table key
 -- Scheme Procedure: hashq-remove! table key
 -- Scheme Procedure: hashv-remove! table key
 -- Scheme Procedure: hashx-remove! hash assoc table key
 -- C Function: scm_hash_remove_x (table, key)
 -- C Function: scm_hashq_remove_x (table, key)
 -- C Function: scm_hashv_remove_x (table, key)
 -- C Function: scm_hashx_remove_x (hash, assoc, table, key)
     Remove any association for KEY in the given hash TABLE.  If KEY is
     not in TABLE then nothing is done.

 -- Scheme Procedure: hash key size
 -- Scheme Procedure: hashq key size
 -- Scheme Procedure: hashv key size
 -- C Function: scm_hash (key, size)
 -- C Function: scm_hashq (key, size)
 -- C Function: scm_hashv (key, size)
     Return a hash value for KEY.  This is a number in the range 0 to
     SIZE-1, which is suitable for use in a hash table of the given
     SIZE.

     Note that `hashq' and `hashv' may use internal addresses of
     objects, so if an object is garbage collected and re-created it can
     have a different hash value, even when the two are notionally
     `eq?'.  For instance with symbols,

          (hashq 'something 123)   => 19
          (gc)
          (hashq 'something 123)   => 62

     In normal use this is not a problem, since an object entered into a
     hash table won't be garbage collected until removed.  It's only if
     hashing calculations are somehow separated from normal references
     that its lifetime needs to be considered.

 -- Scheme Procedure: hash-get-handle table key
 -- Scheme Procedure: hashq-get-handle table key
 -- Scheme Procedure: hashv-get-handle table key
 -- Scheme Procedure: hashx-get-handle hash assoc table key
 -- C Function: scm_hash_get_handle (table, key)
 -- C Function: scm_hashq_get_handle (table, key)
 -- C Function: scm_hashv_get_handle (table, key)
 -- C Function: scm_hashx_get_handle (hash, assoc, table, key)
     Return the `(KEY . VALUE)' pair for KEY in the given hash TABLE,
     or `#f' if KEY is not in TABLE.

 -- Scheme Procedure: hash-create-handle! table key init
 -- Scheme Procedure: hashq-create-handle! table key init
 -- Scheme Procedure: hashv-create-handle! table key init
 -- Scheme Procedure: hashx-create-handle! hash assoc table key init
 -- C Function: scm_hash_create_handle_x (table, key, init)
 -- C Function: scm_hashq_create_handle_x (table, key, init)
 -- C Function: scm_hashv_create_handle_x (table, key, init)
 -- C Function: scm_hashx_create_handle_x (hash, assoc, table, key,
          init)
     Return the `(KEY . VALUE)' pair for KEY in the given hash TABLE.
     If KEY is not in TABLE then create an entry for it with INIT as
     the value, and return that pair.

 -- Scheme Procedure: hash-map->list proc table
 -- Scheme Procedure: hash-for-each proc table
 -- C Function: scm_hash_map_to_list (proc, table)
 -- C Function: scm_hash_for_each (proc, table)
     Apply PROC to the entries in the given hash TABLE.  Each call is
     `(PROC KEY VALUE)'.  `hash-map->list' returns a list of the
     results from these calls, `hash-for-each' discards the results and
     returns an unspecified value.

     Calls are made over the table entries in an unspecified order, and
     for `hash-map->list' the order of the values in the returned list
     is unspecified.  Results will be unpredictable if TABLE is modified
     while iterating.

     For example the following returns a new alist comprising all the
     entries from `mytable', in no particular order.

          (hash-map->list cons mytable)

 -- Scheme Procedure: hash-for-each-handle proc table
 -- C Function: scm_hash_for_each_handle (proc, table)
     Apply PROC to the entries in the given hash TABLE.  Each call is
     `(PROC HANDLE)', where HANDLE is a `(KEY . VALUE)' pair. Return an
     unspecified value.

     `hash-for-each-handle' differs from `hash-for-each' only in the
     argument list of PROC.

 -- Scheme Procedure: hash-fold proc init table
 -- C Function: scm_hash_fold (proc, init, table)
     Accumulate a result by applying PROC to the elements of the given
     hash TABLE.  Each call is `(PROC KEY VALUE PRIOR-RESULT)', where
     KEY and VALUE are from the TABLE and PRIOR-RESULT is the return
     from the previous PROC call.  For the first call, PRIOR-RESULT is
     the given INIT value.

     Calls are made over the table entries in an unspecified order.
     Results will be unpredictable if TABLE is modified while
     `hash-fold' is running.

     For example, the following returns a count of how many keys in
     `mytable' are strings.

          (hash-fold (lambda (key value prior)
                       (if (string? key) (1+ prior) prior))
                     0 mytable)

6.8 Smobs
=========

This chapter contains reference information related to defining and
working with smobs.  See *note Defining New Types (Smobs):: for a
tutorial-like introduction to smobs.

 -- Function: scm_t_bits scm_make_smob_type (const char *name, size_t
          size)
     This function adds a new smob type, named NAME, with instance size
     SIZE, to the system.  The return value is a tag that is used in
     creating instances of the type.

     If SIZE is 0, the default _free_ function will do nothing.

     If SIZE is not 0, the default _free_ function will deallocate the
     memory block pointed to by `SCM_SMOB_DATA' with `scm_gc_free'.
     The WHAT parameter in the call to `scm_gc_free' will be NAME.

     Default values are provided for the _mark_, _free_, _print_, and
     _equalp_ functions, as described in *note Defining New Types
     (Smobs)::.  If you want to customize any of these functions, the
     call to `scm_make_smob_type' should be immediately followed by
     calls to one or several of `scm_set_smob_mark',
     `scm_set_smob_free', `scm_set_smob_print', and/or
     `scm_set_smob_equalp'.

 -- C Function: void scm_set_smob_free (scm_t_bits tc, size_t (*free)
          (SCM obj))
     This function sets the smob freeing procedure (sometimes referred
     to as a "finalizer") for the smob type specified by the tag TC. TC
     is the tag returned by `scm_make_smob_type'.

     The FREE procedure must deallocate all resources that are directly
     associated with the smob instance OBJ.  It must assume that all
     `SCM' values that it references have already been freed and are
     thus invalid.

     It must also not call any libguile function or macro except
     `scm_gc_free', `SCM_SMOB_FLAGS', `SCM_SMOB_DATA',
     `SCM_SMOB_DATA_2', and `SCM_SMOB_DATA_3'.

     The FREE procedure must return 0.

     Note that defining a freeing procedure is not necessary if the
     resources associated with OBJ consists only of memory allocated
     with `scm_gc_malloc' or `scm_gc_malloc_pointerless' because this
     memory is automatically reclaimed by the garbage collector when it
     is no longer needed (*note `scm_gc_malloc': Memory Blocks.).

 -- C Function: void scm_set_smob_mark (scm_t_bits tc, SCM (*mark) (SCM
          obj))
     This function sets the smob marking procedure for the smob type
     specified by the tag TC. TC is the tag returned by
     `scm_make_smob_type'.

     Defining a marking procedure may sometimes be unnecessary because
     large parts of the process' memory (with the exception of
     `scm_gc_malloc_pointerless' regions, and `malloc'- or
     `scm_malloc'-allocated memory) are scanned for live pointers(1).

     The MARK procedure must cause `scm_gc_mark' to be called for every
     `SCM' value that is directly referenced by the smob instance OBJ.
     One of these `SCM' values can be returned from the procedure and
     Guile will call `scm_gc_mark' for it.  This can be used to avoid
     deep recursions for smob instances that form a list.

     It must not call any libguile function or macro except
     `scm_gc_mark', `SCM_SMOB_FLAGS', `SCM_SMOB_DATA',
     `SCM_SMOB_DATA_2', and `SCM_SMOB_DATA_3'.

 -- C Function: void scm_set_smob_print (scm_t_bits tc, int (*print)
          (SCM obj, SCM port, scm_print_state* pstate))
     This function sets the smob printing procedure for the smob type
     specified by the tag TC. TC is the tag returned by
     `scm_make_smob_type'.

     The PRINT procedure should output a textual representation of the
     smob instance OBJ to PORT, using information in PSTATE.

     The textual representation should be of the form `#<name ...>'.
     This ensures that `read' will not interpret it as some other
     Scheme value.

     It is often best to ignore PSTATE and just print to PORT with
     `scm_display', `scm_write', `scm_simple_format', and `scm_puts'.

 -- C Function: void scm_set_smob_equalp (scm_t_bits tc, SCM (*equalp)
          (SCM obj1, SCM obj1))
     This function sets the smob equality-testing predicate for the smob
     type specified by the tag TC. TC is the tag returned by
     `scm_make_smob_type'.

     The EQUALP procedure should return `SCM_BOOL_T' when OBJ1 is
     `equal?' to OBJ2.  Else it should return SCM_BOOL_F.  Both OBJ1
     and OBJ2 are instances of the smob type TC.

 -- C Function: void scm_assert_smob_type (scm_t_bits tag, SCM val)
     When VAL is a smob of the type indicated by TAG, do nothing.
     Else, signal an error.

 -- C Macro: int SCM_SMOB_PREDICATE (scm_t_bits tag, SCM exp)
     Return true iff EXP is a smob instance of the type indicated by
     TAG.  The expression EXP can be evaluated more than once, so it
     shouldn't contain any side effects.

 -- C Macro: void SCM_NEWSMOB (SCM value, scm_t_bits tag, void *data)
 -- C Macro: void SCM_NEWSMOB2 (SCM value, scm_t_bits tag, void *data,
          void *data2)
 -- C Macro: void SCM_NEWSMOB3 (SCM value, scm_t_bits tag, void *data,
          void *data2, void *data3)
     Make VALUE contain a smob instance of the type with tag TAG and
     smob data DATA, DATA2, and DATA3, as appropriate.

     The TAG is what has been returned by `scm_make_smob_type'.  The
     initial values DATA, DATA2, and DATA3 are of type `scm_t_bits';
     when you want to use them for `SCM' values, these values need to
     be converted to a `scm_t_bits' first by using `SCM_UNPACK'.

     The flags of the smob instance start out as zero.

   Since it is often the case (e.g., in smob constructors) that you will
create a smob instance and return it, there is also a slightly
specialized macro for this situation:

 -- C Macro:  SCM_RETURN_NEWSMOB (scm_t_bits tag, void *data)
 -- C Macro:  SCM_RETURN_NEWSMOB2 (scm_t_bits tag, void *data1, void
          *data2)
 -- C Macro:  SCM_RETURN_NEWSMOB3 (scm_t_bits tag, void *data1, void
          *data2, void *data3)
     This macro expands to a block of code that creates a smob instance
     of the type with tag TAG and smob data DATA, DATA2, and DATA3, as
     with `SCM_NEWSMOB', etc., and causes the surrounding function to
     return that `SCM' value.  It should be the last piece of code in a
     block.

 -- C Macro: scm_t_bits SCM_SMOB_FLAGS (SCM obj)
     Return the 16 extra bits of the smob OBJ.  No meaning is
     predefined for these bits, you can use them freely.

 -- C Macro: scm_t_bits SCM_SET_SMOB_FLAGS (SCM obj, scm_t_bits flags)
     Set the 16 extra bits of the smob OBJ to FLAGS.  No meaning is
     predefined for these bits, you can use them freely.

 -- C Macro: scm_t_bits SCM_SMOB_DATA (SCM obj)
 -- C Macro: scm_t_bits SCM_SMOB_DATA_2 (SCM obj)
 -- C Macro: scm_t_bits SCM_SMOB_DATA_3 (SCM obj)
     Return the first (second, third) immediate word of the smob OBJ as
     a `scm_t_bits' value.  When the word contains a `SCM' value, use
     `SCM_SMOB_OBJECT' (etc.) instead.

 -- C Macro: void SCM_SET_SMOB_DATA (SCM obj, scm_t_bits val)
 -- C Macro: void SCM_SET_SMOB_DATA_2 (SCM obj, scm_t_bits val)
 -- C Macro: void SCM_SET_SMOB_DATA_3 (SCM obj, scm_t_bits val)
     Set the first (second, third) immediate word of the smob OBJ to
     VAL.  When the word should be set to a `SCM' value, use
     `SCM_SMOB_SET_OBJECT' (etc.) instead.

 -- C Macro: SCM SCM_SMOB_OBJECT (SCM obj)
 -- C Macro: SCM SCM_SMOB_OBJECT_2 (SCM obj)
 -- C Macro: SCM SCM_SMOB_OBJECT_3 (SCM obj)
     Return the first (second, third) immediate word of the smob OBJ as
     a `SCM' value.  When the word contains a `scm_t_bits' value, use
     `SCM_SMOB_DATA' (etc.) instead.

 -- C Macro: void SCM_SET_SMOB_OBJECT (SCM obj, SCM val)
 -- C Macro: void SCM_SET_SMOB_OBJECT_2 (SCM obj, SCM val)
 -- C Macro: void SCM_SET_SMOB_OBJECT_3 (SCM obj, SCM val)
     Set the first (second, third) immediate word of the smob OBJ to
     VAL.  When the word should be set to a `scm_t_bits' value, use
     `SCM_SMOB_SET_DATA' (etc.) instead.

 -- C Macro: SCM * SCM_SMOB_OBJECT_LOC (SCM obj)
 -- C Macro: SCM * SCM_SMOB_OBJECT_2_LOC (SCM obj)
 -- C Macro: SCM * SCM_SMOB_OBJECT_3_LOC (SCM obj)
     Return a pointer to the first (second, third) immediate word of the
     smob OBJ.  Note that this is a pointer to `SCM'.  If you need to
     work with `scm_t_bits' values, use `SCM_PACK' and `SCM_UNPACK', as
     appropriate.

 -- Function: SCM scm_markcdr (SCM X)
     Mark the references in the smob X, assuming that X's first data
     word contains an ordinary Scheme object, and X refers to no other
     objects.  This function simply returns X's first data word.

   ---------- Footnotes ----------

   (1) Conversely, in Guile up to the 1.8 series, the marking procedure
was always required.  The reason is that Guile's GC would only look for
pointers in the memory area used for built-in types (the "cell heap"),
not in user-allocated or statically allocated memory.  This approach is
often referred to as "precise marking".

6.9 Procedures
==============

6.9.1 Lambda: Basic Procedure Creation
--------------------------------------

A `lambda' expression evaluates to a procedure.  The environment which
is in effect when a `lambda' expression is evaluated is enclosed in the
newly created procedure, this is referred to as a "closure" (*note
About Closure::).

   When a procedure created by `lambda' is called with some actual
arguments, the environment enclosed in the procedure is extended by
binding the variables named in the formal argument list to new locations
and storing the actual arguments into these locations.  Then the body of
the `lambda' expression is evaluated sequentially.  The result of the
last expression in the procedure body is then the result of the
procedure invocation.

   The following examples will show how procedures can be created using
`lambda', and what you can do with these procedures.

     (lambda (x) (+ x x))       => a procedure
     ((lambda (x) (+ x x)) 4)   => 8

   The fact that the environment in effect when creating a procedure is
enclosed in the procedure is shown with this example:

     (define add4
       (let ((x 4))
         (lambda (y) (+ x y))))
     (add4 6)                   => 10

 -- syntax: lambda formals body
     FORMALS should be a formal argument list as described in the
     following table.

    `(VARIABLE1 ...)'
          The procedure takes a fixed number of arguments; when the
          procedure is called, the arguments will be stored into the
          newly created location for the formal variables.

    `VARIABLE'
          The procedure takes any number of arguments; when the
          procedure is called, the sequence of actual arguments will
          converted into a list and stored into the newly created
          location for the formal variable.

    `(VARIABLE1 ... VARIABLEN . VARIABLEN+1)'
          If a space-delimited period precedes the last variable, then
          the procedure takes N or more variables where N is the number
          of formal arguments before the period.  There must be at
          least one argument before the period.  The first N actual
          arguments will be stored into the newly allocated locations
          for the first N formal arguments and the sequence of the
          remaining actual arguments is converted into a list and the
          stored into the location for the last formal argument.  If
          there are exactly N actual arguments, the empty list is
          stored into the location of the last formal argument.

     The list in VARIABLE or VARIABLEN+1 is always newly created and
     the procedure can modify it if desired.  This is the case even
     when the procedure is invoked via `apply', the required part of
     the list argument there will be copied (*note Procedures for On
     the Fly Evaluation: Fly Evaluation.).

     BODY is a sequence of Scheme expressions which are evaluated in
     order when the procedure is invoked.

6.9.2 Primitive Procedures
--------------------------

Procedures written in C can be registered for use from Scheme, provided
they take only arguments of type `SCM' and return `SCM' values.
`scm_c_define_gsubr' is likely to be the most useful mechanism,
combining the process of registration (`scm_c_make_gsubr') and
definition (`scm_define').

 -- Function: SCM scm_c_make_gsubr (const char *name, int req, int opt,
          int rst, fcn)
     Register a C procedure FCN as a "subr" -- a primitive subroutine
     that can be called from Scheme.  It will be associated with the
     given NAME but no environment binding will be created.  The
     arguments REQ, OPT and RST specify the number of required,
     optional and "rest" arguments respectively.  The total number of
     these arguments should match the actual number of arguments to
     FCN, but may not exceed 10.  The number of rest arguments should
     be 0 or 1.  `scm_c_make_gsubr' returns a value of type `SCM' which
     is a "handle" for the procedure.

 -- Function: SCM scm_c_define_gsubr (const char *name, int req, int
          opt, int rst, fcn)
     Register a C procedure FCN, as for `scm_c_make_gsubr' above, and
     additionally create a top-level Scheme binding for the procedure
     in the "current environment" using `scm_define'.
     `scm_c_define_gsubr' returns a handle for the procedure in the
     same way as `scm_c_make_gsubr', which is usually not further
     required.

6.9.3 Compiled Procedures
-------------------------

The evaluation strategy given in *note Lambda:: describes how procedures
are "interpreted". Interpretation operates directly on expanded Scheme
source code, recursively calling the evaluator to obtain the value of
nested expressions.

   Most procedures are compiled, however. This means that Guile has done
some pre-computation on the procedure, to determine what it will need to
do each time the procedure runs. Compiled procedures run faster than
interpreted procedures.

   Loading files is the normal way that compiled procedures come to
being. If Guile sees that a file is uncompiled, or that its compiled
file is out of date, it will attempt to compile the file when it is
loaded, and save the result to disk. Procedures can be compiled at
runtime as well. *Note Read/Load/Eval/Compile::, for more information
on runtime compilation.

   Compiled procedures, also known as "programs", respond all
procedures that operate on procedures. In addition, there are a few
more accessors for low-level details on programs.

   Most people won't need to use the routines described in this section,
but it's good to have them documented. You'll have to include the
appropriate module first, though:

     (use-modules (system vm program))

 -- Scheme Procedure: program? obj
 -- C Function: scm_program_p (obj)
     Returns `#t' iff OBJ is a compiled procedure.

 -- Scheme Procedure: program-objcode program
 -- C Function: scm_program_objcode (program)
     Returns the object code associated with this program. *Note
     Bytecode and Objcode::, for more information.

 -- Scheme Procedure: program-objects program
 -- C Function: scm_program_objects (program)
     Returns the "object table" associated with this program, as a
     vector. *Note VM Programs::, for more information.

 -- Scheme Procedure: program-module program
 -- C Function: scm_program_module (program)
     Returns the module that was current when this program was created.
     Can return `#f' if the compiler could determine that this
     information was unnecessary.

 -- Scheme Procedure: program-free-variables program
 -- C Function: scm_program_free_variables (program)
     Returns the set of free variables that this program captures in its
     closure, as a vector. If a closure is code with data, you can get
     the code from `program-objcode', and the data via
     `program-free-variables'.

     Some of the values captured are actually in variable "boxes".
     *Note Variables and the VM::, for more information.

     Users must not modify the returned value unless they think they're
     really clever.

 -- Scheme Procedure: program-meta program
 -- C Function: scm_program_meta (program)
     Return the metadata thunk of PROGRAM, or `#f' if it has no
     metadata.

     When called, a metadata thunk returns a list of the following form:
     `(BINDINGS SOURCES ARITIES . PROPERTIES)'. The format of each of
     these elements is discussed below.

 -- Scheme Procedure: program-bindings program
 -- Scheme Procedure: make-binding name boxed? index start end
 -- Scheme Procedure: binding:name binding
 -- Scheme Procedure: binding:boxed? binding
 -- Scheme Procedure: binding:index binding
 -- Scheme Procedure: binding:start binding
 -- Scheme Procedure: binding:end binding
     Bindings annotations for programs, along with their accessors.

     Bindings declare names and liveness extents for block-local
     variables.  The best way to see what these are is to play around
     with them at a REPL. *Note VM Concepts::, for more information.

     Note that bindings information is stored in a program as part of
     its metadata thunk, so including it in the generated object code
     does not impose a runtime performance penalty.

 -- Scheme Procedure: program-sources program
 -- Scheme Procedure: source:addr source
 -- Scheme Procedure: source:line source
 -- Scheme Procedure: source:column source
 -- Scheme Procedure: source:file source
     Source location annotations for programs, along with their
     accessors.

     Source location information propagates through the compiler and
     ends up being serialized to the program's metadata. This
     information is keyed by the offset of the instruction pointer
     within the object code of the program. Specifically, it is keyed
     on the `ip' _just following_ an instruction, so that backtraces
     can find the source location of a call that is in progress.

 -- Scheme Procedure: program-arities program
 -- C Function: scm_program_arities (program)
 -- Scheme Procedure: program-arity program ip
 -- Scheme Procedure: arity:start arity
 -- Scheme Procedure: arity:end arity
 -- Scheme Procedure: arity:nreq arity
 -- Scheme Procedure: arity:nopt arity
 -- Scheme Procedure: arity:rest? arity
 -- Scheme Procedure: arity:kw arity
 -- Scheme Procedure: arity:allow-other-keys? arity
     Accessors for a representation of the "arity" of a program.

     The normal case is that a procedure has one arity. For example,
     `(lambda (x) x)', takes one required argument, and that's it. One
     could access that number of required arguments via `(arity:nreq
     (program-arities (lambda (x) x)))'. Similarly, `arity:nopt' gets
     the number of optional arguments, and `arity:rest?' returns a true
     value if the procedure has a rest arg.

     `arity:kw' returns a list of `(KW . IDX)' pairs, if the procedure
     has keyword arguments. The IDX refers to the IDXth local variable;
     *Note Variables and the VM::, for more information. Finally
     `arity:allow-other-keys?' returns a true value if other keys are
     allowed. *Note Optional Arguments::, for more information.

     So what about `arity:start' and `arity:end', then? They return the
     range of bytes in the program's bytecode for which a given arity
     is valid. You see, a procedure can actually have more than one
     arity. The question, "what is a procedure's arity" only really
     makes sense at certain points in the program, delimited by these
     `arity:start' and `arity:end' values.

6.9.4 Optional Arguments
------------------------

Scheme procedures, as defined in R5RS, can either handle a fixed number
of actual arguments, or a fixed number of actual arguments followed by
arbitrarily many additional arguments.  Writing procedures of variable
arity can be useful, but unfortunately, the syntactic means for handling
argument lists of varying length is a bit inconvenient.  It is possible
to give names to the fixed number of arguments, but the remaining
(optional) arguments can be only referenced as a list of values (*note
Lambda::).

   For this reason, Guile provides an extension to `lambda', `lambda*',
which allows the user to define procedures with optional and keyword
arguments. In addition, Guile's virtual machine has low-level support
for optional and keyword argument dispatch.  Calls to procedures with
optional and keyword arguments can be made cheaply, without allocating
a rest list.

6.9.4.1 lambda* and define*.
............................

`lambda*' is like `lambda', except with some extensions to allow
optional and keyword arguments.

 -- library syntax: lambda* ([var...]
          [#:optional vardef...]
          [#:key vardef... [#:allow-other-keys]]
          [#:rest var | . var])
          body

     Create a procedure which takes optional and/or keyword arguments
     specified with `#:optional' and `#:key'.  For example,

          (lambda* (a b #:optional c d . e) '())

     is a procedure with fixed arguments A and B, optional arguments C
     and D, and rest argument E.  If the optional arguments are omitted
     in a call, the variables for them are bound to `#f'.

     Likewise, `define*' is syntactic sugar for defining procedures
     using `lambda*'.

     `lambda*' can also make procedures with keyword arguments. For
     example, a procedure defined like this:

          (define* (sir-yes-sir #:key action how-high)
            (list action how-high))

     can be called as `(sir-yes-sir #:action 'jump)', `(sir-yes-sir
     #:how-high 13)', `(sir-yes-sir #:action 'lay-down #:how-high 0)',
     or just `(sir-yes-sir)'. Whichever arguments are given as keywords
     are bound to values (and those not given are `#f').

     Optional and keyword arguments can also have default values to take
     when not present in a call, by giving a two-element list of
     variable name and expression.  For example in

          (define* (frob foo #:optional (bar 42) #:key (baz 73))
            (list foo bar baz))

     FOO is a fixed argument, BAR is an optional argument with default
     value 42, and baz is a keyword argument with default value 73.
     Default value expressions are not evaluated unless they are needed,
     and until the procedure is called.

     Normally it's an error if a call has keywords other than those
     specified by `#:key', but adding `#:allow-other-keys' to the
     definition (after the keyword argument declarations) will ignore
     unknown keywords.

     If a call has a keyword given twice, the last value is used.  For
     example,

          (define* (flips #:key (heads 0) (tails 0))
            (display (list heads tails)))

          (flips #:heads 37 #:tails 42 #:heads 99)
          -| (99 42)

     `#:rest' is a synonym for the dotted syntax rest argument.  The
     argument lists `(a . b)' and `(a #:rest b)' are equivalent in all
     respects.  This is provided for more similarity to DSSSL,
     MIT-Scheme and Kawa among others, as well as for refugees from
     other Lisp dialects.

     When `#:key' is used together with a rest argument, the keyword
     parameters in a call all remain in the rest list.  This is the
     same as Common Lisp.  For example,

          ((lambda* (#:key (x 0) #:allow-other-keys #:rest r)
             (display r))
           #:x 123 #:y 456)
          -| (#:x 123 #:y 456)

     `#:optional' and `#:key' establish their bindings successively,
     from left to right. This means default expressions can refer back
     to prior parameters, for example

          (lambda* (start #:optional (end (+ 10 start)))
            (do ((i start (1+ i)))
                ((> i end))
              (display i)))

     The exception to this left-to-right scoping rule is the rest
     argument.  If there is a rest argument, it is bound after the
     optional arguments, but before the keyword arguments.

6.9.4.2 (ice-9 optargs)
.......................

Before Guile 2.0, `lambda*' and `define*' were implemented using macros
that processed rest list arguments. This was not optimal, as calling
procedures with optional arguments had to allocate rest lists at every
procedure invocation. Guile 2.0 improved this situation by bringing
optional and keyword arguments into Guile's core.

   However there are occasions in which you have a list and want to
parse it for optional or keyword arguments. Guile's `(ice-9 optargs)'
provides some macros to help with that task.

   The syntax `let-optional' and `let-optional*' are for destructuring
rest argument lists and giving names to the various list elements.
`let-optional' binds all variables simultaneously, while
`let-optional*' binds them sequentially, consistent with `let' and
`let*' (*note Local Bindings::).

 -- library syntax: let-optional rest-arg (binding ...) expr ...
 -- library syntax: let-optional* rest-arg (binding ...) expr ...
     These two macros give you an optional argument interface that is
     very "Schemey" and introduces no fancy syntax. They are compatible
     with the scsh macros of the same name, but are slightly extended.
     Each of BINDING may be of one of the forms VAR or `(VAR
     DEFAULT-VALUE)'. REST-ARG should be the rest-argument of the
     procedures these are used from.  The items in REST-ARG are
     sequentially bound to the variable names are given. When REST-ARG
     runs out, the remaining vars are bound either to the default
     values or `#f' if no default value was specified. REST-ARG remains
     bound to whatever may have been left of REST-ARG.

     After binding the variables, the expressions EXPR ... are
     evaluated in order.

   Similarly, `let-keywords' and `let-keywords*' extract values from
keyword style argument lists, binding local variables to those values
or to defaults.

 -- library syntax: let-keywords args allow-other-keys? (binding ...)
          body ...
 -- library syntax: let-keywords* args allow-other-keys? (binding ...)
          body ...
     ARGS is evaluated and should give a list of the form `(#:keyword1
     value1 #:keyword2 value2 ...)'.  The BINDINGs are variables and
     default expressions, with the variables to be set (by name) from
     the keyword values.  The BODY forms are then evaluated and the
     last is the result.  An example will make the syntax clearest,

          (define args '(#:xyzzy "hello" #:foo "world"))

          (let-keywords args #t
                ((foo  "default for foo")
                 (bar  (string-append "default" "for" "bar")))
            (display foo)
            (display ", ")
            (display bar))
          -| world, defaultforbar

     The binding for `foo' comes from the `#:foo' keyword in `args'.
     But the binding for `bar' is the default in the `let-keywords',
     since there's no `#:bar' in the args.

     ALLOW-OTHER-KEYS? is evaluated and controls whether unknown
     keywords are allowed in the ARGS list.  When true other keys are
     ignored (such as `#:xyzzy' in the example), when `#f' an error is
     thrown for anything unknown.

   `(ice-9 optargs)' also provides some more `define*' sugar, which is
not so useful with modern Guile coding, but still supported:
`define*-public' is the `lambda*' version of `define-public';
`defmacro*' and `defmacro*-public' exist for defining macros with the
improved argument list handling possibilities. The `-public' versions
not only define the procedures/macros, but also export them from the
current module.

 -- library syntax: define*-public formals body
     Like a mix of `define*' and `define-public'.

 -- library syntax: defmacro* name formals body
 -- library syntax: defmacro*-public name formals body
     These are just like `defmacro' and `defmacro-public' except that
     they take `lambda*'-style extended parameter lists, where
     `#:optional', `#:key', `#:allow-other-keys' and `#:rest' are
     allowed with the usual semantics. Here is an example of a macro
     with an optional argument:

          (defmacro* transmogrify (a #:optional b)
            (a 1))

6.9.5 Case-lambda
-----------------

R5RS's rest arguments are indeed useful and very general, but they
often aren't the most appropriate or efficient means to get the job
done. For example, `lambda*' is a much better solution to the optional
argument problem than `lambda' with rest arguments.

   Likewise, `case-lambda' works well for when you want one procedure
to do double duty (or triple, or ...), without the penalty of consing a
rest list.

   For example:

     (define (make-accum n)
       (case-lambda
         (() n)
         ((m) (set! n (+ n m)) n)))

     (define a (make-accum 20))
     (a) => 20
     (a 10) => 30
     (a) => 30

   The value returned by a `case-lambda' form is a procedure which
matches the number of actual arguments against the formals in the
various clauses, in order. The first matching clause is selected, the
corresponding values from the actual parameter list are bound to the
variable names in the clauses and the body of the clause is evaluated.
If no clause matches, an error is signalled.

   The syntax of the `case-lambda' form is defined in the following
EBNF grammar. "Formals" means a formal argument list just like with
`lambda' (*note Lambda::).

     <case-lambda>
        --> (case-lambda <case-lambda-clause>)
     <case-lambda-clause>
        --> (<formals> <definition-or-command>*)
     <formals>
        --> (<identifier>*)
          | (<identifier>* . <identifier>)
          | <identifier>

   Rest lists can be useful with `case-lambda':

     (define plus
       (case-lambda
         (() 0)
         ((a) a)
         ((a b) (+ a b))
         ((a b . rest) (apply plus (+ a b) rest))))
     (plus 1 2 3) => 6

   Also, for completeness. Guile defines `case-lambda*' as well, which
is like `case-lambda', except with `lambda*' clauses.  A `case-lambda*'
clause matches if the arguments fill the required arguments, but are
not too many for the optional and/or rest arguments.

   Keyword arguments are possible with `case-lambda*', but they do not
contribute to the "matching" behavior. That is to say, `case-lambda*'
matches only on required, optional, and rest arguments, and on the
predicate; keyword arguments may be present but do not contribute to
the "success" of a match. In fact a bad keyword argument list may cause
an error to be raised.

6.9.6 Higher-Order Functions
----------------------------

As a functional programming language, Scheme allows the definition of
"higher-order functions", i.e., functions that take functions as
arguments and/or return functions.  Utilities to derive procedures from
other procedures are provided and described below.

 -- Scheme Procedure: const value
     Return a procedure that accepts any number of arguments and returns
     VALUE.

          (procedure? (const 3))        => #t
          ((const 'hello))              => hello
          ((const 'hello) 'world)       => hello

 -- Scheme Procedure: negate proc
     Return a procedure with the same arity as PROC that returns the
     `not' of PROC's result.

          (procedure? (negate number?)) => #t
          ((negate odd?) 2)             => #t
          ((negate real?) 'dream)       => #t
          ((negate string-prefix?) "GNU" "GNU Guile")
                                        => #f
          (filter (negate number?) '(a 2 "b"))
                                        => (a "b")

 -- Scheme Procedure: compose proc rest ...
     Compose PROC with the procedures in REST, such that the last one
     in REST is applied first and PROC last, and return the resulting
     procedure.  The given procedures must have compatible arity.

          (procedure? (compose 1+ 1-)) => #t
          ((compose sqrt 1+ 1+) 2)     => 2.0
          ((compose 1+ sqrt) 3)        => 2.73205080756888
          (eq? (compose 1+) 1+)        => #t

          ((compose zip unzip2) '((1 2) (a b)))
                                       => ((1 2) (a b))

 -- Scheme Procedure: identity x
     Return X.

6.9.7 Procedure Properties and Meta-information
-----------------------------------------------

In addition to the information that is strictly necessary to run,
procedures may have other associated information. For example, the name
of a procedure is information not for the procedure, but about the
procedure. This meta-information can be accessed via the procedure
properties interface.

   The first group of procedures in this meta-interface are predicates
to test whether a Scheme object is a procedure, or a special procedure,
respectively. `procedure?' is the most general predicates, it returns
`#t' for any kind of procedure. `closure?' does not return `#t' for
primitive procedures, and `thunk?' only returns `#t' for procedures
which do not accept any arguments.

 -- Scheme Procedure: procedure? obj
 -- C Function: scm_procedure_p (obj)
     Return `#t' if OBJ is a procedure.

 -- Scheme Procedure: thunk? obj
 -- C Function: scm_thunk_p (obj)
     Return `#t' if OBJ is a thunk.

   Procedure properties are general properties associated with
procedures. These can be the name of a procedure or other relevant
information, such as debug hints.

 -- Scheme Procedure: procedure-name proc
 -- C Function: scm_procedure_name (proc)
     Return the name of the procedure PROC

 -- Scheme Procedure: procedure-source proc
 -- C Function: scm_procedure_source (proc)
     Return the source of the procedure PROC. Returns `#f' if the
     source code is not available.

 -- Scheme Procedure: procedure-properties proc
 -- C Function: scm_procedure_properties (proc)
     Return the properties associated with PROC, as an association list.

 -- Scheme Procedure: procedure-property proc key
 -- C Function: scm_procedure_property (proc, key)
     Return the property of PROC with name KEY.

 -- Scheme Procedure: set-procedure-properties! proc alist
 -- C Function: scm_set_procedure_properties_x (proc, alist)
     Set PROC's property list to ALIST.

 -- Scheme Procedure: set-procedure-property! proc key value
 -- C Function: scm_set_procedure_property_x (proc, key, value)
     In PROC's property list, set the property named KEY to VALUE.

   Documentation for a procedure can be accessed with the procedure
`procedure-documentation'.

 -- Scheme Procedure: procedure-documentation proc
 -- C Function: scm_procedure_documentation (proc)
     Return the documentation string associated with `proc'.  By
     convention, if a procedure contains more than one expression and
     the first expression is a string constant, that string is assumed
     to contain documentation for that procedure.

6.9.8 Procedures with Setters
-----------------------------

A "procedure with setter" is a special kind of procedure which normally
behaves like any accessor procedure, that is a procedure which accesses
a data structure.  The difference is that this kind of procedure has a
so-called "setter" attached, which is a procedure for storing something
into a data structure.

   Procedures with setters are treated specially when the procedure
appears in the special form `set!' (REFFIXME).  How it works is best
shown by example.

   Suppose we have a procedure called `foo-ref', which accepts two
arguments, a value of type `foo' and an integer.  The procedure returns
the value stored at the given index in the `foo' object.  Let `f' be a
variable containing such a `foo' data structure.(1)

     (foo-ref f 0)       => bar
     (foo-ref f 1)       => braz

   Also suppose that a corresponding setter procedure called `foo-set!'
does exist.

     (foo-set! f 0 'bla)
     (foo-ref f 0)       => bla

   Now we could create a new procedure called `foo', which is a
procedure with setter, by calling `make-procedure-with-setter' with the
accessor and setter procedures `foo-ref' and `foo-set!'.  Let us call
this new procedure `foo'.

     (define foo (make-procedure-with-setter foo-ref foo-set!))

   `foo' can from now an be used to either read from the data structure
stored in `f', or to write into the structure.

     (set! (foo f 0) 'dum)
     (foo f 0)          => dum

 -- Scheme Procedure: make-procedure-with-setter procedure setter
 -- C Function: scm_make_procedure_with_setter (procedure, setter)
     Create a new procedure which behaves like PROCEDURE, but with the
     associated setter SETTER.

 -- Scheme Procedure: procedure-with-setter? obj
 -- C Function: scm_procedure_with_setter_p (obj)
     Return `#t' if OBJ is a procedure with an associated setter
     procedure.

 -- Scheme Procedure: procedure proc
 -- C Function: scm_procedure (proc)
     Return the procedure of PROC, which must be an applicable struct.

 -- Scheme Procedure: setter proc
     Return the setter of PROC, which must be either a procedure with
     setter or an operator struct.

   ---------- Footnotes ----------

   (1) Working definitions would be:
     (define foo-ref vector-ref)
     (define foo-set! vector-set!)
     (define f (make-vector 2 #f))

6.9.9 Inlinable Procedures
--------------------------

You can define an "inlinable procedure" by using `define-inlinable'
instead of `define'.  An inlinable procedure behaves the same as a
regular procedure, but direct calls will result in the procedure body
being inlined into the caller.

   Bear in mind that starting from version 2.0.3, Guile has a partial
evaluator that can inline the body of inner procedures when deemed
appropriate:

     scheme@(guile-user)> ,optimize (define (foo x)
                                      (define (bar) (+ x 3))
                                      (* (bar) 2))
     $1 = (define foo
            (lambda (#{x 94}#) (* (+ #{x 94}# 3) 2)))

The partial evaluator does not inline top-level bindings, though, so
this is a situation where you may find it interesting to use
`define-inlinable'.

   Procedures defined with `define-inlinable' are _always_ inlined, at
all direct call sites.  This eliminates function call overhead at the
expense of an increase in code size.  Additionally, the caller will not
transparently use the new definition if the inline procedure is
redefined.  It is not possible to trace an inlined procedures or
install a breakpoint in it (*note Traps::).  For these reasons, you
should not make a procedure inlinable unless it demonstrably improves
performance in a crucial way.

   In general, only small procedures should be considered for inlining,
as making large procedures inlinable will probably result in an
increase in code size.  Additionally, the elimination of the call
overhead rarely matters for large procedures.

 -- Scheme Syntax: define-inlinable (name parameter ...) body ...
     Define NAME as a procedure with parameters PARAMETERs and body
     BODY.

6.10 Macros
===========

At its best, programming in Lisp is an iterative process of building up
a language appropriate to the problem at hand, and then solving the
problem in that language. Defining new procedures is part of that, but
Lisp also allows the user to extend its syntax, with its famous
"macros".

   Macros are syntactic extensions which cause the expression that they
appear in to be transformed in some way _before_ being evaluated. In
expressions that are intended for macro transformation, the identifier
that names the relevant macro must appear as the first element, like
this:

     (MACRO-NAME MACRO-ARGS ...)

   Macro expansion is a separate phase of evaluation, run before code is
interpreted or compiled. A macro is a program that runs on programs,
translating an embedded language into core Scheme(1).

   ---------- Footnotes ----------

   (1) These days such embedded languages are often referred to as
"embedded domain-specific languages", or EDSLs.

6.10.1 Defining Macros
----------------------

A macro is a binding between a keyword and a syntax transformer. Since
it's difficult to discuss `define-syntax' without discussing the format
of transformers, consider the following example macro definition:

     (define-syntax when
       (syntax-rules ()
         ((when condition exp ...)
          (if condition
              (begin exp ...)))))

     (when #t
       (display "hey ho\n")
       (display "let's go\n"))
     -| hey ho
     -| let's go

   In this example, the `when' binding is bound with `define-syntax'.
Syntax transformers are discussed in more depth in *note Syntax Rules::
and *note Syntax Case::.

 -- Syntax: define-syntax keyword transformer
     Bind KEYWORD to the syntax transformer obtained by evaluating
     TRANSFORMER.

     After a macro has been defined, further instances of KEYWORD in
     Scheme source code will invoke the syntax transformer defined by
     TRANSFORMER.

   One can also establish local syntactic bindings with `let-syntax'.

 -- Syntax: let-syntax ((keyword transformer) ...) exp...
     Bind KEYWORD... to TRANSFORMER... while expanding EXP....

     A `let-syntax' binding only exists at expansion-time.

          (let-syntax ((unless
                        (syntax-rules ()
                          ((unless condition exp ...)
                           (if (not condition)
                               (begin exp ...))))))
            (unless #t
              (primitive-exit 1))
            "rock rock rock")
          => "rock rock rock"

   A `define-syntax' form is valid anywhere a definition may appear: at
the top-level, or locally. Just as a local `define' expands out to an
instance of `letrec', a local `define-syntax' expands out to
`letrec-syntax'.

 -- Syntax: letrec-syntax ((keyword transformer) ...) exp...
     Bind KEYWORD... to TRANSFORMER... while expanding EXP....

     In the spirit of `letrec' versus `let', an expansion produced by
     TRANSFORMER may reference a KEYWORD bound by the same
     LETREC-SYNTAX.

          (letrec-syntax ((my-or
                           (syntax-rules ()
                             ((my-or)
                              #t)
                             ((my-or exp)
                              exp)
                             ((my-or exp rest ...)
                              (let ((t exp))
                                (if exp
                                    exp
                                    (my-or rest ...)))))))
            (my-or #f "rockaway beach"))
          => "rockaway beach"

6.10.2 Syntax-rules Macros
--------------------------

`syntax-rules' macros are simple, pattern-driven syntax transformers,
with a beauty worthy of Scheme.

 -- Syntax: syntax-rules literals (pattern template)...
     Create a syntax transformer that will rewrite an expression using
     the rules embodied in the PATTERN and TEMPLATE clauses.

   A `syntax-rules' macro consists of three parts: the literals (if
any), the patterns, and as many templates as there are patterns.

   When the syntax expander sees the invocation of a `syntax-rules'
macro, it matches the expression against the patterns, in order, and
rewrites the expression using the template from the first matching
pattern. If no pattern matches, a syntax error is signalled.

6.10.2.1 Patterns
.................

We have already seen some examples of patterns in the previous section:
`(unless condition exp ...)', `(my-or exp)', and so on. A pattern is
structured like the expression that it is to match. It can have nested
structure as well, like `(let ((var val) ...) exp exp* ...)'. Broadly
speaking, patterns are made of lists, improper lists, vectors,
identifiers, and datums.  Users can match a sequence of patterns using
the ellipsis (`...').

   Identifiers in a pattern are called "literals" if they are present
in the `syntax-rules' literals list, and "pattern variables" otherwise.
When building up the macro output, the expander replaces instances of a
pattern variable in the template with the matched subexpression.

     (define-syntax kwote
       (syntax-rules ()
         ((kwote exp)
          (quote exp))))
     (kwote (foo . bar))
     => (foo . bar)

   An improper list of patterns matches as rest arguments do:

     (define-syntax let1
       (syntax-rules ()
         ((_ (var val) . exps)
          (let ((var val)) . exps))))

   However this definition of `let1' probably isn't what you want, as
the tail pattern EXPS will match non-lists, like `(let1 (foo 'bar) .
baz)'. So often instead of using improper lists as patterns, ellipsized
patterns are better. Instances of a pattern variable in the template
must be followed by an ellipsis.

     (define-syntax let1
       (syntax-rules ()
         ((_ (var val) exp ...)
          (let ((var val)) exp ...))))

   This `let1' probably still doesn't do what we want, because the body
matches sequences of zero expressions, like `(let1 (foo 'bar))'. In this
case we need to assert we have at least one body expression. A common
idiom for this is to name the ellipsized pattern variable with an
asterisk:

     (define-syntax let1
       (syntax-rules ()
         ((_ (var val) exp exp* ...)
          (let ((var val)) exp exp* ...))))

   A vector of patterns matches a vector whose contents match the
patterns, including ellipsizing and tail patterns.

     (define-syntax letv
       (syntax-rules ()
         ((_ #((var val) ...) exp exp* ...)
          (let ((var val) ...) exp exp* ...))))
     (letv #((foo 'bar)) foo)
     => foo

   Literals are used to match specific datums in an expression, like
the use of `=>' and `else' in `cond' expressions.

     (define-syntax cond1
       (syntax-rules (=> else)
         ((cond1 test => fun)
          (let ((exp test))
            (if exp (fun exp) #f)))
         ((cond1 test exp exp* ...)
          (if test (begin exp exp* ...)))
         ((cond1 else exp exp* ...)
          (begin exp exp* ...))))

     (define (square x) (* x x))
     (cond1 10 => square)
     => 100
     (let ((=> #t))
       (cond1 10 => square))
     => #<procedure square (x)>

   A literal matches an input expression if the input expression is an
identifier with the same name as the literal, and both are unbound(1).

   If a pattern is not a list, vector, or an identifier, it matches as
a literal, with `equal?'.

     (define-syntax define-matcher-macro
       (syntax-rules ()
         ((_ name lit)
          (define-syntax name
            (syntax-rules ()
             ((_ lit) #t)
             ((_ else) #f))))))

     (define-matcher-macro is-literal-foo? "foo")

     (is-literal-foo? "foo")
     => #t
     (is-literal-foo? "bar")
     => #f
     (let ((foo "foo"))
       (is-literal-foo? foo))
     => #f

   The last example indicates that matching happens at expansion-time,
not at run-time.

   Syntax-rules macros are always used as `(MACRO . ARGS)', and the
MACRO will always be a symbol. Correspondingly, a `syntax-rules'
pattern must be a list (proper or improper), and the first pattern in
that list must be an identifier. Incidentally it can be any identifier
- it doesn't have to actually be the name of the macro. Thus the
following three are equivalent:

     (define-syntax when
       (syntax-rules ()
         ((when c e ...)
          (if c (begin e ...)))))

     (define-syntax when
       (syntax-rules ()
         ((_ c e ...)
          (if c (begin e ...)))))

     (define-syntax when
       (syntax-rules ()
         ((something-else-entirely c e ...)
          (if c (begin e ...)))))

   For clarity, use one of the first two variants. Also note that since
the pattern variable will always match the macro itself (e.g.,
`cond1'), it is actually left unbound in the template.

6.10.2.2 Hygiene
................

`syntax-rules' macros have a magical property: they preserve referential
transparency. When you read a macro definition, any free bindings in
that macro are resolved relative to the macro definition; and when you
read a macro instantiation, all free bindings in that expression are
resolved relative to the expression.

   This property is sometimes known as "hygiene", and it does aid in
code cleanliness. In your macro definitions, you can feel free to
introduce temporary variables, without worrying about inadvertently
introducing bindings into the macro expansion.

   Consider the definition of `my-or' from the previous section:

     (define-syntax my-or
       (syntax-rules ()
         ((my-or)
          #t)
         ((my-or exp)
          exp)
         ((my-or exp rest ...)
          (let ((t exp))
            (if exp
                exp
                (my-or rest ...))))))

   A naive expansion of `(let ((t #t)) (my-or #f t))' would yield:

     (let ((t #t))
       (let ((t #f))
         (if t t t)))
     => #f

Which clearly is not what we want. Somehow the `t' in the definition is
distinct from the `t' at the site of use; and it is indeed this
distinction that is maintained by the syntax expander, when expanding
hygienic macros.

   This discussion is mostly relevant in the context of traditional
Lisp macros (*note Defmacros::), which do not preserve referential
transparency. Hygiene adds to the expressive power of Scheme.

6.10.2.3 Shorthands
...................

One often ends up writing simple one-clause `syntax-rules' macros.
There is a convenient shorthand for this idiom, in the form of
`define-syntax-rule'.

 -- Syntax: define-syntax-rule (keyword . pattern) [docstring] template
     Define KEYWORD as a new `syntax-rules' macro with one clause.

   Cast into this form, our `when' example is significantly shorter:

     (define-syntax-rule (when c e ...)
       (if c (begin e ...)))

6.10.2.4 Further Information
............................

For a formal definition of `syntax-rules' and its pattern language, see
*Note Macros: (r5rs)Macros.

   `syntax-rules' macros are simple and clean, but do they have
limitations.  They do not lend themselves to expressive error messages:
patterns either match or they don't. Their ability to generate code is
limited to template-driven expansion; often one needs to define a
number of helper macros to get real work done. Sometimes one wants to
introduce a binding into the lexical context of the generated code;
this is impossible with `syntax-rules'. Relatedly, they cannot
programmatically generate identifiers.

   The solution to all of these problems is to use `syntax-case' if you
need its features. But if for some reason you're stuck with
`syntax-rules', you might enjoy Joe Marshall's `syntax-rules' Primer
for the Merely Eccentric
(http://sites.google.com/site/evalapply/eccentric.txt).

   ---------- Footnotes ----------

   (1) Language lawyers probably see the need here for use of
`literal-identifier=?' rather than `free-identifier=?', and would
probably be correct. Patches accepted.

6.10.3 Support for the `syntax-case' System
-------------------------------------------

`syntax-case' macros are procedural syntax transformers, with a power
worthy of Scheme.

 -- Syntax: syntax-case syntax literals (pattern [guard] exp)...
     Match the syntax object SYNTAX against the given patterns, in
     order. If a PATTERN matches, return the result of evaluating the
     associated EXP.

   Compare the following definitions of `when':

     (define-syntax when
       (syntax-rules ()
         ((_ test e e* ...)
          (if test (begin e e* ...)))))

     (define-syntax when
       (lambda (x)
         (syntax-case x ()
           ((_ test e e* ...)
            #'(if test (begin e e* ...))))))

   Clearly, the `syntax-case' definition is similar to its
`syntax-rules' counterpart, and equally clearly there are some
differences. The `syntax-case' definition is wrapped in a `lambda', a
function of one argument; that argument is passed to the `syntax-case'
invocation; and the "return value" of the macro has a `#'' prefix.

   All of these differences stem from the fact that `syntax-case' does
not define a syntax transformer itself - instead, `syntax-case'
expressions provide a way to destructure a "syntax object", and to
rebuild syntax objects as output.

   So the `lambda' wrapper is simply a leaky implementation detail, that
syntax transformers are just functions that transform syntax to syntax.
This should not be surprising, given that we have already described
macros as "programs that write programs". `syntax-case' is simply a way
to take apart and put together program text, and to be a valid syntax
transformer it needs to be wrapped in a procedure.

   Unlike traditional Lisp macros (*note Defmacros::), `syntax-case'
macros transform syntax objects, not raw Scheme forms. Recall the naive
expansion of `my-or' given in the previous section:

     (let ((t #t))
       (my-or #f t))
     ;; naive expansion:
     (let ((t #t))
       (let ((t #f))
         (if t t t)))

   Raw Scheme forms simply don't have enough information to distinguish
the first two `t' instances in `(if t t t)' from the third `t'. So
instead of representing identifiers as symbols, the syntax expander
represents identifiers as annotated syntax objects, attaching such
information to those syntax objects as is needed to maintain
referential transparency.

 -- Syntax: syntax form
     Create a syntax object wrapping FORM within the current lexical
     context.

   Syntax objects are typically created internally to the process of
expansion, but it is possible to create them outside of syntax
expansion:

     (syntax (foo bar baz))
     => #<some representation of that syntax>

However it is more common, and useful, to create syntax objects when
building output from a `syntax-case' expression.

     (define-syntax add1
       (lambda (x)
         (syntax-case x ()
           ((_ exp)
            (syntax (+ exp 1))))))

   It is not strictly necessary for a `syntax-case' expression to
return a syntax object, because `syntax-case' expressions can be used
in helper functions, or otherwise used outside of syntax expansion
itself. However a syntax transformer procedure must return a syntax
object, so most uses of `syntax-case' do end up returning syntax
objects.

   Here in this case, the form that built the return value was `(syntax
(+ exp 1))'. The interesting thing about this is that within a `syntax'
expression, any appearance of a pattern variable is substituted into the
resulting syntax object, carrying with it all relevant metadata from
the source expression, such as lexical identity and source location.

   Indeed, a pattern variable may only be referenced from inside a
`syntax' form. The syntax expander would raise an error when defining
`add1' if it found EXP referenced outside a `syntax' form.

   Since `syntax' appears frequently in macro-heavy code, it has a
special reader macro: `#''. `#'foo' is transformed by the reader into
`(syntax foo)', just as `'foo' is transformed into `(quote foo)'.

   The pattern language used by `syntax-case' is conveniently the same
language used by `syntax-rules'. Given this, Guile actually defines
`syntax-rules' in terms of `syntax-case':

     (define-syntax syntax-rules
       (lambda (x)
         (syntax-case x ()
           ((_ (k ...) ((keyword . pattern) template) ...)
            #'(lambda (x)
                (syntax-case x (k ...)
                  ((dummy . pattern) #'template)
                  ...))))))

   And that's that.

6.10.3.1 Why `syntax-case'?
...........................

The examples we have shown thus far could just as well have been
expressed with `syntax-rules', and have just shown that `syntax-case'
is more verbose, which is true. But there is a difference:
`syntax-case' creates _procedural_ macros, giving the full power of
Scheme to the macro expander.  This has many practical applications.

   A common desire is to be able to match a form only if it is an
identifier. This is impossible with `syntax-rules', given the datum
matching forms. But with `syntax-case' it is easy:

 -- Scheme Procedure: identifier? syntax-object
     Returns `#t' iff SYNTAX-OBJECT is an identifier.

     ;; relying on previous add1 definition
     (define-syntax add1!
       (lambda (x)
         (syntax-case x ()
           ((_ var) (identifier? #'var)
            #'(set! var (add1 var))))))

     (define foo 0)
     (add1! foo)
     foo => 1
     (add1! "not-an-identifier") => error

   With `syntax-rules', the error for `(add1! "not-an-identifier")'
would be something like "invalid `set!'". With `syntax-case', it will
say something like "invalid `add1!'", because we attach the "guard
clause" to the pattern: `(identifier? #'var)'. This becomes more
important with more complicated macros. It is necessary to use
`identifier?', because to the expander, an identifier is more than a
bare symbol.

   Note that even in the guard clause, we reference the VAR pattern
variable within a `syntax' form, via `#'var'.

   Another common desire is to introduce bindings into the lexical
context of the output expression. One example would be in the so-called
"anaphoric macros", like `aif'. Anaphoric macros bind some expression
to a well-known identifier, often `it', within their bodies. For
example, in `(aif (foo) (bar it))', `it' would be bound to the result
of `(foo)'.

   To begin with, we should mention a solution that doesn't work:

     ;; doesn't work
     (define-syntax aif
       (lambda (x)
         (syntax-case x ()
           ((_ test then else)
            #'(let ((it test))
                (if it then else))))))

   The reason that this doesn't work is that, by default, the expander
will preserve referential transparency; the THEN and ELSE expressions
won't have access to the binding of `it'.

   But they can, if we explicitly introduce a binding via
`datum->syntax'.

 -- Scheme Procedure: datum->syntax for-syntax datum
     Create a syntax object that wraps DATUM, within the lexical context
     corresponding to the syntax object FOR-SYNTAX.

   For completeness, we should mention that it is possible to strip the
metadata from a syntax object, returning a raw Scheme datum:

 -- Scheme Procedure: syntax->datum syntax-object
     Strip the metadata from SYNTAX-OBJECT, returning its contents as a
     raw Scheme datum.

   In this case we want to introduce `it' in the context of the whole
expression, so we can create a syntax object as `(datum->syntax x 'it)',
where `x' is the whole expression, as passed to the transformer
procedure.

   Here's another solution that doesn't work:

     ;; doesn't work either
     (define-syntax aif
       (lambda (x)
         (syntax-case x ()
           ((_ test then else)
            (let ((it (datum->syntax x 'it)))
              #'(let ((it test))
                  (if it then else)))))))

   The reason that this one doesn't work is that there are really two
environments at work here - the environment of pattern variables, as
bound by `syntax-case', and the environment of lexical variables, as
bound by normal Scheme. The outer let form establishes a binding in the
environment of lexical variables, but the inner let form is inside a
syntax form, where only pattern variables will be substituted. Here we
need to introduce a piece of the lexical environment into the pattern
variable environment, and we can do so using `syntax-case' itself:

     ;; works, but is obtuse
     (define-syntax aif
       (lambda (x)
         (syntax-case x ()
           ((_ test then else)
            ;; invoking syntax-case on the generated
            ;; syntax object to expose it to `syntax'
            (syntax-case (datum->syntax x 'it) ()
              (it
                #'(let ((it test))
                    (if it then else))))))))

     (aif (getuid) (display it) (display "none")) (newline)
     -| 500

   However there are easier ways to write this. `with-syntax' is often
convenient:

 -- Syntax: with-syntax ((pat val)...) exp...
     Bind patterns PAT from their corresponding values VAL, within the
     lexical context of EXP....

          ;; better
          (define-syntax aif
            (lambda (x)
              (syntax-case x ()
                ((_ test then else)
                 (with-syntax ((it (datum->syntax x 'it)))
                   #'(let ((it test))
                       (if it then else)))))))

   As you might imagine, `with-syntax' is defined in terms of
`syntax-case'. But even that might be off-putting to you if you are an
old Lisp macro hacker, used to building macro output with `quasiquote'.
The issue is that `with-syntax' creates a separation between the point
of definition of a value and its point of substitution.

   So for cases in which a `quasiquote' style makes more sense,
`syntax-case' also defines `quasisyntax', and the related `unsyntax'
and `unsyntax-splicing', abbreviated by the reader as `#`', `#,', and
`#,@', respectively.

   For example, to define a macro that inserts a compile-time timestamp
into a source file, one may write:

     (define-syntax display-compile-timestamp
       (lambda (x)
         (syntax-case x ()
           ((_)
            #`(begin
               (display "The compile timestamp was: ")
               (display #,(current-time))
               (newline))))))

   Readers interested in further information on `syntax-case' macros
should see R. Kent Dybvig's excellent `The Scheme Programming
Language', either edition 3 or 4, in the chapter on syntax. Dybvig was
the primary author of the `syntax-case' system. The book itself is
available online at `http://scheme.com/tspl4/'.

6.10.4 Syntax Transformer Helpers
---------------------------------

As noted in the previous section, Guile's syntax expander operates on
syntax objects.  Procedural macros consume and produce syntax objects.
This section describes some of the auxiliary helpers that procedural
macros can use to compare, generate, and query objects of this data
type.

 -- Scheme Procedure: bound-identifier=? a b
     Return `#t' iff the syntax objects A and B refer to the same
     lexically-bound identifier.

 -- Scheme Procedure: free-identifier=? a b
     Return `#t' iff the syntax objects A and B refer to the same free
     identifier.

 -- Scheme Procedure: generate-temporaries ls
     Return a list of temporary identifiers as long as LS is long.

 -- Scheme Procedure: syntax-source x
     Return the source properties that correspond to the syntax object
     X.  *Note Source Properties::, for more information.

   Guile also offers some more experimental interfaces in a separate
module.  As was the case with the Large Hadron Collider, it is unclear
to our senior macrologists whether adding these interfaces will result
in awesomeness or in the destruction of Guile via the creation of a
singularity.  We will preserve their functionality through the 2.0
series, but we reserve the right to modify them in a future stable
series, to a more than usual degree.

     (use-modules (system syntax))

 -- Scheme Procedure: syntax-module id
     Return the name of the module whose source contains the identifier
     ID.

 -- Scheme Procedure: syntax-local-binding id
     Resolve the identifer ID, a syntax object, within the current
     lexical environment, and return two values, the binding type and a
     binding value.  The binding type is a symbol, which may be one of
     the following:

    `lexical'
          A lexically-bound variable.  The value is a unique token (in
          the sense of `eq?') identifying this binding.

    `macro'
          A syntax transformer, either local or global.  The value is
          the transformer procedure.

    `pattern-variable'
          A pattern variable, bound via syntax-case.  The value is an
          opaque object, internal to the expander.

    `displaced-lexical'
          A lexical variable that has gone out of scope.  This can
          happen if a badly-written procedural macro saves a syntax
          object, then attempts to introduce it in a context in which
          it is unbound.  The value is `#f'.

    `global'
          A global binding.  The value is a pair, whose head is the
          symbol, and whose tail is the name of the module in which to
          resolve the symbol.

    `other'
          Some other binding, like `lambda' or other core bindings.  The
          value is `#f'.

     This is a very low-level procedure, with limited uses.  One case in
     which it is useful is to build abstractions that associate
     auxiliary information with macros:

          (define aux-property (make-object-property))
          (define-syntax-rule (with-aux aux value)
            (let ((trans value))
              (set! (aux-property trans) aux)
              trans))
          (define-syntax retrieve-aux
            (lambda (x)
              (syntax-case x ()
                ((x id)
                 (call-with-values (lambda () (syntax-local-binding #'id))
                   (lambda (type val)
                     (with-syntax ((aux (datum->syntax #'here
                                                       (and (eq? type 'macro)
                                                            (aux-property val)))))
                       #''aux)))))))
          (define-syntax foo
            (with-aux 'bar
              (syntax-rules () ((_) 'foo))))
          (foo)
          => foo
          (retrieve-aux foo)
          => bar

     `syntax-local-binding' must be called within the dynamic extent of
     a syntax transformer; to call it otherwise will signal an error.

 -- Scheme Procedure: syntax-locally-bound-identifiers id
     Return a list of identifiers that were visible lexically when the
     identifier ID was created, in order from outermost to innermost.

     This procedure is intended to be used in specialized procedural
     macros, to provide a macro with the set of bound identifiers that
     the macro can reference.

     As a technical implementation detail, the identifiers returned by
     `syntax-locally-bound-identifiers' will be anti-marked, like the
     syntax object that is given as input to a macro.  This is to
     signal to the macro expander that these bindings were present in
     the original source, and do not need to be hygienically renamed,
     as would be the case with other introduced identifiers.  See the
     discussion of hygiene in section 12.1 of the R6RS, for more
     information on marks.

          (define (local-lexicals id)
            (filter (lambda (x)
                      (eq? (syntax-local-binding x) 'lexical))
                    (syntax-locally-bound-identifiers id)))
          (define-syntax lexicals
            (lambda (x)
              (syntax-case x ()
                ((lexicals) #'(lexicals lexicals))
                ((lexicals scope)
                 (with-syntax (((id ...) (local-lexicals #'scope)))
                   #'(list (cons 'id id) ...))))))

          (let* ((x 10) (x 20)) (lexicals))
          => ((x . 10) (x . 20))

6.10.5 Lisp-style Macro Definitions
-----------------------------------

The traditional way to define macros in Lisp is very similar to
procedure definitions. The key differences are that the macro
definition body should return a list that describes the transformed
expression, and that the definition is marked as a macro definition
(rather than a procedure definition) by the use of a different
definition keyword: in Lisp, `defmacro' rather than `defun', and in
Scheme, `define-macro' rather than `define'.

   Guile supports this style of macro definition using both `defmacro'
and `define-macro'.  The only difference between them is how the macro
name and arguments are grouped together in the definition:

     (defmacro NAME (ARGS ...) BODY ...)

is the same as

     (define-macro (NAME ARGS ...) BODY ...)

The difference is analogous to the corresponding difference between
Lisp's `defun' and Scheme's `define'.

   Having read the previous section on `syntax-case', it's probably
clear that Guile actually implements defmacros in terms of
`syntax-case', applying the transformer on the expression between
invocations of `syntax->datum' and `datum->syntax'. This realization
leads us to the problem with defmacros, that they do not preserve
referential transparency. One can be careful to not introduce bindings
into expanded code, via liberal use of `gensym', but there is no
getting around the lack of referential transparency for free bindings
in the macro itself.

   Even a macro as simple as our `when' from before is difficult to get
right:

     (define-macro (when cond exp . rest)
       `(if ,cond
            (begin ,exp . ,rest)))

     (when #f (display "Launching missiles!\n"))
     => #f

     (let ((if list))
       (when #f (display "Launching missiles!\n")))
     -| Launching missiles!
     => (#f #<unspecified>)

   Guile's perspective is that defmacros have had a good run, but that
modern macros should be written with `syntax-rules' or `syntax-case'.
There are still many uses of defmacros within Guile itself, but we will
be phasing them out over time. Of course we won't take away `defmacro'
or `define-macro' themselves, as there is lots of code out there that
uses them.

6.10.6 Identifier Macros
------------------------

When the syntax expander sees a form in which the first element is a
macro, the whole form gets passed to the macro's syntax transformer.
One may visualize this as:

     (define-syntax foo foo-transformer)
     (foo ARG...)
     ;; expands via
     (foo-transformer #'(foo ARG...))

   If, on the other hand, a macro is referenced in some other part of a
form, the syntax transformer is invoked with only the macro reference,
not the whole form.

     (define-syntax foo foo-transformer)
     foo
     ;; expands via
     (foo-transformer #'foo)

   This allows bare identifier references to be replaced
programmatically via a macro. `syntax-rules' provides some syntax to
effect this transformation more easily.

 -- Syntax: identifier-syntax exp
     Returns a macro transformer that will replace occurrences of the
     macro with EXP.

   For example, if you are importing external code written in terms of
`fx+', the fixnum addition operator, but Guile doesn't have `fx+', you
may use the following to replace `fx+' with `+':

     (define-syntax fx+ (identifier-syntax +))

   There is also special support for recognizing identifiers on the
left-hand side of a `set!' expression, as in the following:

     (define-syntax foo foo-transformer)
     (set! foo VAL)
     ;; expands via
     (foo-transformer #'(set! foo VAL))
     ;; iff foo-transformer is a "variable transformer"

   As the example notes, the transformer procedure must be explicitly
marked as being a "variable transformer", as most macros aren't written
to discriminate on the form in the operator position.

 -- Scheme Procedure: make-variable-transformer transformer
     Mark the TRANSFORMER procedure as being a "variable transformer".
     In practice this means that, when bound to a syntactic keyword, it
     may detect references to that keyword on the left-hand-side of a
     `set!'.

          (define bar 10)
          (define-syntax bar-alias
            (make-variable-transformer
             (lambda (x)
               (syntax-case x (set!)
                 ((set! var val) #'(set! bar val))
                 ((var arg ...) #'(bar arg ...))
                 (var (identifier? #'var) #'bar)))))

          bar-alias => 10
          (set! bar-alias 20)
          bar => 20
          (set! bar 30)
          bar-alias => 30

   There is an extension to identifier-syntax which allows it to handle
the `set!' case as well:

 -- Syntax: identifier-syntax (var exp1) ((set! var val) exp2)
     Create a variable transformer. The first clause is used for
     references to the variable in operator or operand position, and
     the second for appearances of the variable on the left-hand-side
     of an assignment.

     For example, the previous `bar-alias' example could be expressed
     more succinctly like this:

          (define-syntax bar-alias
            (identifier-syntax
              (var bar)
              ((set! var val) (set! bar val))))

     As before, the templates in `identifier-syntax' forms do not need
     wrapping in `#'' syntax forms.

6.10.7 Syntax Parameters
------------------------

Syntax parameters(1) are a mechanism for rebinding a macro definition
within the dynamic extent of a macro expansion.  This provides a
convenient solution to one of the most common types of unhygienic
macro: those that introduce a unhygienic binding each time the macro is
used.  Examples include a `lambda' form with a `return' keyword, or
class macros that introduce a special `self' binding.

   With syntax parameters, instead of introducing the binding
unhygienically each time, we instead create one binding for the keyword,
which we can then adjust later when we want the keyword to have a
different meaning.  As no new bindings are introduced, hygiene is
preserved. This is similar to the dynamic binding mechanisms we have at
run-time (*note parameters: SRFI-39.), except that the dynamic binding
only occurs during macro expansion.  The code after macro expansion
remains lexically scoped.

 -- Syntax: define-syntax-parameter keyword transformer
     Binds KEYWORD to the value obtained by evaluating TRANSFORMER.
     The TRANSFORMER provides the default expansion for the syntax
     parameter, and in the absence of `syntax-parameterize', is
     functionally equivalent to `define-syntax'.  Usually, you will
     just want to have the TRANSFORMER throw a syntax error indicating
     that the KEYWORD is supposed to be used in conjunction with
     another macro, for example:
          (define-syntax-parameter return
            (lambda (stx)
              (syntax-violation 'return "return used outside of a lambda^" stx)))

 -- Syntax: syntax-parameterize ((keyword transformer) ...) exp ...
     Adjusts KEYWORD ... to use the values obtained by evaluating their
     TRANSFORMER ..., in the expansion of the EXP ... forms.  Each
     KEYWORD must be bound to a syntax-parameter.
     `syntax-parameterize' differs from `let-syntax', in that the
     binding is not shadowed, but adjusted, and so uses of the keyword
     in the expansion of EXP ... use the new transformers. This is
     somewhat similar to how `parameterize' adjusts the values of
     regular parameters, rather than creating new bindings.

          (define-syntax lambda^
            (syntax-rules ()
              [(lambda^ argument-list body body* ...)
               (lambda argument-list
                 (call-with-current-continuation
                  (lambda (escape)
                    ;; In the body we adjust the 'return' keyword so that calls
                    ;; to 'return' are replaced with calls to the escape
                    ;; continuation.
                    (syntax-parameterize ([return (syntax-rules ()
                                                    [(return vals (... ...))
                                                     (escape vals (... ...))])])
                      body body* ...))))]))

          ;; Now we can write functions that return early.  Here, 'product' will
          ;; return immediately if it sees any 0 element.
          (define product
            (lambda^ (list)
                     (fold (lambda (n o)
                             (if (zero? n)
                                 (return 0)
                                 (* n o)))
                           1
                           list)))

   ---------- Footnotes ----------

   (1) Described in the paper `Keeping it Clean with Syntax Parameters'
by Barzilay, Culpepper and Flatt.

6.10.8 Eval-when
----------------

As `syntax-case' macros have the whole power of Scheme available to
them, they present a problem regarding time: when a macro runs, what
parts of the program are available for the macro to use?

   The default answer to this question is that when you import a module
(via `define-module' or `use-modules'), that module will be loaded up at
expansion-time, as well as at run-time. Additionally, top-level
syntactic definitions within one compilation unit made by
`define-syntax' are also evaluated at expansion time, in the order that
they appear in the compilation unit (file).

   But if a syntactic definition needs to call out to a normal
procedure at expansion-time, it might well need need special
declarations to indicate that the procedure should be made available at
expansion-time.

   For example, the following code will work at a REPL, but not in a
file:

     ;; incorrect
     (use-modules (srfi srfi-19))
     (define (date) (date->string (current-date)))
     (define-syntax %date (identifier-syntax (date)))
     (define *compilation-date* %date)

   It works at a REPL because the expressions are evaluated one-by-one,
in order, but if placed in a file, the expressions are expanded
one-by-one, but not evaluated until the compiled file is loaded.

   The fix is to use `eval-when'.

     ;; correct: using eval-when
     (use-modules (srfi srfi-19))
     (eval-when (compile load eval)
       (define (date) (date->string (current-date))))
     (define-syntax %date (identifier-syntax (date)))
     (define *compilation-date* %date)

 -- Syntax: eval-when conditions exp...
     Evaluate EXP... under the given CONDITIONS. Valid conditions
     include `eval', `load', and `compile'. If you need to use
     `eval-when', use it with all three conditions, as in the above
     example.  Other uses of `eval-when' may void your warranty or
     poison your cat.

6.10.9 Internal Macros
----------------------

 -- Scheme Procedure: make-syntax-transformer name type binding
     Construct a syntax transformer object. This is part of Guile's
     low-level support for syntax-case.

 -- Scheme Procedure: macro? obj
 -- C Function: scm_macro_p (obj)
     Return `#t' iff OBJ is a syntax transformer.

     Note that it's a bit difficult to actually get a macro as a
     first-class object; simply naming it (like `case') will produce a
     syntax error. But it is possible to get these objects using
     `module-ref':

          (macro? (module-ref (current-module) 'case))
          => #t

 -- Scheme Procedure: macro-type m
 -- C Function: scm_macro_type (m)
     Return the TYPE that was given when M was constructed, via
     `make-syntax-transformer'.

 -- Scheme Procedure: macro-name m
 -- C Function: scm_macro_name (m)
     Return the name of the macro M.

 -- Scheme Procedure: macro-binding m
 -- C Function: scm_macro_binding (m)
     Return the binding of the macro M.

 -- Scheme Procedure: macro-transformer m
 -- C Function: scm_macro_transformer (m)
     Return the transformer of the macro M. This will return a
     procedure, for which one may ask the docstring. That's the whole
     reason this section is documented. Actually a part of the result
     of `macro-binding'.

6.11 General Utility Functions
==============================

This chapter contains information about procedures which are not cleanly
tied to a specific data type.  Because of their wide range of
applications, they are collected in a "utility" chapter.

6.11.1 Equality
---------------

There are three kinds of core equality predicates in Scheme, described
below.  The same kinds of comparisons arise in other functions, like
`memq' and friends (*note List Searching::).

   For all three tests, objects of different types are never equal.  So
for instance a list and a vector are not `equal?', even if their
contents are the same.  Exact and inexact numbers are considered
different types too, and are hence not equal even if their values are
the same.

   `eq?' tests just for the same object (essentially a pointer
comparison).  This is fast, and can be used when searching for a
particular object, or when working with symbols or keywords (which are
always unique objects).

   `eqv?' extends `eq?' to look at the value of numbers and characters.
It can for instance be used somewhat like `=' (*note Comparison::) but
without an error if one operand isn't a number.

   `equal?' goes further, it looks (recursively) into the contents of
lists, vectors, etc.  This is good for instance on lists that have been
read or calculated in various places and are the same, just not made up
of the same pairs.  Such lists look the same (when printed), and
`equal?' will consider them the same.


 -- Scheme Procedure: eq? x y
 -- C Function: scm_eq_p (x, y)
     Return `#t' if X and Y are the same object, except for numbers and
     characters.  For example,

          (define x (vector 1 2 3))
          (define y (vector 1 2 3))

          (eq? x x)  => #t
          (eq? x y)  => #f

     Numbers and characters are not equal to any other object, but the
     problem is they're not necessarily `eq?' to themselves either.
     This is even so when the number comes directly from a variable,

          (let ((n (+ 2 3)))
            (eq? n n))       => *unspecified*

     Generally `eqv?' below should be used when comparing numbers or
     characters.  `=' (*note Comparison::) or `char=?' (*note
     Characters::) can be used too.

     It's worth noting that end-of-list `()', `#t', `#f', a symbol of a
     given name, and a keyword of a given name, are unique objects.
     There's just one of each, so for instance no matter how `()'
     arises in a program, it's the same object and can be compared with
     `eq?',

          (define x (cdr '(123)))
          (define y (cdr '(456)))
          (eq? x y) => #t

          (define x (string->symbol "foo"))
          (eq? x 'foo) => #t

 -- C Function: int scm_is_eq (SCM x, SCM y)
     Return `1' when X and Y are equal in the sense of `eq?', otherwise
     return `0'.

     The `==' operator should not be used on `SCM' values, an `SCM' is
     a C type which cannot necessarily be compared using `==' (*note
     The SCM Type::).


 -- Scheme Procedure: eqv? x y
 -- C Function: scm_eqv_p (x, y)
     Return `#t' if X and Y are the same object, or for characters and
     numbers the same value.

     On objects except characters and numbers, `eqv?' is the same as
     `eq?' above, it's true if X and Y are the same object.

     If X and Y are numbers or characters, `eqv?' compares their type
     and value.  An exact number is not `eqv?' to an inexact number
     (even if their value is the same).

          (eqv? 3 (+ 1 2)) => #t
          (eqv? 1 1.0)     => #f


 -- Scheme Procedure: equal? x y
 -- C Function: scm_equal_p (x, y)
     Return `#t' if X and Y are the same type, and their contents or
     value are equal.

     For a pair, string, vector, array or structure, `equal?' compares
     the contents, and does so using the same `equal?' recursively, so
     a deep structure can be traversed.

          (equal? (list 1 2 3) (list 1 2 3))   => #t
          (equal? (list 1 2 3) (vector 1 2 3)) => #f

     For other objects, `equal?' compares as per `eqv?' above, which
     means characters and numbers are compared by type and value (and
     like `eqv?', exact and inexact numbers are not `equal?', even if
     their value is the same).

          (equal? 3 (+ 1 2)) => #t
          (equal? 1 1.0)     => #f

     Hash tables are currently only compared as per `eq?', so two
     different tables are not `equal?', even if their contents are the
     same.

     `equal?' does not support circular data structures, it may go into
     an infinite loop if asked to compare two circular lists or similar.

     New application-defined object types (*note Defining New Types
     (Smobs)::) have an `equalp' handler which is called by `equal?'.
     This lets an application traverse the contents or control what is
     considered `equal?' for two objects of such a type.  If there's no
     such handler, the default is to just compare as per `eq?'.

6.11.2 Object Properties
------------------------

It's often useful to associate a piece of additional information with a
Scheme object even though that object does not have a dedicated slot
available in which the additional information could be stored.  Object
properties allow you to do just that.

   Guile's representation of an object property is a
procedure-with-setter (*note Procedures with Setters::) that can be
used with the generalized form of `set!' (REFFIXME) to set and retrieve
that property for any Scheme object.  So, setting a property looks like
this:

     (set! (my-property obj1) value-for-obj1)
     (set! (my-property obj2) value-for-obj2)

And retrieving values of the same property looks like this:

     (my-property obj1)
     =>
     value-for-obj1

     (my-property obj2)
     =>
     value-for-obj2

   To create an object property in the first place, use the
`make-object-property' procedure:

     (define my-property (make-object-property))

 -- Scheme Procedure: make-object-property
     Create and return an object property.  An object property is a
     procedure-with-setter that can be called in two ways.  `(set!
     (PROPERTY OBJ) VAL)' sets OBJ's PROPERTY to VAL.  `(PROPERTY OBJ)'
     returns the current setting of OBJ's PROPERTY.

   A single object property created by `make-object-property' can
associate distinct property values with all Scheme values that are
distinguishable by `eq?' (including, for example, integers).

   Internally, object properties are implemented using a weak key hash
table.  This means that, as long as a Scheme value with property values
is protected from garbage collection, its property values are also
protected.  When the Scheme value is collected, its entry in the
property table is removed and so the (ex-) property values are no longer
protected by the table.

   Guile also implements a more traditional Lispy interface to
properties, in which each object has an list of key-value pairs
associated with it.  Properties in that list are keyed by symbols.
This is a legacy interface; you should use weak hash tables or object
properties instead.

 -- Scheme Procedure: object-properties obj
 -- C Function: scm_object_properties (obj)
     Return OBJ's property list.

 -- Scheme Procedure: set-object-properties! obj alist
 -- C Function: scm_set_object_properties_x (obj, alist)
     Set OBJ's property list to ALIST.

 -- Scheme Procedure: object-property obj key
 -- C Function: scm_object_property (obj, key)
     Return the property of OBJ with name KEY.

 -- Scheme Procedure: set-object-property! obj key value
 -- C Function: scm_set_object_property_x (obj, key, value)
     In OBJ's property list, set the property named KEY to VALUE.

6.11.3 Sorting
--------------

Sorting is very important in computer programs.  Therefore, Guile comes
with several sorting procedures built-in.  As always, procedures with
names ending in `!' are side-effecting, that means that they may modify
their parameters in order to produce their results.

   The first group of procedures can be used to merge two lists (which
must be already sorted on their own) and produce sorted lists containing
all elements of the input lists.

 -- Scheme Procedure: merge alist blist less
 -- C Function: scm_merge (alist, blist, less)
     Merge two already sorted lists into one.  Given two lists ALIST
     and BLIST, such that `(sorted? alist less?)' and `(sorted? blist
     less?)', return a new list in which the elements of ALIST and
     BLIST have been stably interleaved so that `(sorted? (merge alist
     blist less?) less?)'.  Note:  this does _not_ accept vectors.

 -- Scheme Procedure: merge! alist blist less
 -- C Function: scm_merge_x (alist, blist, less)
     Takes two lists ALIST and BLIST such that `(sorted? alist less?)'
     and `(sorted? blist less?)' and returns a new list in which the
     elements of ALIST and BLIST have been stably interleaved so that
     `(sorted? (merge alist blist less?) less?)'.  This is the
     destructive variant of `merge' Note:  this does _not_ accept
     vectors.

   The following procedures can operate on sequences which are either
vectors or list.  According to the given arguments, they return sorted
vectors or lists, respectively.  The first of the following procedures
determines whether a sequence is already sorted, the other sort a given
sequence.  The variants with names starting with `stable-' are special
in that they maintain a special property of the input sequences: If two
or more elements are the same according to the comparison predicate,
they are left in the same order as they appeared in the input.

 -- Scheme Procedure: sorted? items less
 -- C Function: scm_sorted_p (items, less)
     Return `#t' iff ITEMS is a list or a vector such that for all 1 <=
     i <= m, the predicate LESS returns true when applied to all
     elements i - 1 and i

 -- Scheme Procedure: sort items less
 -- C Function: scm_sort (items, less)
     Sort the sequence ITEMS, which may be a list or a vector.  LESS is
     used for comparing the sequence elements.  This is not a stable
     sort.

 -- Scheme Procedure: sort! items less
 -- C Function: scm_sort_x (items, less)
     Sort the sequence ITEMS, which may be a list or a vector.  LESS is
     used for comparing the sequence elements.  The sorting is
     destructive, that means that the input sequence is modified to
     produce the sorted result.  This is not a stable sort.

 -- Scheme Procedure: stable-sort items less
 -- C Function: scm_stable_sort (items, less)
     Sort the sequence ITEMS, which may be a list or a vector. LESS is
     used for comparing the sequence elements.  This is a stable sort.

 -- Scheme Procedure: stable-sort! items less
 -- C Function: scm_stable_sort_x (items, less)
     Sort the sequence ITEMS, which may be a list or a vector. LESS is
     used for comparing the sequence elements.  The sorting is
     destructive, that means that the input sequence is modified to
     produce the sorted result.  This is a stable sort.

   The procedures in the last group only accept lists or vectors as
input, as their names indicate.

 -- Scheme Procedure: sort-list items less
 -- C Function: scm_sort_list (items, less)
     Sort the list ITEMS, using LESS for comparing the list elements.
     This is a stable sort.

 -- Scheme Procedure: sort-list! items less
 -- C Function: scm_sort_list_x (items, less)
     Sort the list ITEMS, using LESS for comparing the list elements.
     The sorting is destructive, that means that the input list is
     modified to produce the sorted result.  This is a stable sort.

 -- Scheme Procedure: restricted-vector-sort! vec less startpos endpos
 -- C Function: scm_restricted_vector_sort_x (vec, less, startpos,
          endpos)
     Sort the vector VEC, using LESS for comparing the vector elements.
     STARTPOS (inclusively) and ENDPOS (exclusively) delimit the range
     of the vector which gets sorted.  The return value is not
     specified.

6.11.4 Copying Deep Structures
------------------------------

The procedures for copying lists (*note Lists::) only produce a flat
copy of the input list, and currently Guile does not even contain
procedures for copying vectors.  `copy-tree' can be used for these
application, as it does not only copy the spine of a list, but also
copies any pairs in the cars of the input lists.

 -- Scheme Procedure: copy-tree obj
 -- C Function: scm_copy_tree (obj)
     Recursively copy the data tree that is bound to OBJ, and return
     the new data structure.  `copy-tree' recurses down the contents of
     both pairs and vectors (since both cons cells and vector cells may
     point to arbitrary objects), and stops recursing when it hits any
     other object.

6.11.5 General String Conversion
--------------------------------

When debugging Scheme programs, but also for providing a human-friendly
interface, a procedure for converting any Scheme object into string
format is very useful.  Conversion from/to strings can of course be done
with specialized procedures when the data type of the object to convert
is known, but with this procedure, it is often more comfortable.

   `object->string' converts an object by using a print procedure for
writing to a string port, and then returning the resulting string.
Converting an object back from the string is only possible if the object
type has a read syntax and the read syntax is preserved by the printing
procedure.

 -- Scheme Procedure: object->string obj [printer]
 -- C Function: scm_object_to_string (obj, printer)
     Return a Scheme string obtained by printing OBJ.  Printing
     function can be specified by the optional second argument PRINTER
     (default: `write').

6.11.6 Hooks
------------

A hook is a list of procedures to be called at well defined points in
time.  Typically, an application provides a hook H and promises its
users that it will call all of the procedures in H at a defined point
in the application's processing.  By adding its own procedure to H, an
application user can tap into or even influence the progress of the
application.

   Guile itself provides several such hooks for debugging and
customization purposes: these are listed in a subsection below.

   When an application first creates a hook, it needs to know how many
arguments will be passed to the hook's procedures when the hook is run.
The chosen number of arguments (which may be none) is declared when the
hook is created, and all the procedures that are added to that hook must
be capable of accepting that number of arguments.

   A hook is created using `make-hook'.  A procedure can be added to or
removed from a hook using `add-hook!' or `remove-hook!', and all of a
hook's procedures can be removed together using `reset-hook!'.  When an
application wants to run a hook, it does so using `run-hook'.

6.11.6.1 Hook Usage by Example
..............................

Hook usage is shown by some examples in this section.  First, we will
define a hook of arity 2 -- that is, the procedures stored in the hook
will have to accept two arguments.

     (define hook (make-hook 2))
     hook
     => #<hook 2 40286c90>

   Now we are ready to add some procedures to the newly created hook
with `add-hook!'.  In the following example, two procedures are added,
which print different messages and do different things with their
arguments.

     (add-hook! hook (lambda (x y)
                         (display "Foo: ")
                         (display (+ x y))
                         (newline)))
     (add-hook! hook (lambda (x y)
                         (display "Bar: ")
                         (display (* x y))
                         (newline)))

   Once the procedures have been added, we can invoke the hook using
`run-hook'.

     (run-hook hook 3 4)
     -| Bar: 12
     -| Foo: 7

   Note that the procedures are called in the reverse of the order with
which they were added.  This is because the default behaviour of
`add-hook!' is to add its procedure to the _front_ of the hook's
procedure list.  You can force `add-hook!' to add its procedure to the
_end_ of the list instead by providing a third `#t' argument on the
second call to `add-hook!'.

     (add-hook! hook (lambda (x y)
                         (display "Foo: ")
                         (display (+ x y))
                         (newline)))
     (add-hook! hook (lambda (x y)
                         (display "Bar: ")
                         (display (* x y))
                         (newline))
                         #t)             ; <- Change here!

     (run-hook hook 3 4)
     -| Foo: 7
     -| Bar: 12

6.11.6.2 Hook Reference
.......................

When you create a hook with `make-hook', you must specify the arity of
the procedures which can be added to the hook.  If the arity is not
given explicitly as an argument to `make-hook', it defaults to zero.
All procedures of a given hook must have the same arity, and when the
procedures are invoked using `run-hook', the number of arguments passed
must match the arity specified at hook creation time.

   The order in which procedures are added to a hook matters.  If the
third parameter to `add-hook!' is omitted or is equal to `#f', the
procedure is added in front of the procedures which might already be on
that hook, otherwise the procedure is added at the end.  The procedures
are always called from the front to the end of the list when they are
invoked via `run-hook'.

   The ordering of the list of procedures returned by `hook->list'
matches the order in which those procedures would be called if the hook
was run using `run-hook'.

   Note that the C functions in the following entries are for handling
"Scheme-level" hooks in C.  There are also "C-level" hooks which have
their own interface (*note C Hooks::).

 -- Scheme Procedure: make-hook [n_args]
 -- C Function: scm_make_hook (n_args)
     Create a hook for storing procedure of arity N_ARGS.  N_ARGS
     defaults to zero.  The returned value is a hook object to be used
     with the other hook procedures.

 -- Scheme Procedure: hook? x
 -- C Function: scm_hook_p (x)
     Return `#t' if X is a hook, `#f' otherwise.

 -- Scheme Procedure: hook-empty? hook
 -- C Function: scm_hook_empty_p (hook)
     Return `#t' if HOOK is an empty hook, `#f' otherwise.

 -- Scheme Procedure: add-hook! hook proc [append_p]
 -- C Function: scm_add_hook_x (hook, proc, append_p)
     Add the procedure PROC to the hook HOOK. The procedure is added to
     the end if APPEND_P is true, otherwise it is added to the front.
     The return value of this procedure is not specified.

 -- Scheme Procedure: remove-hook! hook proc
 -- C Function: scm_remove_hook_x (hook, proc)
     Remove the procedure PROC from the hook HOOK.  The return value of
     this procedure is not specified.

 -- Scheme Procedure: reset-hook! hook
 -- C Function: scm_reset_hook_x (hook)
     Remove all procedures from the hook HOOK.  The return value of
     this procedure is not specified.

 -- Scheme Procedure: hook->list hook
 -- C Function: scm_hook_to_list (hook)
     Convert the procedure list of HOOK to a list.

 -- Scheme Procedure: run-hook hook . args
 -- C Function: scm_run_hook (hook, args)
     Apply all procedures from the hook HOOK to the arguments ARGS.
     The order of the procedure application is first to last.  The
     return value of this procedure is not specified.

   If, in C code, you are certain that you have a hook object and well
formed argument list for that hook, you can also use `scm_c_run_hook',
which is identical to `scm_run_hook' but does no type checking.

 -- C Function: void scm_c_run_hook (SCM hook, SCM args)
     The same as `scm_run_hook' but without any type checking to confirm
     that HOOK is actually a hook object and that ARGS is a well-formed
     list matching the arity of the hook.

   For C code, `SCM_HOOKP' is a faster alternative to `scm_hook_p':

 -- C Macro: int SCM_HOOKP (x)
     Return 1 if X is a Scheme-level hook, 0 otherwise.

6.11.6.3 Handling Scheme-level hooks from C code
................................................

Here is an example of how to handle Scheme-level hooks from C code using
the above functions.

     if (scm_is_true (scm_hook_p (obj)))
       /* handle Scheme-level hook using C functions */
       scm_reset_hook_x (obj);
     else
       /* do something else (obj is not a hook) */

6.11.6.4 Hooks For C Code.
..........................

The hooks already described are intended to be populated by Scheme-level
procedures.  In addition to this, the Guile library provides an
independent set of interfaces for the creation and manipulation of hooks
that are designed to be populated by functions implemented in C.

   The original motivation here was to provide a kind of hook that could
safely be invoked at various points during garbage collection.
Scheme-level hooks are unsuitable for this purpose as running them could
itself require memory allocation, which would then invoke garbage
collection recursively ...  However, it is also the case that these
hooks are easier to work with than the Scheme-level ones if you only
want to register C functions with them.  So if that is mainly what your
code needs to do, you may prefer to use this interface.

   To create a C hook, you should allocate storage for a structure of
type `scm_t_c_hook' and then initialize it using `scm_c_hook_init'.

 -- C Type: scm_t_c_hook
     Data type for a C hook.  The internals of this type should be
     treated as opaque.

 -- C Enum: scm_t_c_hook_type
     Enumeration of possible hook types, which are:

    `SCM_C_HOOK_NORMAL'
          Type of hook for which all the registered functions will
          always be called.

    `SCM_C_HOOK_OR'
          Type of hook for which the sequence of registered functions
          will be called only until one of them returns C true (a
          non-NULL pointer).

    `SCM_C_HOOK_AND'
          Type of hook for which the sequence of registered functions
          will be called only until one of them returns C false (a NULL
          pointer).

 -- C Function: void scm_c_hook_init (scm_t_c_hook *hook, void
          *hook_data, scm_t_c_hook_type type)
     Initialize the C hook at memory pointed to by HOOK.  TYPE should
     be one of the values of the `scm_t_c_hook_type' enumeration, and
     controls how the hook functions will be called.  HOOK_DATA is a
     closure parameter that will be passed to all registered hook
     functions when they are called.

   To add or remove a C function from a C hook, use `scm_c_hook_add' or
`scm_c_hook_remove'.  A hook function must expect three `void *'
parameters which are, respectively:

HOOK_DATA
     The hook closure data that was specified at the time the hook was
     initialized by `scm_c_hook_init'.

FUNC_DATA
     The function closure data that was specified at the time that that
     function was registered with the hook by `scm_c_hook_add'.

DATA
     The call closure data specified by the `scm_c_hook_run' call that
     runs the hook.

 -- C Type: scm_t_c_hook_function
     Function type for a C hook function: takes three `void *'
     parameters and returns a `void *' result.

 -- C Function: void scm_c_hook_add (scm_t_c_hook *hook,
          scm_t_c_hook_function func, void *func_data, int appendp)
     Add function FUNC, with function closure data FUNC_DATA, to the C
     hook HOOK.  The new function is appended to the hook's list of
     functions if APPENDP is non-zero, otherwise prepended.

 -- C Function: void scm_c_hook_remove (scm_t_c_hook *hook,
          scm_t_c_hook_function func, void *func_data)
     Remove function FUNC, with function closure data FUNC_DATA, from
     the C hook HOOK.  `scm_c_hook_remove' checks both FUNC and
     FUNC_DATA so as to allow for the same FUNC being registered
     multiple times with different closure data.

   Finally, to invoke a C hook, call the `scm_c_hook_run' function
specifying the hook and the call closure data for this run:

 -- C Function: void * scm_c_hook_run (scm_t_c_hook *hook, void *data)
     Run the C hook HOOK will call closure data DATA.  Subject to the
     variations for hook types `SCM_C_HOOK_OR' and `SCM_C_HOOK_AND',
     `scm_c_hook_run' calls HOOK's registered functions in turn,
     passing them the hook's closure data, each function's closure
     data, and the call closure data.

     `scm_c_hook_run''s return value is the return value of the last
     function to be called.

6.11.6.5 Hooks for Garbage Collection
.....................................

Whenever Guile performs a garbage collection, it calls the following
hooks in the order shown.

 -- C Hook: scm_before_gc_c_hook
     C hook called at the very start of a garbage collection, after
     setting `scm_gc_running_p' to 1, but before entering the GC
     critical section.

     If garbage collection is blocked because `scm_block_gc' is
     non-zero, GC exits early soon after calling this hook, and no
     further hooks will be called.

 -- C Hook: scm_before_mark_c_hook
     C hook called before beginning the mark phase of garbage
     collection, after the GC thread has entered a critical section.

 -- C Hook: scm_before_sweep_c_hook
     C hook called before beginning the sweep phase of garbage
     collection.  This is the same as at the end of the mark phase,
     since nothing else happens between marking and sweeping.

 -- C Hook: scm_after_sweep_c_hook
     C hook called after the end of the sweep phase of garbage
     collection, but while the GC thread is still inside its critical
     section.

 -- C Hook: scm_after_gc_c_hook
     C hook called at the very end of a garbage collection, after the GC
     thread has left its critical section.

 -- Scheme Hook: after-gc-hook
     Scheme hook with arity 0.  This hook is run asynchronously (*note
     Asyncs::) soon after the GC has completed and any other events
     that were deferred during garbage collection have been processed.
     (Also accessible from C with the name `scm_after_gc_hook'.)

   All the C hooks listed here have type `SCM_C_HOOK_NORMAL', are
initialized with hook closure data NULL, are invoked by
`scm_c_hook_run' with call closure data NULL.

   The Scheme hook `after-gc-hook' is particularly useful in
conjunction with guardians (*note Guardians::).  Typically, if you are
using a guardian, you want to call the guardian after garbage collection
to see if any of the objects added to the guardian have been collected.
By adding a thunk that performs this call to `after-gc-hook', you can
ensure that your guardian is tested after every garbage collection
cycle.

6.11.6.6 Hooks into the Guile REPL
..................................

6.12 Definitions and Variable Bindings
======================================

Scheme supports the definition of variables in different contexts.
Variables can be defined at the top level, so that they are visible in
the entire program, and variables can be defined locally to procedures
and expressions.  This is important for modularity and data abstraction.

6.12.1 Top Level Variable Definitions
-------------------------------------

At the top level of a program (i.e., not nested within any other
expression), a definition of the form

     (define a VALUE)

defines a variable called `a' and sets it to the value VALUE.

   If the variable already exists in the current module, because it has
already been created by a previous `define' expression with the same
name, its value is simply changed to the new VALUE.  In this case,
then, the above form is completely equivalent to

     (set! a VALUE)

This equivalence means that `define' can be used interchangeably with
`set!' to change the value of variables at the top level of the REPL or
a Scheme source file.  It is useful during interactive development when
reloading a Scheme file that you have modified, because it allows the
`define' expressions in that file to work as expected both the first
time that the file is loaded and on subsequent occasions.

   Note, though, that `define' and `set!' are not always equivalent.
For example, a `set!' is not allowed if the named variable does not
already exist, and the two expressions can behave differently in the
case where there are imported variables visible from another module.

 -- Scheme Syntax: define name value
     Create a top level variable named NAME with value VALUE.  If the
     named variable already exists, just change its value.  The return
     value of a `define' expression is unspecified.

   The C API equivalents of `define' are `scm_define' and
`scm_c_define', which differ from each other in whether the variable
name is specified as a `SCM' symbol or as a null-terminated C string.

 -- C Function: scm_define (sym, value)
 -- C Function: scm_c_define (const char *name, value)
     C equivalents of `define', with variable name specified either by
     SYM, a symbol, or by NAME, a null-terminated C string.  Both
     variants return the new or preexisting variable object.

   `define' (when it occurs at top level), `scm_define' and
`scm_c_define' all create or set the value of a variable in the top
level environment of the current module.  If there was not already a
variable with the specified name belonging to the current module, but a
similarly named variable from another module was visible through having
been imported, the newly created variable in the current module will
shadow the imported variable, such that the imported variable is no
longer visible.

   Attention: Scheme definitions inside local binding constructs (*note
Local Bindings::) act differently (*note Internal Definitions::).

   Many people end up in a development style of adding and changing
definitions at runtime, building out their program without restarting
it.  (You can do this using `reload-module', the `reload' REPL command,
the `load' procedure, or even just pasting code into a REPL.)  If you
are one of these people, you will find that sometimes you there are
some variables that you _don't_ want to redefine all the time.  For
these, use `define-once'.

 -- Scheme Syntax: define-once name value
     Create a top level variable named NAME with value VALUE, but only
     if NAME is not already bound in the current module.

   Old Lispers probably know `define-once' under its Lisp name,
`defvar'.

6.12.2 Local Variable Bindings
------------------------------

As opposed to definitions at the top level, which creates bindings that
are visible to all code in a module, it is also possible to define
variables which are only visible in a well-defined part of the program.
Normally, this part of a program will be a procedure or a subexpression
of a procedure.

   With the constructs for local binding (`let', `let*', `letrec', and
`letrec*'), the Scheme language has a block structure like most other
programming languages since the days of ALGOL 60.  Readers familiar to
languages like C or Java should already be used to this concept, but
the family of `let' expressions has a few properties which are well
worth knowing.

   The most basic local binding construct is `let'.

 -- syntax: let bindings body
     BINDINGS has the form

          ((VARIABLE1 INIT1) ...)

     that is zero or more two-element lists of a variable and an
     arbitrary expression each.  All VARIABLE names must be distinct.

     A `let' expression is evaluated as follows.

        * All INIT expressions are evaluated.

        * New storage is allocated for the VARIABLES.

        * The values of the INIT expressions are stored into the
          variables.

        * The expressions in BODY are evaluated in order, and the value
          of the last expression is returned as the value of the `let'
          expression.

     The INIT expressions are not allowed to refer to any of the
     VARIABLES.

   The other binding constructs are variations on the same theme:
making new values, binding them to variables, and executing a body in
that new, extended lexical context.

 -- syntax: let* bindings body
     Similar to `let', but the variable bindings are performed
     sequentially, that means that all INIT expression are allowed to
     use the variables defined on their left in the binding list.

     A `let*' expression can always be expressed with nested `let'
     expressions.

          (let* ((a 1) (b a))
             b)
          ==
          (let ((a 1))
            (let ((b a))
              b))

 -- syntax: letrec bindings body
     Similar to `let', but it is possible to refer to the VARIABLE from
     lambda expression created in any of the INITS.  That is,
     procedures created in the INIT expression can recursively refer to
     the defined variables.

          (letrec ((even? (lambda (n)
                            (if (zero? n)
                                #t
                                (odd? (- n 1)))))
                   (odd? (lambda (n)
                            (if (zero? n)
                                #f
                                (even? (- n 1))))))
            (even? 88))
          =>
          #t

     Note that while the INIT expressions may refer to the new
     variables, they may not access their values.  For example, making
     the `even?' function above creates a closure (*note About
     Closure::) referencing the `odd?' variable.  But `odd?' can't be
     called until after execution has entered the body.

 -- syntax: letrec* bindings body
     Similar to `letrec', except the INIT expressions are bound to
     their variables in order.

     `letrec*' thus relaxes the letrec restriction, in that later INIT
     expressions may refer to the values of previously bound variables.

          (letrec ((a 42)
                   (b (+ a 10)))
            (* a b))
          => ;; Error: unbound variable: a

          (letrec* ((a 42)
                    (b (+ a 10)))
            (* a b))
          => 2184

   There is also an alternative form of the `let' form, which is used
for expressing iteration.  Because of the use as a looping construct,
this form (the "named let") is documented in the section about
iteration (*note Iteration: while do.)

6.12.3 Internal definitions
---------------------------

A `define' form which appears inside the body of a `lambda', `let',
`let*', `letrec', `letrec*' or equivalent expression is called an
"internal definition".  An internal definition differs from a top level
definition (*note Top Level::), because the definition is only visible
inside the complete body of the enclosing form.  Let us examine the
following example.

     (let ((frumble "froz"))
        (define banana (lambda () (apple 'peach)))
        (define apple (lambda (x) x))
        (banana))
     =>
     peach

   Here the enclosing form is a `let', so the `define's in the
`let'-body are internal definitions.  Because the scope of the internal
definitions is the *complete* body of the `let'-expression, the
`lambda'-expression which gets bound to the variable `banana' may refer
to the variable `apple', even though its definition appears lexically
_after_ the definition of `banana'.  This is because a sequence of
internal definition acts as if it were a `letrec*' expression.

     (let ()
       (define a 1)
       (define b 2)
       (+ a b))

is equivalent to

     (let ()
       (letrec* ((a 1) (b 2))
         (+ a b)))

   Internal definitions are only allowed at the beginning of the body
of an enclosing expression.  They may not be mixed with other
expressions.

   Another noteworthy difference to top level definitions is that within
one group of internal definitions all variable names must be distinct.
That means where on the top level a second define for a given variable
acts like a `set!', an exception is thrown for internal definitions
with duplicate bindings.

   As a historical note, it used to be that internal bindings were
expanded in terms of `letrec', not `letrec*'. This was the situation
for the R5RS report and before. However with the R6RS, it was recognized
that sequential definition was a more intuitive expansion, as in the
following case:

     (let ()
       (define a 1)
       (define b (+ a a))
       (+ a b))

Guile decided to follow the R6RS in this regard, and now expands
internal definitions using `letrec*'.

6.12.4 Querying variable bindings
---------------------------------

Guile provides a procedure for checking whether a symbol is bound in the
top level environment.

 -- Scheme Procedure: defined? sym [module]
 -- C Function: scm_defined_p (sym, module)
     Return `#t' if SYM is defined in the module MODULE or the current
     module when MODULE is not specified; otherwise return `#f'.

6.13 Controlling the Flow of Program Execution
==============================================

See *note Control Flow:: for a discussion of how the more general
control flow of Scheme affects C code.

6.13.1 Sequencing and Splicing
------------------------------

As an expression, the `begin' syntax is used to evaluate a sequence of
sub-expressions in order.  Consider the conditional expression below:

     (if (> x 0)
         (begin (display "greater") (newline)))

   If the test is true, we want to display "greater" to the current
output port, then display a newline.  We use `begin' to form a compound
expression out of this sequence of sub-expressions.

 -- syntax: begin expr1 expr2 ...
     The expression(s) are evaluated in left-to-right order and the
     value of the last expression is returned as the value of the
     `begin'-expression.  This expression type is used when the
     expressions before the last one are evaluated for their side
     effects.

   The `begin' syntax has another role in definition context (*note
Internal Definitions::).  A `begin' form in a definition context
"splices" its subforms into its place.  For example, consider the
following procedure:

     (define (make-seal)
       (define-sealant seal open)
       (values seal open))

   Let us assume the existence of a `define-sealant' macro that expands
out to some definitions wrapped in a `begin', like so:

     (define (make-seal)
       (begin
         (define seal-tag
           (list 'seal))
         (define (seal x)
           (cons seal-tag x))
         (define (sealed? x)
           (and (pair? x) (eq? (car x) seal-tag)))
         (define (open x)
           (if (sealed? x)
               (cdr x)
               (error "Expected a sealed value:" x))))
       (values seal open))

   Here, because the `begin' is in definition context, its subforms are
"spliced" into the place of the `begin'.  This allows the definitions
created by the macro to be visible to the following expression, the
`values' form.

   It is a fine point, but splicing and sequencing are different.  It
can make sense to splice zero forms, because it can make sense to have
zero internal definitions before the expressions in a procedure or
lexical binding form.  However it does not make sense to have a
sequence of zero expressions, because in that case it would not be
clear what the value of the sequence would be, because in a sequence of
zero expressions, there can be no last value.  Sequencing zero
expressions is an error.

   It would be more elegant in some ways to eliminate splicing from the
Scheme language, and without macros (*note Macros::), that would be a
good idea.  But it is useful to be able to write macros that expand out
to multiple definitions, as in `define-sealant' above, so Scheme abuses
the `begin' form for these two tasks.

6.13.2 Simple Conditional Evaluation
------------------------------------

Guile provides three syntactic constructs for conditional evaluation.
`if' is the normal if-then-else expression (with an optional else
branch), `cond' is a conditional expression with multiple branches and
`case' branches if an expression has one of a set of constant values.

 -- syntax: if test consequent [alternate]
     All arguments may be arbitrary expressions.  First, TEST is
     evaluated.  If it returns a true value, the expression CONSEQUENT
     is evaluated and ALTERNATE is ignored.  If TEST evaluates to `#f',
     ALTERNATE is evaluated instead.  The values of the evaluated
     branch (CONSEQUENT or ALTERNATE) are returned as the values of the
     `if' expression.

     When ALTERNATE is omitted and the TEST evaluates to `#f', the
     value of the expression is not specified.

   When you go to write an `if' without an alternate (a "one-armed
`if'"), part of what you are expressing is that you don't care about
the return value (or values) of the expression.  As such, you are more
interested in the _effect_ of evaluating the consequent expression.
(By convention, we use the word "statement" to refer to an expression
that is evaluated for effect, not for value).

   In such a case, it is considered more clear to express these
intentions with these special forms, `when' and `unless'.  As an added
bonus, these forms accept multiple statements to evaluate, which are
implicitly wrapped in a `begin'.

 -- Scheme Syntax: when test statement1 statement2 ...
 -- Scheme Syntax: unless test statement1 statement2 ...
     The actual definitions of these forms are in many ways their most
     clear documentation:

          (define-syntax-rule (when test stmt stmt* ...)
            (if test (begin stmt stmt* ...)))

          (define-syntax-rule (unless condition stmt stmt* ...)
            (if (not test) (begin stmt stmt* ...)))

     That is to say, `when' evaluates its consequent statements in order
     if TEST is true.  `unless' is the opposite: it evaluates the
     statements if TEST is false.

 -- syntax: cond clause1 clause2 ...
     Each `cond'-clause must look like this:

          (TEST EXPRESSION ...)

     where TEST and EXPRESSION are arbitrary expression, or like this

          (TEST => EXPRESSION)

     where EXPRESSION must evaluate to a procedure.

     The TESTs of the clauses are evaluated in order and as soon as one
     of them evaluates to a true values, the corresponding EXPRESSIONs
     are evaluated in order and the last value is returned as the value
     of the `cond'-expression.  For the `=>' clause type, EXPRESSION is
     evaluated and the resulting procedure is applied to the value of
     TEST.  The result of this procedure application is then the result
     of the `cond'-expression.

     One additional `cond'-clause is available as an extension to
     standard Scheme:

          (TEST GUARD => EXPRESSION)

     where GUARD and EXPRESSION must evaluate to procedures.  For this
     clause type, TEST may return multiple values, and `cond' ignores
     its boolean state; instead, `cond' evaluates GUARD and applies the
     resulting procedure to the value(s) of TEST, as if GUARD were the
     CONSUMER argument of `call-with-values'.  Iff the result of that
     procedure call is a true value, it evaluates EXPRESSION and
     applies the resulting procedure to the value(s) of TEST, in the
     same manner as the GUARD was called.

     The TEST of the last CLAUSE may be the symbol `else'.  Then, if
     none of the preceding TESTs is true, the EXPRESSIONs following the
     `else' are evaluated to produce the result of the
     `cond'-expression.

 -- syntax: case key clause1 clause2 ...
     KEY may be any expression, the CLAUSEs must have the form

          ((DATUM1 ...) EXPR1 EXPR2 ...)

     and the last CLAUSE may have the form

          (else EXPR1 EXPR2 ...)

     All DATUMs must be distinct.  First, KEY is evaluated.  The result
     of this evaluation is compared against all DATUM values using
     `eqv?'.  When this comparison succeeds, the expression(s) following
     the DATUM are evaluated from left to right, returning the value of
     the last expression as the result of the `case' expression.

     If the KEY matches no DATUM and there is an `else'-clause, the
     expressions following the `else' are evaluated.  If there is no
     such clause, the result of the expression is unspecified.

6.13.3 Conditional Evaluation of a Sequence of Expressions
----------------------------------------------------------

`and' and `or' evaluate all their arguments in order, similar to
`begin', but evaluation stops as soon as one of the expressions
evaluates to false or true, respectively.

 -- syntax: and expr ...
     Evaluate the EXPRs from left to right and stop evaluation as soon
     as one expression evaluates to `#f'; the remaining expressions are
     not evaluated.  The value of the last evaluated expression is
     returned.  If no expression evaluates to `#f', the value of the
     last expression is returned.

     If used without expressions, `#t' is returned.

 -- syntax: or expr ...
     Evaluate the EXPRs from left to right and stop evaluation as soon
     as one expression evaluates to a true value (that is, a value
     different from `#f'); the remaining expressions are not evaluated.
     The value of the last evaluated expression is returned.  If all
     expressions evaluate to `#f', `#f' is returned.

     If used without expressions, `#f' is returned.

6.13.4 Iteration mechanisms
---------------------------

Scheme has only few iteration mechanisms, mainly because iteration in
Scheme programs is normally expressed using recursion.  Nevertheless,
R5RS defines a construct for programming loops, calling `do'.  In
addition, Guile has an explicit looping syntax called `while'.

 -- syntax: do ((variable init [step]) ...) (test [expr ...]) body ...
     Bind VARIABLEs and evaluate BODY until TEST is true.  The return
     value is the last EXPR after TEST, if given.  A simple example
     will illustrate the basic form,

          (do ((i 1 (1+ i)))
              ((> i 4))
            (display i))
          -| 1234

     Or with two variables and a final return value,

          (do ((i 1 (1+ i))
               (p 3 (* 3 p)))
              ((> i 4)
               p)
            (format #t "3**~s is ~s\n" i p))
          -|
          3**1 is 3
          3**2 is 9
          3**3 is 27
          3**4 is 81
          =>
          789

     The VARIABLE bindings are established like a `let', in that the
     expressions are all evaluated and then all bindings made.  When
     iterating, the optional STEP expressions are evaluated with the
     previous bindings in scope, then new bindings all made.

     The TEST expression is a termination condition.  Looping stops
     when the TEST is true.  It's evaluated before running the BODY
     each time, so if it's true the first time then BODY is not run at
     all.

     The optional EXPRs after the TEST are evaluated at the end of
     looping, with the final VARIABLE bindings available.  The last
     EXPR gives the return value, or if there are no EXPRs the return
     value is unspecified.

     Each iteration establishes bindings to fresh locations for the
     VARIABLEs, like a new `let' for each iteration.  This is done for
     VARIABLEs without STEP expressions too.  The following illustrates
     this, showing how a new `i' is captured by the `lambda' in each
     iteration (*note The Concept of Closure: About Closure.).

          (define lst '())
          (do ((i 1 (1+ i)))
              ((> i 4))
            (set! lst (cons (lambda () i) lst)))
          (map (lambda (proc) (proc)) lst)
          =>
          (4 3 2 1)

 -- syntax: while cond body ...
     Run a loop executing the BODY forms while COND is true.  COND is
     tested at the start of each iteration, so if it's `#f' the first
     time then BODY is not executed at all.

     Within `while', two extra bindings are provided, they can be used
     from both COND and BODY.

      -- Scheme Procedure: break break-arg...
          Break out of the `while' form.

      -- Scheme Procedure: continue
          Abandon the current iteration, go back to the start and test
          COND again, etc.

     If the loop terminates normally, by the COND evaluating to `#f',
     then the `while' expression as a whole evaluates to `#f'.  If it
     terminates by a call to `break' with some number of arguments,
     those arguments are returned from the `while' expression, as
     multiple values.  Otherwise if it terminates by a call to `break'
     with no arguments, then return value is `#t'.

          (while #f (error "not reached")) => #f
          (while #t (break)) => #t
          (while #t (break 1 2 3)) => 1 2 3

     Each `while' form gets its own `break' and `continue' procedures,
     operating on that `while'.  This means when loops are nested the
     outer `break' can be used to escape all the way out.  For example,

          (while (test1)
            (let ((outer-break break))
              (while (test2)
                (if (something)
                  (outer-break #f))
                ...)))

     Note that each `break' and `continue' procedure can only be used
     within the dynamic extent of its `while'.  Outside the `while'
     their behaviour is unspecified.

   Another very common way of expressing iteration in Scheme programs is
the use of the so-called "named let".

   Named let is a variant of `let' which creates a procedure and calls
it in one step.  Because of the newly created procedure, named let is
more powerful than `do'-it can be used for iteration, but also for
arbitrary recursion.

 -- syntax: let variable bindings body
     For the definition of BINDINGS see the documentation about `let'
     (*note Local Bindings::).

     Named `let' works as follows:

        * A new procedure which accepts as many arguments as are in
          BINDINGS is created and bound locally (using `let') to
          VARIABLE.  The new procedure's formal argument names are the
          name of the VARIABLES.

        * The BODY expressions are inserted into the newly created
          procedure.

        * The procedure is called with the INIT expressions as the
          formal arguments.

     The next example implements a loop which iterates (by recursion)
     1000 times.

          (let lp ((x 1000))
            (if (positive? x)
                (lp (- x 1))
                x))
          =>
          0

6.13.5 Prompts
--------------

Prompts are control-flow barriers between different parts of a program.
In the same way that a user sees a shell prompt (e.g., the Bash prompt)
as a barrier between the operating system and her programs, Scheme
prompts allow the Scheme programmer to treat parts of programs as if
they were running in different operating systems.

   We use this roundabout explanation because, unless you're a
functional programming junkie, you probably haven't heard the term,
"delimited, composable continuation". That's OK; it's a relatively
recent topic, but a very useful one to know about.

6.13.5.1 Prompt Primitives
..........................

Guile's primitive delimited control operators are `call-with-prompt'
and `abort-to-prompt'.

 -- Scheme Procedure: call-with-prompt tag thunk handler
     Set up a prompt, and call THUNK within that prompt.

     During the dynamic extent of the call to THUNK, a prompt named TAG
     will be present in the dynamic context, such that if a user calls
     `abort-to-prompt' (see below) with that tag, control rewinds back
     to the prompt, and the HANDLER is run.

     HANDLER must be a procedure. The first argument to HANDLER will be
     the state of the computation begun when THUNK was called, and
     ending with the call to `abort-to-prompt'. The remaining arguments
     to HANDLER are those passed to `abort-to-prompt'.

 -- Scheme Procedure: make-prompt-tag [stem]
     Make a new prompt tag.  Currently prompt tags are generated
     symbols.  This may change in some future Guile version.

 -- Scheme Procedure: default-prompt-tag
     Return the default prompt tag.  Having a distinguished default
     prompt tag allows some useful prompt and abort idioms, discussed
     in the next section.

 -- Scheme Procedure: abort-to-prompt tag val ...
     Unwind the dynamic and control context to the nearest prompt named
     TAG, also passing the given values.

   C programmers may recognize `call-with-prompt' and `abort-to-prompt'
as a fancy kind of `setjmp' and `longjmp', respectively. Prompts are
indeed quite useful as non-local escape mechanisms. Guile's `catch' and
`throw' are implemented in terms of prompts. Prompts are more convenient
than `longjmp', in that one has the opportunity to pass multiple values
to the jump target.

   Also unlike `longjmp', the prompt handler is given the full state of
the process that was aborted, as the first argument to the prompt's
handler. That state is the "continuation" of the computation wrapped by
the prompt. It is a "delimited continuation", because it is not the
whole continuation of the program; rather, just the computation
initiated by the call to `call-with-prompt'.

   The continuation is a procedure, and may be reinstated simply by
invoking it, with any number of values. Here's where things get
interesting, and complicated as well. Besides being described as
delimited, continuations reified by prompts are also "composable",
because invoking a prompt-saved continuation composes that continuation
with the current one.

   Imagine you have saved a continuation via call-with-prompt:

     (define cont
       (call-with-prompt
        ;; tag
        'foo
        ;; thunk
        (lambda ()
          (+ 34 (abort-to-prompt 'foo)))
        ;; handler
        (lambda (k) k)))

   The resulting continuation is the addition of 34. It's as if you had
written:

     (define cont
       (lambda (x)
         (+ 34 x)))

   So, if we call `cont' with one numeric value, we get that number,
incremented by 34:

     (cont 8)
     => 42
     (* 2 (cont 8))
     => 84

   The last example illustrates what we mean when we say, "composes
with the current continuation". We mean that there is a current
continuation - some remaining things to compute, like `(lambda (x) (* x
2))' - and that calling the saved continuation doesn't wipe out the
current continuation, it composes the saved continuation with the
current one.

   We're belaboring the point here because traditional Scheme
continuations, as discussed in the next section, aren't composable, and
are actually less expressive than continuations captured by prompts.
But there's a place for them both.

   Before moving on, we should mention that if the handler of a prompt
is a `lambda' expression, and the first argument isn't referenced, an
abort to that prompt will not cause a continuation to be reified. This
can be an important efficiency consideration to keep in mind.

6.13.5.2 Shift, Reset, and All That
...................................

There is a whole zoo of delimited control operators, and as it does not
seem to be a bounded set, Guile implements support for them in a
separate module:

     (use-modules (ice-9 control))

   Firstly, we have a helpful abbreviation for the `call-with-prompt'
operator.

 -- Scheme Syntax: % expr
 -- Scheme Syntax: % expr handler
 -- Scheme Syntax: % tag expr handler
     Evaluate EXPR in a prompt, optionally specifying a tag and a
     handler.  If no tag is given, the default prompt tag is used.

     If no handler is given, a default handler is installed.  The
     default handler accepts a procedure of one argument, which will
     called on the captured continuation, within a prompt.

     Sometimes it's easier just to show code, as in this case:

          (define (default-prompt-handler k proc)
            (% (default-prompt-tag)
               (proc k)
               default-prompt-handler))

     The `%' symbol is chosen because it looks like a prompt.

   Likewise there is an abbreviation for `abort-to-prompt', which
assumes the default prompt tag:

 -- Scheme Procedure: abort val...
     Abort to the default prompt tag, passing VAL... to the handler.

   As mentioned before, `(ice-9 control)' also provides other delimited
control operators.  This section is a bit technical, and first-time
users of delimited continuations should probably come back to it after
some practice with `%'.

   Still here?  So, when one implements a delimited control operator
like `call-with-prompt', one needs to make two decisions.  Firstly, does
the handler run within or outside the prompt?  Having the handler run
within the prompt allows an abort inside the handler to return to the
same prompt handler, which is often useful.  However it prevents tail
calls from the handler, so it is less general.

   Similarly, does invoking a captured continuation reinstate a prompt?
Again we have the tradeoff of convenience versus proper tail calls.

   These decisions are captured in the Felleisen "F" operator.  If
neither the continuations nor the handlers implicitly add a prompt, the
operator is known as "-F-".  This is the case for Guile's
`call-with-prompt' and `abort-to-prompt'.

   If both continuation and handler implicitly add prompts, then the
operator is "+F+".  `shift' and `reset' are such operators.

 -- Scheme Syntax: reset body...
     Establish a prompt, and evaluate BODY... within that prompt.

     The prompt handler is designed to work with `shift', described
     below.

 -- Scheme Syntax: shift cont body...
     Abort to the nearest `reset', and evaluate BODY... in a context in
     which the captured continuation is bound to CONT.

     As mentioned above, both the BODY... expression and invocations of
     CONT implicitly establish a prompt.

   Interested readers are invited to explore Oleg Kiselyov's wonderful
web site at `http://okmij.org/ftp/', for more information on these
operators.

6.13.6 Continuations
--------------------

A "continuation" is the code that will execute when a given function or
expression returns.  For example, consider

     (define (foo)
       (display "hello\n")
       (display (bar)) (newline)
       (exit))

   The continuation from the call to `bar' comprises a `display' of the
value returned, a `newline' and an `exit'.  This can be expressed as a
function of one argument.

     (lambda (r)
       (display r) (newline)
       (exit))

   In Scheme, continuations are represented as special procedures just
like this.  The special property is that when a continuation is called
it abandons the current program location and jumps directly to that
represented by the continuation.

   A continuation is like a dynamic label, capturing at run-time a point
in program execution, including all the nested calls that have lead to
it (or rather the code that will execute when those calls return).

   Continuations are created with the following functions.

 -- Scheme Procedure: call-with-current-continuation proc
 -- Scheme Procedure: call/cc proc
     Capture the current continuation and call `(PROC CONT)' with it.
     The return value is the value returned by PROC, or when `(CONT
     VALUE)' is later invoked, the return is the VALUE passed.

     Normally CONT should be called with one argument, but when the
     location resumed is expecting multiple values (*note Multiple
     Values::) then they should be passed as multiple arguments, for
     instance `(CONT X Y Z)'.

     CONT may only be used from the same side of a continuation barrier
     as it was created (*note Continuation Barriers::), and in a
     multi-threaded program only from the thread in which it was
     created.

     The call to PROC is not part of the continuation captured, it runs
     only when the continuation is created.  Often a program will want
     to store CONT somewhere for later use; this can be done in PROC.

     The `call' in the name `call-with-current-continuation' refers to
     the way a call to PROC gives the newly created continuation.  It's
     not related to the way a call is used later to invoke that
     continuation.

     `call/cc' is an alias for `call-with-current-continuation'.  This
     is in common use since the latter is rather long.


Here is a simple example,

     (define kont #f)
     (format #t "the return is ~a\n"
             (call/cc (lambda (k)
                        (set! kont k)
                        1)))
     => the return is 1

     (kont 2)
     => the return is 2

   `call/cc' captures a continuation in which the value returned is
going to be displayed by `format'.  The `lambda' stores this in `kont'
and gives an initial return `1' which is displayed.  The later
invocation of `kont' resumes the captured point, but this time
returning `2', which is displayed.

   When Guile is run interactively, a call to `format' like this has an
implicit return back to the read-eval-print loop.  `call/cc' captures
that like any other return, which is why interactively `kont' will come
back to read more input.


   C programmers may note that `call/cc' is like `setjmp' in the way it
records at runtime a point in program execution.  A call to a
continuation is like a `longjmp' in that it abandons the present
location and goes to the recorded one.  Like `longjmp', the value
passed to the continuation is the value returned by `call/cc' on
resuming there.  However `longjmp' can only go up the program stack,
but the continuation mechanism can go anywhere.

   When a continuation is invoked, `call/cc' and subsequent code
effectively "returns" a second time.  It can be confusing to imagine a
function returning more times than it was called.  It may help instead
to think of it being stealthily re-entered and then program flow going
on as normal.

   `dynamic-wind' (*note Dynamic Wind::) can be used to ensure setup
and cleanup code is run when a program locus is resumed or abandoned
through the continuation mechanism.


   Continuations are a powerful mechanism, and can be used to implement
almost any sort of control structure, such as loops, coroutines, or
exception handlers.

   However the implementation of continuations in Guile is not as
efficient as one might hope, because Guile is designed to cooperate
with programs written in other languages, such as C, which do not know
about continuations.  Basically continuations are captured by a block
copy of the stack, and resumed by copying back.

   For this reason, continuations captured by `call/cc' should be used
only when there is no other simple way to achieve the desired result,
or when the elegance of the continuation mechanism outweighs the need
for performance.

   Escapes upwards from loops or nested functions are generally best
handled with prompts (*note Prompts::).  Coroutines can be efficiently
implemented with cooperating threads (a thread holds a full program
stack but doesn't copy it around the way continuations do).

6.13.7 Returning and Accepting Multiple Values
----------------------------------------------

Scheme allows a procedure to return more than one value to its caller.
This is quite different to other languages which only allow
single-value returns.  Returning multiple values is different from
returning a list (or pair or vector) of values to the caller, because
conceptually not _one_ compound object is returned, but several
distinct values.

   The primitive procedures for handling multiple values are `values'
and `call-with-values'.  `values' is used for returning multiple values
from a procedure.  This is done by placing a call to `values' with zero
or more arguments in tail position in a procedure body.
`call-with-values' combines a procedure returning multiple values with
a procedure which accepts these values as parameters.

 -- Scheme Procedure: values arg1 ... argN
 -- C Function: scm_values (args)
     Delivers all of its arguments to its continuation.  Except for
     continuations created by the `call-with-values' procedure, all
     continuations take exactly one value.  The effect of passing no
     value or more than one value to continuations that were not
     created by `call-with-values' is unspecified.

     For `scm_values', ARGS is a list of arguments and the return is a
     multiple-values object which the caller can return.  In the
     current implementation that object shares structure with ARGS, so
     ARGS should not be modified subsequently.

 -- C Function: scm_c_value_ref (values, idx)
     Returns the value at the position specified by IDX in VALUES.
     Note that VALUES will ordinarily be a multiple-values object, but
     it need not be.  Any other object represents a single value
     (itself), and is handled appropriately.

 -- Scheme Procedure: call-with-values producer consumer
     Calls its PRODUCER argument with no values and a continuation
     that, when passed some values, calls the CONSUMER procedure with
     those values as arguments.  The continuation for the call to
     CONSUMER is the continuation of the call to `call-with-values'.

          (call-with-values (lambda () (values 4 5))
                            (lambda (a b) b))
          => 5

          (call-with-values * -)
          => -1

   In addition to the fundamental procedures described above, Guile has
a module which exports a syntax called `receive', which is much more
convenient.  This is in the `(ice-9 receive)' and is the same as
specified by SRFI-8 (*note SRFI-8::).

     (use-modules (ice-9 receive))

 -- library syntax: receive formals expr body ...
     Evaluate the expression EXPR, and bind the result values (zero or
     more) to the formal arguments in FORMALS.  FORMALS is a list of
     symbols, like the argument list in a `lambda' (*note Lambda::).
     After binding the variables, the expressions in BODY ... are
     evaluated in order, the return value is the result from the last
     expression.

     For example getting results from `partition' in SRFI-1 (*note
     SRFI-1::),

          (receive (odds evens)
              (partition odd? '(7 4 2 8 3))
            (display odds)
            (display " and ")
            (display evens))
          -| (7 3) and (4 2 8)


6.13.8 Exceptions
-----------------

A common requirement in applications is to want to jump "non-locally"
from the depths of a computation back to, say, the application's main
processing loop.  Usually, the place that is the target of the jump is
somewhere in the calling stack of procedures that called the procedure
that wants to jump back.  For example, typical logic for a key press
driven application might look something like this:

     main-loop:
       read the next key press and call dispatch-key

     dispatch-key:
       lookup the key in a keymap and call an appropriate procedure,
       say find-file

     find-file:
       interactively read the required file name, then call
       find-specified-file

     find-specified-file:
       check whether file exists; if not, jump back to main-loop
       ...

   The jump back to `main-loop' could be achieved by returning through
the stack one procedure at a time, using the return value of each
procedure to indicate the error condition, but Guile (like most modern
programming languages) provides an additional mechanism called
"exception handling" that can be used to implement such jumps much more
conveniently.

6.13.8.1 Exception Terminology
..............................

There are several variations on the terminology for dealing with
non-local jumps.  It is useful to be aware of them, and to realize that
they all refer to the same basic mechanism.

   * Actually making a non-local jump may be called "raising an
     exception", "raising a signal", "throwing an exception" or "doing
     a long jump".  When the jump indicates an error condition, people
     may talk about "signalling", "raising" or "throwing" "an error".

   * Handling the jump at its target may be referred to as "catching" or
     "handling" the "exception", "signal" or, where an error condition
     is involved, "error".

   Where "signal" and "signalling" are used, special care is needed to
avoid the risk of confusion with POSIX signals.

   This manual prefers to speak of throwing and catching exceptions,
since this terminology matches the corresponding Guile primitives.

6.13.8.2 Catching Exceptions
............................

`catch' is used to set up a target for a possible non-local jump.  The
arguments of a `catch' expression are a "key", which restricts the set
of exceptions to which this `catch' applies, a thunk that specifies the
code to execute and one or two "handler" procedures that say what to do
if an exception is thrown while executing the code.  If the execution
thunk executes "normally", which means without throwing any exceptions,
the handler procedures are not called at all.

   When an exception is thrown using the `throw' function, the first
argument of the `throw' is a symbol that indicates the type of the
exception.  For example, Guile throws an exception using the symbol
`numerical-overflow' to indicate numerical overflow errors such as
division by zero:

     (/ 1 0)
     =>
     ABORT: (numerical-overflow)

   The KEY argument in a `catch' expression corresponds to this symbol.
KEY may be a specific symbol, such as `numerical-overflow', in which
case the `catch' applies specifically to exceptions of that type; or it
may be `#t', which means that the `catch' applies to all exceptions,
irrespective of their type.

   The second argument of a `catch' expression should be a thunk (i.e.
a procedure that accepts no arguments) that specifies the normal case
code.  The `catch' is active for the execution of this thunk, including
any code called directly or indirectly by the thunk's body.  Evaluation
of the `catch' expression activates the catch and then calls this thunk.

   The third argument of a `catch' expression is a handler procedure.
If an exception is thrown, this procedure is called with exactly the
arguments specified by the `throw'.  Therefore, the handler procedure
must be designed to accept a number of arguments that corresponds to
the number of arguments in all `throw' expressions that can be caught
by this `catch'.

   The fourth, optional argument of a `catch' expression is another
handler procedure, called the "pre-unwind" handler.  It differs from
the third argument in that if an exception is thrown, it is called,
_before_ the third argument handler, in exactly the dynamic context of
the `throw' expression that threw the exception.  This means that it is
useful for capturing or displaying the stack at the point of the
`throw', or for examining other aspects of the dynamic context, such as
fluid values, before the context is unwound back to that of the
prevailing `catch'.

 -- Scheme Procedure: catch key thunk handler [pre-unwind-handler]
 -- C Function: scm_catch_with_pre_unwind_handler (key, thunk, handler,
          pre_unwind_handler)
 -- C Function: scm_catch (key, thunk, handler)
     Invoke THUNK in the dynamic context of HANDLER for exceptions
     matching KEY.  If thunk throws to the symbol KEY, then HANDLER is
     invoked this way:
          (handler key args ...)

     KEY is a symbol or `#t'.

     THUNK takes no arguments.  If THUNK returns normally, that is the
     return value of `catch'.

     Handler is invoked outside the scope of its own `catch'.  If
     HANDLER again throws to the same key, a new handler from further
     up the call chain is invoked.

     If the key is `#t', then a throw to _any_ symbol will match this
     call to `catch'.

     If a PRE-UNWIND-HANDLER is given and THUNK throws an exception
     that matches KEY, Guile calls the PRE-UNWIND-HANDLER before
     unwinding the dynamic state and invoking the main HANDLER.
     PRE-UNWIND-HANDLER should be a procedure with the same signature
     as HANDLER, that is `(lambda (key . args))'.  It is typically used
     to save the stack at the point where the exception occurred, but
     can also query other parts of the dynamic state at that point,
     such as fluid values.

     A PRE-UNWIND-HANDLER can exit either normally or non-locally.  If
     it exits normally, Guile unwinds the stack and dynamic context and
     then calls the normal (third argument) handler.  If it exits
     non-locally, that exit determines the continuation.

   If a handler procedure needs to match a variety of `throw'
expressions with varying numbers of arguments, you should write it like
this:

     (lambda (key . args)
       ...)

The KEY argument is guaranteed always to be present, because a `throw'
without a KEY is not valid.  The number and interpretation of the ARGS
varies from one type of exception to another, but should be specified
by the documentation for each exception type.

   Note that, once the normal (post-unwind) handler procedure is
invoked, the catch that led to the handler procedure being called is no
longer active.  Therefore, if the handler procedure itself throws an
exception, that exception can only be caught by another active catch
higher up the call stack, if there is one.


 -- C Function: SCM scm_c_catch (SCM tag, scm_t_catch_body body, void
          *body_data, scm_t_catch_handler handler, void *handler_data,
          scm_t_catch_handler pre_unwind_handler, void
          *pre_unwind_handler_data)
 -- C Function: SCM scm_internal_catch (SCM tag, scm_t_catch_body body,
          void *body_data, scm_t_catch_handler handler, void
          *handler_data)
     The above `scm_catch_with_pre_unwind_handler' and `scm_catch' take
     Scheme procedures as body and handler arguments.  `scm_c_catch'
     and `scm_internal_catch' are equivalents taking C functions.

     BODY is called as `BODY (BODY_DATA)' with a catch on exceptions of
     the given TAG type.  If an exception is caught, PRE_UNWIND_HANDLER
     and HANDLER are called as `HANDLER (HANDLER_DATA, KEY, ARGS)'.
     KEY and ARGS are the `SCM' key and argument list from the `throw'.

     BODY and HANDLER should have the following prototypes.
     `scm_t_catch_body' and `scm_t_catch_handler' are pointer typedefs
     for these.

          SCM body (void *data);
          SCM handler (void *data, SCM key, SCM args);

     The BODY_DATA and HANDLER_DATA parameters are passed to the
     respective calls so an application can communicate extra
     information to those functions.

     If the data consists of an `SCM' object, care should be taken that
     it isn't garbage collected while still required.  If the `SCM' is
     a local C variable, one way to protect it is to pass a pointer to
     that variable as the data parameter, since the C compiler will
     then know the value must be held on the stack.  Another way is to
     use `scm_remember_upto_here_1' (*note Remembering During
     Operations::).

6.13.8.3 Throw Handlers
.......................

It's sometimes useful to be able to intercept an exception that is being
thrown before the stack is unwound. This could be to clean up some
related state, to print a backtrace, or to pass information about the
exception to a debugger, for example. The `with-throw-handler'
procedure provides a way to do this.

 -- Scheme Procedure: with-throw-handler key thunk handler
 -- C Function: scm_with_throw_handler (key, thunk, handler)
     Add HANDLER to the dynamic context as a throw handler for key KEY,
     then invoke THUNK.

     This behaves exactly like `catch', except that it does not unwind
     the stack before invoking HANDLER. If the HANDLER procedure
     returns normally, Guile rethrows the same exception again to the
     next innermost catch or throw handler. HANDLER may exit
     nonlocally, of course, via an explicit throw or via invoking a
     continuation.

   Typically HANDLER is used to display a backtrace of the stack at the
point where the corresponding `throw' occurred, or to save off this
information for possible display later.

   Not unwinding the stack means that throwing an exception that is
handled via a throw handler is equivalent to calling the throw handler
handler inline instead of each `throw', and then omitting the
surrounding `with-throw-handler'. In other words,

     (with-throw-handler 'key
       (lambda () ... (throw 'key args ...) ...)
       handler)

is mostly equivalent to

     ((lambda () ... (handler 'key args ...) ...))

   In particular, the dynamic context when HANDLER is invoked is that
of the site where `throw' is called. The examples are not quite
equivalent, because the body of a `with-throw-handler' is not in tail
position with respect to the `with-throw-handler', and if HANDLER exits
normally, Guile arranges to rethrow the error, but hopefully the
intention is clear. (For an introduction to what is meant by dynamic
context, *Note Dynamic Wind::.)

 -- C Function: SCM scm_c_with_throw_handler (SCM tag, scm_t_catch_body
          body, void *body_data, scm_t_catch_handler handler, void
          *handler_data, int lazy_catch_p)
     The above `scm_with_throw_handler' takes Scheme procedures as body
     (thunk) and handler arguments.  `scm_c_with_throw_handler' is an
     equivalent taking C functions.  See `scm_c_catch' (*note Catch::)
     for a description of the parameters, the behaviour however of
     course follows `with-throw-handler'.

   If THUNK throws an exception, Guile handles that exception by
invoking the innermost `catch' or throw handler whose key matches that
of the exception.  When the innermost thing is a throw handler, Guile
calls the specified handler procedure using `(apply HANDLER key args)'.
The handler procedure may either return normally or exit non-locally.
If it returns normally, Guile passes the exception on to the next
innermost `catch' or throw handler.  If it exits non-locally, that exit
determines the continuation.

   The behaviour of a throw handler is very similar to that of a
`catch' expression's optional pre-unwind handler.  In particular, a
throw handler's handler procedure is invoked in the exact dynamic
context of the `throw' expression, just as a pre-unwind handler is.
`with-throw-handler' may be seen as a half-`catch': it does everything
that a `catch' would do until the point where `catch' would start
unwinding the stack and dynamic context, but then it rethrows to the
next innermost `catch' or throw handler instead.

   Note also that since the dynamic context is not unwound, if a
`with-throw-handler' handler throws to a key that does not match the
`with-throw-handler' expression's KEY, the new throw may be handled by
a `catch' or throw handler that is _closer_ to the throw than the first
`with-throw-handler'.

   Here is an example to illustrate this behavior:

     (catch 'a
       (lambda ()
         (with-throw-handler 'b
           (lambda ()
             (catch 'a
               (lambda ()
                 (throw 'b))
               inner-handler))
           (lambda (key . args)
             (throw 'a))))
       outer-handler)

This code will call `inner-handler' and then continue with the
continuation of the inner `catch'.

6.13.8.4 Throwing Exceptions
............................

The `throw' primitive is used to throw an exception.  One argument, the
KEY, is mandatory, and must be a symbol; it indicates the type of
exception that is being thrown.  Following the KEY, `throw' accepts any
number of additional arguments, whose meaning depends on the exception
type.  The documentation for each possible type of exception should
specify the additional arguments that are expected for that kind of
exception.

 -- Scheme Procedure: throw key . args
 -- C Function: scm_throw (key, args)
     Invoke the catch form matching KEY, passing ARGS to the HANDLER.

     KEY is a symbol.  It will match catches of the same symbol or of
     `#t'.

     If there is no handler at all, Guile prints an error and then
     exits.

   When an exception is thrown, it will be caught by the innermost
`catch' or throw handler that applies to the type of the thrown
exception; in other words, whose KEY is either `#t' or the same symbol
as that used in the `throw' expression.  Once Guile has identified the
appropriate `catch' or throw handler, it handles the exception by
applying the relevant handler procedure(s) to the arguments of the
`throw'.

   If there is no appropriate `catch' or throw handler for a thrown
exception, Guile prints an error to the current error port indicating an
uncaught exception, and then exits.  In practice, it is quite difficult
to observe this behaviour, because Guile when used interactively
installs a top level `catch' handler that will catch all exceptions and
print an appropriate error message _without_ exiting.  For example,
this is what happens if you try to throw an unhandled exception in the
standard Guile REPL; note that Guile's command loop continues after the
error message:

     guile> (throw 'badex)
     <unnamed port>:3:1: In procedure gsubr-apply ...
     <unnamed port>:3:1: unhandled-exception: badex
     ABORT: (misc-error)
     guile>

   The default uncaught exception behaviour can be observed by
evaluating a `throw' expression from the shell command line:

     $ guile -c "(begin (throw 'badex) (display \"here\\n\"))"
     guile: uncaught throw to badex: ()
     $

That Guile exits immediately following the uncaught exception is shown
by the absence of any output from the `display' expression, because
Guile never gets to the point of evaluating that expression.

6.13.8.5 How Guile Implements Exceptions
........................................

It is traditional in Scheme to implement exception systems using
`call-with-current-continuation'.  Continuations (*note
Continuations::) are such a powerful concept that any other control
mechanism -- including `catch' and `throw' -- can be implemented in
terms of them.

   Guile does not implement `catch' and `throw' like this, though.  Why
not?  Because Guile is specifically designed to be easy to integrate
with applications written in C.  In a mixed Scheme/C environment, the
concept of "continuation" must logically include "what happens next" in
the C parts of the application as well as the Scheme parts, and it
turns out that the only reasonable way of implementing continuations
like this is to save and restore the complete C stack.

   So Guile's implementation of `call-with-current-continuation' is a
stack copying one.  This allows it to interact well with ordinary C
code, but means that creating and calling a continuation is slowed down
by the time that it takes to copy the C stack.

   The more targeted mechanism provided by `catch' and `throw' does not
need to save and restore the C stack because the `throw' always jumps
to a location higher up the stack of the code that executes the
`throw'.  Therefore Guile implements the `catch' and `throw' primitives
independently of `call-with-current-continuation', in a way that takes
advantage of this _upwards only_ nature of exceptions.

6.13.9 Procedures for Signaling Errors
--------------------------------------

Guile provides a set of convenience procedures for signaling error
conditions that are implemented on top of the exception primitives just
described.

 -- Scheme Procedure: error msg args ...
     Raise an error with key `misc-error' and a message constructed by
     displaying MSG and writing ARGS.

 -- Scheme Procedure: scm-error key subr message args data
 -- C Function: scm_error_scm (key, subr, message, args, data)
     Raise an error with key KEY.  SUBR can be a string naming the
     procedure associated with the error, or `#f'.  MESSAGE is the
     error message string, possibly containing `~S' and `~A' escapes.
     When an error is reported, these are replaced by formatting the
     corresponding members of ARGS: `~A' (was `%s' in older versions of
     Guile) formats using `display' and `~S' (was `%S') formats using
     `write'.  DATA is a list or `#f' depending on KEY: if KEY is
     `system-error' then it should be a list containing the Unix
     `errno' value; If KEY is `signal' then it should be a list
     containing the Unix signal number; If KEY is `out-of-range' or
     `wrong-type-arg', it is a list containing the bad value; otherwise
     it will usually be `#f'.

 -- Scheme Procedure: strerror err
 -- C Function: scm_strerror (err)
     Return the Unix error message corresponding to ERR, an integer
     `errno' value.

     When `setlocale' has been called (*note Locales::), the message is
     in the language and charset of `LC_MESSAGES'.  (This is done by
     the C library.)

 -- syntax: false-if-exception expr
     Returns the result of evaluating its argument; however if an
     exception occurs then `#f' is returned instead.

6.13.10 Dynamic Wind
--------------------

For Scheme code, the fundamental procedure to react to non-local entry
and exits of dynamic contexts is `dynamic-wind'.  C code could use
`scm_internal_dynamic_wind', but since C does not allow the convenient
construction of anonymous procedures that close over lexical variables,
this will be, well, inconvenient.

   Therefore, Guile offers the functions `scm_dynwind_begin' and
`scm_dynwind_end' to delimit a dynamic extent.  Within this dynamic
extent, which is called a "dynwind context", you can perform various
"dynwind actions" that control what happens when the dynwind context is
entered or left.  For example, you can register a cleanup routine with
`scm_dynwind_unwind_handler' that is executed when the context is left.
There are several other more specialized dynwind actions as well, for
example to temporarily block the execution of asyncs or to temporarily
change the current output port.  They are described elsewhere in this
manual.

   Here is an example that shows how to prevent memory leaks.


     /* Suppose there is a function called FOO in some library that you
        would like to make available to Scheme code (or to C code that
        follows the Scheme conventions).

        FOO takes two C strings and returns a new string.  When an error has
        occurred in FOO, it returns NULL.
     */

     char *foo (char *s1, char *s2);

     /* SCM_FOO interfaces the C function FOO to the Scheme way of life.
        It takes care to free up all temporary strings in the case of
        non-local exits.
      */

     SCM
     scm_foo (SCM s1, SCM s2)
     {
       char *c_s1, *c_s2, *c_res;

       scm_dynwind_begin (0);

       c_s1 = scm_to_locale_string (s1);

       /* Call 'free (c_s1)' when the dynwind context is left.
       */
       scm_dynwind_unwind_handler (free, c_s1, SCM_F_WIND_EXPLICITLY);

       c_s2 = scm_to_locale_string (s2);

       /* Same as above, but more concisely.
       */
       scm_dynwind_free (c_s2);

       c_res = foo (c_s1, c_s2);
       if (c_res == NULL)
         scm_memory_error ("foo");

       scm_dynwind_end ();

       return scm_take_locale_string (res);
     }

 -- Scheme Procedure: dynamic-wind in_guard thunk out_guard
 -- C Function: scm_dynamic_wind (in_guard, thunk, out_guard)
     All three arguments must be 0-argument procedures.  IN_GUARD is
     called, then THUNK, then OUT_GUARD.

     If, any time during the execution of THUNK, the dynamic extent of
     the `dynamic-wind' expression is escaped non-locally, OUT_GUARD is
     called.  If the dynamic extent of the dynamic-wind is re-entered,
     IN_GUARD is called.  Thus IN_GUARD and OUT_GUARD may be called any
     number of times.

          (define x 'normal-binding)
          => x
          (define a-cont
            (call-with-current-continuation
             (lambda (escape)
               (let ((old-x x))
                 (dynamic-wind
                     ;; in-guard:
                     ;;
                     (lambda () (set! x 'special-binding))

                     ;; thunk
                     ;;
                     (lambda () (display x) (newline)
                                (call-with-current-continuation escape)
                                (display x) (newline)
                                x)

                     ;; out-guard:
                     ;;
                     (lambda () (set! x old-x)))))))
          ;; Prints:
          special-binding
          ;; Evaluates to:
          => a-cont
          x
          => normal-binding
          (a-cont #f)
          ;; Prints:
          special-binding
          ;; Evaluates to:
          => a-cont  ;; the value of the (define a-cont...)
          x
          => normal-binding
          a-cont
          => special-binding

 -- C Type: scm_t_dynwind_flags
     This is an enumeration of several flags that modify the behavior of
     `scm_dynwind_begin'.  The flags are listed in the following table.

    `SCM_F_DYNWIND_REWINDABLE'
          The dynamic context is "rewindable".  This means that it can
          be reentered non-locally (via the invocation of a
          continuation).  The default is that a dynwind context can not
          be reentered non-locally.


 -- C Function: void scm_dynwind_begin (scm_t_dynwind_flags flags)
     The function `scm_dynwind_begin' starts a new dynamic context and
     makes it the `current' one.

     The FLAGS argument determines the default behavior of the context.
     Normally, use 0.  This will result in a context that can not be
     reentered with a captured continuation.  When you are prepared to
     handle reentries, include `SCM_F_DYNWIND_REWINDABLE' in FLAGS.

     Being prepared for reentry means that the effects of unwind
     handlers can be undone on reentry.  In the example above, we want
     to prevent a memory leak on non-local exit and thus register an
     unwind handler that frees the memory.  But once the memory is
     freed, we can not get it back on reentry.  Thus reentry can not be
     allowed.

     The consequence is that continuations become less useful when
     non-reentrant contexts are captured, but you don't need to worry
     about that too much.

     The context is ended either implicitly when a non-local exit
     happens, or explicitly with `scm_dynwind_end'.  You must make sure
     that a dynwind context is indeed ended properly.  If you fail to
     call `scm_dynwind_end' for each `scm_dynwind_begin', the behavior
     is undefined.

 -- C Function: void scm_dynwind_end ()
     End the current dynamic context explicitly and make the previous
     one current.

 -- C Type: scm_t_wind_flags
     This is an enumeration of several flags that modify the behavior of
     `scm_dynwind_unwind_handler' and `scm_dynwind_rewind_handler'.
     The flags are listed in the following table.

    `SCM_F_WIND_EXPLICITLY'
          The registered action is also carried out when the dynwind
          context is entered or left locally.

 -- C Function: void scm_dynwind_unwind_handler (void (*func)(void *),
          void *data, scm_t_wind_flags flags)
 -- C Function: void scm_dynwind_unwind_handler_with_scm (void
          (*func)(SCM), SCM data, scm_t_wind_flags flags)
     Arranges for FUNC to be called with DATA as its arguments when the
     current context ends implicitly.  If FLAGS contains
     `SCM_F_WIND_EXPLICITLY', FUNC is also called when the context ends
     explicitly with `scm_dynwind_end'.

     The function `scm_dynwind_unwind_handler_with_scm' takes care that
     DATA is protected from garbage collection.

 -- C Function: void scm_dynwind_rewind_handler (void (*func)(void *),
          void *data, scm_t_wind_flags flags)
 -- C Function: void scm_dynwind_rewind_handler_with_scm (void
          (*func)(SCM), SCM data, scm_t_wind_flags flags)
     Arrange for FUNC to be called with DATA as its argument when the
     current context is restarted by rewinding the stack.  When FLAGS
     contains `SCM_F_WIND_EXPLICITLY', FUNC is called immediately as
     well.

     The function `scm_dynwind_rewind_handler_with_scm' takes care that
     DATA is protected from garbage collection.

 -- C Function: void scm_dynwind_free (void *mem)
     Arrange for MEM to be freed automatically whenever the current
     context is exited, whether normally or non-locally.
     `scm_dynwind_free (mem)' is an equivalent shorthand for
     `scm_dynwind_unwind_handler (free, mem, SCM_F_WIND_EXPLICITLY)'.

6.13.11 How to Handle Errors
----------------------------

Error handling is based on `catch' and `throw'.  Errors are always
thrown with a KEY and four arguments:

   * KEY: a symbol which indicates the type of error.  The symbols used
     by libguile are listed below.

   * SUBR: the name of the procedure from which the error is thrown, or
     `#f'.

   * MESSAGE: a string (possibly language and system dependent)
     describing the error.  The tokens `~A' and `~S' can be embedded
     within the message: they will be replaced with members of the ARGS
     list when the message is printed.  `~A' indicates an argument
     printed using `display', while `~S' indicates an argument printed
     using `write'.  MESSAGE can also be `#f', to allow it to be
     derived from the KEY by the error handler (may be useful if the
     KEY is to be thrown from both C and Scheme).

   * ARGS: a list of arguments to be used to expand `~A' and `~S'
     tokens in MESSAGE.  Can also be `#f' if no arguments are required.

   * REST: a list of any additional objects required. e.g., when the
     key is `'system-error', this contains the C errno value.  Can also
     be `#f' if no additional objects are required.

   In addition to `catch' and `throw', the following Scheme facilities
are available:

 -- Scheme Procedure: display-error frame port subr message args rest
 -- C Function: scm_display_error (frame, port, subr, message, args,
          rest)
     Display an error message to the output port PORT.  FRAME is the
     frame in which the error occurred, SUBR is the name of the
     procedure in which the error occurred and MESSAGE is the actual
     error message, which may contain formatting instructions. These
     will format the arguments in the list ARGS accordingly.  REST is
     currently ignored.

   The following are the error keys defined by libguile and the
situations in which they are used:

   * `error-signal': thrown after receiving an unhandled fatal signal
     such as SIGSEGV, SIGBUS, SIGFPE etc.  The REST argument in the
     throw contains the coded signal number (at present this is not the
     same as the usual Unix signal number).

   * `system-error': thrown after the operating system indicates an
     error condition.  The REST argument in the throw contains the
     errno value.

   * `numerical-overflow': numerical overflow.

   * `out-of-range': the arguments to a procedure do not fall within the
     accepted domain.

   * `wrong-type-arg': an argument to a procedure has the wrong type.

   * `wrong-number-of-args': a procedure was called with the wrong
     number of arguments.

   * `memory-allocation-error': memory allocation error.

   * `stack-overflow': stack overflow error.

   * `regular-expression-syntax': errors generated by the regular
     expression library.

   * `misc-error': other errors.

6.13.11.1 C Support
...................

In the following C functions, SUBR and MESSAGE parameters can be `NULL'
to give the effect of `#f' described above.

 -- C Function: SCM scm_error (SCM KEY, char *SUBR, char *MESSAGE, SCM
          ARGS, SCM REST)
     Throw an error, as per `scm-error' (*note Error Reporting::).

 -- C Function: void scm_syserror (char *SUBR)
 -- C Function: void scm_syserror_msg (char *SUBR, char *MESSAGE, SCM
          ARGS)
     Throw an error with key `system-error' and supply `errno' in the
     REST argument.  For `scm_syserror' the message is generated using
     `strerror'.

     Care should be taken that any code in between the failing operation
     and the call to these routines doesn't change `errno'.

 -- C Function: void scm_num_overflow (char *SUBR)
 -- C Function: void scm_out_of_range (char *SUBR, SCM BAD_VALUE)
 -- C Function: void scm_wrong_num_args (SCM PROC)
 -- C Function: void scm_wrong_type_arg (char *SUBR, int ARGNUM, SCM
          BAD_VALUE)
 -- C Function: void scm_wrong_type_arg_msg (char *SUBR, int ARGNUM,
          SCM BAD_VALUE, const char *EXPECTED)
 -- C Function: void scm_memory_error (char *SUBR)
     Throw an error with the various keys described above.

 -- C Function: void scm_misc_error (const char *SUBR, const char
          *MESSAGE, SCM ARGS)
     In `scm_wrong_num_args', PROC should be a Scheme symbol which is
     the name of the procedure incorrectly invoked.  The other routines
     take the name of the invoked procedure as a C string.

     In `scm_wrong_type_arg_msg', EXPECTED is a C string describing the
     type of argument that was expected.

     In `scm_misc_error', MESSAGE is the error message string, possibly
     containing `simple-format' escapes (*note Writing::), and the
     corresponding arguments in the ARGS list.

6.13.11.2 Signalling Type Errors
................................

Every function visible at the Scheme level should aggressively check the
types of its arguments, to avoid misinterpreting a value, and perhaps
causing a segmentation fault.  Guile provides some macros to make this
easier.

 -- Macro: void SCM_ASSERT (int TEST, SCM OBJ, unsigned int POSITION,
          const char *SUBR)
 -- Macro: void SCM_ASSERT_TYPE (int TEST, SCM OBJ, unsigned int
          POSITION, const char *SUBR, const char *EXPECTED)
     If TEST is zero, signal a "wrong type argument" error, attributed
     to the subroutine named SUBR, operating on the value OBJ, which is
     the POSITION'th argument of SUBR.

     In `SCM_ASSERT_TYPE', EXPECTED is a C string describing the type
     of argument that was expected.

 -- Macro: int SCM_ARG1
 -- Macro: int SCM_ARG2
 -- Macro: int SCM_ARG3
 -- Macro: int SCM_ARG4
 -- Macro: int SCM_ARG5
 -- Macro: int SCM_ARG6
 -- Macro: int SCM_ARG7
     One of the above values can be used for POSITION to indicate the
     number of the argument of SUBR which is being checked.
     Alternatively, a positive integer number can be used, which allows
     to check arguments after the seventh.  However, for parameter
     numbers up to seven it is preferable to use `SCM_ARGN' instead of
     the corresponding raw number, since it will make the code easier to
     understand.

 -- Macro: int SCM_ARGn
     Passing a value of zero or `SCM_ARGn' for POSITION allows to leave
     it unspecified which argument's type is incorrect.  Again,
     `SCM_ARGn' should be preferred over a raw zero constant.

6.13.12 Continuation Barriers
-----------------------------

The non-local flow of control caused by continuations might sometimes
not be wanted. You can use `with-continuation-barrier' to erect fences
that continuations can not pass.

 -- Scheme Procedure: with-continuation-barrier proc
 -- C Function: scm_with_continuation_barrier (proc)
     Call PROC and return its result.  Do not allow the invocation of
     continuations that would leave or enter the dynamic extent of the
     call to `with-continuation-barrier'.  Such an attempt causes an
     error to be signaled.

     Throws (such as errors) that are not caught from within PROC are
     caught by `with-continuation-barrier'.  In that case, a short
     message is printed to the current error port and `#f' is returned.

     Thus, `with-continuation-barrier' returns exactly once.

 -- C Function: void * scm_c_with_continuation_barrier (void *(*func)
          (void *), void *data)
     Like `scm_with_continuation_barrier' but call FUNC on DATA.  When
     an error is caught, `NULL' is returned.

6.14 Input and Output
=====================

6.14.1 Ports
------------

Sequential input/output in Scheme is represented by operations on a
"port".  This chapter explains the operations that Guile provides for
working with ports.

   Ports are created by opening, for instance `open-file' for a file
(*note File Ports::).  Characters can be read from an input port and
written to an output port, or both on an input/output port.  A port can
be closed (*note Closing::) when no longer required, after which any
attempt to read or write is an error.

   The formal definition of a port is very generic: an input port is
simply "an object which can deliver characters on demand," and an
output port is "an object which can accept characters."  Because this
definition is so loose, it is easy to write functions that simulate
ports in software.  "Soft ports" and "string ports" are two interesting
and powerful examples of this technique.  (*note Soft Ports::, and
*note String Ports::.)

   Ports are garbage collected in the usual way (*note Memory
Management::), and will be closed at that time if not already closed.
In this case any errors occurring in the close will not be reported.
Usually a program will want to explicitly close so as to be sure all
its operations have been successful.  Of course if a program has
abandoned something due to an error or other condition then closing
problems are probably not of interest.

   It is strongly recommended that file ports be closed explicitly when
no longer required.  Most systems have limits on how many files can be
open, both on a per-process and a system-wide basis.  A program that
uses many files should take care not to hit those limits.  The same
applies to similar system resources such as pipes and sockets.

   Note that automatic garbage collection is triggered only by memory
consumption, not by file or other resource usage, so a program cannot
rely on that to keep it away from system limits.  An explicit call to
`gc' can of course be relied on to pick up unreferenced ports.  If
program flow makes it hard to be certain when to close then this may be
an acceptable way to control resource usage.

   All file access uses the "LFS" large file support functions when
available, so files bigger than 2 Gbytes (2^31 bytes) can be read and
written on a 32-bit system.

   Each port has an associated character encoding that controls how
bytes read from the port are converted to characters and string and
controls how characters and strings written to the port are converted
to bytes.  When ports are created, they inherit their character
encoding from the current locale, but, that can be modified after the
port is created.

   Currently, the ports only work with _non-modal_ encodings.  Most
encodings are non-modal, meaning that the conversion of bytes to a
string doesn't depend on its context: the same byte sequence will always
return the same string.  A couple of modal encodings are in common use,
like ISO-2022-JP and ISO-2022-KR, and they are not yet supported.

   Each port also has an associated conversion strategy: what to do when
a Guile character can't be converted to the port's encoded character
representation for output. There are three possible strategies: to
raise an error, to replace the character with a hex escape, or to
replace the character with a substitute character.

 -- Scheme Procedure: input-port? x
 -- C Function: scm_input_port_p (x)
     Return `#t' if X is an input port, otherwise return `#f'.  Any
     object satisfying this predicate also satisfies `port?'.

 -- Scheme Procedure: output-port? x
 -- C Function: scm_output_port_p (x)
     Return `#t' if X is an output port, otherwise return `#f'.  Any
     object satisfying this predicate also satisfies `port?'.

 -- Scheme Procedure: port? x
 -- C Function: scm_port_p (x)
     Return a boolean indicating whether X is a port.  Equivalent to
     `(or (input-port? X) (output-port?  X))'.

 -- Scheme Procedure: set-port-encoding! port enc
 -- C Function: scm_set_port_encoding_x (port, enc)
     Sets the character encoding that will be used to interpret all
     port I/O.  ENC is a string containing the name of an encoding.
     Valid encoding names are those defined by IANA
     (http://www.iana.org/assignments/character-sets).

 -- Scheme Variable: %default-port-encoding
     A fluid containing `#f' or the name of the encoding to be used by
     default for newly created ports (*note Fluids and Dynamic
     States::).  The value `#f' is equivalent to `"ISO-8859-1"'.

     New ports are created with the encoding appropriate for the current
     locale if `setlocale' has been called or the value specified by
     this fluid otherwise.

 -- Scheme Procedure: port-encoding port
 -- C Function: scm_port_encoding
     Returns, as a string, the character encoding that PORT uses to
     interpret its input and output.  The value `#f' is equivalent to
     `"ISO-8859-1"'.

 -- Scheme Procedure: set-port-conversion-strategy! port sym
 -- C Function: scm_set_port_conversion_strategy_x (port, sym)
     Sets the behavior of the interpreter when outputting a character
     that is not representable in the port's current encoding.  SYM can
     be either `'error', `'substitute', or `'escape'.  If it is
     `'error', an error will be thrown when an nonconvertible character
     is encountered.  If it is `'substitute', then nonconvertible
     characters will be replaced with approximate characters, or with
     question marks if no approximately correct character is available.
     If it is `'escape', it will appear as a hex escape when output.

     If PORT is an open port, the conversion error behavior is set for
     that port.  If it is `#f', it is set as the default behavior for
     any future ports that get created in this thread.

 -- Scheme Procedure: port-conversion-strategy port
 -- C Function: scm_port_conversion_strategy (port)
     Returns the behavior of the port when outputting a character that
     is not representable in the port's current encoding.  It returns
     the symbol `error' if unrepresentable characters should cause
     exceptions, `substitute' if the port should try to replace
     unrepresentable characters with question marks or approximate
     characters, or `escape' if unrepresentable characters should be
     converted to string escapes.

     If PORT is `#f', then the current default behavior will be
     returned.  New ports will have this default behavior when they are
     created.

6.14.2 Reading
--------------

[Generic procedures for reading from ports.]

   These procedures pertain to reading characters and strings from
ports. To read general S-expressions from ports, *Note Scheme Read::.

 -- Scheme Procedure: eof-object? x
 -- C Function: scm_eof_object_p (x)
     Return `#t' if X is an end-of-file object; otherwise return `#f'.

 -- Scheme Procedure: char-ready? [port]
 -- C Function: scm_char_ready_p (port)
     Return `#t' if a character is ready on input PORT and return `#f'
     otherwise.  If `char-ready?' returns `#t' then the next
     `read-char' operation on PORT is guaranteed not to hang.  If PORT
     is a file port at end of file then `char-ready?' returns `#t'.

     `char-ready?' exists to make it possible for a program to accept
     characters from interactive ports without getting stuck waiting
     for input.  Any input editors associated with such ports must make
     sure that characters whose existence has been asserted by
     `char-ready?' cannot be rubbed out.  If `char-ready?' were to
     return `#f' at end of file, a port at end of file would be
     indistinguishable from an interactive port that has no ready
     characters.

 -- Scheme Procedure: read-char [port]
 -- C Function: scm_read_char (port)
     Return the next character available from PORT, updating PORT to
     point to the following character.  If no more characters are
     available, the end-of-file object is returned.

     When PORT's data cannot be decoded according to its character
     encoding, a `decoding-error' is raised and PORT points past the
     erroneous byte sequence.

 -- C Function: size_t scm_c_read (SCM port, void *buffer, size_t size)
     Read up to SIZE bytes from PORT and store them in BUFFER.  The
     return value is the number of bytes actually read, which can be
     less than SIZE if end-of-file has been reached.

     Note that this function does not update `port-line' and
     `port-column' below.

 -- Scheme Procedure: peek-char [port]
 -- C Function: scm_peek_char (port)
     Return the next character available from PORT, _without_ updating
     PORT to point to the following character.  If no more characters
     are available, the end-of-file object is returned.

     The value returned by a call to `peek-char' is the same as the
     value that would have been returned by a call to `read-char' on
     the same port.  The only difference is that the very next call to
     `read-char' or `peek-char' on that PORT will return the value
     returned by the preceding call to `peek-char'.  In particular, a
     call to `peek-char' on an interactive port will hang waiting for
     input whenever a call to `read-char' would have hung.

     As for `read-char', a `decoding-error' may be raised if such a
     situation occurs.  However, unlike with `read-char', PORT still
     points at the beginning of the erroneous byte sequence when the
     error is raised.

 -- Scheme Procedure: unread-char cobj [port]
 -- C Function: scm_unread_char (cobj, port)
     Place CHAR in PORT so that it will be read by the next read
     operation.  If called multiple times, the unread characters will
     be read again in last-in first-out order.  If PORT is not
     supplied, the current input port is used.

 -- Scheme Procedure: unread-string str port
 -- C Function: scm_unread_string (str, port)
     Place the string STR in PORT so that its characters will be read
     from left-to-right as the next characters from PORT during
     subsequent read operations.  If called multiple times, the unread
     characters will be read again in last-in first-out order.  If PORT
     is not supplied, the `current-input-port' is used.

 -- Scheme Procedure: drain-input port
 -- C Function: scm_drain_input (port)
     This procedure clears a port's input buffers, similar to the way
     that force-output clears the output buffer.  The contents of the
     buffers are returned as a single string, e.g.,

          (define p (open-input-file ...))
          (drain-input p) => empty string, nothing buffered yet.
          (unread-char (read-char p) p)
          (drain-input p) => initial chars from p, up to the buffer size.

     Draining the buffers may be useful for cleanly finishing buffered
     I/O so that the file descriptor can be used directly for further
     input.

 -- Scheme Procedure: port-column port
 -- Scheme Procedure: port-line port
 -- C Function: scm_port_column (port)
 -- C Function: scm_port_line (port)
     Return the current column number or line number of PORT.  If the
     number is unknown, the result is #f.  Otherwise, the result is a
     0-origin integer - i.e. the first character of the first line is
     line 0, column 0.  (However, when you display a file position, for
     example in an error message, we recommend you add 1 to get
     1-origin integers.  This is because lines and column numbers
     traditionally start with 1, and that is what non-programmers will
     find most natural.)

 -- Scheme Procedure: set-port-column! port column
 -- Scheme Procedure: set-port-line! port line
 -- C Function: scm_set_port_column_x (port, column)
 -- C Function: scm_set_port_line_x (port, line)
     Set the current column or line number of PORT.

6.14.3 Writing
--------------

[Generic procedures for writing to ports.]

   These procedures are for writing characters and strings to ports.
For more information on writing arbitrary Scheme objects to ports,
*Note Scheme Write::.

 -- Scheme Procedure: get-print-state port
 -- C Function: scm_get_print_state (port)
     Return the print state of the port PORT.  If PORT has no
     associated print state, `#f' is returned.

 -- Scheme Procedure: newline [port]
 -- C Function: scm_newline (port)
     Send a newline to PORT.  If PORT is omitted, send to the current
     output port.

 -- Scheme Procedure: port-with-print-state port [pstate]
 -- C Function: scm_port_with_print_state (port, pstate)
     Create a new port which behaves like PORT, but with an included
     print state PSTATE.  PSTATE is optional.  If PSTATE isn't supplied
     and PORT already has a print state, the old print state is reused.

 -- Scheme Procedure: simple-format destination message . args
 -- C Function: scm_simple_format (destination, message, args)
     Write MESSAGE to DESTINATION, defaulting to the current output
     port.  MESSAGE can contain `~A' (was `%s') and `~S' (was `%S')
     escapes.  When printed, the escapes are replaced with
     corresponding members of ARGS: `~A' formats using `display' and
     `~S' formats using `write'.  If DESTINATION is `#t', then use the
     current output port, if DESTINATION is `#f', then return a string
     containing the formatted text. Does not add a trailing newline.

 -- Scheme Procedure: write-char chr [port]
 -- C Function: scm_write_char (chr, port)
     Send character CHR to PORT.

 -- C Function: void scm_c_write (SCM port, const void *buffer, size_t
          size)
     Write SIZE bytes at BUFFER to PORT.

     Note that this function does not update `port-line' and
     `port-column' (*note Reading::).

 -- Scheme Procedure: force-output [port]
 -- C Function: scm_force_output (port)
     Flush the specified output port, or the current output port if PORT
     is omitted.  The current output buffer contents are passed to the
     underlying port implementation (e.g., in the case of fports, the
     data will be written to the file and the output buffer will be
     cleared.)  It has no effect on an unbuffered port.

     The return value is unspecified.

 -- Scheme Procedure: flush-all-ports
 -- C Function: scm_flush_all_ports ()
     Equivalent to calling `force-output' on all open output ports.
     The return value is unspecified.

6.14.4 Closing
--------------

 -- Scheme Procedure: close-port port
 -- C Function: scm_close_port (port)
     Close the specified port object.  Return `#t' if it successfully
     closes a port or `#f' if it was already closed.  An exception may
     be raised if an error occurs, for example when flushing buffered
     output.  See also *note close: Ports and File Descriptors, for a
     procedure which can close file descriptors.

 -- Scheme Procedure: close-input-port port
 -- Scheme Procedure: close-output-port port
 -- C Function: scm_close_input_port (port)
 -- C Function: scm_close_output_port (port)
     Close the specified input or output PORT.  An exception may be
     raised if an error occurs while closing.  If PORT is already
     closed, nothing is done.  The return value is unspecified.

     See also *note close: Ports and File Descriptors, for a procedure
     which can close file descriptors.

 -- Scheme Procedure: port-closed? port
 -- C Function: scm_port_closed_p (port)
     Return `#t' if PORT is closed or `#f' if it is open.

6.14.5 Random Access
--------------------

 -- Scheme Procedure: seek fd_port offset whence
 -- C Function: scm_seek (fd_port, offset, whence)
     Sets the current position of FD/PORT to the integer OFFSET, which
     is interpreted according to the value of WHENCE.

     One of the following variables should be supplied for WHENCE:

      -- Variable: SEEK_SET
          Seek from the beginning of the file.

      -- Variable: SEEK_CUR
          Seek from the current position.

      -- Variable: SEEK_END
          Seek from the end of the file.
     If FD/PORT is a file descriptor, the underlying system call is
     `lseek'.  PORT may be a string port.

     The value returned is the new position in the file.  This means
     that the current position of a port can be obtained using:
          (seek port 0 SEEK_CUR)

 -- Scheme Procedure: ftell fd_port
 -- C Function: scm_ftell (fd_port)
     Return an integer representing the current position of FD/PORT,
     measured from the beginning.  Equivalent to:

          (seek port 0 SEEK_CUR)

 -- Scheme Procedure: truncate-file file [length]
 -- C Function: scm_truncate_file (file, length)
     Truncate FILE to LENGTH bytes.  FILE can be a filename string, a
     port object, or an integer file descriptor.  The return value is
     unspecified.

     For a port or file descriptor LENGTH can be omitted, in which case
     the file is truncated at the current position (per `ftell' above).

     On most systems a file can be extended by giving a length greater
     than the current size, but this is not mandatory in the POSIX
     standard.

6.14.6 Line Oriented and Delimited Text
---------------------------------------

The delimited-I/O module can be accessed with:

     (use-modules (ice-9 rdelim))

   It can be used to read or write lines of text, or read text
delimited by a specified set of characters.  It's similar to the `(scsh
rdelim)' module from guile-scsh, but does not use multiple values or
character sets and has an extra procedure `write-line'.

 -- Scheme Procedure: read-line [port] [handle-delim]
     Return a line of text from PORT if specified, otherwise from the
     value returned by `(current-input-port)'.  Under Unix, a line of
     text is terminated by the first end-of-line character or by
     end-of-file.

     If HANDLE-DELIM is specified, it should be one of the following
     symbols:
    `trim'
          Discard the terminating delimiter.  This is the default, but
          it will be impossible to tell whether the read terminated
          with a delimiter or end-of-file.

    `concat'
          Append the terminating delimiter (if any) to the returned
          string.

    `peek'
          Push the terminating delimiter (if any) back on to the port.

    `split'
          Return a pair containing the string read from the port and the
          terminating delimiter or end-of-file object.

     Like `read-char', this procedure can throw to `decoding-error'
     (*note `read-char': Reading.).

 -- Scheme Procedure: read-line! buf [port]
     Read a line of text into the supplied string BUF and return the
     number of characters added to BUF.  If BUF is filled, then `#f' is
     returned.  Read from PORT if specified, otherwise from the value
     returned by `(current-input-port)'.

 -- Scheme Procedure: read-delimited delims [port] [handle-delim]
     Read text until one of the characters in the string DELIMS is found
     or end-of-file is reached.  Read from PORT if supplied, otherwise
     from the value returned by `(current-input-port)'.  HANDLE-DELIM
     takes the same values as described for `read-line'.

 -- Scheme Procedure: read-delimited! delims buf [port] [handle-delim]
          [start] [end]
     Read text into the supplied string BUF.

     If a delimiter was found, return the number of characters written,
     except if HANDLE-DELIM is `split', in which case the return value
     is a pair, as noted above.

     As a special case, if PORT was already at end-of-stream, the EOF
     object is returned. Also, if no characters were written because the
     buffer was full, `#f' is returned.

     It's something of a wacky interface, to be honest.

 -- Scheme Procedure: write-line obj [port]
 -- C Function: scm_write_line (obj, port)
     Display OBJ and a newline character to PORT.  If PORT is not
     specified, `(current-output-port)' is used.  This function is
     equivalent to:
          (display obj [port])
          (newline [port])

   Some of the aforementioned I/O functions rely on the following C
primitives.  These will mainly be of interest to people hacking Guile
internals.

 -- Scheme Procedure: %read-delimited! delims str gobble [port [start
          [end]]]
 -- C Function: scm_read_delimited_x (delims, str, gobble, port, start,
          end)
     Read characters from PORT into STR until one of the characters in
     the DELIMS string is encountered.  If GOBBLE is true, discard the
     delimiter character; otherwise, leave it in the input stream for
     the next read.  If PORT is not specified, use the value of
     `(current-input-port)'.  If START or END are specified, store data
     only into the substring of STR bounded by START and END (which
     default to the beginning and end of the string, respectively).

     Return a pair consisting of the delimiter that terminated the
     string and the number of characters read.  If reading stopped at
     the end of file, the delimiter returned is the EOF-OBJECT; if the
     string was filled without encountering a delimiter, this value is
     `#f'.

 -- Scheme Procedure: %read-line [port]
 -- C Function: scm_read_line (port)
     Read a newline-terminated line from PORT, allocating storage as
     necessary.  The newline terminator (if any) is removed from the
     string, and a pair consisting of the line and its delimiter is
     returned.  The delimiter may be either a newline or the
     EOF-OBJECT; if `%read-line' is called at the end of file, it
     returns the pair `(#<eof> . #<eof>)'.

6.14.7 Block reading and writing
--------------------------------

The Block-string-I/O module can be accessed with:

     (use-modules (ice-9 rw))

   It currently contains procedures that help to implement the `(scsh
rw)' module in guile-scsh.

 -- Scheme Procedure: read-string!/partial str [port_or_fdes [start
          [end]]]
 -- C Function: scm_read_string_x_partial (str, port_or_fdes, start,
          end)
     Read characters from a port or file descriptor into a string STR.
     A port must have an underlying file descriptor -- a so-called
     fport.  This procedure is scsh-compatible and can efficiently read
     large strings.  It will:

        * attempt to fill the entire string, unless the START and/or
          END arguments are supplied.  i.e., START defaults to 0 and
          END defaults to `(string-length str)'

        * use the current input port if PORT_OR_FDES is not supplied.

        * return fewer than the requested number of characters in some
          cases, e.g., on end of file, if interrupted by a signal, or if
          not all the characters are immediately available.

        * wait indefinitely for some input if no characters are
          currently available, unless the port is in non-blocking mode.

        * read characters from the port's input buffers if available,
          instead from the underlying file descriptor.

        * return `#f' if end-of-file is encountered before reading any
          characters, otherwise return the number of characters read.

        * return 0 if the port is in non-blocking mode and no characters
          are immediately available.

        * return 0 if the request is for 0 bytes, with no end-of-file
          check.

 -- Scheme Procedure: write-string/partial str [port_or_fdes [start
          [end]]]
 -- C Function: scm_write_string_partial (str, port_or_fdes, start, end)
     Write characters from a string STR to a port or file descriptor.
     A port must have an underlying file descriptor -- a so-called
     fport.  This procedure is scsh-compatible and can efficiently
     write large strings.  It will:

        * attempt to write the entire string, unless the START and/or
          END arguments are supplied.  i.e., START defaults to 0 and
          END defaults to `(string-length str)'

        * use the current output port if PORT_OF_FDES is not supplied.

        * in the case of a buffered port, store the characters in the
          port's output buffer, if all will fit.  If they will not fit
          then any existing buffered characters will be flushed before
          attempting to write the new characters directly to the
          underlying file descriptor.  If the port is in non-blocking
          mode and buffered characters can not be flushed immediately,
          then an `EAGAIN' system-error exception will be raised (Note:
          scsh does not support the use of non-blocking buffered ports.)

        * write fewer than the requested number of characters in some
          cases, e.g., if interrupted by a signal or if not all of the
          output can be accepted immediately.

        * wait indefinitely for at least one character from STR to be
          accepted by the port, unless the port is in non-blocking mode.

        * return the number of characters accepted by the port.

        * return 0 if the port is in non-blocking mode and can not
          accept at least one character from STR immediately

        * return 0 immediately if the request size is 0 bytes.

6.14.8 Default Ports for Input, Output and Errors
-------------------------------------------------

 -- Scheme Procedure: current-input-port
 -- C Function: scm_current_input_port ()
     Return the current input port.  This is the default port used by
     many input procedures.

     Initially this is the "standard input" in Unix and C terminology.
     When the standard input is a tty the port is unbuffered, otherwise
     it's fully buffered.

     Unbuffered input is good if an application runs an interactive
     subprocess, since any type-ahead input won't go into Guile's buffer
     and be unavailable to the subprocess.

     Note that Guile buffering is completely separate from the tty "line
     discipline".  In the usual cooked mode on a tty Guile only sees a
     line of input once the user presses <Return>.

 -- Scheme Procedure: current-output-port
 -- C Function: scm_current_output_port ()
     Return the current output port.  This is the default port used by
     many output procedures.

     Initially this is the "standard output" in Unix and C terminology.
     When the standard output is a tty this port is unbuffered,
     otherwise it's fully buffered.

     Unbuffered output to a tty is good for ensuring progress output or
     a prompt is seen.  But an application which always prints whole
     lines could change to line buffered, or an application with a lot
     of output could go fully buffered and perhaps make explicit
     `force-output' calls (*note Writing::) at selected points.

 -- Scheme Procedure: current-error-port
 -- C Function: scm_current_error_port ()
     Return the port to which errors and warnings should be sent.

     Initially this is the "standard error" in Unix and C terminology.
     When the standard error is a tty this port is unbuffered, otherwise
     it's fully buffered.

 -- Scheme Procedure: set-current-input-port port
 -- Scheme Procedure: set-current-output-port port
 -- Scheme Procedure: set-current-error-port port
 -- C Function: scm_set_current_input_port (port)
 -- C Function: scm_set_current_output_port (port)
 -- C Function: scm_set_current_error_port (port)
     Change the ports returned by `current-input-port',
     `current-output-port' and `current-error-port', respectively, so
     that they use the supplied PORT for input or output.

 -- C Function: void scm_dynwind_current_input_port (SCM port)
 -- C Function: void scm_dynwind_current_output_port (SCM port)
 -- C Function: void scm_dynwind_current_error_port (SCM port)
     These functions must be used inside a pair of calls to
     `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic Wind::).
     During the dynwind context, the indicated port is set to PORT.

     More precisely, the current port is swapped with a `backup' value
     whenever the dynwind context is entered or left.  The backup value
     is initialized with the PORT argument.

6.14.9 Types of Port
--------------------

[Types of port; how to make them.]

6.14.9.1 File Ports
...................

The following procedures are used to open file ports.  See also *note
open: Ports and File Descriptors, for an interface to the Unix `open'
system call.

   Most systems have limits on how many files can be open, so it's
strongly recommended that file ports be closed explicitly when no
longer required (*note Ports::).

 -- Scheme Procedure: open-file filename mode
 -- C Function: scm_open_file (filename, mode)
     Open the file whose name is FILENAME, and return a port
     representing that file.  The attributes of the port are determined
     by the MODE string.  The way in which this is interpreted is
     similar to C stdio.  The first character must be one of the
     following:

    `r'
          Open an existing file for input.

    `w'
          Open a file for output, creating it if it doesn't already
          exist or removing its contents if it does.

    `a'
          Open a file for output, creating it if it doesn't already
          exist.  All writes to the port will go to the end of the file.
          The "append mode" can be turned off while the port is in use
          *note fcntl: Ports and File Descriptors.

     The following additional characters can be appended:

    `+'
          Open the port for both input and output.  E.g., `r+': open an
          existing file for both input and output.

    `0'
          Create an "unbuffered" port.  In this case input and output
          operations are passed directly to the underlying port
          implementation without additional buffering.  This is likely
          to slow down I/O operations.  The buffering mode can be
          changed while a port is in use *note setvbuf: Ports and File
          Descriptors.

    `l'
          Add line-buffering to the port.  The port output buffer will
          be automatically flushed whenever a newline character is
          written.

    `b'
          Use binary mode, ensuring that each byte in the file will be
          read as one Scheme character.

          To provide this property, the file will be opened with the
          8-bit character encoding "ISO-8859-1", ignoring any coding
          declaration or port encoding.  *Note Ports::, for more
          information on port encodings.

          Note that while it is possible to read and write binary data
          as characters or strings, it is usually better to treat bytes
          as octets, and byte sequences as bytevectors.  *Note R6RS
          Binary Input::, and *note R6RS Binary Output::, for more.

          This option had another historical meaning, for DOS
          compatibility: in the default (textual) mode, DOS reads a
          CR-LF sequence as one LF byte.  The `b' flag prevents this
          from happening, adding `O_BINARY' to the underlying `open'
          call.  Still, the flag is generally useful because of its
          port encoding ramifications.

     If a file cannot be opened with the access requested, `open-file'
     throws an exception.

     When the file is opened, this procedure will scan for a coding
     declaration (*note Character Encoding of Source Files::). If a
     coding declaration is found, it will be used to interpret the
     file.  Otherwise, the port's encoding will be used.  To suppress
     this behavior, open the file in binary mode and then set the port
     encoding explicitly using `set-port-encoding!'.

     In theory we could create read/write ports which were buffered in
     one direction only.  However this isn't included in the current
     interfaces.

 -- Scheme Procedure: open-input-file filename
     Open FILENAME for input.  Equivalent to
          (open-file FILENAME "r")

 -- Scheme Procedure: open-output-file filename
     Open FILENAME for output.  Equivalent to
          (open-file FILENAME "w")

 -- Scheme Procedure: call-with-input-file filename proc
 -- Scheme Procedure: call-with-output-file filename proc
     Open FILENAME for input or output, and call `(PROC port)' with the
     resulting port.  Return the value returned by PROC.  FILENAME is
     opened as per `open-input-file' or `open-output-file'
     respectively, and an error is signaled if it cannot be opened.

     When PROC returns, the port is closed.  If PROC does not return
     (e.g. if it throws an error), then the port might not be closed
     automatically, though it will be garbage collected in the usual
     way if not otherwise referenced.

 -- Scheme Procedure: with-input-from-file filename thunk
 -- Scheme Procedure: with-output-to-file filename thunk
 -- Scheme Procedure: with-error-to-file filename thunk
     Open FILENAME and call `(THUNK)' with the new port setup as
     respectively the `current-input-port', `current-output-port', or
     `current-error-port'.  Return the value returned by THUNK.
     FILENAME is opened as per `open-input-file' or `open-output-file'
     respectively, and an error is signaled if it cannot be opened.

     When THUNK returns, the port is closed and the previous setting of
     the respective current port is restored.

     The current port setting is managed with `dynamic-wind', so the
     previous value is restored no matter how THUNK exits (eg. an
     exception), and if THUNK is re-entered (via a captured
     continuation) then it's set again to the FILENAME port.

     The port is closed when THUNK returns normally, but not when
     exited via an exception or new continuation.  This ensures it's
     still ready for use if THUNK is re-entered by a captured
     continuation.  Of course the port is always garbage collected and
     closed in the usual way when no longer referenced anywhere.

 -- Scheme Procedure: port-mode port
 -- C Function: scm_port_mode (port)
     Return the port modes associated with the open port PORT.  These
     will not necessarily be identical to the modes used when the port
     was opened, since modes such as "append" which are used only
     during port creation are not retained.

 -- Scheme Procedure: port-filename port
 -- C Function: scm_port_filename (port)
     Return the filename associated with PORT, or `#f' if no filename
     is associated with the port.

     PORT must be open, `port-filename' cannot be used once the port is
     closed.

 -- Scheme Procedure: set-port-filename! port filename
 -- C Function: scm_set_port_filename_x (port, filename)
     Change the filename associated with PORT, using the current input
     port if none is specified.  Note that this does not change the
     port's source of data, but only the value that is returned by
     `port-filename' and reported in diagnostic output.

 -- Scheme Procedure: file-port? obj
 -- C Function: scm_file_port_p (obj)
     Determine whether OBJ is a port that is related to a file.

6.14.9.2 String Ports
.....................

The following allow string ports to be opened by analogy to R4RS file
port facilities:

   With string ports, the port-encoding is treated differently than
other types of ports.  When string ports are created, they do not
inherit a character encoding from the current locale.  They are given a
default locale that allows them to handle all valid string characters.
Typically one should not modify a string port's character encoding away
from its default.

 -- Scheme Procedure: call-with-output-string proc
 -- C Function: scm_call_with_output_string (proc)
     Calls the one-argument procedure PROC with a newly created output
     port.  When the function returns, the string composed of the
     characters written into the port is returned.  PROC should not
     close the port.

     Note that which characters can be written to a string port depend
     on the port's encoding.  The default encoding of string ports is
     specified by the `%default-port-encoding' fluid (*note
     `%default-port-encoding': Ports.).  For instance, it is an error
     to write Greek letter alpha to an ISO-8859-1-encoded string port
     since this character cannot be represented with ISO-8859-1:

          (define alpha (integer->char #x03b1)) ; GREEK SMALL LETTER ALPHA

          (with-fluids ((%default-port-encoding "ISO-8859-1"))
            (call-with-output-string
              (lambda (p)
                (display alpha p))))

          =>
          Throw to key `encoding-error'

     Changing the string port's encoding to a Unicode-capable encoding
     such as UTF-8 solves the problem.

 -- Scheme Procedure: call-with-input-string string proc
 -- C Function: scm_call_with_input_string (string, proc)
     Calls the one-argument procedure PROC with a newly created input
     port from which STRING's contents may be read.  The value yielded
     by the PROC is returned.

 -- Scheme Procedure: with-output-to-string thunk
     Calls the zero-argument procedure THUNK with the current output
     port set temporarily to a new string port.  It returns a string
     composed of the characters written to the current output.

     See `call-with-output-string' above for character encoding
     considerations.

 -- Scheme Procedure: with-input-from-string string thunk
     Calls the zero-argument procedure THUNK with the current input
     port set temporarily to a string port opened on the specified
     STRING.  The value yielded by THUNK is returned.

 -- Scheme Procedure: open-input-string str
 -- C Function: scm_open_input_string (str)
     Take a string and return an input port that delivers characters
     from the string. The port can be closed by `close-input-port',
     though its storage will be reclaimed by the garbage collector if
     it becomes inaccessible.

 -- Scheme Procedure: open-output-string
 -- C Function: scm_open_output_string ()
     Return an output port that will accumulate characters for
     retrieval by `get-output-string'. The port can be closed by the
     procedure `close-output-port', though its storage will be
     reclaimed by the garbage collector if it becomes inaccessible.

 -- Scheme Procedure: get-output-string port
 -- C Function: scm_get_output_string (port)
     Given an output port created by `open-output-string', return a
     string consisting of the characters that have been output to the
     port so far.

     `get-output-string' must be used before closing PORT, once closed
     the string cannot be obtained.

   A string port can be used in many procedures which accept a port but
which are not dependent on implementation details of fports.  E.g.,
seeking and truncating will work on a string port, but trying to
extract the file descriptor number will fail.

6.14.9.3 Soft Ports
...................

A "soft-port" is a port based on a vector of procedures capable of
accepting or delivering characters.  It allows emulation of I/O ports.

 -- Scheme Procedure: make-soft-port pv modes
 -- C Function: scm_make_soft_port (pv, modes)
     Return a port capable of receiving or delivering characters as
     specified by the MODES string (*note open-file: File Ports.).  PV
     must be a vector of length 5 or 6.  Its components are as follows:

       0. procedure accepting one character for output

       1. procedure accepting a string for output

       2. thunk for flushing output

       3. thunk for getting one character

       4. thunk for closing port (not by garbage collection)

       5. (if present and not `#f') thunk for computing the number of
          characters that can be read from the port without blocking.

     For an output-only port only elements 0, 1, 2, and 4 need be
     procedures.  For an input-only port only elements 3 and 4 need be
     procedures.  Thunks 2 and 4 can instead be `#f' if there is no
     useful operation for them to perform.

     If thunk 3 returns `#f' or an `eof-object' (*note eof-object?:
     (r5rs)Input.) it indicates that the port has reached end-of-file.
     For example:

          (define stdout (current-output-port))
          (define p (make-soft-port
                     (vector
                      (lambda (c) (write c stdout))
                      (lambda (s) (display s stdout))
                      (lambda () (display "." stdout))
                      (lambda () (char-upcase (read-char)))
                      (lambda () (display "@" stdout)))
                     "rw"))

          (write p p) => #<input-output: soft 8081e20>

6.14.9.4 Void Ports
...................

This kind of port causes any data to be discarded when written to, and
always returns the end-of-file object when read from.

 -- Scheme Procedure: %make-void-port mode
 -- C Function: scm_sys_make_void_port (mode)
     Create and return a new void port.  A void port acts like
     `/dev/null'.  The MODE argument specifies the input/output modes
     for this port: see the documentation for `open-file' in *note File
     Ports::.

6.14.10 R6RS I/O Ports
----------------------

The I/O port API of the Revised Report^6 on the Algorithmic Language
Scheme (R6RS) (http://www.r6rs.org/) is provided by the `(rnrs io
ports)' module.  It provides features, such as binary I/O and Unicode
string I/O, that complement or refine Guile's historical port API
presented above (*note Input and Output::). Note that R6RS ports are not
disjoint from Guile's native ports, so Guile-specific procedures will
work on ports created using the R6RS API, and vice versa.

   The text in this section is taken from the R6RS standard libraries
document, with only minor adaptions for inclusion in this manual.  The
Guile developers offer their thanks to the R6RS editors for having
provided the report's text under permissive conditions making this
possible.

   _Note_: The implementation of this R6RS API is not complete yet.

   A subset of the `(rnrs io ports)' module is provided by the `(ice-9
binary-ports)' module.  It contains binary input/output procedures and
does not rely on R6RS support.

6.14.10.1 File Names
....................

Some of the procedures described in this chapter accept a file name as
an argument. Valid values for such a file name include strings that
name a file using the native notation of file system paths on an
implementation's underlying operating system, and may include
implementation-dependent values as well.

   A FILENAME parameter name means that the corresponding argument must
be a file name.

6.14.10.2 File Options
......................

When opening a file, the various procedures in this library accept a
`file-options' object that encapsulates flags to specify how the file
is to be opened. A `file-options' object is an enum-set (*note rnrs
enums::) over the symbols constituting valid file options.

   A FILE-OPTIONS parameter name means that the corresponding argument
must be a file-options object.

 -- Scheme Syntax: file-options FILE-OPTIONS-SYMBOL ...
     Each FILE-OPTIONS-SYMBOL must be a symbol.

     The `file-options' syntax returns a file-options object that
     encapsulates the specified options.

     When supplied to an operation that opens a file for output, the
     file-options object returned by `(file-options)' specifies that the
     file is created if it does not exist and an exception with
     condition type `&i/o-file-already-exists' is raised if it does
     exist.  The following standard options can be included to modify
     the default behavior.

    `no-create'
          If the file does not already exist, it is not created;
          instead, an exception with condition type
          `&i/o-file-does-not-exist'       is raised.        If the
          file already exists, the exception with condition type
          `&i/o-file-already-exists' is not raised       and the file
          is truncated to zero length.

    `no-fail'
          If the file already exists, the exception with condition type
               `&i/o-file-already-exists' is not raised,       even if
          `no-create' is not included,       and the file is truncated
          to zero length.

    `no-truncate'
          If the file already exists and the exception with condition
          type       `&i/o-file-already-exists' has been inhibited by
          inclusion of       `no-create' or `no-fail', the file is not
          truncated, but       the port's current position is still set
          to the beginning of the       file.

     These options have no effect when a file is opened only for input.
     Symbols other than those listed above may be used as
     FILE-OPTIONS-SYMBOLs; they have implementation-specific meaning,
     if any.

          Note:   Only the name of FILE-OPTIONS-SYMBOL is significant.

6.14.10.3 Buffer Modes
......................

Each port has an associated buffer mode.  For an output port, the
buffer mode defines when an output operation flushes the buffer
associated with the output port.  For an input port, the buffer mode
defines how much data will be read to satisfy read operations.  The
possible buffer modes are the symbols `none' for no buffering, `line'
for flushing upon line endings and reading up to line endings, or other
implementation-dependent behavior, and `block' for arbitrary buffering.
This section uses the parameter name BUFFER-MODE for arguments that
must be buffer-mode symbols.

   If two ports are connected to the same mutable source, both ports
are unbuffered, and reading a byte or character from that shared source
via one of the two ports would change the bytes or characters seen via
the other port, a lookahead operation on one port will render the
peeked byte or character inaccessible via the other port, while a
subsequent read operation on the peeked port will see the peeked byte
or character even though the port is otherwise unbuffered.

   In other words, the semantics of buffering is defined in terms of
side effects on shared mutable sources, and a lookahead operation has
the same side effect on the shared source as a read operation.

 -- Scheme Syntax: buffer-mode BUFFER-MODE-SYMBOL
     BUFFER-MODE-SYMBOL must be a symbol whose name is one of `none',
     `line', and `block'. The result is the corresponding symbol, and
     specifies the associated buffer mode.

          Note:   Only the name of BUFFER-MODE-SYMBOL is significant.

 -- Scheme Procedure: buffer-mode? obj
     Returns `#t' if the argument is a valid buffer-mode symbol, and
     returns `#f' otherwise.

6.14.10.4 Transcoders
.....................

Several different Unicode encoding schemes describe standard ways to
encode characters and strings as byte sequences and to decode those
sequences. Within this document, a "codec" is an immutable Scheme
object that represents a Unicode or similar encoding scheme.

   An "end-of-line style" is a symbol that, if it is not `none',
describes how a textual port transcodes representations of line endings.

   A "transcoder" is an immutable Scheme object that combines a codec
with an end-of-line style and a method for handling decoding errors.
Each transcoder represents some specific bidirectional (but not
necessarily lossless), possibly stateful translation between byte
sequences and Unicode characters and strings.  Every transcoder can
operate in the input direction (bytes to characters) or in the output
direction (characters to bytes).  A TRANSCODER parameter name means
that the corresponding argument must be a transcoder.

   A "binary port" is a port that supports binary I/O, does not have an
associated transcoder and does not support textual I/O.  A "textual
port" is a port that supports textual I/O, and does not support binary
I/O.  A textual port may or may not have an associated transcoder.

 -- Scheme Procedure: latin-1-codec
 -- Scheme Procedure: utf-8-codec
 -- Scheme Procedure: utf-16-codec
     These are predefined codecs for the ISO 8859-1, UTF-8, and UTF-16
     encoding schemes.

     A call to any of these procedures returns a value that is equal in
     the sense of `eqv?' to the result of any other call to the same
     procedure.

 -- Scheme Syntax: eol-style EOL-STYLE-SYMBOL
     EOL-STYLE-SYMBOL should be a symbol whose name is one of `lf',
     `cr', `crlf', `nel', `crnel', `ls', and `none'.

     The form evaluates to the corresponding symbol.  If the name of
     EOL-STYLE-SYMBOL is not one of these symbols, the effect and
     result are implementation-dependent; in particular, the result may
     be an eol-style symbol acceptable as an EOL-STYLE argument to
     `make-transcoder'.  Otherwise, an exception is raised.

     All eol-style symbols except `none' describe a specific
     line-ending encoding:

    `lf'
          linefeed

    `cr'
          carriage return

    `crlf'
          carriage return, linefeed

    `nel'
          next line

    `crnel'
          carriage return, next line

    `ls'
          line separator

     For a textual port with a transcoder, and whose transcoder has an
     eol-style symbol `none', no conversion occurs.  For a textual input
     port, any eol-style symbol other than `none' means that all of the
     above line-ending encodings are recognized and are translated into
     a single linefeed.  For a textual output port, `none' and `lf' are
     equivalent.  Linefeed characters are encoded according to the
     specified eol-style symbol, and all other characters that
     participate in possible line endings are encoded as is.

          Note:   Only the name of EOL-STYLE-SYMBOL is significant.

 -- Scheme Procedure: native-eol-style
     Returns the default end-of-line style of the underlying platform,
     e.g., `lf' on Unix and `crlf' on Windows.

 -- Condition Type: &i/o-decoding
 -- Scheme Procedure: make-i/o-decoding-error port
 -- Scheme Procedure: i/o-decoding-error? obj
     This condition type could be defined by

          (define-condition-type &i/o-decoding &i/o-port
            make-i/o-decoding-error i/o-decoding-error?)

     An exception with this type is raised when one of the operations
     for textual input from a port encounters a sequence of bytes that
     cannot be translated into a character or string by the input
     direction of the port's transcoder.

     When such an exception is raised, the port's position is past the
     invalid encoding.

 -- Condition Type: &i/o-encoding
 -- Scheme Procedure: make-i/o-encoding-error port char
 -- Scheme Procedure: i/o-encoding-error? obj
 -- Scheme Procedure: i/o-encoding-error-char condition
     This condition type could be defined by

          (define-condition-type &i/o-encoding &i/o-port
            make-i/o-encoding-error i/o-encoding-error?
            (char i/o-encoding-error-char))

     An exception with this type is raised when one of the operations
     for textual output to a port encounters a character that cannot be
     translated into bytes by the output direction of the port's
     transcoder.  CHAR is the character that could not be encoded.

 -- Scheme Syntax: error-handling-mode ERROR-HANDLING-MODE-SYMBOL
     ERROR-HANDLING-MODE-SYMBOL should be a symbol whose name is one of
     `ignore', `raise', and `replace'. The form evaluates to the
     corresponding symbol.  If ERROR-HANDLING-MODE-SYMBOL is not one of
     these identifiers, effect and result are implementation-dependent:
     The result may be an error-handling-mode symbol acceptable as a
     HANDLING-MODE argument to `make-transcoder'.  If it is not
     acceptable as a HANDLING-MODE argument to `make-transcoder', an
     exception is raised.

          Note:   Only the name of ERROR-HANDLING-STYLE-SYMBOL is
          significant.

     The error-handling mode of a transcoder specifies the behavior of
     textual I/O operations in the presence of encoding or decoding
     errors.

     If a textual input operation encounters an invalid or incomplete
     character encoding, and the error-handling mode is `ignore', an
     appropriate number of bytes of the invalid encoding are ignored and
     decoding continues with the following bytes.

     If the error-handling mode is `replace', the replacement character
     U+FFFD is injected into the data stream, an appropriate number of
     bytes are ignored, and decoding continues with the following bytes.

     If the error-handling mode is `raise', an exception with condition
     type `&i/o-decoding' is raised.

     If a textual output operation encounters a character it cannot
     encode, and the error-handling mode is `ignore', the character is
     ignored and encoding continues with the next character.  If the
     error-handling mode is `replace', a codec-specific replacement
     character is emitted by the transcoder, and encoding continues
     with the next character.  The replacement character is U+FFFD for
     transcoders whose codec is one of the Unicode encodings, but is
     the `?'  character for the Latin-1 encoding.  If the
     error-handling mode is `raise', an exception with condition type
     `&i/o-encoding' is raised.

 -- Scheme Procedure: make-transcoder codec
 -- Scheme Procedure: make-transcoder codec eol-style
 -- Scheme Procedure: make-transcoder codec eol-style handling-mode
     CODEC must be a codec; EOL-STYLE, if present, an eol-style symbol;
     and HANDLING-MODE, if present, an error-handling-mode symbol.

     EOL-STYLE may be omitted, in which case it defaults to the native
     end-of-line style of the underlying platform.  HANDLING-MODE may
     be omitted, in which case it defaults to `replace'.  The result is
     a transcoder with the behavior specified by its arguments.

 -- Scheme procedure: native-transcoder
     Returns an implementation-dependent transcoder that represents a
     possibly locale-dependent "native" transcoding.

 -- Scheme Procedure: transcoder-codec transcoder
 -- Scheme Procedure: transcoder-eol-style transcoder
 -- Scheme Procedure: transcoder-error-handling-mode transcoder
     These are accessors for transcoder objects; when applied to a
     transcoder returned by `make-transcoder', they return the CODEC,
     EOL-STYLE, and HANDLING-MODE arguments, respectively.

 -- Scheme Procedure: bytevector->string bytevector transcoder
     Returns the string that results from transcoding the BYTEVECTOR
     according to the input direction of the transcoder.

 -- Scheme Procedure: string->bytevector string transcoder
     Returns the bytevector that results from transcoding the STRING
     according to the output direction of the transcoder.

6.14.10.5 The End-of-File Object
................................

R5RS' `eof-object?' procedure is provided by the `(rnrs io ports)'
module:

 -- Scheme Procedure: eof-object? obj
 -- C Function: scm_eof_object_p (obj)
     Return true if OBJ is the end-of-file (EOF) object.

   In addition, the following procedure is provided:

 -- Scheme Procedure: eof-object
 -- C Function: scm_eof_object ()
     Return the end-of-file (EOF) object.

          (eof-object? (eof-object))
          => #t

6.14.10.6 Port Manipulation
...........................

The procedures listed below operate on any kind of R6RS I/O port.

 -- Scheme Procedure: port? obj
     Returns `#t' if the argument is a port, and returns `#f' otherwise.

 -- Scheme Procedure: port-transcoder port
     Returns the transcoder associated with PORT if PORT is textual and
     has an associated transcoder, and returns `#f' if PORT is binary
     or does not have an associated transcoder.

 -- Scheme Procedure: binary-port? port
     Return `#t' if PORT is a "binary port", suitable for binary data
     input/output.

     Note that internally Guile does not differentiate between binary
     and textual ports, unlike the R6RS.  Thus, this procedure returns
     true when PORT does not have an associated encoding--i.e., when
     `(port-encoding PORT)' is `#f' (*note port-encoding: Ports.).
     This is the case for ports returned by R6RS procedures such as
     `open-bytevector-input-port' and `make-custom-binary-output-port'.

     However, Guile currently does not prevent use of textual I/O
     procedures such as `display' or `read-char' with binary ports.
     Doing so "upgrades" the port from binary to textual, under the
     ISO-8859-1 encoding.  Likewise, Guile does not prevent use of
     `set-port-encoding!' on a binary port, which also turns it into a
     "textual" port.

 -- Scheme Procedure: textual-port? port
     Always return #T, as all ports can be used for textual I/O in
     Guile.

 -- Scheme Procedure: transcoded-port obj
     The `transcoded-port' procedure returns a new textual port with
     the specified TRANSCODER.  Otherwise the new textual port's state
     is largely the same as that of BINARY-PORT.  If BINARY-PORT is an
     input port, the new textual port will be an input port and will
     transcode the bytes that have not yet been read from BINARY-PORT.
     If BINARY-PORT is an output port, the new textual port will be an
     output port and will transcode output characters into bytes that
     are written to the byte sink represented by BINARY-PORT.

     As a side effect, however, `transcoded-port' closes BINARY-PORT in
     a special way that allows the new textual port to continue to use
     the byte source or sink represented by BINARY-PORT, even though
     BINARY-PORT itself is closed and cannot be used by the input and
     output operations described in this chapter.

 -- Scheme Procedure: port-position port
     If PORT supports it (see below), return the offset (an integer)
     indicating where the next octet will be read from/written to in
     PORT.  If PORT does not support this operation, an error condition
     is raised.

     This is similar to Guile's `seek' procedure with the `SEEK_CUR'
     argument (*note Random Access::).

 -- Scheme Procedure: port-has-port-position? port
     Return `#t' is PORT supports `port-position'.

 -- Scheme Procedure: set-port-position! port offset
     If PORT supports it (see below), set the position where the next
     octet will be read from/written to PORT to OFFSET (an integer).
     If PORT does not support this operation, an error condition is
     raised.

     This is similar to Guile's `seek' procedure with the `SEEK_SET'
     argument (*note Random Access::).

 -- Scheme Procedure: port-has-set-port-position!? port
     Return `#t' is PORT supports `set-port-position!'.

 -- Scheme Procedure: call-with-port port proc
     Call PROC, passing it PORT and closing PORT upon exit of PROC.
     Return the return values of PROC.

6.14.10.7 Input Ports
.....................

 -- Scheme Procedure: input-port? obj Returns `#t' if the argument is
          an input port (or a combined input
     and output port), and returns `#f' otherwise.

 -- Scheme Procedure: port-eof? port
     Returns `#t' if the `lookahead-u8' procedure (if INPUT-PORT is a
     binary port) or the `lookahead-char' procedure (if INPUT-PORT is a
     textual port) would return the end-of-file object, and `#f'
     otherwise.  The operation may block indefinitely if no data is
     available but the port cannot be determined to be at end of file.

 -- Scheme Procedure: open-file-input-port filename
 -- Scheme Procedure: open-file-input-port filename file-options
 -- Scheme Procedure: open-file-input-port filename file-options
          buffer-mode
 -- Scheme Procedure: open-file-input-port filename file-options
          buffer-mode maybe-transcoder
     MAYBE-TRANSCODER must be either a transcoder or `#f'.

     The `open-file-input-port' procedure returns an input port for the
     named file. The FILE-OPTIONS and MAYBE-TRANSCODER arguments are
     optional.

     The FILE-OPTIONS argument, which may determine various aspects of
     the returned port (*note R6RS File Options::), defaults to the
     value of `(file-options)'.

     The BUFFER-MODE argument, if supplied, must be one of the symbols
     that name a buffer mode.  The BUFFER-MODE argument defaults to
     `block'.

     If MAYBE-TRANSCODER is a transcoder, it becomes the transcoder
     associated with the returned port.

     If MAYBE-TRANSCODER is `#f' or absent, the port will be a binary
     port and will support the `port-position' and `set-port-position!'
     operations.  Otherwise the port will be a textual port, and
     whether it supports the `port-position' and `set-port-position!'
     operations is implementation-dependent (and possibly
     transcoder-dependent).

 -- Scheme Procedure: standard-input-port
     Returns a fresh binary input port connected to standard input.
     Whether the port supports the `port-position' and
     `set-port-position!' operations is implementation-dependent.

 -- Scheme Procedure: current-input-port
     This returns a default textual port for input.  Normally, this
     default port is associated with standard input, but can be
     dynamically re-assigned using the `with-input-from-file' procedure
     from the `io simple (6)' library (*note rnrs io simple::).  The
     port may or may not have an associated transcoder; if it does, the
     transcoder is implementation-dependent.

6.14.10.8 Binary Input
......................

R6RS binary input ports can be created with the procedures described
below.

 -- Scheme Procedure: open-bytevector-input-port bv [transcoder]
 -- C Function: scm_open_bytevector_input_port (bv, transcoder)
     Return an input port whose contents are drawn from bytevector BV
     (*note Bytevectors::).

     The TRANSCODER argument is currently not supported.

 -- Scheme Procedure: make-custom-binary-input-port id read!
          get-position set-position! close
 -- C Function: scm_make_custom_binary_input_port (id, read!,
          get-position, set-position!, close)
     Return a new custom binary input port(1) named ID (a string) whose
     input is drained by invoking READ! and passing it a bytevector, an
     index where bytes should be written, and the number of bytes to
     read.  The `read!'  procedure must return an integer indicating
     the number of bytes read, or `0' to indicate the end-of-file.

     Optionally, if GET-POSITION is not `#f', it must be a thunk that
     will be called when PORT-POSITION is invoked on the custom binary
     port and should return an integer indicating the position within
     the underlying data stream; if GET-POSITION was not supplied, the
     returned port does not support PORT-POSITION.

     Likewise, if SET-POSITION! is not `#f', it should be a
     one-argument procedure.  When SET-PORT-POSITION! is invoked on the
     custom binary input port, SET-POSITION! is passed an integer
     indicating the position of the next byte is to read.

     Finally, if CLOSE is not `#f', it must be a thunk.  It is invoked
     when the custom binary input port is closed.

     Using a custom binary input port, the `open-bytevector-input-port'
     procedure could be implemented as follows:

          (define (open-bytevector-input-port source)
            (define position 0)
            (define length (bytevector-length source))

            (define (read! bv start count)
              (let ((count (min count (- length position))))
                (bytevector-copy! source position
                                  bv start count)
                (set! position (+ position count))
                count))

            (define (get-position) position)

            (define (set-position! new-position)
              (set! position new-position))

            (make-custom-binary-input-port "the port" read!
                                            get-position
                                            set-position!))

          (read (open-bytevector-input-port (string->utf8 "hello")))
          => hello

   Binary input is achieved using the procedures below:

 -- Scheme Procedure: get-u8 port
 -- C Function: scm_get_u8 (port)
     Return an octet read from PORT, a binary input port, blocking as
     necessary, or the end-of-file object.

 -- Scheme Procedure: lookahead-u8 port
 -- C Function: scm_lookahead_u8 (port)
     Like `get-u8' but does not update PORT's position to point past
     the octet.

 -- Scheme Procedure: get-bytevector-n port count
 -- C Function: scm_get_bytevector_n (port, count)
     Read COUNT octets from PORT, blocking as necessary and return a
     bytevector containing the octets read.  If fewer bytes are
     available, a bytevector smaller than COUNT is returned.

 -- Scheme Procedure: get-bytevector-n! port bv start count
 -- C Function: scm_get_bytevector_n_x (port, bv, start, count)
     Read COUNT bytes from PORT and store them in BV starting at index
     START.  Return either the number of bytes actually read or the
     end-of-file object.

 -- Scheme Procedure: get-bytevector-some port
 -- C Function: scm_get_bytevector_some (port)
     Read from PORT, blocking as necessary, until data are available or
     and end-of-file is reached.  Return either a new bytevector
     containing the data read or the end-of-file object.

 -- Scheme Procedure: get-bytevector-all port
 -- C Function: scm_get_bytevector_all (port)
     Read from PORT, blocking as necessary, until the end-of-file is
     reached.  Return either a new bytevector containing the data read
     or the end-of-file object (if no data were available).

   ---------- Footnotes ----------

   (1) This is similar in spirit to Guile's "soft ports" (*note Soft
Ports::).

6.14.10.9 Textual Input
.......................

 -- Scheme Procedure: get-char port
     Reads from TEXTUAL-INPUT-PORT, blocking as necessary, until a
     complete character is available from TEXTUAL-INPUT-PORT, or until
     an end of file is reached.

     If a complete character is available before the next end of file,
     `get-char' returns that character and updates the input port to
     point past the character. If an end of file is reached before any
     character is read, `get-char' returns the end-of-file object.

 -- Scheme Procedure: lookahead-char port
     The `lookahead-char' procedure is like `get-char', but it does not
     update TEXTUAL-INPUT-PORT to point past the character.

 -- Scheme Procedure: get-string-n port count
     COUNT must be an exact, non-negative integer object, representing
     the number of characters to be read.

     The `get-string-n' procedure reads from TEXTUAL-INPUT-PORT,
     blocking as necessary, until COUNT characters are available, or
     until an end of file is reached.

     If COUNT characters are available before end of file,
     `get-string-n' returns a string consisting of those COUNT
     characters. If fewer characters are available before an end of
     file, but one or more characters can be read, `get-string-n'
     returns a string containing those characters. In either case, the
     input port is updated to point just past the characters read. If
     no characters can be read before an end of file, the end-of-file
     object is returned.

 -- Scheme Procedure: get-string-n! port string start count
     START and COUNT must be exact, non-negative integer objects, with
     COUNT representing the number of characters to be read.  STRING
     must be a string with at least $START + COUNT$ characters.

     The `get-string-n!' procedure reads from TEXTUAL-INPUT-PORT in the
     same manner as `get-string-n'.  If COUNT characters are available
     before an end of file, they are written into STRING starting at
     index START, and COUNT is returned. If fewer characters are
     available before an end of file, but one or more can be read,
     those characters are written into STRING starting at index START
     and the number of characters actually read is returned as an exact
     integer object. If no characters can be read before an end of
     file, the end-of-file object is returned.

 -- Scheme Procedure: get-string-all port count
     Reads from TEXTUAL-INPUT-PORT until an end of file, decoding
     characters in the same manner as `get-string-n' and
     `get-string-n!'.

     If characters are available before the end of file, a string
     containing all the characters decoded from that data are returned.
     If no character precedes the end of file, the end-of-file object
     is returned.

 -- Scheme Procedure: get-line port
     Reads from TEXTUAL-INPUT-PORT up to and including the linefeed
     character or end of file, decoding characters in the same manner as
     `get-string-n' and `get-string-n!'.

     If a linefeed character is read, a string containing all of the
     text up to (but not including) the linefeed character is returned,
     and the port is updated to point just past the linefeed character.
     If an end of file is encountered before any linefeed character is
     read, but some characters have been read and decoded as
     characters, a string containing those characters is returned. If
     an end of file is encountered before any characters are read, the
     end-of-file object is returned.

          Note:   The end-of-line style, if not `none', will cause all
          line endings   to be read as linefeed characters.  *Note R6RS
          Transcoders::.

 -- Scheme Procedure: get-datum port count
     Reads an external representation from TEXTUAL-INPUT-PORT and
     returns the datum it represents.  The `get-datum' procedure
     returns the next datum that can be parsed from the given
     TEXTUAL-INPUT-PORT, updating TEXTUAL-INPUT-PORT to point exactly
     past the end of the external representation of the object.

     Any _interlexeme space_ (comment or whitespace, *note Scheme
     Syntax::) in the input is first skipped.  If an end of file occurs
     after the interlexeme space, the end-of-file object (*note R6RS
     End-of-File::) is returned.

     If a character inconsistent with an external representation is
     encountered in the input, an exception with condition types
     `&lexical' and `&i/o-read' is raised.  Also, if the end of file is
     encountered after the beginning of an external representation, but
     the external representation is incomplete and therefore cannot be
     parsed, an exception with condition types `&lexical' and
     `&i/o-read' is raised.

6.14.10.10 Output Ports
.......................

 -- Scheme Procedure: output-port? obj
     Returns `#t' if the argument is an output port (or a combined
     input and output port), `#f' otherwise.

 -- Scheme Procedure: flush-output-port port
     Flushes any buffered output from the buffer of OUTPUT-PORT to the
     underlying file, device, or object. The `flush-output-port'
     procedure returns an unspecified values.

 -- Scheme Procedure: open-file-output-port filename
 -- Scheme Procedure: open-file-output-port filename file-options
 -- Scheme Procedure: open-file-output-port filename file-options
          buffer-mode
 -- Scheme Procedure: open-file-output-port filename file-options
          buffer-mode maybe-transcoder
     MAYBE-TRANSCODER must be either a transcoder or `#f'.

     The `open-file-output-port' procedure returns an output port for
     the named file.

     The FILE-OPTIONS argument, which may determine various aspects of
     the returned port (*note R6RS File Options::), defaults to the
     value of `(file-options)'.

     The BUFFER-MODE argument, if supplied, must be one of the symbols
     that name a buffer mode.  The BUFFER-MODE argument defaults to
     `block'.

     If MAYBE-TRANSCODER is a transcoder, it becomes the transcoder
     associated with the port.

     If MAYBE-TRANSCODER is `#f' or absent, the port will be a binary
     port and will support the `port-position' and `set-port-position!'
     operations.  Otherwise the port will be a textual port, and
     whether it supports the `port-position' and `set-port-position!'
     operations is implementation-dependent (and possibly
     transcoder-dependent).

 -- Scheme Procedure: standard-output-port
 -- Scheme Procedure: standard-error-port
     Returns a fresh binary output port connected to the standard
     output or standard error respectively.  Whether the port supports
     the `port-position' and `set-port-position!' operations is
     implementation-dependent.

 -- Scheme Procedure: current-output-port
 -- Scheme Procedure: current-error-port
     These return default textual ports for regular output and error
     output.  Normally, these default ports are associated with
     standard output, and standard error, respectively.  The return
     value of `current-output-port' can be dynamically re-assigned
     using the `with-output-to-file' procedure from the `io simple (6)'
     library (*note rnrs io simple::).  A port returned by one of these
     procedures may or may not have an associated transcoder; if it
     does, the transcoder is implementation-dependent.

6.14.10.11 Binary Output
........................

Binary output ports can be created with the procedures below.

 -- Scheme Procedure: open-bytevector-output-port [transcoder]
 -- C Function: scm_open_bytevector_output_port (transcoder)
     Return two values: a binary output port and a procedure.  The
     latter should be called with zero arguments to obtain a bytevector
     containing the data accumulated by the port, as illustrated below.

          (call-with-values
            (lambda ()
              (open-bytevector-output-port))
            (lambda (port get-bytevector)
              (display "hello" port)
              (get-bytevector)))

          => #vu8(104 101 108 108 111)

     The TRANSCODER argument is currently not supported.

 -- Scheme Procedure: make-custom-binary-output-port id write!
          get-position set-position! close
 -- C Function: scm_make_custom_binary_output_port (id, write!,
          get-position, set-position!, close)
     Return a new custom binary output port named ID (a string) whose
     output is sunk by invoking WRITE! and passing it a bytevector, an
     index where bytes should be read from this bytevector, and the
     number of bytes to be "written".  The `write!'  procedure must
     return an integer indicating the number of bytes actually written;
     when it is passed `0' as the number of bytes to write, it should
     behave as though an end-of-file was sent to the byte sink.

     The other arguments are as for `make-custom-binary-input-port'
     (*note `make-custom-binary-input-port': R6RS Binary Input.).

   Writing to a binary output port can be done using the following
procedures:

 -- Scheme Procedure: put-u8 port octet
 -- C Function: scm_put_u8 (port, octet)
     Write OCTET, an integer in the 0-255 range, to PORT, a binary
     output port.

 -- Scheme Procedure: put-bytevector port bv [start [count]]
 -- C Function: scm_put_bytevector (port, bv, start, count)
     Write the contents of BV to PORT, optionally starting at index
     START and limiting to COUNT octets.

6.14.10.12 Textual Output
.........................

 -- Scheme Procedure: put-char port char
     Writes CHAR to the port. The `put-char' procedure returns

 -- Scheme Procedure: put-string port string
 -- Scheme Procedure: put-string port string start
 -- Scheme Procedure: put-string port string start count
     START and COUNT must be non-negative exact integer objects.
     STRING must have a length of at least START + COUNT.  START
     defaults to 0.  COUNT defaults to `(string-length STRING)' -
     START$. The `put-string' procedure writes the COUNT characters of
     STRING starting at index START to the port.  The `put-string'
     procedure returns an unspecified value.

 -- Scheme Procedure: put-datum port datum
     DATUM should be a datum value.  The `put-datum' procedure writes
     an external representation of DATUM to TEXTUAL-OUTPUT-PORT.  The
     specific external representation is implementation-dependent.
     However, whenever possible, an implementation should produce a
     representation for which `get-datum', when reading the
     representation, will return an object equal (in the sense of
     `equal?') to DATUM.

          Note:   Not all datums may allow producing an external
          representation for which   `get-datum' will produce an object
          that is equal to the   original.  Specifically, NaNs
          contained in DATUM may make   this impossible.

          Note:   The `put-datum' procedure merely writes the external
          representation, but no trailing delimiter.  If `put-datum' is
           used to write several subsequent external representations to
          an   output port, care should be taken to delimit them
          properly so they can   be read back in by subsequent calls to
          `get-datum'.

6.14.11 Using and Extending Ports in C
--------------------------------------

6.14.11.1 C Port Interface
..........................

This section describes how to use Scheme ports from C.

Port basics
...........

There are two main data structures.  A port type object (ptob) is of
type `scm_ptob_descriptor'.  A port instance is of type `scm_port'.
Given an `SCM' variable which points to a port, the corresponding C
port object can be obtained using the `SCM_PTAB_ENTRY' macro.  The ptob
can be obtained by using `SCM_PTOBNUM' to give an index into the
`scm_ptobs' global array.

Port buffers
............

An input port always has a read buffer and an output port always has a
write buffer.  However the size of these buffers is not guaranteed to be
more than one byte (e.g., the `shortbuf' field in `scm_port' which is
used when no other buffer is allocated).  The way in which the buffers
are allocated depends on the implementation of the ptob.  For example
in the case of an fport, buffers may be allocated with malloc when the
port is created, but in the case of an strport the underlying string is
used as the buffer.

The `rw_random' flag
....................

Special treatment is required for ports which can be seeked at random.
Before various operations, such as seeking the port or changing from
input to output on a bidirectional port or vice versa, the port
implementation must be given a chance to update its state.  The write
buffer is updated by calling the `flush' ptob procedure and the input
buffer is updated by calling the `end_input' ptob procedure.  In the
case of an fport, `flush' causes buffered output to be written to the
file descriptor, while `end_input' causes the descriptor position to be
adjusted to account for buffered input which was never read.

   The special treatment must be performed if the `rw_random' flag in
the port is non-zero.

The `rw_active' variable
........................

The `rw_active' variable in the port is only used if `rw_random' is
set.  It's defined as an enum with the following values:

`SCM_PORT_READ'
     the read buffer may have unread data.

`SCM_PORT_WRITE'
     the write buffer may have unwritten data.

`SCM_PORT_NEITHER'
     neither the write nor the read buffer has data.

Reading from a port.
....................

To read from a port, it's possible to either call existing libguile
procedures such as `scm_getc' and `scm_read_line' or to read data from
the read buffer directly.  Reading from the buffer involves the
following steps:

  1. Flush output on the port, if `rw_active' is `SCM_PORT_WRITE'.

  2. Fill the read buffer, if it's empty, using `scm_fill_input'.

  3. Read the data from the buffer and update the read position in the
     buffer.  Steps 2) and 3) may be repeated as many times as required.

  4. Set rw_active to `SCM_PORT_READ' if `rw_random' is set.

  5. update the port's line and column counts.

Writing to a port.
..................

To write data to a port, calling `scm_lfwrite' should be sufficient for
most purposes.  This takes care of the following steps:

  1. End input on the port, if `rw_active' is `SCM_PORT_READ'.

  2. Pass the data to the ptob implementation using the `write' ptob
     procedure.  The advantage of using the ptob `write' instead of
     manipulating the write buffer directly is that it allows the data
     to be written in one operation even if the port is using the
     single-byte `shortbuf'.

  3. Set `rw_active' to `SCM_PORT_WRITE' if `rw_random' is set.

6.14.11.2 Port Implementation
.............................

This section describes how to implement a new port type in C.

   As described in the previous section, a port type object (ptob) is a
structure of type `scm_ptob_descriptor'.  A ptob is created by calling
`scm_make_port_type'.

 -- Function: scm_t_bits scm_make_port_type (char *name, int
          (*fill_input) (SCM port), void (*write) (SCM port, const void
          *data, size_t size))
     Return a new port type object.  The NAME, FILL_INPUT and WRITE
     parameters are initial values for those port type fields, as
     described below.  The other fields are initialized with default
     values and can be changed later.

   All of the elements of the ptob, apart from `name', are procedures
which collectively implement the port behaviour.  Creating a new port
type mostly involves writing these procedures.

`name'
     A pointer to a NUL terminated string: the name of the port type.
     This is the only element of `scm_ptob_descriptor' which is not a
     procedure.  Set via the first argument to `scm_make_port_type'.

`mark'
     Called during garbage collection to mark any SCM objects that a
     port object may contain.  It doesn't need to be set unless the
     port has `SCM' components.  Set using

      -- Function: void scm_set_port_mark (scm_t_bits tc, SCM (*mark)
               (SCM port))

`free'
     Called when the port is collected during gc.  It should free any
     resources used by the port.  Set using

      -- Function: void scm_set_port_free (scm_t_bits tc, size_t
               (*free) (SCM port))

`print'
     Called when `write' is called on the port object, to print a port
     description.  E.g., for an fport it may produce something like:
     `#<input: /etc/passwd 3>'.   Set using

      -- Function: void scm_set_port_print (scm_t_bits tc, int (*print)
               (SCM port, SCM dest_port, scm_print_state *pstate))
          The first argument PORT is the object being printed, the
          second argument DEST_PORT is where its description should go.

`equalp'
     Not used at present.  Set using

      -- Function: void scm_set_port_equalp (scm_t_bits tc, SCM
               (*equalp) (SCM, SCM))

`close'
     Called when the port is closed, unless it was collected during gc.
     It should free any resources used by the port.  Set using

      -- Function: void scm_set_port_close (scm_t_bits tc, int (*close)
               (SCM port))

`write'
     Accept data which is to be written using the port.  The port
     implementation may choose to buffer the data instead of processing
     it directly.  Set via the third argument to `scm_make_port_type'.

`flush'
     Complete the processing of buffered output data.  Reset the value
     of `rw_active' to `SCM_PORT_NEITHER'.  Set using

      -- Function: void scm_set_port_flush (scm_t_bits tc, void
               (*flush) (SCM port))

`end_input'
     Perform any synchronization required when switching from input to
     output on the port.  Reset the value of `rw_active' to
     `SCM_PORT_NEITHER'.  Set using

      -- Function: void scm_set_port_end_input (scm_t_bits tc, void
               (*end_input) (SCM port, int offset))

`fill_input'
     Read new data into the read buffer and return the first character.
     It can be assumed that the read buffer is empty when this
     procedure is called.  Set via the second argument to
     `scm_make_port_type'.

`input_waiting'
     Return a lower bound on the number of bytes that could be read
     from the port without blocking.  It can be assumed that the
     current state of `rw_active' is `SCM_PORT_NEITHER'.  Set using

      -- Function: void scm_set_port_input_waiting (scm_t_bits tc, int
               (*input_waiting) (SCM port))

`seek'
     Set the current position of the port.  The procedure can not make
     any assumptions about the value of `rw_active' when it's called.
     It can reset the buffers first if desired by using something like:

          if (pt->rw_active == SCM_PORT_READ)
            scm_end_input (port);
          else if (pt->rw_active == SCM_PORT_WRITE)
            ptob->flush (port);

     However note that this will have the side effect of discarding any
     data in the unread-char buffer, in addition to any side effects
     from the `end_input' and `flush' ptob procedures.  This is
     undesirable when seek is called to measure the current position of
     the port, i.e., `(seek p 0 SEEK_CUR)'.  The libguile fport and
     string port implementations take care to avoid this problem.

     The procedure is set using

      -- Function: void scm_set_port_seek (scm_t_bits tc, scm_t_off
               (*seek) (SCM port, scm_t_off offset, int whence))

`truncate'
     Truncate the port data to be specified length.  It can be assumed
     that the current state of `rw_active' is `SCM_PORT_NEITHER'.  Set
     using

      -- Function: void scm_set_port_truncate (scm_t_bits tc, void
               (*truncate) (SCM port, scm_t_off length))


6.15 Regular Expressions
========================

A "regular expression" (or "regexp") is a pattern that describes a
whole class of strings.  A full description of regular expressions and
their syntax is beyond the scope of this manual; an introduction can be
found in the Emacs manual (*note Syntax of Regular Expressions:
(emacs)Regexps.), or in many general Unix reference books.

   If your system does not include a POSIX regular expression library,
and you have not linked Guile with a third-party regexp library such as
Rx, these functions will not be available.  You can tell whether your
Guile installation includes regular expression support by checking
whether `(provided? 'regex)' returns true.

   The following regexp and string matching features are provided by the
`(ice-9 regex)' module.  Before using the described functions, you
should load this module by executing `(use-modules (ice-9 regex))'.

6.15.1 Regexp Functions
-----------------------

By default, Guile supports POSIX extended regular expressions.  That
means that the characters `(', `)', `+' and `?' are special, and must
be escaped if you wish to match the literal characters.

   This regular expression interface was modeled after that implemented
by SCSH, the Scheme Shell.  It is intended to be upwardly compatible
with SCSH regular expressions.

   Zero bytes (`#\nul') cannot be used in regex patterns or input
strings, since the underlying C functions treat that as the end of
string.  If there's a zero byte an error is thrown.

   Patterns and input strings are treated as being in the locale
character set if `setlocale' has been called (*note Locales::), and in
a multibyte locale this includes treating multi-byte sequences as a
single character.  (Guile strings are currently merely bytes, though
this may change in the future, *Note Conversion to/from C::.)

 -- Scheme Procedure: string-match pattern str [start]
     Compile the string PATTERN into a regular expression and compare
     it with STR.  The optional numeric argument START specifies the
     position of STR at which to begin matching.

     `string-match' returns a "match structure" which describes what,
     if anything, was matched by the regular expression.  *Note Match
     Structures::.  If STR does not match PATTERN at all,
     `string-match' returns `#f'.

   Two examples of a match follow.  In the first example, the pattern
matches the four digits in the match string.  In the second, the pattern
matches nothing.

     (string-match "[0-9][0-9][0-9][0-9]" "blah2002")
     => #("blah2002" (4 . 8))

     (string-match "[A-Za-z]" "123456")
     => #f

   Each time `string-match' is called, it must compile its PATTERN
argument into a regular expression structure.  This operation is
expensive, which makes `string-match' inefficient if the same regular
expression is used several times (for example, in a loop).  For better
performance, you can compile a regular expression in advance and then
match strings against the compiled regexp.

 -- Scheme Procedure: make-regexp pat flag...
 -- C Function: scm_make_regexp (pat, flaglst)
     Compile the regular expression described by PAT, and return the
     compiled regexp structure.  If PAT does not describe a legal
     regular expression, `make-regexp' throws a
     `regular-expression-syntax' error.

     The FLAG arguments change the behavior of the compiled regular
     expression.  The following values may be supplied:

      -- Variable: regexp/icase
          Consider uppercase and lowercase letters to be the same when
          matching.

      -- Variable: regexp/newline
          If a newline appears in the target string, then permit the
          `^' and `$' operators to match immediately after or
          immediately before the newline, respectively.  Also, the `.'
          and `[^...]' operators will never match a newline character.
          The intent of this flag is to treat the target string as a
          buffer containing many lines of text, and the regular
          expression as a pattern that may match a single one of those
          lines.

      -- Variable: regexp/basic
          Compile a basic ("obsolete") regexp instead of the extended
          ("modern") regexps that are the default.  Basic regexps do
          not consider `|', `+' or `?' to be special characters, and
          require the `{...}' and `(...)' metacharacters to be
          backslash-escaped (*note Backslash Escapes::).  There are
          several other differences between basic and extended regular
          expressions, but these are the most significant.

      -- Variable: regexp/extended
          Compile an extended regular expression rather than a basic
          regexp.  This is the default behavior; this flag will not
          usually be needed.  If a call to `make-regexp' includes both
          `regexp/basic' and `regexp/extended' flags, the one which
          comes last will override the earlier one.

 -- Scheme Procedure: regexp-exec rx str [start [flags]]
 -- C Function: scm_regexp_exec (rx, str, start, flags)
     Match the compiled regular expression RX against `str'.  If the
     optional integer START argument is provided, begin matching from
     that position in the string.  Return a match structure describing
     the results of the match, or `#f' if no match could be found.

     The FLAGS argument changes the matching behavior.  The following
     flag values may be supplied, use `logior' (*note Bitwise
     Operations::) to combine them,

      -- Variable: regexp/notbol
          Consider that the START offset into STR is not the beginning
          of a line and should not match operator `^'.

          If RX was created with the `regexp/newline' option above, `^'
          will still match after a newline in STR.

      -- Variable: regexp/noteol
          Consider that the end of STR is not the end of a line and
          should not match operator `$'.

          If RX was created with the `regexp/newline' option above, `$'
          will still match before a newline in STR.

     ;; Regexp to match uppercase letters
     (define r (make-regexp "[A-Z]*"))

     ;; Regexp to match letters, ignoring case
     (define ri (make-regexp "[A-Z]*" regexp/icase))

     ;; Search for bob using regexp r
     (match:substring (regexp-exec r "bob"))
     => ""                  ; no match

     ;; Search for bob using regexp ri
     (match:substring (regexp-exec ri "Bob"))
     => "Bob"               ; matched case insensitive

 -- Scheme Procedure: regexp? obj
 -- C Function: scm_regexp_p (obj)
     Return `#t' if OBJ is a compiled regular expression, or `#f'
     otherwise.


 -- Scheme Procedure: list-matches regexp str [flags]
     Return a list of match structures which are the non-overlapping
     matches of REGEXP in STR.  REGEXP can be either a pattern string
     or a compiled regexp.  The FLAGS argument is as per `regexp-exec'
     above.

          (map match:substring (list-matches "[a-z]+" "abc 42 def 78"))
          => ("abc" "def")

 -- Scheme Procedure: fold-matches regexp str init proc [flags]
     Apply PROC to the non-overlapping matches of REGEXP in STR, to
     build a result.  REGEXP can be either a pattern string or a
     compiled regexp.  The FLAGS argument is as per `regexp-exec' above.

     PROC is called as `(PROC match prev)' where MATCH is a match
     structure and PREV is the previous return from PROC.  For the
     first call PREV is the given INIT parameter.  `fold-matches'
     returns the final value from PROC.

     For example to count matches,

          (fold-matches "[a-z][0-9]" "abc x1 def y2" 0
                        (lambda (match count)
                          (1+ count)))
          => 2


   Regular expressions are commonly used to find patterns in one string
and replace them with the contents of another string.  The following
functions are convenient ways to do this.

 -- Scheme Procedure: regexp-substitute port match [item...]
     Write to PORT selected parts of the match structure MATCH.  Or if
     PORT is `#f' then form a string from those parts and return that.

     Each ITEM specifies a part to be written, and may be one of the
     following,

        * A string.  String arguments are written out verbatim.

        * An integer.  The submatch with that number is written
          (`match:substring').  Zero is the entire match.

        * The symbol `pre'.  The portion of the matched string preceding
          the regexp match is written (`match:prefix').

        * The symbol `post'.  The portion of the matched string
          following the regexp match is written (`match:suffix').

     For example, changing a match and retaining the text before and
     after,

          (regexp-substitute #f (string-match "[0-9]+" "number 25 is good")
                             'pre "37" 'post)
          => "number 37 is good"

     Or matching a YYYYMMDD format date such as `20020828' and
     re-ordering and hyphenating the fields.

          (define date-regex
             "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
          (define s "Date 20020429 12am.")
          (regexp-substitute #f (string-match date-regex s)
                             'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
          => "Date 04-29-2002 12am. (20020429)"

 -- Scheme Procedure: regexp-substitute/global port regexp target
          [item...]
     Write to PORT selected parts of matches of REGEXP in TARGET.  If
     PORT is `#f' then form a string from those parts and return that.
     REGEXP can be a string or a compiled regex.

     This is similar to `regexp-substitute', but allows global
     substitutions on TARGET.  Each ITEM behaves as per
     `regexp-substitute', with the following differences,

        * A function.  Called as `(ITEM match)' with the match
          structure for the REGEXP match, it should return a string to
          be written to PORT.

        * The symbol `post'.  This doesn't output anything, but instead
          causes `regexp-substitute/global' to recurse on the unmatched
          portion of TARGET.

          This _must_ be supplied to perform a global search and
          replace on TARGET; without it `regexp-substitute/global'
          returns after a single match and output.

     For example, to collapse runs of tabs and spaces to a single hyphen
     each,

          (regexp-substitute/global #f "[ \t]+"  "this   is   the text"
                                    'pre "-" 'post)
          => "this-is-the-text"

     Or using a function to reverse the letters in each word,

          (regexp-substitute/global #f "[a-z]+"  "to do and not-do"
            'pre (lambda (m) (string-reverse (match:substring m))) 'post)
          => "ot od dna ton-od"

     Without the `post' symbol, just one regexp match is made.  For
     example the following is the date example from `regexp-substitute'
     above, without the need for the separate `string-match' call.

          (define date-regex
             "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
          (define s "Date 20020429 12am.")
          (regexp-substitute/global #f date-regex s
                                    'pre 2 "-" 3 "-" 1 'post " (" 0 ")")

          => "Date 04-29-2002 12am. (20020429)"

6.15.2 Match Structures
-----------------------

A "match structure" is the object returned by `string-match' and
`regexp-exec'.  It describes which portion of a string, if any, matched
the given regular expression.  Match structures include: a reference to
the string that was checked for matches; the starting and ending
positions of the regexp match; and, if the regexp included any
parenthesized subexpressions, the starting and ending positions of each
submatch.

   In each of the regexp match functions described below, the `match'
argument must be a match structure returned by a previous call to
`string-match' or `regexp-exec'.  Most of these functions return some
information about the original target string that was matched against a
regular expression; we will call that string TARGET for easy reference.

 -- Scheme Procedure: regexp-match? obj
     Return `#t' if OBJ is a match structure returned by a previous
     call to `regexp-exec', or `#f' otherwise.

 -- Scheme Procedure: match:substring match [n]
     Return the portion of TARGET matched by subexpression number N.
     Submatch 0 (the default) represents the entire regexp match.  If
     the regular expression as a whole matched, but the subexpression
     number N did not match, return `#f'.

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:substring s)
     => "2002"

     ;; match starting at offset 6 in the string
     (match:substring
       (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
     => "7654"

 -- Scheme Procedure: match:start match [n]
     Return the starting position of submatch number N.

   In the following example, the result is 4, since the match starts at
character index 4:

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:start s)
     => 4

 -- Scheme Procedure: match:end match [n]
     Return the ending position of submatch number N.

   In the following example, the result is 8, since the match runs
between characters 4 and 8 (i.e. the "2002").

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:end s)
     => 8

 -- Scheme Procedure: match:prefix match
     Return the unmatched portion of TARGET preceding the regexp match.

          (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
          (match:prefix s)
          => "blah"

 -- Scheme Procedure: match:suffix match
     Return the unmatched portion of TARGET following the regexp match.

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:suffix s)
     => "foo"

 -- Scheme Procedure: match:count match
     Return the number of parenthesized subexpressions from MATCH.
     Note that the entire regular expression match itself counts as a
     subexpression, and failed submatches are included in the count.

 -- Scheme Procedure: match:string match
     Return the original TARGET string.

     (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
     (match:string s)
     => "blah2002foo"

6.15.3 Backslash Escapes
------------------------

Sometimes you will want a regexp to match characters like `*' or `$'
exactly.  For example, to check whether a particular string represents
a menu entry from an Info node, it would be useful to match it against
a regexp like `^* [^:]*::'.  However, this won't work; because the
asterisk is a metacharacter, it won't match the `*' at the beginning of
the string.  In this case, we want to make the first asterisk un-magic.

   You can do this by preceding the metacharacter with a backslash
character `\'.  (This is also called "quoting" the metacharacter, and
is known as a "backslash escape".)  When Guile sees a backslash in a
regular expression, it considers the following glyph to be an ordinary
character, no matter what special meaning it would ordinarily have.
Therefore, we can make the above example work by changing the regexp to
`^\* [^:]*::'.  The `\*' sequence tells the regular expression engine
to match only a single asterisk in the target string.

   Since the backslash is itself a metacharacter, you may force a
regexp to match a backslash in the target string by preceding the
backslash with itself.  For example, to find variable references in a
TeX program, you might want to find occurrences of the string `\let\'
followed by any number of alphabetic characters.  The regular expression
`\\let\\[A-Za-z]*' would do this: the double backslashes in the regexp
each match a single backslash in the target string.

 -- Scheme Procedure: regexp-quote str
     Quote each special character found in STR with a backslash, and
     return the resulting string.

   *Very important:* Using backslash escapes in Guile source code (as
in Emacs Lisp or C) can be tricky, because the backslash character has
special meaning for the Guile reader.  For example, if Guile encounters
the character sequence `\n' in the middle of a string while processing
Scheme code, it replaces those characters with a newline character.
Similarly, the character sequence `\t' is replaced by a horizontal tab.
Several of these "escape sequences" are processed by the Guile reader
before your code is executed.  Unrecognized escape sequences are
ignored: if the characters `\*' appear in a string, they will be
translated to the single character `*'.

   This translation is obviously undesirable for regular expressions,
since we want to be able to include backslashes in a string in order to
escape regexp metacharacters.  Therefore, to make sure that a backslash
is preserved in a string in your Guile program, you must use _two_
consecutive backslashes:

     (define Info-menu-entry-pattern (make-regexp "^\\* [^:]*"))

   The string in this example is preprocessed by the Guile reader before
any code is executed.  The resulting argument to `make-regexp' is the
string `^\* [^:]*', which is what we really want.

   This also means that in order to write a regular expression that
matches a single backslash character, the regular expression string in
the source code must include _four_ backslashes.  Each consecutive pair
of backslashes gets translated by the Guile reader to a single
backslash, and the resulting double-backslash is interpreted by the
regexp engine as matching a single backslash character.  Hence:

     (define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*"))

   The reason for the unwieldiness of this syntax is historical.  Both
regular expression pattern matchers and Unix string processing systems
have traditionally used backslashes with the special meanings described
above.  The POSIX regular expression specification and ANSI C standard
both require these semantics.  Attempting to abandon either convention
would cause other kinds of compatibility problems, possibly more severe
ones.  Therefore, without extending the Scheme reader to support
strings with different quoting conventions (an ungainly and confusing
extension when implemented in other languages), we must adhere to this
cumbersome escape syntax.

6.16 LALR(1) Parsing
====================

The `(system base lalr)' module provides the `lalr-scm' LALR(1) parser
generator by Dominique Boucher (http://code.google.com/p/lalr-scm/).
`lalr-scm' uses the same algorithm as GNU Bison (*note Introduction to
Bison: (bison)Introduction.).  Parsers are defined using the
`lalr-parser' macro.

 -- Scheme Syntax: lalr-parser [OPTIONS] TOKENS RULES...
     Generate an LALR(1) syntax analyzer.  TOKENS is a list of symbols
     representing the terminal symbols of the grammar.  RULES are the
     grammar production rules.

     Each rule has the form `(NON-TERMINAL (RHS ...) : ACTION ...)',
     where NON-TERMINAL is the name of the rule, RHS are the right-hand
     sides, i.e., the production rule, and ACTION is a semantic action
     associated with the rule.

     The generated parser is a two-argument procedure that takes a
     "tokenizer" and a "syntax error procedure".  The tokenizer should
     be a thunk that returns lexical tokens as produced by
     `make-lexical-token'.  The syntax error procedure may be called
     with at least an error message (a string), and optionally the
     lexical token that caused the error.

   Please refer to the `lalr-scm' documentation for details.

6.17 Reading and Evaluating Scheme Code
=======================================

This chapter describes Guile functions that are concerned with reading,
loading, evaluating, and compiling Scheme code at run time.

6.17.1 Scheme Syntax: Standard and Guile Extensions
---------------------------------------------------

6.17.1.1 Expression Syntax
..........................

An expression to be evaluated takes one of the following forms.

SYMBOL
     A symbol is evaluated by dereferencing.  A binding of that symbol
     is sought and the value there used.  For example,

          (define x 123)
          x => 123

(PROC ARGS...)
     A parenthesised expression is a function call.  PROC and each
     argument are evaluated, then the function (which PROC evaluated
     to) is called with those arguments.

     The order in which PROC and the arguments are evaluated is
     unspecified, so be careful when using expressions with side
     effects.

          (max 1 2 3) => 3

          (define (get-some-proc)  min)
          ((get-some-proc) 1 2 3) => 1

     The same sort of parenthesised form is used for a macro invocation,
     but in that case the arguments are not evaluated.  See the
     descriptions of macros for more on this (*note Macros::, and *note
     Syntax Rules::).

CONSTANT
     Number, string, character and boolean constants evaluate "to
     themselves", so can appear as literals.

          123     => 123
          99.9    => 99.9
          "hello" => "hello"
          #\z     => #\z
          #t      => #t

     Note that an application must not attempt to modify literal
     strings, since they may be in read-only memory.

(quote DATA)
'DATA
     Quoting is used to obtain a literal symbol (instead of a variable
     reference), a literal list (instead of a function call), or a
     literal vector.  ' is simply a shorthand for a `quote' form.  For
     example,

          'x                   => x
          '(1 2 3)             => (1 2 3)
          '#(1 (2 3) 4)        => #(1 (2 3) 4)
          (quote x)            => x
          (quote (1 2 3))      => (1 2 3)
          (quote #(1 (2 3) 4)) => #(1 (2 3) 4)

     Note that an application must not attempt to modify literal lists
     or vectors obtained from a `quote' form, since they may be in
     read-only memory.

(quasiquote DATA)
`DATA
     Backquote quasi-quotation is like `quote', but selected
     sub-expressions are evaluated.  This is a convenient way to
     construct a list or vector structure most of which is constant,
     but at certain points should have expressions substituted.

     The same effect can always be had with suitable `list', `cons' or
     `vector' calls, but quasi-quoting is often easier.

    (unquote EXPR)
    ,EXPR
          Within the quasiquote DATA, `unquote' or `,' indicates an
          expression to be evaluated and inserted.  The comma syntax `,'
          is simply a shorthand for an `unquote' form.  For example,

               `(1 2 ,(* 9 9) 3 4)      => (1 2 81 3 4)
               `(1 (unquote (+ 1 1)) 3) => (1 2 3)
               `#(1 ,(/ 12 2))          => #(1 6)

    (unquote-splicing EXPR)
    ,@EXPR
          Within the quasiquote DATA, `unquote-splicing' or `,@'
          indicates an expression to be evaluated and the elements of
          the returned list inserted.  EXPR must evaluate to a list.
          The "comma-at" syntax `,@' is simply a shorthand for an
          `unquote-splicing' form.

               (define x '(2 3))
               `(1 ,@x 4)                         => (1 2 3 4)
               `(1 (unquote-splicing (map 1+ x))) => (1 3 4)
               `#(9 ,@x 9)                        => #(9 2 3 9)

          Notice `,@' differs from plain `,' in the way one level of
          nesting is stripped.  For `,@' the elements of a returned list
          are inserted, whereas with `,' it would be the list itself
          inserted.


6.17.1.2 Comments
.................

Comments in Scheme source files are written by starting them with a
semicolon character (`;').  The comment then reaches up to the end of
the line.  Comments can begin at any column, and the may be inserted on
the same line as Scheme code.

     ; Comment
     ;; Comment too
     (define x 1)        ; Comment after expression
     (let ((y 1))
       ;; Display something.
       (display y)
     ;;; Comment at left margin.
       (display (+ y 1)))

   It is common to use a single semicolon for comments following
expressions on a line, to use two semicolons for comments which are
indented like code, and three semicolons for comments which start at
column 0, even if they are inside an indented code block.  This
convention is used when indenting code in Emacs' Scheme mode.

6.17.1.3 Block Comments
.......................

In addition to the standard line comments defined by R5RS, Guile has
another comment type for multiline comments, called "block comments".
This type of comment begins with the character sequence `#!' and ends
with the characters `!#', which must appear on a line of their own.
These comments are compatible with the block comments in the Scheme
Shell `scsh' (*note The Scheme shell (scsh)::).  The characters `#!'
were chosen because they are the magic characters used in shell scripts
for indicating that the name of the program for executing the script
follows on the same line.

   Thus a Guile script often starts like this.

     #! /usr/local/bin/guile -s
     !#

   More details on Guile scripting can be found in the scripting section
(*note Guile Scripting::).

   Similarly, Guile (starting from version 2.0) supports nested block
comments as specified by R6RS and SRFI-30
(http://srfi.schemers.org/srfi-30/srfi-30.html):

     (+  #| this is a #| nested |# block comment |# 2)
     => 3

   For backward compatibility, this syntax can be overridden with
`read-hash-extend' (*note `read-hash-extend': Reader Extensions.).

   There is one special case where the contents of a comment can
actually affect the interpretation of code.  When a character encoding
declaration, such as `coding: utf-8' appears in one of the first few
lines of a source file, it indicates to Guile's default reader that
this source code file is not ASCII.  For details see *note Character
Encoding of Source Files::.

6.17.1.4 Case Sensitivity
.........................

Scheme as defined in R5RS is not case sensitive when reading symbols.
Guile, on the contrary is case sensitive by default, so the identifiers

     guile-whuzzy
     Guile-Whuzzy

   are the same in R5RS Scheme, but are different in Guile.

   It is possible to turn off case sensitivity in Guile by setting the
reader option `case-insensitive'.  For more information on reader
options, *Note Scheme Read::.

     (read-enable 'case-insensitive)

   Note that this is seldom a problem, because Scheme programmers tend
not to use uppercase letters in their identifiers anyway.

6.17.1.5 Keyword Syntax
.......................

6.17.1.6 Reader Extensions
..........................

 -- Scheme Procedure: read-hash-extend chr proc
 -- C Function: scm_read_hash_extend (chr, proc)
     Install the procedure PROC for reading expressions starting with
     the character sequence `#' and CHR.  PROC will be called with two
     arguments:  the character CHR and the port to read further data
     from. The object returned will be the return value of `read'.
     Passing `#f' for PROC will remove a previous setting.


6.17.2 Reading Scheme Code
--------------------------

 -- Scheme Procedure: read [port]
 -- C Function: scm_read (port)
     Read an s-expression from the input port PORT, or from the current
     input port if PORT is not specified.  Any whitespace before the
     next token is discarded.

   The behaviour of Guile's Scheme reader can be modified by
manipulating its read options.

 -- Scheme Procedure: read-options [setting]
     Display the current settings of the read options.  If SETTING is
     omitted, only a short form of the current read options is printed.
     Otherwise if SETTING is the symbol `help', a complete options
     description is displayed.

   The set of available options, and their default values, may be had by
invoking `read-options' at the prompt.

     scheme@(guile-user)> (read-options)
     (square-brackets keywords #f positions)
     scheme@(guile-user)> (read-options 'help)
     copy              no    Copy source code expressions.
     positions         yes   Record positions of source code expressions.
     case-insensitive  no    Convert symbols to lower case.
     keywords          #f    Style of keyword recognition: #f, 'prefix or 'postfix.
     r6rs-hex-escapes  no    Use R6RS variable-length character and string hex escapes.
     square-brackets   yes   Treat `[' and `]' as parentheses, for R6RS compatibility.
     hungry-eol-escapes no   In strings, consume leading whitespace after an
                             escaped end-of-line.

   The boolean options may be toggled with `read-enable' and
`read-disable'. The non-boolean `keywords' option must be set using
`read-set!'.

 -- Scheme Procedure: read-enable option-name
 -- Scheme Procedure: read-disable option-name
 -- Scheme Syntax: read-set! option-name value
     Modify the read options.  `read-enable' should be used with boolean
     options and switches them on, `read-disable' switches them off.

     `read-set!' can be used to set an option to a specific value.  Due
     to historical oddities, it is a macro that expects an unquoted
     option name.

   For example, to make `read' fold all symbols to their lower case
(perhaps for compatibility with older Scheme code), you can enter:

     (read-enable 'case-insensitive)

   For more information on the effect of the `r6rs-hex-escapes' and
`hungry-eol-escapes' options, see (*note String Syntax::).

6.17.3 Writing Scheme Values
----------------------------

Any scheme value may be written to a port. Not all values may be read
back in (*note Scheme Read::), however.

 -- Scheme Procedure: write obj [port]
     Send a representation of OBJ to PORT or to the current output port
     if not given.

     The output is designed to be machine readable, and can be read back
     with `read' (*note Scheme Read::).  Strings are printed in double
     quotes, with escapes if necessary, and characters are printed in
     `#\' notation.

 -- Scheme Procedure: display obj [port]
     Send a representation of OBJ to PORT or to the current output port
     if not given.

     The output is designed for human readability, it differs from
     `write' in that strings are printed without double quotes and
     escapes, and characters are printed as per `write-char', not in
     `#\' form.

   As was the case with the Scheme reader, there are a few options that
affect the behavior of the Scheme printer.

 -- Scheme Procedure: print-options [setting]
     Display the current settings of the read options.  If SETTING is
     omitted, only a short form of the current read options is printed.
     Otherwise if SETTING is the symbol `help', a complete options
     description is displayed.

   The set of available options, and their default values, may be had by
invoking `print-options' at the prompt.

     scheme@(guile-user)> (print-options)
     (quote-keywordish-symbols reader highlight-suffix "}" highlight-prefix "{")
     scheme@(guile-user)> (print-options 'help)
     highlight-prefix          {       The string to print before highlighted values.
     highlight-suffix          }       The string to print after highlighted values.
     quote-keywordish-symbols  reader  How to print symbols that have a colon
                                       as their first or last character. The
                                       value '#f' does not quote the colons;
                                       '#t' quotes them; 'reader' quotes them
                                       when the reader option 'keywords' is
                                       not '#f'.
     escape-newlines           yes     Render newlines as \n when printing
                                       using `write'.

   These options may be modified with the print-set! syntax.

 -- Scheme Syntax: print-set! option-name value
     Modify the print options.  Due to historical oddities, `print-set!'
     is a macro that expects an unquoted option name.

6.17.4 Procedures for On the Fly Evaluation
-------------------------------------------

Scheme has the lovely property that its expressions may be represented
as data.  The `eval' procedure takes a Scheme datum and evaluates it as
code.

 -- Scheme Procedure: eval exp module_or_state
 -- C Function: scm_eval (exp, module_or_state)
     Evaluate EXP, a list representing a Scheme expression, in the
     top-level environment specified by MODULE.  While EXP is evaluated
     (using `primitive-eval'), MODULE is made the current module.  The
     current module is reset to its previous value when EVAL returns.
     XXX - dynamic states.  Example: (eval '(+ 1 2)
     (interaction-environment))

 -- Scheme Procedure: interaction-environment
 -- C Function: scm_interaction_environment ()
     Return a specifier for the environment that contains
     implementation-defined bindings, typically a superset of those
     listed in the report.  The intent is that this procedure will
     return the environment in which the implementation would evaluate
     expressions dynamically typed by the user.

   *Note Environments::, for other environments.

   One does not always receive code as Scheme data, of course, and this
is especially the case for Guile's other language implementations
(*note Other Languages::).  For the case in which all you have is a
string, we have `eval-string'.  There is a legacy version of this
procedure in the default environment, but you really want the one from
`(ice-9 eval-string)', so load it up:

     (use-modules (ice-9 eval-string))

 -- Scheme Procedure: eval-string string [module=#f] [file=#f]
          [line=#f] [column=#f] [lang=(current-language)] [compile?=#f]
     Parse STRING according to the current language, normally Scheme.
     Evaluate or compile the expressions it contains, in order,
     returning the last expression.

     If the MODULE keyword argument is set, save a module excursion
     (*note Module System Reflection::) and set the current module to
     MODULE before evaluation.

     The FILE, LINE, and COLUMN keyword arguments can be used to
     indicate that the source string begins at a particular source
     location.

     Finally, LANG is a language, defaulting to the current language,
     and the expression is compiled if COMPILE? is true or there is no
     evaluator for the given language.

 -- C Function: scm_eval_string (string)
 -- C Function: scm_eval_string_in_module (string, module)
     These C bindings call `eval-string' from `(ice-9 eval-string)',
     evaluating within MODULE or the current module.

 -- C Function: SCM scm_c_eval_string (const char *string)
     `scm_eval_string', but taking a C string in locale encoding instead
     of an `SCM'.

 -- Scheme Procedure: apply proc arg1 ... argN arglst
 -- C Function: scm_apply_0 (proc, arglst)
 -- C Function: scm_apply_1 (proc, arg1, arglst)
 -- C Function: scm_apply_2 (proc, arg1, arg2, arglst)
 -- C Function: scm_apply_3 (proc, arg1, arg2, arg3, arglst)
 -- C Function: scm_apply (proc, arg, rest)
     Call PROC with arguments ARG1 ... ARGN plus the elements of the
     ARGLST list.

     `scm_apply' takes parameters corresponding to a Scheme level
     `(lambda (proc arg . rest) ...)'.  So ARG and all but the last
     element of the REST list make up ARG1...ARGN and the last element
     of REST is the ARGLST list.  Or if REST is the empty list `SCM_EOL'
     then there's no ARG1...ARGN and ARG is the ARGLST.

     ARGLST is not modified, but the REST list passed to `scm_apply' is
     modified.

 -- C Function: scm_call_0 (proc)
 -- C Function: scm_call_1 (proc, arg1)
 -- C Function: scm_call_2 (proc, arg1, arg2)
 -- C Function: scm_call_3 (proc, arg1, arg2, arg3)
 -- C Function: scm_call_4 (proc, arg1, arg2, arg3, arg4)
 -- C Function: scm_call_5 (proc, arg1, arg2, arg3, arg4, arg5)
 -- C Function: scm_call_6 (proc, arg1, arg2, arg3, arg4, arg5, arg6)
 -- C Function: scm_call_7 (proc, arg1, arg2, arg3, arg4, arg5, arg6,
          arg7)
 -- C Function: scm_call_8 (proc, arg1, arg2, arg3, arg4, arg5, arg6,
          arg7, arg8)
 -- C Function: scm_call_9 (proc, arg1, arg2, arg3, arg4, arg5, arg6,
          arg7, arg8, arg9)
     Call PROC with the given arguments.

 -- C Function: scm_call (proc, ...)
     Call PROC with any number of arguments.  The argument list must be
     terminated by `SCM_UNDEFINED'.  For example:

          scm_call (scm_c_public_ref ("guile", "+"),
                    scm_from_int (1),
                    scm_from_int (2),
                    SCM_UNDEFINED);

 -- C Function: scm_call_n (proc, argv, nargs)
     Call PROC with the array of arguments ARGV, as a `SCM*'.  The
     length of the arguments should be passed in NARGS, as a `size_t'.

 -- Scheme Procedure: apply:nconc2last lst
 -- C Function: scm_nconc2last (lst)
     LST should be a list (ARG1 ... ARGN ARGLST), with ARGLST being a
     list.  This function returns a list comprising ARG1 to ARGN plus
     the elements of ARGLST.  LST is modified to form the return.
     ARGLST is not modified, though the return does share structure
     with it.

     This operation collects up the arguments from a list which is
     `apply' style parameters.

 -- Scheme Procedure: primitive-eval exp
 -- C Function: scm_primitive_eval (exp)
     Evaluate EXP in the top-level environment specified by the current
     module.

6.17.5 Compiling Scheme Code
----------------------------

The `eval' procedure directly interprets the S-expression
representation of Scheme. An alternate strategy for evaluation is to
determine ahead of time what computations will be necessary to evaluate
the expression, and then use that recipe to produce the desired
results. This is known as "compilation".

   While it is possible to compile simple Scheme expressions such as
`(+ 2 2)' or even `"Hello world!"', compilation is most interesting in
the context of procedures. Compiling a lambda expression produces a
compiled procedure, which is just like a normal procedure except
typically much faster, because it can bypass the generic interpreter.

   Functions from system modules in a Guile installation are normally
compiled already, so they load and run quickly.

   Note that well-written Scheme programs will not typically call the
procedures in this section, for the same reason that it is often bad
taste to use `eval'.  By default, Guile automatically compiles any
files it encounters that have not been compiled yet (*note
`--auto-compile': Invoking Guile.).  The compiler can also be invoked
explicitly from the shell as `guild compile foo.scm'.

   (Why are calls to `eval' and `compile' usually in bad taste?
Because they are limited, in that they can only really make sense for
top-level expressions. Also, most needs for "compile-time" computation
are fulfilled by macros and closures. Of course one good counterexample
is the REPL itself, or any code that reads expressions from a port.)

   Automatic compilation generally works transparently, without any need
for user intervention.  However Guile does not yet do proper dependency
tracking, so that if file `A.scm' uses macros from `B.scm', and B.SCM
changes, `A.scm' would not be automatically recompiled.  To forcibly
invalidate the auto-compilation cache, pass the `--fresh-auto-compile'
option to Guile, or set the `GUILE_AUTO_COMPILE' environment variable to
`fresh' (instead of to `0' or `1').

   For more information on the compiler itself, see *note Compiling to
the Virtual Machine::. For information on the virtual machine, see
*note A Virtual Machine for Guile::.

   The command-line interface to Guile's compiler is the `guild
compile' command:

 -- Command: guild compile [`option'...] FILE...
     Compile FILE, a source file, and store bytecode in the compilation
     cache or in the file specified by the `-o' option.  The following
     options are available:

    `-L DIR'
    `--load-path=DIR'
          Add DIR to the front of the module load path.

    `-o OFILE'
    `--output=OFILE'
          Write output bytecode to OFILE.  By convention, bytecode file
          names end in `.go'.  When `-o' is omitted, the output file
          name is as for `compile-file' (see below).

    `-W WARNING'
    `--warn=WARNING'
          Emit warnings of type WARNING; use `--warn=help' for a list
          of available warnings and their description.  Currently
          recognized warnings include `unused-variable',
          `unused-toplevel', `unbound-variable', `arity-mismatch', and
          `format'.

    `-f LANG'
    `--from=LANG'
          Use LANG as the source language of FILE.  If this option is
          omitted, `scheme' is assumed.

    `-t LANG'
    `--to=LANG'
          Use LANG as the target language of FILE.  If this option is
          omitted, `objcode' is assumed.

    `-T TARGET'
    `--target=TARGET'
          Produce bytecode for TARGET instead of %HOST-TYPE (*note
          %host-type: Build Config.).  Target must be a valid GNU
          triplet, such as `armv5tel-unknown-linux-gnueabi' (*note
          Specifying Target Triplets: (autoconf)Specifying Target
          Triplets.).


     Each FILE is assumed to be UTF-8-encoded, unless it contains a
     coding declaration as recognized by `file-encoding' (*note
     Character Encoding of Source Files::).

   The compiler can also be invoked directly by Scheme code using the
procedures below:

 -- Scheme Procedure: compile exp [env=#f] [from=(current-language)]
          [to=value] [opts=()]
     Compile the expression EXP in the environment ENV. If EXP is a
     procedure, the result will be a compiled procedure; otherwise
     `compile' is mostly equivalent to `eval'.

     For a discussion of languages and compiler options, *Note
     Compiling to the Virtual Machine::.

 -- Scheme Procedure: compile-file file [output-file=#f]
          [from=(current-language)] [to='objcode]
          [env=(default-environment from)] [opts='()] [canonicalization
          'relative]
     Compile the file named FILE.

     Output will be written to a OUTPUT-FILE.   If you do not supply an
     output file name, output is written to a file in the cache
     directory, as computed by `(compiled-file-name FILE)'.

     FROM and TO specify the source and target languages.  *Note
     Compiling to the Virtual Machine::, for more information on these
     options, and on ENV and OPTS.

     As with `guild compile', FILE is assumed to be UTF-8-encoded
     unless it contains a coding declaration.

 -- Scheme Procedure: compiled-file-name file
     Compute a cached location for a compiled version of a Scheme file
     named FILE.

     This file will usually be below the `$HOME/.cache/guile/ccache'
     directory, depending on the value of the `XDG_CACHE_HOME'
     environment variable.  The intention is that `compiled-file-name'
     provides a fallback location for caching auto-compiled files.  If
     you want to place a compile file in the `%load-compiled-path', you
     should pass the OUTPUT-FILE option to `compile-file', explicitly.

 -- Scheme Variable: %auto-compilation-options
     This variable contains the options passed to the `compile-file'
     procedure when auto-compiling source files.  By default, it enables
     useful compilation warnings.  It can be customized from `~/.guile'.

6.17.6 Loading Scheme Code from File
------------------------------------

 -- Scheme Procedure: load filename [reader]
     Load FILENAME and evaluate its contents in the top-level
     environment.

     READER if provided should be either `#f', or a procedure with the
     signature `(lambda (port) ...)' which reads the next expression
     from PORT.  If READER is `#f' or absent, Guile's built-in `read'
     procedure is used (*note Scheme Read::).

     The READER argument takes effect by setting the value of the
     `current-reader' fluid (see below) before loading the file, and
     restoring its previous value when loading is complete.  The Scheme
     code inside FILENAME can itself change the current reader
     procedure on the fly by setting `current-reader' fluid.

     If the variable `%load-hook' is defined, it should be bound to a
     procedure that will be called before any code is loaded.  See
     documentation for `%load-hook' later in this section.

 -- Scheme Procedure: load-compiled filename
     Load the compiled file named FILENAME.

     Compiling a source file (*note Read/Load/Eval/Compile::) and then
     calling `load-compiled' on the resulting file is equivalent to
     calling `load' on the source file.

 -- Scheme Procedure: primitive-load filename
 -- C Function: scm_primitive_load (filename)
     Load the file named FILENAME and evaluate its contents in the
     top-level environment.  FILENAME must either be a full pathname or
     be a pathname relative to the current directory.  If the variable
     `%load-hook' is defined, it should be bound to a procedure that
     will be called before any code is loaded.  See the documentation
     for `%load-hook' later in this section.

 -- C Function: SCM scm_c_primitive_load (const char *filename)
     `scm_primitive_load', but taking a C string instead of an `SCM'.

 -- Variable: current-reader
     `current-reader' holds the read procedure that is currently being
     used by the above loading procedures to read expressions (from the
     file that they are loading).  `current-reader' is a fluid, so it
     has an independent value in each dynamic root and should be read
     and set using `fluid-ref' and `fluid-set!' (*note Fluids and
     Dynamic States::).

     Changing `current-reader' is typically useful to introduce local
     syntactic changes, such that code following the `fluid-set!' call
     is read using the newly installed reader.  The `current-reader'
     change should take place at evaluation time when the code is
     evaluated, or at compilation time when the code is compiled:

          (eval-when (compile eval)
            (fluid-set! current-reader my-own-reader))

     The `eval-when' form above ensures that the `current-reader'
     change occurs at the right time.

 -- Variable: %load-hook
     A procedure to be called `(%load-hook FILENAME)' whenever a file
     is loaded, or `#f' for no such call.  `%load-hook' is used by all
     of the loading functions (`load' and `primitive-load', and
     `load-from-path' and `primitive-load-path' documented in the next
     section).

     For example an application can set this to show what's loaded,

          (set! %load-hook (lambda (filename)
                             (format #t "Loading ~a ...\n" filename)))
          (load-from-path "foo.scm")
          -| Loading /usr/local/share/guile/site/foo.scm ...

 -- Scheme Procedure: current-load-port
 -- C Function: scm_current_load_port ()
     Return the current-load-port.  The load port is used internally by
     `primitive-load'.

6.17.7 Load Paths
-----------------

The procedure in the previous section look for Scheme code in the file
system at specific location.  Guile also has some procedures to search
the load path for code.

 -- Variable: %load-path
     List of directories which should be searched for Scheme modules and
     libraries.  `%load-path' is initialized when Guile starts up to
     `(list (%site-dir) (%library-dir) (%package-data-dir))', prepended
     with the contents of the `GUILE_LOAD_PATH' environment variable, if
     it is set.  *Note Build Config::, for more on `%site-dir' and
     related procedures.

 -- Scheme Procedure: load-from-path filename
     Similar to `load', but searches for FILENAME in the load paths.
     Preferentially loads a compiled version of the file, if it is
     available and up-to-date.

   A user can extend the load path by calling `add-to-load-path'.

 -- Scheme Syntax: add-to-load-path dir
     Add DIR to the load path.

   For example, a script might include this form to add the directory
that it is in to the load path:

     (add-to-load-path (dirname (current-filename)))

   It's better to use `add-to-load-path' than to modify `%load-path'
directly, because `add-to-load-path' takes care of modifying the path
both at compile-time and at run-time.

 -- Scheme Procedure: primitive-load-path filename
          [exception-on-not-found]
 -- C Function: scm_primitive_load_path (filename)
     Search `%load-path' for the file named FILENAME and load it into
     the top-level environment.  If FILENAME is a relative pathname and
     is not found in the list of search paths, an error is signalled.
     Preferentially loads a compiled version of the file, if it is
     available and up-to-date.

     By default or if EXCEPTION-ON-NOT-FOUND is true, an exception is
     raised if FILENAME is not found.  If EXCEPTION-ON-NOT-FOUND is
     `#f' and FILENAME is not found, no exception is raised and `#f' is
     returned.  For compatibility with Guile 1.8 and earlier, the C
     function takes only one argument, which can be either a string
     (the file name) or an argument list.

 -- Scheme Procedure: %search-load-path filename
 -- C Function: scm_sys_search_load_path (filename)
     Search `%load-path' for the file named FILENAME, which must be
     readable by the current user.  If FILENAME is found in the list of
     paths to search or is an absolute pathname, return its full
     pathname.  Otherwise, return `#f'.  Filenames may have any of the
     optional extensions in the `%load-extensions' list;
     `%search-load-path' will try each extension automatically.

 -- Variable: %load-extensions
     A list of default file extensions for files containing Scheme code.
     `%search-load-path' tries each of these extensions when looking for
     a file to load.  By default, `%load-extensions' is bound to the
     list `("" ".scm")'.

   As mentioned above, when Guile searches the `%load-path' for a
source file, it will also search the `%load-compiled-path' for a
corresponding compiled file.  If the compiled file is as new or newer
than the source file, it will be loaded instead of the source file,
using `load-compiled'.

 -- Variable: %load-compiled-path
     Like `%load-path', but for compiled files.  By default, this path
     has two entries: one for compiled files from Guile itself, and one
     for site packages.

   When `primitive-load-path' searches the `%load-compiled-path' for a
corresponding compiled file for a relative path it does so by appending
`.go' to the relative path.  For example, searching for `ice-9/popen'
could find `/usr/lib/guile/2.0/ccache/ice-9/popen.go', and use it
instead of `/usr/share/guile/2.0/ice-9/popen.scm'.

   If `primitive-load-path' does not find a corresponding `.go' file in
the `%load-compiled-path', or the `.go' file is out of date, it will
search for a corresponding auto-compiled file in the fallback path,
possibly creating one if one does not exist.

   *Note Installing Site Packages::, for more on how to correctly
install site packages.  *Note Modules and the File System::, for more
on the relationship between load paths and modules.  *Note
Compilation::, for more on the fallback path and auto-compilation.

   Finally, there are a couple of helper procedures for general path
manipulation.

 -- Scheme Procedure: parse-path path [tail]
 -- C Function: scm_parse_path (path, tail)
     Parse PATH, which is expected to be a colon-separated string, into
     a list and return the resulting list with TAIL appended. If PATH
     is `#f', TAIL is returned.

 -- Scheme Procedure: search-path path filename [extensions
          [require-exts?]]
 -- C Function: scm_search_path (path, filename, rest)
     Search PATH for a directory containing a file named FILENAME. The
     file must be readable, and not a directory.  If we find one,
     return its full filename; otherwise, return `#f'.  If FILENAME is
     absolute, return it unchanged.  If given, EXTENSIONS is a list of
     strings; for each directory in PATH, we search for FILENAME
     concatenated with each EXTENSION.  If REQUIRE-EXTS?  is true,
     require that the returned file name have one of the given
     extensions; if REQUIRE-EXTS? is not given, it defaults to `#f'.

     For compatibility with Guile 1.8 and earlier, the C function takes
     only three arguments.

6.17.8 Character Encoding of Source Files
-----------------------------------------

Scheme source code files are usually encoded in ASCII, but, the
built-in reader can interpret other character encodings.  The procedure
`primitive-load', and by extension the functions that call it, such as
`load', first scan the top 500 characters of the file for a coding
declaration.

   A coding declaration has the form `coding: XXXXXX', where `XXXXXX'
is the name of a character encoding in which the source code file has
been encoded.  The coding declaration must appear in a scheme comment.
It can either be a semicolon-initiated comment or a block `#!' comment.

   The name of the character encoding in the coding declaration is
typically lower case and containing only letters, numbers, and hyphens,
as recognized by `set-port-encoding!' (*note `set-port-encoding!':
Ports.).  Common examples of character encoding names are `utf-8' and
`iso-8859-1', as defined by IANA
(http://www.iana.org/assignments/character-sets).  Thus, the coding
declaration is mostly compatible with Emacs.

   However, there are some differences in encoding names recognized by
Emacs and encoding names defined by IANA, the latter being essentially a
subset of the former.  For instance, `latin-1' is a valid encoding name
for Emacs, but it's not according to the IANA standard, which Guile
follows; instead, you should use `iso-8859-1', which is both understood
by Emacs and dubbed by IANA (IANA writes it uppercase but Emacs wants
it lowercase and Guile is case insensitive.)

   For source code, only a subset of all possible character encodings
can be interpreted by the built-in source code reader.  Only those
character encodings in which ASCII text appears unmodified can be used.
This includes `UTF-8' and `ISO-8859-1' through `ISO-8859-15'.  The
multi-byte character encodings `UTF-16' and `UTF-32' may not be used
because they are not compatible with ASCII.

   There might be a scenario in which one would want to read non-ASCII
code from a port, such as with the function `read', instead of with
`load'.  If the port's character encoding is the same as the encoding
of the code to be read by the port, not other special handling is
necessary.  The port will automatically do the character encoding
conversion.  The functions `setlocale' or by `set-port-encoding!' are
used to set port encodings (*note Ports::).

   If a port is used to read code of unknown character encoding, it can
accomplish this in three steps.  First, the character encoding of the
port should be set to ISO-8859-1 using `set-port-encoding!'.  Then, the
procedure `file-encoding', described below, is used to scan for a
coding declaration when reading from the port.  As a side effect, it
rewinds the port after its scan is complete. After that, the port's
character encoding should be set to the encoding returned by
`file-encoding', if any, again by using `set-port-encoding!'.  Then the
code can be read as normal.

 -- Scheme Procedure: file-encoding port
 -- C Function: scm_file_encoding port
     Scan the port for an Emacs-like character coding declaration near
     the top of the contents of a port with random-accessible contents
     (*note how Emacs recognizes file encoding: (emacs)Recognize
     Coding.).  The coding declaration is of the form `coding: XXXXX'
     and must appear in a Scheme comment.  Return a string containing
     the character encoding of the file if a declaration was found, or
     `#f' otherwise.  The port is rewound.

6.17.9 Delayed Evaluation
-------------------------

Promises are a convenient way to defer a calculation until its result
is actually needed, and to run such a calculation only once.

 -- syntax: delay expr
     Return a promise object which holds the given EXPR expression,
     ready to be evaluated by a later `force'.

 -- Scheme Procedure: promise? obj
 -- C Function: scm_promise_p (obj)
     Return true if OBJ is a promise.

 -- Scheme Procedure: force p
 -- C Function: scm_force (p)
     Return the value obtained from evaluating the EXPR in the given
     promise P.  If P has previously been forced then its EXPR is not
     evaluated again, instead the value obtained at that time is simply
     returned.

     During a `force', an EXPR can call `force' again on its own
     promise, resulting in a recursive evaluation of that EXPR.  The
     first evaluation to return gives the value for the promise.
     Higher evaluations run to completion in the normal way, but their
     results are ignored, `force' always returns the first value.

6.17.10 Local Evaluation
------------------------

Guile includes a facility to capture a lexical environment, and later
evaluate a new expression within that environment.  This code is
implemented in a module.

     (use-modules (ice-9 local-eval))

 -- syntax: the-environment
     Captures and returns a lexical environment for use with
     `local-eval' or `local-compile'.

 -- Scheme Procedure: local-eval exp env
 -- C Function: scm_local_eval (exp, env)
 -- Scheme Procedure: local-compile exp env [opts=()]
     Evaluate or compile the expression EXP in the lexical environment
     ENV.

   Here is a simple example, illustrating that it is the variable that
gets captured, not just its value at one point in time.

     (define e (let ((x 100)) (the-environment)))
     (define fetch-x (local-eval '(lambda () x) e))
     (fetch-x)
     => 100
     (local-eval '(set! x 42) e)
     (fetch-x)
     => 42

   While EXP is evaluated within the lexical environment of
`(the-environment)', it has the dynamic environment of the call to
`local-eval'.

   `local-eval' and `local-compile' can only evaluate expressions, not
definitions.

     (local-eval '(define foo 42)
                 (let ((x 100)) (the-environment)))
     => syntax error: definition in expression context

   Note that the current implementation of `(the-environment)' only
captures "normal" lexical bindings, and pattern variables bound by
`syntax-case'.  It does not currently capture local syntax transformers
bound by `let-syntax', `letrec-syntax' or non-top-level `define-syntax'
forms.  Any attempt to reference such captured syntactic keywords via
`local-eval' or `local-compile' produces an error.

6.17.11 Local Inclusion
-----------------------

This section has discussed various means of linking Scheme code
together: fundamentally, loading up files at run-time using `load' and
`load-compiled'.  Guile provides another option to compose parts of
programs together at expansion-time instead of at run-time.

 -- Scheme Syntax: include file-name
     Open FILE-NAME, at expansion-time, and read the Scheme forms that
     it contains, splicing them into the location of the `include',
     within a `begin'.

   If you are a C programmer, if `load' in Scheme is like `dlopen' in
C, consider `include' to be like the C preprocessor's `#include'.  When
you use `include', it is as if the contents of the included file were
typed in instead of the `include' form.

   Because the code is included at compile-time, it is available to the
macroexpander.  Syntax definitions in the included file are available to
later code in the form in which the `include' appears, without the need
for `eval-when'.  (*Note Eval When::.)

   For the same reason, compiling a form that uses `include' results in
one compilation unit, composed of multiple files.  Loading the compiled
file is one `stat' operation for the compilation unit, instead of `2*N'
in the case of `load' (once for each loaded source file, and once each
corresponding compiled file, in the best case).

   Unlike `load', `include' also works within nested lexical contexts.
It so happens that the optimizer works best within a lexical context,
because all of the uses of bindings in a lexical context are visible,
so composing files by including them within a `(let () ...)' can
sometimes lead to important speed improvements.

   On the other hand, `include' does have all the disadvantages of
early binding: once the code with the `include' is compiled, no change
to the included file is reflected in the future behavior of the
including form.

   Also, the particular form of `include', which requires an absolute
path, or a path relative to the current directory at compile-time, is
not very amenable to compiling the source in one place, but then
installing the source to another place.  For this reason, Guile provides
another form, `include-from-path', which looks for the source file to
include within a load path.

 -- Scheme Syntax: include-from-path file-name
     Like `include', but instead of expecting `file-name' to be an
     absolute file name, it is expected to be a relative path to search
     in the `%load-path'.

   `include-from-path' is more useful when you want to install all of
the source files for a package (as you should!).  It makes it possible
to evaluate an installed file from source, instead of relying on the
`.go' file being up to date.

6.18 Memory Management and Garbage Collection
=============================================

Guile uses a _garbage collector_ to manage most of its objects.  While
the garbage collector is designed to be mostly invisible, you sometimes
need to interact with it explicitly.

   See *note Garbage Collection:: for a general discussion of how
garbage collection relates to using Guile from C.

6.18.1 Function related to Garbage Collection
---------------------------------------------

 -- Scheme Procedure: gc
 -- C Function: scm_gc ()
     Scans all of SCM objects and reclaims for further use those that
     are no longer accessible.  You normally don't need to call this
     function explicitly.  It is called automatically when appropriate.

 -- C Function: SCM scm_gc_protect_object (SCM OBJ)
     Protects OBJ from being freed by the garbage collector, when it
     otherwise might be.  When you are done with the object, call
     `scm_gc_unprotect_object' on the object. Calls to
     `scm_gc_protect'/`scm_gc_unprotect_object' can be nested, and the
     object remains protected until it has been unprotected as many
     times as it was protected. It is an error to unprotect an object
     more times than it has been protected. Returns the SCM object it
     was passed.

     Note that storing OBJ in a C global variable has the same
     effect(1).

 -- C Function: SCM scm_gc_unprotect_object (SCM OBJ)
     Unprotects an object from the garbage collector which was
     protected by `scm_gc_unprotect_object'. Returns the SCM object it
     was passed.

 -- C Function: SCM scm_permanent_object (SCM OBJ)
     Similar to `scm_gc_protect_object' in that it causes the collector
     to always mark the object, except that it should not be nested
     (only call `scm_permanent_object' on an object once), and it has
     no corresponding unpermanent function. Once an object is declared
     permanent, it will never be freed. Returns the SCM object it was
     passed.

 -- C Macro: void scm_remember_upto_here_1 (SCM obj)
 -- C Macro: void scm_remember_upto_here_2 (SCM obj1, SCM obj2)
     Create a reference to the given object or objects, so they're
     certain to be present on the stack or in a register and hence will
     not be freed by the garbage collector before this point.

     Note that these functions can only be applied to ordinary C local
     variables (ie. "automatics").  Objects held in global or static
     variables or some malloced block or the like cannot be protected
     with this mechanism.

 -- Scheme Procedure: gc-stats
 -- C Function: scm_gc_stats ()
     Return an association list of statistics about Guile's current use
     of storage.

 -- Scheme Procedure: gc-live-object-stats
 -- C Function: scm_gc_live_object_stats ()
     Return an alist of statistics of the current live objects.

 -- Function: void scm_gc_mark (SCM X)
     Mark the object X, and recurse on any objects X refers to.  If X's
     mark bit is already set, return immediately.  This function must
     only be called during the mark-phase of garbage collection,
     typically from a smob _mark_ function.

   ---------- Footnotes ----------

   (1) In Guile up to version 1.8, C global variables were not scanned
by the garbage collector; hence, `scm_gc_protect_object' was the only
way in C to prevent a Scheme object from being freed.

6.18.2 Memory Blocks
--------------------

In C programs, dynamic management of memory blocks is normally done
with the functions malloc, realloc, and free.  Guile has additional
functions for dynamic memory allocation that are integrated into the
garbage collector and the error reporting system.

   Memory blocks that are associated with Scheme objects (for example a
smob) should be allocated with `scm_gc_malloc' or
`scm_gc_malloc_pointerless'.  These two functions will either return a
valid pointer or signal an error.  Memory blocks allocated this way can
be freed with `scm_gc_free'; however, this is not strictly needed:
memory allocated with `scm_gc_malloc' or `scm_gc_malloc_pointerless' is
automatically reclaimed when the garbage collector no longer sees any
live reference to it(1).

   Memory allocated with `scm_gc_malloc' is scanned for live pointers.
This means that if `scm_gc_malloc'-allocated memory contains a pointer
to some other part of the memory, the garbage collector notices it and
prevents it from being reclaimed(2).  Conversely, memory allocated with
`scm_gc_malloc_pointerless' is assumed to be "pointer-less" and is not
scanned.

   For memory that is not associated with a Scheme object, you can use
`scm_malloc' instead of `malloc'.  Like `scm_gc_malloc', it will either
return a valid pointer or signal an error.  However, it will not assume
that the new memory block can be freed by a garbage collection.  The
memory must be explicitly freed with `free'.

   There is also `scm_gc_realloc' and `scm_realloc', to be used in
place of `realloc' when appropriate, and `scm_gc_calloc' and
`scm_calloc', to be used in place of `calloc' when appropriate.

   The function `scm_dynwind_free' can be useful when memory should be
freed with libc's `free' when leaving a dynwind context, *Note Dynamic
Wind::.

 -- C Function: void * scm_malloc (size_t SIZE)
 -- C Function: void * scm_calloc (size_t SIZE)
     Allocate SIZE bytes of memory and return a pointer to it.  When
     SIZE is 0, return `NULL'.  When not enough memory is available,
     signal an error.  This function runs the GC to free up some memory
     when it deems it appropriate.

     The memory is allocated by the libc `malloc' function and can be
     freed with `free'.  There is no `scm_free' function to go with
     `scm_malloc' to make it easier to pass memory back and forth
     between different modules.

     The function `scm_calloc' is similar to `scm_malloc', but
     initializes the block of memory to zero as well.

     These functions will (indirectly) call
     `scm_gc_register_allocation'.

 -- C Function: void * scm_realloc (void *MEM, size_t NEW_SIZE)
     Change the size of the memory block at MEM to NEW_SIZE and return
     its new location.  When NEW_SIZE is 0, this is the same as calling
     `free' on MEM and `NULL' is returned.  When MEM is `NULL', this
     function behaves like `scm_malloc' and allocates a new block of
     size NEW_SIZE.

     When not enough memory is available, signal an error.  This
     function runs the GC to free up some memory when it deems it
     appropriate.

     This function will call `scm_gc_register_allocation'.

 -- C Function: void * scm_gc_malloc (size_t SIZE, const char *WHAT)
 -- C Function: void * scm_gc_malloc_pointerless (size_t SIZE, const
          char *WHAT)
 -- C Function: void * scm_gc_realloc (void *MEM, size_t OLD_SIZE,
          size_t NEW_SIZE, const char *WHAT);
 -- C Function: void * scm_gc_calloc (size_t SIZE, const char *WHAT)
     Allocate SIZE bytes of automatically-managed memory.  The memory
     is automatically freed when no longer referenced from any live
     memory block.

     Memory allocated with `scm_gc_malloc' or `scm_gc_calloc' is
     scanned for pointers.  Memory allocated by
     `scm_gc_malloc_pointerless' is not scanned.

     The `scm_gc_realloc' call preserves the "pointerlessness" of the
     memory area pointed to by MEM.  Note that you need to pass the old
     size of a reallocated memory block as well.  See below for a
     motivation.

 -- C Function: void scm_gc_free (void *MEM, size_t SIZE, const char
          *WHAT)
     Explicitly free the memory block pointed to by MEM, which was
     previously allocated by one of the above `scm_gc' functions.

     Note that you need to explicitly pass the SIZE parameter.  This is
     done since it should normally be easy to provide this parameter
     (for memory that is associated with GC controlled objects) and
     help keep the memory management overhead very low.  However, in
     Guile 2.x, SIZE is always ignored.

 -- C Function: void scm_gc_register_allocation (size_t SIZE)
     Informs the garbage collector that SIZE bytes have been allocated,
     which the collector would otherwise not have known about.

     In general, Scheme will decide to collect garbage only after some
     amount of memory has been allocated.  Calling this function will
     make the Scheme garbage collector know about more allocation, and
     thus run more often (as appropriate).

     It is especially important to call this function when large
     unmanaged allocations, like images, may be freed by small Scheme
     allocations, like SMOBs.

 -- C Function: void scm_dynwind_free (void *mem)
     Equivalent to `scm_dynwind_unwind_handler (free, MEM,
     SCM_F_WIND_EXPLICITLY)'.  That is, the memory block at MEM will be
     freed (using `free' from the C library) when the current dynwind is
     left.

 -- Scheme Procedure: malloc-stats
     Return an alist ((WHAT . N) ...) describing number of malloced
     objects.  WHAT is the second argument to `scm_gc_malloc', N is the
     number of objects of that type currently allocated.

     This function is only available if the `GUILE_DEBUG_MALLOC'
     preprocessor macro was defined when Guile was compiled.

6.18.2.1 Upgrading from scm_must_malloc et al.
..............................................

Version 1.6 of Guile and earlier did not have the functions from the
previous section.  In their place, it had the functions
`scm_must_malloc', `scm_must_realloc' and `scm_must_free'.  This
section explains why we want you to stop using them, and how to do this.

   The functions `scm_must_malloc' and `scm_must_realloc' behaved like
`scm_gc_malloc' and `scm_gc_realloc' do now, respectively.  They would
inform the GC about the newly allocated memory via the internal
equivalent of `scm_gc_register_allocation'.  However, `scm_must_free'
did not unregister the memory it was about to free.  The usual way to
unregister memory was to return its size from a smob free function.

   This disconnectedness of the actual freeing of memory and reporting
this to the GC proved to be bad in practice.  It was easy to make
mistakes and report the wrong size because allocating and freeing was
not done with symmetric code, and because it is cumbersome to compute
the total size of nested data structures that were freed with multiple
calls to `scm_must_free'.  Additionally, there was no equivalent to
`scm_malloc', and it was tempting to just use `scm_must_malloc' and
never to tell the GC that the memory has been freed.

   The effect was that the internal statistics kept by the GC drifted
out of sync with reality and could even overflow in long running
programs.  When this happened, the result was a dramatic increase in
(senseless) GC activity which would effectively stop the program dead.

   The functions `scm_done_malloc' and `scm_done_free' were introduced
to help restore balance to the force, but existing bugs did not
magically disappear, of course.

   Therefore we decided to force everybody to review their code by
deprecating the existing functions and introducing new ones in their
place that are hopefully easier to use correctly.

   For every use of `scm_must_malloc' you need to decide whether to use
`scm_malloc' or `scm_gc_malloc' in its place.  When the memory block is
not part of a smob or some other Scheme object whose lifetime is
ultimately managed by the garbage collector, use `scm_malloc' and
`free'.  When it is part of a smob, use `scm_gc_malloc' and change the
smob free function to use `scm_gc_free' instead of `scm_must_free' or
`free' and make it return zero.

   The important thing is to always pair `scm_malloc' with `free'; and
to always pair `scm_gc_malloc' with `scm_gc_free'.

   The same reasoning applies to `scm_must_realloc' and `scm_realloc'
versus `scm_gc_realloc'.

   ---------- Footnotes ----------

   (1) In Guile up to version 1.8, memory allocated with `scm_gc_malloc'
_had_ to be freed with `scm_gc_free'.

   (2) In Guile up to 1.8, memory allocated with `scm_gc_malloc' was
_not_ scanned.  Consequently, the GC had to be told explicitly about
pointers to live objects contained in the memory block, e.g., via SMOB
mark functions (*note `scm_set_smob_mark': Smobs.)

6.18.3 Weak References
----------------------

[FIXME: This chapter is based on Mikael Djurfeldt's answer to a
question by Michael Livshin. Any mistakes are not theirs, of course. ]

   Weak references let you attach bookkeeping information to data so
that the additional information automatically disappears when the
original data is no longer in use and gets garbage collected. In a weak
key hash, the hash entry for that key disappears as soon as the key is
no longer referenced from anywhere else. For weak value hashes, the
same happens as soon as the value is no longer in use. Entries in a
doubly weak hash disappear when either the key or the value are not
used anywhere else anymore.

   Object properties offer the same kind of functionality as weak key
hashes in many situations. (*note Object Properties::)

   Here's an example (a little bit strained perhaps, but one of the
examples is actually used in Guile):

   Assume that you're implementing a debugging system where you want to
associate information about filename and position of source code
expressions with the expressions themselves.

   Hashtables can be used for that, but if you use ordinary hash tables
it will be impossible for the scheme interpreter to "forget" old source
when, for example, a file is reloaded.

   To implement the mapping from source code expressions to positional
information it is necessary to use weak-key tables since we don't want
the expressions to be remembered just because they are in our table.

   To implement a mapping from source file line numbers to source code
expressions you would use a weak-value table.

   To implement a mapping from source code expressions to the procedures
they constitute a doubly-weak table has to be used.

6.18.3.1 Weak hash tables
.........................

 -- Scheme Procedure: make-weak-key-hash-table size
 -- Scheme Procedure: make-weak-value-hash-table size
 -- Scheme Procedure: make-doubly-weak-hash-table size
 -- C Function: scm_make_weak_key_hash_table (size)
 -- C Function: scm_make_weak_value_hash_table (size)
 -- C Function: scm_make_doubly_weak_hash_table (size)
     Return a weak hash table with SIZE buckets. As with any hash
     table, choosing a good size for the table requires some caution.

     You can modify weak hash tables in exactly the same way you would
     modify regular hash tables. (*note Hash Tables::)

 -- Scheme Procedure: weak-key-hash-table? obj
 -- Scheme Procedure: weak-value-hash-table? obj
 -- Scheme Procedure: doubly-weak-hash-table? obj
 -- C Function: scm_weak_key_hash_table_p (obj)
 -- C Function: scm_weak_value_hash_table_p (obj)
 -- C Function: scm_doubly_weak_hash_table_p (obj)
     Return `#t' if OBJ is the specified weak hash table. Note that a
     doubly weak hash table is neither a weak key nor a weak value hash
     table.

6.18.3.2 Weak vectors
.....................

Weak vectors are mainly useful in Guile's implementation of weak hash
tables.

 -- Scheme Procedure: make-weak-vector size [fill]
 -- C Function: scm_make_weak_vector (size, fill)
     Return a weak vector with SIZE elements. If the optional argument
     FILL is given, all entries in the vector will be set to FILL. The
     default value for FILL is the empty list.

 -- Scheme Procedure: weak-vector . l
 -- Scheme Procedure: list->weak-vector l
 -- C Function: scm_weak_vector (l)
     Construct a weak vector from a list: `weak-vector' uses the list
     of its arguments while `list->weak-vector' uses its only argument
     L (a list) to construct a weak vector the same way `list->vector'
     would.

 -- Scheme Procedure: weak-vector? obj
 -- C Function: scm_weak_vector_p (obj)
     Return `#t' if OBJ is a weak vector. Note that all weak hashes are
     also weak vectors.

6.18.4 Guardians
----------------

Guardians provide a way to be notified about objects that would
otherwise be collected as garbage.  Guarding them prevents the objects
from being collected and cleanup actions can be performed on them, for
example.

   See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians
in a Generation-Based Garbage Collector".  ACM SIGPLAN Conference on
Programming Language Design and Implementation, June 1993.

 -- Scheme Procedure: make-guardian
 -- C Function: scm_make_guardian ()
     Create a new guardian.  A guardian protects a set of objects from
     garbage collection, allowing a program to apply cleanup or other
     actions.

     `make-guardian' returns a procedure representing the guardian.
     Calling the guardian procedure with an argument adds the argument
     to the guardian's set of protected objects.  Calling the guardian
     procedure without an argument returns one of the protected objects
     which are ready for garbage collection, or `#f' if no such object
     is available.  Objects which are returned in this way are removed
     from the guardian.

     You can put a single object into a guardian more than once and you
     can put a single object into more than one guardian.  The object
     will then be returned multiple times by the guardian procedures.

     An object is eligible to be returned from a guardian when it is no
     longer referenced from outside any guardian.

     There is no guarantee about the order in which objects are returned
     from a guardian.  If you want to impose an order on finalization
     actions, for example, you can do that by keeping objects alive in
     some global data structure until they are no longer needed for
     finalizing other objects.

     Being an element in a weak vector, a key in a hash table with weak
     keys, or a value in a hash table with weak values does not prevent
     an object from being returned by a guardian.  But as long as an
     object can be returned from a guardian it will not be removed from
     such a weak vector or hash table.  In other words, a weak link
     does not prevent an object from being considered collectable, but
     being inside a guardian prevents a weak link from being broken.

     A key in a weak key hash table can be thought of as having a strong
     reference to its associated value as long as the key is accessible.
     Consequently, when the key is only accessible from within a
     guardian, the reference from the key to the value is also
     considered to be coming from within a guardian.  Thus, if there is
     no other reference to the value, it is eligible to be returned
     from a guardian.

6.19 Modules
============

When programs become large, naming conflicts can occur when a function
or global variable defined in one file has the same name as a function
or global variable in another file.  Even just a _similarity_ between
function names can cause hard-to-find bugs, since a programmer might
type the wrong function name.

   The approach used to tackle this problem is called _information
encapsulation_, which consists of packaging functional units into a
given name space that is clearly separated from other name spaces.  

   The language features that allow this are usually called _the module
system_ because programs are broken up into modules that are compiled
separately (or loaded separately in an interpreter).

   Older languages, like C, have limited support for name space
manipulation and protection.  In C a variable or function is public by
default, and can be made local to a module with the `static' keyword.
But you cannot reference public variables and functions from another
module with different names.

   More advanced module systems have become a common feature in recently
designed languages: ML, Python, Perl, and Modula 3 all allow the
_renaming_ of objects from a foreign module, so they will not clutter
the global name space.  

   In addition, Guile offers variables as first-class objects.  They can
be used for interacting with the module system.

6.19.1 General Information about Modules
----------------------------------------

A Guile module can be thought of as a collection of named procedures,
variables and macros.  More precisely, it is a set of "bindings" of
symbols (names) to Scheme objects.

   Within a module, all bindings are visible.  Certain bindings can be
declared "public", in which case they are added to the module's
so-called "export list"; this set of public bindings is called the
module's "public interface" (*note Creating Guile Modules::).

   A client module "uses" a providing module's bindings by either
accessing the providing module's public interface, or by building a
custom interface (and then accessing that).  In a custom interface, the
client module can "select" which bindings to access and can also
algorithmically "rename" bindings.  In contrast, when using the
providing module's public interface, the entire export list is available
without renaming (*note Using Guile Modules::).

   All Guile modules have a unique "module name", for example `(ice-9
popen)' or `(srfi srfi-11)'.  Module names are lists of one or more
symbols.

   When Guile goes to use an interface from a module, for example
`(ice-9 popen)', Guile first looks to see if it has loaded `(ice-9
popen)' for any reason.  If the module has not been loaded yet, Guile
searches a "load path" for a file that might define it, and loads that
file.

   The following subsections go into more detail on using, creating,
installing, and otherwise manipulating modules and the module system.

6.19.2 Using Guile Modules
--------------------------

To use a Guile module is to access either its public interface or a
custom interface (*note General Information about Modules::).  Both
types of access are handled by the syntactic form `use-modules', which
accepts one or more interface specifications and, upon evaluation,
arranges for those interfaces to be available to the current module.
This process may include locating and loading code for a given module if
that code has not yet been loaded, following `%load-path' (*note
Modules and the File System::).

   An "interface specification" has one of two forms.  The first
variation is simply to name the module, in which case its public
interface is the one accessed.  For example:

     (use-modules (ice-9 popen))

   Here, the interface specification is `(ice-9 popen)', and the result
is that the current module now has access to `open-pipe', `close-pipe',
`open-input-pipe', and so on (*note Included Guile Modules::).

   Note in the previous example that if the current module had already
defined `open-pipe', that definition would be overwritten by the
definition in `(ice-9 popen)'.  For this reason (and others), there is
a second variation of interface specification that not only names a
module to be accessed, but also selects bindings from it and renames
them to suit the current module's needs.  For example:

     (use-modules ((ice-9 popen)
                   #:select ((open-pipe . pipe-open) close-pipe)
                   #:renamer (symbol-prefix-proc 'unixy:)))

   Here, the interface specification is more complex than before, and
the result is that a custom interface with only two bindings is created
and subsequently accessed by the current module.  The mapping of old to
new names is as follows:

     (ice-9 popen) sees:             current module sees:
     open-pipe                       unixy:pipe-open
     close-pipe                      unixy:close-pipe

   This example also shows how to use the convenience procedure
`symbol-prefix-proc'.

   You can also directly refer to bindings in a module by using the `@'
syntax.  For example, instead of using the `use-modules' statement from
above and writing `unixy:pipe-open' to refer to the `pipe-open' from the
`(ice-9 popen)', you could also write `(@ (ice-9 popen) open-pipe)'.
Thus an alternative to the complete `use-modules' statement would be

     (define unixy:pipe-open (@ (ice-9 popen) open-pipe))
     (define unixy:close-pipe (@ (ice-9 popen) close-pipe))

   There is also `@@', which can be used like `@', but does not check
whether the variable that is being accessed is actually exported.
Thus, `@@' can be thought of as the impolite version of `@' and should
only be used as a last resort or for debugging, for example.

   Note that just as with a `use-modules' statement, any module that
has not yet been loaded yet will be loaded when referenced by a `@' or
`@@' form.

   You can also use the `@' and `@@' syntaxes as the target of a `set!'
when the binding refers to a variable.

 -- Scheme Procedure: symbol-prefix-proc prefix-sym
     Return a procedure that prefixes its arg (a symbol) with
     PREFIX-SYM.

 -- syntax: use-modules spec ...
     Resolve each interface specification SPEC into an interface and
     arrange for these to be accessible by the current module.  The
     return value is unspecified.

     SPEC can be a list of symbols, in which case it names a module
     whose public interface is found and used.

     SPEC can also be of the form:

           (MODULE-NAME [#:select SELECTION] [#:renamer RENAMER])

     in which case a custom interface is newly created and used.
     MODULE-NAME is a list of symbols, as above; SELECTION is a list of
     selection-specs; and RENAMER is a procedure that takes a symbol
     and returns its new name.  A selection-spec is either a symbol or
     a pair of symbols `(ORIG . SEEN)', where ORIG is the name in the
     used module and SEEN is the name in the using module.  Note that
     SEEN is also passed through RENAMER.

     The `#:select' and `#:renamer' clauses are optional.  If both are
     omitted, the returned interface has no bindings.  If the `#:select'
     clause is omitted, RENAMER operates on the used module's public
     interface.

     In addition to the above, SPEC can also include a `#:version'
     clause, of the form:

           #:version VERSION-SPEC

     where VERSION-SPEC is an R6RS-compatible version reference.  An
     error will be signaled in the case in which a module with the same
     name has already been loaded, if that module specifies a version
     and that version is not compatible with VERSION-SPEC.  *Note R6RS
     Version References::, for more on version references.

     If the module name is not resolvable, `use-modules' will signal an
     error.

 -- syntax: @ module-name binding-name
     Refer to the binding named BINDING-NAME in module MODULE-NAME.
     The binding must have been exported by the module.

 -- syntax: @@ module-name binding-name
     Refer to the binding named BINDING-NAME in module MODULE-NAME.
     The binding must not have been exported by the module.  This
     syntax is only intended for debugging purposes or as a last resort.

6.19.3 Creating Guile Modules
-----------------------------

When you want to create your own modules, you have to take the following
steps:

   * Create a Scheme source file and add all variables and procedures
     you wish to export, or which are required by the exported
     procedures.

   * Add a `define-module' form at the beginning.

   * Export all bindings which should be in the public interface, either
     by using `define-public' or `export' (both documented below).

 -- syntax: define-module module-name [options ...]
     MODULE-NAME is a list of one or more symbols.

          (define-module (ice-9 popen))

     `define-module' makes this module available to Guile programs under
     the given MODULE-NAME.

     The OPTIONS are keyword/value pairs which specify more about the
     defined module.  The recognized options and their meaning is shown
     in the following table.

    `#:use-module INTERFACE-SPECIFICATION'
          Equivalent to a `(use-modules INTERFACE-SPECIFICATION)'
          (*note Using Guile Modules::).

    `#:autoload MODULE SYMBOL-LIST'
          Load MODULE when any of SYMBOL-LIST are accessed.  For
          example,

               (define-module (my mod)
                 #:autoload (srfi srfi-1) (partition delete-duplicates))
               ...
               (if something
                   (set! foo (delete-duplicates ...)))

          When a module is autoloaded, all its bindings become
          available.  SYMBOL-LIST is just those that will first trigger
          the load.

          An autoload is a good way to put off loading a big module
          until it's really needed, for instance for faster startup or
          if it will only be needed in certain circumstances.

          `@' can do a similar thing (*note Using Guile Modules::), but
          in that case an `@' form must be written every time a binding
          from the module is used.

    `#:export LIST'
          Export all identifiers in LIST which must be a list of symbols
          or pairs of symbols.  This is equivalent to `(export LIST)'
          in the module body.

    `#:re-export LIST'
          Re-export all identifiers in LIST which must be a list of
          symbols or pairs of symbols.  The symbols in LIST must be
          imported by the current module from other modules.  This is
          equivalent to `re-export' below.

    `#:replace LIST'
          Export all identifiers in LIST (a list of symbols or pairs of
          symbols) and mark them as "replacing bindings".  In the module
          user's name space, this will have the effect of replacing any
          binding with the same name that is not also "replacing".
          Normally a replacement results in an "override" warning
          message, `#:replace' avoids that.

          In general, a module that exports a binding for which the
          `(guile)' module already has a definition should use
          `#:replace' instead of `#:export'.  `#:replace', in a sense,
          lets Guile know that the module _purposefully_ replaces a
          core binding.  It is important to note, however, that this
          binding replacement is confined to the name space of the
          module user.  In other words, the value of the core binding
          in question remains unchanged for other modules.

          Note that although it is often a good idea for the replaced
          binding to remain compatible with a binding in `(guile)', to
          avoid surprising the user, sometimes the bindings will be
          incompatible.  For example, SRFI-19 exports its own version
          of `current-time' (*note SRFI-19 Time::) which is not
          compatible with the core `current-time' function (*note
          Time::).  Guile assumes that a user importing a module knows
          what she is doing, and uses `#:replace' for this binding
          rather than `#:export'.

          A `#:replace' clause is equivalent to `(export! LIST)' in the
          module body.

          The `#:duplicates' (see below) provides fine-grain control
          about duplicate binding handling on the module-user side.

    `#:version LIST'
          Specify a version for the module in the form of LIST, a list
          of zero or more exact, nonnegative integers.  The
          corresponding `#:version' option in the `use-modules' form
          allows callers to restrict the value of this option in
          various ways.

    `#:duplicates LIST'
          Tell Guile to handle duplicate bindings for the bindings
          imported by the current module according to the policy
          defined by LIST, a list of symbols.  LIST must contain
          symbols representing a duplicate binding handling policy
          chosen among the following:

         `check'
               Raises an error when a binding is imported from more
               than one place.

         `warn'
               Issue a warning when a binding is imported from more
               than one place and leave the responsibility of actually
               handling the duplication to the next duplicate binding
               handler.

         `replace'
               When a new binding is imported that has the same name as
               a previously imported binding, then do the following:

                 1. If the old binding was said to be "replacing" (via
                    the `#:replace' option above) and the new binding
                    is not replacing, the keep the old binding.

                 2. If the old binding was not said to be replacing and
                    the new binding is replacing, then replace the old
                    binding with the new one.

                 3. If neither the old nor the new binding is
                    replacing, then keep the old one.

         `warn-override-core'
               Issue a warning when a core binding is being overwritten
               and actually override the core binding with the new one.

         `first'
               In case of duplicate bindings, the firstly imported
               binding is always the one which is kept.

         `last'
               In case of duplicate bindings, the lastly imported
               binding is always the one which is kept.

         `noop'
               In case of duplicate bindings, leave the responsibility
               to the next duplicate handler.

          If LIST contains more than one symbol, then the duplicate
          binding handlers which appear first will be used first when
          resolving a duplicate binding situation.  As mentioned above,
          some resolution policies may explicitly leave the
          responsibility of handling the duplication to the next
          handler in LIST.

          If GOOPS has been loaded before the `#:duplicates' clause is
          processed, there are additional strategies available for
          dealing with generic functions.  *Note Merging Generics::,
          for more information.

          The default duplicate binding resolution policy is given by
          the `default-duplicate-binding-handler' procedure, and is

               (replace warn-override-core warn last)

    `#:pure'
          Create a "pure" module, that is a module which does not
          contain any of the standard procedure bindings except for the
          syntax forms.  This is useful if you want to create "safe"
          modules, that is modules which do not know anything about
          dangerous procedures.


 -- syntax: export variable ...
     Add all VARIABLEs (which must be symbols or pairs of symbols) to
     the list of exported bindings of the current module.  If VARIABLE
     is a pair, its `car' gives the name of the variable as seen by the
     current module and its `cdr' specifies a name for the binding in
     the current module's public interface.

 -- syntax: define-public ...
     Equivalent to `(begin (define foo ...) (export foo))'.

 -- syntax: re-export variable ...
     Add all VARIABLEs (which must be symbols or pairs of symbols) to
     the list of re-exported bindings of the current module.  Pairs of
     symbols are handled as in `export'.  Re-exported bindings must be
     imported by the current module from some other module.

 -- syntax: export! variable ...
     Like `export', but marking the exported variables as replacing.
     Using a module with replacing bindings will cause any existing
     bindings to be replaced without issuing any warnings.  See the
     discussion of `#:replace' above.

6.19.4 Modules and the File System
----------------------------------

Typical programs only use a small subset of modules installed on a Guile
system.  In order to keep startup time down, Guile only loads modules
when a program uses them, on demand.

   When a program evaluates `(use-modules (ice-9 popen))', and the
module is not loaded, Guile searches for a conventionally-named file
from in the "load path".

   In this case, loading `(ice-9 popen)' will eventually cause Guile to
run `(primitive-load-path "ice-9/popen")'.  `primitive-load-path' will
search for a file `ice-9/popen' in the `%load-path' (*note Load
Paths::).  For each directory in `%load-path', Guile will try to find
the file name, concatenated with the extensions from
`%load-extensions'.  By default, this will cause Guile to `stat'
`ice-9/popen.scm', and then `ice-9/popen'.  *Note Load Paths::, for
more on `primitive-load-path'.

   If a corresponding compiled `.go' file is found in the
`%load-compiled-path' or in the fallback path, and is as fresh as the
source file, it will be loaded instead of the source file.  If no
compiled file is found, Guile may try to compile the source file and
cache away the resulting `.go' file.  *Note Compilation::, for more on
compilation.

   Once Guile finds a suitable source or compiled file is found, the
file will be loaded.  If, after loading the file, the module under
consideration is still not defined, Guile will signal an error.

   For more information on where and how to install Scheme modules,
*Note Installing Site Packages::.

6.19.5 R6RS Version References
------------------------------

Guile's module system includes support for locating modules based on a
declared version specifier of the same form as the one described in
R6RS (*note R6RS Library Form: (r6rs)Library form.).  By using the
`#:version' keyword in a `define-module' form, a module may specify a
version as a list of zero or more exact, nonnegative integers.

   This version can then be used to locate the module during the module
search process.  Client modules and callers of the `use-modules'
function may specify constraints on the versions of target modules by
providing a "version reference", which has one of the following forms:

      (SUB-VERSION-REFERENCE ...)
      (and VERSION-REFERENCE ...)
      (or VERSION-REFERENCE ...)
      (not VERSION-REFERENCE)

   in which SUB-VERSION-REFERENCE is in turn one of:

      (SUB-VERSION)
      (>= SUB-VERSION)
      (<= SUB-VERSION)
      (and SUB-VERSION-REFERENCE ...)
      (or SUB-VERSION-REFERENCE ...)
      (not SUB-VERSION-REFERENCE)

   in which SUB-VERSION is an exact, nonnegative integer as above. A
version reference matches a declared module version if each element of
the version reference matches a corresponding element of the module
version, according to the following rules:

   * The `and' sub-form matches a version or version element if every
     element in the tail of the sub-form matches the specified version
     or version element.

   * The `or' sub-form matches a version or version element if any
     element in the tail of the sub-form matches the specified version
     or version element.

   * The `not' sub-form matches a version or version element if the tail
     of the sub-form does not match the version or version element.

   * The `>=' sub-form matches a version element if the element is
     greater than or equal to the SUB-VERSION in the tail of the
     sub-form.

   * The `<=' sub-form matches a version element if the version is less
     than or equal to the SUB-VERSION in the tail of the sub-form.

   * A SUB-VERSION matches a version element if one is EQV? to the
     other.

   For example, a module declared as:

      (define-module (mylib mymodule) #:version (1 2 0))

   would be successfully loaded by any of the following `use-modules'
expressions:

      (use-modules ((mylib mymodule) #:version (1 2 (>= 0))))
      (use-modules ((mylib mymodule) #:version (or (1 2 0) (1 2 1))))
      (use-modules ((mylib mymodule) #:version ((and (>= 1) (not 2)) 2 0)))

6.19.6 R6RS Libraries
---------------------

In addition to the API described in the previous sections, you also
have the option to create modules using the portable `library' form
described in R6RS (*note R6RS Library Form: (r6rs)Library form.), and
to import libraries created in this format by other programmers.
Guile's R6RS library implementation takes advantage of the flexibility
built into the module system by expanding the R6RS library form into a
corresponding Guile `define-module' form that specifies equivalent
import and export requirements and includes the same body expressions.
The library expression:

       (library (mylib (1 2))
         (import (otherlib (3)))
         (export mybinding))

   is equivalent to the module definition:

       (define-module (mylib)
         #:version (1 2)
         #:use-module ((otherlib) #:version (3))
         #:export (mybinding))

   Central to the mechanics of R6RS libraries is the concept of import
and export "levels", which control the visibility of bindings at
various phases of a library's lifecycle -- macros necessary to expand
forms in the library's body need to be available at expand time;
variables used in the body of a procedure exported by the library must
be available at runtime.  R6RS specifies the optional `for' sub-form of
an _import set_ specification (see below) as a mechanism by which a
library author can indicate that a particular library import should
take place at a particular phase with respect to the lifecycle of the
importing library.

   Guile's library implementation uses a technique called "implicit
phasing" (first described by Abdulaziz Ghuloum and R.  Kent Dybvig),
which allows the expander and compiler to automatically determine the
necessary visibility of a binding imported from another library.  As
such, the `for' sub-form described below is ignored by Guile (but may
be required by Schemes in which phasing is explicit).

 -- Scheme Syntax: library name (export export-spec ...) (import
          import-spec ...) body ...
     Defines a new library with the specified name, exports, and
     imports, and evaluates the specified body expressions in this
     library's environment.

     The library NAME is a non-empty list of identifiers, optionally
     ending with a version specification of the form described above
     (*note Creating Guile Modules::).

     Each EXPORT-SPEC is the name of a variable defined or imported by
     the library, or must take the form `(rename (internal-name
     external-name) ...)', where the identifier INTERNAL-NAME names a
     variable defined or imported by the library and EXTERNAL-NAME is
     the name by which the variable is seen by importing libraries.

     Each IMPORT-SPEC must be either an "import set" (see below) or
     must be of the form `(for import-set import-level ...)', where
     each IMPORT-LEVEL is one of:

            run
            expand
            (meta LEVEL)

     where LEVEL is an integer.  Note that since Guile does not require
     explicit phase specification, any IMPORT-SETs found inside of
     `for' sub-forms will be "unwrapped" during expansion and processed
     as if they had been specified directly.

     Import sets in turn take one of the following forms:

            LIBRARY-REFERENCE
            (library LIBRARY-REFERENCE)
            (only IMPORT-SET IDENTIFIER ...)
            (except IMPORT-SET IDENTIFIER ...)
            (prefix IMPORT-SET IDENTIFIER)
            (rename IMPORT-SET (INTERNAL-IDENTIFIER EXTERNAL-IDENTIFIER) ...)

     where LIBRARY-REFERENCE is a non-empty list of identifiers ending
     with an optional version reference (*note R6RS Version
     References::), and the other sub-forms have the following
     semantics, defined recursively on nested IMPORT-SETs:

        * The `library' sub-form is used to specify libraries for import
          whose names begin with the identifier "library."

        * The `only' sub-form imports only the specified IDENTIFIERs
          from the given IMPORT-SET.

        * The `except' sub-form imports all of the bindings exported by
          IMPORT-SET except for those that appear in the specified list
          of IDENTIFIERs.

        * The `prefix' sub-form imports all of the bindings exported by
          IMPORT-SET, first prefixing them with the specified
          IDENTIFIER.

        * The `rename' sub-form imports all of the identifiers exported
          by IMPORT-SET.  The binding for each INTERNAL-IDENTIFIER
          among these identifiers is made visible to the importing
          library as the corresponding EXTERNAL-IDENTIFIER; all other
          bindings are imported using the names provided by IMPORT-SET.


     Note that because Guile translates R6RS libraries into module
     definitions, an import specification may be used to declare a
     dependency on a native Guile module -- although doing so may make
     your libraries less portable to other Schemes.


 -- Scheme Syntax: import import-spec ...
     Import into the current environment the libraries specified by the
     given import specifications, where each IMPORT-SPEC takes the same
     form as in the `library' form described above.

6.19.7 Variables
----------------

Each module has its own hash table, sometimes known as an "obarray",
that maps the names defined in that module to their corresponding
variable objects.

   A variable is a box-like object that can hold any Scheme value.  It
is said to be "undefined" if its box holds a special Scheme value that
denotes undefined-ness (which is different from all other Scheme values,
including for example `#f'); otherwise the variable is "defined".

   On its own, a variable object is anonymous.  A variable is said to be
"bound" when it is associated with a name in some way, usually a symbol
in a module obarray.  When this happens, the name is said to be bound
to the variable, in that module.

   (That's the theory, anyway.  In practice, defined-ness and bound-ness
sometimes get confused, because Lisp and Scheme implementations have
often conflated -- or deliberately drawn no distinction between -- a
name that is unbound and a name that is bound to a variable whose value
is undefined.  We will try to be clear about the difference and explain
any confusion where it is unavoidable.)

   Variables do not have a read syntax.  Most commonly they are created
and bound implicitly by `define' expressions: a top-level `define'
expression of the form

     (define NAME VALUE)

creates a variable with initial value VALUE and binds it to the name
NAME in the current module.  But they can also be created dynamically
by calling one of the constructor procedures `make-variable' and
`make-undefined-variable'.

 -- Scheme Procedure: make-undefined-variable
 -- C Function: scm_make_undefined_variable ()
     Return a variable that is initially unbound.

 -- Scheme Procedure: make-variable init
 -- C Function: scm_make_variable (init)
     Return a variable initialized to value INIT.

 -- Scheme Procedure: variable-bound? var
 -- C Function: scm_variable_bound_p (var)
     Return `#t' iff VAR is bound to a value.  Throws an error if VAR
     is not a variable object.

 -- Scheme Procedure: variable-ref var
 -- C Function: scm_variable_ref (var)
     Dereference VAR and return its value.  VAR must be a variable
     object; see `make-variable' and `make-undefined-variable'.

 -- Scheme Procedure: variable-set! var val
 -- C Function: scm_variable_set_x (var, val)
     Set the value of the variable VAR to VAL.  VAR must be a variable
     object, VAL can be any value. Return an unspecified value.

 -- Scheme Procedure: variable-unset! var
 -- C Function: scm_variable_unset_x (var)
     Unset the value of the variable VAR, leaving VAR unbound.

 -- Scheme Procedure: variable? obj
 -- C Function: scm_variable_p (obj)
     Return `#t' iff OBJ is a variable object, else return `#f'.

6.19.8 Module System Reflection
-------------------------------

The previous sections have described a declarative view of the module
system.  You can also work with it programmatically by accessing and
modifying various parts of the Scheme objects that Guile uses to
implement the module system.

   At any time, there is a "current module".  This module is the one
where a top-level `define' and similar syntax will add new bindings.
You can find other module objects with `resolve-module', for example.

   These module objects can be used as the second argument to `eval'.

 -- Scheme Procedure: current-module
 -- C Function: scm_current_module ()
     Return the current module object.

 -- Scheme Procedure: set-current-module module
 -- C Function: scm_set_current_module (module)
     Set the current module to MODULE and return the previous current
     module.

 -- Scheme Procedure: save-module-excursion thunk
     Call THUNK within a `dynamic-wind' such that the module that is
     current at invocation time is restored when THUNK's dynamic extent
     is left (*note Dynamic Wind::).

     More precisely, if THUNK escapes non-locally, the current module
     (at the time of escape) is saved, and the original current module
     (at the time THUNK's dynamic extent was last entered) is restored.
     If THUNK's dynamic extent is re-entered, then the current module is
     saved, and the previously saved inner module is set current again.

 -- Scheme Procedure: resolve-module name [autoload=#t] [version=#f]
          [#:ensure=#t]
 -- C Function: scm_resolve_module (name)
     Find the module named NAME and return it.  When it has not already
     been defined and AUTOLOAD is true, try to auto-load it.  When it
     can't be found that way either, create an empty module if ENSURE
     is true, otherwise return `#f'.  If VERSION is true, ensure that
     the resulting module is compatible with the given version reference
     (*note R6RS Version References::).  The name is a list of symbols.

 -- Scheme Procedure: resolve-interface name [#:select=#f] [#:hide='()]
          [#:select=()] [#:prefix=#f] [#:renamer] [#:version=#f]
     Find the module named NAME as with `resolve-module' and return its
     interface.  The interface of a module is also a module object, but
     it contains only the exported bindings.

 -- Scheme Procedure: module-uses module
     Return a list of the interfaces used by MODULE.

 -- Scheme Procedure: module-use! module interface
     Add INTERFACE to the front of the use-list of MODULE.  Both
     arguments should be module objects, and INTERFACE should very
     likely be a module returned by `resolve-interface'.

 -- Scheme Procedure: reload-module module
     Revisit the source file that corresponds to MODULE.  Raises an
     error if no source file is associated with the given module.

   As mentioned in the previous section, modules contain a mapping
between identifiers (as symbols) and storage locations (as variables).
Guile defines a number of procedures to allow access to this mapping.
If you are programming in C, *note Accessing Modules from C::.

 -- Scheme Procedure: module-variable module name
     Return the variable bound to NAME (a symbol) in MODULE, or `#f' if
     NAME is unbound.

 -- Scheme Procedure: module-add! module name var
     Define a new binding between NAME (a symbol) and VAR (a variable)
     in MODULE.

 -- Scheme Procedure: module-ref module name
     Look up the value bound to NAME in MODULE.  Like
     `module-variable', but also does a `variable-ref' on the resulting
     variable, raising an error if NAME is unbound.

 -- Scheme Procedure: module-define! module name value
     Locally bind NAME to VALUE in MODULE.  If NAME was already locally
     bound in MODULE, i.e., defined locally and not by an imported
     module, the value stored in the existing variable will be updated.
     Otherwise, a new variable will be added to the module, via
     `module-add!'.

 -- Scheme Procedure: module-set! module name value
     Update the binding of NAME in MODULE to VALUE, raising an error if
     NAME is not already bound in MODULE.

   There are many other reflective procedures available in the default
environment.  If you find yourself using one of them, please contact the
Guile developers so that we can commit to stability for that interface.

6.19.9 Accessing Modules from C
-------------------------------

The last sections have described how modules are used in Scheme code,
which is the recommended way of creating and accessing modules.  You
can also work with modules from C, but it is more cumbersome.

   The following procedures are available.

 -- C Function: SCM scm_c_call_with_current_module (SCM MODULE, SCM
          (*FUNC)(void *), void *DATA)
     Call FUNC and make MODULE the current module during the call.  The
     argument DATA is passed to FUNC.  The return value of
     `scm_c_call_with_current_module' is the return value of FUNC.

 -- C Function: SCM scm_public_variable (SCM MODULE_NAME, SCM NAME)
 -- C Function: SCM scm_c_public_variable (const char *MODULE_NAME,
          const char *NAME)
     Find a the variable bound to the symbol NAME in the public
     interface of the module named MODULE_NAME.

     MODULE_NAME should be a list of symbols, when represented as a
     Scheme object, or a space-separated string, in the `const char *'
     case.  See `scm_c_define_module' below, for more examples.

     Signals an error if no module was found with the given name.  If
     NAME is not bound in the module, just returns `#f'.

 -- C Function: SCM scm_private_variable (SCM MODULE_NAME, SCM NAME)
 -- C Function: SCM scm_c_private_variable (const char *MODULE_NAME,
          const char *NAME)
     Like `scm_public_variable', but looks in the internals of the
     module named MODULE_NAME instead of the public interface.
     Logically, these procedures should only be called on modules you
     write.

 -- C Function: SCM scm_public_lookup (SCM MODULE_NAME, SCM NAME)
 -- C Function: SCM scm_c_public_lookup (const char *MODULE_NAME, const
          char *NAME)
 -- C Function: SCM scm_private_lookup (SCM MODULE_NAME, SCM NAME)
 -- C Function: SCM scm_c_private_lookup (const char *MODULE_NAME,
          const char *NAME)
     Like `scm_public_variable' or `scm_private_variable', but if the
     NAME is not bound in the module, signals an error.  Returns a
     variable, always.

          SCM my_eval_string (SCM str)
          {
            static SCM eval_string_var = SCM_BOOL_F;

            if (scm_is_false (eval_string_var))
              eval_string_var =
                scm_c_public_lookup ("ice-9 eval-string", "eval-string");

            return scm_call_1 (scm_variable_ref (eval_string_var), str);
          }

 -- C Function: SCM scm_public_ref (SCM MODULE_NAME, SCM NAME)
 -- C Function: SCM scm_c_public_ref (const char *MODULE_NAME, const
          char *NAME)
 -- C Function: SCM scm_private_ref (SCM MODULE_NAME, SCM NAME)
 -- C Function: SCM scm_c_private_ref (const char *MODULE_NAME, const
          char *NAME)
     Like `scm_public_lookup' or `scm_private_lookup', but additionally
     dereferences the variable.  If the variable object is unbound,
     signals an error.  Returns the value bound to NAME in MODULE.

   In addition, there are a number of other lookup-related procedures.
We suggest that you use the `scm_public_' and `scm_private_' family of
procedures instead, if possible.

 -- C Function: SCM scm_c_lookup (const char *NAME)
     Return the variable bound to the symbol indicated by NAME in the
     current module.  If there is no such binding or the symbol is not
     bound to a variable, signal an error.

 -- C Function: SCM scm_lookup (SCM NAME)
     Like `scm_c_lookup', but the symbol is specified directly.

 -- C Function: SCM scm_c_module_lookup (SCM MODULE, const char *NAME)
 -- C Function: SCM scm_module_lookup (SCM MODULE, SCM NAME)
     Like `scm_c_lookup' and `scm_lookup', but the specified module is
     used instead of the current one.

 -- C Function: SCM scm_module_variable (SCM MODULE, SCM NAME)
     Like `scm_module_lookup', but if the binding does not exist, just
     returns `#f' instead of raising an error.

   To define a value, use `scm_define':

 -- C Function: SCM scm_c_define (const char *NAME, SCM VAL)
     Bind the symbol indicated by NAME to a variable in the current
     module and set that variable to VAL.  When NAME is already bound
     to a variable, use that.  Else create a new variable.

 -- C Function: SCM scm_define (SCM NAME, SCM VAL)
     Like `scm_c_define', but the symbol is specified directly.

 -- C Function: SCM scm_c_module_define (SCM MODULE, const char *NAME,
          SCM VAL)
 -- C Function: SCM scm_module_define (SCM MODULE, SCM NAME, SCM VAL)
     Like `scm_c_define' and `scm_define', but the specified module is
     used instead of the current one.

 -- C Function: SCM scm_module_reverse_lookup (SCM MODULE, SCM VARIABLE)
     Find the symbol that is bound to VARIABLE in MODULE.  When no such
     binding is found, return #F.

 -- C Function: SCM scm_c_define_module (const char *NAME, void
          (*INIT)(void *), void *DATA)
     Define a new module named NAME and make it current while INIT is
     called, passing it DATA.  Return the module.

     The parameter NAME is a string with the symbols that make up the
     module name, separated by spaces.  For example, `"foo bar"' names
     the module `(foo bar)'.

     When there already exists a module named NAME, it is used
     unchanged, otherwise, an empty module is created.

 -- C Function: SCM scm_c_resolve_module (const char *NAME)
     Find the module name NAME and return it.  When it has not already
     been defined, try to auto-load it.  When it can't be found that
     way either, create an empty module.  The name is interpreted as
     for `scm_c_define_module'.

 -- C Function: SCM scm_c_use_module (const char *NAME)
     Add the module named NAME to the uses list of the current module,
     as with `(use-modules NAME)'.  The name is interpreted as for
     `scm_c_define_module'.

 -- C Function: SCM scm_c_export (const char *NAME, ...)
     Add the bindings designated by NAME, ... to the public interface
     of the current module.  The list of names is terminated by `NULL'.

6.19.10 Included Guile Modules
------------------------------

Some modules are included in the Guile distribution; here are references
to the entries in this manual which describe them in more detail:

*boot-9*
     boot-9 is Guile's initialization module, and it is always loaded
     when Guile starts up.

*(ice-9 expect)*
     Actions based on matching input from a port (*note Expect::).

*(ice-9 format)*
     Formatted output in the style of Common Lisp (*note Formatted
     Output::).

*(ice-9 ftw)*
     File tree walker (*note File Tree Walk::).

*(ice-9 getopt-long)*
     Command line option processing (*note getopt-long::).

*(ice-9 history)*
     Refer to previous interactive expressions (*note Value History::).

*(ice-9 popen)*
     Pipes to and from child processes (*note Pipes::).

*(ice-9 pretty-print)*
     Nicely formatted output of Scheme expressions and objects (*note
     Pretty Printing::).

*(ice-9 q)*
     First-in first-out queues (*note Queues::).

*(ice-9 rdelim)*
     Line- and character-delimited input (*note Line/Delimited::).

*(ice-9 readline)*
     `readline' interactive command line editing (*note Readline
     Support::).

*(ice-9 receive)*
     Multiple-value handling with `receive' (*note Multiple Values::).

*(ice-9 regex)*
     Regular expression matching (*note Regular Expressions::).

*(ice-9 rw)*
     Block string input/output (*note Block Reading and Writing::).

*(ice-9 streams)*
     Sequence of values calculated on-demand (*note Streams::).

*(ice-9 syncase)*
     R5RS `syntax-rules' macro system (*note Syntax Rules::).

*(ice-9 threads)*
     Guile's support for multi threaded execution (*note Scheduling::).

*(ice-9 documentation)*
     Online documentation (REFFIXME).

*(srfi srfi-1)*
     A library providing a lot of useful list and pair processing
     procedures (*note SRFI-1::).

*(srfi srfi-2)*
     Support for `and-let*' (*note SRFI-2::).

*(srfi srfi-4)*
     Support for homogeneous numeric vectors (*note SRFI-4::).

*(srfi srfi-6)*
     Support for some additional string port procedures (*note
     SRFI-6::).

*(srfi srfi-8)*
     Multiple-value handling with `receive' (*note SRFI-8::).

*(srfi srfi-9)*
     Record definition with `define-record-type' (*note SRFI-9::).

*(srfi srfi-10)*
     Read hash extension `#,()' (*note SRFI-10::).

*(srfi srfi-11)*
     Multiple-value handling with `let-values' and `let*-values' (*note
     SRFI-11::).

*(srfi srfi-13)*
     String library (*note SRFI-13::).

*(srfi srfi-14)*
     Character-set library (*note SRFI-14::).

*(srfi srfi-16)*
     `case-lambda' procedures of variable arity (*note SRFI-16::).

*(srfi srfi-17)*
     Getter-with-setter support (*note SRFI-17::).

*(srfi srfi-19)*
     Time/Date library (*note SRFI-19::).

*(srfi srfi-26)*
     Convenient syntax for partial application (*note SRFI-26::)

*(srfi srfi-31)*
     `rec' convenient recursive expressions (*note SRFI-31::)

*(ice-9 slib)*
     This module contains hooks for using Aubrey Jaffer's portable
     Scheme library SLIB from Guile (*note SLIB::).

6.19.11 provide and require
---------------------------

Aubrey Jaffer, mostly to support his portable Scheme library SLIB,
implemented a provide/require mechanism for many Scheme implementations.
Library files in SLIB _provide_ a feature, and when user programs
_require_ that feature, the library file is loaded in.

   For example, the file `random.scm' in the SLIB package contains the
line

     (provide 'random)

   so to use its procedures, a user would type

     (require 'random)

   and they would magically become available, _but still have the same
names!_  So this method is nice, but not as good as a full-featured
module system.

   When SLIB is used with Guile, provide and require can be used to
access its facilities.

6.19.12 Environments
--------------------

Scheme, as defined in R5RS, does _not_ have a full module system.
However it does define the concept of a top-level "environment".  Such
an environment maps identifiers (symbols) to Scheme objects such as
procedures and lists: *note About Closure::.  In other words, it
implements a set of "bindings".

   Environments in R5RS can be passed as the second argument to `eval'
(*note Fly Evaluation::).  Three procedures are defined to return
environments: `scheme-report-environment', `null-environment' and
`interaction-environment' (*note Fly Evaluation::).

   In addition, in Guile any module can be used as an R5RS environment,
i.e., passed as the second argument to `eval'.

   Note: the following two procedures are available only when the
`(ice-9 r5rs)' module is loaded:

     (use-modules (ice-9 r5rs))

 -- Scheme Procedure: scheme-report-environment version
 -- Scheme Procedure: null-environment version
     VERSION must be the exact integer `5', corresponding to revision 5
     of the Scheme report (the Revised^5 Report on Scheme).
     `scheme-report-environment' returns a specifier for an environment
     that is empty except for all bindings defined in the report that
     are either required or both optional and supported by the
     implementation. `null-environment' returns a specifier for an
     environment that is empty except for the (syntactic) bindings for
     all syntactic keywords defined in the report that are either
     required or both optional and supported by the implementation.

     Currently Guile does not support values of VERSION for other
     revisions of the report.

     The effect of assigning (through the use of `eval') a variable
     bound in a `scheme-report-environment' (for example `car') is
     unspecified.  Currently the environments specified by
     `scheme-report-environment' are not immutable in Guile.

6.20 Foreign Function Interface
===============================

The more one hacks in Scheme, the more one realizes that there are
actually two computational worlds: one which is warm and alive, that
land of parentheses, and one cold and dead, the land of C and its ilk.

   But yet we as programmers live in both worlds, and Guile itself is
half implemented in C. So it is that Guile's living half pays respect
to its dead counterpart, via a spectrum of interfaces to C ranging from
dynamic loading of Scheme primitives to dynamic binding of stock C
library procedures.

6.20.1 Foreign Libraries
------------------------

Most modern Unices have something called "shared libraries".  This
ordinarily means that they have the capability to share the executable
image of a library between several running programs to save memory and
disk space.  But generally, shared libraries give a lot of additional
flexibility compared to the traditional static libraries.  In fact,
calling them `dynamic' libraries is as correct as calling them `shared'.

   Shared libraries really give you a lot of flexibility in addition to
the memory and disk space savings.  When you link a program against a
shared library, that library is not closely incorporated into the final
executable.  Instead, the executable of your program only contains
enough information to find the needed shared libraries when the program
is actually run.  Only then, when the program is starting, is the final
step of the linking process performed.  This means that you need not
recompile all programs when you install a new, only slightly modified
version of a shared library.  The programs will pick up the changes
automatically the next time they are run.

   Now, when all the necessary machinery is there to perform part of the
linking at run-time, why not take the next step and allow the programmer
to explicitly take advantage of it from within his program?  Of course,
many operating systems that support shared libraries do just that, and
chances are that Guile will allow you to access this feature from within
your Scheme programs.  As you might have guessed already, this feature
is called "dynamic linking".(1)

   We titled this section "foreign libraries" because although the name
"foreign" doesn't leak into the API, the world of C really is foreign
to Scheme - and that estrangement extends to components of foreign
libraries as well, as we see in future sections.

 -- Scheme Procedure: dynamic-link [library]
 -- C Function: scm_dynamic_link (library)
     Find the shared library denoted by LIBRARY (a string) and link it
     into the running Guile application.  When everything works out,
     return a Scheme object suitable for representing the linked object
     file.  Otherwise an error is thrown.  How object files are
     searched is system dependent.

     Normally, LIBRARY is just the name of some shared library file
     that will be searched for in the places where shared libraries
     usually reside, such as in `/usr/lib' and `/usr/local/lib'.

     LIBRARY should not contain an extension such as `.so'.  The
     correct file name extension for the host operating system is
     provided automatically, according to libltdl's rules (*note
     lt_dlopenext: (libtool)Libltdl interface.).

     When LIBRARY is omitted, a "global symbol handle" is returned.
     This handle provides access to the symbols available to the
     program at run-time, including those exported by the program
     itself and the shared libraries already loaded.

 -- Scheme Procedure: dynamic-object? obj
 -- C Function: scm_dynamic_object_p (obj)
     Return `#t' if OBJ is a dynamic library handle, or `#f' otherwise.

 -- Scheme Procedure: dynamic-unlink dobj
 -- C Function: scm_dynamic_unlink (dobj)
     Unlink the indicated object file from the application.  The
     argument DOBJ must have been obtained by a call to `dynamic-link'.
     After `dynamic-unlink' has been called on DOBJ, its content is no
     longer accessible.

     (define libgl-obj (dynamic-link "libGL"))
     libgl-obj
     => #<dynamic-object "libGL">
     (dynamic-unlink libGL-obj)
     libGL-obj
     => #<dynamic-object "libGL" (unlinked)>

   As you can see, after calling `dynamic-unlink' on a dynamically
linked library, it is marked as `(unlinked)' and you are no longer able
to use it with `dynamic-call', etc.  Whether the library is really
removed from you program is system-dependent and will generally not
happen when some other parts of your program still use it.

   When dynamic linking is disabled or not supported on your system,
the above functions throw errors, but they are still available.

   ---------- Footnotes ----------

   (1) Some people also refer to the final linking stage at program
startup as `dynamic linking', so if you want to make yourself perfectly
clear, it is probably best to use the more technical term "dlopening",
as suggested by Gordon Matzigkeit in his libtool documentation.

6.20.2 Foreign Functions
------------------------

The most natural thing to do with a dynamic library is to grovel around
in it for a function pointer: a "foreign function".  `dynamic-func'
exists for that purpose.

 -- Scheme Procedure: dynamic-func name dobj
 -- C Function: scm_dynamic_func (name, dobj)
     Return a "handle" for the func NAME in the shared object referred
     to by DOBJ. The handle can be passed to `dynamic-call' to actually
     call the function.

     Regardless whether your C compiler prepends an underscore `_' to
     the global names in a program, you should *not* include this
     underscore in NAME since it will be added automatically when
     necessary.

   Guile has static support for calling functions with no arguments,
`dynamic-call'.

 -- Scheme Procedure: dynamic-call func dobj
 -- C Function: scm_dynamic_call (func, dobj)
     Call the C function indicated by FUNC and DOBJ.  The function is
     passed no arguments and its return value is ignored.  When
     FUNCTION is something returned by `dynamic-func', call that
     function and ignore DOBJ.  When FUNC is a string , look it up in
     DYNOBJ; this is equivalent to
          (dynamic-call (dynamic-func FUNC DOBJ) #f)

     Interrupts are deferred while the C function is executing (with
     `SCM_DEFER_INTS'/`SCM_ALLOW_INTS').

   `dynamic-call' is not very powerful. It is mostly intended to be
used for calling specially written initialization functions that will
then add new primitives to Guile. For example, we do not expect that you
will dynamically link `libX11' with `dynamic-link' and then construct a
beautiful graphical user interface just by using `dynamic-call'.
Instead, the usual way would be to write a special Guile-to-X11 glue
library that has intimate knowledge about both Guile and X11 and does
whatever is necessary to make them inter-operate smoothly. This glue
library could then be dynamically linked into a vanilla Guile
interpreter and activated by calling its initialization function. That
function would add all the new types and primitives to the Guile
interpreter that it has to offer.

   (There is actually another, better option: simply to create a
`libX11' wrapper in Scheme via the dynamic FFI. *Note Dynamic FFI::,
for more information.)

   Given some set of C extensions to Guile, the next logical step is to
integrate these glue libraries into the module system of Guile so that
you can load new primitives into a running system just as you can load
new Scheme code.

 -- Scheme Procedure: load-extension lib init
 -- C Function: scm_load_extension (lib, init)
     Load and initialize the extension designated by LIB and INIT.
     When there is no pre-registered function for LIB/INIT, this is
     equivalent to

          (dynamic-call INIT (dynamic-link LIB))

     When there is a pre-registered function, that function is called
     instead.

     Normally, there is no pre-registered function.  This option exists
     only for situations where dynamic linking is unavailable or
     unwanted.  In that case, you would statically link your program
     with the desired library, and register its init function right
     after Guile has been initialized.

     As for `dynamic-link', LIB should not contain any suffix such as
     `.so' (*note dynamic-link: Foreign Libraries.).  It should also
     not contain any directory components.  Libraries that implement
     Guile Extensions should be put into the normal locations for
     shared libraries.  We recommend to use the naming convention
     `libguile-bla-blum' for a extension related to a module `(bla
     blum)'.

     The normal way for a extension to be used is to write a small
     Scheme file that defines a module, and to load the extension into
     this module.  When the module is auto-loaded, the extension is
     loaded as well.  For example,

          (define-module (bla blum))

          (load-extension "libguile-bla-blum" "bla_init_blum")

6.20.3 C Extensions
-------------------

The most interesting application of dynamically linked libraries is
probably to use them for providing _compiled code modules_ to Scheme
programs.  As much fun as programming in Scheme is, every now and then
comes the need to write some low-level C stuff to make Scheme even more
fun.

   Not only can you put these new primitives into their own module (see
the previous section), you can even put them into a shared library that
is only then linked to your running Guile image when it is actually
needed.

   An example will hopefully make everything clear.  Suppose we want to
make the Bessel functions of the C library available to Scheme in the
module `(math bessel)'.  First we need to write the appropriate glue
code to convert the arguments and return values of the functions from
Scheme to C and back.  Additionally, we need a function that will add
them to the set of Guile primitives.  Because this is just an example,
we will only implement this for the `j0' function.

     #include <math.h>
     #include <libguile.h>

     SCM
     j0_wrapper (SCM x)
     {
       return scm_from_double (j0 (scm_to_double (x, "j0")));
     }

     void
     init_math_bessel ()
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
     }

   We can already try to bring this into action by manually calling the
low level functions for performing dynamic linking.  The C source file
needs to be compiled into a shared library.  Here is how to do it on
GNU/Linux, please refer to the `libtool' documentation for how to
create dynamically linkable libraries portably.

     gcc -shared -o libbessel.so -fPIC bessel.c

   Now fire up Guile:

     (define bessel-lib (dynamic-link "./libbessel.so"))
     (dynamic-call "init_math_bessel" bessel-lib)
     (j0 2)
     => 0.223890779141236

   The filename `./libbessel.so' should be pointing to the shared
library produced with the `gcc' command above, of course.  The second
line of the Guile interaction will call the `init_math_bessel' function
which in turn will register the C function `j0_wrapper' with the Guile
interpreter under the name `j0'.  This function becomes immediately
available and we can call it from Scheme.

   Fun, isn't it?  But we are only half way there.  This is what
`apropos' has to say about `j0':

     (apropos "j0")
     -| (guile-user): j0     #<primitive-procedure j0>

   As you can see, `j0' is contained in the root module, where all the
other Guile primitives like `display', etc live.  In general, a
primitive is put into whatever module is the "current module" at the
time `scm_c_define_gsubr' is called.

   A compiled module should have a specially named "module init
function".  Guile knows about this special name and will call that
function automatically after having linked in the shared library.  For
our example, we replace `init_math_bessel' with the following code in
`bessel.c':

     void
     init_math_bessel (void *unused)
     {
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
       scm_c_export ("j0", NULL);
     }

     void
     scm_init_math_bessel_module ()
     {
       scm_c_define_module ("math bessel", init_math_bessel, NULL);
     }

   The general pattern for the name of a module init function is:
`scm_init_', followed by the name of the module where the individual
hierarchical components are concatenated with underscores, followed by
`_module'.

   After `libbessel.so' has been rebuilt, we need to place the shared
library into the right place.

   Once the module has been correctly installed, it should be possible
to use it like this:

     guile> (load-extension "./libbessel.so" "scm_init_math_bessel_module")
     guile> (use-modules (math bessel))
     guile> (j0 2)
     0.223890779141236
     guile> (apropos "j0")
     -| (math bessel): j0      #<primitive-procedure j0>

   That's it!

6.20.4 Modules and Extensions
-----------------------------

The new primitives that you add to Guile with `scm_c_define_gsubr'
(*note Primitive Procedures::) or with any of the other mechanisms are
placed into the module that is current when the `scm_c_define_gsubr' is
executed. Extensions loaded from the REPL, for example, will be placed
into the `(guile-user)' module, if the REPL module was not changed.

   To define C primitives within a specific module, the simplest way is:

     (define-module (foo bar))
     (load-extension "foobar-c-code" "foo_bar_init")

   When loaded with `(use-modules (foo bar))', the `load-extension'
call looks for the `foobar-c-code.so' (etc) object file in Guile's
`extensiondir', which is usually a subdirectory of the `libdir'. For
example, if your libdir is `/usr/lib', the `extensiondir' for the Guile
2.0.X series will be `/usr/lib/guile/2.0/'.

   The extension path includes the major and minor version of Guile (the
"effective version"), because Guile guarantees compatibility within a
given effective version. This allows you to install different versions
of the same extension for different versions of Guile.

   If the extension is not found in the `extensiondir', Guile will also
search the standard system locations, such as `/usr/lib' or
`/usr/local/lib'. It is preferable, however, to keep your extension out
of the system library path, to prevent unintended interference with
other dynamically-linked C libraries.

   If someone installs your module to a non-standard location then the
object file won't be found.  You can address this by inserting the
install location in the `foo/bar.scm' file.  This is convenient for the
user and also guarantees the intended object is read, even if stray
older or newer versions are in the loader's path.

   The usual way to specify an install location is with a `prefix' at
the configure stage, for instance `./configure prefix=/opt' results in
library files as say `/opt/lib/foobar-c-code.so'.  When using Autoconf
(*note Introduction: (autoconf)Top.), the library location is in a
`libdir' variable.  Its value is intended to be expanded by `make', and
can by substituted into a source file like `foo.scm.in'

     (define-module (foo bar))
     (load-extension "XXextensiondirXX/foobar-c-code" "foo_bar_init")

with the following in a `Makefile', using `sed' (*note Introduction:
(sed)Top. A Stream Editor),

     foo.scm: foo.scm.in
             sed 's|XXextensiondirXX|$(libdir)/guile/2.0|' <foo.scm.in >foo.scm

   The actual pattern `XXextensiondirXX' is arbitrary, it's only
something which doesn't otherwise occur.  If several modules need the
value, it can be easier to create one `foo/config.scm' with a define of
the `extensiondir' location, and use that as required.

     (define-module (foo config))
     (define-public foo-config-extensiondir "XXextensiondirXX"")

   Such a file might have other locations too, for instance a data
directory for auxiliary files, or `localedir' if the module has its own
`gettext' message catalogue (*note Internationalization::).

   It will be noted all of the above requires that the Scheme code to be
found in `%load-path' (*note Load Paths::).  Presently it's left up to
the system administrator or each user to augment that path when
installing Guile modules in non-default locations.  But having reached
the Scheme code, that code should take care of hitting any of its own
private files etc.

6.20.5 Foreign Pointers
-----------------------

The previous sections have shown how Guile can be extended at runtime by
loading compiled C extensions. This approach is all well and good, but
wouldn't it be nice if we didn't have to write any C at all? This
section takes up the problem of accessing C values from Scheme, and the
next discusses C functions.

6.20.5.1 Foreign Types
......................

The first impedance mismatch that one sees between C and Scheme is that
in C, the storage locations (variables) are typed, but in Scheme types
are associated with values, not variables. *Note Values and Variables::.

   So when describing a C function or a C structure so that it can be
accessed from Scheme, the data types of the parameters or fields must be
passed explicitly.

   These "C type values" may be constructed using the constants and
procedures from the `(system foreign)' module, which may be loaded like
this:

     (use-modules (system foreign))

   `(system foreign)' exports a number of values expressing the basic C
types:

 -- Scheme Variable: int8
 -- Scheme Variable: uint8
 -- Scheme Variable: uint16
 -- Scheme Variable: int16
 -- Scheme Variable: uint32
 -- Scheme Variable: int32
 -- Scheme Variable: uint64
 -- Scheme Variable: int64
 -- Scheme Variable: float
 -- Scheme Variable: double
     These values represent the C numeric types of the specified sizes
     and signednesses.

   In addition there are some convenience bindings for indicating types
of platform-dependent size:

 -- Scheme Variable: int
 -- Scheme Variable: unsigned-int
 -- Scheme Variable: long
 -- Scheme Variable: unsigned-long
 -- Scheme Variable: size_t
     Values exported by the `(system foreign)' module, representing C
     numeric types. For example, `long' may be `equal?' to `int64' on a
     64-bit platform.

 -- Scheme Variable: void
     The `void' type.  It can be used as the first argument to
     `pointer->procedure' to wrap a C function that returns nothing.

   In addition, the symbol `*' is used by convention to denote pointer
types.  Procedures detailed in the following sections, such as
`pointer->procedure', accept it as a type descriptor.

6.20.5.2 Foreign Variables
..........................

Pointers to variables in the current address space may be looked up
dynamically using `dynamic-pointer'.

 -- Scheme Procedure: dynamic-pointer name dobj
 -- C Function: scm_dynamic_pointer (name, dobj)
     Return a "wrapped pointer" for the symbol NAME in the shared
     object referred to by DOBJ.  The returned pointer points to a C
     object.

     Regardless whether your C compiler prepends an underscore `_' to
     the global names in a program, you should *not* include this
     underscore in NAME since it will be added automatically when
     necessary.

   For example, currently Guile has a variable, `scm_numptob', as part
of its API. It is declared as a C `long'. So, to create a handle
pointing to that foreign value, we do:

     (use-modules (system foreign))
     (define numptob (dynamic-pointer "scm_numptob" (dynamic-link)))
     numptob
     => #<pointer 0x7fb35b1b4688>

   (The next section discusses ways to dereference pointers.)

   A value returned by `dynamic-pointer' is a Scheme wrapper for a C
pointer.

 -- Scheme Procedure: pointer-address pointer
 -- C Function: scm_pointer_address pointer
     Return the numerical value of POINTER.

          (pointer-address numptob)
          => 139984413364296 ; YMMV

 -- Scheme Procedure: make-pointer address [finalizer]
     Return a foreign pointer object pointing to ADDRESS.  If FINALIZER
     is passed, it should be a pointer to a one-argument C function
     that will be called when the pointer object becomes unreachable.

 -- Scheme Procedure: pointer? obj
     Return `#t' if OBJ is a pointer object, `#f' otherwise.

 -- Scheme Variable: %null-pointer
     A foreign pointer whose value is 0.

 -- Scheme Procedure: null-pointer? pointer
     Return `#t' if POINTER is the null pointer, `#f' otherwise.

   For the purpose of passing SCM values directly to foreign functions,
and allowing them to return SCM values, Guile also supports some unsafe
casting operators.

 -- Scheme Procedure: scm->pointer scm
     Return a foreign pointer object with the `object-address' of SCM.

 -- Scheme Procedure: pointer->scm pointer
     Unsafely cast POINTER to a Scheme object.  Cross your fingers!

6.20.5.3 Void Pointers and Byte Access
......................................

Wrapped pointers are untyped, so they are essentially equivalent to C
`void' pointers.  As in C, the memory region pointed to by a pointer
can be accessed at the byte level.  This is achieved using
_bytevectors_ (*note Bytevectors::).  The `(rnrs bytevector)' module
contains procedures that can be used to convert byte sequences to
Scheme objects such as strings, floating point numbers, or integers.

 -- Scheme Procedure: pointer->bytevector pointer len [offset
          [uvec_type]]
 -- C Function: scm_foreign_to_bytevector pointer len offset uvec_type
     Return a bytevector aliasing the LEN bytes pointed to by POINTER.

     The user may specify an alternate default interpretation for the
     memory by passing the UVEC_TYPE argument, to indicate that the
     memory is an array of elements of that type.  UVEC_TYPE should be
     something that `uniform-vector-element-type' would return, like
     `f32' or `s16'.

     When OFFSET is passed, it specifies the offset in bytes relative
     to POINTER of the memory region aliased by the returned bytevector.

     Mutating the returned bytevector mutates the memory pointed to by
     POINTER, so buckle your seatbelts.

 -- Scheme Procedure: bytevector->pointer bv [offset]
 -- C Function: scm_bytevector_to_pointer bv offset
     Return a pointer pointer aliasing the memory pointed to by BV or
     OFFSET bytes after BV when OFFSET is passed.

   In addition to these primitives, convenience procedures are
available:

 -- Scheme Procedure: dereference-pointer pointer
     Assuming POINTER points to a memory region that holds a pointer,
     return this pointer.

 -- Scheme Procedure: string->pointer string [encoding]
     Return a foreign pointer to a nul-terminated copy of STRING in the
     given ENCODING, defaulting to the current locale encoding.  The C
     string is freed when the returned foreign pointer becomes
     unreachable.

     This is the Scheme equivalent of `scm_to_stringn'.

 -- Scheme Procedure: pointer->string pointer [length] [encoding]
     Return the string representing the C string pointed to by POINTER.
     If LENGTH is omitted or `-1', the string is assumed to be
     nul-terminated.  Otherwise LENGTH is the number of bytes in memory
     pointed to by POINTER.  The C string is assumed to be in the given
     ENCODING, defaulting to the current locale encoding.

     This is the Scheme equivalent of `scm_from_stringn'.

   Most object-oriented C libraries use pointers to specific data
structures to identify objects.  It is useful in such cases to reify the
different pointer types as disjoint Scheme types.  The
`define-wrapped-pointer-type' macro simplifies this.

 -- Scheme Syntax: define-wrapped-pointer-type type-name pred wrap
          unwrap print
     Define helper procedures to wrap pointer objects into Scheme
     objects with a disjoint type.  Specifically, this macro defines:

        * PRED, a predicate for the new Scheme type;

        * WRAP, a procedure that takes a pointer object and returns an
          object that satisfies PRED;

        * UNWRAP, which does the reverse.

     WRAP preserves pointer identity, for two pointer objects P1 and P2
     that are `equal?', `(eq? (WRAP P1) (WRAP P2)) => #t'.

     Finally, PRINT should name a user-defined procedure to print such
     objects.  The procedure is passed the wrapped object and a port to
     write to.

     For example, assume we are wrapping a C library that defines a
     type, `bottle_t', and functions that can be passed `bottle_t *'
     pointers to manipulate them.  We could write:

          (define-wrapped-pointer-type bottle
            bottle?
            wrap-bottle unwrap-bottle
            (lambda (b p)
              (format p "#<bottle of ~a ~x>"
                      (bottle-contents b)
                      (pointer-address (unwrap-bottle b)))))

          (define grab-bottle
            ;; Wrapper for `bottle_t *grab (void)'.
            (let ((grab (pointer->procedure '*
                                            (dynamic-func "grab_bottle" libbottle)
                                            '())))
              (lambda ()
                "Return a new bottle."
                (wrap-bottle (grab)))))

          (define bottle-contents
            ;; Wrapper for `const char *bottle_contents (bottle_t *)'.
            (let ((contents (pointer->procedure '*
                                                (dynamic-func "bottle_contents"
                                                               libbottle)
                                                '(*))))
              (lambda (b)
                "Return the contents of B."
                (pointer->string (contents (unwrap-bottle b))))))

          (write (grab-bottle))
          => #<bottle of Château Haut-Brion 803d36>

     In this example, `grab-bottle' is guaranteed to return a genuine
     `bottle' object satisfying `bottle?'.  Likewise, `bottle-contents'
     errors out when its argument is not a genuine `bottle' object.

   Going back to the `scm_numptob' example above, here is how we can
read its value as a C `long' integer:

     (use-modules (rnrs bytevectors))

     (bytevector-uint-ref (pointer->bytevector numptob (sizeof long))
                          0 (native-endianness)
                          (sizeof long))
     => 8

   If we wanted to corrupt Guile's internal state, we could set
`scm_numptob' to another value; but we shouldn't, because that variable
is not meant to be set.  Indeed this point applies more widely: the C
API is a dangerous place to be.  Not only might setting a value crash
your program, simply accessing the data pointed to by a dangling
pointer or similar can prove equally disastrous.

6.20.5.4 Foreign Structs
........................

Finally, one last note on foreign values before moving on to actually
calling foreign functions. Sometimes you need to deal with C structs,
which requires interpreting each element of the struct according to the
its type, offset, and alignment. Guile has some primitives to support
this.

 -- Scheme Procedure: sizeof type
 -- C Function: scm_sizeof type
     Return the size of TYPE, in bytes.

     TYPE should be a valid C type, like `int'.  Alternately TYPE may
     be the symbol `*', in which case the size of a pointer is
     returned. TYPE may also be a list of types, in which case the size
     of a `struct' with ABI-conventional packing is returned.

 -- Scheme Procedure: alignof type
 -- C Function: scm_alignof type
     Return the alignment of TYPE, in bytes.

     TYPE should be a valid C type, like `int'.  Alternately TYPE may
     be the symbol `*', in which case the alignment of a pointer is
     returned. TYPE may also be a list of types, in which case the
     alignment of a `struct' with ABI-conventional packing is returned.

   Guile also provides some convenience methods to pack and unpack
foreign pointers wrapping C structs.

 -- Scheme Procedure: make-c-struct types vals
     Create a foreign pointer to a C struct containing VALS with types
     `types'.

     VALS and `types' should be lists of the same length.

 -- Scheme Procedure: parse-c-struct foreign types
     Parse a foreign pointer to a C struct, returning a list of values.

     `types' should be a list of C types.

   For example, to create and parse the equivalent of a `struct {
int64_t a; uint8_t b; }':

     (parse-c-struct (make-c-struct (list int64 uint8)
                                    (list 300 43))
                     (list int64 uint8))
     => (300 43)

   As yet, Guile only has convenience routines to support
conventionally-packed structs. But given the `bytevector->foreign' and
`foreign->bytevector' routines, one can create and parse tightly packed
structs and unions by hand. See the code for `(system foreign)' for
details.

6.20.6 Dynamic FFI
------------------

Of course, the land of C is not all nouns and no verbs: there are
functions too, and Guile allows you to call them.

 -- Scheme Procedure: pointer->procedure return_type func_ptr arg_types
 -- C Procedure: scm_pointer_to_procedure return_type func_ptr arg_types
     Make a foreign function.

     Given the foreign void pointer FUNC_PTR, its argument and return
     types ARG_TYPES and RETURN_TYPE, return a procedure that will pass
     arguments to the foreign function and return appropriate values.

     ARG_TYPES should be a list of foreign types.  `return_type' should
     be a foreign type. *Note Foreign Types::, for more information on
     foreign types.

   Here is a better definition of `(math bessel)':

     (define-module (math bessel)
       #:use-module (system foreign)
       #:export (j0))

     (define libm (dynamic-link "libm"))

     (define j0
       (pointer->procedure double
                           (dynamic-func "j0" libm)
                           (list double)))

   That's it! No C at all.

   Numeric arguments and return values from foreign functions are
represented as Scheme values. For example, `j0' in the above example
takes a Scheme number as its argument, and returns a Scheme number.

   Pointers may be passed to and returned from foreign functions as
well.  In that case the type of the argument or return value should be
the symbol `*', indicating a pointer. For example, the following code
makes `memcpy' available to Scheme:

     (define memcpy
       (let ((this (dynamic-link)))
         (pointer->procedure '*
                             (dynamic-func "memcpy" this)
                             (list '* '* size_t))))

   To invoke `memcpy', one must pass it foreign pointers:

     (use-modules (rnrs bytevectors))

     (define src-bits
       (u8-list->bytevector '(0 1 2 3 4 5 6 7)))
     (define src
       (bytevector->pointer src-bits))
     (define dest
       (bytevector->pointer (make-bytevector 16 0)))

     (memcpy dest src (bytevector-length src-bits))

     (bytevector->u8-list (pointer->bytevector dest 16))
     => (0 1 2 3 4 5 6 7 0 0 0 0 0 0 0 0)

   One may also pass structs as values, passing structs as foreign
pointers. *Note Foreign Structs::, for more information on how to
express struct types and struct values.

   "Out" arguments are passed as foreign pointers. The memory pointed to
by the foreign pointer is mutated in place.

     ;; struct timeval {
     ;;      time_t      tv_sec;     /* seconds */
     ;;      suseconds_t tv_usec;    /* microseconds */
     ;; };
     ;; assuming fields are of type "long"

     (define gettimeofday
       (let ((f (pointer->procedure
                 int
                 (dynamic-func "gettimeofday" (dynamic-link))
                 (list '* '*)))
             (tv-type (list long long)))
         (lambda ()
           (let* ((timeval (make-c-struct tv-type (list 0 0)))
                  (ret (f timeval %null-pointer)))
             (if (zero? ret)
                 (apply values (parse-c-struct timeval tv-type))
                 (error "gettimeofday returned an error" ret))))))

     (gettimeofday)
     => 1270587589
     => 499553

   As you can see, this interface to foreign functions is at a very low,
somewhat dangerous level(1).

   The FFI can also work in the opposite direction: making Scheme
procedures callable from C.  This makes it possible to use Scheme
procedures as "callbacks" expected by C function.

 -- Scheme Procedure: procedure->pointer return-type proc arg-types
 -- C Function: scm_procedure_to_pointer (return_type, proc, arg_types)
     Return a pointer to a C function of type RETURN-TYPE taking
     arguments of types ARG-TYPES (a list) and behaving as a proxy to
     procedure PROC.  Thus PROC's arity, supported argument types, and
     return type should match RETURN-TYPE and ARG-TYPES.

   As an example, here's how the C library's `qsort' array sorting
function can be made accessible to Scheme (*note `qsort': (libc)Array
Sort Function.):

     (define qsort!
       (let ((qsort (pointer->procedure void
                                        (dynamic-func "qsort"
                                                      (dynamic-link))
                                        (list '* size_t size_t '*))))
         (lambda (bv compare)
           ;; Sort bytevector BV in-place according to comparison
           ;; procedure COMPARE.
           (let ((ptr (procedure->pointer int
                                          (lambda (x y)
                                            ;; X and Y are pointers so,
                                            ;; for convenience, dereference
                                            ;; them before calling COMPARE.
                                            (compare (dereference-uint8* x)
                                                     (dereference-uint8* y)))
                                          (list '* '*))))
             (qsort (bytevector->pointer bv)
                    (bytevector-length bv) 1 ;; we're sorting bytes
                    ptr)))))

     (define (dereference-uint8* ptr)
       ;; Helper function: dereference the byte pointed to by PTR.
       (let ((b (pointer->bytevector ptr 1)))
         (bytevector-u8-ref b 0)))

     (define bv
       ;; An unsorted array of bytes.
       (u8-list->bytevector '(7 1 127 3 5 4 77 2 9 0)))

     ;; Sort BV.
     (qsort! bv (lambda (x y) (- x y)))

     ;; Let's see what the sorted array looks like:
     (bytevector->u8-list bv)
     => (0 1 2 3 4 5 7 9 77 127)

   And voilà!

   Note that `procedure->pointer' is not supported (and not defined) on
a few exotic architectures.  Thus, user code may need to check
`(defined? 'procedure->pointer)'.  Nevertheless, it is available on
many architectures, including (as of libffi 3.0.9) x86, ia64, SPARC,
PowerPC, ARM, and MIPS, to name a few.

   ---------- Footnotes ----------

   (1) A contribution to Guile in the form of a high-level FFI would be
most welcome.

6.21 Threads, Mutexes, Asyncs and Dynamic Roots
===============================================

6.21.1 Arbiters
---------------

Arbiters are synchronization objects, they can be used by threads to
control access to a shared resource.  An arbiter can be locked to
indicate a resource is in use, and unlocked when done.

   An arbiter is like a light-weight mutex (*note Mutexes and Condition
Variables::).  It uses less memory and may be faster, but there's no
way for a thread to block waiting on an arbiter, it can only test and
get the status returned.

 -- Scheme Procedure: make-arbiter name
 -- C Function: scm_make_arbiter (name)
     Return an object of type arbiter and name NAME. Its state is
     initially unlocked.  Arbiters are a way to achieve process
     synchronization.

 -- Scheme Procedure: try-arbiter arb
 -- C Function: scm_try_arbiter (arb)
     If ARB is unlocked, then lock it and return `#t'.  If ARB is
     already locked, then do nothing and return `#f'.

 -- Scheme Procedure: release-arbiter arb
 -- C Function: scm_release_arbiter (arb)
     If ARB is locked, then unlock it and return `#t'.  If ARB is
     already unlocked, then do nothing and return `#f'.

     Typical usage is for the thread which locked an arbiter to later
     release it, but that's not required, any thread can release it.

6.21.2 Asyncs
-------------

Asyncs are a means of deferring the execution of Scheme code until it is
safe to do so.

   Guile provides two kinds of asyncs that share the basic concept but
are otherwise quite different: system asyncs and user asyncs.  System
asyncs are integrated into the core of Guile and are executed
automatically when the system is in a state to allow the execution of
Scheme code.  For example, it is not possible to execute Scheme code in
a POSIX signal handler, but such a signal handler can queue a system
async to be executed in the near future, when it is safe to do so.

   System asyncs can also be queued for threads other than the current
one.  This way, you can cause threads to asynchronously execute
arbitrary code.

   User asyncs offer a convenient means of queuing procedures for future
execution and triggering this execution.  They will not be executed
automatically.

6.21.2.1 System asyncs
......................

To cause the future asynchronous execution of a procedure in a given
thread, use `system-async-mark'.

   Automatic invocation of system asyncs can be temporarily disabled by
calling `call-with-blocked-asyncs'.  This function works by temporarily
increasing the _async blocking level_ of the current thread while a
given procedure is running.  The blocking level starts out at zero, and
whenever a safe point is reached, a blocking level greater than zero
will prevent the execution of queued asyncs.

   Analogously, the procedure `call-with-unblocked-asyncs' will
temporarily decrease the blocking level of the current thread.  You can
use it when you want to disable asyncs by default and only allow them
temporarily.

   In addition to the C versions of `call-with-blocked-asyncs' and
`call-with-unblocked-asyncs', C code can use `scm_dynwind_block_asyncs'
and `scm_dynwind_unblock_asyncs' inside a "dynamic context" (*note
Dynamic Wind::) to block or unblock system asyncs temporarily.

 -- Scheme Procedure: system-async-mark proc [thread]
 -- C Function: scm_system_async_mark (proc)
 -- C Function: scm_system_async_mark_for_thread (proc, thread)
     Mark PROC (a procedure with zero arguments) for future execution
     in THREAD.  When PROC has already been marked for THREAD but has
     not been executed yet, this call has no effect.  When THREAD is
     omitted, the thread that called `system-async-mark' is used.

     This procedure is not safe to be called from signal handlers.  Use
     `scm_sigaction' or `scm_sigaction_for_thread' to install signal
     handlers.

 -- Scheme Procedure: call-with-blocked-asyncs proc
 -- C Function: scm_call_with_blocked_asyncs (proc)
     Call PROC and block the execution of system asyncs by one level
     for the current thread while it is running.  Return the value
     returned by PROC.  For the first two variants, call PROC with no
     arguments; for the third, call it with DATA.

 -- C Function: void * scm_c_call_with_blocked_asyncs (void * (*proc)
          (void *data), void *data)
     The same but with a C function PROC instead of a Scheme thunk.

 -- Scheme Procedure: call-with-unblocked-asyncs proc
 -- C Function: scm_call_with_unblocked_asyncs (proc)
     Call PROC and unblock the execution of system asyncs by one level
     for the current thread while it is running.  Return the value
     returned by PROC.  For the first two variants, call PROC with no
     arguments; for the third, call it with DATA.

 -- C Function: void * scm_c_call_with_unblocked_asyncs (void *(*proc)
          (void *data), void *data)
     The same but with a C function PROC instead of a Scheme thunk.

 -- C Function: void scm_dynwind_block_asyncs ()
     During the current dynwind context, increase the blocking of
     asyncs by one level.  This function must be used inside a pair of
     calls to `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic
     Wind::).

 -- C Function: void scm_dynwind_unblock_asyncs ()
     During the current dynwind context, decrease the blocking of
     asyncs by one level.  This function must be used inside a pair of
     calls to `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic
     Wind::).

6.21.2.2 User asyncs
....................

A user async is a pair of a thunk (a parameterless procedure) and a
mark.  Setting the mark on a user async will cause the thunk to be
executed when the user async is passed to `run-asyncs'.  Setting the
mark more than once is satisfied by one execution of the thunk.

   User asyncs are created with `async'.  They are marked with
`async-mark'.

 -- Scheme Procedure: async thunk
 -- C Function: scm_async (thunk)
     Create a new user async for the procedure THUNK.

 -- Scheme Procedure: async-mark a
 -- C Function: scm_async_mark (a)
     Mark the user async A for future execution.

 -- Scheme Procedure: run-asyncs list_of_a
 -- C Function: scm_run_asyncs (list_of_a)
     Execute all thunks from the marked asyncs of the list LIST_OF_A.

6.21.3 Threads
--------------

Guile supports POSIX threads, unless it was configured with
`--without-threads' or the host lacks POSIX thread support.  When
thread support is available, the `threads' feature is provided (*note
`provided?': Feature Manipulation.).

   The procedures below manipulate Guile threads, which are wrappers
around the system's POSIX threads.  For application-level parallelism,
using higher-level constructs, such as futures, is recommended (*note
Futures::).

 -- Scheme Procedure: all-threads
 -- C Function: scm_all_threads ()
     Return a list of all threads.

 -- Scheme Procedure: current-thread
 -- C Function: scm_current_thread ()
     Return the thread that called this function.

 -- Scheme Procedure: call-with-new-thread thunk [handler]
     Call `thunk' in a new thread and with a new dynamic state,
     returning the new thread.  The procedure THUNK is called via
     `with-continuation-barrier'.

     When HANDLER is specified, then THUNK is called from within a
     `catch' with tag `#t' that has HANDLER as its handler.  This catch
     is established inside the continuation barrier.

     Once THUNK or HANDLER returns, the return value is made the _exit
     value_ of the thread and the thread is terminated.

 -- C Function: SCM scm_spawn_thread (scm_t_catch_body body, void
          *body_data, scm_t_catch_handler handler, void *handler_data)
     Call BODY in a new thread, passing it BODY_DATA, returning the new
     thread.  The function BODY is called via
     `scm_c_with_continuation_barrier'.

     When HANDLER is non-`NULL', BODY is called via
     `scm_internal_catch' with tag `SCM_BOOL_T' that has HANDLER and
     HANDLER_DATA as the handler and its data.  This catch is
     established inside the continuation barrier.

     Once BODY or HANDLER returns, the return value is made the _exit
     value_ of the thread and the thread is terminated.

 -- Scheme Procedure: thread? obj
 -- C Function: scm_thread_p (obj)
     Return `#t' iff OBJ is a thread; otherwise, return `#f'.

 -- Scheme Procedure: join-thread thread [timeout [timeoutval]]
 -- C Function: scm_join_thread (thread)
 -- C Function: scm_join_thread_timed (thread, timeout, timeoutval)
     Wait for THREAD to terminate and return its exit value.  Threads
     that have not been created with `call-with-new-thread' or
     `scm_spawn_thread' have an exit value of `#f'.  When TIMEOUT is
     given, it specifies a point in time where the waiting should be
     aborted.  It can be either an integer as returned by
     `current-time' or a pair as returned by `gettimeofday'.  When the
     waiting is aborted, TIMEOUTVAL is returned (if it is specified;
     `#f' is returned otherwise).

 -- Scheme Procedure: thread-exited? thread
 -- C Function: scm_thread_exited_p (thread)
     Return `#t' iff THREAD has exited.

 -- Scheme Procedure: yield
     If one or more threads are waiting to execute, calling yield
     forces an immediate context switch to one of them. Otherwise,
     yield has no effect.

 -- Scheme Procedure: cancel-thread thread
 -- C Function: scm_cancel_thread (thread)
     Asynchronously notify THREAD to exit.  Immediately after receiving
     this notification, THREAD will call its cleanup handler (if one
     has been set) and then terminate, aborting any evaluation that is
     in progress.

     Because Guile threads are isomorphic with POSIX threads, THREAD
     will not receive its cancellation signal until it reaches a
     cancellation point.  See your operating system's POSIX threading
     documentation for more information on cancellation points; note
     that in Guile, unlike native POSIX threads, a thread can receive a
     cancellation notification while attempting to lock a mutex.

 -- Scheme Procedure: set-thread-cleanup! thread proc
 -- C Function: scm_set_thread_cleanup_x (thread, proc)
     Set PROC as the cleanup handler for the thread THREAD.  PROC,
     which must be a thunk, will be called when THREAD exits, either
     normally or by being canceled.  Thread cleanup handlers can be
     used to perform useful tasks like releasing resources, such as
     locked mutexes, when thread exit cannot be predicted.

     The return value of PROC will be set as the _exit value_ of THREAD.

     To remove a cleanup handler, pass `#f' for PROC.

 -- Scheme Procedure: thread-cleanup thread
 -- C Function: scm_thread_cleanup (thread)
     Return the cleanup handler currently installed for the thread
     THREAD.  If no cleanup handler is currently installed,
     thread-cleanup returns `#f'.

   Higher level thread procedures are available by loading the `(ice-9
threads)' module.  These provide standardized thread creation.

 -- macro: make-thread proc [args...]
     Apply PROC to ARGS in a new thread formed by
     `call-with-new-thread' using a default error handler that display
     the error to the current error port.  The ARGS...  expressions are
     evaluated in the new thread.

 -- macro: begin-thread first [rest...]
     Evaluate forms FIRST and REST in a new thread formed by
     `call-with-new-thread' using a default error handler that display
     the error to the current error port.

6.21.4 Mutexes and Condition Variables
--------------------------------------

A mutex is a thread synchronization object, it can be used by threads
to control access to a shared resource.  A mutex can be locked to
indicate a resource is in use, and other threads can then block on the
mutex to wait for the resource (or can just test and do something else
if not available).  "Mutex" is short for "mutual exclusion".

   There are two types of mutexes in Guile, "standard" and "recursive".
They're created by `make-mutex' and `make-recursive-mutex'
respectively, the operation functions are then common to both.

   Note that for both types of mutex there's no protection against a
"deadly embrace".  For instance if one thread has locked mutex A and is
waiting on mutex B, but another thread owns B and is waiting on A, then
an endless wait will occur (in the current implementation).  Acquiring
requisite mutexes in a fixed order (like always A before B) in all
threads is one way to avoid such problems.


 -- Scheme Procedure: make-mutex . flags
 -- C Function: scm_make_mutex ()
 -- C Function: scm_make_mutex_with_flags (SCM flags)
     Return a new mutex.  It is initially unlocked.  If FLAGS is
     specified, it must be a list of symbols specifying configuration
     flags for the newly-created mutex.  The supported flags are:
    `unchecked-unlock'
          Unless this flag is present, a call to `unlock-mutex' on the
          returned mutex when it is already unlocked will cause an
          error to be signalled.

    `allow-external-unlock'
          Allow the returned mutex to be unlocked by the calling thread
          even if it was originally locked by a different thread.

    `recursive'
          The returned mutex will be recursive.


 -- Scheme Procedure: mutex? obj
 -- C Function: scm_mutex_p (obj)
     Return `#t' iff OBJ is a mutex; otherwise, return `#f'.

 -- Scheme Procedure: make-recursive-mutex
 -- C Function: scm_make_recursive_mutex ()
     Create a new recursive mutex.  It is initially unlocked.  Calling
     this function is equivalent to calling `make-mutex' and specifying
     the `recursive' flag.

 -- Scheme Procedure: lock-mutex mutex [timeout [owner]]
 -- C Function: scm_lock_mutex (mutex)
 -- C Function: scm_lock_mutex_timed (mutex, timeout, owner)
     Lock MUTEX.  If the mutex is already locked, then block and return
     only when MUTEX has been acquired.

     When TIMEOUT is given, it specifies a point in time where the
     waiting should be aborted.  It can be either an integer as returned
     by `current-time' or a pair as returned by `gettimeofday'.  When
     the waiting is aborted, `#f' is returned.

     When OWNER is given, it specifies an owner for MUTEX other than
     the calling thread.  OWNER may also be `#f', indicating that the
     mutex should be locked but left unowned.

     For standard mutexes (`make-mutex'), and error is signalled if the
     thread has itself already locked MUTEX.

     For a recursive mutex (`make-recursive-mutex'), if the thread has
     itself already locked MUTEX, then a further `lock-mutex' call
     increments the lock count.  An additional `unlock-mutex' will be
     required to finally release.

     If MUTEX was locked by a thread that exited before unlocking it,
     the next attempt to lock MUTEX will succeed, but
     `abandoned-mutex-error' will be signalled.

     When a system async (*note System asyncs::) is activated for a
     thread blocked in `lock-mutex', the wait is interrupted and the
     async is executed.  When the async returns, the wait resumes.

 -- C Function: void scm_dynwind_lock_mutex (SCM mutex)
     Arrange for MUTEX to be locked whenever the current dynwind
     context is entered and to be unlocked when it is exited.

 -- Scheme Procedure: try-mutex mx
 -- C Function: scm_try_mutex (mx)
     Try to lock MUTEX as per `lock-mutex'.  If MUTEX can be acquired
     immediately then this is done and the return is `#t'.  If MUTEX is
     locked by some other thread then nothing is done and the return is
     `#f'.

 -- Scheme Procedure: unlock-mutex mutex [condvar [timeout]]
 -- C Function: scm_unlock_mutex (mutex)
 -- C Function: scm_unlock_mutex_timed (mutex, condvar, timeout)
     Unlock MUTEX.  An error is signalled if MUTEX is not locked and
     was not created with the `unchecked-unlock' flag set, or if MUTEX
     is locked by a thread other than the calling thread and was not
     created with the `allow-external-unlock' flag set.

     If CONDVAR is given, it specifies a condition variable upon which
     the calling thread will wait to be signalled before returning.
     (This behavior is very similar to that of
     `wait-condition-variable', except that the mutex is left in an
     unlocked state when the function returns.)

     When TIMEOUT is also given, it specifies a point in time where the
     waiting should be aborted.  It can be either an integer as
     returned by `current-time' or a pair as returned by
     `gettimeofday'.  When the waiting is aborted, `#f' is returned.
     Otherwise the function returns `#t'.

 -- Scheme Procedure: mutex-owner mutex
 -- C Function: scm_mutex_owner (mutex)
     Return the current owner of MUTEX, in the form of a thread or `#f'
     (indicating no owner).  Note that a mutex may be unowned but still
     locked.

 -- Scheme Procedure: mutex-level mutex
 -- C Function: scm_mutex_level (mutex)
     Return the current lock level of MUTEX.  If MUTEX is currently
     unlocked, this value will be 0; otherwise, it will be the number
     of times MUTEX has been recursively locked by its current owner.

 -- Scheme Procedure: mutex-locked? mutex
 -- C Function: scm_mutex_locked_p (mutex)
     Return `#t' if MUTEX is locked, regardless of ownership;
     otherwise, return `#f'.

 -- Scheme Procedure: make-condition-variable
 -- C Function: scm_make_condition_variable ()
     Return a new condition variable.

 -- Scheme Procedure: condition-variable? obj
 -- C Function: scm_condition_variable_p (obj)
     Return `#t' iff OBJ is a condition variable; otherwise, return
     `#f'.

 -- Scheme Procedure: wait-condition-variable condvar mutex [time]
 -- C Function: scm_wait_condition_variable (condvar, mutex, time)
     Wait until CONDVAR has been signalled.  While waiting, MUTEX is
     atomically unlocked (as with `unlock-mutex') and is locked again
     when this function returns.  When TIME is given, it specifies a
     point in time where the waiting should be aborted.  It can be
     either a integer as returned by `current-time' or a pair as
     returned by `gettimeofday'.  When the waiting is aborted, `#f' is
     returned.  When the condition variable has in fact been signalled,
     `#t' is returned.  The mutex is re-locked in any case before
     `wait-condition-variable' returns.

     When a system async is activated for a thread that is blocked in a
     call to `wait-condition-variable', the waiting is interrupted, the
     mutex is locked, and the async is executed.  When the async
     returns, the mutex is unlocked again and the waiting is resumed.
     When the thread block while re-acquiring the mutex, execution of
     asyncs is blocked.

 -- Scheme Procedure: signal-condition-variable condvar
 -- C Function: scm_signal_condition_variable (condvar)
     Wake up one thread that is waiting for CONDVAR.

 -- Scheme Procedure: broadcast-condition-variable condvar
 -- C Function: scm_broadcast_condition_variable (condvar)
     Wake up all threads that are waiting for CONDVAR.


   The following are higher level operations on mutexes.  These are
available from

     (use-modules (ice-9 threads))

 -- macro: with-mutex mutex [body...]
     Lock MUTEX, evaluate the BODY forms, then unlock MUTEX.  The
     return value is the return from the last BODY form.

     The lock, body and unlock form the branches of a `dynamic-wind'
     (*note Dynamic Wind::), so MUTEX is automatically unlocked if an
     error or new continuation exits BODY, and is re-locked if BODY is
     re-entered by a captured continuation.

 -- macro: monitor body...
     Evaluate the BODY forms, with a mutex locked so only one thread
     can execute that code at any one time.  The return value is the
     return from the last BODY form.

     Each `monitor' form has its own private mutex and the locking and
     evaluation is as per `with-mutex' above.  A standard mutex
     (`make-mutex') is used, which means BODY must not recursively
     re-enter the `monitor' form.

     The term "monitor" comes from operating system theory, where it
     means a particular bit of code managing access to some resource and
     which only ever executes on behalf of one process at any one time.

6.21.5 Blocking in Guile Mode
-----------------------------

Up to Guile version 1.8, a thread blocked in guile mode would prevent
the garbage collector from running.  Thus threads had to explicitly
leave guile mode with `scm_without_guile ()' before making a
potentially blocking call such as a mutex lock, a `select ()' system
call, etc.  The following functions could be used to temporarily leave
guile mode or to perform some common blocking operations in a supported
way.

   Starting from Guile 2.0, blocked threads no longer hinder garbage
collection.  Thus, the functions below are not needed anymore.  They can
still be used to inform the GC that a thread is about to block, giving
it a (small) optimization opportunity for "stop the world" garbage
collections, should they occur while the thread is blocked.

 -- C Function: void * scm_without_guile (void *(*func) (void *), void
          *data)
     Leave guile mode, call FUNC on DATA, enter guile mode and return
     the result of calling FUNC.

     While a thread has left guile mode, it must not call any libguile
     functions except `scm_with_guile' or `scm_without_guile' and must
     not use any libguile macros.  Also, local variables of type `SCM'
     that are allocated while not in guile mode are not protected from
     the garbage collector.

     When used from non-guile mode, calling `scm_without_guile' is
     still allowed: it simply calls FUNC.  In that way, you can leave
     guile mode without having to know whether the current thread is in
     guile mode or not.

 -- C Function: int scm_pthread_mutex_lock (pthread_mutex_t *mutex)
     Like `pthread_mutex_lock', but leaves guile mode while waiting for
     the mutex.

 -- C Function: int scm_pthread_cond_wait (pthread_cond_t *cond,
          pthread_mutex_t *mutex)
 -- C Function: int scm_pthread_cond_timedwait (pthread_cond_t *cond,
          pthread_mutex_t *mutex, struct timespec *abstime)
     Like `pthread_cond_wait' and `pthread_cond_timedwait', but leaves
     guile mode while waiting for the condition variable.

 -- C Function: int scm_std_select (int nfds, fd_set *readfds, fd_set
          *writefds, fd_set *exceptfds, struct timeval *timeout)
     Like `select' but leaves guile mode while waiting.  Also, the
     delivery of a system async causes this function to be interrupted
     with error code `EINTR'.

 -- C Function: unsigned int scm_std_sleep (unsigned int seconds)
     Like `sleep', but leaves guile mode while sleeping.  Also, the
     delivery of a system async causes this function to be interrupted.

 -- C Function: unsigned long scm_std_usleep (unsigned long usecs)
     Like `usleep', but leaves guile mode while sleeping.  Also, the
     delivery of a system async causes this function to be interrupted.

6.21.6 Critical Sections
------------------------

 -- C Macro: SCM_CRITICAL_SECTION_START
 -- C Macro: SCM_CRITICAL_SECTION_END
     These two macros can be used to delimit a critical section.
     Syntactically, they are both statements and need to be followed
     immediately by a semicolon.

     Executing `SCM_CRITICAL_SECTION_START' will lock a recursive mutex
     and block the executing of system asyncs.  Executing
     `SCM_CRITICAL_SECTION_END' will unblock the execution of system
     asyncs and unlock the mutex.  Thus, the code that executes between
     these two macros can only be executed in one thread at any one time
     and no system asyncs will run.  However, because the mutex is a
     recursive one, the code might still be reentered by the same
     thread.  You must either allow for this or avoid it, both by
     careful coding.

     On the other hand, critical sections delimited with these macros
     can be nested since the mutex is recursive.

     You must make sure that for each `SCM_CRITICAL_SECTION_START', the
     corresponding `SCM_CRITICAL_SECTION_END' is always executed.  This
     means that no non-local exit (such as a signalled error) might
     happen, for example.

 -- C Function: void scm_dynwind_critical_section (SCM mutex)
     Call `scm_dynwind_lock_mutex' on MUTEX and call
     `scm_dynwind_block_asyncs'.  When MUTEX is false, a recursive
     mutex provided by Guile is used instead.

     The effect of a call to `scm_dynwind_critical_section' is that the
     current dynwind context (*note Dynamic Wind::) turns into a
     critical section.  Because of the locked mutex, no second thread
     can enter it concurrently and because of the blocked asyncs, no
     system async can reenter it from the current thread.

     When the current thread reenters the critical section anyway, the
     kind of MUTEX determines what happens: When MUTEX is recursive,
     the reentry is allowed.  When it is a normal mutex, an error is
     signalled.

6.21.7 Fluids and Dynamic States
--------------------------------

A _fluid_ is an object that can store one value per _dynamic state_.
Each thread has a current dynamic state, and when accessing a fluid,
this current dynamic state is used to provide the actual value.  In
this way, fluids can be used for thread local storage, but they are in
fact more flexible: dynamic states are objects of their own and can be
made current for more than one thread at the same time, or only be made
current temporarily, for example.

   Fluids can also be used to simulate the desirable effects of
dynamically scoped variables.  Dynamically scoped variables are useful
when you want to set a variable to a value during some dynamic extent
in the execution of your program and have them revert to their original
value when the control flow is outside of this dynamic extent.  See the
description of `with-fluids' below for details.

   New fluids are created with `make-fluid' and `fluid?' is used for
testing whether an object is actually a fluid.  The values stored in a
fluid can be accessed with `fluid-ref' and `fluid-set!'.

 -- Scheme Procedure: make-fluid [dflt]
 -- C Function: scm_make_fluid ()
 -- C Function: scm_make_fluid_with_default (dflt)
     Return a newly created fluid, whose initial value is DFLT, or `#f'
     if DFLT is not given.  Fluids are objects that can hold one value
     per dynamic state.  That is, modifications to this value are only
     visible to code that executes with the same dynamic state as the
     modifying code.  When a new dynamic state is constructed, it
     inherits the values from its parent.  Because each thread normally
     executes with its own dynamic state, you can use fluids for thread
     local storage.

 -- Scheme Procedure: make-unbound-fluid
 -- C Function: scm_make_unbound_fluid ()
     Return a new fluid that is initially unbound (instead of being
     implicitly bound to some definite value).

 -- Scheme Procedure: fluid? obj
 -- C Function: scm_fluid_p (obj)
     Return `#t' iff OBJ is a fluid; otherwise, return `#f'.

 -- Scheme Procedure: fluid-ref fluid
 -- C Function: scm_fluid_ref (fluid)
     Return the value associated with FLUID in the current dynamic
     root.  If FLUID has not been set, then return its default value.
     Calling `fluid-ref' on an unbound fluid produces a runtime error.

 -- Scheme Procedure: fluid-set! fluid value
 -- C Function: scm_fluid_set_x (fluid, value)
     Set the value associated with FLUID in the current dynamic root.

 -- Scheme Procedure: fluid-unset! fluid
 -- C Function: scm_fluid_unset_x (fluid)
     Disassociate the given fluid from any value, making it unbound.

 -- Scheme Procedure: fluid-bound? fluid
 -- C Function: scm_fluid_bound_p (fluid)
     Returns `#t' iff the given fluid is bound to a value, otherwise
     `#f'.

   `with-fluids*' temporarily changes the values of one or more fluids,
so that the given procedure and each procedure called by it access the
given values.  After the procedure returns, the old values are restored.

 -- Scheme Procedure: with-fluid* fluid value thunk
 -- C Function: scm_with_fluid (fluid, value, thunk)
     Set FLUID to VALUE temporarily, and call THUNK.  THUNK must be a
     procedure with no argument.

 -- Scheme Procedure: with-fluids* fluids values thunk
 -- C Function: scm_with_fluids (fluids, values, thunk)
     Set FLUIDS to VALUES temporary, and call THUNK.  FLUIDS must be a
     list of fluids and VALUES must be the same number of their values
     to be applied.  Each substitution is done in the order given.
     THUNK must be a procedure with no argument.  It is called inside a
     `dynamic-wind' and the fluids are set/restored when control enter
     or leaves the established dynamic extent.

 -- Scheme Macro: with-fluids ((fluid value) ...) body...
     Execute BODY... while each FLUID is set to the corresponding
     VALUE.  Both FLUID and VALUE are evaluated and FLUID must yield a
     fluid.  BODY... is executed inside a `dynamic-wind' and the fluids
     are set/restored when control enter or leaves the established
     dynamic extent.

 -- C Function: SCM scm_c_with_fluids (SCM fluids, SCM vals, SCM
          (*cproc)(void *), void *data)
 -- C Function: SCM scm_c_with_fluid (SCM fluid, SCM val, SCM
          (*cproc)(void *), void *data)
     The function `scm_c_with_fluids' is like `scm_with_fluids' except
     that it takes a C function to call instead of a Scheme thunk.

     The function `scm_c_with_fluid' is similar but only allows one
     fluid to be set instead of a list.

 -- C Function: void scm_dynwind_fluid (SCM fluid, SCM val)
     This function must be used inside a pair of calls to
     `scm_dynwind_begin' and `scm_dynwind_end' (*note Dynamic Wind::).
     During the dynwind context, the fluid FLUID is set to VAL.

     More precisely, the value of the fluid is swapped with a `backup'
     value whenever the dynwind context is entered or left.  The backup
     value is initialized with the VAL argument.

 -- Scheme Procedure: make-dynamic-state [parent]
 -- C Function: scm_make_dynamic_state (parent)
     Return a copy of the dynamic state object PARENT or of the current
     dynamic state when PARENT is omitted.

 -- Scheme Procedure: dynamic-state? obj
 -- C Function: scm_dynamic_state_p (obj)
     Return `#t' if OBJ is a dynamic state object; return `#f'
     otherwise.

 -- C Procedure: int scm_is_dynamic_state (SCM obj)
     Return non-zero if OBJ is a dynamic state object; return zero
     otherwise.

 -- Scheme Procedure: current-dynamic-state
 -- C Function: scm_current_dynamic_state ()
     Return the current dynamic state object.

 -- Scheme Procedure: set-current-dynamic-state state
 -- C Function: scm_set_current_dynamic_state (state)
     Set the current dynamic state object to STATE and return the
     previous current dynamic state object.

 -- Scheme Procedure: with-dynamic-state state proc
 -- C Function: scm_with_dynamic_state (state, proc)
     Call PROC while STATE is the current dynamic state object.

 -- C Procedure: void scm_dynwind_current_dynamic_state (SCM state)
     Set the current dynamic state to STATE for the current dynwind
     context.

 -- C Procedure: void * scm_c_with_dynamic_state (SCM state, void
          *(*func)(void *), void *data)
     Like `scm_with_dynamic_state', but call FUNC with DATA.

6.21.8 Parameters
-----------------

A parameter object is a procedure.  Calling it with no arguments returns
its value.  Calling it with one argument sets the value.

     (define my-param (make-parameter 123))
     (my-param) => 123
     (my-param 456)
     (my-param) => 456

   The `parameterize' special form establishes new locations for
parameters, those new locations having effect within the dynamic scope
of the `parameterize' body.  Leaving restores the previous locations.
Re-entering (through a saved continuation) will again use the new
locations.

     (parameterize ((my-param 789))
       (my-param)) => 789
     (my-param) => 456

   Parameters are like dynamically bound variables in other Lisp
dialects.  They allow an application to establish parameter settings
(as the name suggests) just for the execution of a particular bit of
code, restoring when done.  Examples of such parameters might be
case-sensitivity for a search, or a prompt for user input.

   Global variables are not as good as parameter objects for this sort
of thing.  Changes to them are visible to all threads, but in Guile
parameter object locations are per-thread, thereby truly limiting the
effect of `parameterize' to just its dynamic execution.

   Passing arguments to functions is thread-safe, but that soon becomes
tedious when there's more than a few or when they need to pass down
through several layers of calls before reaching the point they should
affect.  And introducing a new setting to existing code is often easier
with a parameter object than adding arguments.

 -- Function: make-parameter init [converter]
     Return a new parameter object, with initial value INIT.

     If a CONVERTER is given, then a call `(CONVERTER val)' is made for
     each value set, its return is the value stored.  Such a call is
     made for the INIT initial value too.

     A CONVERTER allows values to be validated, or put into a canonical
     form.  For example,

          (define my-param (make-parameter 123
                             (lambda (val)
                               (if (not (number? val))
                                   (error "must be a number"))
                               (inexact->exact val))))
          (my-param 0.75)
          (my-param) => 3/4

 -- Scheme Syntax: parameterize ((param value) ...) body ...
     Establish a new dynamic scope with the given PARAMs bound to new
     locations and set to the given VALUEs.  BODY is evaluated in that
     environment, the result is the return from the last form in BODY.

     Each PARAM is an expression which is evaluated to get the
     parameter object.  Often this will just be the name of a variable
     holding the object, but it can be anything that evaluates to a
     parameter.

     The PARAM expressions and VALUE expressions are all evaluated
     before establishing the new dynamic bindings, and they're
     evaluated in an unspecified order.

     For example,

          (define prompt (make-parameter "Type something: "))
          (define (get-input)
            (display (prompt))
            ...)

          (parameterize ((prompt "Type a number: "))
            (get-input)
            ...)

   Parameter objects are implemented using fluids (*note Fluids and
Dynamic States::), so each dynamic state has its own parameter
locations.  That includes the separate locations when outside any
`parameterize' form.  When a parameter is created it gets a separate
initial location in each dynamic state, all initialized to the given
INIT value.

   As alluded to above, because each thread usually has a separate
dynamic state, each thread has its own locations behind parameter
objects, and changes in one thread are not visible to any other.  When
a new dynamic state or thread is created, the values of parameters in
the originating context are copied, into new locations.

   Guile's parameters conform to SRFI-39 (*note SRFI-39::).

6.21.9 Futures
--------------

The `(ice-9 futures)' module provides "futures", a construct for
fine-grain parallelism.  A future is a wrapper around an expression
whose computation may occur in parallel with the code of the calling
thread, and possibly in parallel with other futures.  Like promises,
futures are essentially proxies that can be queried to obtain the value
of the enclosed expression:

     (touch (future (+ 2 3)))
     => 5

   However, unlike promises, the expression associated with a future
may be evaluated on another CPU core, should one be available.  This
supports "fine-grain parallelism", because even relatively small
computations can be embedded in futures.  Consider this sequential code:

     (define (find-prime lst1 lst2)
       (or (find prime? lst1)
           (find prime? lst2)))

   The two arms of `or' are potentially computation-intensive.  They
are independent of one another, yet, they are evaluated sequentially
when the first one returns `#f'.  Using futures, one could rewrite it
like this:

     (define (find-prime lst1 lst2)
       (let ((f (future (find prime? lst2))))
         (or (find prime? lst1)
             (touch f))))

   This preserves the semantics of `find-prime'.  On a multi-core
machine, though, the computation of `(find prime? lst2)' may be done in
parallel with that of the other `find' call, which can reduce the
execution time of `find-prime'.

   Note that futures are intended for the evaluation of purely
functional expressions.  Expressions that have side-effects or rely on
I/O may require additional care, such as explicit synchronization
(*note Mutexes and Condition Variables::).

   Guile's futures are implemented on top of POSIX threads (*note
Threads::).  Internally, a fixed-size pool of threads is used to
evaluate futures, such that offloading the evaluation of an expression
to another thread doesn't incur thread creation costs.  By default, the
pool contains one thread per available CPU core, minus one, to account
for the main thread.  The number of available CPU cores is determined
using `current-processor-count' (*note Processes::).

 -- Scheme Syntax: future exp
     Return a future for expression EXP.  This is equivalent to:

          (make-future (lambda () exp))

 -- Scheme Procedure: make-future thunk
     Return a future for THUNK, a zero-argument procedure.

     This procedure returns immediately.  Execution of THUNK may begin
     in parallel with the calling thread's computations, if idle CPU
     cores are available, or it may start when `touch' is invoked on the
     returned future.

     If the execution of THUNK throws an exception, that exception will
     be re-thrown when `touch' is invoked on the returned future.

 -- Scheme Procedure: future? obj
     Return `#t' if OBJ is a future.

 -- Scheme Procedure: touch f
     Return the result of the expression embedded in future F.

     If the result was already computed in parallel, `touch' returns
     instantaneously.  Otherwise, it waits for the computation to
     complete, if it already started, or initiates it.

6.21.10 Parallel forms
----------------------

The functions described in this section are available from

     (use-modules (ice-9 threads))

   They provide high-level parallel constructs.  The following functions
are implemented in terms of futures (*note Futures::).  Thus they are
relatively cheap as they re-use existing threads, and portable, since
they automatically use one thread per available CPU core.

 -- syntax: parallel expr1 ... exprN
     Evaluate each EXPR expression in parallel, each in its own thread.
     Return the results as a set of N multiple values (*note Multiple
     Values::).

 -- syntax: letpar ((var1 expr1) ... (varN exprN)) body...
     Evaluate each EXPR in parallel, each in its own thread, then bind
     the results to the corresponding VAR variables and evaluate BODY.

     `letpar' is like `let' (*note Local Bindings::), but all the
     expressions for the bindings are evaluated in parallel.

 -- Scheme Procedure: par-map proc lst1 ... lstN
 -- Scheme Procedure: par-for-each proc lst1 ... lstN
     Call PROC on the elements of the given lists.  `par-map' returns a
     list comprising the return values from PROC.  `par-for-each'
     returns an unspecified value, but waits for all calls to complete.

     The PROC calls are `(PROC ELEM1 ...  ELEMN)', where each ELEM is
     from the corresponding LST.  Each LST must be the same length.
     The calls are potentially made in parallel, depending on the
     number of CPU cores available.

     These functions are like `map' and `for-each' (*note List
     Mapping::), but make their PROC calls in parallel.

   Unlike those above, the functions described below take a number of
threads as an argument.  This makes them inherently non-portable since
the specified number of threads may differ from the number of available
CPU cores as returned by `current-processor-count' (*note Processes::).
In addition, these functions create the specified number of threads
when they are called and terminate them upon completion, which makes
them quite expensive.

   Therefore, they should be avoided.

 -- Scheme Procedure: n-par-map n proc lst1 ... lstN
 -- Scheme Procedure: n-par-for-each n proc lst1 ... lstN
     Call PROC on the elements of the given lists, in the same way as
     `par-map' and `par-for-each' above, but use no more than N threads
     at any one time.  The order in which calls are initiated within
     that threads limit is unspecified.

     These functions are good for controlling resource consumption if
     PROC calls might be costly, or if there are many to be made.  On a
     dual-CPU system for instance N=4 might be enough to keep the CPUs
     utilized, and not consume too much memory.

 -- Scheme Procedure: n-for-each-par-map n sproc pproc lst1 ... lstN
     Apply PPROC to the elements of the given lists, and apply SPROC to
     each result returned by PPROC.  The final return value is
     unspecified, but all calls will have been completed before
     returning.

     The calls made are `(SPROC (PPROC ELEM1 ...  ELEMN))', where each
     ELEM is from the corresponding LST.  Each LST must have the same
     number of elements.

     The PPROC calls are made in parallel, in separate threads.  No more
     than N threads are used at any one time.  The order in which PPROC
     calls are initiated within that limit is unspecified.

     The SPROC calls are made serially, in list element order, one at a
     time.  PPROC calls on later elements may execute in parallel with
     the SPROC calls.  Exactly which thread makes each SPROC call is
     unspecified.

     This function is designed for individual calculations that can be
     done in parallel, but with results needing to be handled serially,
     for instance to write them to a file.  The N limit on threads
     controls system resource usage when there are many calculations or
     when they might be costly.

     It will be seen that `n-for-each-par-map' is like a combination of
     `n-par-map' and `for-each',

          (for-each sproc (n-par-map n pproc lst1 ... lstN))

     But the actual implementation is more efficient since each SPROC
     call, in turn, can be initiated once the relevant PPROC call has
     completed, it doesn't need to wait for all to finish.

6.22 Configuration, Features and Runtime Options
================================================

Why is my Guile different from your Guile?  There are three kinds of
possible variation:

   * build differences -- different versions of the Guile source code,
     installation directories, configuration flags that control pieces
     of functionality being included or left out, etc.

   * differences in dynamically loaded code -- behaviour and features
     provided by modules that can be dynamically loaded into a running
     Guile

   * different runtime options -- some of the options that are provided
     for controlling Guile's behaviour may be set differently.

   Guile provides "introspective" variables and procedures to query all
of these possible variations at runtime.  For runtime options, it also
provides procedures to change the settings of options and to obtain
documentation on what the options mean.

6.22.1 Configuration, Build and Installation
--------------------------------------------

The following procedures and variables provide information about how
Guile was configured, built and installed on your system.

 -- Scheme Procedure: version
 -- Scheme Procedure: effective-version
 -- Scheme Procedure: major-version
 -- Scheme Procedure: minor-version
 -- Scheme Procedure: micro-version
 -- C Function: scm_version ()
 -- C Function: scm_effective_version ()
 -- C Function: scm_major_version ()
 -- C Function: scm_minor_version ()
 -- C Function: scm_micro_version ()
     Return a string describing Guile's full version number, effective
     version number, major, minor or micro version number, respectively.
     The `effective-version' function returns the version name that
     should remain unchanged during a stable series.  Currently that
     means that it omits the micro version.  The effective version
     should be used for items like the versioned share directory name
     i.e. `/usr/share/guile/2.0/'

          (version) => "2.0.4"
          (effective-version) => "2.0"
          (major-version) => "2"
          (minor-version) => "0"
          (micro-version) => "4"

 -- Scheme Procedure: %package-data-dir
 -- C Function: scm_sys_package_data_dir ()
     Return the name of the directory under which Guile Scheme files in
     general are stored.  On Unix-like systems, this is usually
     `/usr/local/share/guile' or `/usr/share/guile'.

 -- Scheme Procedure: %library-dir
 -- C Function: scm_sys_library_dir ()
     Return the name of the directory where the Guile Scheme files that
     belong to the core Guile installation (as opposed to files from a
     3rd party package) are installed.  On Unix-like systems this is
     usually `/usr/local/share/guile/GUILE_EFFECTIVE_VERSION' or
     `/usr/share/guile/GUILE_EFFECTIVE_VERSION';

     for example `/usr/local/share/guile/2.0'.

 -- Scheme Procedure: %site-dir
 -- C Function: scm_sys_site_dir ()
     Return the name of the directory where Guile Scheme files specific
     to your site should be installed.  On Unix-like systems, this is
     usually `/usr/local/share/guile/site' or `/usr/share/guile/site'.

 -- Variable: %guile-build-info
     Alist of information collected during the building of a particular
     Guile.  Entries can be grouped into one of several categories:
     directories, env vars, and versioning info.

     Briefly, here are the keys in `%guile-build-info', by group:

    directories
          srcdir, top_srcdir, prefix, exec_prefix, bindir, sbindir,
          libexecdir, datadir, sysconfdir, sharedstatedir,
          localstatedir, libdir, infodir, mandir, includedir,
          pkgdatadir, pkglibdir, pkgincludedir 

    env vars
          LIBS 

    versioning info
          guileversion, libguileinterface, buildstamp

     Values are all strings.  The value for `LIBS' is typically found
     also as a part of `pkg-config --libs guile-2.0' output.  The value
     for `guileversion' has form X.Y.Z, and should be the same as
     returned by `(version)'.  The value for `libguileinterface' is
     libtool compatible and has form CURRENT:REVISION:AGE (*note
     Library interface versions: (libtool)Versioning.).  The value for
     `buildstamp' is the output of the command `date -u +'%Y-%m-%d %T''
     (UTC).

     In the source, `%guile-build-info' is initialized from
     libguile/libpath.h, which is completely generated, so deleting
     this file before a build guarantees up-to-date values for that
     build.

 -- Variable: %host-type
     The canonical host type (GNU triplet) of the host Guile was
     configured for, e.g., `"x86_64-unknown-linux-gnu"' (*note
     Canonicalizing: (autoconf)Canonicalizing.).

6.22.2 Feature Tracking
-----------------------

Guile has a Scheme level variable `*features*' that keeps track to some
extent of the features that are available in a running Guile.
`*features*' is a list of symbols, for example `threads', each of which
describes a feature of the running Guile process.

 -- Variable: *features*
     A list of symbols describing available features of the Guile
     process.

   You shouldn't modify the `*features*' variable directly using
`set!'.  Instead, see the procedures that are provided for this purpose
in the following subsection.

6.22.2.1 Feature Manipulation
.............................

To check whether a particular feature is available, use the `provided?'
procedure:

 -- Scheme Procedure: provided? feature
 -- Deprecated Scheme Procedure: feature? feature
     Return `#t' if the specified FEATURE is available, otherwise `#f'.

   To advertise a feature from your own Scheme code, you can use the
`provide' procedure:

 -- Scheme Procedure: provide feature
     Add FEATURE to the list of available features in this Guile
     process.

   For C code, the equivalent function takes its feature name as a
`char *' argument for convenience:

 -- C Function: void scm_add_feature (const char *str)
     Add a symbol with name STR to the list of available features in
     this Guile process.

6.22.2.2 Common Feature Symbols
...............................

In general, a particular feature may be available for one of two
reasons.  Either because the Guile library was configured and compiled
with that feature enabled -- i.e. the feature is built into the library
on your system.  Or because some C or Scheme code that was dynamically
loaded by Guile has added that feature to the list.

   In the first category, here are the features that the current
version of Guile may define (depending on how it is built), and what
they mean.

`array'
     Indicates support for arrays (*note Arrays::).

`array-for-each'
     Indicates availability of `array-for-each' and other array mapping
     procedures (*note Arrays::).

`char-ready?'
     Indicates that the `char-ready?' function is available (*note
     Reading::).

`complex'
     Indicates support for complex numbers.

`current-time'
     Indicates availability of time-related functions: `times',
     `get-internal-run-time' and so on (*note Time::).

`debug-extensions'
     Indicates that the debugging evaluator is available, together with
     the options for controlling it.

`delay'
     Indicates support for promises (*note Delayed Evaluation::).

`EIDs'
     Indicates that the `geteuid' and `getegid' really return effective
     user and group IDs (*note Processes::).

`inexact'
     Indicates support for inexact numbers.

`i/o-extensions'
     Indicates availability of the following extended I/O procedures:
     `ftell', `redirect-port', `dup->fdes', `dup2', `fileno',
     `isatty?', `fdopen', `primitive-move->fdes' and `fdes->ports'
     (*note Ports and File Descriptors::).

`net-db'
     Indicates availability of network database functions:
     `scm_gethost', `scm_getnet', `scm_getproto', `scm_getserv',
     `scm_sethost', `scm_setnet', `scm_setproto', `scm_setserv', and
     their `byXXX' variants (*note Network Databases::).

`posix'
     Indicates support for POSIX functions: `pipe', `getgroups',
     `kill', `execl' and so on (*note POSIX::).

`random'
     Indicates availability of random number generation functions:
     `random', `copy-random-state', `random-uniform' and so on (*note
     Random::).

`reckless'
     Indicates that Guile was built with important checks omitted -- you
     should never see this!

`regex'
     Indicates support for POSIX regular expressions using
     `make-regexp', `regexp-exec' and friends (*note Regexp
     Functions::).

`socket'
     Indicates availability of socket-related functions: `socket',
     `bind', `connect' and so on (*note Network Sockets and
     Communication::).

`sort'
     Indicates availability of sorting and merging functions (*note
     Sorting::).

`system'
     Indicates that the `system' function is available (*note
     Processes::).

`threads'
     Indicates support for multithreading (*note Threads::).

`values'
     Indicates support for multiple return values using `values' and
     `call-with-values' (*note Multiple Values::).

   Available features in the second category depend, by definition, on
what additional code your Guile process has loaded in.  The following
table lists features that you might encounter for this reason.

`defmacro'
     Indicates that the `defmacro' macro is available (*note Macros::).

`describe'
     Indicates that the `(oop goops describe)' module has been loaded,
     which provides a procedure for describing the contents of GOOPS
     instances.

`readline'
     Indicates that Guile has loaded in Readline support, for command
     line editing (*note Readline Support::).

`record'
     Indicates support for record definition using `make-record-type'
     and friends (*note Records::).

   Although these tables may seem exhaustive, it is probably unwise in
practice to rely on them, as the correspondences between feature symbols
and available procedures/behaviour are not strictly defined.  If you are
writing code that needs to check for the existence of some procedure, it
is probably safer to do so directly using the `defined?' procedure than
to test for the corresponding feature using `provided?'.

6.22.3 Runtime Options
----------------------

There are a number of runtime options available for paramaterizing
built-in procedures, like `read', and built-in behavior, like what
happens on an uncaught error.

   For more information on reader options, *Note Scheme Read::.

   For more information on print options, *Note Scheme Write::.

   Finally, for more information on debugger options, *Note Debug
Options::.

6.22.3.1 Examples of option use
...............................

Here is an example of a session in which some read and debug option
handling procedures are used.  In this example, the user

  1. Notices that the symbols `abc' and `aBc' are not the same

  2. Examines the `read-options', and sees that `case-insensitive' is
     set to "no".

  3. Enables `case-insensitive'

  4. Quits the recursive prompt

  5. Verifies that now `aBc' and `abc' are the same

     scheme@(guile-user)> (define abc "hello")
     scheme@(guile-user)> abc
     $1 = "hello"
     scheme@(guile-user)> aBc
     <unknown-location>: warning: possibly unbound variable `aBc'
     ERROR: In procedure module-lookup:
     ERROR: Unbound variable: aBc
     Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
     scheme@(guile-user) [1]> (read-options 'help)
     copy              no    Copy source code expressions.
     positions         yes   Record positions of source code expressions.
     case-insensitive  no    Convert symbols to lower case.
     keywords          #f    Style of keyword recognition: #f, 'prefix or 'postfix.
     r6rs-hex-escapes  no    Use R6RS variable-length character and string hex escapes.
     square-brackets   yes   Treat `[' and `]' as parentheses, for R6RS compatibility.
     hungry-eol-escapes no   In strings, consume leading whitespace after an
                             escaped end-of-line.
     scheme@(guile-user) [1]> (read-enable 'case-insensitive)
     $2 = (square-brackets keywords #f case-insensitive positions)
     scheme@(guile-user) [1]> ,q
     scheme@(guile-user)> aBc
     $3 = "hello"

6.23 Support for Other Languages
================================

In addition to Scheme, a user may write a Guile program in an increasing
number of other languages. Currently supported languages include Emacs
Lisp and ECMAScript.

   Guile is still fundamentally a Scheme, but it tries to support a wide
variety of language building-blocks, so that other languages can be
implemented on top of Guile. This allows users to write or extend
applications in languages other than Scheme, too. This section describes
the languages that have been implemented.

   (For details on how to implement a language, *Note Compiling to the
Virtual Machine::.)

6.23.1 Using Other Languages
----------------------------

There are currently only two ways to access other languages from within
Guile: at the REPL, and programmatically, via `compile',
`read-and-compile', and `compile-file'.

   The REPL is Guile's command prompt (*note Using Guile
Interactively::).  The REPL has a concept of the "current language",
which defaults to Scheme. The user may change that language, via the
meta-command `,language'.

   For example, the following meta-command enables Emacs Lisp input:

     scheme@(guile-user)> ,language elisp
     Happy hacking with Emacs Lisp!  To switch back, type `,L scheme'.
     elisp@(guile-user)> (eq 1 2)
     $1 = #nil

   Each language has its short name: for example, `elisp', for Elisp.
The same short name may be used to compile source code programmatically,
via `compile':

     elisp@(guile-user)> ,L scheme
     Happy hacking with Guile Scheme!  To switch back, type `,L elisp'.
     scheme@(guile-user)> (compile '(eq 1 2) #:from 'elisp)
     $2 = #nil

   Granted, as the input to `compile' is a datum, this works best for
Lispy languages, which have a straightforward datum representation.
Other languages that need more parsing are better dealt with as strings.

   The easiest way to deal with syntax-heavy language is with files, via
`compile-file' and friends. However it is possible to invoke a
language's reader on a port, and then compile the resulting expression
(which is a datum at that point). For more information, *Note
Compilation::.

   For more details on introspecting aspects of different languages,
*Note Compiler Tower::.

6.23.2 Emacs Lisp
-----------------

Emacs Lisp (Elisp) is a dynamically-scoped Lisp dialect used in the
Emacs editor. *Note Overview: (elisp)top, for more information on Emacs
Lisp.

   We hope that eventually Guile's implementation of Elisp will be good
enough to replace Emacs' own implementation of Elisp. For that reason,
we have thought long and hard about how to support the various features
of Elisp in a performant and compatible manner.

   Readers familiar with Emacs Lisp might be curious about how exactly
these various Elisp features are supported in Guile. The rest of this
section focuses on addressing these concerns of the Elisp elect.

6.23.2.1 Nil
............

`nil' in ELisp is an amalgam of Scheme's `#f' and `'()'.  It is false,
and it is the end-of-list; thus it is a boolean, and a list as well.

   Guile has chosen to support `nil' as a separate value, distinct from
`#f' and `'()'. This allows existing Scheme and Elisp code to maintain
their current semantics. `nil', which in Elisp would just be written
and read as `nil', in Scheme has the external representation `#nil'.

   This decision to have `nil' as a low-level distinct value
facilitates interoperability between the two languages. Guile has chosen
to have Scheme deal with `nil' as follows:

     (boolean? #nil) => #t
     (not #nil) => #t
     (null? #nil) => #t

   And in C, one has:

     scm_is_bool (SCM_ELISP_NIL) => 1
     scm_is_false (SCM_ELISP_NIL) => 1
     scm_is_null (SCM_ELISP_NIL) => 1

   In this way, a version of `fold' written in Scheme can correctly
fold a function written in Elisp (or in fact any other language) over a
nil-terminated list, as Elisp makes. The converse holds as well; a
version of `fold' written in Elisp can fold over a `'()'-terminated
list, as made by Scheme.

   On a low level, the bit representations for `#f', `#t', `nil', and
`'()' are made in such a way that they differ by only one bit, and so a
test for, for example, `#f'-or-`nil' may be made very efficiently. See
`libguile/boolean.h', for more information.

6.23.2.2 Equality
.................

Since Scheme's `equal?' must be transitive, and `'()' is not `equal?'
to `#f', to Scheme `nil' is not `equal?' to `#f' or `'()'.

     (eq? #f '()) => #f
     (eq? #nil '()) => #f
     (eq? #nil #f) => #f
     (eqv? #f '()) => #f
     (eqv? #nil '()) => #f
     (eqv? #nil #f) => #f
     (equal? #f '()) => #f
     (equal? #nil '()) => #f
     (equal? #nil #f) => #f

   However, in Elisp, `'()', `#f', and `nil' are all `equal' (though
not `eq').

     (defvar f (make-scheme-false))
     (defvar eol (make-scheme-null))
     (eq f eol) => nil
     (eq nil eol) => nil
     (eq nil f) => nil
     (equal f eol) => t
     (equal nil eol) => t
     (equal nil f) => t

   These choices facilitate interoperability between Elisp and Scheme
code, but they are not perfect. Some code that is correct standard
Scheme is not correct in the presence of a second false and null value.
For example:

     (define (truthiness x)
       (if (eq? x #f)
           #f
           #t))

   This code seems to be meant to test a value for truth, but now that
there are two false values, `#f' and `nil', it is no longer correct.

   Similarly, there is the loop:

     (define (my-length l)
       (let lp ((l l) (len 0))
         (if (eq? l '())
             len
             (lp (cdr l) (1+ len)))))

   Here, `my-length' will raise an error if L is a `nil'-terminated
list.

   Both of these examples are correct standard Scheme, but, depending on
what they really want to do, they are not correct Guile Scheme.
Correctly written, they would test the _properties_ of falsehood or
nullity, not the individual members of that set. That is to say, they
should use `not' or `null?' to test for falsehood or nullity, not `eq?'
or `memv' or the like.

   Fortunately, using `not' and `null?' is in good style, so all
well-written standard Scheme programs are correct, in Guile Scheme.

   Here are correct versions of the above examples:

     (define (truthiness* x)
       (if (not x)
           #f
           #t))
     ;; or: (define (t* x) (not (not x)))
     ;; or: (define (t** x) x)

     (define (my-length* l)
       (let lp ((l l) (len 0))
         (if (null? l)
             len
             (lp (cdr l) (1+ len)))))

   This problem has a mirror-image case in Elisp:

     (deffn my-falsep (x)
       (if (eq x nil)
           t
           nil))

   Guile can warn when compiling code that has equality comparisons with
`#f', `'()', or `nil'. *Note Compilation::, for details.

6.23.2.3 Dynamic Binding
........................

In contrast to Scheme, which uses "lexical scoping", Emacs Lisp scopes
its variables dynamically. Guile supports dynamic scoping with its
"fluids" facility. *Note Fluids and Dynamic States::, for more
information.

6.23.2.4 Other Elisp Features
.............................

Buffer-local and mode-local variables should be mentioned here, along
with buckybits on characters, Emacs primitive data types, the
Lisp-2-ness of Elisp, and other things. Contributions to the
documentation are most welcome!

6.23.3 ECMAScript
-----------------

ECMAScript
(http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf)
was not the first non-Schemey language implemented by Guile, but it was
the first implemented for Guile's bytecode compiler. The goal was to
support ECMAScript version 3.1, a relatively small language, but the
implementor was completely irresponsible and got distracted by other
things before finishing the standard library, and even some bits of the
syntax. So, ECMAScript does deserve a mention in the manual, but it
doesn't deserve an endorsement until its implementation is completed,
perhaps by some more responsible hacker.

   In the meantime, the charitable user might investigate such
invocations as `,L ecmascript' and `cat
test-suite/tests/ecmascript.test'.

6.24 Support for Internationalization
=====================================

Guile provides internationalization(1) support for Scheme programs in
two ways.  First, procedures to manipulate text and data in a way that
conforms to particular cultural conventions (i.e., in a
"locale-dependent" way) are provided in the `(ice-9 i18n)'.  Second,
Guile allows the use of GNU `gettext' to translate program message
strings.

   ---------- Footnotes ----------

   (1) For concision and style, programmers often like to refer to
internationalization as "i18n".

6.24.1 Internationalization with Guile
--------------------------------------

In order to make use of the functions described thereafter, the `(ice-9
i18n)' module must be imported in the usual way:

     (use-modules (ice-9 i18n))

   The `(ice-9 i18n)' module provides procedures to manipulate text and
other data in a way that conforms to the cultural conventions chosen by
the user.  Each region of the world or language has its own customs to,
for instance, represent real numbers, classify characters, collate
text, etc.  All these aspects comprise the so-called "cultural
conventions" of that region or language.

   Computer systems typically refer to a set of cultural conventions as
a "locale".  For each particular aspect that comprise those cultural
conventions, a "locale category" is defined.  For instance, the way
characters are classified is defined by the `LC_CTYPE' category, while
the language in which program messages are issued to the user is
defined by the `LC_MESSAGES' category (*note General Locale
Information: Locales. for details).

   The procedures provided by this module allow the development of
programs that adapt automatically to any locale setting.  As we will
see later, many of these procedures can optionally take a "locale
object" argument.  This additional argument defines the locale settings
that must be followed by the invoked procedure.  When it is omitted,
then the current locale settings of the process are followed (*note
`setlocale': Locales.).

   The following procedures allow the manipulation of such locale
objects.

 -- Scheme Procedure: make-locale category-list locale-name
          [base-locale]
 -- C Function: scm_make_locale (category_list, locale_name,
          base_locale)
     Return a reference to a data structure representing a set of locale
     datasets.  LOCALE-NAME should be a string denoting a particular
     locale (e.g., `"aa_DJ"') and CATEGORY-LIST should be either a list
     of locale categories or a single category as used with `setlocale'
     (*note `setlocale': Locales.).  Optionally, if `base-locale' is
     passed, it should be a locale object denoting settings for
     categories not listed in CATEGORY-LIST.

     The following invocation creates a locale object that combines the
     use of Swedish for messages and character classification with the
     default settings for the other categories (i.e., the settings of
     the default `C' locale which usually represents conventions in use
     in the USA):

          (make-locale (list LC_MESSAGE LC_CTYPE) "sv_SE")

     The following example combines the use of Esperanto messages and
     conventions with monetary conventions from Croatia:

          (make-locale LC_MONETARY "hr_HR"
                       (make-locale LC_ALL "eo_EO"))

     A `system-error' exception (*note Handling Errors::) is raised by
     `make-locale' when LOCALE-NAME does not match any of the locales
     compiled on the system.  Note that on non-GNU systems, this error
     may be raised later, when the locale object is actually used.


 -- Scheme Procedure: locale? obj
 -- C Function: scm_locale_p (obj)
     Return true if OBJ is a locale object.

 -- Scheme Variable: %global-locale
 -- C Variable: scm_global_locale
     This variable is bound to a locale object denoting the current
     process locale as installed using `setlocale ()' (*note
     Locales::).  It may be used like any other locale object,
     including as a third argument to `make-locale', for instance.

6.24.2 Text Collation
---------------------

The following procedures provide support for text collation, i.e.,
locale-dependent string and character sorting.

 -- Scheme Procedure: string-locale<? s1 s2 [locale]
 -- C Function: scm_string_locale_lt (s1, s2, locale)
 -- Scheme Procedure: string-locale>? s1 s2 [locale]
 -- C Function: scm_string_locale_gt (s1, s2, locale)
 -- Scheme Procedure: string-locale-ci<? s1 s2 [locale]
 -- C Function: scm_string_locale_ci_lt (s1, s2, locale)
 -- Scheme Procedure: string-locale-ci>? s1 s2 [locale]
 -- C Function: scm_string_locale_ci_gt (s1, s2, locale)
     Compare strings S1 and S2 in a locale-dependent way.  If LOCALE is
     provided, it should be locale object (as returned by
     `make-locale') and will be used to perform the comparison;
     otherwise, the current system locale is used.  For the `-ci'
     variants, the comparison is made in a case-insensitive way.

 -- Scheme Procedure: string-locale-ci=? s1 s2 [locale]
 -- C Function: scm_string_locale_ci_eq (s1, s2, locale)
     Compare strings S1 and S2 in a case-insensitive, and
     locale-dependent way.  If LOCALE is provided, it should be a
     locale object (as returned by `make-locale') and will be used to
     perform the comparison; otherwise, the current system locale is
     used.

 -- Scheme Procedure: char-locale<? c1 c2 [locale]
 -- C Function: scm_char_locale_lt (c1, c2, locale)
 -- Scheme Procedure: char-locale>? c1 c2 [locale]
 -- C Function: scm_char_locale_gt (c1, c2, locale)
 -- Scheme Procedure: char-locale-ci<? c1 c2 [locale]
 -- C Function: scm_char_locale_ci_lt (c1, c2, locale)
 -- Scheme Procedure: char-locale-ci>? c1 c2 [locale]
 -- C Function: scm_char_locale_ci_gt (c1, c2, locale)
     Compare characters C1 and C2 according to either LOCALE (a locale
     object as returned by `make-locale') or the current locale.  For
     the `-ci' variants, the comparison is made in a case-insensitive
     way.

 -- Scheme Procedure: char-locale-ci=? c1 c2 [locale]
 -- C Function: scm_char_locale_ci_eq (c1, c2, locale)
     Return true if character C1 is equal to C2, in a case insensitive
     way according to LOCALE or to the current locale.

6.24.3 Character Case Mapping
-----------------------------

The procedures below provide support for "character case mapping",
i.e., to convert characters or strings to their upper-case or
lower-case equivalent.  Note that SRFI-13 provides procedures that look
similar (*note Alphabetic Case Mapping::).  However, the SRFI-13
procedures are locale-independent.  Therefore, they do not take into
account specificities of the customs in use in a particular language or
region of the world.  For instance, while most languages using the
Latin alphabet map lower-case letter "i" to upper-case letter "I",
Turkish maps lower-case "i" to "Latin capital letter I with dot above".
The following procedures allow programmers to provide idiomatic
character mapping.

 -- Scheme Procedure: char-locale-downcase chr [locale]
 -- C Function: scm_char_locale_upcase (chr, locale)
     Return the lowercase character that corresponds to CHR according
     to either LOCALE or the current locale.

 -- Scheme Procedure: char-locale-upcase chr [locale]
 -- C Function: scm_char_locale_downcase (chr, locale)
     Return the uppercase character that corresponds to CHR according
     to either LOCALE or the current locale.

 -- Scheme Procedure: char-locale-titlecase chr [locale]
 -- C Function: scm_char_locale_titlecase (chr, locale)
     Return the titlecase character that corresponds to CHR according
     to either LOCALE or the current locale.

 -- Scheme Procedure: string-locale-upcase str [locale]
 -- C Function: scm_string_locale_upcase (str, locale)
     Return a new string that is the uppercase version of STR according
     to either LOCALE or the current locale.

 -- Scheme Procedure: string-locale-downcase str [locale]
 -- C Function: scm_string_locale_downcase (str, locale)
     Return a new string that is the down-case version of STR according
     to either LOCALE or the current locale.

 -- Scheme Procedure: string-locale-titlecase str [locale]
 -- C Function: scm_string_locale_titlecase (str, locale)
     Return a new string that is the titlecase version of STR according
     to either LOCALE or the current locale.

6.24.4 Number Input and Output
------------------------------

The following procedures allow programs to read and write numbers
written according to a particular locale.  As an example, in English,
"ten thousand and a half" is usually written `10,000.5' while in French
it is written `10 000,5'.  These procedures allow such differences to
be taken into account.

 -- Scheme Procedure: locale-string->integer str [base [locale]]
 -- C Function: scm_locale_string_to_integer (str, base, locale)
     Convert string STR into an integer according to either LOCALE (a
     locale object as returned by `make-locale') or the current process
     locale.  If BASE is specified, then it determines the base of the
     integer being read (e.g., `16' for an hexadecimal number, `10' for
     a decimal number); by default, decimal numbers are read.  Return
     two values (*note Multiple Values::): an integer (on success) or
     `#f', and the number of characters read from STR (`0' on failure).

     This function is based on the C library's `strtol' function (*note
     `strtol': (libc)Parsing of Integers.).

 -- Scheme Procedure: locale-string->inexact str [locale]
 -- C Function: scm_locale_string_to_inexact (str, locale)
     Convert string STR into an inexact number according to either
     LOCALE (a locale object as returned by `make-locale') or the
     current process locale.  Return two values (*note Multiple
     Values::): an inexact number (on success) or `#f', and the number
     of characters read from STR (`0' on failure).

     This function is based on the C library's `strtod' function (*note
     `strtod': (libc)Parsing of Floats.).

 -- Scheme Procedure: number->locale-string number [fraction-digits
          [locale]]
     Convert NUMBER (an inexact) into a string according to the
     cultural conventions of either LOCALE (a locale object) or the
     current locale.  Optionally, FRACTION-DIGITS may be bound to an
     integer specifying the number of fractional digits to be displayed.

 -- Scheme Procedure: monetary-amount->locale-string amount intl?
          [locale]
     Convert AMOUNT (an inexact denoting a monetary amount) into a
     string according to the cultural conventions of either LOCALE (a
     locale object) or the current locale.  If INTL? is true, then the
     international monetary format for the given locale is used (*note
     international and locale monetary formats: (libc)Currency Symbol.).

6.24.5 Accessing Locale Information
-----------------------------------

It is sometimes useful to obtain very specific information about a
locale such as the word it uses for days or months, its format for
representing floating-point figures, etc.  The `(ice-9 i18n)' module
provides support for this in a way that is similar to the libc
functions `nl_langinfo ()' and `localeconv ()' (*note accessing locale
information from C: (libc)Locale Information.).  The available functions
are listed below.

 -- Scheme Procedure: locale-encoding [locale]
     Return the name of the encoding (a string whose interpretation is
     system-dependent) of either LOCALE or the current locale.

   The following functions deal with dates and times.

 -- Scheme Procedure: locale-day day [locale]
 -- Scheme Procedure: locale-day-short day [locale]
 -- Scheme Procedure: locale-month month [locale]
 -- Scheme Procedure: locale-month-short month [locale]
     Return the word (a string) used in either LOCALE or the current
     locale to name the day (or month) denoted by DAY (or MONTH), an
     integer between 1 and 7 (or 1 and 12).  The `-short' variants
     provide an abbreviation instead of a full name.

 -- Scheme Procedure: locale-am-string [locale]
 -- Scheme Procedure: locale-pm-string [locale]
     Return a (potentially empty) string that is used to denote ante
     meridiem (or post meridiem) hours in 12-hour format.

 -- Scheme Procedure: locale-date+time-format [locale]
 -- Scheme Procedure: locale-date-format [locale]
 -- Scheme Procedure: locale-time-format [locale]
 -- Scheme Procedure: locale-time+am/pm-format [locale]
 -- Scheme Procedure: locale-era-date-format [locale]
 -- Scheme Procedure: locale-era-date+time-format [locale]
 -- Scheme Procedure: locale-era-time-format [locale]
     These procedures return format strings suitable to `strftime'
     (*note Time::) that may be used to display (part of) a date/time
     according to certain constraints and to the conventions of either
     LOCALE or the current locale (*note the `nl_langinfo ()' items:
     (libc)The Elegant and Fast Way.).

 -- Scheme Procedure: locale-era [locale]
 -- Scheme Procedure: locale-era-year [locale]
     These functions return, respectively, the era and the year of the
     relevant era used in LOCALE or the current locale.  Most locales
     do not define this value.  In this case, the empty string is
     returned.  An example of a locale that does define this value is
     the Japanese one.

   The following procedures give information about number
representation.

 -- Scheme Procedure: locale-decimal-point [locale]
 -- Scheme Procedure: locale-thousands-separator [locale]
     These functions return a string denoting the representation of the
     decimal point or that of the thousand separator (respectively) for
     either LOCALE or the current locale.

 -- Scheme Procedure: locale-digit-grouping [locale]
     Return a (potentially circular) list of integers denoting how
     digits of the integer part of a number are to be grouped, starting
     at the decimal point and going to the left.  The list contains
     integers indicating the size of the successive groups, from right
     to left.  If the list is non-circular, then no grouping occurs for
     digits beyond the last group.

     For instance, if the returned list is a circular list that contains
     only `3' and the thousand separator is `","' (as is the case with
     English locales), then the number `12345678' should be printed
     `12,345,678'.

   The following procedures deal with the representation of monetary
amounts.  Some of them take an additional INTL? argument (a boolean)
that tells whether the international or local monetary conventions for
the given locale are to be used.

 -- Scheme Procedure: locale-monetary-decimal-point [locale]
 -- Scheme Procedure: locale-monetary-thousands-separator [locale]
 -- Scheme Procedure: locale-monetary-grouping [locale]
     These are the monetary counterparts of the above procedures.  These
     procedures apply to monetary amounts.

 -- Scheme Procedure: locale-currency-symbol intl? [locale]
     Return the currency symbol (a string) of either LOCALE or the
     current locale.

     The following example illustrates the difference between the local
     and international monetary formats:

          (define us (make-locale LC_MONETARY "en_US"))
          (locale-currency-symbol #f us)
          => "-$"
          (locale-currency-symbol #t us)
          => "USD "

 -- Scheme Procedure: locale-monetary-fractional-digits intl? [locale]
     Return the number of fractional digits to be used when printing
     monetary amounts according to either LOCALE or the current locale.
     If the locale does not specify it, then `#f' is returned.

 -- Scheme Procedure: locale-currency-symbol-precedes-positive? intl?
          [locale]
 -- Scheme Procedure: locale-currency-symbol-precedes-negative? intl?
          [locale]
 -- Scheme Procedure: locale-positive-separated-by-space? intl? [locale]
 -- Scheme Procedure: locale-negative-separated-by-space? intl? [locale]
     These procedures return a boolean indicating whether the currency
     symbol should precede a positive/negative number, and whether a
     whitespace should be inserted between the currency symbol and a
     positive/negative amount.

 -- Scheme Procedure: locale-monetary-positive-sign [locale]
 -- Scheme Procedure: locale-monetary-negative-sign [locale]
     Return a string denoting the positive (respectively negative) sign
     that should be used when printing a monetary amount.

 -- Scheme Procedure: locale-positive-sign-position
 -- Scheme Procedure: locale-negative-sign-position
     These functions return a symbol telling where a sign of a
     positive/negative monetary amount is to appear when printing it.
     The possible values are:

    `parenthesize'
          The currency symbol and quantity should be surrounded by
          parentheses.

    `sign-before'
          Print the sign string before the quantity and currency symbol.

    `sign-after'
          Print the sign string after the quantity and currency symbol.

    `sign-before-currency-symbol'
          Print the sign string right before the currency symbol.

    `sign-after-currency-symbol'
          Print the sign string right after the currency symbol.

    `unspecified'
          Unspecified.  We recommend you print the sign after the
          currency symbol.


   Finally, the two following procedures may be helpful when programming
user interfaces:

 -- Scheme Procedure: locale-yes-regexp [locale]
 -- Scheme Procedure: locale-no-regexp [locale]
     Return a string that can be used as a regular expression to
     recognize a positive (respectively, negative) response to a yes/no
     question.  For the C locale, the default values are typically
     `"^[yY]"' and `"^[nN]"', respectively.

     Here is an example:

          (use-modules (ice-9 rdelim))
          (format #t "Does Guile rock?~%")
          (let lp ((answer (read-line)))
            (cond ((string-match (locale-yes-regexp) answer)
                   (format #t "High fives!~%"))
                  ((string-match (locale-no-regexp) answer)
                   (format #t "How about now? Does it rock yet?~%")
                   (lp (read-line)))
                  (else
                   (format #t "What do you mean?~%")
                   (lp (read-line)))))

     For an internationalized yes/no string output, `gettext' should be
     used (*note Gettext Support::).

   Example uses of some of these functions are the implementation of the
`number->locale-string' and `monetary-amount->locale-string' procedures
(*note Number Input and Output::), as well as that the SRFI-19 date and
time conversion to/from strings (*note SRFI-19::).

6.24.6 Gettext Support
----------------------

Guile provides an interface to GNU `gettext' for translating message
strings (*note Introduction: (gettext)Introduction.).

   Messages are collected in domains, so different libraries and
programs maintain different message catalogues.  The DOMAIN parameter in
the functions below is a string (it becomes part of the message catalog
filename).

   When `gettext' is not available, or if Guile was configured
`--without-nls', dummy functions doing no translation are provided.
When `gettext' support is available in Guile, the `i18n' feature is
provided (*note Feature Tracking::).

 -- Scheme Procedure: gettext msg [domain [category]]
 -- C Function: scm_gettext (msg, domain, category)
     Return the translation of MSG in DOMAIN.  DOMAIN is optional and
     defaults to the domain set through `textdomain' below.  CATEGORY
     is optional and defaults to `LC_MESSAGES' (*note Locales::).

     Normal usage is for MSG to be a literal string.  `xgettext' can
     extract those from the source to form a message catalogue ready
     for translators (*note Invoking the `xgettext' Program:
     (gettext)xgettext Invocation.).

          (display (gettext "You are in a maze of twisty passages."))

     `_' is a commonly used shorthand, an application can make that an
     alias for `gettext'.  Or a library can make a definition that uses
     its specific DOMAIN (so an application can change the default
     without affecting the library).

          (define (_ msg) (gettext msg "mylibrary"))
          (display (_ "File not found."))

     `_' is also a good place to perhaps strip disambiguating extra
     text from the message string, as for instance in *note How to use
     `gettext' in GUI programs: (gettext)GUI program problems.

 -- Scheme Procedure: ngettext msg msgplural n [domain [category]]
 -- C Function: scm_ngettext (msg, msgplural, n, domain, category)
     Return the translation of MSG/MSGPLURAL in DOMAIN, with a plural
     form chosen appropriately for the number N.  DOMAIN is optional
     and defaults to the domain set through `textdomain' below.
     CATEGORY is optional and defaults to `LC_MESSAGES' (*note
     Locales::).

     MSG is the singular form, and MSGPLURAL the plural.  When no
     translation is available, MSG is used if N = 1, or MSGPLURAL
     otherwise.  When translated, the message catalogue can have a
     different rule, and can have more than two possible forms.

     As per `gettext' above, normal usage is for MSG and MSGPLURAL to
     be literal strings, since `xgettext' can extract them from the
     source to build a message catalogue.  For example,

          (define (done n)
            (format #t (ngettext "~a file processed\n"
                                 "~a files processed\n" n)
                       n))

          (done 1) -| 1 file processed
          (done 3) -| 3 files processed

     It's important to use `ngettext' rather than plain `gettext' for
     plurals, since the rules for singular and plural forms in English
     are not the same in other languages.  Only `ngettext' will allow
     translators to give correct forms (*note Additional functions for
     plural forms: (gettext)Plural forms.).

 -- Scheme Procedure: textdomain [domain]
 -- C Function: scm_textdomain (domain)
     Get or set the default gettext domain.  When called with no
     parameter the current domain is returned.  When called with a
     parameter, DOMAIN is set as the current domain, and that new value
     returned.  For example,

          (textdomain "myprog")
          => "myprog"

 -- Scheme Procedure: bindtextdomain domain [directory]
 -- C Function: scm_bindtextdomain (domain, directory)
     Get or set the directory under which to find message files for
     DOMAIN.  When called without a DIRECTORY the current setting is
     returned.  When called with a DIRECTORY, DIRECTORY is set for
     DOMAIN and that new setting returned.  For example,

          (bindtextdomain "myprog" "/my/tree/share/locale")
          => "/my/tree/share/locale"

     When using Autoconf/Automake, an application should arrange for the
     configured `localedir' to get into the program (by substituting,
     or by generating a config file) and set that for its domain.  This
     ensures the catalogue can be found even when installed in a
     non-standard location.

 -- Scheme Procedure: bind-textdomain-codeset domain [encoding]
 -- C Function: scm_bind_textdomain_codeset (domain, encoding)
     Get or set the text encoding to be used by `gettext' for messages
     from DOMAIN.  ENCODING is a string, the name of a coding system,
     for instance "8859_1".  (On a Unix/POSIX system the `iconv'
     program can list all available encodings.)

     When called without an ENCODING the current setting is returned,
     or `#f' if none yet set.  When called with an ENCODING, it is set
     for DOMAIN and that new setting returned.  For example,

          (bind-textdomain-codeset "myprog")
          => #f
          (bind-textdomain-codeset "myprog" "latin-9")
          => "latin-9"

     The encoding requested can be different from the translated data
     file, messages will be recoded as necessary.  But note that when
     there is no translation, `gettext' returns its MSG unchanged, ie.
     without any recoding.  For that reason source message strings are
     best as plain ASCII.

     Currently Guile has no understanding of multi-byte characters, and
     string functions won't recognise character boundaries in multi-byte
     strings.  An application will at least be able to pass such strings
     through to some output though.  Perhaps this will change in the
     future.

6.25 Debugging Infrastructure
=============================

In order to understand Guile's debugging facilities, you first need to
understand a little about how Guile represent the Scheme control stack.
With that in place we explain the low level trap calls that the virtual
machine can be configured to make, and the trap and breakpoint
infrastructure that builds on top of those calls.

6.25.1 Evaluation and the Scheme Stack
--------------------------------------

The idea of the Scheme stack is central to a lot of debugging.  The
Scheme stack is a reified representation of the pending function returns
in an expression's continuation.  As Guile implements function calls
using a stack, this reification takes the form of a number of nested
stack frames, each of which corresponds to the application of a
procedure to a set of arguments.

   A Scheme stack always exists implicitly, and can be summoned into
concrete existence as a first-class Scheme value by the `make-stack'
call, so that an introspective Scheme program - such as a debugger -
can present it in some way and allow the user to query its details. The
first thing to understand, therefore, is how Guile's function call
convention creates the stack.

   Broadly speaking, Guile represents all control flow on a stack.
Calling a function involves pushing an empty frame on the stack, then
evaluating the procedure and its arguments, then fixing up the new
frame so that it points to the old one. Frames on the stack are thus
linked together. A tail call is the same, except it reuses the existing
frame instead of pushing on a new one.

   In this way, the only frames that are on the stack are "active"
frames, frames which need to do some work before the computation is
complete. On the other hand, a function that has tail-called another
function will not be on the stack, as it has no work left to do.

   Therefore, when an error occurs in a running program, or the program
hits a breakpoint, or in fact at any point that the programmer chooses,
its state at that point can be represented by a "stack" of all the
procedure applications that are logically in progress at that time, each
of which is known as a "frame".  The programmer can learn more about
the program's state at that point by inspecting the stack and its
frames.

6.25.1.1 Stack Capture
......................

A Scheme program can use the `make-stack' primitive anywhere in its
code, with first arg `#t', to construct a Scheme value that describes
the Scheme stack at that point.

     (make-stack #t)
     =>
     #<stack 25205a0>

   Use `start-stack' to limit the stack extent captured by future
`make-stack' calls.

 -- Scheme Procedure: make-stack obj . args
 -- C Function: scm_make_stack (obj, args)
     Create a new stack. If OBJ is `#t', the current evaluation stack
     is used for creating the stack frames, otherwise the frames are
     taken from OBJ (which must be a continuation or a frame object).

     ARGS should be a list containing any combination of integer,
     procedure, prompt tag and `#t' values.

     These values specify various ways of cutting away uninteresting
     stack frames from the top and bottom of the stack that
     `make-stack' returns.  They come in pairs like this: `(INNER_CUT_1
     OUTER_CUT_1 INNER_CUT_2 OUTER_CUT_2 ...)'.

     Each INNER_CUT_N can be `#t', an integer, a prompt tag, or a
     procedure.  `#t' means to cut away all frames up to but excluding
     the first user module frame.  An integer means to cut away exactly
     that number of frames.  A prompt tag means to cut away all frames
     that are inside a prompt with the given tag. A procedure means to
     cut away all frames up to but excluding the application frame
     whose procedure matches the specified one.

     Each OUTER_CUT_N can be an integer, a prompt tag, or a procedure.
     An integer means to cut away that number of frames.  A prompt tag
     means to cut away all frames that are outside a prompt with the
     given tag. A procedure means to cut away frames down to but
     excluding the application frame whose procedure matches the
     specified one.

     If the OUTER_CUT_N of the last pair is missing, it is taken as 0.

 -- Scheme Syntax: start-stack id exp
     Evaluate EXP on a new calling stack with identity ID.  If EXP is
     interrupted during evaluation, backtraces will not display frames
     farther back than EXP's top-level form.  This macro is a way of
     artificially limiting backtraces and stack procedures, largely as
     a convenience to the user.

6.25.1.2 Stacks
...............

 -- Scheme Procedure: stack? obj
 -- C Function: scm_stack_p (obj)
     Return `#t' if OBJ is a calling stack.

 -- Scheme Procedure: stack-id stack
 -- C Function: scm_stack_id (stack)
     Return the identifier given to STACK by `start-stack'.

 -- Scheme Procedure: stack-length stack
 -- C Function: scm_stack_length (stack)
     Return the length of STACK.

 -- Scheme Procedure: stack-ref stack index
 -- C Function: scm_stack_ref (stack, index)
     Return the INDEX'th frame from STACK.

 -- Scheme Procedure: display-backtrace stack port [first [depth
          [highlights]]]
 -- C Function: scm_display_backtrace_with_highlights (stack, port,
          first, depth, highlights)
 -- C Function: scm_display_backtrace (stack, port, first, depth)
     Display a backtrace to the output port PORT.  STACK is the stack
     to take the backtrace from, FIRST specifies where in the stack to
     start and DEPTH how many frames to display.  FIRST and DEPTH can
     be `#f', which means that default values will be used.  If
     HIGHLIGHTS is given it should be a list; the elements of this list
     will be highlighted wherever they appear in the backtrace.

6.25.1.3 Frames
...............

 -- Scheme Procedure: frame? obj
 -- C Function: scm_frame_p (obj)
     Return `#t' if OBJ is a stack frame.

 -- Scheme Procedure: frame-previous frame
 -- C Function: scm_frame_previous (frame)
     Return the previous frame of FRAME, or `#f' if FRAME is the first
     frame in its stack.

 -- Scheme Procedure: frame-procedure frame
 -- C Function: scm_frame_procedure (frame)
     Return the procedure for FRAME, or `#f' if no procedure is
     associated with FRAME.

 -- Scheme Procedure: frame-arguments frame
 -- C Function: scm_frame_arguments (frame)
     Return the arguments of FRAME.

 -- Scheme Procedure: frame-address frame
 -- Scheme Procedure: frame-instruction-pointer frame
 -- Scheme Procedure: frame-stack-pointer frame
     Accessors for the three VM registers associated with this frame:
     the frame pointer (fp), instruction pointer (ip), and stack
     pointer (sp), respectively. *Note VM Concepts::, for more
     information.

 -- Scheme Procedure: frame-dynamic-link frame
 -- Scheme Procedure: frame-return-address frame
 -- Scheme Procedure: frame-mv-return-address frame
     Accessors for the three saved VM registers in a frame: the previous
     frame pointer, the single-value return address, and the
     multiple-value return address. *Note Stack Layout::, for more
     information.

 -- Scheme Procedure: frame-num-locals frame
 -- Scheme Procedure: frame-local-ref frame i
 -- Scheme Procedure: frame-local-set! frame i val
     Accessors for the temporary values corresponding to FRAME's
     procedure application. The first local is the first argument given
     to the procedure. After the arguments, there are the local
     variables, and after that temporary values. *Note Stack Layout::,
     for more information.

 -- Scheme Procedure: display-application frame [port [indent]]
 -- C Function: scm_display_application (frame, port, indent)
     Display a procedure application FRAME to the output port PORT.
     INDENT specifies the indentation of the output.

   Additionally, the `(system vm frame)' module defines a number of
higher-level introspective procedures, for example to retrieve the names
of local variables, and the source location to correspond to a frame.
See its source code for more details.

6.25.2 Source Properties
------------------------

As Guile reads in Scheme code from file or from standard input, it
remembers the file name, line number and column number where each
expression begins. These pieces of information are known as the "source
properties" of the expression. Syntax expanders and the compiler
propagate these source properties to compiled procedures, so that, if
an error occurs when evaluating the transformed expression, Guile's
debugger can point back to the file and location where the expression
originated.

   The way that source properties are stored means that Guile can only
associate source properties with parenthesized expressions, and not, for
example, with individual symbols, numbers or strings.  The difference
can be seen by typing `(xxx)' and `xxx' at the Guile prompt (where the
variable `xxx' has not been defined):

     scheme@(guile-user)> (xxx)
     <unnamed port>:4:1: In procedure module-lookup:
     <unnamed port>:4:1: Unbound variable: xxx

     scheme@(guile-user)> xxx
     ERROR: In procedure module-lookup:
     ERROR: Unbound variable: xxx

In the latter case, no source properties were stored, so the error
doesn't have any source information.

   The recording of source properties is controlled by the read option
named "positions" (*note Scheme Read::).  This option is switched _on_
by default.

   The following procedures can be used to access and set the source
properties of read expressions.

 -- Scheme Procedure: set-source-properties! obj alist
 -- C Function: scm_set_source_properties_x (obj, alist)
     Install the association list ALIST as the source property list for
     OBJ.

 -- Scheme Procedure: set-source-property! obj key datum
 -- C Function: scm_set_source_property_x (obj, key, datum)
     Set the source property of object OBJ, which is specified by KEY
     to DATUM.  Normally, the key will be a symbol.

 -- Scheme Procedure: source-properties obj
 -- C Function: scm_source_properties (obj)
     Return the source property association list of OBJ.

 -- Scheme Procedure: source-property obj key
 -- C Function: scm_source_property (obj, key)
     Return the property specified by KEY from OBJ's source properties.

   If the `positions' reader option is enabled, each parenthesized
expression will have values set for the `filename', `line' and `column'
properties.

   Source properties are also associated with syntax objects.
Procedural macros can get at the source location of their input using
the `syntax-source' accessor.  *Note Syntax Transformer Helpers::, for
more.

   Guile also defines a couple of convenience macros built on
`syntax-source':

 -- Scheme Syntax: current-source-location
     Expands to the source properties corresponding to the location of
     the `(current-source-location)' form.

 -- Scheme Syntax: current-filename
     Expands to the current filename: the filename that the
     `(current-filename)' form appears in.  Expands to `#f' if this
     information is unavailable.

   If you're stuck with defmacros (*note Defmacros::), and want to
preserve source information, the following helper function might be
useful to you:

 -- Scheme Procedure: cons-source xorig x y
 -- C Function: scm_cons_source (xorig, x, y)
     Create and return a new pair whose car and cdr are X and Y.  Any
     source properties associated with XORIG are also associated with
     the new pair.

6.25.3 Programmatic Error Handling
----------------------------------

For better or for worse, all programs have bugs, and dealing with bugs
is part of programming. This section deals with that class of bugs that
causes an exception to be raised - from your own code, from within a
library, or from Guile itself.

6.25.3.1 Catching Exceptions
............................

A common requirement is to be able to show as much useful context as
possible when a Scheme program hits an error.  The most immediate
information about an error is the kind of error that it is - such as
"division by zero" - and any parameters that the code which signalled
the error chose explicitly to provide.  This information originates with
the `error' or `throw' call (or their C code equivalents, if the error
is detected by C code) that signals the error, and is passed
automatically to the handler procedure of the innermost applicable
`catch' or `with-throw-handler' expression.

   Therefore, to catch errors that occur within a chunk of Scheme code,
and to intercept basic information about those errors, you need to
execute that code inside the dynamic context of a `catch' or
`with-throw-handler' expression, or the equivalent in C. In Scheme,
this means you need something like this:

     (catch #t
            (lambda ()
              ;; Execute the code in which
              ;; you want to catch errors here.
              ...)
            (lambda (key . parameters)
              ;; Put the code which you want
              ;; to handle an error here.
              ...))

The `catch' here can also be `with-throw-handler'; see *note Throw
Handlers:: for information on the when you might want to use
`with-throw-handler' instead of `catch'.

   For example, to print out a message and return #f when an error
occurs, you might use:

     (define (catch-all thunk)
       (catch #t
         thunk
         (lambda (key . parameters)
           (format (current-error-port)
                   "Uncaught throw to '~a: ~a\n" key parameters)
           #f)))

     (catch-all
      (lambda () (error "Not a vegetable: tomato")))
     -| Uncaught throw to 'misc-error: (#f ~A (Not a vegetable: tomato) #f)
     => #f

   The `#t' means that the catch is applicable to all kinds of error.
If you want to restrict your catch to just one kind of error, you can
put the symbol for that kind of error instead of `#t'. The equivalent
to this in C would be something like this:

     SCM my_body_proc (void *body_data)
     {
       /* Execute the code in which
          you want to catch errors here. */
       ...
     }

     SCM my_handler_proc (void *handler_data,
                          SCM key,
                          SCM parameters)
     {
       /* Put the code which you want
          to handle an error here. */
       ...
     }

     {
       ...
       scm_c_catch (SCM_BOOL_T,
                    my_body_proc, body_data,
                    my_handler_proc, handler_data,
                    NULL, NULL);
       ...
     }

Again, as with the Scheme version, `scm_c_catch' could be replaced by
`scm_c_with_throw_handler', and `SCM_BOOL_T' could instead be the
symbol for a particular kind of error.

6.25.3.2 Capturing the full error stack
.......................................

The other interesting information about an error is the full Scheme
stack at the point where the error occurred; in other words what
innermost expression was being evaluated, what was the expression that
called that one, and so on.  If you want to write your code so that it
captures and can display this information as well, there are a couple
important things to understand.

   Firstly, the stack at the point of the error needs to be explicitly
captured by a `make-stack' call (or the C equivalent `scm_make_stack').
The Guile library does not do this "automatically" for you, so you will
need to write code with a `make-stack' or `scm_make_stack' call
yourself.  (We emphasise this point because some people are misled by
the fact that the Guile interactive REPL code _does_ capture and
display the stack automatically.  But the Guile interactive REPL is
itself a Scheme program(1) running on top of the Guile library, and
which uses `catch' and `make-stack' in the way we are about to describe
to capture the stack when an error occurs.)

   And secondly, in order to capture the stack effectively at the point
where the error occurred, the `make-stack' call must be made before
Guile unwinds the stack back to the location of the prevailing catch
expression. This means that the `make-stack' call must be made within
the handler of a `with-throw-handler' expression, or the optional
"pre-unwind" handler of a `catch'. (For the full story of how these
alternatives differ from each other, see *note Exceptions::. The main
difference is that `catch' terminates the error, whereas
`with-throw-handler' only intercepts it temporarily and then allow it
to continue propagating up to the next innermost handler.)

   So, here are some examples of how to do all this in Scheme and in C.
For the purpose of these examples we assume that the captured stack
should be stored in a variable, so that it can be displayed or
arbitrarily processed later on.  In Scheme:

     (let ((captured-stack #f))
       (catch #t
              (lambda ()
                ;; Execute the code in which
                ;; you want to catch errors here.
                ...)
              (lambda (key . parameters)
                ;; Put the code which you want
                ;; to handle an error after the
                ;; stack has been unwound here.
                ...)
              (lambda (key . parameters)
                ;; Capture the stack here:
                (set! captured-stack (make-stack #t))))
       ...
       (if captured-stack
           (begin
             ;; Display or process the captured stack.
             ...))
       ...)

And in C:

     SCM my_body_proc (void *body_data)
     {
       /* Execute the code in which
          you want to catch errors here. */
       ...
     }

     SCM my_handler_proc (void *handler_data,
                          SCM key,
                          SCM parameters)
     {
       /* Put the code which you want
          to handle an error after the
          stack has been unwound here. */
       ...
     }

     SCM my_preunwind_proc (void *handler_data,
                            SCM key,
                            SCM parameters)
     {
       /* Capture the stack here: */
       *(SCM *)handler_data = scm_make_stack (SCM_BOOL_T, SCM_EOL);
     }

     {
       SCM captured_stack = SCM_BOOL_F;
       ...
       scm_c_catch (SCM_BOOL_T,
                    my_body_proc, body_data,
                    my_handler_proc, handler_data,
                    my_preunwind_proc, &captured_stack);
       ...
       if (captured_stack != SCM_BOOL_F)
       {
         /* Display or process the captured stack. */
         ...
       }
       ...
     }

   Once you have a captured stack, you can interrogate and display its
details in any way that you want, using the `stack-...' and `frame-...'
API described in *note Stacks:: and *note Frames::.

   If you want to print out a backtrace in the same format that the
Guile REPL does, you can use the `display-backtrace' procedure to do so.
You can also use `display-application' to display an individual frame
in the Guile REPL format.

   ---------- Footnotes ----------

   (1) In effect, it is the default program which is run when no
commands or script file are specified on the Guile command line.

6.25.3.3 Pre-Unwind Debugging
.............................

Instead of saving a stack away and waiting for the `catch' to return,
you can handle errors directly, from within the pre-unwind handler.

   For example, to show a backtrace when an error is thrown, you might
want to use a procedure like this:

     (define (with-backtrace thunk)
       (with-throw-handler #t
                           thunk
                           (lambda args (backtrace))))
     (with-backtrace (lambda () (error "Not a vegetable: tomato")))

   Since we used `with-throw-handler' here, we didn't actually catch
the error. *Note Throw Handlers::, for more information. However, we did
print out a context at the time of the error, using the built-in
procedure, `backtrace'.

 -- Scheme Procedure: backtrace [highlights]
 -- C Function: scm_backtrace_with_highlights (highlights)
 -- C Function: scm_backtrace ()
     Display a backtrace of the current stack to the current output
     port.  If HIGHLIGHTS is given it should be a list; the elements of
     this list will be highlighted wherever they appear in the
     backtrace.

   The Guile REPL code (in `system/repl/repl.scm' and related files)
uses a `catch' with a pre-unwind handler to capture the stack when an
error occurs in an expression that was typed into the REPL, and debug
that stack interactively in the context of the error.

   These procedures are available for use by user programs, in the
`(system repl error-handling)' module.

     (use-modules (system repl error-handling))

 -- Scheme Procedure: call-with-error-handling thunk [#:on-error
          on-error='debug] [#:post-error post-error='catch]
          [#:pass-keys pass-keys='(quit)] [#:trap-handler
          trap-handler='debug]
     Call a thunk in a context in which errors are handled.

     There are four keyword arguments:

    ON-ERROR
          Specifies what to do before the stack is unwound.

          Valid options are `debug' (the default), which will enter a
          debugger; `pass', in which case nothing is done, and the
          exception is rethrown; or a procedure, which will be the
          pre-unwind handler.

    POST-ERROR
          Specifies what to do after the stack is unwound.

          Valid options are `catch' (the default), which will silently
          catch errors, returning the unspecified value; `report',
          which prints out a description of the error (via
          `display-error'), and then returns the unspecified value; or
          a procedure, which will be the catch handler.

    TRAP-HANDLER
          Specifies a trap handler: what to do when a breakpoint is hit.

          Valid options are `debug', which will enter the debugger;
          `pass', which does nothing; or `disabled', which disables
          traps entirely.  *Note Traps::, for more information.

    PASS-KEYS
          A set of keys to ignore, as a list.

6.25.3.4 Debug options
......................

The behavior of the `backtrace' procedure and of the default error
handler can be parameterized via the debug options.

 -- Scheme Procedure: debug-options [setting]
     Display the current settings of the debug options.  If SETTING is
     omitted, only a short form of the current read options is printed.
     Otherwise if SETTING is the symbol `help', a complete options
     description is displayed.

   The set of available options, and their default values, may be had by
invoking `debug-options' at the prompt.

     scheme@(guile-user)>
     backwards       no      Display backtrace in anti-chronological order.
     width           79      Maximal width of backtrace.
     depth           20      Maximal length of printed backtrace.
     backtrace       yes     Show backtrace on error.
     stack           1048576 Stack size limit (measured in words;
                             0 = no check).
     show-file-name  #t      Show file names and line numbers in backtraces
                             when not `#f'.  A value of `base' displays only
                             base names, while `#t' displays full names.
     warn-deprecated no      Warn when deprecated features are used.

   The boolean options may be toggled with `debug-enable' and
`debug-disable'. The non-boolean `keywords' option must be set using
`debug-set!'.

 -- Scheme Procedure: debug-enable option-name
 -- Scheme Procedure: debug-disable option-name
 -- Scheme Syntax: debug-set! option-name value
     Modify the debug options.  `debug-enable' should be used with
     boolean options and switches them on, `debug-disable' switches
     them off.

     `debug-set!' can be used to set an option to a specific value.  Due
     to historical oddities, it is a macro that expects an unquoted
     option name.

Stack overflow
..............

Stack overflow errors are caused by a computation trying to use more
stack space than has been enabled by the `stack' option.  There are
actually two kinds of stack that can overflow, the C stack and the
Scheme stack.

   Scheme stack overflows can occur if Scheme procedures recurse too far
deeply. An example would be the following recursive loop:

     scheme@(guile-user)> (let lp () (+ 1 (lp)))
     <unnamed port>:8:17: In procedure vm-run:
     <unnamed port>:8:17: VM: Stack overflow

   The default stack size should allow for about 10000 frames or so, so
one usually doesn't hit this level of recursion. Unfortunately there is
no way currently to make a VM with a bigger stack. If you are in this
unfortunate situation, please file a bug, and in the meantime, rewrite
your code to be tail-recursive (*note Tail Calls::).

   The other limit you might hit would be C stack overflows. If you
call a primitive procedure which then calls a Scheme procedure in a
loop, you will consume C stack space. Guile tries to detect excessive
consumption of C stack space, throwing an error when you have hit 80%
of the process' available stack (as allocated by the operating system),
or 160 kilowords in the absence of a strict limit.

   For example, looping through `call-with-vm', a primitive that calls
a thunk, gives us the following:

     scheme@(guile-user)> (use-modules (system vm vm))
     scheme@(guile-user)> (debug-set! stack 10000)
     scheme@(guile-user)> (let lp () (call-with-vm (the-vm) lp))
     ERROR: In procedure call-with-vm:
     ERROR: Stack overflow

   If you get an error like this, you can either try rewriting your
code to use less stack space, or increase the maximum stack size.  To
increase the maximum stack size, use `debug-set!', for example:

     (debug-set! stack 200000)

   But of course it's better to have your code operate without so much
resource consumption, avoiding loops through C trampolines.

6.25.4 Traps
------------

Guile's virtual machine can be configured to call out at key points to
arbitrary user-specified procedures.

   In principle, these "hooks" allow Scheme code to implement any model
it chooses for examining the evaluation stack as program execution
proceeds, and for suspending execution to be resumed later.

   VM hooks are very low-level, though, and so Guile also has a library
of higher-level "traps" on top of the VM hooks. A trap is an execution
condition that, when fulfilled, will fire a handler. For example, Guile
defines a trap that fires when control reaches a certain source
location.

   Finally, Guile also defines a third level of abstractions: per-thread
"trap states". A trap state exists to give names to traps, and to hold
on to the set of traps so that they can be enabled, disabled, or
removed. The trap state infrastructure defines the most useful
abstractions for most cases. For example, Guile's REPL uses trap state
functions to set breakpoints and tracepoints.

   The following subsections describe all this in detail, for both the
user wanting to use traps, and the developer interested in
understanding how the interface hangs together.

6.25.4.1 VM Hooks
.................

Everything that runs in Guile runs on its virtual machine, a C program
that defines a number of operations that Scheme programs can perform.

   Note that there are multiple VM "engines" for Guile. Only some of
them have support for hooks compiled in. Normally the deal is that you
get hooks if you are running interactively, and otherwise they are
disabled, as they do have some overhead (about 10 or 20 percent).

   To ensure that you are running with hooks, pass `--debug' to Guile
when running your program, or otherwise use the `call-with-vm' and
`set-vm-engine!'  procedures to ensure that you are running in a VM
with the `debug' engine.

   To digress, Guile's VM has 6 different hooks (*note Hooks::) that
can be fired at different times, which may be accessed with the
following procedures.

   All hooks are called with one argument, the frame in question. *Note
Frames::.  Since these hooks may be fired very frequently, Guile does a
terrible thing: it allocates the frames on the C stack instead of the
garbage-collected heap.

   The upshot here is that the frames are only valid within the dynamic
extent of the call to the hook. If a hook procedure keeps a reference to
the frame outside the extent of the hook, bad things will happen.

   The interface to hooks is provided by the `(system vm vm)' module:

     (use-modules (system vm vm))

The result of calling `the-vm' is usually passed as the VM argument to
all of these procedures.

 -- Scheme Procedure: vm-next-hook vm
     The hook that will be fired before an instruction is retired (and
     executed).

 -- Scheme Procedure: vm-push-continuation-hook vm
     The hook that will be fired after preparing a new frame. Fires just
     before applying a procedure in a non-tail context, just before the
     corresponding apply-hook.

 -- Scheme Procedure: vm-pop-continuation-hook vm
     The hook that will be fired before returning from a frame.

     This hook is a bit trickier than the rest, in that there is a
     particular interpretation of the values on the stack.
     Specifically, the top value on the stack is the number of values
     being returned, and the next N values are the actual values being
     returned, with the last value highest on the stack.

 -- Scheme Procedure: vm-apply-hook vm
     The hook that will be fired before a procedure is applied. The
     frame's procedure will have already been set to the new procedure.

     Note that procedure application is somewhat orthogonal to
     continuation pushes and pops. A non-tail call to a procedure will
     result first in a firing of the push-continuation hook, then this
     application hook, whereas a tail call will run without having
     fired a push-continuation hook.

 -- Scheme Procedure: vm-abort-continuation-hook vm
     The hook that will be called after aborting to a prompt. *Note
     Prompts::. The stack will be in the same state as for
     `vm-pop-continuation-hook'.

 -- Scheme Procedure: vm-restore-continuation-hook vm
     The hook that will be called after restoring an undelimited
     continuation. Unfortunately it's not currently possible to
     introspect on the values that were given to the continuation.

   These hooks do impose a performance penalty, if they are on.
Obviously, the `vm-next-hook' has quite an impact, performance-wise.
Therefore Guile exposes a single, heavy-handed knob to turn hooks on or
off, the "VM trace level". If the trace level is positive, hooks run;
otherwise they don't.

   For convenience, when the VM fires a hook, it does so with the trap
level temporarily set to 0.  That way the hooks don't fire while you're
handling a hook.  The trace level is restored to whatever it was once
the hook procedure finishes.

 -- Scheme Procedure: vm-trace-level vm
     Retrieve the "trace level" of the VM. If positive, the trace hooks
     associated with VM will be run. The initial trace level is 0.

 -- Scheme Procedure: set-vm-trace-level! vm level
     Set the "trace level" of the VM.

   *Note A Virtual Machine for Guile::, for more information on Guile's
virtual machine.

6.25.4.2 Trap Interface
.......................

The capabilities provided by hooks are great, but hooks alone rarely
correspond to what users want to do.

   For example, if a user wants to break when and if control reaches a
certain source location, how do you do it?  If you install a "next"
hook, you get unacceptable overhead for the execution of the entire
program. It would be possible to install an "apply" hook, then if the
procedure encompasses those source locations, install a "next" hook,
but already you're talking about one concept that might be implemented
by a varying number of lower-level concepts.

   It's best to be clear about things and define one abstraction for all
such conditions: the "trap".

   Considering the myriad capabilities offered by the hooks though,
there is only a minimum of functionality shared by all traps. Guile's
current take is to reduce this to the absolute minimum, and have the
only standard interface of a trap be "turn yourself on" or "turn
yourself off".

   This interface sounds a bit strange, but it is useful to procedurally
compose higher-level traps from lower-level building blocks. For
example, Guile defines a trap that calls one handler when control enters
a procedure, and another when control leaves the procedure. Given that
trap, one can define a trap that adds to the next-hook only when within
a given procedure. Building further, one can define a trap that fires
when control reaches particular instructions within a procedure.

   Or of course you can stop at any of these intermediate levels. For
example, one might only be interested in calls to a given procedure. But
the point is that a simple enable/disable interface is all the
commonality that exists between the various kinds of traps, and
furthermore that such an interface serves to allow "higher-level" traps
to be composed from more primitive ones.

   Specifically, a trap, in Guile, is a procedure. When a trap is
created, by convention the trap is enabled; therefore, the procedure
that is the trap will, when called, disable the trap, and return a
procedure that will enable the trap, and so on.

   Trap procedures take one optional argument: the current frame. (A
trap may want to add to different sets of hooks depending on the frame
that is current at enable-time.)

   If this all sounds very complicated, it's because it is. Some of it
is essential, but probably most of it is not. The advantage of using
this minimal interface is that composability is more lexically apparent
than when, for example, using a stateful interface based on GOOPS. But
perhaps this reflects the cognitive limitations of the programmer who
made the current interface more than anything else.

6.25.4.3 Low-Level Traps
........................

To summarize the last sections, traps are enabled or disabled, and when
they are enabled, they add to various VM hooks.

   Note, however, that _traps do not increase the VM trace level_. So
if you create a trap, it will be enabled, but unless something else
increases the VM's trace level (*note VM Hooks::), the trap will not
fire.  It turns out that getting the VM trace level right is tricky
without a global view of what traps are enabled.  *Note Trap States::,
for Guile's answer to this problem.

   Traps are created by calling procedures. Most of these procedures
share a set of common keyword arguments, so rather than document them
separately, we discuss them all together here:

`#:vm'
     The VM to instrument. Defaults to the current thread's VM.

`#:closure?'
     For traps that depend on the current frame's procedure, this
     argument specifies whether to trap on the only the specific
     procedure given, or on any closure that has the given procedure's
     code. Defaults to `#f'.

`#:current-frame'
     For traps that enable more hooks depending on their dynamic
     context, this argument gives the current frame that the trap is
     running in.  Defaults to `#f'.

   To have access to these procedures, you'll need to have imported the
`(system vm traps)' module:

     (use-modules (system vm traps))

 -- Scheme Procedure: trap-at-procedure-call proc handler [#:vm]
          [#:closure?]
     A trap that calls HANDLER when PROC is applied.

 -- Scheme Procedure: trap-in-procedure proc enter-handler exit-handler
          [#:current-frame] [#:vm] [#:closure?]
     A trap that calls ENTER-HANDLER when control enters PROC, and
     EXIT-HANDLER when control leaves PROC.

     Control can enter a procedure via:
        * A procedure call.

        * A return to a procedure's frame on the stack.

        * A continuation returning directly to an application of this
          procedure.

     Control can leave a procedure via:
        * A normal return from the procedure.

        * An application of another procedure.

        * An invocation of a continuation.

        * An abort.

 -- Scheme Procedure: trap-instructions-in-procedure proc next-handler
          exit-handler [#:current-frame] [#:vm] [#:closure?]
     A trap that calls NEXT-HANDLER for every instruction executed in
     PROC, and EXIT-HANDLER when execution leaves PROC.

 -- Scheme Procedure: trap-at-procedure-ip-in-range proc range handler
          [#:current-frame] [#:vm] [#:closure?]
     A trap that calls HANDLER when execution enters a range of
     instructions in PROC. RANGE is a simple of pairs, `((START . END)
     ...)'. The START addresses are inclusive, and END addresses are
     exclusive.

 -- Scheme Procedure: trap-at-source-location file user-line handler
          [#:current-frame] [#:vm]
     A trap that fires when control reaches a given source location.
     The USER-LINE parameter is one-indexed, as a user counts lines,
     instead of zero-indexed, as Guile counts lines.

 -- Scheme Procedure: trap-frame-finish frame return-handler
          abort-handler [#:vm]
     A trap that fires when control leaves the given frame. FRAME
     should be a live frame in the current continuation. RETURN-HANDLER
     will be called on a normal return, and ABORT-HANDLER on a nonlocal
     exit.

 -- Scheme Procedure: trap-in-dynamic-extent proc enter-handler
          return-handler abort-handler [#:vm] [#:closure?]
     A more traditional dynamic-wind trap, which fires ENTER-HANDLER
     when control enters PROC, RETURN-HANDLER on a normal return, and
     ABORT-HANDLER on a nonlocal exit.

     Note that rewinds are not handled, so there is no rewind handler.

 -- Scheme Procedure: trap-calls-in-dynamic-extent proc apply-handler
          return-handler [#:current-frame] [#:vm] [#:closure?]
     A trap that calls APPLY-HANDLER every time a procedure is applied,
     and RETURN-HANDLER for returns, but only during the dynamic extent
     of an application of PROC.

 -- Scheme Procedure: trap-instructions-in-dynamic-extent proc
          next-handler [#:current-frame] [#:vm] [#:closure?]
     A trap that calls NEXT-HANDLER for all retired instructions within
     the dynamic extent of a call to PROC.

 -- Scheme Procedure: trap-calls-to-procedure proc apply-handler
          return-handler [#:vm]
     A trap that calls APPLY-HANDLER whenever PROC is applied, and
     RETURN-HANDLER when it returns, but with an additional argument,
     the call depth.

     That is to say, the handlers will get two arguments: the frame in
     question, and the call depth (a non-negative integer).

 -- Scheme Procedure: trap-matching-instructions frame-pred handler
          [#:vm]
     A trap that calls FRAME-PRED at every instruction, and if
     FRAME-PRED returns a true value, calls HANDLER on the frame.

6.25.4.4 Tracing Traps
......................

The `(system vm trace)' module defines a number of traps for tracing of
procedure applications.  When a procedure is "traced", it means that
every call to that procedure is reported to the user during a program
run.  The idea is that you can mark a collection of procedures for
tracing, and Guile will subsequently print out a line of the form

     |  |  (PROCEDURE ARGS ...)

   whenever a marked procedure is about to be applied to its arguments.
This can help a programmer determine whether a function is being called
at the wrong time or with the wrong set of arguments.

   In addition, the indentation of the output is useful for
demonstrating how the traced applications are or are not tail recursive
with respect to each other.  Thus, a trace of a non-tail recursive
factorial implementation looks like this:

     scheme@(guile-user)> (define (fact1 n)
                            (if (zero? n) 1
                                (* n (fact1 (1- n)))))
     scheme@(guile-user)> ,trace (fact1 4)
     trace: (fact1 4)
     trace: |  (fact1 3)
     trace: |  |  (fact1 2)
     trace: |  |  |  (fact1 1)
     trace: |  |  |  |  (fact1 0)
     trace: |  |  |  |  1
     trace: |  |  |  1
     trace: |  |  2
     trace: |  6
     trace: 24

   While a typical tail recursive implementation would look more like
this:

     scheme@(guile-user)> (define (facti acc n)
                            (if (zero? n) acc
                                (facti (* n acc) (1- n))))
     scheme@(guile-user)> (define (fact2 n) (facti 1 n))
     scheme@(guile-user)> ,trace (fact2 4)
     trace: (fact2 4)
     trace: (facti 1 4)
     trace: (facti 4 3)
     trace: (facti 12 2)
     trace: (facti 24 1)
     trace: (facti 24 0)
     trace: 24

   The low-level traps below (*note Low-Level Traps::) share some common
options:

`#:width'
     The maximum width of trace output. Trace printouts will try not to
     exceed this column, but for highly nested procedure calls, it may
     be unavoidable. Defaults to 80.

`#:vm'
     The VM on which to add the traps. Defaults to the current thread's
     VM.

`#:prefix'
     A string to print out before each trace line. As seen above in the
     examples, defaults to `"trace: "'.

   To have access to these procedures, you'll need to have imported the
`(system vm trace)' module:

     (use-modules (system vm trace))

 -- Scheme Procedure: trace-calls-to-procedure proc [#:width] [#:vm]
          [#:prefix]
     Print a trace at applications of and returns from PROC.

 -- Scheme Procedure: trace-calls-in-procedure proc [#:width] [#:vm]
          [#:prefix]
     Print a trace at all applications and returns within the dynamic
     extent of calls to PROC.

 -- Scheme Procedure: trace-instructions-in-procedure proc [#:width]
          [#:vm]
     Print a trace at all instructions executed in the dynamic extent of
     calls to PROC.

   In addition, Guile defines a procedure to call a thunk, tracing all
procedure calls and returns within the thunk.

 -- Scheme Procedure: call-with-trace thunk #:key (calls? #t)
          (instructions? #f) (width 80) (vm (the-vm))
     Call THUNK, tracing all execution within its dynamic extent.

     If CALLS? is true, Guile will print a brief report at each
     procedure call and return, as given above.

     If INSTRUCTIONS? is true, Guile will also print a message each
     time an instruction is executed.  This is a lot of output, but it
     is sometimes useful when doing low-level optimization.

     Note that because this procedure manipulates the VM trace level
     directly, it doesn't compose well with traps at the REPL.

   *Note Profile Commands::, for more information on tracing at the
REPL.

6.25.4.5 Trap States
....................

When multiple traps are present in a system, we begin to have a
bookkeeping problem. How are they named? How does one disable, enable,
or delete them?

   Guile's answer to this is to keep an implicit per-thread "trap
state". The trap state object is not exposed to the user; rather, API
that works on trap states fetches the current trap state from the
dynamic environment.

   Traps are identified by integers. A trap can be enabled, disabled, or
removed, and can have an associated user-visible name.

   These procedures have their own module:

     (use-modules (system vm trap-state))

 -- Scheme Procedure: add-trap! trap name
     Add a trap to the current trap state, associating the given NAME
     with it. Returns a fresh trap identifier (an integer).

     Note that usually the more specific functions detailed in *note
     High-Level Traps:: are used in preference to this one.

 -- Scheme Procedure: list-traps
     List the current set of traps, both enabled and disabled. Returns
     a list of integers.

 -- Scheme Procedure: trap-name idx
     Returns the name associated with trap IDX, or `#f' if there is no
     such trap.

 -- Scheme Procedure: trap-enabled? idx
     Returns `#t' if trap IDX is present and enabled, or `#f' otherwise.

 -- Scheme Procedure: enable-trap! idx
     Enables trap IDX.

 -- Scheme Procedure: disable-trap! idx
     Disables trap IDX.

 -- Scheme Procedure: delete-trap! idx
     Removes trap IDX, disabling it first, if necessary.

6.25.4.6 High-Level Traps
.........................

The low-level trap API allows one to make traps that call procedures,
and the trap state API allows one to keep track of what traps are
there.  But neither of these APIs directly helps you when you want to
set a breakpoint, because it's unclear what to do when the trap fires.
Do you enter a debugger, or mail a summary of the situation to your
great-aunt, or what?

   So for the common case in which you just want to install breakpoints,
and then have them all result in calls to one parameterizable procedure,
we have the high-level trap interface.

   Perhaps we should have started this section with this interface, as
it's clearly the one most people should use.  But as its capabilities
and limitations proceed from the lower layers, we felt that the
character-building exercise of building a mental model might be helpful.

   These procedures share a module with trap states:

     (use-modules (system vm trap-state))

 -- Scheme Procedure: with-default-trap-handler handler thunk
     Call THUNK in a dynamic context in which HANDLER is the current
     trap handler.

     Additionally, during the execution of THUNK, the VM trace level
     (*note VM Hooks::) is set to the number of enabled traps. This
     ensures that traps will in fact fire.

     HANDLER may be `#f', in which case VM hooks are not enabled as
     they otherwise would be, as there is nothing to handle the traps.

   The trace-level-setting behavior of `with-default-trap-handler' is
one of its more useful aspects, but if you are willing to forgo that,
and just want to install a global trap handler, there's a function for
that too:

 -- Scheme Procedure: install-trap-handler! handler
     Set the current thread's trap handler to HANDLER.

   Trap handlers are called when traps installed by procedures from this
module fire.  The current "consumer" of this API is Guile's REPL, but
one might easily imagine other trap handlers being used to integrate
with other debugging tools.

 -- Scheme Procedure: add-trap-at-procedure-call! proc
     Install a trap that will fire when PROC is called.

     This is a breakpoint.

 -- Scheme Procedure: add-trace-at-procedure-call! proc
     Install a trap that will print a tracing message when PROC is
     called. *Note Tracing Traps::, for more information.

     This is a tracepoint.

 -- Scheme Procedure: add-trap-at-source-location! file user-line
     Install a trap that will fire when control reaches the given source
     location. USER-LINE is one-indexed, as users count lines, instead
     of zero-indexed, as Guile counts lines.

     This is a source breakpoint.

 -- Scheme Procedure: add-ephemeral-trap-at-frame-finish! frame handler
     Install a trap that will call HANDLER when FRAME finishes
     executing. The trap will be removed from the trap state after
     firing, or on nonlocal exit.

     This is a finish trap, used to implement the "finish" REPL command.

 -- Scheme Procedure: add-ephemeral-stepping-trap! frame handler
          [#:into?] [#:instruction?]
     Install a trap that will call HANDLER after stepping to a
     different source line or instruction.  The trap will be removed
     from the trap state after firing, or on nonlocal exit.

     If INSTRUCTION? is false (the default), the trap will fire when
     control reaches a new source line. Otherwise it will fire when
     control reaches a new instruction.

     Additionally, if INTO? is false (not the default), the trap will
     only fire for frames at or prior to the given frame. If INTO? is
     true (the default), the trap may step into nested procedure
     invocations.

     This is a stepping trap, used to implement the "step", "next",
     "step-instruction", and "next-instruction" REPL commands.

6.26 Code Coverage Reports
==========================

When writing a test suite for a program or library, it is desirable to
know what part of the code is "covered" by the test suite.  The
`(system vm coverage)' module provides tools to gather code coverage
data and to present them, as detailed below.

 -- Scheme Procedure: with-code-coverage vm thunk
     Run THUNK, a zero-argument procedure, using VM; instrument VM to
     collect code coverage data.  Return code coverage data and the
     values returned by THUNK.

 -- Scheme Procedure: coverage-data? obj
     Return `#t' if OBJ is a "coverage data" object as returned by
     `with-code-coverage'.

 -- Scheme Procedure: coverage-data->lcov data port #:key modules
     Traverse code coverage information DATA, as obtained with
     `with-code-coverage', and write coverage information to port in the
     `.info' format used by LCOV
     (http://ltp.sourceforge.net/coverage/lcov.php).  The report will
     include all of MODULES (or, by default, all the currently loaded
     modules) even if their code was not executed.

     The generated data can be fed to LCOV's `genhtml' command to
     produce an HTML report, which aids coverage data visualization.

   Here's an example use:

     (use-modules (system vm coverage)
                  (system vm vm))

     (call-with-values (lambda ()
                         (with-code-coverage (the-vm)
                           (lambda ()
                             (do-something-tricky))))
       (lambda (data result)
         (let ((port (open-output-file "lcov.info")))
           (coverage-data->lcov data port)
           (close file))))

   In addition, the module provides low-level procedures that would
make it possible to write other user interfaces to the coverage data.

 -- Scheme Procedures: instrumented-source-files data
     Return the list of "instrumented" source files, i.e., source files
     whose code was loaded at the time DATA was collected.

 -- Scheme Procedures: line-execution-counts data file
     Return a list of line number/execution count pairs for FILE, or
     `#f' if FILE is not among the files covered by DATA.  This
     includes lines with zero count.

 -- Scheme Procedures: instrumented/executed-lines data file
     Return the number of instrumented and the number of executed
     source lines in FILE according to DATA.

 -- Scheme Procedures: procedure-execution-count data proc
     Return the number of times PROC's code was executed, according to
     DATA, or `#f' if PROC was not executed.  When PROC is a closure,
     the number of times its code was executed is returned, not the
     number of times this code associated with this particular closure
     was executed.

7 Guile Modules
***************

7.1 SLIB
========

SLIB is a portable library of Scheme packages which can be used with
Guile and other Scheme implementations.  SLIB is not included in the
Guile distribution, but can be installed separately (*note SLIB
installation::).  It is available from
`http://people.csail.mit.edu/jaffer/SLIB.html'.

   After SLIB is installed, the following Scheme expression must be
executed before the SLIB facilities can be used:

     (use-modules (ice-9 slib))

`require' can then be used in the usual way (*note Require:
(slib)Require.).  For example,

     (use-modules (ice-9 slib))
     (require 'primes)
     (prime? 13)
     => #t

   A few Guile core functions are overridden by the SLIB setups; for
example the SLIB version of `delete-file' returns a boolean indicating
success or failure, whereas the Guile core version throws an error for
failure.  In general (and as might be expected) when SLIB is loaded
it's the SLIB specifications that are followed.

7.1.1 SLIB installation
-----------------------

The following procedure works, e.g., with SLIB version 3a3 (*note SLIB
installation: (slib)Installation.):

  1. Unpack SLIB and install it using `make install' from its directory.
     By default, this will install SLIB in `/usr/local/lib/slib/'.
     Running `make install-info' installs its documentation, by default
     under `/usr/local/info/'.

  2. Define the `SCHEME_LIBRARY_PATH' environment variable:

          $ SCHEME_LIBRARY_PATH=/usr/local/lib/slib/
          $ export SCHEME_LIBRARY_PATH

     Alternatively, you can create a symlink in the Guile directory to
     SLIB, e.g.:

          ln -s /usr/local/lib/slib /usr/local/share/guile/2.0/slib

  3. Use Guile to create the catalog file, e.g.,:

          # guile
          guile> (use-modules (ice-9 slib))
          guile> (require 'new-catalog)
          guile> (quit)

     The catalog data should now be in
     `/usr/local/share/guile/2.0/slibcat'.

     If instead you get an error such as:

          Unbound variable: scheme-implementation-type

     then a solution is to get a newer version of Guile, or to modify
     `ice-9/slib.scm' to use `define-public' for the offending
     variables.


7.1.2 JACAL
-----------

Jacal is a symbolic math package written in Scheme by Aubrey Jaffer.
It is usually installed as an extra package in SLIB.

   You can use Guile's interface to SLIB to invoke Jacal:

     (use-modules (ice-9 slib))
     (slib:load "math")
     (math)

For complete documentation on Jacal, please read the Jacal manual.  If
it has been installed on line, you can look at *note Jacal: (jacal)Top.
Otherwise you can find it on the web at
`http://www-swiss.ai.mit.edu/~jaffer/JACAL.html'

7.2 POSIX System Calls and Networking
=====================================

7.2.1 POSIX Interface Conventions
---------------------------------

These interfaces provide access to operating system facilities.  They
provide a simple wrapping around the underlying C interfaces to make
usage from Scheme more convenient.  They are also used to implement the
Guile port of scsh (*note The Scheme shell (scsh)::).

   Generally there is a single procedure for each corresponding Unix
facility.  There are some exceptions, such as procedures implemented for
speed and convenience in Scheme with no primitive Unix equivalent, e.g.
`copy-file'.

   The interfaces are intended as far as possible to be portable across
different versions of Unix.  In some cases procedures which can't be
implemented on particular systems may become no-ops, or perform limited
actions.  In other cases they may throw errors.

   General naming conventions are as follows:

   * The Scheme name is often identical to the name of the underlying
     Unix facility.

   * Underscores in Unix procedure names are converted to hyphens.

   * Procedures which destructively modify Scheme data have exclamation
     marks appended, e.g., `recv!'.

   * Predicates (returning only `#t' or `#f') have question marks
     appended, e.g., `access?'.

   * Some names are changed to avoid conflict with dissimilar interfaces
     defined by scsh, e.g., `primitive-fork'.

   * Unix preprocessor names such as `EPERM' or `R_OK' are converted to
     Scheme variables of the same name (underscores are not replaced
     with hyphens).

   Unexpected conditions are generally handled by raising exceptions.
There are a few procedures which return a special value if they don't
succeed, e.g., `getenv' returns `#f' if it the requested string is not
found in the environment.  These cases are noted in the documentation.

   For ways to deal with exceptions, see *note Exceptions::.

   Errors which the C library would report by returning a null pointer
or through some other means are reported by raising a `system-error'
exception with `scm-error' (*note Error Reporting::).  The DATA
parameter is a list containing the Unix `errno' value (an integer).
For example,

     (define (my-handler key func fmt fmtargs data)
       (display key) (newline)
       (display func) (newline)
       (apply format #t fmt fmtargs) (newline)
       (display data) (newline))

     (catch 'system-error
       (lambda () (dup2 -123 -456))
       my-handler)

     -|
     system-error
     dup2
     Bad file descriptor
     (9)


 -- Function: system-error-errno arglist
     Return the `errno' value from a list which is the arguments to an
     exception handler.  If the exception is not a `system-error', then
     the return is `#f'.  For example,

          (catch
           'system-error
           (lambda ()
             (mkdir "/this-ought-to-fail-if-I'm-not-root"))
           (lambda stuff
             (let ((errno (system-error-errno stuff)))
               (cond
                ((= errno EACCES)
                 (display "You're not allowed to do that."))
                ((= errno EEXIST)
                 (display "Already exists."))
                (#t
                 (display (strerror errno))))
               (newline))))

7.2.2 Ports and File Descriptors
--------------------------------

Conventions generally follow those of scsh, *note The Scheme shell
(scsh)::.

   File ports are implemented using low-level operating system I/O
facilities, with optional buffering to improve efficiency; see *note
File Ports::.

   Note that some procedures (e.g., `recv!') will accept ports as
arguments, but will actually operate directly on the file descriptor
underlying the port.  Any port buffering is ignored, including the
buffer which implements `peek-char' and `unread-char'.

   The `force-output' and `drain-input' procedures can be used to clear
the buffers.

   Each open file port has an associated operating system file
descriptor.  File descriptors are generally not useful in Scheme
programs; however they may be needed when interfacing with foreign code
and the Unix environment.

   A file descriptor can be extracted from a port and a new port can be
created from a file descriptor.  However a file descriptor is just an
integer and the garbage collector doesn't recognize it as a reference
to the port.  If all other references to the port were dropped, then
it's likely that the garbage collector would free the port, with the
side-effect of closing the file descriptor prematurely.

   To assist the programmer in avoiding this problem, each port has an
associated "revealed count" which can be used to keep track of how many
times the underlying file descriptor has been stored in other places.
If a port's revealed count is greater than zero, the file descriptor
will not be closed when the port is garbage collected.  A programmer
can therefore ensure that the revealed count will be greater than zero
if the file descriptor is needed elsewhere.

   For the simple case where a file descriptor is "imported" once to
become a port, it does not matter if the file descriptor is closed when
the port is garbage collected.  There is no need to maintain a revealed
count.  Likewise when "exporting" a file descriptor to the external
environment, setting the revealed count is not required provided the
port is kept open (i.e., is pointed to by a live Scheme binding) while
the file descriptor is in use.

   To correspond with traditional Unix behaviour, three file descriptors
(0, 1, and 2) are automatically imported when a program starts up and
assigned to the initial values of the current/standard input, output,
and error ports, respectively.  The revealed count for each is
initially set to one, so that dropping references to one of these ports
will not result in its garbage collection: it could be retrieved with
`fdopen' or `fdes->ports'.

 -- Scheme Procedure: port-revealed port
 -- C Function: scm_port_revealed (port)
     Return the revealed count for PORT.

 -- Scheme Procedure: set-port-revealed! port rcount
 -- C Function: scm_set_port_revealed_x (port, rcount)
     Sets the revealed count for a PORT to RCOUNT.  The return value is
     unspecified.

 -- Scheme Procedure: fileno port
 -- C Function: scm_fileno (port)
     Return the integer file descriptor underlying PORT.  Does not
     change its revealed count.

 -- Scheme Procedure: port->fdes port
     Returns the integer file descriptor underlying PORT.  As a side
     effect the revealed count of PORT is incremented.

 -- Scheme Procedure: fdopen fdes modes
 -- C Function: scm_fdopen (fdes, modes)
     Return a new port based on the file descriptor FDES.  Modes are
     given by the string MODES.  The revealed count of the port is
     initialized to zero.  The MODES string is the same as that
     accepted by `open-file' (*note open-file: File Ports.).

 -- Scheme Procedure: fdes->ports fd
 -- C Function: scm_fdes_to_ports (fd)
     Return a list of existing ports which have FDES as an underlying
     file descriptor, without changing their revealed counts.

 -- Scheme Procedure: fdes->inport fdes
     Returns an existing input port which has FDES as its underlying
     file descriptor, if one exists, and increments its revealed count.
     Otherwise, returns a new input port with a revealed count of 1.

 -- Scheme Procedure: fdes->outport fdes
     Returns an existing output port which has FDES as its underlying
     file descriptor, if one exists, and increments its revealed count.
     Otherwise, returns a new output port with a revealed count of 1.

 -- Scheme Procedure: primitive-move->fdes port fd
 -- C Function: scm_primitive_move_to_fdes (port, fd)
     Moves the underlying file descriptor for PORT to the integer value
     FDES without changing the revealed count of PORT.  Any other ports
     already using this descriptor will be automatically shifted to new
     descriptors and their revealed counts reset to zero.  The return
     value is `#f' if the file descriptor already had the required
     value or `#t' if it was moved.

 -- Scheme Procedure: move->fdes port fdes
     Moves the underlying file descriptor for PORT to the integer value
     FDES and sets its revealed count to one.  Any other ports already
     using this descriptor will be automatically shifted to new
     descriptors and their revealed counts reset to zero.  The return
     value is unspecified.

 -- Scheme Procedure: release-port-handle port
     Decrements the revealed count for a port.

 -- Scheme Procedure: fsync object
 -- C Function: scm_fsync (object)
     Copies any unwritten data for the specified output file descriptor
     to disk.  If PORT/FD is a port, its buffer is flushed before the
     underlying file descriptor is fsync'd.  The return value is
     unspecified.

 -- Scheme Procedure: open path flags [mode]
 -- C Function: scm_open (path, flags, mode)
     Open the file named by PATH for reading and/or writing.  FLAGS is
     an integer specifying how the file should be opened.  MODE is an
     integer specifying the permission bits of the file, if it needs to
     be created, before the umask (*note Processes::) is applied.  The
     default is 666 (Unix itself has no default).

     FLAGS can be constructed by combining variables using `logior'.
     Basic flags are:

      -- Variable: O_RDONLY
          Open the file read-only.

      -- Variable: O_WRONLY
          Open the file write-only.

      -- Variable: O_RDWR
          Open the file read/write.

      -- Variable: O_APPEND
          Append to the file instead of truncating.

      -- Variable: O_CREAT
          Create the file if it does not already exist.

     *Note File Status Flags: (libc)File Status Flags, for additional
     flags.

 -- Scheme Procedure: open-fdes path flags [mode]
 -- C Function: scm_open_fdes (path, flags, mode)
     Similar to `open' but return a file descriptor instead of a port.

 -- Scheme Procedure: close fd_or_port
 -- C Function: scm_close (fd_or_port)
     Similar to `close-port' (*note close-port: Closing.), but also
     works on file descriptors.  A side effect of closing a file
     descriptor is that any ports using that file descriptor are moved
     to a different file descriptor and have their revealed counts set
     to zero.

 -- Scheme Procedure: close-fdes fd
 -- C Function: scm_close_fdes (fd)
     A simple wrapper for the `close' system call.  Close file
     descriptor FD, which must be an integer.  Unlike `close', the file
     descriptor will be closed even if a port is using it.  The return
     value is unspecified.

 -- Scheme Procedure: unread-char char [port]
 -- C Function: scm_unread_char (char, port)
     Place CHAR in PORT so that it will be read by the next read
     operation on that port.  If called multiple times, the unread
     characters will be read again in "last-in, first-out" order (i.e.
     a stack).  If PORT is not supplied, the current input port is used.

 -- Scheme Procedure: unread-string str port
     Place the string STR in PORT so that its characters will be read
     in subsequent read operations.  If called multiple times, the
     unread characters will be read again in last-in first-out order.
     If PORT is not supplied, the current-input-port is used.

 -- Scheme Procedure: pipe
 -- C Function: scm_pipe ()
     Return a newly created pipe: a pair of ports which are linked
     together on the local machine.  The CAR is the input port and the
     CDR is the output port.  Data written (and flushed) to the output
     port can be read from the input port.  Pipes are commonly used for
     communication with a newly forked child process.  The need to
     flush the output port can be avoided by making it unbuffered using
     `setvbuf'.

      -- Variable: PIPE_BUF
          A write of up to `PIPE_BUF' many bytes to a pipe is atomic,
          meaning when done it goes into the pipe instantaneously and
          as a contiguous block (*note Atomicity of Pipe I/O:
          (libc)Pipe Atomicity.).

     Note that the output port is likely to block if too much data has
     been written but not yet read from the input port.  Typically the
     capacity is `PIPE_BUF' bytes.

   The next group of procedures perform a `dup2' system call, if NEWFD
(an integer) is supplied, otherwise a `dup'.  The file descriptor to be
duplicated can be supplied as an integer or contained in a port.  The
type of value returned varies depending on which procedure is used.

   All procedures also have the side effect when performing `dup2' that
any ports using NEWFD are moved to a different file descriptor and have
their revealed counts set to zero.

 -- Scheme Procedure: dup->fdes fd_or_port [fd]
 -- C Function: scm_dup_to_fdes (fd_or_port, fd)
     Return a new integer file descriptor referring to the open file
     designated by FD_OR_PORT, which must be either an open file port
     or a file descriptor.

 -- Scheme Procedure: dup->inport port/fd [newfd]
     Returns a new input port using the new file descriptor.

 -- Scheme Procedure: dup->outport port/fd [newfd]
     Returns a new output port using the new file descriptor.

 -- Scheme Procedure: dup port/fd [newfd]
     Returns a new port if PORT/FD is a port, with the same mode as the
     supplied port, otherwise returns an integer file descriptor.

 -- Scheme Procedure: dup->port port/fd mode [newfd]
     Returns a new port using the new file descriptor.  MODE supplies a
     mode string for the port (*note open-file: File Ports.).

 -- Scheme Procedure: duplicate-port port modes
     Returns a new port which is opened on a duplicate of the file
     descriptor underlying PORT, with mode string MODES as for *note
     open-file: File Ports.  The two ports will share a file position
     and file status flags.

     Unexpected behaviour can result if both ports are subsequently used
     and the original and/or duplicate ports are buffered.  The mode
     string can include `0' to obtain an unbuffered duplicate port.

     This procedure is equivalent to `(dup->port PORT MODES)'.

 -- Scheme Procedure: redirect-port old new
 -- C Function: scm_redirect_port (old, new)
     This procedure takes two ports and duplicates the underlying file
     descriptor from OLD-PORT into NEW-PORT.  The current file
     descriptor in NEW-PORT will be closed.  After the redirection the
     two ports will share a file position and file status flags.

     The return value is unspecified.

     Unexpected behaviour can result if both ports are subsequently used
     and the original and/or duplicate ports are buffered.

     This procedure does not have any side effects on other ports or
     revealed counts.

 -- Scheme Procedure: dup2 oldfd newfd
 -- C Function: scm_dup2 (oldfd, newfd)
     A simple wrapper for the `dup2' system call.  Copies the file
     descriptor OLDFD to descriptor number NEWFD, replacing the
     previous meaning of NEWFD.  Both OLDFD and NEWFD must be integers.
     Unlike for `dup->fdes' or `primitive-move->fdes', no attempt is
     made to move away ports which are using NEWFD.  The return value
     is unspecified.

 -- Scheme Procedure: port-mode port
     Return the port modes associated with the open port PORT.  These
     will not necessarily be identical to the modes used when the port
     was opened, since modes such as "append" which are used only
     during port creation are not retained.

 -- Scheme Procedure: port-for-each proc
 -- C Function: scm_port_for_each (SCM proc)
 -- C Function: scm_c_port_for_each (void (*proc)(void *, SCM), void
          *data)
     Apply PROC to each port in the Guile port table (FIXME: what is
     the Guile port table?)  in turn.  The return value is unspecified.
     More specifically, PROC is applied exactly once to every port that
     exists in the system at the time `port-for-each' is invoked.
     Changes to the port table while `port-for-each' is running have no
     effect as far as `port-for-each' is concerned.

     The C function `scm_port_for_each' takes a Scheme procedure
     encoded as a `SCM' value, while `scm_c_port_for_each' takes a
     pointer to a C function and passes along a arbitrary DATA cookie.

 -- Scheme Procedure: setvbuf port mode [size]
 -- C Function: scm_setvbuf (port, mode, size)
     Set the buffering mode for PORT.  MODE can be:

      -- Variable: _IONBF
          non-buffered

      -- Variable: _IOLBF
          line buffered

      -- Variable: _IOFBF
          block buffered, using a newly allocated buffer of SIZE bytes.
          If SIZE is omitted, a default size will be used.

 -- Scheme Procedure: fcntl port/fd cmd [value]
 -- C Function: scm_fcntl (object, cmd, value)
     Apply CMD on PORT/FD, either a port or file descriptor.  The VALUE
     argument is used by the `SET' commands described below, it's an
     integer value.

     Values for CMD are:

      -- Variable: F_DUPFD
          Duplicate the file descriptor, the same as `dup->fdes' above
          does.

      -- Variable: F_GETFD
      -- Variable: F_SETFD
          Get or set flags associated with the file descriptor.  The
          only flag is the following,

           -- Variable: FD_CLOEXEC
               "Close on exec", meaning the file descriptor will be
               closed on an `exec' call (a successful such call).  For
               example to set that flag,

                    (fcntl port F_SETFD FD_CLOEXEC)

               Or better, set it but leave any other possible future
               flags unchanged,

                    (fcntl port F_SETFD (logior FD_CLOEXEC
                                                (fcntl port F_GETFD)))

      -- Variable: F_GETFL
      -- Variable: F_SETFL
          Get or set flags associated with the open file.  These flags
          are `O_RDONLY' etc described under `open' above.

          A common use is to set `O_NONBLOCK' on a network socket.  The
          following sets that flag, and leaves other flags unchanged.

               (fcntl sock F_SETFL (logior O_NONBLOCK
                                           (fcntl sock F_GETFL)))

      -- Variable: F_GETOWN
      -- Variable: F_SETOWN
          Get or set the process ID of a socket's owner, for `SIGIO'
          signals.

 -- Scheme Procedure: flock file operation
 -- C Function: scm_flock (file, operation)
     Apply or remove an advisory lock on an open file.  OPERATION
     specifies the action to be done:

      -- Variable: LOCK_SH
          Shared lock.  More than one process may hold a shared lock
          for a given file at a given time.

      -- Variable: LOCK_EX
          Exclusive lock.  Only one process may hold an exclusive lock
          for a given file at a given time.

      -- Variable: LOCK_UN
          Unlock the file.

      -- Variable: LOCK_NB
          Don't block when locking.  This is combined with one of the
          other operations using `logior' (*note Bitwise Operations::).
          If `flock' would block an `EWOULDBLOCK' error is thrown
          (*note Conventions::).

     The return value is not specified. FILE may be an open file
     descriptor or an open file descriptor port.

     Note that `flock' does not lock files across NFS.

 -- Scheme Procedure: select reads writes excepts [secs [usecs]]
 -- C Function: scm_select (reads, writes, excepts, secs, usecs)
     This procedure has a variety of uses: waiting for the ability to
     provide input, accept output, or the existence of exceptional
     conditions on a collection of ports or file descriptors, or
     waiting for a timeout to occur.  It also returns if interrupted by
     a signal.

     READS, WRITES and EXCEPTS can be lists or vectors, with each
     member a port or a file descriptor.  The value returned is a list
     of three corresponding lists or vectors containing only the
     members which meet the specified requirement.  The ability of port
     buffers to provide input or accept output is taken into account.
     Ordering of the input lists or vectors is not preserved.

     The optional arguments SECS and USECS specify the timeout.  Either
     SECS can be specified alone, as either an integer or a real
     number, or both SECS and USECS can be specified as integers, in
     which case USECS is an additional timeout expressed in
     microseconds.  If SECS is omitted or is `#f' then select will wait
     for as long as it takes for one of the other conditions to be
     satisfied.

     The scsh version of `select' differs as follows: Only vectors are
     accepted for the first three arguments.  The USECS argument is not
     supported.  Multiple values are returned instead of a list.
     Duplicates in the input vectors appear only once in output.  An
     additional `select!' interface is provided.

7.2.3 File System
-----------------

These procedures allow querying and setting file system attributes
(such as owner, permissions, sizes and types of files); deleting,
copying, renaming and linking files; creating and removing directories
and querying their contents; syncing the file system and creating
special files.

 -- Scheme Procedure: access? path how
 -- C Function: scm_access (path, how)
     Test accessibility of a file under the real UID and GID of the
     calling process.  The return is `#t' if PATH exists and the
     permissions requested by HOW are all allowed, or `#f' if not.

     HOW is an integer which is one of the following values, or a
     bitwise-OR (`logior') of multiple values.

      -- Variable: R_OK
          Test for read permission.

      -- Variable: W_OK
          Test for write permission.

      -- Variable: X_OK
          Test for execute permission.

      -- Variable: F_OK
          Test for existence of the file.  This is implied by each of
          the other tests, so there's no need to combine it with them.

     It's important to note that `access?' does not simply indicate
     what will happen on attempting to read or write a file.  In normal
     circumstances it does, but in a set-UID or set-GID program it
     doesn't because `access?' tests the real ID, whereas an open or
     execute attempt uses the effective ID.

     A program which will never run set-UID/GID can ignore the
     difference between real and effective IDs, but for maximum
     generality, especially in library functions, it's best not to use
     `access?' to predict the result of an open or execute, instead
     simply attempt that and catch any exception.

     The main use for `access?' is to let a set-UID/GID program
     determine what the invoking user would have been allowed to do,
     without the greater (or perhaps lesser) privileges afforded by the
     effective ID.  For more on this, see *note Testing File Access:
     (libc)Testing File Access.

 -- Scheme Procedure: stat object
 -- C Function: scm_stat (object)
     Return an object containing various information about the file
     determined by OBJ.  OBJ can be a string containing a file name or
     a port or integer file descriptor which is open on a file (in
     which case `fstat' is used as the underlying system call).

     The object returned by `stat' can be passed as a single parameter
     to the following procedures, all of which return integers:

      -- Scheme Procedure: stat:dev st
          The device number containing the file.

      -- Scheme Procedure: stat:ino st
          The file serial number, which distinguishes this file from all
          other files on the same device.

      -- Scheme Procedure: stat:mode st
          The mode of the file.  This is an integer which incorporates
          file type information and file permission bits.  See also
          `stat:type' and `stat:perms' below.

      -- Scheme Procedure: stat:nlink st
          The number of hard links to the file.

      -- Scheme Procedure: stat:uid st
          The user ID of the file's owner.

      -- Scheme Procedure: stat:gid st
          The group ID of the file.

      -- Scheme Procedure: stat:rdev st
          Device ID; this entry is defined only for character or block
          special files.  On some systems this field is not available
          at all, in which case `stat:rdev' returns `#f'.

      -- Scheme Procedure: stat:size st
          The size of a regular file in bytes.

      -- Scheme Procedure: stat:atime st
          The last access time for the file, in seconds.

      -- Scheme Procedure: stat:mtime st
          The last modification time for the file, in seconds.

      -- Scheme Procedure: stat:ctime st
          The last modification time for the attributes of the file, in
          seconds.

      -- Scheme Procedure: stat:atimensec st
      -- Scheme Procedure: stat:mtimensec st
      -- Scheme Procedure: stat:ctimensec st
          The fractional part of a file's access, modification, or
          attribute modification time, in nanoseconds. Nanosecond
          timestamps are only available on some operating systems and
          file systems. If Guile cannot retrieve nanosecond-level
          timestamps for a file, these fields will be set to 0.

      -- Scheme Procedure: stat:blksize st
          The optimal block size for reading or writing the file, in
          bytes.  On some systems this field is not available, in which
          case `stat:blksize' returns a sensible suggested block size.

      -- Scheme Procedure: stat:blocks st
          The amount of disk space that the file occupies measured in
          units of 512 byte blocks.  On some systems this field is not
          available, in which case `stat:blocks' returns `#f'.

     In addition, the following procedures return the information from
     `stat:mode' in a more convenient form:

      -- Scheme Procedure: stat:type st
          A symbol representing the type of file.  Possible values are
          `regular', `directory', `symlink', `block-special',
          `char-special', `fifo', `socket', and `unknown'.

      -- Scheme Procedure: stat:perms st
          An integer representing the access permission bits.

 -- Scheme Procedure: lstat str
 -- C Function: scm_lstat (str)
     Similar to `stat', but does not follow symbolic links, i.e., it
     will return information about a symbolic link itself, not the file
     it points to.  PATH must be a string.

 -- Scheme Procedure: readlink path
 -- C Function: scm_readlink (path)
     Return the value of the symbolic link named by PATH (a string),
     i.e., the file that the link points to.

 -- Scheme Procedure: chown object owner group
 -- C Function: scm_chown (object, owner, group)
     Change the ownership and group of the file referred to by OBJECT
     to the integer values OWNER and GROUP.  OBJECT can be a string
     containing a file name or, if the platform supports `fchown'
     (*note File Owner: (libc)File Owner.), a port or integer file
     descriptor which is open on the file.  The return value is
     unspecified.

     If OBJECT is a symbolic link, either the ownership of the link or
     the ownership of the referenced file will be changed depending on
     the operating system (lchown is unsupported at present).  If OWNER
     or GROUP is specified as `-1', then that ID is not changed.

 -- Scheme Procedure: chmod object mode
 -- C Function: scm_chmod (object, mode)
     Changes the permissions of the file referred to by OBJ.  OBJ can
     be a string containing a file name or a port or integer file
     descriptor which is open on a file (in which case `fchmod' is used
     as the underlying system call).  MODE specifies the new
     permissions as a decimal number, e.g., `(chmod "foo" #o755)'.  The
     return value is unspecified.

 -- Scheme Procedure: utime pathname [actime [modtime [actimens
          [modtimens [flags]]]]]
 -- C Function: scm_utime (pathname, actime, modtime, actimens,
          modtimens, flags)
     `utime' sets the access and modification times for the file named
     by PATH.  If ACTIME or MODTIME is not supplied, then the current
     time is used.  ACTIME and MODTIME must be integer time values as
     returned by the `current-time' procedure.

     The optional ACTIMENS and MODTIMENS are nanoseconds to add ACTIME
     and MODTIME. Nanosecond precision is only supported on some
     combinations of file systems and operating systems.
          (utime "foo" (- (current-time) 3600))
     will set the access time to one hour in the past and the
     modification time to the current time.

 -- Scheme Procedure: delete-file str
 -- C Function: scm_delete_file (str)
     Deletes (or "unlinks") the file whose path is specified by STR.

 -- Scheme Procedure: copy-file oldfile newfile
 -- C Function: scm_copy_file (oldfile, newfile)
     Copy the file specified by OLDFILE to NEWFILE.  The return value
     is unspecified.

 -- Scheme Procedure: rename-file oldname newname
 -- C Function: scm_rename (oldname, newname)
     Renames the file specified by OLDNAME to NEWNAME.  The return
     value is unspecified.

 -- Scheme Procedure: link oldpath newpath
 -- C Function: scm_link (oldpath, newpath)
     Creates a new name NEWPATH in the file system for the file named
     by OLDPATH.  If OLDPATH is a symbolic link, the link may or may
     not be followed depending on the system.

 -- Scheme Procedure: symlink oldpath newpath
 -- C Function: scm_symlink (oldpath, newpath)
     Create a symbolic link named NEWPATH with the value (i.e.,
     pointing to) OLDPATH.  The return value is unspecified.

 -- Scheme Procedure: mkdir path [mode]
 -- C Function: scm_mkdir (path, mode)
     Create a new directory named by PATH.  If MODE is omitted then the
     permissions of the directory file are set using the current umask
     (*note Processes::).  Otherwise they are set to the decimal value
     specified with MODE.  The return value is unspecified.

 -- Scheme Procedure: rmdir path
 -- C Function: scm_rmdir (path)
     Remove the existing directory named by PATH.  The directory must
     be empty for this to succeed.  The return value is unspecified.

 -- Scheme Procedure: opendir dirname
 -- C Function: scm_opendir (dirname)
     Open the directory specified by DIRNAME and return a directory
     stream.

     Before using this and the procedures below, make sure to see the
     higher-level procedures for directory traversal that are available
     (*note File Tree Walk::).

 -- Scheme Procedure: directory-stream? object
 -- C Function: scm_directory_stream_p (object)
     Return a boolean indicating whether OBJECT is a directory stream
     as returned by `opendir'.

 -- Scheme Procedure: readdir stream
 -- C Function: scm_readdir (stream)
     Return (as a string) the next directory entry from the directory
     stream STREAM.  If there is no remaining entry to be read then the
     end of file object is returned.

 -- Scheme Procedure: rewinddir stream
 -- C Function: scm_rewinddir (stream)
     Reset the directory port STREAM so that the next call to `readdir'
     will return the first directory entry.

 -- Scheme Procedure: closedir stream
 -- C Function: scm_closedir (stream)
     Close the directory stream STREAM.  The return value is
     unspecified.

   Here is an example showing how to display all the entries in a
directory:

     (define dir (opendir "/usr/lib"))
     (do ((entry (readdir dir) (readdir dir)))
         ((eof-object? entry))
       (display entry)(newline))
     (closedir dir)

 -- Scheme Procedure: sync
 -- C Function: scm_sync ()
     Flush the operating system disk buffers.  The return value is
     unspecified.

 -- Scheme Procedure: mknod path type perms dev
 -- C Function: scm_mknod (path, type, perms, dev)
     Creates a new special file, such as a file corresponding to a
     device.  PATH specifies the name of the file.  TYPE should be one
     of the following symbols: `regular', `directory', `symlink',
     `block-special', `char-special', `fifo', or `socket'.  PERMS (an
     integer) specifies the file permissions.  DEV (an integer)
     specifies which device the special file refers to.  Its exact
     interpretation depends on the kind of special file being created.

     E.g.,
          (mknod "/dev/fd0" 'block-special #o660 (+ (* 2 256) 2))

     The return value is unspecified.

 -- Scheme Procedure: tmpnam
 -- C Function: scm_tmpnam ()
     Return an auto-generated name of a temporary file, a file which
     doesn't already exist.  The name includes a path, it's usually in
     `/tmp' but that's system dependent.

     Care must be taken when using `tmpnam'.  In between choosing the
     name and creating the file another program might use that name, or
     an attacker might even make it a symlink pointing at something
     important and causing you to overwrite that.

     The safe way is to create the file using `open' with `O_EXCL' to
     avoid any overwriting.  A loop can try again with another name if
     the file exists (error `EEXIST').  `mkstemp!' below does that.

 -- Scheme Procedure: mkstemp! tmpl
 -- C Function: scm_mkstemp (tmpl)
     Create a new unique file in the file system and return a new
     buffered port open for reading and writing to the file.

     TMPL is a string specifying where the file should be created: it
     must end with `XXXXXX' and those `X's will be changed in the
     string to return the name of the file.  (`port-filename' on the
     port also gives the name.)

     POSIX doesn't specify the permissions mode of the file, on GNU and
     most systems it's `#o600'.  An application can use `chmod' to
     relax that if desired.  For example `#o666' less `umask', which is
     usual for ordinary file creation,

          (let ((port (mkstemp! (string-copy "/tmp/myfile-XXXXXX"))))
            (chmod port (logand #o666 (lognot (umask))))
            ...)

 -- Scheme Procedure: tmpfile
 -- C Function: scm_tmpfile
     Return an input/output port to a unique temporary file named using
     the path prefix `P_tmpdir' defined in `stdio.h'.  The file is
     automatically deleted when the port is closed or the program
     terminates.

 -- Scheme Procedure: dirname filename
 -- C Function: scm_dirname (filename)
     Return the directory name component of the file name FILENAME. If
     FILENAME does not contain a directory component, `.' is returned.

 -- Scheme Procedure: basename filename [suffix]
 -- C Function: scm_basename (filename, suffix)
     Return the base name of the file name FILENAME. The base name is
     the file name without any directory components.  If SUFFIX is
     provided, and is equal to the end of BASENAME, it is removed also.

          (basename "/tmp/test.xml" ".xml")
          => "test"

 -- Scheme Procedure: file-exists? filename
     Return `#t' if the file named FILENAME exists, `#f' if not.

7.2.4 User Information
----------------------

The facilities in this section provide an interface to the user and
group database.  They should be used with care since they are not
reentrant.

   The following functions accept an object representing user
information and return a selected component:

 -- Scheme Procedure: passwd:name pw
     The name of the userid.

 -- Scheme Procedure: passwd:passwd pw
     The encrypted passwd.

 -- Scheme Procedure: passwd:uid pw
     The user id number.

 -- Scheme Procedure: passwd:gid pw
     The group id number.

 -- Scheme Procedure: passwd:gecos pw
     The full name.

 -- Scheme Procedure: passwd:dir pw
     The home directory.

 -- Scheme Procedure: passwd:shell pw
     The login shell.


 -- Scheme Procedure: getpwuid uid
     Look up an integer userid in the user database.

 -- Scheme Procedure: getpwnam name
     Look up a user name string in the user database.

 -- Scheme Procedure: setpwent
     Initializes a stream used by `getpwent' to read from the user
     database.  The next use of `getpwent' will return the first entry.
     The return value is unspecified.

 -- Scheme Procedure: getpwent
     Read the next entry in the user database stream.  The return is a
     passwd user object as above, or `#f' when no more entries.

 -- Scheme Procedure: endpwent
     Closes the stream used by `getpwent'.  The return value is
     unspecified.

 -- Scheme Procedure: setpw [arg]
 -- C Function: scm_setpwent (arg)
     If called with a true argument, initialize or reset the password
     data stream.  Otherwise, close the stream.  The `setpwent' and
     `endpwent' procedures are implemented on top of this.

 -- Scheme Procedure: getpw [user]
 -- C Function: scm_getpwuid (user)
     Look up an entry in the user database.  OBJ can be an integer, a
     string, or omitted, giving the behaviour of getpwuid, getpwnam or
     getpwent respectively.

   The following functions accept an object representing group
information and return a selected component:

 -- Scheme Procedure: group:name gr
     The group name.

 -- Scheme Procedure: group:passwd gr
     The encrypted group password.

 -- Scheme Procedure: group:gid gr
     The group id number.

 -- Scheme Procedure: group:mem gr
     A list of userids which have this group as a supplementary group.


 -- Scheme Procedure: getgrgid gid
     Look up an integer group id in the group database.

 -- Scheme Procedure: getgrnam name
     Look up a group name in the group database.

 -- Scheme Procedure: setgrent
     Initializes a stream used by `getgrent' to read from the group
     database.  The next use of `getgrent' will return the first entry.
     The return value is unspecified.

 -- Scheme Procedure: getgrent
     Return the next entry in the group database, using the stream set
     by `setgrent'.

 -- Scheme Procedure: endgrent
     Closes the stream used by `getgrent'.  The return value is
     unspecified.

 -- Scheme Procedure: setgr [arg]
 -- C Function: scm_setgrent (arg)
     If called with a true argument, initialize or reset the group data
     stream.  Otherwise, close the stream.  The `setgrent' and
     `endgrent' procedures are implemented on top of this.

 -- Scheme Procedure: getgr [name]
 -- C Function: scm_getgrgid (name)
     Look up an entry in the group database.  OBJ can be an integer, a
     string, or omitted, giving the behaviour of getgrgid, getgrnam or
     getgrent respectively.

   In addition to the accessor procedures for the user database, the
following shortcut procedure is also available.

 -- Scheme Procedure: getlogin
 -- C Function: scm_getlogin ()
     Return a string containing the name of the user logged in on the
     controlling terminal of the process, or `#f' if this information
     cannot be obtained.

7.2.5 Time
----------

 -- Scheme Procedure: current-time
 -- C Function: scm_current_time ()
     Return the number of seconds since 1970-01-01 00:00:00 UTC,
     excluding leap seconds.

 -- Scheme Procedure: gettimeofday
 -- C Function: scm_gettimeofday ()
     Return a pair containing the number of seconds and microseconds
     since 1970-01-01 00:00:00 UTC, excluding leap seconds.  Note:
     whether true microsecond resolution is available depends on the
     operating system.

   The following procedures either accept an object representing a
broken down time and return a selected component, or accept an object
representing a broken down time and a value and set the component to
the value.  The numbers in parentheses give the usual range.

 -- Scheme Procedure: tm:sec tm
 -- Scheme Procedure: set-tm:sec tm val
     Seconds (0-59).

 -- Scheme Procedure: tm:min tm
 -- Scheme Procedure: set-tm:min tm val
     Minutes (0-59).

 -- Scheme Procedure: tm:hour tm
 -- Scheme Procedure: set-tm:hour tm val
     Hours (0-23).

 -- Scheme Procedure: tm:mday tm
 -- Scheme Procedure: set-tm:mday tm val
     Day of the month (1-31).

 -- Scheme Procedure: tm:mon tm
 -- Scheme Procedure: set-tm:mon tm val
     Month (0-11).

 -- Scheme Procedure: tm:year tm
 -- Scheme Procedure: set-tm:year tm val
     Year (70-), the year minus 1900.

 -- Scheme Procedure: tm:wday tm
 -- Scheme Procedure: set-tm:wday tm val
     Day of the week (0-6) with Sunday represented as 0.

 -- Scheme Procedure: tm:yday tm
 -- Scheme Procedure: set-tm:yday tm val
     Day of the year (0-364, 365 in leap years).

 -- Scheme Procedure: tm:isdst tm
 -- Scheme Procedure: set-tm:isdst tm val
     Daylight saving indicator (0 for "no", greater than 0 for "yes",
     less than 0 for "unknown").

 -- Scheme Procedure: tm:gmtoff tm
 -- Scheme Procedure: set-tm:gmtoff tm val
     Time zone offset in seconds west of UTC (-46800 to 43200).  For
     example on East coast USA (zone `EST+5') this would be 18000 (ie.
     5*60*60) in winter, or 14400 (ie. 4*60*60) during daylight savings.

     Note `tm:gmtoff' is not the same as `tm_gmtoff' in the C `tm'
     structure.  `tm_gmtoff' is seconds east and hence the negative of
     the value here.

 -- Scheme Procedure: tm:zone tm
 -- Scheme Procedure: set-tm:zone tm val
     Time zone label (a string), not necessarily unique.


 -- Scheme Procedure: localtime time [zone]
 -- C Function: scm_localtime (time, zone)
     Return an object representing the broken down components of TIME,
     an integer like the one returned by `current-time'.  The time zone
     for the calculation is optionally specified by ZONE (a string),
     otherwise the `TZ' environment variable or the system default is
     used.

 -- Scheme Procedure: gmtime time
 -- C Function: scm_gmtime (time)
     Return an object representing the broken down components of TIME,
     an integer like the one returned by `current-time'.  The values
     are calculated for UTC.

 -- Scheme Procedure: mktime sbd-time [zone]
 -- C Function: scm_mktime (sbd_time, zone)
     For a broken down time object SBD-TIME, return a pair the `car' of
     which is an integer time like `current-time', and the `cdr' of
     which is a new broken down time with normalized fields.

     ZONE is a timezone string, or the default is the `TZ' environment
     variable or the system default (*note Specifying the Time Zone
     with `TZ': (libc)TZ Variable.).  SBD-TIME is taken to be in that
     ZONE.

     The following fields of SBD-TIME are used: `tm:year', `tm:mon',
     `tm:mday', `tm:hour', `tm:min', `tm:sec', `tm:isdst'.  The values
     can be outside their usual ranges.  For example `tm:hour' normally
     goes up to 23, but a value say 33 would mean 9 the following day.

     `tm:isdst' in SBD-TIME says whether the time given is with
     daylight savings or not.  This is ignored if ZONE doesn't have any
     daylight savings adjustment amount.

     The broken down time in the return normalizes the values of
     SBD-TIME by bringing them into their usual ranges, and using the
     actual daylight savings rule for that time in ZONE (which may
     differ from what SBD-TIME had).  The easiest way to think of this
     is that SBD-TIME plus ZONE converts to the integer UTC time, then
     a `localtime' is applied to get the normal presentation of that
     time, in ZONE.

 -- Scheme Procedure: tzset
 -- C Function: scm_tzset ()
     Initialize the timezone from the `TZ' environment variable or the
     system default.  It's not usually necessary to call this procedure
     since it's done automatically by other procedures that depend on
     the timezone.

 -- Scheme Procedure: strftime format tm
 -- C Function: scm_strftime (format, tm)
     Return a string which is broken-down time structure TM formatted
     according to the given FORMAT string.

     FORMAT contains field specifications introduced by a `%'
     character.  See *note Formatting Calendar Time: (libc)Formatting
     Calendar Time, or `man 3 strftime', for the available formatting.

          (strftime "%c" (localtime (current-time)))
          => "Mon Mar 11 20:17:43 2002"

     If `setlocale' has been called (*note Locales::), month and day
     names are from the current locale and in the locale character set.

 -- Scheme Procedure: strptime format string
 -- C Function: scm_strptime (format, string)
     Performs the reverse action to `strftime', parsing STRING
     according to the specification supplied in TEMPLATE.  The
     interpretation of month and day names is dependent on the current
     locale.  The value returned is a pair.  The CAR has an object with
     time components in the form returned by `localtime' or `gmtime',
     but the time zone components are not usefully set.  The CDR
     reports the number of characters from STRING which were used for
     the conversion.

 -- Variable: internal-time-units-per-second
     The value of this variable is the number of time units per second
     reported by the following procedures.

 -- Scheme Procedure: times
 -- C Function: scm_times ()
     Return an object with information about real and processor time.
     The following procedures accept such an object as an argument and
     return a selected component:

      -- Scheme Procedure: tms:clock tms
          The current real time, expressed as time units relative to an
          arbitrary base.

      -- Scheme Procedure: tms:utime tms
          The CPU time units used by the calling process.

      -- Scheme Procedure: tms:stime tms
          The CPU time units used by the system on behalf of the calling
          process.

      -- Scheme Procedure: tms:cutime tms
          The CPU time units used by terminated child processes of the
          calling process, whose status has been collected (e.g., using
          `waitpid').

      -- Scheme Procedure: tms:cstime tms
          Similarly, the CPU times units used by the system on behalf of
          terminated child processes.

 -- Scheme Procedure: get-internal-real-time
 -- C Function: scm_get_internal_real_time ()
     Return the number of time units since the interpreter was started.

 -- Scheme Procedure: get-internal-run-time
 -- C Function: scm_get_internal_run_time ()
     Return the number of time units of processor time used by the
     interpreter.  Both _system_ and _user_ time are included but
     subprocesses are not.

7.2.6 Runtime Environment
-------------------------

 -- Scheme Procedure: program-arguments
 -- Scheme Procedure: command-line
 -- Scheme Procedure: set-program-arguments
 -- C Function: scm_program_arguments ()
 -- C Function: scm_set_program_arguments_scm (lst)
     Get the command line arguments passed to Guile, or set new
     arguments.

     The arguments are a list of strings, the first of which is the
     invoked program name.  This is just "guile" (or the executable
     path) when run interactively, or it's the script name when running
     a script with `-s' (*note Invoking Guile::).

          guile -L /my/extra/dir -s foo.scm abc def

          (program-arguments) => ("foo.scm" "abc" "def")

     `set-program-arguments' allows a library module or similar to
     modify the arguments, for example to strip options it recognises,
     leaving the rest for the mainline.

     The argument list is held in a fluid, which means it's separate for
     each thread.  Neither the list nor the strings within it are
     copied at any point and normally should not be mutated.

     The two names `program-arguments' and `command-line' are an
     historical accident, they both do exactly the same thing.  The name
     `scm_set_program_arguments_scm' has an extra `_scm' on the end to
     avoid clashing with the C function below.

 -- C Function: void scm_set_program_arguments (int argc, char **argv,
          char *first)
     Set the list of command line arguments for `program-arguments' and
     `command-line' above.

     ARGV is an array of null-terminated strings, as in a C `main'
     function.  ARGC is the number of strings in ARGV, or if it's
     negative then a `NULL' in ARGV marks its end.

     FIRST is an extra string put at the start of the arguments, or
     `NULL' for no such extra.  This is a convenient way to pass the
     program name after advancing ARGV to strip option arguments.  Eg.

          {
            char *progname = argv[0];
            for (argv++; argv[0] != NULL && argv[0][0] == '-'; argv++)
              {
                /* munch option ... */
              }
            /* remaining args for scheme level use */
            scm_set_program_arguments (-1, argv, progname);
          }

     This sort of thing is often done at startup under `scm_boot_guile'
     with options handled at the C level removed.  The given strings
     are all copied, so the C data is not accessed again once
     `scm_set_program_arguments' returns.

 -- Scheme Procedure: getenv nam
 -- C Function: scm_getenv (nam)
     Looks up the string NAME in the current environment.  The return
     value is `#f' unless a string of the form `NAME=VALUE' is found,
     in which case the string `VALUE' is returned.

 -- Scheme Procedure: setenv name value
     Modifies the environment of the current process, which is also the
     default environment inherited by child processes.

     If VALUE is `#f', then NAME is removed from the environment.
     Otherwise, the string NAME=VALUE is added to the environment,
     replacing any existing string with name matching NAME.

     The return value is unspecified.

 -- Scheme Procedure: unsetenv name
     Remove variable NAME from the environment.  The name can not
     contain a `=' character.

 -- Scheme Procedure: environ [env]
 -- C Function: scm_environ (env)
     If ENV is omitted, return the current environment (in the Unix
     sense) as a list of strings.  Otherwise set the current
     environment, which is also the default environment for child
     processes, to the supplied list of strings.  Each member of ENV
     should be of the form NAME=VALUE and values of NAME should not be
     duplicated.  If ENV is supplied then the return value is
     unspecified.

 -- Scheme Procedure: putenv str
 -- C Function: scm_putenv (str)
     Modifies the environment of the current process, which is also the
     default environment inherited by child processes.

     If STRING is of the form `NAME=VALUE' then it will be written
     directly into the environment, replacing any existing environment
     string with name matching `NAME'.  If STRING does not contain an
     equal sign, then any existing string with name matching STRING will
     be removed.

     The return value is unspecified.

7.2.7 Processes
---------------

 -- Scheme Procedure: chdir str
 -- C Function: scm_chdir (str)
     Change the current working directory to PATH.  The return value is
     unspecified.

 -- Scheme Procedure: getcwd
 -- C Function: scm_getcwd ()
     Return the name of the current working directory.

 -- Scheme Procedure: umask [mode]
 -- C Function: scm_umask (mode)
     If MODE is omitted, returns a decimal number representing the
     current file creation mask.  Otherwise the file creation mask is
     set to MODE and the previous value is returned. *Note Assigning
     File Permissions: (libc)Setting Permissions, for more on how to
     use umasks.

     E.g., `(umask #o022)' sets the mask to octal 22/decimal 18.

 -- Scheme Procedure: chroot path
 -- C Function: scm_chroot (path)
     Change the root directory to that specified in PATH.  This
     directory will be used for path names beginning with `/'.  The
     root directory is inherited by all children of the current
     process.  Only the superuser may change the root directory.

 -- Scheme Procedure: getpid
 -- C Function: scm_getpid ()
     Return an integer representing the current process ID.

 -- Scheme Procedure: getgroups
 -- C Function: scm_getgroups ()
     Return a vector of integers representing the current supplementary
     group IDs.

 -- Scheme Procedure: getppid
 -- C Function: scm_getppid ()
     Return an integer representing the process ID of the parent
     process.

 -- Scheme Procedure: getuid
 -- C Function: scm_getuid ()
     Return an integer representing the current real user ID.

 -- Scheme Procedure: getgid
 -- C Function: scm_getgid ()
     Return an integer representing the current real group ID.

 -- Scheme Procedure: geteuid
 -- C Function: scm_geteuid ()
     Return an integer representing the current effective user ID.  If
     the system does not support effective IDs, then the real ID is
     returned.  `(provided? 'EIDs)' reports whether the system supports
     effective IDs.

 -- Scheme Procedure: getegid
 -- C Function: scm_getegid ()
     Return an integer representing the current effective group ID.  If
     the system does not support effective IDs, then the real ID is
     returned.  `(provided? 'EIDs)' reports whether the system supports
     effective IDs.

 -- Scheme Procedure: setgroups vec
 -- C Function: scm_setgroups (vec)
     Set the current set of supplementary group IDs to the integers in
     the given vector VEC.  The return value is unspecified.

     Generally only the superuser can set the process group IDs (*note
     Setting the Group IDs: (libc)Setting Groups.).

 -- Scheme Procedure: setuid id
 -- C Function: scm_setuid (id)
     Sets both the real and effective user IDs to the integer ID,
     provided the process has appropriate privileges.  The return value
     is unspecified.

 -- Scheme Procedure: setgid id
 -- C Function: scm_setgid (id)
     Sets both the real and effective group IDs to the integer ID,
     provided the process has appropriate privileges.  The return value
     is unspecified.

 -- Scheme Procedure: seteuid id
 -- C Function: scm_seteuid (id)
     Sets the effective user ID to the integer ID, provided the process
     has appropriate privileges.  If effective IDs are not supported,
     the real ID is set instead--`(provided? 'EIDs)' reports whether the
     system supports effective IDs.  The return value is unspecified.

 -- Scheme Procedure: setegid id
 -- C Function: scm_setegid (id)
     Sets the effective group ID to the integer ID, provided the process
     has appropriate privileges.  If effective IDs are not supported,
     the real ID is set instead--`(provided? 'EIDs)' reports whether the
     system supports effective IDs.  The return value is unspecified.

 -- Scheme Procedure: getpgrp
 -- C Function: scm_getpgrp ()
     Return an integer representing the current process group ID.  This
     is the POSIX definition, not BSD.

 -- Scheme Procedure: setpgid pid pgid
 -- C Function: scm_setpgid (pid, pgid)
     Move the process PID into the process group PGID.  PID or PGID
     must be integers: they can be zero to indicate the ID of the
     current process.  Fails on systems that do not support job control.
     The return value is unspecified.

 -- Scheme Procedure: setsid
 -- C Function: scm_setsid ()
     Creates a new session.  The current process becomes the session
     leader and is put in a new process group.  The process will be
     detached from its controlling terminal if it has one.  The return
     value is an integer representing the new process group ID.

 -- Scheme Procedure: getsid pid
 -- C Function: scm_getsid (pid)
     Returns the session ID of process PID.  (The session ID of a
     process is the process group ID of its session leader.)

 -- Scheme Procedure: waitpid pid [options]
 -- C Function: scm_waitpid (pid, options)
     This procedure collects status information from a child process
     which has terminated or (optionally) stopped.  Normally it will
     suspend the calling process until this can be done.  If more than
     one child process is eligible then one will be chosen by the
     operating system.

     The value of PID determines the behaviour:

    PID greater than 0
          Request status information from the specified child process.

    PID equal to -1 or `WAIT_ANY'
          Request status information for any child process.

    PID equal to 0 or `WAIT_MYPGRP'
          Request status information for any child process in the
          current process group.

    PID less than -1
          Request status information for any child process whose
          process group ID is -PID.

     The OPTIONS argument, if supplied, should be the bitwise OR of the
     values of zero or more of the following variables:

      -- Variable: WNOHANG
          Return immediately even if there are no child processes to be
          collected.

      -- Variable: WUNTRACED
          Report status information for stopped processes as well as
          terminated processes.

     The return value is a pair containing:

       1. The process ID of the child process, or 0 if `WNOHANG' was
          specified and no process was collected.

       2. The integer status value.

   The following three functions can be used to decode the process
status code returned by `waitpid'.

 -- Scheme Procedure: status:exit-val status
 -- C Function: scm_status_exit_val (status)
     Return the exit status value, as would be set if a process ended
     normally through a call to `exit' or `_exit', if any, otherwise
     `#f'.

 -- Scheme Procedure: status:term-sig status
 -- C Function: scm_status_term_sig (status)
     Return the signal number which terminated the process, if any,
     otherwise `#f'.

 -- Scheme Procedure: status:stop-sig status
 -- C Function: scm_status_stop_sig (status)
     Return the signal number which stopped the process, if any,
     otherwise `#f'.

 -- Scheme Procedure: system [cmd]
 -- C Function: scm_system (cmd)
     Execute CMD using the operating system's "command processor".
     Under Unix this is usually the default shell `sh'.  The value
     returned is CMD's exit status as returned by `waitpid', which can
     be interpreted using the functions above.

     If `system' is called without arguments, return a boolean
     indicating whether the command processor is available.

 -- Scheme Procedure: system* . args
 -- C Function: scm_system_star (args)
     Execute the command indicated by ARGS.  The first element must be
     a string indicating the command to be executed, and the remaining
     items must be strings representing each of the arguments to that
     command.

     This function returns the exit status of the command as provided by
     `waitpid'.  This value can be handled with `status:exit-val' and
     the related functions.

     `system*' is similar to `system', but accepts only one string
     per-argument, and performs no shell interpretation.  The command
     is executed using fork and execlp.  Accordingly this function may
     be safer than `system' in situations where shell interpretation is
     not required.

     Example: (system* "echo" "foo" "bar")

 -- Scheme Procedure: primitive-exit [status]
 -- Scheme Procedure: primitive-_exit [status]
 -- C Function: scm_primitive_exit (status)
 -- C Function: scm_primitive__exit (status)
     Terminate the current process without unwinding the Scheme stack.
     The exit status is STATUS if supplied, otherwise zero.

     `primitive-exit' uses the C `exit' function and hence runs usual C
     level cleanups (flush output streams, call `atexit' functions,
     etc, see *note Normal Termination: (libc)Normal Termination.)).

     `primitive-_exit' is the `_exit' system call (*note Termination
     Internals: (libc)Termination Internals.).  This terminates the
     program immediately, with neither Scheme-level nor C-level
     cleanups.

     The typical use for `primitive-_exit' is from a child process
     created with `primitive-fork'.  For example in a Gdk program the
     child process inherits the X server connection and a C-level
     `atexit' cleanup which will close that connection.  But closing in
     the child would upset the protocol in the parent, so
     `primitive-_exit' should be used to exit without that.

 -- Scheme Procedure: execl filename . args
 -- C Function: scm_execl (filename, args)
     Executes the file named by PATH as a new process image.  The
     remaining arguments are supplied to the process; from a C program
     they are accessible as the `argv' argument to `main'.
     Conventionally the first ARG is the same as PATH.  All arguments
     must be strings.

     If ARG is missing, PATH is executed with a null argument list,
     which may have system-dependent side-effects.

     This procedure is currently implemented using the `execv' system
     call, but we call it `execl' because of its Scheme calling
     interface.

 -- Scheme Procedure: execlp filename . args
 -- C Function: scm_execlp (filename, args)
     Similar to `execl', however if FILENAME does not contain a slash
     then the file to execute will be located by searching the
     directories listed in the `PATH' environment variable.

     This procedure is currently implemented using the `execvp' system
     call, but we call it `execlp' because of its Scheme calling
     interface.

 -- Scheme Procedure: execle filename env . args
 -- C Function: scm_execle (filename, env, args)
     Similar to `execl', but the environment of the new process is
     specified by ENV, which must be a list of strings as returned by
     the `environ' procedure.

     This procedure is currently implemented using the `execve' system
     call, but we call it `execle' because of its Scheme calling
     interface.

 -- Scheme Procedure: primitive-fork
 -- C Function: scm_fork ()
     Creates a new "child" process by duplicating the current "parent"
     process.  In the child the return value is 0.  In the parent the
     return value is the integer process ID of the child.

     This procedure has been renamed from `fork' to avoid a naming
     conflict with the scsh fork.

 -- Scheme Procedure: nice incr
 -- C Function: scm_nice (incr)
     Increment the priority of the current process by INCR.  A higher
     priority value means that the process runs less often.  The return
     value is unspecified.

 -- Scheme Procedure: setpriority which who prio
 -- C Function: scm_setpriority (which, who, prio)
     Set the scheduling priority of the process, process group or user,
     as indicated by WHICH and WHO. WHICH is one of the variables
     `PRIO_PROCESS', `PRIO_PGRP' or `PRIO_USER', and WHO is interpreted
     relative to WHICH (a process identifier for `PRIO_PROCESS',
     process group identifier for `PRIO_PGRP', and a user identifier
     for `PRIO_USER'.  A zero value of WHO denotes the current process,
     process group, or user.  PRIO is a value in the range [-20,20].
     The default priority is 0; lower priorities (in numerical terms)
     cause more favorable scheduling.  Sets the priority of all of the
     specified processes.  Only the super-user may lower priorities.
     The return value is not specified.

 -- Scheme Procedure: getpriority which who
 -- C Function: scm_getpriority (which, who)
     Return the scheduling priority of the process, process group or
     user, as indicated by WHICH and WHO. WHICH is one of the variables
     `PRIO_PROCESS', `PRIO_PGRP' or `PRIO_USER', and WHO should be
     interpreted depending on WHICH (a process identifier for
     `PRIO_PROCESS', process group identifier for `PRIO_PGRP', and a
     user identifier for `PRIO_USER').  A zero value of WHO denotes the
     current process, process group, or user.  Return the highest
     priority (lowest numerical value) of any of the specified
     processes.

 -- Scheme Procedure: getaffinity pid
 -- C Function: scm_getaffinity (pid)
     Return a bitvector representing the CPU affinity mask for process
     PID.  Each CPU the process has affinity with has its corresponding
     bit set in the returned bitvector.  The number of bits set is a
     good estimate of how many CPUs Guile can use without stepping on
     other processes' toes.

     Currently this procedure is only defined on GNU variants (*note
     `sched_getaffinity': (libc)CPU Affinity.).

 -- Scheme Procedure: setaffinity pid mask
 -- C Function: scm_setaffinity (pid, mask)
     Install the CPU affinity mask MASK, a bitvector, for the process
     or thread with ID PID.  The return value is unspecified.

     Currently this procedure is only defined on GNU variants (*note
     `sched_setaffinity': (libc)CPU Affinity.).

 -- Scheme Procedure: total-processor-count
 -- C Function: scm_total_processor_count ()
     Return the total number of processors of the machine, which is
     guaranteed to be at least 1.  A "processor" here is a thread
     execution unit, which can be either:

        * an execution core in a (possibly multi-core) chip, in a
          (possibly multi- chip) module, in a single computer, or

        * a thread execution unit inside a core in the case of
          "hyper-threaded" CPUs.

     Which of the two definitions is used, is unspecified.

 -- Scheme Procedure: current-processor-count
 -- C Function: scm_current_processor_count ()
     Like `total-processor-count', but return the number of processors
     available to the current process.  See `setaffinity' and
     `getaffinity' for more information.

7.2.8 Signals
-------------

The following procedures raise, handle and wait for signals.

   Scheme code signal handlers are run via a system async (*note System
asyncs::), so they're called in the handler's thread at the next safe
opportunity.  Generally this is after any currently executing primitive
procedure finishes (which could be a long time for primitives that wait
for an external event).

 -- Scheme Procedure: kill pid sig
 -- C Function: scm_kill (pid, sig)
     Sends a signal to the specified process or group of processes.

     PID specifies the processes to which the signal is sent:

    PID greater than 0
          The process whose identifier is PID.

    PID equal to 0
          All processes in the current process group.

    PID less than -1
          The process group whose identifier is -PID

    PID equal to -1
          If the process is privileged, all processes except for some
          special system processes.  Otherwise, all processes with the
          current effective user ID.

     SIG should be specified using a variable corresponding to the Unix
     symbolic name, e.g.,

      -- Variable: SIGHUP
          Hang-up signal.

      -- Variable: SIGINT
          Interrupt signal.

     A full list of signals on the GNU system may be found in *note
     Standard Signals: (libc)Standard Signals.

 -- Scheme Procedure: raise sig
 -- C Function: scm_raise (sig)
     Sends a specified signal SIG to the current process, where SIG is
     as described for the `kill' procedure.

 -- Scheme Procedure: sigaction signum [handler [flags [thread]]]
 -- C Function: scm_sigaction (signum, handler, flags)
 -- C Function: scm_sigaction_for_thread (signum, handler, flags,
          thread)
     Install or report the signal handler for a specified signal.

     SIGNUM is the signal number, which can be specified using the value
     of variables such as `SIGINT'.

     If HANDLER is omitted, `sigaction' returns a pair: the CAR is the
     current signal hander, which will be either an integer with the
     value `SIG_DFL' (default action) or `SIG_IGN' (ignore), or the
     Scheme procedure which handles the signal, or `#f' if a non-Scheme
     procedure handles the signal.  The CDR contains the current
     `sigaction' flags for the handler.

     If HANDLER is provided, it is installed as the new handler for
     SIGNUM.  HANDLER can be a Scheme procedure taking one argument, or
     the value of `SIG_DFL' (default action) or `SIG_IGN' (ignore), or
     `#f' to restore whatever signal handler was installed before
     `sigaction' was first used.  When a scheme procedure has been
     specified, that procedure will run in the given THREAD.  When no
     thread has been given, the thread that made this call to
     `sigaction' is used.

     FLAGS is a `logior' (*note Bitwise Operations::) of the following
     (where provided by the system), or `0' for none.

      -- Variable: SA_NOCLDSTOP
          By default, `SIGCHLD' is signalled when a child process stops
          (ie. receives `SIGSTOP'), and when a child process terminates.
          With the `SA_NOCLDSTOP' flag, `SIGCHLD' is only signalled for
          termination, not stopping.

          `SA_NOCLDSTOP' has no effect on signals other than `SIGCHLD'.

      -- Variable: SA_RESTART
          If a signal occurs while in a system call, deliver the signal
          then restart the system call (as opposed to returning an
          `EINTR' error from that call).

     The return value is a pair with information about the old handler
     as described above.

     This interface does not provide access to the "signal blocking"
     facility.  Maybe this is not needed, since the thread support may
     provide solutions to the problem of consistent access to data
     structures.

 -- Scheme Procedure: restore-signals
 -- C Function: scm_restore_signals ()
     Return all signal handlers to the values they had before any call
     to `sigaction' was made.  The return value is unspecified.

 -- Scheme Procedure: alarm i
 -- C Function: scm_alarm (i)
     Set a timer to raise a `SIGALRM' signal after the specified number
     of seconds (an integer).  It's advisable to install a signal
     handler for `SIGALRM' beforehand, since the default action is to
     terminate the process.

     The return value indicates the time remaining for the previous
     alarm, if any.  The new value replaces the previous alarm.  If
     there was no previous alarm, the return value is zero.

 -- Scheme Procedure: pause
 -- C Function: scm_pause ()
     Pause the current process (thread?) until a signal arrives whose
     action is to either terminate the current process or invoke a
     handler procedure.  The return value is unspecified.

 -- Scheme Procedure: sleep secs
 -- Scheme Procedure: usleep usecs
 -- C Function: scm_sleep (secs)
 -- C Function: scm_usleep (usecs)
     Wait the given period SECS seconds or USECS microseconds (both
     integers).  If a signal arrives the wait stops and the return
     value is the time remaining, in seconds or microseconds
     respectively.  If the period elapses with no signal the return is
     zero.

     On most systems the process scheduler is not microsecond accurate
     and the actual period slept by `usleep' might be rounded to a
     system clock tick boundary, which might be 10 milliseconds for
     instance.

     See `scm_std_sleep' and `scm_std_usleep' for equivalents at the C
     level (*note Blocking::).

 -- Scheme Procedure: getitimer which_timer
 -- Scheme Procedure: setitimer which_timer interval_seconds
          interval_microseconds periodic_seconds periodic_microseconds
 -- C Function: scm_getitimer (which_timer)
 -- C Function: scm_setitimer (which_timer, interval_seconds,
          interval_microseconds, periodic_seconds,
          periodic_microseconds)
     Get or set the periods programmed in certain system timers.  These
     timers have a current interval value which counts down and on
     reaching zero raises a signal.  An optional periodic value can be
     set to restart from there each time, for periodic operation.
     WHICH_TIMER is one of the following values

      -- Variable: ITIMER_REAL
          A real-time timer, counting down elapsed real time.  At zero
          it raises `SIGALRM'.  This is like `alarm' above, but with a
          higher resolution period.

      -- Variable: ITIMER_VIRTUAL
          A virtual-time timer, counting down while the current process
          is actually using CPU.  At zero it raises `SIGVTALRM'.

      -- Variable: ITIMER_PROF
          A profiling timer, counting down while the process is running
          (like `ITIMER_VIRTUAL') and also while system calls are
          running on the process's behalf.  At zero it raises a
          `SIGPROF'.

          This timer is intended for profiling where a program is
          spending its time (by looking where it is when the timer goes
          off).

     `getitimer' returns the current timer value and its programmed
     restart value, as a list containing two pairs.  Each pair is a
     time in seconds and microseconds: `((INTERVAL_SECS .
     INTERVAL_USECS) (PERIODIC_SECS . PERIODIC_USECS))'.

     `setitimer' sets the timer values similarly, in seconds and
     microseconds (which must be integers).  The periodic value can be
     zero to have the timer run down just once.  The return value is
     the timer's previous setting, in the same form as `getitimer'
     returns.

          (setitimer ITIMER_REAL
                     5 500000     ;; first SIGALRM in 5.5 seconds time
                     2 0)         ;; then repeat every 2 seconds

     Although the timers are programmed in microseconds, the actual
     accuracy might not be that high.

7.2.9 Terminals and Ptys
------------------------

 -- Scheme Procedure: isatty? port
 -- C Function: scm_isatty_p (port)
     Return `#t' if PORT is using a serial non-file device, otherwise
     `#f'.

 -- Scheme Procedure: ttyname port
 -- C Function: scm_ttyname (port)
     Return a string with the name of the serial terminal device
     underlying PORT.

 -- Scheme Procedure: ctermid
 -- C Function: scm_ctermid ()
     Return a string containing the file name of the controlling
     terminal for the current process.

 -- Scheme Procedure: tcgetpgrp port
 -- C Function: scm_tcgetpgrp (port)
     Return the process group ID of the foreground process group
     associated with the terminal open on the file descriptor
     underlying PORT.

     If there is no foreground process group, the return value is a
     number greater than 1 that does not match the process group ID of
     any existing process group.  This can happen if all of the
     processes in the job that was formerly the foreground job have
     terminated, and no other job has yet been moved into the
     foreground.

 -- Scheme Procedure: tcsetpgrp port pgid
 -- C Function: scm_tcsetpgrp (port, pgid)
     Set the foreground process group ID for the terminal used by the
     file descriptor underlying PORT to the integer PGID.  The calling
     process must be a member of the same session as PGID and must have
     the same controlling terminal.  The return value is unspecified.

7.2.10 Pipes
------------

The following procedures are similar to the `popen' and `pclose' system
routines.  The code is in a separate "popen" module:

     (use-modules (ice-9 popen))

 -- Scheme Procedure: open-pipe command mode
 -- Scheme Procedure: open-pipe* mode prog [args...]
     Execute a command in a subprocess, with a pipe to it or from it, or
     with pipes in both directions.

     `open-pipe' runs the shell COMMAND using `/bin/sh -c'.
     `open-pipe*' executes PROG directly, with the optional ARGS
     arguments (all strings).

     MODE should be one of the following values.  `OPEN_READ' is an
     input pipe, ie. to read from the subprocess.  `OPEN_WRITE' is an
     output pipe, ie. to write to it.

      -- Variable: OPEN_READ
      -- Variable: OPEN_WRITE
      -- Variable: OPEN_BOTH

     For an input pipe, the child's standard output is the pipe and
     standard input is inherited from `current-input-port'.  For an
     output pipe, the child's standard input is the pipe and standard
     output is inherited from `current-output-port'.  In all cases
     cases the child's standard error is inherited from
     `current-error-port' (*note Default Ports::).

     If those `current-X-ports' are not files of some kind, and hence
     don't have file descriptors for the child, then `/dev/null' is
     used instead.

     Care should be taken with `OPEN_BOTH', a deadlock will occur if
     both parent and child are writing, and waiting until the write
     completes before doing any reading.  Each direction has `PIPE_BUF'
     bytes of buffering (*note Ports and File Descriptors::), which
     will be enough for small writes, but not for say putting a big
     file through a filter.

 -- Scheme Procedure: open-input-pipe command
     Equivalent to `open-pipe' with mode `OPEN_READ'.

          (let* ((port (open-input-pipe "date --utc"))
                 (str  (read-line port)))
            (close-pipe port)
            str)
          => "Mon Mar 11 20:10:44 UTC 2002"

 -- Scheme Procedure: open-output-pipe command
     Equivalent to `open-pipe' with mode `OPEN_WRITE'.

          (let ((port (open-output-pipe "lpr")))
            (display "Something for the line printer.\n" port)
            (if (not (eqv? 0 (status:exit-val (close-pipe port))))
                (error "Cannot print")))

 -- Scheme Procedure: open-input-output-pipe command
     Equivalent to `open-pipe' with mode `OPEN_BOTH'.

 -- Scheme Procedure: close-pipe port
     Close a pipe created by `open-pipe', wait for the process to
     terminate, and return the wait status code.  The status is as per
     `waitpid' and can be decoded with `status:exit-val' etc (*note
     Processes::)


   `waitpid WAIT_ANY' should not be used when pipes are open, since it
can reap a pipe's child process, causing an error from a subsequent
`close-pipe'.

   `close-port' (*note Closing::) can close a pipe, but it doesn't reap
the child process.

   The garbage collector will close a pipe no longer in use, and reap
the child process with `waitpid'.  If the child hasn't yet terminated
the garbage collector doesn't block, but instead checks again in the
next GC.

   Many systems have per-user and system-wide limits on the number of
processes, and a system-wide limit on the number of pipes, so pipes
should be closed explicitly when no longer needed, rather than letting
the garbage collector pick them up at some later time.

7.2.11 Networking
-----------------

7.2.11.1 Network Address Conversion
...................................

This section describes procedures which convert internet addresses
between numeric and string formats.

IPv4 Address Conversion
.......................

An IPv4 Internet address is a 4-byte value, represented in Guile as an
integer in host byte order, so that say "0.0.0.1" is 1, or "1.0.0.0" is
16777216.

   Some underlying C functions use network byte order for addresses,
Guile converts as necessary so that at the Scheme level its host byte
order everywhere.

 -- Variable: INADDR_ANY
     For a server, this can be used with `bind' (*note Network Sockets
     and Communication::) to allow connections from any interface on
     the machine.

 -- Variable: INADDR_BROADCAST
     The broadcast address on the local network.

 -- Variable: INADDR_LOOPBACK
     The address of the local host using the loopback device, ie.
     `127.0.0.1'.

 -- Scheme Procedure: inet-aton address
 -- C Function: scm_inet_aton (address)
     This function is deprecated in favor of `inet-pton'.

     Convert an IPv4 Internet address from printable string (dotted
     decimal notation) to an integer.  E.g.,

          (inet-aton "127.0.0.1") => 2130706433

 -- Scheme Procedure: inet-ntoa inetid
 -- C Function: scm_inet_ntoa (inetid)
     This function is deprecated in favor of `inet-ntop'.

     Convert an IPv4 Internet address to a printable (dotted decimal
     notation) string.  E.g.,

          (inet-ntoa 2130706433) => "127.0.0.1"

 -- Scheme Procedure: inet-netof address
 -- C Function: scm_inet_netof (address)
     Return the network number part of the given IPv4 Internet address.
     E.g.,

          (inet-netof 2130706433) => 127

 -- Scheme Procedure: inet-lnaof address
 -- C Function: scm_lnaof (address)
     Return the local-address-with-network part of the given IPv4
     Internet address, using the obsolete class A/B/C system.  E.g.,

          (inet-lnaof 2130706433) => 1

 -- Scheme Procedure: inet-makeaddr net lna
 -- C Function: scm_inet_makeaddr (net, lna)
     Make an IPv4 Internet address by combining the network number NET
     with the local-address-within-network number LNA.  E.g.,

          (inet-makeaddr 127 1) => 2130706433

IPv6 Address Conversion
.......................

An IPv6 Internet address is a 16-byte value, represented in Guile as an
integer in host byte order, so that say "::1" is 1.

 -- Scheme Procedure: inet-ntop family address
 -- C Function: scm_inet_ntop (family, address)
     Convert a network address from an integer to a printable string.
     FAMILY can be `AF_INET' or `AF_INET6'.  E.g.,

          (inet-ntop AF_INET 2130706433) => "127.0.0.1"
          (inet-ntop AF_INET6 (- (expt 2 128) 1))
            => "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff"

 -- Scheme Procedure: inet-pton family address
 -- C Function: scm_inet_pton (family, address)
     Convert a string containing a printable network address to an
     integer address.  FAMILY can be `AF_INET' or `AF_INET6'.  E.g.,

          (inet-pton AF_INET "127.0.0.1") => 2130706433
          (inet-pton AF_INET6 "::1") => 1

7.2.11.2 Network Databases
..........................

This section describes procedures which query various network databases.
Care should be taken when using the database routines since they are not
reentrant.

`getaddrinfo'
.............

The `getaddrinfo' procedure maps host and service names to socket
addresses and associated information in a protocol-independent way.

 -- Scheme Procedure: getaddrinfo name service [hint_flags [hint_family
          [hint_socktype [hint_protocol]]]]
 -- C Function: scm_getaddrinfo (name, service, hint_flags,
          hint_family, hint_socktype, hint_protocol)
     Return a list of `addrinfo' structures containing a socket address
     and associated information for host NAME and/or SERVICE to be used
     in creating a socket with which to address the specified service.

          (let* ((ai (car (getaddrinfo "www.gnu.org" "http")))
                 (s  (socket (addrinfo:fam ai) (addrinfo:socktype ai)
                             (addrinfo:protocol ai))))
            (connect s (addrinfo:addr ai))
            s)

     When SERVICE is omitted or is `#f', return network-level addresses
     for NAME.  When NAME is `#f' SERVICE must be provided and service
     locations local to the caller are returned.

     Additional hints can be provided.  When specified, HINT_FLAGS
     should be a bitwise-or of zero or more constants among the
     following:

    `AI_PASSIVE'
          Socket address is intended for `bind'.

    `AI_CANONNAME'
          Request for canonical host name, available via
          `addrinfo:canonname'.  This makes sense mainly when DNS
          lookups are involved.

    `AI_NUMERICHOST'
          Specifies that NAME is a numeric host address string (e.g.,
          `"127.0.0.1"'), meaning that name resolution will not be used.

    `AI_NUMERICSERV'
          Likewise, specifies that SERVICE is a numeric port string
          (e.g., `"80"').

    `AI_ADDRCONFIG'
          Return only addresses configured on the local system  It is
          highly recommended to provide this flag when the returned
          socket addresses are to be used to make connections;
          otherwise, some of the returned addresses could be unreachable
          or use a protocol that is not supported.

    `AI_V4MAPPED'
          When looking up IPv6 addresses, return mapped IPv4 addresses
          if there is no IPv6 address available at all.

    `AI_ALL'
          If this flag is set along with `AI_V4MAPPED' when looking up
          IPv6 addresses, return all IPv6 addresses as well as all IPv4
          addresses, the latter mapped to IPv6 format.

     When given, HINT_FAMILY should specify the requested address
     family, e.g., `AF_INET6'.  Similarly, HINT_SOCKTYPE should specify
     the requested socket type (e.g., `SOCK_DGRAM'), and HINT_PROTOCOL
     should specify the requested protocol (its value is interpreted as
     in calls to `socket').

     On error, an exception with key `getaddrinfo-error' is thrown,
     with an error code (an integer) as its argument:

          (catch 'getaddrinfo-error
            (lambda ()
              (getaddrinfo "www.gnu.org" "gopher"))
            (lambda (key errcode)
              (cond ((= errcode EAI_SERVICE)
          	   (display "doesn't know about Gopher!\n"))
          	  ((= errcode EAI_NONAME)
          	   (display "www.gnu.org not found\\n"))
          	  (else
          	   (format #t "something wrong: ~a\n"
          		   (gai-strerror errcode))))))

     Error codes are:

    `EAI_AGAIN'
          The name or service could not be resolved at this time. Future
          attempts may succeed.

    `EAI_BADFLAGS'
          HINT_FLAGS contains an invalid value.

    `EAI_FAIL'
          A non-recoverable error occurred when attempting to resolve
          the name.

    `EAI_FAMILY'
          HINT_FAMILY was not recognized.

    `EAI_NONAME'
          Either NAME does not resolve for the supplied parameters, or
          neither NAME nor SERVICE were supplied.

    `EAI_NODATA'
          This non-POSIX error code can be returned on GNU systems when
          a request was actually made but returned no data, meaning
          that no address is associated with NAME.  Error handling code
          should be prepared to handle it when it is defined.

    `EAI_SERVICE'
          SERVICE was not recognized for the specified socket type.

    `EAI_SOCKTYPE'
          HINT_SOCKTYPE was not recognized.

    `EAI_SYSTEM'
          A system error occurred; the error code can be found in
          `errno'.

     Users are encouraged to read the "POSIX specification
     (http://www.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html)
     for more details.

   The following procedures take an `addrinfo' object as returned by
`getaddrinfo':

 -- Scheme Procedure: addrinfo:flags ai
     Return flags for AI as a bitwise or of `AI_' values (see above).

 -- Scheme Procedure: addrinfo:fam ai
     Return the address family of AI (a `AF_' value).

 -- Scheme Procedure: addrinfo:socktype ai
     Return the socket type for AI (a `SOCK_' value).

 -- Scheme Procedure: addrinfo:protocol ai
     Return the protocol of AI.

 -- Scheme Procedure: addrinfo:addr ai
     Return the socket address associated with AI as a `sockaddr'
     object (*note Network Socket Address::).

 -- Scheme Procedure: addrinfo:canonname ai
     Return a string for the canonical name associated with AI if the
     `AI_CANONNAME' flag was supplied.

The Host Database
.................

A "host object" is a structure that represents what is known about a
network host, and is the usual way of representing a system's network
identity inside software.

   The following functions accept a host object and return a selected
component:

 -- Scheme Procedure: hostent:name host
     The "official" hostname for HOST.

 -- Scheme Procedure: hostent:aliases host
     A list of aliases for HOST.

 -- Scheme Procedure: hostent:addrtype host
     The host address type, one of the `AF' constants, such as
     `AF_INET' or `AF_INET6'.

 -- Scheme Procedure: hostent:length host
     The length of each address for HOST, in bytes.

 -- Scheme Procedure: hostent:addr-list host
     The list of network addresses associated with HOST.  For `AF_INET'
     these are integer IPv4 address (*note Network Address
     Conversion::).

   The following procedures can be used to search the host database.
However, `getaddrinfo' should be preferred over them since it's more
generic and thread-safe.

 -- Scheme Procedure: gethost [host]
 -- Scheme Procedure: gethostbyname hostname
 -- Scheme Procedure: gethostbyaddr address
 -- C Function: scm_gethost (host)
     Look up a host by name or address, returning a host object.  The
     `gethost' procedure will accept either a string name or an integer
     address; if given no arguments, it behaves like `gethostent' (see
     below).  If a name or address is supplied but the address can not
     be found, an error will be thrown to one of the keys:
     `host-not-found', `try-again', `no-recovery' or `no-data',
     corresponding to the equivalent `h_error' values.  Unusual
     conditions may result in errors thrown to the `system-error' or
     `misc_error' keys.

          (gethost "www.gnu.org")
          => #("www.gnu.org" () 2 4 (3353880842))

          (gethostbyname "www.emacs.org")
          => #("emacs.org" ("www.emacs.org") 2 4 (1073448978))

   The following procedures may be used to step through the host
database from beginning to end.

 -- Scheme Procedure: sethostent [stayopen]
     Initialize an internal stream from which host objects may be read.
     This procedure must be called before any calls to `gethostent',
     and may also be called afterward to reset the host entry stream.
     If STAYOPEN is supplied and is not `#f', the database is not
     closed by subsequent `gethostbyname' or `gethostbyaddr' calls,
     possibly giving an efficiency gain.

 -- Scheme Procedure: gethostent
     Return the next host object from the host database, or `#f' if
     there are no more hosts to be found (or an error has been
     encountered).  This procedure may not be used before `sethostent'
     has been called.

 -- Scheme Procedure: endhostent
     Close the stream used by `gethostent'.  The return value is
     unspecified.

 -- Scheme Procedure: sethost [stayopen]
 -- C Function: scm_sethost (stayopen)
     If STAYOPEN is omitted, this is equivalent to `endhostent'.
     Otherwise it is equivalent to `sethostent stayopen'.

The Network Database
....................

The following functions accept an object representing a network and
return a selected component:

 -- Scheme Procedure: netent:name net
     The "official" network name.

 -- Scheme Procedure: netent:aliases net
     A list of aliases for the network.

 -- Scheme Procedure: netent:addrtype net
     The type of the network number.  Currently, this returns only
     `AF_INET'.

 -- Scheme Procedure: netent:net net
     The network number.

   The following procedures are used to search the network database:

 -- Scheme Procedure: getnet [net]
 -- Scheme Procedure: getnetbyname net-name
 -- Scheme Procedure: getnetbyaddr net-number
 -- C Function: scm_getnet (net)
     Look up a network by name or net number in the network database.
     The NET-NAME argument must be a string, and the NET-NUMBER
     argument must be an integer.  `getnet' will accept either type of
     argument, behaving like `getnetent' (see below) if no arguments are
     given.

   The following procedures may be used to step through the network
database from beginning to end.

 -- Scheme Procedure: setnetent [stayopen]
     Initialize an internal stream from which network objects may be
     read.  This procedure must be called before any calls to
     `getnetent', and may also be called afterward to reset the net
     entry stream.  If STAYOPEN is supplied and is not `#f', the
     database is not closed by subsequent `getnetbyname' or
     `getnetbyaddr' calls, possibly giving an efficiency gain.

 -- Scheme Procedure: getnetent
     Return the next entry from the network database.

 -- Scheme Procedure: endnetent
     Close the stream used by `getnetent'.  The return value is
     unspecified.

 -- Scheme Procedure: setnet [stayopen]
 -- C Function: scm_setnet (stayopen)
     If STAYOPEN is omitted, this is equivalent to `endnetent'.
     Otherwise it is equivalent to `setnetent stayopen'.

The Protocol Database
.....................

The following functions accept an object representing a protocol and
return a selected component:

 -- Scheme Procedure: protoent:name protocol
     The "official" protocol name.

 -- Scheme Procedure: protoent:aliases protocol
     A list of aliases for the protocol.

 -- Scheme Procedure: protoent:proto protocol
     The protocol number.

   The following procedures are used to search the protocol database:

 -- Scheme Procedure: getproto [protocol]
 -- Scheme Procedure: getprotobyname name
 -- Scheme Procedure: getprotobynumber number
 -- C Function: scm_getproto (protocol)
     Look up a network protocol by name or by number.  `getprotobyname'
     takes a string argument, and `getprotobynumber' takes an integer
     argument.  `getproto' will accept either type, behaving like
     `getprotoent' (see below) if no arguments are supplied.

   The following procedures may be used to step through the protocol
database from beginning to end.

 -- Scheme Procedure: setprotoent [stayopen]
     Initialize an internal stream from which protocol objects may be
     read.  This procedure must be called before any calls to
     `getprotoent', and may also be called afterward to reset the
     protocol entry stream.  If STAYOPEN is supplied and is not `#f',
     the database is not closed by subsequent `getprotobyname' or
     `getprotobynumber' calls, possibly giving an efficiency gain.

 -- Scheme Procedure: getprotoent
     Return the next entry from the protocol database.

 -- Scheme Procedure: endprotoent
     Close the stream used by `getprotoent'.  The return value is
     unspecified.

 -- Scheme Procedure: setproto [stayopen]
 -- C Function: scm_setproto (stayopen)
     If STAYOPEN is omitted, this is equivalent to `endprotoent'.
     Otherwise it is equivalent to `setprotoent stayopen'.

The Service Database
....................

The following functions accept an object representing a service and
return a selected component:

 -- Scheme Procedure: servent:name serv
     The "official" name of the network service.

 -- Scheme Procedure: servent:aliases serv
     A list of aliases for the network service.

 -- Scheme Procedure: servent:port serv
     The Internet port used by the service.

 -- Scheme Procedure: servent:proto serv
     The protocol used by the service.  A service may be listed many
     times in the database under different protocol names.

   The following procedures are used to search the service database:

 -- Scheme Procedure: getserv [name [protocol]]
 -- Scheme Procedure: getservbyname name protocol
 -- Scheme Procedure: getservbyport port protocol
 -- C Function: scm_getserv (name, protocol)
     Look up a network service by name or by service number, and return
     a network service object.  The PROTOCOL argument specifies the name
     of the desired protocol; if the protocol found in the network
     service database does not match this name, a system error is
     signalled.

     The `getserv' procedure will take either a service name or number
     as its first argument; if given no arguments, it behaves like
     `getservent' (see below).

          (getserv "imap" "tcp")
          => #("imap2" ("imap") 143 "tcp")

          (getservbyport 88 "udp")
          => #("kerberos" ("kerberos5" "krb5") 88 "udp")

   The following procedures may be used to step through the service
database from beginning to end.

 -- Scheme Procedure: setservent [stayopen]
     Initialize an internal stream from which service objects may be
     read.  This procedure must be called before any calls to
     `getservent', and may also be called afterward to reset the
     service entry stream.  If STAYOPEN is supplied and is not `#f',
     the database is not closed by subsequent `getservbyname' or
     `getservbyport' calls, possibly giving an efficiency gain.

 -- Scheme Procedure: getservent
     Return the next entry from the services database.

 -- Scheme Procedure: endservent
     Close the stream used by `getservent'.  The return value is
     unspecified.

 -- Scheme Procedure: setserv [stayopen]
 -- C Function: scm_setserv (stayopen)
     If STAYOPEN is omitted, this is equivalent to `endservent'.
     Otherwise it is equivalent to `setservent stayopen'.

7.2.11.3 Network Socket Address
...............................

A "socket address" object identifies a socket endpoint for
communication.  In the case of `AF_INET' for instance, the socket
address object comprises the host address (or interface on the host)
and a port number which specifies a particular open socket in a running
client or server process.  A socket address object can be created with,

 -- Scheme Procedure: make-socket-address AF_INET ipv4addr port
 -- Scheme Procedure: make-socket-address AF_INET6 ipv6addr port
          [flowinfo [scopeid]]
 -- Scheme Procedure: make-socket-address AF_UNIX path
 -- C Function: scm_make_socket_address family address arglist
     Return a new socket address object.  The first argument is the
     address family, one of the `AF' constants, then the arguments vary
     according to the family.

     For `AF_INET' the arguments are an IPv4 network address number
     (*note Network Address Conversion::), and a port number.

     For `AF_INET6' the arguments are an IPv6 network address number
     and a port number.  Optional FLOWINFO and SCOPEID arguments may be
     given (both integers, default 0).

     For `AF_UNIX' the argument is a filename (a string).

     The C function `scm_make_socket_address' takes the FAMILY and
     ADDRESS arguments directly, then ARGLIST is a list of further
     arguments, being the port for IPv4, port and optional flowinfo and
     scopeid for IPv6, or the empty list `SCM_EOL' for Unix domain.

The following functions access the fields of a socket address object,

 -- Scheme Procedure: sockaddr:fam sa
     Return the address family from socket address object SA.  This is
     one of the `AF' constants (e.g. `AF_INET').

 -- Scheme Procedure: sockaddr:path sa
     For an `AF_UNIX' socket address object SA, return the filename.

 -- Scheme Procedure: sockaddr:addr sa
     For an `AF_INET' or `AF_INET6' socket address object SA, return
     the network address number.

 -- Scheme Procedure: sockaddr:port sa
     For an `AF_INET' or `AF_INET6' socket address object SA, return
     the port number.

 -- Scheme Procedure: sockaddr:flowinfo sa
     For an `AF_INET6' socket address object SA, return the flowinfo
     value.

 -- Scheme Procedure: sockaddr:scopeid sa
     For an `AF_INET6' socket address object SA, return the scope ID
     value.

   The functions below convert to and from the C `struct sockaddr'
(*note Address Formats: (libc)Address Formats.).  That structure is a
generic type, an application can cast to or from `struct sockaddr_in',
`struct sockaddr_in6' or `struct sockaddr_un' according to the address
family.

   In a `struct sockaddr' taken or returned, the byte ordering in the
fields follows the C conventions (*note Byte Order Conversion:
(libc)Byte Order.).  This means network byte order for `AF_INET' host
address (`sin_addr.s_addr') and port number (`sin_port'), and
`AF_INET6' port number (`sin6_port').  But at the Scheme level these
values are taken or returned in host byte order, so the port is an
ordinary integer, and the host address likewise is an ordinary integer
(as described in *note Network Address Conversion::).

 -- C Function: struct sockaddr * scm_c_make_socket_address (SCM
          family, SCM address, SCM args, size_t *outsize)
     Return a newly-`malloc'ed `struct sockaddr' created from arguments
     like those taken by `scm_make_socket_address' above.

     The size (in bytes) of the `struct sockaddr' return is stored into
     `*OUTSIZE'.  An application must call `free' to release the
     returned structure when no longer required.

 -- C Function: SCM scm_from_sockaddr (const struct sockaddr *address,
          unsigned address_size)
     Return a Scheme socket address object from the C ADDRESS
     structure.  ADDRESS_SIZE is the size in bytes of ADDRESS.

 -- C Function: struct sockaddr * scm_to_sockaddr (SCM address, size_t
          *address_size)
     Return a newly-`malloc'ed `struct sockaddr' from a Scheme level
     socket address object.

     The size (in bytes) of the `struct sockaddr' return is stored into
     `*OUTSIZE'.  An application must call `free' to release the
     returned structure when no longer required.

7.2.11.4 Network Sockets and Communication
..........................................

Socket ports can be created using `socket' and `socketpair'.  The ports
are initially unbuffered, to make reading and writing to the same port
more reliable.  A buffer can be added to the port using `setvbuf'; see
*note Ports and File Descriptors::.

   Most systems have limits on how many files and sockets can be open,
so it's strongly recommended that socket ports be closed explicitly when
no longer required (*note Ports::).

   Some of the underlying C functions take values in network byte order,
but the convention in Guile is that at the Scheme level everything is
ordinary host byte order and conversions are made automatically where
necessary.

 -- Scheme Procedure: socket family style proto
 -- C Function: scm_socket (family, style, proto)
     Return a new socket port of the type specified by FAMILY, STYLE
     and PROTO.  All three parameters are integers.  The possible
     values for FAMILY are as follows, where supported by the system,

      -- Variable: PF_UNIX
      -- Variable: PF_INET
      -- Variable: PF_INET6

     The possible values for STYLE are as follows, again where
     supported by the system,

      -- Variable: SOCK_STREAM
      -- Variable: SOCK_DGRAM
      -- Variable: SOCK_RAW
      -- Variable: SOCK_RDM
      -- Variable: SOCK_SEQPACKET

     PROTO can be obtained from a protocol name using `getprotobyname'
     (*note Network Databases::).  A value of zero means the default
     protocol, which is usually right.

     A socket cannot by used for communication until it has been
     connected somewhere, usually with either `connect' or `accept'
     below.

 -- Scheme Procedure: socketpair family style proto
 -- C Function: scm_socketpair (family, style, proto)
     Return a pair, the `car' and `cdr' of which are two unnamed socket
     ports connected to each other.  The connection is full-duplex, so
     data can be transferred in either direction between the two.

     FAMILY, STYLE and PROTO are as per `socket' above.  But many
     systems only support socket pairs in the `PF_UNIX' family.  Zero
     is likely to be the only meaningful value for PROTO.

 -- Scheme Procedure: getsockopt sock level optname
 -- Scheme Procedure: setsockopt sock level optname value
 -- C Function: scm_getsockopt (sock, level, optname)
 -- C Function: scm_setsockopt (sock, level, optname, value)
     Get or set an option on socket port SOCK.  `getsockopt' returns
     the current value.  `setsockopt' sets a value and the return is
     unspecified.

     LEVEL is an integer specifying a protocol layer, either
     `SOL_SOCKET' for socket level options, or a protocol number from
     the `IPPROTO' constants or `getprotoent' (*note Network
     Databases::).

      -- Variable: SOL_SOCKET
      -- Variable: IPPROTO_IP
      -- Variable: IPPROTO_TCP
      -- Variable: IPPROTO_UDP

     OPTNAME is an integer specifying an option within the protocol
     layer.

     For `SOL_SOCKET' level the following OPTNAMEs are defined (when
     provided by the system).  For their meaning see *note Socket-Level
     Options: (libc)Socket-Level Options, or `man 7 socket'.

      -- Variable: SO_DEBUG
      -- Variable: SO_REUSEADDR
      -- Variable: SO_STYLE
      -- Variable: SO_TYPE
      -- Variable: SO_ERROR
      -- Variable: SO_DONTROUTE
      -- Variable: SO_BROADCAST
      -- Variable: SO_SNDBUF
      -- Variable: SO_RCVBUF
      -- Variable: SO_KEEPALIVE
      -- Variable: SO_OOBINLINE
      -- Variable: SO_NO_CHECK
      -- Variable: SO_PRIORITY
          The VALUE taken or returned is an integer.

      -- Variable: SO_LINGER
          The VALUE taken or returned is a pair of integers `(ENABLE .
          TIMEOUT)'.  On old systems without timeout support (ie.
          without `struct linger'), only ENABLE has an effect but the
          value in Guile is always a pair.

     For IP level (`IPPROTO_IP') the following OPTNAMEs are defined
     (when provided by the system).  See `man ip' for what they mean.

      -- Variable: IP_MULTICAST_IF
          This sets the source interface used by multicast traffic.

      -- Variable: IP_MULTICAST_TTL
          This sets the default TTL for multicast traffic. This defaults
          to 1 and should be increased to allow traffic to pass beyond
          the local network.

      -- Variable: IP_ADD_MEMBERSHIP
      -- Variable: IP_DROP_MEMBERSHIP
          These can be used only with `setsockopt', not `getsockopt'.
          VALUE is a pair `(MULTIADDR . INTERFACEADDR)' of integer IPv4
          addresses (*note Network Address Conversion::).  MULTIADDR is
          a multicast address to be added to or dropped from the
          interface INTERFACEADDR.  INTERFACEADDR can be `INADDR_ANY'
          to have the system select the interface.  INTERFACEADDR can
          also be an interface index number, on systems supporting that.

 -- Scheme Procedure: shutdown sock how
 -- C Function: scm_shutdown (sock, how)
     Sockets can be closed simply by using `close-port'.  The
     `shutdown' procedure allows reception or transmission on a
     connection to be shut down individually, according to the parameter
     HOW:

    0
          Stop receiving data for this socket.  If further data
          arrives, reject it.

    1
          Stop trying to transmit data from this socket.  Discard any
          data waiting to be sent.  Stop looking for acknowledgement of
          data already sent; don't retransmit it if it is lost.

    2
          Stop both reception and transmission.

     The return value is unspecified.

 -- Scheme Procedure: connect sock sockaddr
 -- Scheme Procedure: connect sock AF_INET ipv4addr port
 -- Scheme Procedure: connect sock AF_INET6 ipv6addr port [flowinfo
          [scopeid]]
 -- Scheme Procedure: connect sock AF_UNIX path
 -- C Function: scm_connect (sock, fam, address, args)
     Initiate a connection on socket port SOCK to a given address.  The
     destination is either a socket address object, or arguments the
     same as `make-socket-address' would take to make such an object
     (*note Network Socket Address::).  The return value is unspecified.

          (connect sock AF_INET INADDR_LOOPBACK 23)
          (connect sock (make-socket-address AF_INET INADDR_LOOPBACK 23))

 -- Scheme Procedure: bind sock sockaddr
 -- Scheme Procedure: bind sock AF_INET ipv4addr port
 -- Scheme Procedure: bind sock AF_INET6 ipv6addr port [flowinfo
          [scopeid]]
 -- Scheme Procedure: bind sock AF_UNIX path
 -- C Function: scm_bind (sock, fam, address, args)
     Bind socket port SOCK to the given address.  The address is either
     a socket address object, or arguments the same as
     `make-socket-address' would take to make such an object (*note
     Network Socket Address::).  The return value is unspecified.

     Generally a socket is only explicitly bound to a particular address
     when making a server, i.e. to listen on a particular port.  For an
     outgoing connection the system will assign a local address
     automatically, if not already bound.

          (bind sock AF_INET INADDR_ANY 12345)
          (bind sock (make-socket-address AF_INET INADDR_ANY 12345))

 -- Scheme Procedure: listen sock backlog
 -- C Function: scm_listen (sock, backlog)
     Enable SOCK to accept connection requests.  BACKLOG is an integer
     specifying the maximum length of the queue for pending connections.
     If the queue fills, new clients will fail to connect until the
     server calls `accept' to accept a connection from the queue.

     The return value is unspecified.

 -- Scheme Procedure: accept sock
 -- C Function: scm_accept (sock)
     Accept a connection from socket port SOCK which has been enabled
     for listening with `listen' above.  If there are no incoming
     connections in the queue, wait until one is available (unless
     `O_NONBLOCK' has been set on the socket, *note `fcntl': Ports and
     File Descriptors.).

     The return value is a pair.  The `car' is a new socket port,
     connected and ready to communicate.  The `cdr' is a socket address
     object (*note Network Socket Address::) which is where the remote
     connection is from (like `getpeername' below).

     All communication takes place using the new socket returned.  The
     given SOCK remains bound and listening, and `accept' may be called
     on it again to get another incoming connection when desired.

 -- Scheme Procedure: getsockname sock
 -- C Function: scm_getsockname (sock)
     Return a socket address object which is the where SOCK is bound
     locally.  SOCK may have obtained its local address from `bind'
     (above), or if a `connect' is done with an otherwise unbound
     socket (which is usual) then the system will have assigned an
     address.

     Note that on many systems the address of a socket in the `AF_UNIX'
     namespace cannot be read.

 -- Scheme Procedure: getpeername sock
 -- C Function: scm_getpeername (sock)
     Return a socket address object which is where SOCK is connected
     to, i.e. the remote endpoint.

     Note that on many systems the address of a socket in the `AF_UNIX'
     namespace cannot be read.

 -- Scheme Procedure: recv! sock buf [flags]
 -- C Function: scm_recv (sock, buf, flags)
     Receive data from a socket port.  SOCK must already be bound to
     the address from which data is to be received.  BUF is a
     bytevector into which the data will be written.  The size of BUF
     limits the amount of data which can be received: in the case of
     packet protocols, if a packet larger than this limit is encountered
     then some data will be irrevocably lost.

     The optional FLAGS argument is a value or bitwise OR of `MSG_OOB',
     `MSG_PEEK', `MSG_DONTROUTE' etc.

     The value returned is the number of bytes read from the socket.

     Note that the data is read directly from the socket file
     descriptor: any unread buffered port data is ignored.

 -- Scheme Procedure: send sock message [flags]
 -- C Function: scm_send (sock, message, flags)
     Transmit bytevector MESSAGE on socket port SOCK.  SOCK must
     already be bound to a destination address.  The value returned is
     the number of bytes transmitted--it's possible for this to be less
     than the length of MESSAGE if the socket is set to be
     non-blocking.  The optional FLAGS argument is a value or bitwise
     OR of `MSG_OOB', `MSG_PEEK', `MSG_DONTROUTE' etc.

     Note that the data is written directly to the socket file
     descriptor: any unflushed buffered port data is ignored.

 -- Scheme Procedure: recvfrom! sock buf [flags [start [end]]]
 -- C Function: scm_recvfrom (sock, buf, flags, start, end)
     Receive data from socket port SOCK, returning the originating
     address as well as the data.  This function is usually for datagram
     sockets, but can be used on stream-oriented sockets too.

     The data received is stored in bytevector BUF, using either the
     whole bytevector or just the region between the optional START and
     END positions.  The size of BUF limits the amount of data that can
     be received.  For datagram protocols if a packet larger than this
     is received then excess bytes are irrevocably lost.

     The return value is a pair.  The `car' is the number of bytes
     read.  The `cdr' is a socket address object (*note Network Socket
     Address::) which is where the data came from, or `#f' if the
     origin is unknown.

     The optional FLAGS argument is a or bitwise-OR (`logior') of
     `MSG_OOB', `MSG_PEEK', `MSG_DONTROUTE' etc.

     Data is read directly from the socket file descriptor, any buffered
     port data is ignored.

     On a GNU/Linux system `recvfrom!' is not multi-threading, all
     threads stop while a `recvfrom!' call is in progress.  An
     application may need to use `select', `O_NONBLOCK' or
     `MSG_DONTWAIT' to avoid this.

 -- Scheme Procedure: sendto sock message sockaddr [flags]
 -- Scheme Procedure: sendto sock message AF_INET ipv4addr port [flags]
 -- Scheme Procedure: sendto sock message AF_INET6 ipv6addr port
          [flowinfo [scopeid [flags]]]
 -- Scheme Procedure: sendto sock message AF_UNIX path [flags]
 -- C Function: scm_sendto (sock, message, fam, address, args_and_flags)
     Transmit bytevector MESSAGE as a datagram socket port SOCK.  The
     destination is specified either as a socket address object, or as
     arguments the same as would be taken by `make-socket-address' to
     create such an object (*note Network Socket Address::).

     The destination address may be followed by an optional FLAGS
     argument which is a `logior' (*note Bitwise Operations::) of
     `MSG_OOB', `MSG_PEEK', `MSG_DONTROUTE' etc.

     The value returned is the number of bytes transmitted - it's
     possible for this to be less than the length of MESSAGE if the
     socket is set to be non-blocking.  Note that the data is written
     directly to the socket file descriptor: any unflushed buffered
     port data is ignored.

   The following functions can be used to convert short and long
integers between "host" and "network" order.  Although the procedures
above do this automatically for addresses, the conversion will still
need to be done when sending or receiving encoded integer data from the
network.

 -- Scheme Procedure: htons value
 -- C Function: scm_htons (value)
     Convert a 16 bit quantity from host to network byte ordering.
     VALUE is packed into 2 bytes, which are then converted and
     returned as a new integer.

 -- Scheme Procedure: ntohs value
 -- C Function: scm_ntohs (value)
     Convert a 16 bit quantity from network to host byte ordering.
     VALUE is packed into 2 bytes, which are then converted and
     returned as a new integer.

 -- Scheme Procedure: htonl value
 -- C Function: scm_htonl (value)
     Convert a 32 bit quantity from host to network byte ordering.
     VALUE is packed into 4 bytes, which are then converted and
     returned as a new integer.

 -- Scheme Procedure: ntohl value
 -- C Function: scm_ntohl (value)
     Convert a 32 bit quantity from network to host byte ordering.
     VALUE is packed into 4 bytes, which are then converted and
     returned as a new integer.

   These procedures are inconvenient to use at present, but consider:

     (define write-network-long
       (lambda (value port)
         (let ((v (make-uniform-vector 1 1 0)))
           (uniform-vector-set! v 0 (htonl value))
           (uniform-vector-write v port))))

     (define read-network-long
       (lambda (port)
         (let ((v (make-uniform-vector 1 1 0)))
           (uniform-vector-read! v port)
           (ntohl (uniform-vector-ref v 0)))))

7.2.11.5 Network Socket Examples
................................

The following give examples of how to use network sockets.

Internet Socket Client Example
..............................

The following example demonstrates an Internet socket client.  It
connects to the HTTP daemon running on the local machine and returns
the contents of the root index URL.

     (let ((s (socket PF_INET SOCK_STREAM 0)))
       (connect s AF_INET (inet-pton AF_INET "127.0.0.1") 80)
       (display "GET / HTTP/1.0\r\n\r\n" s)

       (do ((line (read-line s) (read-line s)))
           ((eof-object? line))
         (display line)
         (newline)))

Internet Socket Server Example
..............................

The following example shows a simple Internet server which listens on
port 2904 for incoming connections and sends a greeting back to the
client.

     (let ((s (socket PF_INET SOCK_STREAM 0)))
       (setsockopt s SOL_SOCKET SO_REUSEADDR 1)
       ;; Specific address?
       ;; (bind s AF_INET (inet-pton AF_INET "127.0.0.1") 2904)
       (bind s AF_INET INADDR_ANY 2904)
       (listen s 5)

       (simple-format #t "Listening for clients in pid: ~S" (getpid))
       (newline)

       (while #t
         (let* ((client-connection (accept s))
                (client-details (cdr client-connection))
                (client (car client-connection)))
           (simple-format #t "Got new client connection: ~S"
                          client-details)
           (newline)
           (simple-format #t "Client address: ~S"
                          (gethostbyaddr
                           (sockaddr:addr client-details)))
           (newline)
           ;; Send back the greeting to the client port
           (display "Hello client\r\n" client)
           (close client))))

7.2.12 System Identification
----------------------------

This section lists the various procedures Guile provides for accessing
information about the system it runs on.

 -- Scheme Procedure: uname
 -- C Function: scm_uname ()
     Return an object with some information about the computer system
     the program is running on.

     The following procedures accept an object as returned by `uname'
     and return a selected component (all of which are strings).

      -- Scheme Procedure: utsname:sysname un
          The name of the operating system.

      -- Scheme Procedure: utsname:nodename un
          The network name of the computer.

      -- Scheme Procedure: utsname:release un
          The current release level of the operating system
          implementation.

      -- Scheme Procedure: utsname:version un
          The current version level within the release of the operating
          system.

      -- Scheme Procedure: utsname:machine un
          A description of the hardware.

 -- Scheme Procedure: gethostname
 -- C Function: scm_gethostname ()
     Return the host name of the current processor.

 -- Scheme Procedure: sethostname name
 -- C Function: scm_sethostname (name)
     Set the host name of the current processor to NAME. May only be
     used by the superuser.  The return value is not specified.

7.2.13 Locales
--------------

 -- Scheme Procedure: setlocale category [locale]
 -- C Function: scm_setlocale (category, locale)
     Get or set the current locale, used for various
     internationalizations.  Locales are strings, such as `sv_SE'.

     If LOCALE is given then the locale for the given CATEGORY is set
     and the new value returned.  If LOCALE is not given then the
     current value is returned.  CATEGORY should be one of the
     following values (*note Categories of Activities that Locales
     Affect: (libc)Locale Categories.):

      -- Variable: LC_ALL
      -- Variable: LC_COLLATE
      -- Variable: LC_CTYPE
      -- Variable: LC_MESSAGES
      -- Variable: LC_MONETARY
      -- Variable: LC_NUMERIC
      -- Variable: LC_TIME

     A common usage is `(setlocale LC_ALL "")', which initializes all
     categories based on standard environment variables (`LANG' etc).
     For full details on categories and locale names *note Locales and
     Internationalization: (libc)Locales.

     Note that `setlocale' affects locale settings for the whole
     process.  *Note locale objects and `make-locale': i18n
     Introduction, for a thread-safe alternative.

7.2.14 Encryption
-----------------

Please note that the procedures in this section are not suited for
strong encryption, they are only interfaces to the well-known and
common system library functions of the same name.  They are just as good
(or bad) as the underlying functions, so you should refer to your system
documentation before using them (*note Encrypting Passwords:
(libc)crypt.).

 -- Scheme Procedure: crypt key salt
 -- C Function: scm_crypt (key, salt)
     Encrypt KEY, with the addition of SALT (both strings), using the
     `crypt' C library call.

   Although `getpass' is not an encryption procedure per se, it appears
here because it is often used in combination with `crypt':

 -- Scheme Procedure: getpass prompt
 -- C Function: scm_getpass (prompt)
     Display PROMPT to the standard error output and read a password
     from `/dev/tty'.  If this file is not accessible, it reads from
     standard input.  The password may be up to 127 characters in
     length.  Additional characters and the terminating newline
     character are discarded.  While reading the password, echoing and
     the generation of signals by special characters is disabled.

7.3 HTTP, the Web, and All That
===============================

It has always been possible to connect computers together and share
information between them, but the rise of the World-Wide Web over the
last couple of decades has made it much easier to do so.  The result is
a richly connected network of computation, in which Guile forms a part.

   By "the web", we mean the HTTP protocol(1) as handled by servers,
clients, proxies, caches, and the various kinds of messages and message
components that can be sent and received by that protocol, notably HTML.

   On one level, the web is text in motion: the protocols themselves are
textual (though the payload may be binary), and it's possible to create
a socket and speak text to the web.  But such an approach is obviously
primitive.  This section details the higher-level data types and
operations provided by Guile: URIs, HTTP request and response records,
and a conventional web server implementation.

   The material in this section is arranged in ascending order, in which
later concepts build on previous ones.  If you prefer to start with the
highest-level perspective, *note Web Examples::, and work your way back.

   ---------- Footnotes ----------

   (1) Yes, the P is for protocol, but this phrase appears repeatedly
in RFC 2616.

7.3.1 Types and the Web
-----------------------

It is a truth universally acknowledged, that a program with good use of
data types, will be free from many common bugs.  Unfortunately, the
common practice in web programming seems to ignore this maxim.  This
subsection makes the case for expressive data types in web programming.

   By "expressive data types", we mean that the data types _say_
something about how a program solves a problem.  For example, if we
choose to represent dates using SRFI 19 date records (*note SRFI-19::),
this indicates that there is a part of the program that will always have
valid dates.  Error handling for a number of basic cases, like invalid
dates, occurs on the boundary in which we produce a SRFI 19 date record
from other types, like strings.

   With regards to the web, data types are helpful in the two broad
phases of HTTP messages: parsing and generation.

   Consider a server, which has to parse a request, and produce a
response.  Guile will parse the request into an HTTP request object
(*note Requests::), with each header parsed into an appropriate Scheme
data type.  This transition from an incoming stream of characters to
typed data is a state change in a program--the strings might parse, or
they might not, and something has to happen if they do not.  (Guile
throws an error in this case.)  But after you have the parsed request,
"client" code (code built on top of the Guile web framework) will not
have to check for syntactic validity.  The types already make this
information manifest.

   This state change on the parsing boundary makes programs more robust,
as they themselves are freed from the need to do a number of common
error checks, and they can use normal Scheme procedures to handle a
request instead of ad-hoc string parsers.

   The need for types on the response generation side (in a server) is
more subtle, though not less important.  Consider the example of a POST
handler, which prints out the text that a user submits from a form.
Such a handler might include a procedure like this:

     ;; First, a helper procedure
     (define (para . contents)
       (string-append "<p>" (string-concatenate contents) "</p>"))

     ;; Now the meat of our simple web application
     (define (you-said text)
       (para "You said: " text))

     (display (you-said "Hi!"))
     -| <p>You said: Hi!</p>

   This is a perfectly valid implementation, provided that the incoming
text does not contain the special HTML characters `<', `>', or `&'.
But this provision of a restricted character set is not reflected
anywhere in the program itself: we must _assume_ that the programmer
understands this, and performs the check elsewhere.

   Unfortunately, the short history of the practice of programming does
not bear out this assumption.  A "cross-site scripting" (XSS)
vulnerability is just such a common error in which unfiltered user input
is allowed into the output.  A user could submit a crafted comment to
your web site which results in visitors running malicious Javascript,
within the security context of your domain:

     (display (you-said "<script src=\"http://bad.com/nasty.js\" />"))
     -| <p>You said: <script src="http://bad.com/nasty.js" /></p>

   The fundamental problem here is that both user data and the program
template are represented using strings.  This identity means that types
can't help the programmer to make a distinction between these two, so
they get confused.

   There are a number of possible solutions, but perhaps the best is to
treat HTML not as strings, but as native s-expressions: as SXML.  The
basic idea is that HTML is either text, represented by a string, or an
element, represented as a tagged list.  So `foo' becomes `"foo"', and
`<b>foo</b>' becomes `(b "foo")'.  Attributes, if present, go in a
tagged list headed by `@', like `(img (@ (src
"http://example.com/foo.png")))'.  *Note sxml simple::, for more
information.

   The good thing about SXML is that HTML elements cannot be confused
with text.  Let's make a new definition of `para':

     (define (para . contents)
       `(p ,@contents))

     (use-modules (sxml simple))
     (sxml->xml (you-said "Hi!"))
     -| <p>You said: Hi!</p>

     (sxml->xml (you-said "<i>Rats, foiled again!</i>"))
     -| <p>You said: &lt;i&gt;Rats, foiled again!&lt;/i&gt;</p>

   So we see in the second example that HTML elements cannot be
unwittingly introduced into the output.  However it is now perfectly
acceptable to pass SXML to `you-said'; in fact, that is the big
advantage of SXML over everything-as-a-string.

     (sxml->xml (you-said (you-said "<Hi!>")))
     -| <p>You said: <p>You said: &lt;Hi!&gt;</p></p>

   The SXML types allow procedures to _compose_.  The types make
manifest which parts are HTML elements, and which are text.  So you
needn't worry about escaping user input; the type transition back to a
string handles that for you.  XSS vulnerabilities are a thing of the
past.

   Well.  That's all very nice and opinionated and such, but how do I
use the thing?  Read on!

7.3.2 Universal Resource Identifiers
------------------------------------

Guile provides a standard data type for Universal Resource Identifiers
(URIs), as defined in RFC 3986.

   The generic URI syntax is as follows:

     URI := scheme ":" ["//" [userinfo "@"] host [":" port]] path \
            [ "?" query ] [ "#" fragment ]

   For example, in the URI, <http://www.gnu.org/help/>, the scheme is
`http', the host is `www.gnu.org', the path is `/help/', and there is
no userinfo, port, query, or path.  All URIs have a scheme and a path
(though the path might be empty).  Some URIs have a host, and some of
those have ports and userinfo.  Any URI might have a query part or a
fragment.

   Userinfo is something of an abstraction, as some legacy URI schemes
allowed userinfo of the form `USERNAME:PASSWD'.  But since passwords do
not belong in URIs, the RFC does not want to condone this practice, so
it calls anything before the `@' sign "userinfo".

   Properly speaking, a fragment is not part of a URI.  For example,
when a web browser follows a link to <http://example.com/#foo>, it
sends a request for <http://example.com/>, then looks in the resulting
page for the fragment identified `foo' reference.  A fragment
identifies a part of a resource, not the resource itself.  But it is
useful to have a fragment field in the URI record itself, so we hope
you will forgive the inconsistency.

     (use-modules (web uri))

   The following procedures can be found in the `(web uri)' module.
Load it into your Guile, using a form like the above, to have access to
them.

 -- Scheme Procedure: build-uri scheme [#:userinfo=`#f'] [#:host=`#f']
          [#:port=`#f'] [#:path=`""'] [#:query=`#f'] [#:fragment=`#f']
          [#:validate?=`#t']
     Construct a URI object.  SCHEME should be a symbol, and the rest
     of the fields are either strings or `#f'.  If VALIDATE? is true,
     also run some consistency checks to make sure that the constructed
     URI is valid.

 -- Scheme Procedure: uri? x
 -- Scheme Procedure: uri-scheme uri
 -- Scheme Procedure: uri-userinfo uri
 -- Scheme Procedure: uri-host uri
 -- Scheme Procedure: uri-port uri
 -- Scheme Procedure: uri-path uri
 -- Scheme Procedure: uri-query uri
 -- Scheme Procedure: uri-fragment uri
     A predicate and field accessors for the URI record type.  The URI
     scheme will be a symbol, and the rest either strings or `#f' if not
     present.

 -- Scheme Procedure: string->uri string
     Parse STRING into a URI object.  Return `#f' if the string could
     not be parsed.

 -- Scheme Procedure: uri->string uri
     Serialize URI to a string.  If the URI has a port that is the
     default port for its scheme, the port is not included in the
     serialization.

 -- Scheme Procedure: declare-default-port! scheme port
     Declare a default port for the given URI scheme.

 -- Scheme Procedure: uri-decode str [#:encoding=`"utf-8"']
     Percent-decode the given STR, according to ENCODING, which should
     be the name of a character encoding.

     Note that this function should not generally be applied to a full
     URI string. For paths, use split-and-decode-uri-path instead. For
     query strings, split the query on `&' and `=' boundaries, and
     decode the components separately.

     Note also that percent-encoded strings encode _bytes_, not
     characters.  There is no guarantee that a given byte sequence is a
     valid string encoding. Therefore this routine may signal an error
     if the decoded bytes are not valid for the given encoding. Pass
     `#f' for ENCODING if you want decoded bytes as a bytevector
     directly.  *Note `set-port-encoding!': Ports, for more information
     on character encodings.

     Returns a string of the decoded characters, or a bytevector if
     ENCODING was `#f'.

   Fixme: clarify return type. indicate default values. type of
unescaped-chars.

 -- Scheme Procedure: uri-encode str [#:encoding=`"utf-8"']
          [#:unescaped-chars]
     Percent-encode any character not in the character set,
     UNESCAPED-CHARS.

     The default character set includes alphanumerics from ASCII, as
     well as the special characters `-', `.', `_', and `~'.  Any other
     character will be percent-encoded, by writing out the character to
     a bytevector within the given ENCODING, then encoding each byte as
     `%HH', where HH is the hexadecimal representation of the byte.

 -- Scheme Procedure: split-and-decode-uri-path path
     Split PATH into its components, and decode each component,
     removing empty components.

     For example, `"/foo/bar%20baz/"' decodes to the two-element list,
     `("foo" "bar baz")'.

 -- Scheme Procedure: encode-and-join-uri-path parts
     URI-encode each element of PARTS, which should be a list of
     strings, and join the parts together with `/' as a delimiter.

     For example, the list `("scrambled eggs" "biscuits&gravy")' encodes
     as `"scrambled%20eggs/biscuits%26gravy"'.

7.3.3 The Hyper-Text Transfer Protocol
--------------------------------------

The initial motivation for including web functionality in Guile, rather
than rely on an external package, was to establish a standard base on
which people can share code.  To that end, we continue the focus on data
types by providing a number of low-level parsers and unparsers for
elements of the HTTP protocol.

   If you are want to skip the low-level details for now and move on to
web pages, *note Web Client::, and *note Web Server::.  Otherwise, load
the HTTP module, and read on.

     (use-modules (web http))

   The focus of the `(web http)' module is to parse and unparse
standard HTTP headers, representing them to Guile as native data
structures.  For example, a `Date:' header will be represented as a
SRFI-19 date record (*note SRFI-19::), rather than as a string.

   Guile tries to follow RFCs fairly strictly--the road to perdition
being paved with compatibility hacks--though some allowances are made
for not-too-divergent texts.

   Header names are represented as lower-case symbols.

 -- Scheme Procedure: string->header name
     Parse NAME to a symbolic header name.

 -- Scheme Procedure: header->string sym
     Return the string form for the header named SYM.

   For example:

     (string->header "Content-Length")
     => content-length
     (header->string 'content-length)
     => "Content-Length"

     (string->header "FOO")
     => foo
     (header->string 'foo)
     => "Foo"

   Guile keeps a registry of known headers, their string names, and some
parsing and serialization procedures.  If a header is unknown, its
string name is simply its symbol name in title-case.

 -- Scheme Procedure: known-header? sym
     Return `#t' iff SYM is a known header, with associated parsers and
     serialization procedures.

 -- Scheme Procedure: header-parser sym
     Return the value parser for headers named SYM.  The result is a
     procedure that takes one argument, a string, and returns the parsed
     value.  If the header isn't known to Guile, a default parser is
     returned that passes through the string unchanged.

 -- Scheme Procedure: header-validator sym
     Return a predicate which returns `#t' if the given value is valid
     for headers named SYM.  The default validator for unknown headers
     is `string?'.

 -- Scheme Procedure: header-writer sym
     Return a procedure that writes values for headers named SYM to a
     port.  The resulting procedure takes two arguments: a value and a
     port.  The default writer is `display'.

   For more on the set of headers that Guile knows about out of the box,
*note HTTP Headers::.  To add your own, use the `declare-header!'
procedure:

 -- Scheme Procedure: declare-header! name parser validator writer
          [#:multiple?=`#f']
     Declare a parser, validator, and writer for a given header.

   For example, let's say you are running a web server behind some sort
of proxy, and your proxy adds an `X-Client-Address' header, indicating
the IPv4 address of the original client.  You would like for the HTTP
request record to parse out this header to a Scheme value, instead of
leaving it as a string.  You could register this header with Guile's
HTTP stack like this:

     (declare-header! "X-Client-Address"
       (lambda (str)
         (inet-aton str))
       (lambda (ip)
         (and (integer? ip) (exact? ip) (<= 0 ip #xffffffff)))
       (lambda (ip port)
         (display (inet-ntoa ip) port)))

 -- Scheme Procedure: valid-header? sym val
     Return a true value iff VAL is a valid Scheme value for the header
     with name SYM.

   Now that we have a generic interface for reading and writing
headers, we do just that.

 -- Scheme Procedure: read-header port
     Read one HTTP header from PORT. Return two values: the header name
     and the parsed Scheme value. May raise an exception if the header
     was known but the value was invalid.

     Returns the end-of-file object for both values if the end of the
     message body was reached (i.e., a blank line).

 -- Scheme Procedure: parse-header name val
     Parse VAL, a string, with the parser for the header named NAME.
     Returns the parsed value.

 -- Scheme Procedure: write-header name val port
     Write the given header name and value to PORT, using the writer
     from `header-writer'.

 -- Scheme Procedure: read-headers port
     Read the headers of an HTTP message from PORT, returning the
     headers as an ordered alist.

 -- Scheme Procedure: write-headers headers port
     Write the given header alist to PORT. Doesn't write the final
     `\r\n', as the user might want to add another header.

   The `(web http)' module also has some utility procedures to read and
write request and response lines.

 -- Scheme Procedure: parse-http-method str [start] [end]
     Parse an HTTP method from STR. The result is an upper-case symbol,
     like `GET'.

 -- Scheme Procedure: parse-http-version str [start] [end]
     Parse an HTTP version from STR, returning it as a major-minor
     pair. For example, `HTTP/1.1' parses as the pair of integers, `(1
     . 1)'.

 -- Scheme Procedure: parse-request-uri str [start] [end]
     Parse a URI from an HTTP request line. Note that URIs in requests
     do not have to have a scheme or host name. The result is a URI
     object.

 -- Scheme Procedure: read-request-line port
     Read the first line of an HTTP request from PORT, returning three
     values: the method, the URI, and the version.

 -- Scheme Procedure: write-request-line method uri version port
     Write the first line of an HTTP request to PORT.

 -- Scheme Procedure: read-response-line port
     Read the first line of an HTTP response from PORT, returning three
     values: the HTTP version, the response code, and the "reason
     phrase".

 -- Scheme Procedure: write-response-line version code reason-phrase
          port
     Write the first line of an HTTP response to PORT.

7.3.4 HTTP Headers
------------------

In addition to defining the infrastructure to parse headers, the `(web
http)' module defines specific parsers and unparsers for all headers
defined in the HTTP/1.1 standard.

   For example, if you receive a header named `Accept-Language' with a
value `en, es;q=0.8', Guile parses it as a quality list (defined below):

     (parse-header 'accept-language "en, es;q=0.8")
     => ((1000 . "en") (800 . "es"))

   The format of the value for `Accept-Language' headers is defined
below, along with all other headers defined in the HTTP standard.  (If
the header were unknown, the value would have been returned as a
string.)

   For brevity, the header definitions below are given in the form,
TYPE `NAME', indicating that values for the header `NAME' will be of
the given TYPE.  Since Guile internally treats header names in lower
case, in this document we give types title-cased names.  A short
description of the each header's purpose and an example follow.

   For full details on the meanings of all of these headers, see the
HTTP 1.1 standard, RFC 2616.

7.3.4.1 HTTP Header Types
.........................

Here we define the types that are used below, when defining headers.

 -- HTTP Header Type: Date
     A SRFI-19 date.

 -- HTTP Header Type: KVList
     A list whose elements are keys or key-value pairs.  Keys are
     parsed to symbols.  Values are strings by default.  Non-string
     values are the exception, and are mentioned explicitly below, as
     appropriate.

 -- HTTP Header Type: SList
     A list of strings.

 -- HTTP Header Type: Quality
     An exact integer between 0 and 1000.  Qualities are used to express
     preference, given multiple options.  An option with a quality of
     870, for example, is preferred over an option with quality 500.

     (Qualities are written out over the wire as numbers between 0.0 and
     1.0, but since the standard only allows three digits after the
     decimal, it's equivalent to integers between 0 and 1000, so that's
     what Guile uses.)

 -- HTTP Header Type: QList
     A quality list: a list of pairs, the car of which is a quality,
     and the cdr a string.  Used to express a list of options, along
     with their qualities.

 -- HTTP Header Type: ETag
     An entity tag, represented as a pair.  The car of the pair is an
     opaque string, and the cdr is `#t' if the entity tag is a "strong"
     entity tag, and `#f' otherwise.

7.3.4.2 General Headers
.......................

General HTTP headers may be present in any HTTP message.

 -- HTTP Header: KVList cache-control
     A key-value list of cache-control directives.  See RFC 2616, for
     more details.

     If present, parameters to `max-age', `max-stale', `min-fresh', and
     `s-maxage' are all parsed as non-negative integers.

     If present, parameters to `private' and `no-cache' are parsed as
     lists of header names, as symbols.

          (parse-header 'cache-control "no-cache,no-store"
          => (no-cache no-store)
          (parse-header 'cache-control "no-cache=\"Authorization,Date\",no-store"
          => ((no-cache . (authorization date)) no-store)
          (parse-header 'cache-control "no-cache=\"Authorization,Date\",max-age=10"
          => ((no-cache . (authorization date)) (max-age . 10))

 -- HTTP Header: List connection
     A list of header names that apply only to this HTTP connection, as
     symbols.  Additionally, the symbol `close' may be present, to
     indicate that the server should close the connection after
     responding to the request.
          (parse-header 'connection "close")
          => (close)

 -- HTTP Header: Date date
     The date that a given HTTP message was originated.
          (parse-header 'date "Tue, 15 Nov 1994 08:12:31 GMT")
          => #<date ...>

 -- HTTP Header: KVList pragma
     A key-value list of implementation-specific directives.
          (parse-header 'pragma "no-cache, broccoli=tasty")
          => (no-cache (broccoli . "tasty"))

 -- HTTP Header: List trailer
     A list of header names which will appear after the message body,
     instead of with the message headers.
          (parse-header 'trailer "ETag")
          => (etag)

 -- HTTP Header: List transfer-encoding
     A list of transfer codings, expressed as key-value lists.  The only
     transfer coding defined by the specification is `chunked'.
          (parse-header 'transfer-encoding "chunked")
          => ((chunked))

 -- HTTP Header: List upgrade
     A list of strings, indicating additional protocols that a server
     could use in response to a request.
          (parse-header 'upgrade "WebSocket")
          => ("WebSocket")

   FIXME: parse out more fully?

 -- HTTP Header: List via
     A list of strings, indicating the protocol versions and hosts of
     intermediate servers and proxies.  There may be multiple `via'
     headers in one message.
          (parse-header 'via "1.0 venus, 1.1 mars")
          => ("1.0 venus" "1.1 mars")

 -- HTTP Header: List warning
     A list of warnings given by a server or intermediate proxy.  Each
     warning is a itself a list of four elements: a code, as an exact
     integer between 0 and 1000, a host as a string, the warning text
     as a string, and either `#f' or a SRFI-19 date.

     There may be multiple `warning' headers in one message.
          (parse-header 'warning "123 foo \"core breach imminent\"")
          => ((123 "foo" "core-breach imminent" #f))

7.3.4.3 Entity Headers
......................

Entity headers may be present in any HTTP message, and refer to the
resource referenced in the HTTP request or response.

 -- HTTP Header: List allow
     A list of allowed methods on a given resource, as symbols.
          (parse-header 'allow "GET, HEAD")
          => (GET HEAD)

 -- HTTP Header: List content-encoding
     A list of content codings, as symbols.
          (parse-header 'content-encoding "gzip")
          => (GET HEAD)

 -- HTTP Header: List content-language
     The languages that a resource is in, as strings.
          (parse-header 'content-language "en")
          => ("en")

 -- HTTP Header: UInt content-length
     The number of bytes in a resource, as an exact, non-negative
     integer.
          (parse-header 'content-length "300")
          => 300

 -- HTTP Header: URI content-location
     The canonical URI for a resource, in the case that it is also
     accessible from a different URI.
          (parse-header 'content-location "http://example.com/foo")
          => #<<uri> ...>

 -- HTTP Header: String content-md5
     The MD5 digest of a resource.
          (parse-header 'content-md5 "ffaea1a79810785575e29e2bd45e2fa5")
          => "ffaea1a79810785575e29e2bd45e2fa5"

 -- HTTP Header: List content-range
     A range specification, as a list of three elements: the symbol
     `bytes', either the symbol `*' or a pair of integers, indicating
     the byte rage, and either `*' or an integer, for the instance
     length.  Used to indicate that a response only includes part of a
     resource.
          (parse-header 'content-range "bytes 10-20/*")
          => (bytes (10 . 20) *)

 -- HTTP Header: List content-type
     The MIME type of a resource, as a symbol, along with any
     parameters.
          (parse-header 'content-length "text/plain")
          => (text/plain)
          (parse-header 'content-length "text/plain;charset=utf-8")
          => (text/plain (charset . "utf-8"))
     Note that the `charset' parameter is something is a misnomer, and
     the HTTP specification admits this.  It specifies the _encoding_ of
     the characters, not the character set.

 -- HTTP Header: Date expires
     The date/time after which the resource given in a response is
     considered stale.
          (parse-header 'expires "Tue, 15 Nov 1994 08:12:31 GMT")
          => #<date ...>

 -- HTTP Header: Date last-modified
     The date/time on which the resource given in a response was last
     modified.
          (parse-header 'expires "Tue, 15 Nov 1994 08:12:31 GMT")
          => #<date ...>

7.3.4.4 Request Headers
.......................

Request headers may only appear in an HTTP request, not in a response.

 -- HTTP Header: List accept
     A list of preferred media types for a response.  Each element of
     the list is itself a list, in the same format as `content-type'.
          (parse-header 'accept "text/html,text/plain;charset=utf-8")
          => ((text/html) (text/plain (charset . "utf-8")))
     Preference is expressed with quality values:
          (parse-header 'accept "text/html;q=0.8,text/plain;q=0.6")
          => ((text/html (q . 800)) (text/plain (q . 600)))

 -- HTTP Header: QList accept-charset
     A quality list of acceptable charsets.  Note again that what HTTP
     calls a "charset" is what Guile calls a "character encoding".
          (parse-header 'accept-charset "iso-8859-5, unicode-1-1;q=0.8")
          => ((1000 . "iso-8859-5") (800 . "unicode-1-1"))

 -- HTTP Header: QList accept-encoding
     A quality list of acceptable content codings.
          (parse-header 'accept-encoding "gzip,identity=0.8")
          => ((1000 . "gzip") (800 . "identity"))

 -- HTTP Header: QList accept-language
     A quality list of acceptable languages.
          (parse-header 'accept-language "cn,en=0.75")
          => ((1000 . "cn") (750 . "en"))

 -- HTTP Header: Pair authorization
     Authorization credentials.  The car of the pair indicates the
     authentication scheme, like `basic'.  For basic authentication, the
     cdr of the pair will be the base64-encoded `USER:PASS' string.
     For other authentication schemes, like `digest', the cdr will be a
     key-value list of credentials.
          (parse-header 'authorization "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ=="
          => (basic . "QWxhZGRpbjpvcGVuIHNlc2FtZQ==")

 -- HTTP Header: List expect
     A list of expectations that a client has of a server.  The
     expectations are key-value lists.
          (parse-header 'expect "100-continue")
          => ((100-continue))

 -- HTTP Header: String from
     The email address of a user making an HTTP request.
          (parse-header 'from "bob@example.com")
          => "bob@example.com"

 -- HTTP Header: Pair host
     The host for the resource being requested, as a hostname-port
     pair.  If no port is given, the port is `#f'.
          (parse-header 'host "gnu.org:80")
          => ("gnu.org" . 80)
          (parse-header 'host "gnu.org")
          => ("gnu.org" . #f)

 -- HTTP Header: *|List if-match
     A set of etags, indicating that the request should proceed if and
     only if the etag of the resource is in that set.  Either the
     symbol `*', indicating any etag, or a list of entity tags.
          (parse-header 'if-match "*")
          => *
          (parse-header 'if-match "asdfadf")
          => (("asdfadf" . #t))
          (parse-header 'if-match W/"asdfadf")
          => (("asdfadf" . #f))

 -- HTTP Header: Date if-modified-since
     Indicates that a response should proceed if and only if the
     resource has been modified since the given date.
          (parse-header 'if-modified-since "Tue, 15 Nov 1994 08:12:31 GMT")
          => #<date ...>

 -- HTTP Header: *|List if-none-match
     A set of etags, indicating that the request should proceed if and
     only if the etag of the resource is not in the set.  Either the
     symbol `*', indicating any etag, or a list of entity tags.
          (parse-header 'if-none-match "*")
          => *

 -- HTTP Header: ETag|Date if-range
     Indicates that the range request should proceed if and only if the
     resource matches a modification date or an etag.  Either an entity
     tag, or a SRFI-19 date.
          (parse-header 'if-range "\"original-etag\"")
          => ("original-etag" . #t)

 -- HTTP Header: Date if-unmodified-since
     Indicates that a response should proceed if and only if the
     resource has not been modified since the given date.
          (parse-header 'if-not-modified-since "Tue, 15 Nov 1994 08:12:31 GMT")
          => #<date ...>

 -- HTTP Header: UInt max-forwards
     The maximum number of proxy or gateway hops that a request should
     be subject to.
          (parse-header 'max-forwards "10")
          => 10

 -- HTTP Header: Pair proxy-authorization
     Authorization credentials for a proxy connection.  See the
     documentation for `authorization' above for more information on
     the format.
          (parse-header 'proxy-authorization "Digest foo=bar,baz=qux"
          => (digest (foo . "bar") (baz . "qux"))

 -- HTTP Header: Pair range
     A range request, indicating that the client wants only part of a
     resource.  The car of the pair is the symbol `bytes', and the cdr
     is a list of pairs. Each element of the cdr indicates a range; the
     car is the first byte position and the cdr is the last byte
     position, as integers, or `#f' if not given.
          (parse-header 'range "bytes=10-30,50-")
          => (bytes (10 . 30) (50 . #f))

 -- HTTP Header: URI referer
     The URI of the resource that referred the user to this resource.
     The name of the header is a misspelling, but we are stuck with it.
          (parse-header 'referer "http://www.gnu.org/")
          => #<uri ...>

 -- HTTP Header: List te
     A list of transfer codings, expressed as key-value lists.  A common
     transfer coding is `trailers'.
          (parse-header 'te "trailers")
          => ((trailers))

 -- HTTP Header: String user-agent
     A string indicating the user agent making the request.  The
     specification defines a structured format for this header, but it
     is widely disregarded, so Guile does not attempt to parse strictly.
          (parse-header 'user-agent "Mozilla/5.0")
          => "Mozilla/5.0"

7.3.4.5 Response Headers
........................

 -- HTTP Header: List accept-ranges
     A list of range units that the server supports, as symbols.
          (parse-header 'accept-ranges "bytes")
          => (bytes)

 -- HTTP Header: UInt age
     The age of a cached response, in seconds.
          (parse-header 'age "3600")
          => 3600

 -- HTTP Header: ETag etag
     The entity-tag of the resource.
          (parse-header 'etag "\"foo\"")
          => ("foo" . #t)

 -- HTTP Header: URI location
     A URI on which a request may be completed.  Used in combination
     with a redirecting status code to perform client-side redirection.
          (parse-header 'location "http://example.com/other")
          => #<uri ...>

 -- HTTP Header: List proxy-authenticate
     A list of challenges to a proxy, indicating the need for
     authentication.
          (parse-header 'proxy-authenticate "Basic realm=\"foo\"")
          => ((basic (realm . "foo")))

 -- HTTP Header: UInt|Date retry-after
     Used in combination with a server-busy status code, like 503, to
     indicate that a client should retry later.  Either a number of
     seconds, or a date.
          (parse-header 'retry-after "60")
          => 60

 -- HTTP Header: String server
     A string identifying the server.
          (parse-header 'server "My first web server")
          => "My first web server"

 -- HTTP Header: *|List vary
     A set of request headers that were used in computing this response.
     Used to indicate that server-side content negotiation was
     performed, for example in response to the `accept-language'
     header.  Can also be the symbol `*', indicating that all headers
     were considered.
          (parse-header 'vary "Accept-Language, Accept")
          => (accept-language accept)

 -- HTTP Header: List www-authenticate
     A list of challenges to a user, indicating the need for
     authentication.
          (parse-header 'www-authenticate "Basic realm=\"foo\"")
          => ((basic (realm . "foo")))

7.3.5 HTTP Requests
-------------------

     (use-modules (web request))

   The request module contains a data type for HTTP requests.

7.3.5.1 An Important Note on Character Sets
...........................................

HTTP requests consist of two parts: the request proper, consisting of a
request line and a set of headers, and (optionally) a body.  The body
might have a binary content-type, and even in the textual case its
length is specified in bytes, not characters.

   Therefore, HTTP is a fundamentally binary protocol.  However the
request line and headers are specified to be in a subset of ASCII, so
they can be treated as text, provided that the port's encoding is set
to an ASCII-compatible one-byte-per-character encoding.  ISO-8859-1
(latin-1) is just such an encoding, and happens to be very efficient
for Guile.

   So what Guile does when reading requests from the wire, or writing
them out, is to set the port's encoding to latin-1, and treating the
request headers as text.

   The request body is another issue.  For binary data, the data is
probably in a bytevector, so we use the R6RS binary output procedures to
write out the binary payload.  Textual data usually has to be written
out to some character encoding, usually UTF-8, and then the resulting
bytevector is written out to the port.

   In summary, Guile reads and writes HTTP over latin-1 sockets, without
any loss of generality.

7.3.5.2 Request API
...................

 -- Scheme Procedure: request?
 -- Scheme Procedure: request-method
 -- Scheme Procedure: request-uri
 -- Scheme Procedure: request-version
 -- Scheme Procedure: request-headers
 -- Scheme Procedure: request-meta
 -- Scheme Procedure: request-port
     A predicate and field accessors for the request type.  The fields
     are as follows:
    `method'
          The HTTP method, for example, `GET'.

    `uri'
          The URI as a URI record.

    `version'
          The HTTP version pair, like `(1 . 1)'.

    `headers'
          The request headers, as an alist of parsed values.

    `meta'
          An arbitrary alist of other data, for example information
          returned in the `sockaddr' from `accept' (*note Network
          Sockets and Communication::).

    `port'
          The port on which to read or write a request body, if any.

 -- Scheme Procedure: read-request port [meta='()]
     Read an HTTP request from PORT, optionally attaching the given
     metadata, META.

     As a side effect, sets the encoding on PORT to ISO-8859-1
     (latin-1), so that reading one character reads one byte. See the
     discussion of character sets above, for more information.

     Note that the body is not part of the request.  Once you have read
     a request, you may read the body separately, and likewise for
     writing requests.

 -- Scheme Procedure: build-request uri [#:method='GET] [#:version='(1
          . 1)] [#:headers='()] [#:port=#f] [#:meta='()]
          [#:validate-headers?=#t]
     Construct an HTTP request object. If VALIDATE-HEADERS? is true,
     the headers are each run through their respective validators.

 -- Scheme Procedure: write-request r port
     Write the given HTTP request to PORT.

     Return a new request, whose `request-port' will continue writing
     on PORT, perhaps using some transfer encoding.

 -- Scheme Procedure: read-request-body r
     Reads the request body from R, as a bytevector.  Return `#f' if
     there was no request body.

 -- Scheme Procedure: write-request-body r bv
     Write BODY, a bytevector, to the port corresponding to the HTTP
     request R.

   The various headers that are typically associated with HTTP requests
may be accessed with these dedicated accessors.  *Note HTTP Headers::,
for more information on the format of parsed headers.

 -- Scheme Procedure: request-accept request [default='()]
 -- Scheme Procedure: request-accept-charset request [default='()]
 -- Scheme Procedure: request-accept-encoding request [default='()]
 -- Scheme Procedure: request-accept-language request [default='()]
 -- Scheme Procedure: request-allow request [default='()]
 -- Scheme Procedure: request-authorization request [default=#f]
 -- Scheme Procedure: request-cache-control request [default='()]
 -- Scheme Procedure: request-connection request [default='()]
 -- Scheme Procedure: request-content-encoding request [default='()]
 -- Scheme Procedure: request-content-language request [default='()]
 -- Scheme Procedure: request-content-length request [default=#f]
 -- Scheme Procedure: request-content-location request [default=#f]
 -- Scheme Procedure: request-content-md5 request [default=#f]
 -- Scheme Procedure: request-content-range request [default=#f]
 -- Scheme Procedure: request-content-type request [default=#f]
 -- Scheme Procedure: request-date request [default=#f]
 -- Scheme Procedure: request-expect request [default='()]
 -- Scheme Procedure: request-expires request [default=#f]
 -- Scheme Procedure: request-from request [default=#f]
 -- Scheme Procedure: request-host request [default=#f]
 -- Scheme Procedure: request-if-match request [default=#f]
 -- Scheme Procedure: request-if-modified-since request [default=#f]
 -- Scheme Procedure: request-if-none-match request [default=#f]
 -- Scheme Procedure: request-if-range request [default=#f]
 -- Scheme Procedure: request-if-unmodified-since request [default=#f]
 -- Scheme Procedure: request-last-modified request [default=#f]
 -- Scheme Procedure: request-max-forwards request [default=#f]
 -- Scheme Procedure: request-pragma request [default='()]
 -- Scheme Procedure: request-proxy-authorization request [default=#f]
 -- Scheme Procedure: request-range request [default=#f]
 -- Scheme Procedure: request-referer request [default=#f]
 -- Scheme Procedure: request-te request [default=#f]
 -- Scheme Procedure: request-trailer request [default='()]
 -- Scheme Procedure: request-transfer-encoding request [default='()]
 -- Scheme Procedure: request-upgrade request [default='()]
 -- Scheme Procedure: request-user-agent request [default=#f]
 -- Scheme Procedure: request-via request [default='()]
 -- Scheme Procedure: request-warning request [default='()]
     Return the given request header, or DEFAULT if none was present.

 -- Scheme Procedure: request-absolute-uri r [default-host=#f]
          [default-port=#f]
     A helper routine to determine the absolute URI of a request, using
     the `host' header and the default host and port.

7.3.6 HTTP Responses
--------------------

     (use-modules (web response))

   As with requests (*note Requests::), Guile offers a data type for
HTTP responses.  Again, the body is represented separately from the
request.

 -- Scheme Procedure: response?
 -- Scheme Procedure: response-version
 -- Scheme Procedure: response-code
 -- Scheme Procedure: response-reason-phrase response
 -- Scheme Procedure: response-headers
 -- Scheme Procedure: response-port
     A predicate and field accessors for the response type.  The fields
     are as follows:
    `version'
          The HTTP version pair, like `(1 . 1)'.

    `code'
          The HTTP response code, like `200'.

    `reason-phrase'
          The reason phrase, or the standard reason phrase for the
          response's code.

    `headers'
          The response headers, as an alist of parsed values.

    `port'
          The port on which to read or write a response body, if any.

 -- Scheme Procedure: read-response port
     Read an HTTP response from PORT.

     As a side effect, sets the encoding on PORT to ISO-8859-1
     (latin-1), so that reading one character reads one byte. See the
     discussion of character sets in *note Responses::, for more
     information.

 -- Scheme Procedure: build-response [#:version='(1 . 1)] [#:code=200]
          [#:reason-phrase=#f] [#:headers='()] [#:port=#f]
          [#:validate-headers=#t]
     Construct an HTTP response object. If VALIDATE-HEADERS? is true,
     the headers are each run through their respective validators.

 -- Scheme Procedure: adapt-response-version response version
     Adapt the given response to a different HTTP version.  Return a
     new HTTP response.

     The idea is that many applications might just build a response for
     the default HTTP version, and this method could handle a number of
     programmatic transformations to respond to older HTTP versions
     (0.9 and 1.0). But currently this function is a bit heavy-handed,
     just updating the version field.

 -- Scheme Procedure: write-response r port
     Write the given HTTP response to PORT.

     Return a new response, whose `response-port' will continue writing
     on PORT, perhaps using some transfer encoding.

 -- Scheme Procedure: read-response-body r
     Read the response body from R, as a bytevector.  Returns `#f' if
     there was no response body.

 -- Scheme Procedure: write-response-body r bv
     Write BODY, a bytevector, to the port corresponding to the HTTP
     response R.

   As with requests, the various headers that are typically associated
with HTTP responses may be accessed with these dedicated accessors.
*Note HTTP Headers::, for more information on the format of parsed
headers.

 -- Scheme Procedure: response-accept-ranges response [default=#f]
 -- Scheme Procedure: response-age response [default='()]
 -- Scheme Procedure: response-allow response [default='()]
 -- Scheme Procedure: response-cache-control response [default='()]
 -- Scheme Procedure: response-connection response [default='()]
 -- Scheme Procedure: response-content-encoding response [default='()]
 -- Scheme Procedure: response-content-language response [default='()]
 -- Scheme Procedure: response-content-length response [default=#f]
 -- Scheme Procedure: response-content-location response [default=#f]
 -- Scheme Procedure: response-content-md5 response [default=#f]
 -- Scheme Procedure: response-content-range response [default=#f]
 -- Scheme Procedure: response-content-type response [default=#f]
 -- Scheme Procedure: response-date response [default=#f]
 -- Scheme Procedure: response-etag response [default=#f]
 -- Scheme Procedure: response-expires response [default=#f]
 -- Scheme Procedure: response-last-modified response [default=#f]
 -- Scheme Procedure: response-location response [default=#f]
 -- Scheme Procedure: response-pragma response [default='()]
 -- Scheme Procedure: response-proxy-authenticate response [default=#f]
 -- Scheme Procedure: response-retry-after response [default=#f]
 -- Scheme Procedure: response-server response [default=#f]
 -- Scheme Procedure: response-trailer response [default='()]
 -- Scheme Procedure: response-transfer-encoding response [default='()]
 -- Scheme Procedure: response-upgrade response [default='()]
 -- Scheme Procedure: response-vary response [default='()]
 -- Scheme Procedure: response-via response [default='()]
 -- Scheme Procedure: response-warning response [default='()]
 -- Scheme Procedure: response-www-authenticate response [default=#f]
     Return the given response header, or DEFAULT if none was present.

7.3.7 Web Client
----------------

`(web client)' provides a simple, synchronous HTTP client, built on the
lower-level HTTP, request, and response modules.

 -- Scheme Procedure: open-socket-for-uri uri

 -- Scheme Procedure: http-get uri [#:port=(open-socket-for-uri uri)]
          [#:version='(1 . 1)] [#:keep-alive?=#f] [#:extra-headers='()]
          [#:decode-body=#t]
     Connect to the server corresponding to URI and ask for the
     resource, using the `GET' method.  If you already have a port open,
     pass it as PORT.  The port will be closed at the end of the
     request unless KEEP-ALIVE? is true.  Any extra headers in the
     alist EXTRA-HEADERS will be added to the request.

     If DECODE-BODY? is true, as is the default, the body of the
     response will be decoded to string, if it is a textual
     content-type.  Otherwise it will be returned as a bytevector.

   `http-get' is useful for making one-off requests to web sites.  If
you are writing a web spider or some other client that needs to handle a
number of requests in parallel, it's better to build an event-driven URL
fetcher, similar in structure to the web server (*note Web Server::).

   Another option, good but not as performant, would be to use threads,
possibly via par-map or futures.

   More helper procedures for the other common HTTP verbs would be a
good addition to this module.  Send your code to <guile-user@gnu.org>.

7.3.8 Web Server
----------------

`(web server)' is a generic web server interface, along with a main
loop implementation for web servers controlled by Guile.

     (use-modules (web server))

   The lowest layer is the `<server-impl>' object, which defines a set
of hooks to open a server, read a request from a client, write a
response to a client, and close a server.  These hooks - `open',
`read', `write', and `close', respectively - are bound together in a
`<server-impl>' object.  Procedures in this module take a
`<server-impl>' object, if needed.

   A `<server-impl>' may also be looked up by name.  If you pass the
`http' symbol to `run-server', Guile looks for a variable named `http'
in the `(web server http)' module, which should be bound to a
`<server-impl>' object.  Such a binding is made by instantiation of the
`define-server-impl' syntax.  In this way the run-server loop can
automatically load other backends if available.

   The life cycle of a server goes as follows:

  1. The `open' hook is called, to open the server. `open' takes 0 or
     more arguments, depending on the backend, and returns an opaque
     server socket object, or signals an error.

  2. The `read' hook is called, to read a request from a new client.
     The `read' hook takes one argument, the server socket.  It should
     return three values: an opaque client socket, the request, and the
     request body. The request should be a `<request>' object, from
     `(web request)'.  The body should be a string or a bytevector, or
     `#f' if there is no body.

     If the read failed, the `read' hook may return #f for the client
     socket, request, and body.

  3. A user-provided handler procedure is called, with the request and
     body as its arguments.  The handler should return two values: the
     response, as a `<response>' record from `(web response)', and the
     response body as bytevector, or `#f' if not present.

     The respose and response body are run through `sanitize-response',
     documented below.  This allows the handler writer to take some
     convenient shortcuts: for example, instead of a `<response>', the
     handler can simply return an alist of headers, in which case a
     default response object is constructed with those headers.
     Instead of a bytevector for the body, the handler can return a
     string, which will be serialized into an appropriate encoding; or
     it can return a procedure, which will be called on a port to write
     out the data.  See the `sanitize-response' documentation, for more.

  4. The `write' hook is called with three arguments: the client
     socket, the response, and the body.  The `write' hook returns no
     values.

  5. At this point the request handling is complete. For a loop, we
     loop back and try to read a new request.

  6. If the user interrupts the loop, the `close' hook is called on the
     server socket.

   A user may define a server implementation with the following form:

 -- Scheme Procedure: define-server-impl name open read write close
     Make a `<server-impl>' object with the hooks OPEN, READ, WRITE,
     and CLOSE, and bind it to the symbol NAME in the current module.

 -- Scheme Procedure: lookup-server-impl impl
     Look up a server implementation. If IMPL is a server
     implementation already, it is returned directly. If it is a
     symbol, the binding named IMPL in the `(web server IMPL)' module is
     looked up. Otherwise an error is signaled.

     Currently a server implementation is a somewhat opaque type,
     useful only for passing to other procedures in this module, like
     `read-client'.

   The `(web server)' module defines a number of routines that use
`<server-impl>' objects to implement parts of a web server.  Given that
we don't expose the accessors for the various fields of a
`<server-impl>', indeed these routines are the only procedures with any
access to the impl objects.

 -- Scheme Procedure: open-server impl open-params
     Open a server for the given implementation.  Return one value, the
     new server object. The implementation's `open' procedure is
     applied to OPEN-PARAMS, which should be a list.

 -- Scheme Procedure: read-client impl server
     Read a new client from SERVER, by applying the implementation's
     `read' procedure to the server.  If successful, return three
     values: an object corresponding to the client, a request object,
     and the request body. If any exception occurs, return `#f' for all
     three values.

 -- Scheme Procedure: handle-request handler request body state
     Handle a given request, returning the response and body.

     The response and response body are produced by calling the given
     HANDLER with REQUEST and BODY as arguments.

     The elements of STATE are also passed to HANDLER as arguments, and
     may be returned as additional values. The new STATE, collected
     from the HANDLER's return values, is then returned as a list. The
     idea is that a server loop receives a handler from the user, along
     with whatever state values the user is interested in, allowing the
     user's handler to explicitly manage its state.

 -- Scheme Procedure: sanitize-response request response body
     "Sanitize" the given response and body, making them appropriate
     for the given request.

     As a convenience to web handler authors, RESPONSE may be given as
     an alist of headers, in which case it is used to construct a
     default response. Ensures that the response version corresponds to
     the request version. If BODY is a string, encodes the string to a
     bytevector, in an encoding appropriate for RESPONSE. Adds a
     `content-length' and `content-type' header, as necessary.

     If BODY is a procedure, it is called with a port as an argument,
     and the output collected as a bytevector. In the future we might
     try to instead use a compressing, chunk-encoded port, and call
     this procedure later, in the write-client procedure. Authors are
     advised not to rely on the procedure being called at any
     particular time.

 -- Scheme Procedure: write-client impl server client response body
     Write an HTTP response and body to CLIENT. If the server and
     client support persistent connections, it is the implementation's
     responsibility to keep track of the client thereafter, presumably
     by attaching it to the SERVER argument somehow.

 -- Scheme Procedure: close-server impl server
     Release resources allocated by a previous invocation of
     `open-server'.

   Given the procedures above, it is a small matter to make a web
server:

 -- Scheme Procedure: serve-one-client handler impl server state
     Read one request from SERVER, call HANDLER on the request and
     body, and write the response to the client.  Return the new state
     produced by the handler procedure.

 -- Scheme Procedure: run-server handler [impl='http] [open-params='()]
          . state
     Run Guile's built-in web server.

     HANDLER should be a procedure that takes two or more arguments,
     the HTTP request and request body, and returns two or more values,
     the response and response body.

     For examples, skip ahead to the next section, *note Web Examples::.

     The response and body will be run through `sanitize-response'
     before sending back to the client.

     Additional arguments to HANDLER are taken from STATE.  Additional
     return values are accumulated into a new STATE, which will be used
     for subsequent requests. In this way a handler can explicitly
     manage its state.

   The default web server implementation is `http', which binds to a
socket, listening for request on that port.

 -- HTTP Implementation: http [#:host=#f] [#:family=AF_INET]
          [#:addr=INADDR_LOOPBACK] [#:port 8080] [#:socket]
     The default HTTP implementation.  We document it as a function with
     keyword arguments, because that is precisely the way that it is -
     all of the OPEN-PARAMS to `run-server' get passed to the
     implementation's open function.

          ;; The defaults: localhost:8080
          (run-server handler)
          ;; Same thing
          (run-server handler 'http '())
          ;; On a different port
          (run-server handler 'http '(#:port 8081))
          ;; IPv6
          (run-server handler 'http '(#:family AF_INET6 #:port 8081))
          ;; Custom socket
          (run-server handler 'http `(#:socket ,(sudo-make-me-a-socket)))

7.3.9 Web Examples
------------------

Well, enough about the tedious internals.  Let's make a web application!

7.3.9.1 Hello, World!
.....................

The first program we have to write, of course, is "Hello, World!".
This means that we have to implement a web handler that does what we
want.

   Now we define a handler, a function of two arguments and two return
values:

     (define (handler request request-body)
       (values RESPONSE RESPONSE-BODY))

   In this first example, we take advantage of a short-cut, returning an
alist of headers instead of a proper response object. The response body
is our payload:

     (define (hello-world-handler request request-body)
       (values '((content-type . (text/plain)))
               "Hello World!"))

   Now let's test it, by running a server with this handler. Load up the
web server module if you haven't yet done so, and run a server with this
handler:

     (use-modules (web server))
     (run-server hello-world-handler)

   By default, the web server listens for requests on `localhost:8080'.
Visit that address in your web browser to test.  If you see the string,
`Hello World!', sweet!

7.3.9.2 Inspecting the Request
..............................

The Hello World program above is a general greeter, responding to all
URIs.  To make a more exclusive greeter, we need to inspect the request
object, and conditionally produce different results.  So let's load up
the request, response, and URI modules, and do just that.

     (use-modules (web server)) ; you probably did this already
     (use-modules (web request)
                  (web response)
                  (web uri))

     (define (request-path-components request)
       (split-and-decode-uri-path (uri-path (request-uri request))))

     (define (hello-hacker-handler request body)
       (if (equal? (request-path-components request)
                   '("hacker"))
           (values '((content-type . (text/plain)))
                   "Hello hacker!")
           (not-found request)))

     (run-server hello-hacker-handler)

   Here we see that we have defined a helper to return the components of
the URI path as a list of strings, and used that to check for a request
to `/hacker/'. Then the success case is just as before - visit
`http://localhost:8080/hacker/' in your browser to check.

   You should always match against URI path components as decoded by
`split-and-decode-uri-path'. The above example will work for
`/hacker/', `//hacker///', and `/h%61ck%65r'.

   But we forgot to define `not-found'!  If you are pasting these
examples into a REPL, accessing any other URI in your web browser will
drop your Guile console into the debugger:

     <unnamed port>:38:7: In procedure module-lookup:
     <unnamed port>:38:7: Unbound variable: not-found

     Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
     scheme@(guile-user) [1]>

   So let's define the function, right there in the debugger.  As you
probably know, we'll want to return a 404 response.

     ;; Paste this in your REPL
     (define (not-found request)
       (values (build-response #:code 404)
               (string-append "Resource not found: "
                              (uri->string (request-uri request)))))

     ;; Now paste this to let the web server keep going:
     ,continue

   Now if you access `http://localhost/foo/', you get this error
message.  (Note that some popular web browsers won't show
server-generated 404 messages, showing their own instead, unless the 404
message body is long enough.)

7.3.9.3 Higher-Level Interfaces
...............................

The web handler interface is a common baseline that all kinds of Guile
web applications can use.  You will usually want to build something on
top of it, however, especially when producing HTML.  Here is a simple
example that builds up HTML output using SXML (*note sxml simple::).

   First, load up the modules:

     (use-modules (web server)
                  (web request)
                  (web response)
                  (sxml simple))

   Now we define a simple templating function that takes a list of HTML
body elements, as SXML, and puts them in our super template:

     (define (templatize title body)
       `(html (head (title ,title))
              (body ,@body)))

   For example, the simplest Hello HTML can be produced like this:

     (sxml->xml (templatize "Hello!" '((b "Hi!"))))
     -|
     <html><head><title>Hello!</title></head><body><b>Hi!</b></body></html>

   Much better to work with Scheme data types than to work with HTML as
strings. Now we define a little response helper:

     (define* (respond #:optional body #:key
                       (status 200)
                       (title "Hello hello!")
                       (doctype "<!DOCTYPE html>\n")
                       (content-type-params '((charset . "utf-8")))
                       (content-type 'text/html)
                       (extra-headers '())
                       (sxml (and body (templatize title body))))
       (values (build-response
                #:code status
                #:headers `((content-type
                             . (,content-type ,@content-type-params))
                            ,@extra-headers))
               (lambda (port)
                 (if sxml
                     (begin
                       (if doctype (display doctype port))
                       (sxml->xml sxml port))))))

   Here we see the power of keyword arguments with default
initializers. By the time the arguments are fully parsed, the `sxml'
local variable will hold the templated SXML, ready for sending out to
the client.

   Also, instead of returning the body as a string, `respond' gives a
procedure, which will be called by the web server to write out the
response to the client.

   Now, a simple example using this responder, which lays out the
incoming headers in an HTML table.

     (define (debug-page request body)
       (respond
        `((h1 "hello world!")
          (table
           (tr (th "header") (th "value"))
           ,@(map (lambda (pair)
                    `(tr (td (tt ,(with-output-to-string
                                    (lambda () (display (car pair))))))
                         (td (tt ,(with-output-to-string
                                    (lambda ()
                                      (write (cdr pair))))))))
                  (request-headers request))))))

     (run-server debug-page)

   Now if you visit any local address in your web browser, we actually
see some HTML, finally.

7.3.9.4 Conclusion
..................

Well, this is about as far as Guile's built-in web support goes, for
now.  There are many ways to make a web application, but hopefully by
standardizing the most fundamental data types, users will be able to
choose the approach that suits them best, while also being able to
switch between implementations of the server.  This is a relatively new
part of Guile, so if you have feedback, let us know, and we can take it
into account.  Happy hacking on the web!

7.4 The (ice-9 getopt-long) Module
==================================

The `(ice-9 getopt-long)' module exports two procedures: `getopt-long'
and `option-ref'.

   * `getopt-long' takes a list of strings -- the command line
     arguments -- an "option specification", and some optional keyword
     parameters.  It parses the command line arguments according to the
     option specification and keyword parameters, and returns a data
     structure that encapsulates the results of the parsing.

   * `option-ref' then takes the parsed data structure and a specific
     option's name, and returns information about that option in
     particular.

   To make these procedures available to your Guile script, include the
expression `(use-modules (ice-9 getopt-long))' somewhere near the top,
before the first usage of `getopt-long' or `option-ref'.

7.4.1 A Short getopt-long Example
---------------------------------

This section illustrates how `getopt-long' is used by presenting and
dissecting a simple example.  The first thing that we need is an
"option specification" that tells `getopt-long' how to parse the
command line.  This specification is an association list with the long
option name as the key.  Here is how such a specification might look:

     (define option-spec
       '((version (single-char #\v) (value #f))
         (help    (single-char #\h) (value #f))))

   This alist tells `getopt-long' that it should accept two long
options, called _version_ and _help_, and that these options can also
be selected by the single-letter abbreviations _v_ and _h_,
respectively.  The `(value #f)' clauses indicate that neither of the
options accepts a value.

   With this specification we can use `getopt-long' to parse a given
command line:

     (define options (getopt-long (command-line) option-spec))

   After this call, `options' contains the parsed command line and is
ready to be examined by `option-ref'.  `option-ref' is called like this:

     (option-ref options 'help #f)

It expects the parsed command line, a symbol indicating the option to
examine, and a default value.  The default value is returned if the
option was not present in the command line, or if the option was present
but without a value; otherwise the value from the command line is
returned.  Usually `option-ref' is called once for each possible option
that a script supports.

   The following example shows a main program which puts all this
together to parse its command line and figure out what the user wanted.

     (define (main args)
       (let* ((option-spec '((version (single-char #\v) (value #f))
                             (help    (single-char #\h) (value #f))))
              (options (getopt-long args option-spec))
              (help-wanted (option-ref options 'help #f))
              (version-wanted (option-ref options 'version #f)))
         (if (or version-wanted help-wanted)
             (begin
               (if version-wanted
                   (display "getopt-long-example version 0.3\n"))
               (if help-wanted
                   (display "\
     getopt-long-example [options]
       -v, --version    Display version
       -h, --help       Display this help
     ")))
             (begin
               (display "Hello, World!") (newline)))))

7.4.2 How to Write an Option Specification
------------------------------------------

An option specification is an association list (*note Association
Lists::) with one list element for each supported option. The key of
each list element is a symbol that names the option, while the value is
a list of option properties:

     OPTION-SPEC ::=  '( (OPT-NAME1 (PROP-NAME PROP-VALUE) ...)
                         (OPT-NAME2 (PROP-NAME PROP-VALUE) ...)
                         (OPT-NAME3 (PROP-NAME PROP-VALUE) ...)
                         ...
                       )

   Each OPT-NAME specifies the long option name for that option.  For
example, a list element with OPT-NAME `background' specifies an option
that can be specified on the command line using the long option
`--background'.  Further information about the option -- whether it
takes a value, whether it is required to be present in the command
line, and so on -- is specified by the option properties.

   In the example of the preceding section, we already saw that a long
option name can have a equivalent "short option" character.  The
equivalent short option character can be set for an option by specifying
a `single-char' property in that option's property list.  For example,
a list element like `'(output (single-char #\o) ...)' specifies an
option with long name `--output' that can also be specified by the
equivalent short name `-o'.

   The `value' property specifies whether an option requires or accepts
a value.  If the `value' property is set to `#t', the option requires a
value: `getopt-long' will signal an error if the option name is present
without a corresponding value.  If set to `#f', the option does not
take a value; in this case, a non-option word that follows the option
name in the command line will be treated as a non-option argument.  If
set to the symbol `optional', the option accepts a value but does not
require one: a non-option word that follows the option name in the
command line will be interpreted as that option's value.  If the option
name for an option with `'(value optional)' is immediately followed in
the command line by _another_ option name, the value for the first
option is implicitly `#t'.

   The `required?' property indicates whether an option is required to
be present in the command line.  If the `required?'  property is set to
`#t', `getopt-long' will signal an error if the option is not specified.

   Finally, the `predicate' property can be used to constrain the
possible values of an option.  If used, the `predicate' property should
be set to a procedure that takes one argument -- the proposed option
value as a string -- and returns either `#t' or `#f' according as the
proposed value is or is not acceptable.  If the predicate procedure
returns `#f', `getopt-long' will signal an error.

   By default, options do not have single-character equivalents, are not
required, and do not take values.  Where the list element for an option
includes a `value' property but no `predicate' property, the option
values are unconstrained.

7.4.3 Expected Command Line Format
----------------------------------

In order for `getopt-long' to correctly parse a command line, that
command line must conform to a standard set of rules for how command
line options are specified.  This section explains what those rules are.

   `getopt-long' splits a given command line into several pieces.  All
elements of the argument list are classified to be either options or
normal arguments.  Options consist of two dashes and an option name
(so-called "long" options), or of one dash followed by a single letter
("short" options).

   Options can behave as switches, when they are given without a value,
or they can be used to pass a value to the program.  The value for an
option may be specified using an equals sign, or else is simply the next
word in the command line, so the following two invocations are
equivalent:

     $ ./foo.scm --output=bar.txt
     $ ./foo.scm --output bar.txt

   Short options can be used instead of their long equivalents and can
be grouped together after a single dash.  For example, the following
commands are equivalent.

     $ ./foo.scm --version --help
     $ ./foo.scm -v --help
     $ ./foo.scm -vh

   If an option requires a value, it can only be grouped together with
other short options if it is the last option in the group; the value is
the next argument.  So, for example, with the following option
specification --

     ((apples    (single-char #\a))
      (blimps    (single-char #\b) (value #t))
      (catalexis (single-char #\c) (value #t)))

-- the following command lines would all be acceptable:

     $ ./foo.scm -a -b bang -c couth
     $ ./foo.scm -ab bang -c couth
     $ ./foo.scm -ac couth -b bang

   But the next command line is an error, because `-b' is not the last
option in its combination, and because a group of short options cannot
include two options that both require values:

     $ ./foo.scm -abc couth bang

   If an option's value is optional, `getopt-long' decides whether the
option has a value by looking at what follows it in the argument list.
If the next element is a string, and it does not appear to be an option
itself, then that string is the option's value.

   If the option `--' appears in the argument list, argument parsing
stops there and subsequent arguments are returned as ordinary arguments,
even if they resemble options.  So, with the command line

     $ ./foo.scm --apples "Granny Smith" -- --blimp Goodyear

`getopt-long' will recognize the `--apples' option as having the value
"Granny Smith", but will not treat `--blimp' as an option.  The strings
`--blimp' and `Goodyear' will be returned as ordinary argument strings.

7.4.4 Reference Documentation for `getopt-long'
-----------------------------------------------

 -- Scheme Procedure: getopt-long args grammar
          [#:stop-at-first-non-option #t]
     Parse the command line given in ARGS (which must be a list of
     strings) according to the option specification GRAMMAR.

     The GRAMMAR argument is expected to be a list of this form:

     `((OPTION (PROPERTY VALUE) ...) ...)'

     where each OPTION is a symbol denoting the long option, but
     without the two leading dashes (e.g. `version' if the option is
     called `--version').

     For each option, there may be list of arbitrarily many
     property/value pairs.  The order of the pairs is not important,
     but every property may only appear once in the property list.  The
     following table lists the possible properties:

    `(single-char CHAR)'
          Accept `-CHAR' as a single-character equivalent to
          `--OPTION'.  This is how to specify traditional Unix-style
          flags.

    `(required? BOOL)'
          If BOOL is true, the option is required.  `getopt-long' will
          raise an error if it is not found in ARGS.

    `(value BOOL)'
          If BOOL is `#t', the option accepts a value; if it is `#f',
          it does not; and if it is the symbol `optional', the option
          may appear in ARGS with or without a value.

    `(predicate FUNC)'
          If the option accepts a value (i.e. you specified `(value
          #t)' for this option), then `getopt-long' will apply FUNC to
          the value, and throw an exception if it returns `#f'.  FUNC
          should be a procedure which accepts a string and returns a
          boolean value; you may need to use quasiquotes to get it into
          GRAMMAR.

     The `#:stop-at-first-non-option' keyword, if specified with any
     true value, tells `getopt-long' to stop when it gets to the first
     non-option in the command line.  That is, at the first word which
     is neither an option itself, nor the value of an option.
     Everything in the command line from that word onwards will be
     returned as non-option arguments.

   `getopt-long''s ARGS parameter is expected to be a list of strings
like the one returned by `command-line', with the first element being
the name of the command.  Therefore `getopt-long' ignores the first
element in ARGS and starts argument interpretation with the second
element.

   `getopt-long' signals an error if any of the following conditions
hold.

   * The option grammar has an invalid syntax.

   * One of the options in the argument list was not specified by the
     grammar.

   * A required option is omitted.

   * An option which requires an argument did not get one.

   * An option that doesn't accept an argument does get one (this can
     only happen using the long option `--opt=VALUE' syntax).

   * An option predicate fails.

   `#:stop-at-first-non-option' is useful for command line invocations
like `guild [--help | --version] [script [script-options]]' and `cvs
[general-options] command [command-options]', where there are options
at two levels: some generic and understood by the outer command, and
some that are specific to the particular script or command being
invoked.  To use `getopt-long' in such cases, you would call it twice:
firstly with `#:stop-at-first-non-option #t', so as to parse any
generic options and identify the wanted script or sub-command;
secondly, and after trimming off the initial generic command words, with
a script- or sub-command-specific option grammar, so as to process those
specific options.

7.4.5 Reference Documentation for `option-ref'
----------------------------------------------

 -- Scheme Procedure: option-ref options key default
     Search OPTIONS for a command line option named KEY and return its
     value, if found.  If the option has no value, but was given,
     return `#t'.  If the option was not given, return DEFAULT.
     OPTIONS must be the result of a call to `getopt-long'.

   `option-ref' always succeeds, either by returning the requested
option value from the command line, or the default value.

   The special key `'()' can be used to get a list of all non-option
arguments.

7.5 SRFI Support Modules
========================

SRFI is an acronym for Scheme Request For Implementation.  The SRFI
documents define a lot of syntactic and procedure extensions to standard
Scheme as defined in R5RS.

   Guile has support for a number of SRFIs.  This chapter gives an
overview over the available SRFIs and some usage hints.  For complete
documentation, design rationales and further examples, we advise you to
get the relevant SRFI documents from the SRFI home page
`http://srfi.schemers.org/'.

7.5.1 About SRFI Usage
----------------------

SRFI support in Guile is currently implemented partly in the core
library, and partly as add-on modules.  That means that some SRFIs are
automatically available when the interpreter is started, whereas the
other SRFIs require you to use the appropriate support module
explicitly.

   There are several reasons for this inconsistency.  First, the feature
checking syntactic form `cond-expand' (*note SRFI-0::) must be
available immediately, because it must be there when the user wants to
check for the Scheme implementation, that is, before she can know that
it is safe to use `use-modules' to load SRFI support modules.  The
second reason is that some features defined in SRFIs had been
implemented in Guile before the developers started to add SRFI
implementations as modules (for example SRFI-6 (*note SRFI-6::)).  In
the future, it is possible that SRFIs in the core library might be
factored out into separate modules, requiring explicit module loading
when they are needed.  So you should be prepared to have to use
`use-modules' someday in the future to access SRFI-6 bindings.  If you
want, you can do that already.  We have included the module `(srfi
srfi-6)' in the distribution, which currently does nothing, but ensures
that you can write future-safe code.

   Generally, support for a specific SRFI is made available by using
modules named `(srfi srfi-NUMBER)', where NUMBER is the number of the
SRFI needed.  Another possibility is to use the command line option
`--use-srfi', which will load the necessary modules automatically
(*note Invoking Guile::).

7.5.2 SRFI-0 - cond-expand
--------------------------

This SRFI lets a portable Scheme program test for the presence of
certain features, and adapt itself by using different blocks of code,
or fail if the necessary features are not available.  There's no module
to load, this is in the Guile core.

   A program designed only for Guile will generally not need this
mechanism, such a program can of course directly use the various
documented parts of Guile.

 -- syntax: cond-expand (feature body...) ...
     Expand to the BODY of the first clause whose FEATURE specification
     is satisfied.  It is an error if no FEATURE is satisfied.

     Features are symbols such as `srfi-1', and a feature specification
     can use `and', `or' and `not' forms to test combinations.  The
     last clause can be an `else', to be used if no other passes.

     For example, define a private version of `alist-cons' if SRFI-1 is
     not available.

          (cond-expand (srfi-1
                        )
                       (else
                        (define (alist-cons key val alist)
                          (cons (cons key val) alist))))

     Or demand a certain set of SRFIs (list operations, string ports,
     `receive' and string operations), failing if they're not available.

          (cond-expand ((and srfi-1 srfi-6 srfi-8 srfi-13)
                        ))

The Guile core has the following features,

     guile
     guile-2  ;; starting from Guile 2.x
     r5rs
     srfi-0
     srfi-4
     srfi-6
     srfi-13
     srfi-14

   Other SRFI feature symbols are defined once their code has been
loaded with `use-modules', since only then are their bindings available.

   The `--use-srfi' command line option (*note Invoking Guile::) is a
good way to load SRFIs to satisfy `cond-expand' when running a portable
program.

   Testing the `guile' feature allows a program to adapt itself to the
Guile module system, but still run on other Scheme systems.  For
example the following demands SRFI-8 (`receive'), but also knows how to
load it with the Guile mechanism.

     (cond-expand (srfi-8
                   )
                  (guile
                   (use-modules (srfi srfi-8))))

   Likewise, testing the `guile-2' feature allows code to be portable
between Guile 2.0 and previous versions of Guile.  For instance, it
makes it possible to write code that accounts for Guile 2.0's compiler,
yet be correctly interpreted on 1.8 and earlier versions:

     (cond-expand (guile-2 (eval-when (compile)
                             ;; This must be evaluated at compile time.
                             (fluid-set! current-reader my-reader)))
                  (guile
                           ;; Earlier versions of Guile do not have a
                           ;; separate compilation phase.
                           (fluid-set! current-reader my-reader)))

   It should be noted that `cond-expand' is separate from the
`*features*' mechanism (*note Feature Tracking::), feature symbols in
one are unrelated to those in the other.

7.5.3 SRFI-1 - List library
---------------------------

The list library defined in SRFI-1 contains a lot of useful list
processing procedures for construction, examining, destructuring and
manipulating lists and pairs.

   Since SRFI-1 also defines some procedures which are already contained
in R5RS and thus are supported by the Guile core library, some list and
pair procedures which appear in the SRFI-1 document may not appear in
this section.  So when looking for a particular list/pair processing
procedure, you should also have a look at the sections *note Lists::
and *note Pairs::.

7.5.3.1 Constructors
....................

New lists can be constructed by calling one of the following procedures.

 -- Scheme Procedure: xcons d a
     Like `cons', but with interchanged arguments.  Useful mostly when
     passed to higher-order procedures.

 -- Scheme Procedure: list-tabulate n init-proc
     Return an N-element list, where each list element is produced by
     applying the procedure INIT-PROC to the corresponding list index.
     The order in which INIT-PROC is applied to the indices is not
     specified.

 -- Scheme Procedure: list-copy lst
     Return a new list containing the elements of the list LST.

     This function differs from the core `list-copy' (*note List
     Constructors::) in accepting improper lists too.  And if LST is
     not a pair at all then it's treated as the final tail of an
     improper list and simply returned.

 -- Scheme Procedure: circular-list elt1 elt2 ...
     Return a circular list containing the given arguments ELT1 ELT2
     ....

 -- Scheme Procedure: iota count [start step]
     Return a list containing COUNT numbers, starting from START and
     adding STEP each time.  The default START is 0, the default STEP
     is 1.  For example,

          (iota 6)        => (0 1 2 3 4 5)
          (iota 4 2.5 -2) => (2.5 0.5 -1.5 -3.5)

     This function takes its name from the corresponding primitive in
     the APL language.

7.5.3.2 Predicates
..................

The procedures in this section test specific properties of lists.

 -- Scheme Procedure: proper-list? obj
     Return `#t' if OBJ is a proper list, or `#f' otherwise.  This is
     the same as the core `list?' (*note List Predicates::).

     A proper list is a list which ends with the empty list `()' in the
     usual way.  The empty list `()' itself is a proper list too.

          (proper-list? '(1 2 3))  => #t
          (proper-list? '())       => #t

 -- Scheme Procedure: circular-list? obj
     Return `#t' if OBJ is a circular list, or `#f' otherwise.

     A circular list is a list where at some point the `cdr' refers
     back to a previous pair in the list (either the start or some later
     point), so that following the `cdr's takes you around in a circle,
     with no end.

          (define x (list 1 2 3 4))
          (set-cdr! (last-pair x) (cddr x))
          x => (1 2 3 4 3 4 3 4 ...)
          (circular-list? x)  => #t

 -- Scheme Procedure: dotted-list? obj
     Return `#t' if OBJ is a dotted list, or `#f' otherwise.

     A dotted list is a list where the `cdr' of the last pair is not
     the empty list `()'.  Any non-pair OBJ is also considered a dotted
     list, with length zero.

          (dotted-list? '(1 2 . 3))  => #t
          (dotted-list? 99)          => #t

   It will be noted that any Scheme object passes exactly one of the
above three tests `proper-list?', `circular-list?' and `dotted-list?'.
Non-lists are `dotted-list?', finite lists are either `proper-list?' or
`dotted-list?', and infinite lists are `circular-list?'.


 -- Scheme Procedure: null-list? lst
     Return `#t' if LST is the empty list `()', `#f' otherwise.  If
     something else than a proper or circular list is passed as LST, an
     error is signalled.  This procedure is recommended for checking
     for the end of a list in contexts where dotted lists are not
     allowed.

 -- Scheme Procedure: not-pair? obj
     Return `#t' is OBJ is not a pair, `#f' otherwise.  This is
     shorthand notation `(not (pair? OBJ))' and is supposed to be used
     for end-of-list checking in contexts where dotted lists are
     allowed.

 -- Scheme Procedure: list= elt= list1 ...
     Return `#t' if all argument lists are equal, `#f' otherwise.  List
     equality is determined by testing whether all lists have the same
     length and the corresponding elements are equal in the sense of the
     equality predicate ELT=.  If no or only one list is given, `#t' is
     returned.

7.5.3.3 Selectors
.................

 -- Scheme Procedure: first pair
 -- Scheme Procedure: second pair
 -- Scheme Procedure: third pair
 -- Scheme Procedure: fourth pair
 -- Scheme Procedure: fifth pair
 -- Scheme Procedure: sixth pair
 -- Scheme Procedure: seventh pair
 -- Scheme Procedure: eighth pair
 -- Scheme Procedure: ninth pair
 -- Scheme Procedure: tenth pair
     These are synonyms for `car', `cadr', `caddr', ....

 -- Scheme Procedure: car+cdr pair
     Return two values, the CAR and the CDR of PAIR.

 -- Scheme Procedure: take lst i
 -- Scheme Procedure: take! lst i
     Return a list containing the first I elements of LST.

     `take!' may modify the structure of the argument list LST in order
     to produce the result.

 -- Scheme Procedure: drop lst i
     Return a list containing all but the first I elements of LST.

 -- Scheme Procedure: take-right lst i
     Return a list containing the I last elements of LST.  The return
     shares a common tail with LST.

 -- Scheme Procedure: drop-right lst i
 -- Scheme Procedure: drop-right! lst i
     Return a list containing all but the I last elements of LST.

     `drop-right' always returns a new list, even when I is zero.
     `drop-right!' may modify the structure of the argument list LST in
     order to produce the result.

 -- Scheme Procedure: split-at lst i
 -- Scheme Procedure: split-at! lst i
     Return two values, a list containing the first I elements of the
     list LST and a list containing the remaining elements.

     `split-at!' may modify the structure of the argument list LST in
     order to produce the result.

 -- Scheme Procedure: last lst
     Return the last element of the non-empty, finite list LST.

7.5.3.4 Length, Append, Concatenate, etc.
.........................................

 -- Scheme Procedure: length+ lst
     Return the length of the argument list LST.  When LST is a
     circular list, `#f' is returned.

 -- Scheme Procedure: concatenate list-of-lists
 -- Scheme Procedure: concatenate! list-of-lists
     Construct a list by appending all lists in LIST-OF-LISTS.

     `concatenate!' may modify the structure of the given lists in
     order to produce the result.

     `concatenate' is the same as `(apply append LIST-OF-LISTS)'.  It
     exists because some Scheme implementations have a limit on the
     number of arguments a function takes, which the `apply' might
     exceed.  In Guile there is no such limit.

 -- Scheme Procedure: append-reverse rev-head tail
 -- Scheme Procedure: append-reverse! rev-head tail
     Reverse REV-HEAD, append TAIL to it, and return the result.  This
     is equivalent to `(append (reverse REV-HEAD) TAIL)', but its
     implementation is more efficient.

          (append-reverse '(1 2 3) '(4 5 6)) => (3 2 1 4 5 6)

     `append-reverse!' may modify REV-HEAD in order to produce the
     result.

 -- Scheme Procedure: zip lst1 lst2 ...
     Return a list as long as the shortest of the argument lists, where
     each element is a list.  The first list contains the first
     elements of the argument lists, the second list contains the
     second elements, and so on.

 -- Scheme Procedure: unzip1 lst
 -- Scheme Procedure: unzip2 lst
 -- Scheme Procedure: unzip3 lst
 -- Scheme Procedure: unzip4 lst
 -- Scheme Procedure: unzip5 lst
     `unzip1' takes a list of lists, and returns a list containing the
     first elements of each list, `unzip2' returns two lists, the first
     containing the first elements of each lists and the second
     containing the second elements of each lists, and so on.

 -- Scheme Procedure: count pred lst1 ... lstN
     Return a count of the number of times PRED returns true when
     called on elements from the given lists.

     PRED is called with N parameters `(PRED ELEM1 ... ELEMN)', each
     element being from the corresponding LST1 ... LSTN.  The first
     call is with the first element of each list, the second with the
     second element from each, and so on.

     Counting stops when the end of the shortest list is reached.  At
     least one list must be non-circular.

7.5.3.5 Fold, Unfold & Map
..........................

 -- Scheme Procedure: fold proc init lst1 ... lstN
 -- Scheme Procedure: fold-right proc init lst1 ... lstN
     Apply PROC to the elements of LST1 ... LSTN to build a result, and
     return that result.

     Each PROC call is `(PROC ELEM1 ...  ELEMN PREVIOUS)', where ELEM1
     is from LST1, through ELEMN from LSTN.  PREVIOUS is the return
     from the previous call to PROC, or the given INIT for the first
     call.  If any list is empty, just INIT is returned.

     `fold' works through the list elements from first to last.  The
     following shows a list reversal and the calls it makes,

          (fold cons '() '(1 2 3))

          (cons 1 '())
          (cons 2 '(1))
          (cons 3 '(2 1)
          => (3 2 1)

     `fold-right' works through the list elements from last to first,
     ie. from the right.  So for example the following finds the longest
     string, and the last among equal longest,

          (fold-right (lambda (str prev)
                        (if (> (string-length str) (string-length prev))
                            str
                            prev))
                      ""
                      '("x" "abc" "xyz" "jk"))
          => "xyz"

     If LST1 through LSTN have different lengths, `fold' stops when the
     end of the shortest is reached; `fold-right' commences at the last
     element of the shortest.  Ie. elements past the length of the
     shortest are ignored in the other LSTs.  At least one LST must be
     non-circular.

     `fold' should be preferred over `fold-right' if the order of
     processing doesn't matter, or can be arranged either way, since
     `fold' is a little more efficient.

     The way `fold' builds a result from iterating is quite general, it
     can do more than other iterations like say `map' or `filter'.  The
     following for example removes adjacent duplicate elements from a
     list,

          (define (delete-adjacent-duplicates lst)
            (fold-right (lambda (elem ret)
                          (if (equal? elem (first ret))
                              ret
                              (cons elem ret)))
                        (list (last lst))
                        lst))
          (delete-adjacent-duplicates '(1 2 3 3 4 4 4 5))
          => (1 2 3 4 5)

     Clearly the same sort of thing can be done with a `for-each' and a
     variable in which to build the result, but a self-contained PROC
     can be re-used in multiple contexts, where a `for-each' would have
     to be written out each time.

 -- Scheme Procedure: pair-fold proc init lst1 ... lstN
 -- Scheme Procedure: pair-fold-right proc init lst1 ... lstN
     The same as `fold' and `fold-right', but apply PROC to the pairs
     of the lists instead of the list elements.

 -- Scheme Procedure: reduce proc default lst
 -- Scheme Procedure: reduce-right proc default lst
     `reduce' is a variant of `fold', where the first call to PROC is
     on two elements from LST, rather than one element and a given
     initial value.

     If LST is empty, `reduce' returns DEFAULT (this is the only use
     for DEFAULT).  If LST has just one element then that's the return
     value.  Otherwise PROC is called on the elements of LST.

     Each PROC call is `(PROC ELEM PREVIOUS)', where ELEM is from LST
     (the second and subsequent elements of LST), and PREVIOUS is the
     return from the previous call to PROC.  The first element of LST
     is the PREVIOUS for the first call to PROC.

     For example, the following adds a list of numbers, the calls made
     to `+' are shown.  (Of course `+' accepts multiple arguments and
     can add a list directly, with `apply'.)

          (reduce + 0 '(5 6 7)) => 18

          (+ 6 5)  => 11
          (+ 7 11) => 18

     `reduce' can be used instead of `fold' where the INIT value is an
     "identity", meaning a value which under PROC doesn't change the
     result, in this case 0 is an identity since `(+ 5 0)' is just 5.
     `reduce' avoids that unnecessary call.

     `reduce-right' is a similar variation on `fold-right', working
     from the end (ie. the right) of LST.  The last element of LST is
     the PREVIOUS for the first call to PROC, and the ELEM values go
     from the second last.

     `reduce' should be preferred over `reduce-right' if the order of
     processing doesn't matter, or can be arranged either way, since
     `reduce' is a little more efficient.

 -- Scheme Procedure: unfold p f g seed [tail-gen]
     `unfold' is defined as follows:

          (unfold p f g seed) =
             (if (p seed) (tail-gen seed)
                 (cons (f seed)
                       (unfold p f g (g seed))))

    P
          Determines when to stop unfolding.

    F
          Maps each seed value to the corresponding list element.

    G
          Maps each seed value to next seed value.

    SEED
          The state value for the unfold.

    TAIL-GEN
          Creates the tail of the list; defaults to `(lambda (x) '())'.

     G produces a series of seed values, which are mapped to list
     elements by F.  These elements are put into a list in
     left-to-right order, and P tells when to stop unfolding.

 -- Scheme Procedure: unfold-right p f g seed [tail]
     Construct a list with the following loop.

          (let lp ((seed seed) (lis tail))
             (if (p seed) lis
                 (lp (g seed)
                     (cons (f seed) lis))))

    P
          Determines when to stop unfolding.

    F
          Maps each seed value to the corresponding list element.

    G
          Maps each seed value to next seed value.

    SEED
          The state value for the unfold.

    TAIL-GEN
          Creates the tail of the list; defaults to `(lambda (x) '())'.


 -- Scheme Procedure: map f lst1 lst2 ...
     Map the procedure over the list(s) LST1, LST2, ... and return a
     list containing the results of the procedure applications.  This
     procedure is extended with respect to R5RS, because the argument
     lists may have different lengths.  The result list will have the
     same length as the shortest argument lists.  The order in which F
     will be applied to the list element(s) is not specified.

 -- Scheme Procedure: for-each f lst1 lst2 ...
     Apply the procedure F to each pair of corresponding elements of
     the list(s) LST1, LST2, ....  The return value is not specified.
     This procedure is extended with respect to R5RS, because the
     argument lists may have different lengths.  The shortest argument
     list determines the number of times F is called.  F will be
     applied to the list elements in left-to-right order.


 -- Scheme Procedure: append-map f lst1 lst2 ...
 -- Scheme Procedure: append-map! f lst1 lst2 ...
     Equivalent to

          (apply append (map f clist1 clist2 ...))

     and

          (apply append! (map f clist1 clist2 ...))

     Map F over the elements of the lists, just as in the `map'
     function. However, the results of the applications are appended
     together to make the final result. `append-map' uses `append' to
     append the results together; `append-map!' uses `append!'.

     The dynamic order in which the various applications of F are made
     is not specified.

 -- Scheme Procedure: map! f lst1 lst2 ...
     Linear-update variant of `map' - `map!' is allowed, but not
     required, to alter the cons cells of LST1 to construct the result
     list.

     The dynamic order in which the various applications of F are made
     is not specified. In the n-ary case, LST2, LST3, ... must have at
     least as many elements as LST1.

 -- Scheme Procedure: pair-for-each f lst1 lst2 ...
     Like `for-each', but applies the procedure F to the pairs from
     which the argument lists are constructed, instead of the list
     elements.  The return value is not specified.

 -- Scheme Procedure: filter-map f lst1 lst2 ...
     Like `map', but only results from the applications of F which are
     true are saved in the result list.

7.5.3.6 Filtering and Partitioning
..................................

Filtering means to collect all elements from a list which satisfy a
specific condition.  Partitioning a list means to make two groups of
list elements, one which contains the elements satisfying a condition,
and the other for the elements which don't.

   The `filter' and `filter!' functions are implemented in the Guile
core, *Note List Modification::.

 -- Scheme Procedure: partition pred lst
 -- Scheme Procedure: partition! pred lst
     Split LST into those elements which do and don't satisfy the
     predicate PRED.

     The return is two values (*note Multiple Values::), the first
     being a list of all elements from LST which satisfy PRED, the
     second a list of those which do not.

     The elements in the result lists are in the same order as in LST
     but the order in which the calls `(PRED elem)' are made on the
     list elements is unspecified.

     `partition' does not change LST, but one of the returned lists may
     share a tail with it.  `partition!' may modify LST to construct
     its return.

 -- Scheme Procedure: remove pred lst
 -- Scheme Procedure: remove! pred lst
     Return a list containing all elements from LST which do not
     satisfy the predicate PRED.  The elements in the result list have
     the same order as in LST.  The order in which PRED is applied to
     the list elements is not specified.

     `remove!' is allowed, but not required to modify the structure of
     the input list.

7.5.3.7 Searching
.................

The procedures for searching elements in lists either accept a
predicate or a comparison object for determining which elements are to
be searched.

 -- Scheme Procedure: find pred lst
     Return the first element of LST which satisfies the predicate PRED
     and `#f' if no such element is found.

 -- Scheme Procedure: find-tail pred lst
     Return the first pair of LST whose CAR satisfies the predicate
     PRED and `#f' if no such element is found.

 -- Scheme Procedure: take-while pred lst
 -- Scheme Procedure: take-while! pred lst
     Return the longest initial prefix of LST whose elements all
     satisfy the predicate PRED.

     `take-while!' is allowed, but not required to modify the input
     list while producing the result.

 -- Scheme Procedure: drop-while pred lst
     Drop the longest initial prefix of LST whose elements all satisfy
     the predicate PRED.

 -- Scheme Procedure: span pred lst
 -- Scheme Procedure: span! pred lst
 -- Scheme Procedure: break pred lst
 -- Scheme Procedure: break! pred lst
     `span' splits the list LST into the longest initial prefix whose
     elements all satisfy the predicate PRED, and the remaining tail.
     `break' inverts the sense of the predicate.

     `span!' and `break!' are allowed, but not required to modify the
     structure of the input list LST in order to produce the result.

     Note that the name `break' conflicts with the `break' binding
     established by `while' (*note while do::).  Applications wanting
     to use `break' from within a `while' loop will need to make a new
     define under a different name.

 -- Scheme Procedure: any pred lst1 lst2 ... lstN
     Test whether any set of elements from LST1 ... lstN satisfies
     PRED.  If so the return value is the return from the successful
     PRED call, or if not the return is `#f'.

     Each PRED call is `(PRED ELEM1 ...  ELEMN)' taking an element from
     each LST.  The calls are made successively for the first, second,
     etc elements of the lists, stopping when PRED returns non-`#f', or
     when the end of the shortest list is reached.

     The PRED call on the last set of elements (ie. when the end of the
     shortest list has been reached), if that point is reached, is a
     tail call.

 -- Scheme Procedure: every pred lst1 lst2 ... lstN
     Test whether every set of elements from LST1 ... lstN satisfies
     PRED.  If so the return value is the return from the final PRED
     call, or if not the return is `#f'.

     Each PRED call is `(PRED ELEM1 ...  ELEMN)' taking an element from
     each LST.  The calls are made successively for the first, second,
     etc elements of the lists, stopping if PRED returns `#f', or when
     the end of any of the lists is reached.

     The PRED call on the last set of elements (ie. when the end of the
     shortest list has been reached) is a tail call.

     If one of LST1 ... LSTN is empty then no calls to PRED are made,
     and the return is `#t'.

 -- Scheme Procedure: list-index pred lst1 ... lstN
     Return the index of the first set of elements, one from each of
     LST1...LSTN, which satisfies PRED.

     PRED is called as `(PRED elem1 ... elemN)'.  Searching stops when
     the end of the shortest LST is reached.  The return index starts
     from 0 for the first set of elements.  If no set of elements pass
     then the return is `#f'.

          (list-index odd? '(2 4 6 9))      => 3
          (list-index = '(1 2 3) '(3 1 2))  => #f

 -- Scheme Procedure: member x lst [=]
     Return the first sublist of LST whose CAR is equal to X.  If X
     does not appear in LST, return `#f'.

     Equality is determined by `equal?', or by the equality predicate =
     if given.  = is called `(= X elem)', ie. with the given X first,
     so for example to find the first element greater than 5,

          (member 5 '(3 5 1 7 2 9) <) => (7 2 9)

     This version of `member' extends the core `member' (*note List
     Searching::) by accepting an equality predicate.

7.5.3.8 Deleting
................

 -- Scheme Procedure: delete x lst [=]
 -- Scheme Procedure: delete! x lst [=]
     Return a list containing the elements of LST but with those equal
     to X deleted.  The returned elements will be in the same order as
     they were in LST.

     Equality is determined by the = predicate, or `equal?' if not
     given.  An equality call is made just once for each element, but
     the order in which the calls are made on the elements is
     unspecified.

     The equality calls are always `(= x elem)', ie. the given X is
     first.  This means for instance elements greater than 5 can be
     deleted with `(delete 5 lst <)'.

     `delete' does not modify LST, but the return might share a common
     tail with LST.  `delete!' may modify the structure of LST to
     construct its return.

     These functions extend the core `delete' and `delete!' (*note List
     Modification::) in accepting an equality predicate.  See also
     `lset-difference' (*note SRFI-1 Set Operations::) for deleting
     multiple elements from a list.

 -- Scheme Procedure: delete-duplicates lst [=]
 -- Scheme Procedure: delete-duplicates! lst [=]
     Return a list containing the elements of LST but without
     duplicates.

     When elements are equal, only the first in LST is retained.  Equal
     elements can be anywhere in LST, they don't have to be adjacent.
     The returned list will have the retained elements in the same
     order as they were in LST.

     Equality is determined by the = predicate, or `equal?' if not
     given.  Calls `(= x y)' are made with element X being before Y in
     LST.  A call is made at most once for each combination, but the
     sequence of the calls across the elements is unspecified.

     `delete-duplicates' does not modify LST, but the return might
     share a common tail with LST.  `delete-duplicates!' may modify the
     structure of LST to construct its return.

     In the worst case, this is an O(N^2) algorithm because it must
     check each element against all those preceding it.  For long lists
     it is more efficient to sort and then compare only adjacent
     elements.

7.5.3.9 Association Lists
.........................

Association lists are described in detail in section *note Association
Lists::.  The present section only documents the additional procedures
for dealing with association lists defined by SRFI-1.

 -- Scheme Procedure: assoc key alist [=]
     Return the pair from ALIST which matches KEY.  This extends the
     core `assoc' (*note Retrieving Alist Entries::) by taking an
     optional = comparison procedure.

     The default comparison is `equal?'.  If an = parameter is given
     it's called `(= KEY ALISTCAR)', i.e. the given target KEY is the
     first argument, and a `car' from ALIST is second.

     For example a case-insensitive string lookup,

          (assoc "yy" '(("XX" . 1) ("YY" . 2)) string-ci=?)
          => ("YY" . 2)

 -- Scheme Procedure: alist-cons key datum alist
     Cons a new association KEY and DATUM onto ALIST and return the
     result.  This is equivalent to

          (cons (cons KEY DATUM) ALIST)

     `acons' (*note Adding or Setting Alist Entries::) in the Guile
     core does the same thing.

 -- Scheme Procedure: alist-copy alist
     Return a newly allocated copy of ALIST, that means that the spine
     of the list as well as the pairs are copied.

 -- Scheme Procedure: alist-delete key alist [=]
 -- Scheme Procedure: alist-delete! key alist [=]
     Return a list containing the elements of ALIST but with those
     elements whose keys are equal to KEY deleted.  The returned
     elements will be in the same order as they were in ALIST.

     Equality is determined by the = predicate, or `equal?' if not
     given.  The order in which elements are tested is unspecified, but
     each equality call is made `(= key alistkey)', i.e. the given KEY
     parameter is first and the key from ALIST second.  This means for
     instance all associations with a key greater than 5 can be removed
     with `(alist-delete 5 alist <)'.

     `alist-delete' does not modify ALIST, but the return might share a
     common tail with ALIST.  `alist-delete!' may modify the list
     structure of ALIST to construct its return.

7.5.3.10 Set Operations on Lists
................................

Lists can be used to represent sets of objects.  The procedures in this
section operate on such lists as sets.

   Note that lists are not an efficient way to implement large sets.
The procedures here typically take time MxN when operating on M and N
element lists.  Other data structures like trees, bitsets (*note Bit
Vectors::) or hash tables (*note Hash Tables::) are faster.

   All these procedures take an equality predicate as the first
argument.  This predicate is used for testing the objects in the list
sets for sameness.  This predicate must be consistent with `eq?' (*note
Equality::) in the sense that if two list elements are `eq?' then they
must also be equal under the predicate.  This simply means a given
object must be equal to itself.

 -- Scheme Procedure: lset<= = list1 list2 ...
     Return `#t' if each list is a subset of the one following it.  Ie.
     LIST1 a subset of LIST2, LIST2 a subset of LIST3, etc, for as many
     lists as given.  If only one list or no lists are given then the
     return is `#t'.

     A list X is a subset of Y if each element of X is equal to some
     element in Y.  Elements are compared using the given = procedure,
     called as `(= xelem yelem)'.

          (lset<= eq?)                      => #t
          (lset<= eqv? '(1 2 3) '(1))       => #f
          (lset<= eqv? '(1 3 2) '(4 3 1 2)) => #t

 -- Scheme Procedure: lset= = list1 list2 ...
     Return `#t' if all argument lists are set-equal.  LIST1 is
     compared to LIST2, LIST2 to LIST3, etc, for as many lists as
     given.  If only one list or no lists are given then the return is
     `#t'.

     Two lists X and Y are set-equal if each element of X is equal to
     some element of Y and conversely each element of Y is equal to
     some element of X.  The order of the elements in the lists doesn't
     matter.  Element equality is determined with the given =
     procedure, called as `(= xelem yelem)', but exactly which calls
     are made is unspecified.

          (lset= eq?)                      => #t
          (lset= eqv? '(1 2 3) '(3 2 1))   => #t
          (lset= string-ci=? '("a" "A" "b") '("B" "b" "a")) => #t

 -- Scheme Procedure: lset-adjoin = list elem1 ...
     Add to LIST any of the given ELEMs not already in the list.  ELEMs
     are `cons'ed onto the start of LIST (so the return shares a common
     tail with LIST), but the order they're added is unspecified.

     The given = procedure is used for comparing elements, called as
     `(= listelem elem)', ie. the second argument is one of the given
     ELEM parameters.

          (lset-adjoin eqv? '(1 2 3) 4 1 5) => (5 4 1 2 3)

 -- Scheme Procedure: lset-union = list1 list2 ...
 -- Scheme Procedure: lset-union! = list1 list2 ...
     Return the union of the argument list sets.  The result is built by
     taking the union of LIST1 and LIST2, then the union of that with
     LIST3, etc, for as many lists as given.  For one list argument
     that list itself is the result, for no list arguments the result
     is the empty list.

     The union of two lists X and Y is formed as follows.  If X is
     empty then the result is Y.  Otherwise start with X as the result
     and consider each Y element (from first to last).  A Y element not
     equal to something already in the result is `cons'ed onto the
     result.

     The given = procedure is used for comparing elements, called as
     `(= relem yelem)'.  The first argument is from the result
     accumulated so far, and the second is from the list being union-ed
     in.  But exactly which calls are made is otherwise unspecified.

     Notice that duplicate elements in LIST1 (or the first non-empty
     list) are preserved, but that repeated elements in subsequent lists
     are only added once.

          (lset-union eqv?)                          => ()
          (lset-union eqv? '(1 2 3))                 => (1 2 3)
          (lset-union eqv? '(1 2 1 3) '(2 4 5) '(5)) => (5 4 1 2 1 3)

     `lset-union' doesn't change the given lists but the result may
     share a tail with the first non-empty list.  `lset-union!' can
     modify all of the given lists to form the result.

 -- Scheme Procedure: lset-intersection = list1 list2 ...
 -- Scheme Procedure: lset-intersection! = list1 list2 ...
     Return the intersection of LIST1 with the other argument lists,
     meaning those elements of LIST1 which are also in all of LIST2
     etc.  For one list argument, just that list is returned.

     The test for an element of LIST1 to be in the return is simply
     that it's equal to some element in each of LIST2 etc.  Notice this
     means an element appearing twice in LIST1 but only once in each of
     LIST2 etc will go into the return twice.  The return has its
     elements in the same order as they were in LIST1.

     The given = procedure is used for comparing elements, called as
     `(= elem1 elemN)'.  The first argument is from LIST1 and the
     second is from one of the subsequent lists.  But exactly which
     calls are made and in what order is unspecified.

          (lset-intersection eqv? '(x y))                        => (x y)
          (lset-intersection eqv? '(1 2 3) '(4 3 2))             => (2 3)
          (lset-intersection eqv? '(1 1 2 2) '(1 2) '(2 1) '(2)) => (2 2)

     The return from `lset-intersection' may share a tail with LIST1.
     `lset-intersection!' may modify LIST1 to form its result.

 -- Scheme Procedure: lset-difference = list1 list2 ...
 -- Scheme Procedure: lset-difference! = list1 list2 ...
     Return LIST1 with any elements in LIST2, LIST3 etc removed (ie.
     subtracted).  For one list argument, just that list is returned.

     The given = procedure is used for comparing elements, called as
     `(= elem1 elemN)'.  The first argument is from LIST1 and the
     second from one of the subsequent lists.  But exactly which calls
     are made and in what order is unspecified.

          (lset-difference eqv? '(x y))             => (x y)
          (lset-difference eqv? '(1 2 3) '(3 1))    => (2)
          (lset-difference eqv? '(1 2 3) '(3) '(2)) => (1)

     The return from `lset-difference' may share a tail with LIST1.
     `lset-difference!' may modify LIST1 to form its result.

 -- Scheme Procedure: lset-diff+intersection = list1 list2 ...
 -- Scheme Procedure: lset-diff+intersection! = list1 list2 ...
     Return two values (*note Multiple Values::), the difference and
     intersection of the argument lists as per `lset-difference' and
     `lset-intersection' above.

     For two list arguments this partitions LIST1 into those elements
     of LIST1 which are in LIST2 and not in LIST2.  (But for more than
     two arguments there can be elements of LIST1 which are neither
     part of the difference nor the intersection.)

     One of the return values from `lset-diff+intersection' may share a
     tail with LIST1.  `lset-diff+intersection!' may modify LIST1 to
     form its results.

 -- Scheme Procedure: lset-xor = list1 list2 ...
 -- Scheme Procedure: lset-xor! = list1 list2 ...
     Return an XOR of the argument lists.  For two lists this means
     those elements which are in exactly one of the lists.  For more
     than two lists it means those elements which appear in an odd
     number of the lists.

     To be precise, the XOR of two lists X and Y is formed by taking
     those elements of X not equal to any element of Y, plus those
     elements of Y not equal to any element of X.  Equality is
     determined with the given = procedure, called as `(= e1 e2)'.  One
     argument is from X and the other from Y, but which way around is
     unspecified.  Exactly which calls are made is also unspecified, as
     is the order of the elements in the result.

          (lset-xor eqv? '(x y))             => (x y)
          (lset-xor eqv? '(1 2 3) '(4 3 2))  => (4 1)

     The return from `lset-xor' may share a tail with one of the list
     arguments.  `lset-xor!' may modify LIST1 to form its result.

7.5.4 SRFI-2 - and-let*
-----------------------

The following syntax can be obtained with

     (use-modules (srfi srfi-2))

 -- library syntax: and-let* (clause ...) body ...
     A combination of `and' and `let*'.

     Each CLAUSE is evaluated in turn, and if `#f' is obtained then
     evaluation stops and `#f' is returned.  If all are non-`#f' then
     BODY is evaluated and the last form gives the return value, or if
     BODY is empty then the result is `#t'.  Each CLAUSE should be one
     of the following,

    `(symbol expr)'
          Evaluate EXPR, check for `#f', and bind it to SYMBOL.  Like
          `let*', that binding is available to subsequent clauses.

    `(expr)'
          Evaluate EXPR and check for `#f'.

    `symbol'
          Get the value bound to SYMBOL and check for `#f'.

     Notice that `(expr)' has an "extra" pair of parentheses, for
     instance `((eq? x y))'.  One way to remember this is to imagine
     the `symbol' in `(symbol expr)' is omitted.

     `and-let*' is good for calculations where a `#f' value means
     termination, but where a non-`#f' value is going to be needed in
     subsequent expressions.

     The following illustrates this, it returns text between brackets
     `[...]' in a string, or `#f' if there are no such brackets (ie.
     either `string-index' gives `#f').

          (define (extract-brackets str)
            (and-let* ((start (string-index str #\[))
                       (end   (string-index str #\] start)))
              (substring str (1+ start) end)))

     The following shows plain variables and expressions tested too.
     `diagnostic-levels' is taken to be an alist associating a
     diagnostic type with a level.  `str' is printed only if the type
     is known and its level is high enough.

          (define (show-diagnostic type str)
            (and-let* (want-diagnostics
                       (level (assq-ref diagnostic-levels type))
                       ((>= level current-diagnostic-level)))
              (display str)))

     The advantage of `and-let*' is that an extended sequence of
     expressions and tests doesn't require lots of nesting as would
     arise from separate `and' and `let*', or from `cond' with `=>'.


7.5.5 SRFI-4 - Homogeneous numeric vector datatypes
---------------------------------------------------

SRFI-4 provides an interface to uniform numeric vectors: vectors whose
elements are all of a single numeric type. Guile offers uniform numeric
vectors for signed and unsigned 8-bit, 16-bit, 32-bit, and 64-bit
integers, two sizes of floating point values, and, as an extension to
SRFI-4, complex floating-point numbers of these two sizes.

   The standard SRFI-4 procedures and data types may be included via
loading the appropriate module:

     (use-modules (srfi srfi-4))

   This module is currently a part of the default Guile environment,
but it is a good practice to explicitly import the module. In the
future, using SRFI-4 procedures without importing the SRFI-4 module
will cause a deprecation message to be printed. (Of course, one may
call the C functions at any time. Would that C had modules!)

7.5.5.1 SRFI-4 - Overview
.........................

Uniform numeric vectors can be useful since they consume less memory
than the non-uniform, general vectors.  Also, since the types they can
store correspond directly to C types, it is easier to work with them
efficiently on a low level.  Consider image processing as an example,
where you want to apply a filter to some image.  While you could store
the pixels of an image in a general vector and write a general
convolution function, things are much more efficient with uniform
vectors: the convolution function knows that all pixels are unsigned
8-bit values (say), and can use a very tight inner loop.

   This is implemented in Scheme by having the compiler notice calls to
the SRFI-4 accessors, and inline them to appropriate compiled code.
From C you have access to the raw array; functions for efficiently
working with uniform numeric vectors from C are listed at the end of
this section.

   Uniform numeric vectors are the special case of one dimensional
uniform numeric arrays.

   There are 12 standard kinds of uniform numeric vectors, and they all
have their own complement of constructors, accessors, and so on.
Procedures that operate on a specific kind of uniform numeric vector
have a "tag" in their name, indicating the element type.

u8
     unsigned 8-bit integers

s8
     signed 8-bit integers

u16
     unsigned 16-bit integers

s16
     signed 16-bit integers

u32
     unsigned 32-bit integers

s32
     signed 32-bit integers

u64
     unsigned 64-bit integers

s64
     signed 64-bit integers

f32
     the C type `float'

f64
     the C type `double'


   In addition, Guile supports uniform arrays of complex numbers, with
the nonstandard tags:

c32
     complex numbers in rectangular form with the real and imaginary
     part being a `float'

c64
     complex numbers in rectangular form with the real and imaginary
     part being a `double'


   The external representation (ie. read syntax) for these vectors is
similar to normal Scheme vectors, but with an additional tag from the
tables above indicating the vector's type.  For example,

     #u16(1 2 3)
     #f64(3.1415 2.71)

   Note that the read syntax for floating-point here conflicts with
`#f' for false.  In Standard Scheme one can write `(1 #f3)' for a three
element list `(1 #f 3)', but for Guile `(1 #f3)' is invalid.  `(1 #f
3)' is almost certainly what one should write anyway to make the
intention clear, so this is rarely a problem.

7.5.5.2 SRFI-4 - API
....................

Note that the c32 and c64 functions are only available from (srfi
srfi-4 gnu).

 -- Scheme Procedure: u8vector? obj
 -- Scheme Procedure: s8vector? obj
 -- Scheme Procedure: u16vector? obj
 -- Scheme Procedure: s16vector? obj
 -- Scheme Procedure: u32vector? obj
 -- Scheme Procedure: s32vector? obj
 -- Scheme Procedure: u64vector? obj
 -- Scheme Procedure: s64vector? obj
 -- Scheme Procedure: f32vector? obj
 -- Scheme Procedure: f64vector? obj
 -- Scheme Procedure: c32vector? obj
 -- Scheme Procedure: c64vector? obj
 -- C Function: scm_u8vector_p (obj)
 -- C Function: scm_s8vector_p (obj)
 -- C Function: scm_u16vector_p (obj)
 -- C Function: scm_s16vector_p (obj)
 -- C Function: scm_u32vector_p (obj)
 -- C Function: scm_s32vector_p (obj)
 -- C Function: scm_u64vector_p (obj)
 -- C Function: scm_s64vector_p (obj)
 -- C Function: scm_f32vector_p (obj)
 -- C Function: scm_f64vector_p (obj)
 -- C Function: scm_c32vector_p (obj)
 -- C Function: scm_c64vector_p (obj)
     Return `#t' if OBJ is a homogeneous numeric vector of the
     indicated type.

 -- Scheme Procedure: make-u8vector n [value]
 -- Scheme Procedure: make-s8vector n [value]
 -- Scheme Procedure: make-u16vector n [value]
 -- Scheme Procedure: make-s16vector n [value]
 -- Scheme Procedure: make-u32vector n [value]
 -- Scheme Procedure: make-s32vector n [value]
 -- Scheme Procedure: make-u64vector n [value]
 -- Scheme Procedure: make-s64vector n [value]
 -- Scheme Procedure: make-f32vector n [value]
 -- Scheme Procedure: make-f64vector n [value]
 -- Scheme Procedure: make-c32vector n [value]
 -- Scheme Procedure: make-c64vector n [value]
 -- C Function: scm_make_u8vector n [value]
 -- C Function: scm_make_s8vector n [value]
 -- C Function: scm_make_u16vector n [value]
 -- C Function: scm_make_s16vector n [value]
 -- C Function: scm_make_u32vector n [value]
 -- C Function: scm_make_s32vector n [value]
 -- C Function: scm_make_u64vector n [value]
 -- C Function: scm_make_s64vector n [value]
 -- C Function: scm_make_f32vector n [value]
 -- C Function: scm_make_f64vector n [value]
 -- C Function: scm_make_c32vector n [value]
 -- C Function: scm_make_c64vector n [value]
     Return a newly allocated homogeneous numeric vector holding N
     elements of the indicated type.  If VALUE is given, the vector is
     initialized with that value, otherwise the contents are
     unspecified.

 -- Scheme Procedure: u8vector value ...
 -- Scheme Procedure: s8vector value ...
 -- Scheme Procedure: u16vector value ...
 -- Scheme Procedure: s16vector value ...
 -- Scheme Procedure: u32vector value ...
 -- Scheme Procedure: s32vector value ...
 -- Scheme Procedure: u64vector value ...
 -- Scheme Procedure: s64vector value ...
 -- Scheme Procedure: f32vector value ...
 -- Scheme Procedure: f64vector value ...
 -- Scheme Procedure: c32vector value ...
 -- Scheme Procedure: c64vector value ...
 -- C Function: scm_u8vector (values)
 -- C Function: scm_s8vector (values)
 -- C Function: scm_u16vector (values)
 -- C Function: scm_s16vector (values)
 -- C Function: scm_u32vector (values)
 -- C Function: scm_s32vector (values)
 -- C Function: scm_u64vector (values)
 -- C Function: scm_s64vector (values)
 -- C Function: scm_f32vector (values)
 -- C Function: scm_f64vector (values)
 -- C Function: scm_c32vector (values)
 -- C Function: scm_c64vector (values)
     Return a newly allocated homogeneous numeric vector of the
     indicated type, holding the given parameter VALUEs.  The vector
     length is the number of parameters given.

 -- Scheme Procedure: u8vector-length vec
 -- Scheme Procedure: s8vector-length vec
 -- Scheme Procedure: u16vector-length vec
 -- Scheme Procedure: s16vector-length vec
 -- Scheme Procedure: u32vector-length vec
 -- Scheme Procedure: s32vector-length vec
 -- Scheme Procedure: u64vector-length vec
 -- Scheme Procedure: s64vector-length vec
 -- Scheme Procedure: f32vector-length vec
 -- Scheme Procedure: f64vector-length vec
 -- Scheme Procedure: c32vector-length vec
 -- Scheme Procedure: c64vector-length vec
 -- C Function: scm_u8vector_length (vec)
 -- C Function: scm_s8vector_length (vec)
 -- C Function: scm_u16vector_length (vec)
 -- C Function: scm_s16vector_length (vec)
 -- C Function: scm_u32vector_length (vec)
 -- C Function: scm_s32vector_length (vec)
 -- C Function: scm_u64vector_length (vec)
 -- C Function: scm_s64vector_length (vec)
 -- C Function: scm_f32vector_length (vec)
 -- C Function: scm_f64vector_length (vec)
 -- C Function: scm_c32vector_length (vec)
 -- C Function: scm_c64vector_length (vec)
     Return the number of elements in VEC.

 -- Scheme Procedure: u8vector-ref vec i
 -- Scheme Procedure: s8vector-ref vec i
 -- Scheme Procedure: u16vector-ref vec i
 -- Scheme Procedure: s16vector-ref vec i
 -- Scheme Procedure: u32vector-ref vec i
 -- Scheme Procedure: s32vector-ref vec i
 -- Scheme Procedure: u64vector-ref vec i
 -- Scheme Procedure: s64vector-ref vec i
 -- Scheme Procedure: f32vector-ref vec i
 -- Scheme Procedure: f64vector-ref vec i
 -- Scheme Procedure: c32vector-ref vec i
 -- Scheme Procedure: c64vector-ref vec i
 -- C Function: scm_u8vector_ref (vec i)
 -- C Function: scm_s8vector_ref (vec i)
 -- C Function: scm_u16vector_ref (vec i)
 -- C Function: scm_s16vector_ref (vec i)
 -- C Function: scm_u32vector_ref (vec i)
 -- C Function: scm_s32vector_ref (vec i)
 -- C Function: scm_u64vector_ref (vec i)
 -- C Function: scm_s64vector_ref (vec i)
 -- C Function: scm_f32vector_ref (vec i)
 -- C Function: scm_f64vector_ref (vec i)
 -- C Function: scm_c32vector_ref (vec i)
 -- C Function: scm_c64vector_ref (vec i)
     Return the element at index I in VEC.  The first element in VEC is
     index 0.

 -- Scheme Procedure: u8vector-set! vec i value
 -- Scheme Procedure: s8vector-set! vec i value
 -- Scheme Procedure: u16vector-set! vec i value
 -- Scheme Procedure: s16vector-set! vec i value
 -- Scheme Procedure: u32vector-set! vec i value
 -- Scheme Procedure: s32vector-set! vec i value
 -- Scheme Procedure: u64vector-set! vec i value
 -- Scheme Procedure: s64vector-set! vec i value
 -- Scheme Procedure: f32vector-set! vec i value
 -- Scheme Procedure: f64vector-set! vec i value
 -- Scheme Procedure: c32vector-set! vec i value
 -- Scheme Procedure: c64vector-set! vec i value
 -- C Function: scm_u8vector_set_x (vec i value)
 -- C Function: scm_s8vector_set_x (vec i value)
 -- C Function: scm_u16vector_set_x (vec i value)
 -- C Function: scm_s16vector_set_x (vec i value)
 -- C Function: scm_u32vector_set_x (vec i value)
 -- C Function: scm_s32vector_set_x (vec i value)
 -- C Function: scm_u64vector_set_x (vec i value)
 -- C Function: scm_s64vector_set_x (vec i value)
 -- C Function: scm_f32vector_set_x (vec i value)
 -- C Function: scm_f64vector_set_x (vec i value)
 -- C Function: scm_c32vector_set_x (vec i value)
 -- C Function: scm_c64vector_set_x (vec i value)
     Set the element at index I in VEC to VALUE.  The first element in
     VEC is index 0.  The return value is unspecified.

 -- Scheme Procedure: u8vector->list vec
 -- Scheme Procedure: s8vector->list vec
 -- Scheme Procedure: u16vector->list vec
 -- Scheme Procedure: s16vector->list vec
 -- Scheme Procedure: u32vector->list vec
 -- Scheme Procedure: s32vector->list vec
 -- Scheme Procedure: u64vector->list vec
 -- Scheme Procedure: s64vector->list vec
 -- Scheme Procedure: f32vector->list vec
 -- Scheme Procedure: f64vector->list vec
 -- Scheme Procedure: c32vector->list vec
 -- Scheme Procedure: c64vector->list vec
 -- C Function: scm_u8vector_to_list (vec)
 -- C Function: scm_s8vector_to_list (vec)
 -- C Function: scm_u16vector_to_list (vec)
 -- C Function: scm_s16vector_to_list (vec)
 -- C Function: scm_u32vector_to_list (vec)
 -- C Function: scm_s32vector_to_list (vec)
 -- C Function: scm_u64vector_to_list (vec)
 -- C Function: scm_s64vector_to_list (vec)
 -- C Function: scm_f32vector_to_list (vec)
 -- C Function: scm_f64vector_to_list (vec)
 -- C Function: scm_c32vector_to_list (vec)
 -- C Function: scm_c64vector_to_list (vec)
     Return a newly allocated list holding all elements of VEC.

 -- Scheme Procedure: list->u8vector lst
 -- Scheme Procedure: list->s8vector lst
 -- Scheme Procedure: list->u16vector lst
 -- Scheme Procedure: list->s16vector lst
 -- Scheme Procedure: list->u32vector lst
 -- Scheme Procedure: list->s32vector lst
 -- Scheme Procedure: list->u64vector lst
 -- Scheme Procedure: list->s64vector lst
 -- Scheme Procedure: list->f32vector lst
 -- Scheme Procedure: list->f64vector lst
 -- Scheme Procedure: list->c32vector lst
 -- Scheme Procedure: list->c64vector lst
 -- C Function: scm_list_to_u8vector (lst)
 -- C Function: scm_list_to_s8vector (lst)
 -- C Function: scm_list_to_u16vector (lst)
 -- C Function: scm_list_to_s16vector (lst)
 -- C Function: scm_list_to_u32vector (lst)
 -- C Function: scm_list_to_s32vector (lst)
 -- C Function: scm_list_to_u64vector (lst)
 -- C Function: scm_list_to_s64vector (lst)
 -- C Function: scm_list_to_f32vector (lst)
 -- C Function: scm_list_to_f64vector (lst)
 -- C Function: scm_list_to_c32vector (lst)
 -- C Function: scm_list_to_c64vector (lst)
     Return a newly allocated homogeneous numeric vector of the
     indicated type, initialized with the elements of the list LST.

 -- C Function: SCM scm_take_u8vector (const scm_t_uint8 *data, size_t
          len)
 -- C Function: SCM scm_take_s8vector (const scm_t_int8 *data, size_t
          len)
 -- C Function: SCM scm_take_u16vector (const scm_t_uint16 *data,
          size_t len)
 -- C Function: SCM scm_take_s16vector (const scm_t_int16 *data, size_t
          len)
 -- C Function: SCM scm_take_u32vector (const scm_t_uint32 *data,
          size_t len)
 -- C Function: SCM scm_take_s32vector (const scm_t_int32 *data, size_t
          len)
 -- C Function: SCM scm_take_u64vector (const scm_t_uint64 *data,
          size_t len)
 -- C Function: SCM scm_take_s64vector (const scm_t_int64 *data, size_t
          len)
 -- C Function: SCM scm_take_f32vector (const float *data, size_t len)
 -- C Function: SCM scm_take_f64vector (const double *data, size_t len)
 -- C Function: SCM scm_take_c32vector (const float *data, size_t len)
 -- C Function: SCM scm_take_c64vector (const double *data, size_t len)
     Return a new uniform numeric vector of the indicated type and
     length that uses the memory pointed to by DATA to store its
     elements.  This memory will eventually be freed with `free'.  The
     argument LEN specifies the number of elements in DATA, not its size
     in bytes.

     The `c32' and `c64' variants take a pointer to a C array of
     `float's or `double's.  The real parts of the complex numbers are
     at even indices in that array, the corresponding imaginary parts
     are at the following odd index.

 -- C Function: const scm_t_uint8 * scm_u8vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int8 * scm_s8vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_uint16 * scm_u16vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int16 * scm_s16vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_uint32 * scm_u32vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int32 * scm_s32vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_uint64 * scm_u64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const scm_t_int64 * scm_s64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const float * scm_f32vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const double * scm_f64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const float * scm_c32vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: const double * scm_c64vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_elements' (*note Vector Accessing from C::), but
     returns a pointer to the elements of a uniform numeric vector of
     the indicated kind.

 -- C Function: scm_t_uint8 * scm_u8vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int8 * scm_s8vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_uint16 * scm_u16vector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int16 * scm_s16vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_uint32 * scm_u32vector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int32 * scm_s32vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_uint64 * scm_u64vector_writable_elements (SCM
          vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: scm_t_int64 * scm_s64vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: float * scm_f32vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: double * scm_f64vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: float * scm_c32vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
 -- C Function: double * scm_c64vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_writable_elements' (*note Vector Accessing from
     C::), but returns a pointer to the elements of a uniform numeric
     vector of the indicated kind.

7.5.5.3 SRFI-4 - Generic operations
...................................

Guile also provides procedures that operate on all types of uniform
numeric vectors.  In what is probably a bug, these procedures are
currently available in the default environment as well; however prudent
hackers will make sure to import `(srfi srfi-4 gnu)' before using these.

 -- C Function: int scm_is_uniform_vector (SCM uvec)
     Return non-zero when UVEC is a uniform numeric vector, zero
     otherwise.

 -- C Function: size_t scm_c_uniform_vector_length (SCM uvec)
     Return the number of elements of UVEC as a `size_t'.

 -- Scheme Procedure: uniform-vector? obj
 -- C Function: scm_uniform_vector_p (obj)
     Return `#t' if OBJ is a homogeneous numeric vector of the
     indicated type.

 -- Scheme Procedure: uniform-vector-length vec
 -- C Function: scm_uniform_vector_length (vec)
     Return the number of elements in VEC.

 -- Scheme Procedure: uniform-vector-ref vec i
 -- C Function: scm_uniform_vector_ref (vec i)
     Return the element at index I in VEC.  The first element in VEC is
     index 0.

 -- Scheme Procedure: uniform-vector-set! vec i value
 -- C Function: scm_uniform_vector_set_x (vec i value)
     Set the element at index I in VEC to VALUE.  The first element in
     VEC is index 0.  The return value is unspecified.

 -- Scheme Procedure: uniform-vector->list vec
 -- C Function: scm_uniform_vector_to_list (vec)
     Return a newly allocated list holding all elements of VEC.

 -- C Function: const void * scm_uniform_vector_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_elements' (*note Vector Accessing from C::), but
     returns a pointer to the elements of a uniform numeric vector.

 -- C Function: void * scm_uniform_vector_writable_elements (SCM vec,
          scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
     Like `scm_vector_writable_elements' (*note Vector Accessing from
     C::), but returns a pointer to the elements of a uniform numeric
     vector.

   Unless you really need to the limited generality of these functions,
it is best to use the type-specific functions, or the generalized
vector accessors.

7.5.5.4 SRFI-4 - Relation to bytevectors
........................................

Guile implements SRFI-4 vectors using bytevectors (*note
Bytevectors::). Often when you have a numeric vector, you end up
wanting to write its bytes somewhere, or have access to the underlying
bytes, or read in bytes from somewhere else.  Bytevectors are very good
at this sort of thing. But the SRFI-4 APIs are nicer to use when doing
number-crunching, because they are addressed by element and not by byte.

   So as a compromise, Guile allows all bytevector functions to operate
on numeric vectors. They address the underlying bytes in the native
endianness, as one would expect.

   Following the same reasoning, that it's just bytes underneath, Guile
also allows uniform vectors of a given type to be accessed as if they
were of any type. One can fill a u32vector, and access its elements with
u8vector-ref. One can use f64vector-ref on bytevectors. It's all the
same to Guile.

   In this way, uniform numeric vectors may be written to and read from
input/output ports using the procedures that operate on bytevectors.

   *Note Bytevectors::, for more information.

7.5.5.5 SRFI-4 - Guile extensions
.................................

Guile defines some useful extensions to SRFI-4, which are not available
in the default Guile environment. They may be imported by loading the
extensions module:

     (use-modules (srfi srfi-4 gnu))

 -- Scheme Procedure: any->u8vector obj
 -- Scheme Procedure: any->s8vector obj
 -- Scheme Procedure: any->u16vector obj
 -- Scheme Procedure: any->s16vector obj
 -- Scheme Procedure: any->u32vector obj
 -- Scheme Procedure: any->s32vector obj
 -- Scheme Procedure: any->u64vector obj
 -- Scheme Procedure: any->s64vector obj
 -- Scheme Procedure: any->f32vector obj
 -- Scheme Procedure: any->f64vector obj
 -- Scheme Procedure: any->c32vector obj
 -- Scheme Procedure: any->c64vector obj
 -- C Function: scm_any_to_u8vector (obj)
 -- C Function: scm_any_to_s8vector (obj)
 -- C Function: scm_any_to_u16vector (obj)
 -- C Function: scm_any_to_s16vector (obj)
 -- C Function: scm_any_to_u32vector (obj)
 -- C Function: scm_any_to_s32vector (obj)
 -- C Function: scm_any_to_u64vector (obj)
 -- C Function: scm_any_to_s64vector (obj)
 -- C Function: scm_any_to_f32vector (obj)
 -- C Function: scm_any_to_f64vector (obj)
 -- C Function: scm_any_to_c32vector (obj)
 -- C Function: scm_any_to_c64vector (obj)
     Return a (maybe newly allocated) uniform numeric vector of the
     indicated type, initialized with the elements of OBJ, which must
     be a list, a vector, or a uniform vector.  When OBJ is already a
     suitable uniform numeric vector, it is returned unchanged.

7.5.6 SRFI-6 - Basic String Ports
---------------------------------

SRFI-6 defines the procedures `open-input-string', `open-output-string'
and `get-output-string'.  These procedures are included in the Guile
core, so using this module does not make any difference at the moment.
But it is possible that support for SRFI-6 will be factored out of the
core library in the future, so using this module does not hurt, after
all.

7.5.7 SRFI-8 - receive
----------------------

`receive' is a syntax for making the handling of multiple-value
procedures easier.  It is documented in *Note Multiple Values::.

7.5.8 SRFI-9 - define-record-type
---------------------------------

This SRFI is a syntax for defining new record types and creating
predicate, constructor, and field getter and setter functions.  In
Guile this is simply an alternate interface to the core record
functionality (*note Records::).  It can be used with,

     (use-modules (srfi srfi-9))

 -- library syntax: define-record-type type
          (constructor fieldname ...)
          predicate
          (fieldname accessor [modifier]) ...

     Create a new record type, and make various `define's for using it.
     This syntax can only occur at the top-level, not nested within
     some other form.

     TYPE is bound to the record type, which is as per the return from
     the core `make-record-type'.  TYPE also provides the name for the
     record, as per `record-type-name'.

     CONSTRUCTOR is bound to a function to be called as `(CONSTRUCTOR
     fieldval ...)' to create a new record of this type.  The arguments
     are initial values for the fields, one argument for each field, in
     the order they appear in the `define-record-type' form.

     The FIELDNAMEs provide the names for the record fields, as per the
     core `record-type-fields' etc, and are referred to in the
     subsequent accessor/modifier forms.

     PREDICATE is bound to a function to be called as `(PREDICATE
     obj)'.  It returns `#t' or `#f' according to whether OBJ is a
     record of this type.

     Each ACCESSOR is bound to a function to be called `(ACCESSOR
     record)' to retrieve the respective field from a RECORD.
     Similarly each MODIFIER is bound to a function to be called
     `(MODIFIER record val)' to set the respective field in a RECORD.

An example will illustrate typical usage,

     (define-record-type employee-type
       (make-employee name age salary)
       employee?
       (name    get-employee-name)
       (age     get-employee-age    set-employee-age)
       (salary  get-employee-salary set-employee-salary))

   This creates a new employee data type, with name, age and salary
fields.  Accessor functions are created for each field, but no modifier
function for the name (the intention in this example being that it's
established only when an employee object is created).  These can all
then be used as for example,

     employee-type => #<record-type employee-type>

     (define fred (make-employee "Fred" 45 20000.00))

     (employee? fred)        => #t
     (get-employee-age fred) => 45
     (set-employee-salary fred 25000.00)  ;; pay rise

   The functions created by `define-record-type' are ordinary top-level
`define's.  They can be redefined or `set!' as desired, exported from a
module, etc.

Non-toplevel Record Definitions
...............................

The SRFI-9 specification explicitly disallows record definitions in a
non-toplevel context, such as inside `lambda' body or inside a LET
block.  However, Guile's implementation does not enforce that
restriction.

Custom Printers
...............

You may use `set-record-type-printer!' to customize the default printing
behavior of records.  This is a Guile extension and is not part of
SRFI-9.  It is located in the (srfi srfi-9 gnu) module.

 -- Scheme Syntax: set-record-type-printer! name thunk
     Where TYPE corresponds to the first argument of
     `define-record-type', and THUNK is a procedure accepting two
     arguments, the record to print, and an output port.

This example prints the employee's name in brackets, for instance
`[Fred]'.

     (set-record-type-printer! employee-type
       (lambda (record port)
         (write-char #\[ port)
         (display (get-employee-name record) port)
         (write-char #\] port)))

7.5.9 SRFI-10 - Hash-Comma Reader Extension
-------------------------------------------

This SRFI implements a reader extension `#,()' called hash-comma.  It
allows the reader to give new kinds of objects, for use both in data
and as constants or literals in source code.  This feature is available
with

     (use-modules (srfi srfi-10))

The new read syntax is of the form

     #,(TAG ARG...)

where TAG is a symbol and the ARGs are objects taken as parameters.
TAGs are registered with the following procedure.

 -- Scheme Procedure: define-reader-ctor tag proc
     Register PROC as the constructor for a hash-comma read syntax
     starting with symbol TAG, i.e. #,(TAG arg...).  PROC is called
     with the given arguments `(PROC arg...)' and the object it returns
     is the result of the read.

For example, a syntax giving a list of N copies of an object.

     (define-reader-ctor 'repeat
       (lambda (obj reps)
         (make-list reps obj)))

     (display '#,(repeat 99 3))
     -| (99 99 99)

   Notice the quote ' when the #,( ) is used.  The `repeat' handler
returns a list and the program must quote to use it literally, the same
as any other list.  Ie.

     (display '#,(repeat 99 3))
     =>
     (display '(99 99 99))

   When a handler returns an object which is self-evaluating, like a
number or a string, then there's no need for quoting, just as there's
no need when giving those directly as literals.  For example an
addition,

     (define-reader-ctor 'sum
       (lambda (x y)
         (+ x y)))
     (display #,(sum 123 456)) -| 579

   A typical use for #,() is to get a read syntax for objects which
don't otherwise have one.  For example, the following allows a hash
table to be given literally, with tags and values, ready for fast
lookup.

     (define-reader-ctor 'hash
       (lambda elems
         (let ((table (make-hash-table)))
           (for-each (lambda (elem)
                       (apply hash-set! table elem))
                     elems)
           table)))

     (define (animal->family animal)
       (hash-ref '#,(hash ("tiger" "cat")
                          ("lion"  "cat")
                          ("wolf"  "dog"))
                 animal))

     (animal->family "lion") => "cat"

   Or for example the following is a syntax for a compiled regular
expression (*note Regular Expressions::).

     (use-modules (ice-9 regex))

     (define-reader-ctor 'regexp make-regexp)

     (define (extract-angs str)
       (let ((match (regexp-exec '#,(regexp "<([A-Z0-9]+)>") str)))
         (and match
              (match:substring match 1))))

     (extract-angs "foo <BAR> quux") => "BAR"


   #,() is somewhat similar to `define-macro' (*note Macros::) in that
handler code is run to produce a result, but #,() operates at the read
stage, so it can appear in data for `read' (*note Scheme Read::), not
just in code to be executed.

   Because #,() is handled at read-time it has no direct access to
variables etc.  A symbol in the arguments is just a symbol, not a
variable reference.  The arguments are essentially constants, though
the handler procedure can use them in any complicated way it might want.

   Once `(srfi srfi-10)' has loaded, #,() is available globally,
there's no need to use `(srfi srfi-10)' in later modules.  Similarly
the tags registered are global and can be used anywhere once registered.

   There's no attempt to record what previous #,() forms have been
seen, if two identical forms occur then two calls are made to the
handler procedure.  The handler might like to maintain a cache or
similar to avoid making copies of large objects, depending on expected
usage.

   In code the best uses of #,() are generally when there's a lot of
objects of a particular kind as literals or constants.  If there's just
a few then some local variables and initializers are fine, but that
becomes tedious and error prone when there's a lot, and the anonymous
and compact syntax of #,() is much better.

7.5.10 SRFI-11 - let-values
---------------------------

This module implements the binding forms for multiple values
`let-values' and `let*-values'.  These forms are similar to `let' and
`let*' (*note Local Bindings::), but they support binding of the values
returned by multiple-valued expressions.

   Write `(use-modules (srfi srfi-11))' to make the bindings available.

     (let-values (((x y) (values 1 2))
                  ((z f) (values 3 4)))
        (+ x y z f))
     =>
     10

   `let-values' performs all bindings simultaneously, which means that
no expression in the binding clauses may refer to variables bound in the
same clause list.  `let*-values', on the other hand, performs the
bindings sequentially, just like `let*' does for single-valued
expressions.

7.5.11 SRFI-13 - String Library
-------------------------------

The SRFI-13 procedures are always available, *Note Strings::.

7.5.12 SRFI-14 - Character-set Library
--------------------------------------

The SRFI-14 data type and procedures are always available, *Note
Character Sets::.

7.5.13 SRFI-16 - case-lambda
----------------------------

SRFI-16 defines a variable-arity `lambda' form, `case-lambda'. This
form is available in the default Guile environment. *Note
Case-lambda::, for more information.

7.5.14 SRFI-17 - Generalized set!
---------------------------------

This SRFI implements a generalized `set!', allowing some "referencing"
functions to be used as the target location of a `set!'.  This feature
is available from

     (use-modules (srfi srfi-17))

For example `vector-ref' is extended so that

     (set! (vector-ref vec idx) new-value)

is equivalent to

     (vector-set! vec idx new-value)

   The idea is that a `vector-ref' expression identifies a location,
which may be either fetched or stored.  The same form is used for the
location in both cases, encouraging visual clarity.  This is similar to
the idea of an "lvalue" in C.

   The mechanism for this kind of `set!' is in the Guile core (*note
Procedures with Setters::).  This module adds definitions of the
following functions as procedures with setters, allowing them to be
targets of a `set!',

     car, cdr, caar, cadr, cdar, cddr, caaar, caadr, cadar, caddr,
     cdaar, cdadr, cddar, cdddr, caaaar, caaadr, caadar, caaddr,
     cadaar, cadadr, caddar, cadddr, cdaaar, cdaadr, cdadar, cdaddr,
     cddaar, cddadr, cdddar, cddddr

     string-ref, vector-ref

   The SRFI specifies `setter' (*note Procedures with Setters::) as a
procedure with setter, allowing the setter for a procedure to be
changed, eg. `(set! (setter foo) my-new-setter-handler)'.  Currently
Guile does not implement this, a setter can only be specified on
creation (`getter-with-setter' below).

 -- Function: getter-with-setter
     The same as the Guile core `make-procedure-with-setter' (*note
     Procedures with Setters::).

7.5.15 SRFI-18 - Multithreading support
---------------------------------------

This is an implementation of the SRFI-18 threading and synchronization
library.  The functions and variables described here are provided by

     (use-modules (srfi srfi-18))

   As a general rule, the data types and functions in this SRFI-18
implementation are compatible with the types and functions in Guile's
core threading code.  For example, mutexes created with the SRFI-18
`make-mutex' function can be passed to the built-in Guile function
`lock-mutex' (*note Mutexes and Condition Variables::), and mutexes
created with the built-in Guile function `make-mutex' can be passed to
the SRFI-18 function `mutex-lock!'.  Cases in which this does not hold
true are noted in the following sections.

7.5.15.1 SRFI-18 Threads
........................

Threads created by SRFI-18 differ in two ways from threads created by
Guile's built-in thread functions.  First, a thread created by SRFI-18
`make-thread' begins in a blocked state and will not start execution
until `thread-start!' is called on it.  Second, SRFI-18 threads are
constructed with a top-level exception handler that captures any
exceptions that are thrown on thread exit.  In all other regards,
SRFI-18 threads are identical to normal Guile threads.

 -- Function: current-thread
     Returns the thread that called this function.  This is the same
     procedure as the same-named built-in procedure `current-thread'
     (*note Threads::).

 -- Function: thread? obj
     Returns `#t' if OBJ is a thread, `#f' otherwise.  This is the same
     procedure as the same-named built-in procedure `thread?' (*note
     Threads::).

 -- Function: make-thread thunk [name]
     Call `thunk' in a new thread and with a new dynamic state,
     returning the new thread and optionally assigning it the object
     name NAME, which may be any Scheme object.

     Note that the name `make-thread' conflicts with the `(ice-9
     threads)' function `make-thread'.  Applications wanting to use
     both of these functions will need to refer to them by different
     names.

 -- Function: thread-name thread
     Returns the name assigned to THREAD at the time of its creation,
     or `#f' if it was not given a name.

 -- Function: thread-specific thread
 -- Function: thread-specific-set! thread obj
     Get or set the "object-specific" property of THREAD.  In Guile's
     implementation of SRFI-18, this value is stored as an object
     property, and will be `#f' if not set.

 -- Function: thread-start! thread
     Unblocks THREAD and allows it to begin execution if it has not
     done so already.

 -- Function: thread-yield!
     If one or more threads are waiting to execute, calling
     `thread-yield!' forces an immediate context switch to one of them.
     Otherwise, `thread-yield!' has no effect.  `thread-yield!' behaves
     identically to the Guile built-in function `yield'.

 -- Function: thread-sleep! timeout
     The current thread waits until the point specified by the time
     object TIMEOUT is reached (*note SRFI-18 Time::).  This blocks the
     thread only if TIMEOUT represents a point in the future.  it is an
     error for TIMEOUT to be `#f'.

 -- Function: thread-terminate! thread
     Causes an abnormal termination of THREAD.  If THREAD is not
     already terminated, all mutexes owned by THREAD become
     unlocked/abandoned.  If THREAD is the current thread,
     `thread-terminate!' does not return.  Otherwise
     `thread-terminate!' returns an unspecified value; the termination
     of THREAD will occur before `thread-terminate!' returns.
     Subsequent attempts to join on THREAD will cause a "terminated
     thread exception" to be raised.

     `thread-terminate!' is compatible with the thread cancellation
     procedures in the core threads API (*note Threads::) in that if a
     cleanup handler has been installed for the target thread, it will
     be called before the thread exits and its return value (or
     exception, if any) will be stored for later retrieval via a call to
     `thread-join!'.

 -- Function: thread-join! thread [timeout [timeout-val]]
     Wait for THREAD to terminate and return its exit value.  When a
     time value TIMEOUT is given, it specifies a point in time where
     the waiting should be aborted.  When the waiting is aborted,
     TIMEOUTVAL is returned if it is specified; otherwise, a
     `join-timeout-exception' exception is raised (*note SRFI-18
     Exceptions::).  Exceptions may also be raised if the thread was
     terminated by a call to `thread-terminate!'
     (`terminated-thread-exception' will be raised) or if the thread
     exited by raising an exception that was handled by the top-level
     exception handler (`uncaught-exception' will be raised; the
     original exception can be retrieved using
     `uncaught-exception-reason').

7.5.15.2 SRFI-18 Mutexes
........................

The behavior of Guile's built-in mutexes is parameterized via a set of
flags passed to the `make-mutex' procedure in the core (*note Mutexes
and Condition Variables::).  To satisfy the requirements for mutexes
specified by SRFI-18, the `make-mutex' procedure described below sets
the following flags:
   * `recursive': the mutex can be locked recursively

   * `unchecked-unlock': attempts to unlock a mutex that is already
     unlocked will not raise an exception

   * `allow-external-unlock': the mutex can be unlocked by any thread,
     not just the thread that locked it originally

 -- Function: make-mutex [name]
     Returns a new mutex, optionally assigning it the object name NAME,
     which may be any Scheme object.  The returned mutex will be
     created with the configuration described above.  Note that the name
     `make-mutex' conflicts with Guile core function `make-mutex'.
     Applications wanting to use both of these functions will need to
     refer to them by different names.

 -- Function: mutex-name mutex
     Returns the name assigned to MUTEX at the time of its creation, or
     `#f' if it was not given a name.

 -- Function: mutex-specific mutex
 -- Function: mutex-specific-set! mutex obj
     Get or set the "object-specific" property of MUTEX.  In Guile's
     implementation of SRFI-18, this value is stored as an object
     property, and will be `#f' if not set.

 -- Function: mutex-state mutex
     Returns information about the state of MUTEX.  Possible values are:
        * thread `T': the mutex is in the locked/owned state and thread
          T is the owner of the mutex

        * symbol `not-owned': the mutex is in the locked/not-owned state

        * symbol `abandoned': the mutex is in the unlocked/abandoned
          state

        * symbol `not-abandoned': the mutex is in the
          unlocked/not-abandoned state

 -- Function: mutex-lock! mutex [timeout [thread]]
     Lock MUTEX, optionally specifying a time object TIMEOUT after
     which to abort the lock attempt and a thread THREAD giving a new
     owner for MUTEX different than the current thread.  This procedure
     has the same behavior as the `lock-mutex' procedure in the core
     library.

 -- Function: mutex-unlock! mutex [condition-variable [timeout]]
     Unlock MUTEX, optionally specifying a condition variable
     CONDITION-VARIABLE on which to wait, either indefinitely or,
     optionally, until the time object TIMEOUT has passed, to be
     signalled.  This procedure has the same behavior as the
     `unlock-mutex' procedure in the core library.

7.5.15.3 SRFI-18 Condition variables
....................................

SRFI-18 does not specify a "wait" function for condition variables.
Waiting on a condition variable can be simulated using the SRFI-18
`mutex-unlock!' function described in the previous section, or Guile's
built-in `wait-condition-variable' procedure can be used.

 -- Function: condition-variable? obj
     Returns `#t' if OBJ is a condition variable, `#f' otherwise.  This
     is the same procedure as the same-named built-in procedure (*note
     `condition-variable?': Mutexes and Condition Variables.).

 -- Function: make-condition-variable [name]
     Returns a new condition variable, optionally assigning it the
     object name NAME, which may be any Scheme object.  This procedure
     replaces a procedure of the same name in the core library.

 -- Function: condition-variable-name condition-variable
     Returns the name assigned to THREAD at the time of its creation,
     or `#f' if it was not given a name.

 -- Function: condition-variable-specific condition-variable
 -- Function: condition-variable-specific-set! condition-variable obj
     Get or set the "object-specific" property of CONDITION-VARIABLE.
     In Guile's implementation of SRFI-18, this value is stored as an
     object property, and will be `#f' if not set.

 -- Function: condition-variable-signal! condition-variable
 -- Function: condition-variable-broadcast! condition-variable
     Wake up one thread that is waiting for CONDITION-VARIABLE, in the
     case of `condition-variable-signal!', or all threads waiting for
     it, in the case of `condition-variable-broadcast!'.  The behavior
     of these procedures is equivalent to that of the procedures
     `signal-condition-variable' and `broadcast-condition-variable' in
     the core library.

7.5.15.4 SRFI-18 Time
.....................

The SRFI-18 time functions manipulate time in two formats: a "time
object" type that represents an absolute point in time in some
implementation-specific way; and the number of seconds since some
unspecified "epoch".  In Guile's implementation, the epoch is the Unix
epoch, 00:00:00 UTC, January 1, 1970.

 -- Function: current-time
     Return the current time as a time object.  This procedure replaces
     the procedure of the same name in the core library, which returns
     the current time in seconds since the epoch.

 -- Function: time? obj
     Returns `#t' if OBJ is a time object, `#f' otherwise.

 -- Function: time->seconds time
 -- Function: seconds->time seconds
     Convert between time objects and numerical values representing the
     number of seconds since the epoch.  When converting from a time
     object to seconds, the return value is the number of seconds
     between TIME and the epoch.  When converting from seconds to a time
     object, the return value is a time object that represents a time
     SECONDS seconds after the epoch.

7.5.15.5 SRFI-18 Exceptions
...........................

SRFI-18 exceptions are identical to the exceptions provided by Guile's
implementation of SRFI-34.  The behavior of exception handlers invoked
to handle exceptions thrown from SRFI-18 functions, however, differs
from the conventional behavior of SRFI-34 in that the continuation of
the handler is the same as that of the call to the function.  Handlers
are called in a tail-recursive manner; the exceptions do not "bubble
up".

 -- Function: current-exception-handler
     Returns the current exception handler.

 -- Function: with-exception-handler handler thunk
     Installs HANDLER as the current exception handler and calls the
     procedure THUNK with no arguments, returning its value as the
     value of the exception.  HANDLER must be a procedure that accepts
     a single argument. The current exception handler at the time this
     procedure is called will be restored after the call returns.

 -- Function: raise obj
     Raise OBJ as an exception.  This is the same procedure as the
     same-named procedure defined in SRFI 34.

 -- Function: join-timeout-exception? obj
     Returns `#t' if OBJ is an exception raised as the result of
     performing a timed join on a thread that does not exit within the
     specified timeout, `#f' otherwise.

 -- Function: abandoned-mutex-exception? obj
     Returns `#t' if OBJ is an exception raised as the result of
     attempting to lock a mutex that has been abandoned by its owner
     thread, `#f' otherwise.

 -- Function: terminated-thread-exception? obj
     Returns `#t' if OBJ is an exception raised as the result of
     joining on a thread that exited as the result of a call to
     `thread-terminate!'.

 -- Function: uncaught-exception? obj
 -- Function: uncaught-exception-reason exc
     `uncaught-exception?' returns `#t' if OBJ is an exception thrown
     as the result of joining a thread that exited by raising an
     exception that was handled by the top-level exception handler
     installed by `make-thread'.  When this occurs, the original
     exception is preserved as part of the exception thrown by
     `thread-join!' and can be accessed by calling
     `uncaught-exception-reason' on that exception.  Note that because
     this exception-preservation mechanism is a side-effect of
     `make-thread', joining on threads that exited as described above
     but were created by other means will not raise this
     `uncaught-exception' error.

7.5.16 SRFI-19 - Time/Date Library
----------------------------------

This is an implementation of the SRFI-19 time/date library.  The
functions and variables described here are provided by

     (use-modules (srfi srfi-19))

   *Caution*: The current code in this module incorrectly extends the
Gregorian calendar leap year rule back prior to the introduction of
those reforms in 1582 (or the appropriate year in various countries).
The Julian calendar was used prior to 1582, and there were 10 days
skipped for the reform, but the code doesn't implement that.

   This will be fixed some time.  Until then calculations for 1583
onwards are correct, but prior to that any day/month/year and day of
the week calculations are wrong.

7.5.16.1 SRFI-19 Introduction
.............................

This module implements time and date representations and calculations,
in various time systems, including universal time (UTC) and atomic time
(TAI).

   For those not familiar with these time systems, TAI is based on a
fixed length second derived from oscillations of certain atoms.  UTC
differs from TAI by an integral number of seconds, which is increased
or decreased at announced times to keep UTC aligned to a mean solar day
(the orbit and rotation of the earth are not quite constant).

   So far, only increases in the TAI <-> UTC difference have been
needed.  Such an increase is a "leap second", an extra second of TAI
introduced at the end of a UTC day.  When working entirely within UTC
this is never seen, every day simply has 86400 seconds.  But when
converting from TAI to a UTC date, an extra 23:59:60 is present, where
normally a day would end at 23:59:59.  Effectively the UTC second from
23:59:59 to 00:00:00 has taken two TAI seconds.

   In the current implementation, the system clock is assumed to be UTC,
and a table of leap seconds in the code converts to TAI.  See comments
in `srfi-19.scm' for how to update this table.

   Also, for those not familiar with the terminology, a "Julian Day" is
a real number which is a count of days and fraction of a day, in UTC,
starting from -4713-01-01T12:00:00Z, ie. midday Monday 1 Jan 4713 B.C.
A "Modified Julian Day" is the same, but starting from
1858-11-17T00:00:00Z, ie. midnight 17 November 1858 UTC.  That time is
julian day 2400000.5.

7.5.16.2 SRFI-19 Time
.....................

A "time" object has type, seconds and nanoseconds fields representing a
point in time starting from some epoch.  This is an arbitrary point in
time, not just a time of day.  Although times are represented in
nanoseconds, the actual resolution may be lower.

   The following variables hold the possible time types.  For instance
`(current-time time-process)' would give the current CPU process time.

 -- Variable: time-utc
     Universal Coordinated Time (UTC).  

 -- Variable: time-tai
     International Atomic Time (TAI).  

 -- Variable: time-monotonic
     Monotonic time, meaning a monotonically increasing time starting
     from an unspecified epoch.

     Note that in the current implementation `time-monotonic' is the
     same as `time-tai', and unfortunately is therefore affected by
     adjustments to the system clock.  Perhaps this will change in the
     future.

 -- Variable: time-duration
     A duration, meaning simply a difference between two times.

 -- Variable: time-process
     CPU time spent in the current process, starting from when the
     process began.  

 -- Variable: time-thread
     CPU time spent in the current thread.  Not currently implemented.  


 -- Function: time? obj
     Return `#t' if OBJ is a time object, or `#f' if not.

 -- Function: make-time type nanoseconds seconds
     Create a time object with the given TYPE, SECONDS and NANOSECONDS.

 -- Function: time-type time
 -- Function: time-nanosecond time
 -- Function: time-second time
 -- Function: set-time-type! time type
 -- Function: set-time-nanosecond! time nsec
 -- Function: set-time-second! time sec
     Get or set the type, seconds or nanoseconds fields of a time
     object.

     `set-time-type!' merely changes the field, it doesn't convert the
     time value.  For conversions, see *note SRFI-19 Time/Date
     conversions::.

 -- Function: copy-time time
     Return a new time object, which is a copy of the given TIME.

 -- Function: current-time [type]
     Return the current time of the given TYPE.  The default TYPE is
     `time-utc'.

     Note that the name `current-time' conflicts with the Guile core
     `current-time' function (*note Time::) as well as the SRFI-18
     `current-time' function (*note SRFI-18 Time::).  Applications
     wanting to use more than one of these functions will need to refer
     to them by different names.

 -- Function: time-resolution [type]
     Return the resolution, in nanoseconds, of the given time TYPE.
     The default TYPE is `time-utc'.

 -- Function: time<=? t1 t2
 -- Function: time<? t1 t2
 -- Function: time=? t1 t2
 -- Function: time>=? t1 t2
 -- Function: time>? t1 t2
     Return `#t' or `#f' according to the respective relation between
     time objects T1 and T2.  T1 and T2 must be the same time type.

 -- Function: time-difference t1 t2
 -- Function: time-difference! t1 t2
     Return a time object of type `time-duration' representing the
     period between T1 and T2.  T1 and T2 must be the same time type.

     `time-difference' returns a new time object, `time-difference!'
     may modify T1 to form its return.

 -- Function: add-duration time duration
 -- Function: add-duration! time duration
 -- Function: subtract-duration time duration
 -- Function: subtract-duration! time duration
     Return a time object which is TIME with the given DURATION added
     or subtracted.  DURATION must be a time object of type
     `time-duration'.

     `add-duration' and `subtract-duration' return a new time object.
     `add-duration!' and `subtract-duration!' may modify the given TIME
     to form their return.

7.5.16.3 SRFI-19 Date
.....................

A "date" object represents a date in the Gregorian calendar and a time
of day on that date in some timezone.

   The fields are year, month, day, hour, minute, second, nanoseconds
and timezone.  A date object is immutable, its fields can be read but
they cannot be modified once the object is created.

 -- Function: date? obj
     Return `#t' if OBJ is a date object, or `#f' if not.

 -- Function: make-date nsecs seconds minutes hours date month year
          zone-offset
     Create a new date object.

 -- Function: date-nanosecond date
     Nanoseconds, 0 to 999999999.

 -- Function: date-second date
     Seconds, 0 to 59, or 60 for a leap second.  60 is never seen when
     working entirely within UTC, it's only when converting to or from
     TAI.

 -- Function: date-minute date
     Minutes, 0 to 59.

 -- Function: date-hour date
     Hour, 0 to 23.

 -- Function: date-day date
     Day of the month, 1 to 31 (or less, according to the month).

 -- Function: date-month date
     Month, 1 to 12.

 -- Function: date-year date
     Year, eg. 2003.  Dates B.C. are negative, eg. -46 is 46 B.C.
     There is no year 0, year -1 is followed by year 1.

 -- Function: date-zone-offset date
     Time zone, an integer number of seconds east of Greenwich.

 -- Function: date-year-day date
     Day of the year, starting from 1 for 1st January.

 -- Function: date-week-day date
     Day of the week, starting from 0 for Sunday.

 -- Function: date-week-number date dstartw
     Week of the year, ignoring a first partial week.  DSTARTW is the
     day of the week which is taken to start a week, 0 for Sunday, 1 for
     Monday, etc.

 -- Function: current-date [tz-offset]
     Return a date object representing the current date/time, in UTC
     offset by TZ-OFFSET.  TZ-OFFSET is seconds east of Greenwich and
     defaults to the local timezone.

 -- Function: current-julian-day
     Return the current Julian Day.

 -- Function: current-modified-julian-day
     Return the current Modified Julian Day.

7.5.16.4 SRFI-19 Time/Date conversions
......................................

 -- Function: date->julian-day date
 -- Function: date->modified-julian-day date
 -- Function: date->time-monotonic date
 -- Function: date->time-tai date
 -- Function: date->time-utc date

 -- Function: julian-day->date jdn [tz-offset]
 -- Function: julian-day->time-monotonic jdn
 -- Function: julian-day->time-tai jdn
 -- Function: julian-day->time-utc jdn

 -- Function: modified-julian-day->date jdn [tz-offset]
 -- Function: modified-julian-day->time-monotonic jdn
 -- Function: modified-julian-day->time-tai jdn
 -- Function: modified-julian-day->time-utc jdn

 -- Function: time-monotonic->date time [tz-offset]
 -- Function: time-monotonic->time-tai time
 -- Function: time-monotonic->time-tai! time
 -- Function: time-monotonic->time-utc time
 -- Function: time-monotonic->time-utc! time

 -- Function: time-tai->date time [tz-offset]
 -- Function: time-tai->julian-day time
 -- Function: time-tai->modified-julian-day time
 -- Function: time-tai->time-monotonic time
 -- Function: time-tai->time-monotonic! time
 -- Function: time-tai->time-utc time
 -- Function: time-tai->time-utc! time

 -- Function: time-utc->date time [tz-offset]
 -- Function: time-utc->julian-day time
 -- Function: time-utc->modified-julian-day time
 -- Function: time-utc->time-monotonic time
 -- Function: time-utc->time-monotonic! time
 -- Function: time-utc->time-tai time
 -- Function: time-utc->time-tai! time

     Convert between dates, times and days of the respective types.  For
     instance `time-tai->time-utc' accepts a TIME object of type
     `time-tai' and returns an object of type `time-utc'.

     The `!' variants may modify their TIME argument to form their
     return.  The plain functions create a new object.

     For conversions to dates, TZ-OFFSET is seconds east of Greenwich.
     The default is the local timezone, at the given time, as provided
     by the system, using `localtime' (*note Time::).

     On 32-bit systems, `localtime' is limited to a 32-bit `time_t', so
     a default TZ-OFFSET is only available for times between Dec 1901
     and Jan 2038.  For prior dates an application might like to use
     the value in 1902, though some locations have zone changes prior
     to that.  For future dates an application might like to assume
     today's rules extend indefinitely.  But for correct daylight
     savings transitions it will be necessary to take an offset for the
     same day and time but a year in range and which has the same
     starting weekday and same leap/non-leap (to support rules like
     last Sunday in October).

7.5.16.5 SRFI-19 Date to string
...............................

 -- Function: date->string date [format]
     Convert a date to a string under the control of a format.  FORMAT
     should be a string containing `~' escapes, which will be expanded
     as per the following conversion table.  The default FORMAT is
     `~c', a locale-dependent date and time.

     Many of these conversion characters are the same as POSIX
     `strftime' (*note Time::), but there are some extras and some
     variations.

     ~~     literal ~
     ~a     locale abbreviated weekday, eg. `Sun'
     ~A     locale full weekday, eg. `Sunday'
     ~b     locale abbreviated month, eg. `Jan'
     ~B     locale full month, eg. `January'
     ~c     locale date and time, eg.
            `Fri Jul 14 20:28:42-0400 2000'
     ~d     day of month, zero padded, `01' to `31'
     ~e     day of month, blank padded, ` 1' to `31'
     ~f     seconds and fractional seconds, with locale decimal
            point, eg. `5.2'
     ~h     same as ~b
     ~H     hour, 24-hour clock, zero padded, `00' to `23'
     ~I     hour, 12-hour clock, zero padded, `01' to `12'
     ~j     day of year, zero padded, `001' to `366'
     ~k     hour, 24-hour clock, blank padded, ` 0' to `23'
     ~l     hour, 12-hour clock, blank padded, ` 1' to `12'
     ~m     month, zero padded, `01' to `12'
     ~M     minute, zero padded, `00' to `59'
     ~n     newline
     ~N     nanosecond, zero padded, `000000000' to `999999999'
     ~p     locale AM or PM
     ~r     time, 12 hour clock, `~I:~M:~S ~p'
     ~s     number of full seconds since "the epoch" in UTC
     ~S     second, zero padded `00' to `60'
            (usual limit is 59, 60 is a leap second)
     ~t     horizontal tab character
     ~T     time, 24 hour clock, `~H:~M:~S'
     ~U     week of year, Sunday first day of week, `00' to `52'
     ~V     week of year, Monday first day of week, `01' to `53'
     ~w     day of week, 0 for Sunday, `0' to `6'
     ~W     week of year, Monday first day of week, `00' to `52'
     ~y     year, two digits, `00' to `99'
     ~Y     year, full, eg. `2003'
     ~z     time zone, RFC-822 style
     ~Z     time zone symbol (not currently implemented)
     ~1     ISO-8601 date, `~Y-~m-~d'
     ~2     ISO-8601 time+zone, `~k:~M:~S~z'
     ~3     ISO-8601 time, `~k:~M:~S'
     ~4     ISO-8601 date/time+zone, `~Y-~m-~dT~k:~M:~S~z'
     ~5     ISO-8601 date/time, `~Y-~m-~dT~k:~M:~S'

Conversions `~D', `~x' and `~X' are not currently described here, since
the specification and reference implementation differ.

   Conversion is locale-dependent on systems that support it (*note
Accessing Locale Information::).  *Note `setlocale': Locales, for
information on how to change the current locale.

7.5.16.6 SRFI-19 String to date
...............................

 -- Function: string->date input template
     Convert an INPUT string to a date under the control of a TEMPLATE
     string.  Return a newly created date object.

     Literal characters in TEMPLATE must match characters in INPUT and
     `~' escapes must match the input forms described in the table
     below.  "Skip to" means characters up to one of the given type are
     ignored, or "no skip" for no skipping.  "Read" is what's then
     read, and "Set" is the field affected in the date object.

     For example `~Y' skips input characters until a digit is reached,
     at which point it expects a year and stores that to the year field
     of the date.

            Skip to            Read                        Set
     ~~     no skip            literal ~                   nothing
     ~a     char-alphabetic?   locale abbreviated weekday  nothing
                               name                        
     ~A     char-alphabetic?   locale full weekday name    nothing
     ~b     char-alphabetic?   locale abbreviated month    date-month
                               name                        
     ~B     char-alphabetic?   locale full month name      date-month
     ~d     char-numeric?      day of month                date-day
     ~e     no skip            day of month, blank padded  date-day
     ~h     same as `~b'                                   
     ~H     char-numeric?      hour                        date-hour
     ~k     no skip            hour, blank padded          date-hour
     ~m     char-numeric?      month                       date-month
     ~M     char-numeric?      minute                      date-minute
     ~S     char-numeric?      second                      date-second
     ~y     no skip            2-digit year                date-year within
                                                           50 years
     ~Y     char-numeric?      year                        date-year
     ~z     no skip            time zone                   date-zone-offset

     Notice that the weekday matching forms don't affect the date object
     returned, instead the weekday will be derived from the day, month
     and year.

     Conversion is locale-dependent on systems that support it (*note
     Accessing Locale Information::).  *Note `setlocale': Locales, for
     information on how to change the current locale.

7.5.17 SRFI-23 - Error Reporting
--------------------------------

The SRFI-23 `error' procedure is always available.

7.5.18 SRFI-26 - specializing parameters
----------------------------------------

This SRFI provides a syntax for conveniently specializing selected
parameters of a function.  It can be used with,

     (use-modules (srfi srfi-26))

 -- library syntax: cut slot ...
 -- library syntax: cute slot ...
     Return a new procedure which will make a call (SLOT ...) but with
     selected parameters specialized to given expressions.

     An example will illustrate the idea.  The following is a
     specialization of `write', sending output to `my-output-port',

          (cut write <> my-output-port)
          =>
          (lambda (obj) (write obj my-output-port))

     The special symbol `<>' indicates a slot to be filled by an
     argument to the new procedure.  `my-output-port' on the other hand
     is an expression to be evaluated and passed, ie. it specializes
     the behaviour of `write'.

    <>
          A slot to be filled by an argument from the created procedure.
          Arguments are assigned to `<>' slots in the order they appear
          in the `cut' form, there's no way to re-arrange arguments.

          The first argument to `cut' is usually a procedure (or
          expression giving a procedure), but `<>' is allowed there
          too.  For example,

               (cut <> 1 2 3)
               =>
               (lambda (proc) (proc 1 2 3))

    <...>
          A slot to be filled by all remaining arguments from the new
          procedure.  This can only occur at the end of a `cut' form.

          For example, a procedure taking a variable number of
          arguments like `max' but in addition enforcing a lower bound,

               (define my-lower-bound 123)

               (cut max my-lower-bound <...>)
               =>
               (lambda arglist (apply max my-lower-bound arglist))

     For `cut' the specializing expressions are evaluated each time the
     new procedure is called.  For `cute' they're evaluated just once,
     when the new procedure is created.  The name `cute' stands for
     "`cut' with evaluated arguments".  In all cases the evaluations
     take place in an unspecified order.

     The following illustrates the difference between `cut' and `cute',

          (cut format <> "the time is ~s" (current-time))
          =>
          (lambda (port) (format port "the time is ~s" (current-time)))

          (cute format <> "the time is ~s" (current-time))
          =>
          (let ((val (current-time)))
            (lambda (port) (format port "the time is ~s" val))

     (There's no provision for a mixture of `cut' and `cute' where some
     expressions would be evaluated every time but others evaluated
     only once.)

     `cut' is really just a shorthand for the sort of `lambda' forms
     shown in the above examples.  But notice `cut' avoids the need to
     name unspecialized parameters, and is more compact.  Use in
     functional programming style or just with `map', `for-each' or
     similar is typical.

          (map (cut * 2 <>) '(1 2 3 4))

          (for-each (cut write <> my-port) my-list)

7.5.19 SRFI-27 - Sources of Random Bits
---------------------------------------

This subsection is based on the specification of SRFI-27
(http://srfi.schemers.org/srfi-27/srfi-27.html) written by Sebastian
Egner.

   This SRFI provides access to a (pseudo) random number generator; for
Guile's built-in random number facilities, which SRFI-27 is implemented
upon, *Note Random::.  With SRFI-27, random numbers are obtained from a
_random source_, which encapsulates a random number generation
algorithm and its state.

7.5.19.1 The Default Random Source
..................................

 -- Function: random-integer n
     Return a random number between zero (inclusive) and N (exclusive),
     using the default random source.  The numbers returned have a
     uniform distribution.

 -- Function: random-real
     Return a random number in (0,1), using the default random source.
     The numbers returned have a uniform distribution.

 -- Function: default-random-source
     A random source from which `random-integer' and `random-real' have
     been derived using `random-source-make-integers' and
     `random-source-make-reals' (*note SRFI-27 Random Number
     Generators:: for those procedures).  Note that an assignment to
     `default-random-source' does not change `random-integer' or
     `random-real'; it is also strongly recommended not to assign a new
     value.

7.5.19.2 Random Sources
.......................

 -- Function: make-random-source
     Create a new random source.  The stream of random numbers obtained
     from each random source created by this procedure will be
     identical, unless its state is changed by one of the procedures
     below.

 -- Function: random-source? object
     Tests whether OBJECT is a random source.  Random sources are a
     disjoint type.

 -- Function: random-source-randomize! source
     Attempt to set the state of the random source to a truly random
     value.  The current implementation uses a seed based on the
     current system time.

 -- Function: random-source-pseudo-randomize! source i j
     Changes the state of the random source s into the initial state of
     the (I, J)-th independent random source, where I and J are
     non-negative integers.  This procedure provides a mechanism to
     obtain a large number of independent random sources (usually all
     derived from the same backbone generator), indexed by two
     integers. In contrast to `random-source-randomize!', this
     procedure is entirely deterministic.

   The state associated with a random state can be obtained an
reinstated with the following procedures:

 -- Function: random-source-state-ref source
 -- Function: random-source-state-set! source state
     Get and set the state of a random source.  No assumptions should
     be made about the nature of the state object, besides it having an
     external representation (i.e. it can be passed to `write' and
     subsequently `read' back).

7.5.19.3 Obtaining random number generator procedures
.....................................................

 -- Function: random-source-make-integers source
     Obtains a procedure to generate random integers using the random
     source SOURCE.  The returned procedure takes a single argument N,
     which must be a positive integer, and returns the next uniformly
     distributed random integer from the interval {0, ..., N-1} by
     advancing the state of SOURCE.

     If an application obtains and uses several generators for the same
     random source SOURCE, a call to any of these generators advances
     the state of SOURCE.  Hence, the generators do not produce the
     same sequence of random integers each but rather share a state.
     This also holds for all other types of generators derived from a
     fixed random sources.

     While the SRFI text specifies that "Implementations that support
     concurrency make sure that the state of a generator is properly
     advanced", this is currently not the case in Guile's
     implementation of SRFI-27, as it would cause a severe performance
     penalty.  So in multi-threaded programs, you either must perform
     locking on random sources shared between threads yourself, or use
     different random sources for multiple threads.

 -- Function: random-source-make-reals source
 -- Function: random-source-make-reals source unit
     Obtains a procedure to generate random real numbers 0 < x < 1
     using the random source SOURCE.  The procedure rand is called
     without arguments.

     The optional parameter UNIT determines the type of numbers being
     produced by the returned procedure and the quantization of the
     output.  UNIT must be a number such that 0 < UNIT < 1.  The
     numbers created by the returned procedure are of the same
     numerical type as UNIT and the potential output values are spaced
     by at most UNIT.  One can imagine rand to create numbers as X *
     UNIT where X is a random integer in {1, ..., floor(1/unit)-1}.
     Note, however, that this need not be the way the values are
     actually created and that the actual resolution of rand can be
     much higher than unit. In case UNIT is absent it defaults to a
     reasonably small value (related to the width of the mantissa of an
     efficient number format).

7.5.20 SRFI-30 - Nested Multi-line Comments
-------------------------------------------

Starting from version 2.0, Guile's `read' supports SRFI-30/R6RS nested
multi-line comments by default, *note Block Comments::.

7.5.21 SRFI-31 - A special form `rec' for recursive evaluation
--------------------------------------------------------------

SRFI-31 defines a special form that can be used to create
self-referential expressions more conveniently.  The syntax is as
follows:

     <rec expression> --> (rec <variable> <expression>)
     <rec expression> --> (rec (<variable>+) <body>)

   The first syntax can be used to create self-referential expressions,
for example:

       guile> (define tmp (rec ones (cons 1 (delay ones))))

   The second syntax can be used to create anonymous recursive
functions:

       guile> (define tmp (rec (display-n item n)
                            (if (positive? n)
                                (begin (display n) (display-n (- n 1))))))
       guile> (tmp 42 3)
       424242
       guile>

7.5.22 SRFI-34 - Exception handling for programs
------------------------------------------------

Guile provides an implementation of SRFI-34's exception handling
mechanisms (http://srfi.schemers.org/srfi-34/srfi-34.html) as an
alternative to its own built-in mechanisms (*note Exceptions::).  It
can be made available as follows:

     (use-modules (srfi srfi-34))

7.5.23 SRFI-35 - Conditions
---------------------------

SRFI-35 (http://srfi.schemers.org/srfi-35/srfi-35.html) implements
"conditions", a data structure akin to records designed to convey
information about exceptional conditions between parts of a program.  It
is normally used in conjunction with SRFI-34's `raise':

     (raise (condition (&message
                         (message "An error occurred"))))

   Users can define "condition types" containing arbitrary information.
Condition types may inherit from one another.  This allows the part of
the program that handles (or "catches") conditions to get accurate
information about the exceptional condition that arose.

   SRFI-35 conditions are made available using:

     (use-modules (srfi srfi-35))

   The procedures available to manipulate condition types are the
following:

 -- Scheme Procedure: make-condition-type id parent field-names
     Return a new condition type named ID, inheriting from PARENT, and
     with the fields whose names are listed in FIELD-NAMES.
     FIELD-NAMES must be a list of symbols and must not contain names
     already used by PARENT or one of its supertypes.

 -- Scheme Procedure: condition-type? obj
     Return true if OBJ is a condition type.

   Conditions can be created and accessed with the following procedures:

 -- Scheme Procedure: make-condition type . field+value
     Return a new condition of type TYPE with fields initialized as
     specified by FIELD+VALUE, a sequence of field names (symbols) and
     values as in the following example:

          (let ((&ct (make-condition-type 'foo &condition '(a b c))))
            (make-condition &ct 'a 1 'b 2 'c 3))

     Note that all fields of TYPE and its supertypes must be specified.

 -- Scheme Procedure: make-compound-condition . conditions
     Return a new compound condition composed of CONDITIONS.  The
     returned condition has the type of each condition of CONDITIONS
     (per `condition-has-type?').

 -- Scheme Procedure: condition-has-type? c type
     Return true if condition C has type TYPE.

 -- Scheme Procedure: condition-ref c field-name
     Return the value of the field named FIELD-NAME from condition C.

     If C is a compound condition and several underlying condition
     types contain a field named FIELD-NAME, then the value of the
     first such field is returned, using the order in which conditions
     were passed to MAKE-COMPOUND-CONDITION.

 -- Scheme Procedure: extract-condition c type
     Return a condition of condition type TYPE with the field values
     specified by C.

     If C is a compound condition, extract the field values from the
     subcondition belonging to TYPE that appeared first in the call to
     `make-compound-condition' that created the condition.

   Convenience macros are also available to create condition types and
conditions.

 -- library syntax: define-condition-type type supertype predicate
          field-spec...
     Define a new condition type named TYPE that inherits from
     SUPERTYPE.  In addition, bind PREDICATE to a type predicate that
     returns true when passed a condition of type TYPE or any of its
     subtypes.  FIELD-SPEC must have the form `(field accessor)' where
     FIELD is the name of field of TYPE and ACCESSOR is the name of a
     procedure to access field FIELD in conditions of type TYPE.

     The example below defines condition type `&foo', inheriting from
     `&condition' with fields `a', `b' and `c':

          (define-condition-type &foo &condition
            foo-condition?
            (a  foo-a)
            (b  foo-b)
            (c  foo-c))

 -- library syntax: condition type-field-bindings...
     Return a new condition, or compound condition, initialized
     according to TYPE-FIELD-BINDINGS.  Each TYPE-FIELD-BINDING must
     have the form `(type field-specs...)', where TYPE is the name of a
     variable bound to condition type; each FIELD-SPEC must have the
     form `(field-name value)' where FIELD-NAME is a symbol denoting
     the field being initialized to VALUE.  As for `make-condition',
     all fields must be specified.

     The following example returns a simple condition:

          (condition (&message (message "An error occurred")))

     The one below returns a compound condition:

          (condition (&message (message "An error occurred"))
                     (&serious))

   Finally, SRFI-35 defines a several standard condition types.

 -- Variable: &condition
     This condition type is the root of all condition types.  It has no
     fields.

 -- Variable: &message
     A condition type that carries a message describing the nature of
     the condition to humans.

 -- Scheme Procedure: message-condition? c
     Return true if C is of type `&message' or one of its subtypes.

 -- Scheme Procedure: condition-message c
     Return the message associated with message condition C.

 -- Variable: &serious
     This type describes conditions serious enough that they cannot
     safely be ignored.  It has no fields.

 -- Scheme Procedure: serious-condition? c
     Return true if C is of type `&serious' or one of its subtypes.

 -- Variable: &error
     This condition describes errors, typically caused by something
     that has gone wrong in the interaction of the program with the
     external world or the user.

 -- Scheme Procedure: error? c
     Return true if C is of type `&error' or one of its subtypes.

7.5.24 SRFI-37 - args-fold
--------------------------

This is a processor for GNU `getopt_long'-style program arguments.  It
provides an alternative, less declarative interface than `getopt-long'
in `(ice-9 getopt-long)' (*note The (ice-9 getopt-long) Module:
getopt-long.).  Unlike `getopt-long', it supports repeated options and
any number of short and long names per option.  Access it with:

     (use-modules (srfi srfi-37))

   SRFI-37 principally provides an `option' type and the `args-fold'
function.  To use the library, create a set of options with `option'
and use it as a specification for invoking `args-fold'.

   Here is an example of a simple argument processor for the typical
`--version' and `--help' options, which returns a backwards list of
files given on the command line:

     (args-fold (cdr (program-arguments))
                (let ((display-and-exit-proc
                       (lambda (msg)
                         (lambda (opt name arg loads)
                           (display msg) (quit)))))
                  (list (option '(#\v "version") #f #f
                                (display-and-exit-proc "Foo version 42.0\n"))
                        (option '(#\h "help") #f #f
                                (display-and-exit-proc
                                 "Usage: foo scheme-file ..."))))
                (lambda (opt name arg loads)
                  (error "Unrecognized option `~A'" name))
                (lambda (op loads) (cons op loads))
                '())

 -- Scheme Procedure: option names required-arg? optional-arg? processor
     Return an object that specifies a single kind of program option.

     NAMES is a list of command-line option names, and should consist of
     characters for traditional `getopt' short options and strings for
     `getopt_long'-style long options.

     REQUIRED-ARG? and OPTIONAL-ARG? are mutually exclusive; one or
     both must be `#f'.  If REQUIRED-ARG?, the option must be followed
     by an argument on the command line, such as `--opt=value' for long
     options, or an error will be signalled.  If OPTIONAL-ARG?, an
     argument will be taken if available.

     PROCESSOR is a procedure that takes at least 3 arguments, called
     when `args-fold' encounters the option: the containing option
     object, the name used on the command line, and the argument given
     for the option (or `#f' if none).  The rest of the arguments are
     `args-fold' "seeds", and the PROCESSOR should return seeds as well.

 -- Scheme Procedure: option-names opt
 -- Scheme Procedure: option-required-arg? opt
 -- Scheme Procedure: option-optional-arg? opt
 -- Scheme Procedure: option-processor opt
     Return the specified field of OPT, an option object, as described
     above for `option'.

 -- Scheme Procedure: args-fold args options unrecognized-option-proc
          operand-proc seeds ...
     Process ARGS, a list of program arguments such as that returned by
     `(cdr (program-arguments))', in order against OPTIONS, a list of
     option objects as described above.  All functions called take the
     "seeds", or the last multiple-values as multiple arguments,
     starting with SEEDS, and must return the new seeds.  Return the
     final seeds.

     Call `unrecognized-option-proc', which is like an option object's
     processor, for any options not found in OPTIONS.

     Call `operand-proc' with any items on the command line that are
     not named options.  This includes arguments after `--'.  It is
     called with the argument in question, as well as the seeds.

7.5.25 SRFI-38 - External Representation for Data With Shared Structure
-----------------------------------------------------------------------

This subsection is based on the specification of SRFI-38
(http://srfi.schemers.org/srfi-38/srfi-38.html) written by Ray
Dillinger.

   This SRFI creates an alternative external representation for data
written and read using `write-with-shared-structure' and
`read-with-shared-structure'.  It is identical to the grammar for
external representation for data written and read with `write' and
`read' given in section 7 of R5RS, except that the single production

     <datum> --> <simple datum> | <compound datum>

   is replaced by the following five productions:

     <datum> --> <defining datum> | <nondefining datum> | <defined datum>
     <defining datum> -->  #<indexnum>=<nondefining datum>
     <defined datum> --> #<indexnum>#
     <nondefining datum> --> <simple datum> | <compound datum>
     <indexnum> --> <digit 10>+

 -- Scheme procedure: write-with-shared-structure obj
 -- Scheme procedure: write-with-shared-structure obj port
 -- Scheme procedure: write-with-shared-structure obj port optarg
     Writes an external representation of OBJ to the given port.
     Strings that appear in the written representation are enclosed in
     doublequotes, and within those strings backslash and doublequote
     characters are escaped by backslashes.  Character objects are
     written using the `#\' notation.

     Objects which denote locations rather than values (cons cells,
     vectors, and non-zero-length strings in R5RS scheme; also Guile's
     structs, bytevectors and ports and hash-tables), if they appear at
     more than one point in the data being written, are preceded by
     `#N=' the first time they are written and replaced by `#N#' all
     subsequent times they are written, where N is a natural number
     used to identify that particular object.  If objects which denote
     locations occur only once in the structure, then
     `write-with-shared-structure' must produce the same external
     representation for those objects as `write'.

     `write-with-shared-structure' terminates in finite time and
     produces a finite representation when writing finite data.

     `write-with-shared-structure' returns an unspecified value. The
     PORT argument may be omitted, in which case it defaults to the
     value returned by `(current-output-port)'.  The OPTARG argument
     may also be omitted.  If present, its effects on the output and
     return value are unspecified but `write-with-shared-structure' must
     still write a representation that can be read by
     `read-with-shared-structure'.  Some implementations may wish to use
     OPTARG to specify formatting conventions, numeric radixes, or
     return values.  Guile's implementation ignores OPTARG.

     For example, the code

          (begin (define a (cons 'val1 'val2))
                 (set-cdr! a a)
                 (write-with-shared-structure a))

     should produce the output `#1=(val1 . #1#)'.  This shows a cons
     cell whose `cdr' contains itself.


 -- Scheme procedure: read-with-shared-structure
 -- Scheme procedure: read-with-shared-structure port
     `read-with-shared-structure' converts the external representations
     of Scheme objects produced by `write-with-shared-structure' into
     Scheme objects.  That is, it is a parser for the nonterminal
     `<datum>' in the augmented external representation grammar defined
     above.  `read-with-shared-structure' returns the next object
     parsable from the given input port, updating PORT to point to the
     first character past the end of the external representation of the
     object.

     If an end-of-file is encountered in the input before any
     characters are found that can begin an object, then an end-of-file
     object is returned.  The port remains open, and further attempts
     to read it (by `read-with-shared-structure' or `read' will also
     return an end-of-file object.  If an end of file is encountered
     after the beginning of an object's external representation, but
     the external representation is incomplete and therefore not
     parsable, an error is signalled.

     The PORT argument may be omitted, in which case it defaults to the
     value returned by `(current-input-port)'.  It is an error to read
     from a closed port.


7.5.26 SRFI-39 - Parameters
---------------------------

This SRFI adds support for dynamically-scoped parameters.  SRFI 39 is
implemented in the Guile core; there's no module needed to get SRFI-39
itself.  Parameters are documented in *note Parameters::.

   This module does export one extra function: `with-parameters*'.
This is a Guile-specific addition to the SRFI, similar to the core
`with-fluids*' (*note Fluids and Dynamic States::).

 -- Function: with-parameters* param-list value-list thunk
     Establish a new dynamic scope, as per `parameterize' above, taking
     parameters from PARAM-LIST and corresponding values from
     VALUES-LIST.  A call `(THUNK)' is made in the new scope and the
     result from that THUNK is the return from `with-parameters*'.

7.5.27 SRFI-42 - Eager Comprehensions
-------------------------------------

See the specification of SRFI-42
(http://srfi.schemers.org/srfi-42/srfi-42.html).

7.5.28 SRFI-45 - Primitives for Expressing Iterative Lazy Algorithms
--------------------------------------------------------------------

This subsection is based on the specification of SRFI-45
(http://srfi.schemers.org/srfi-45/srfi-45.html) written by André van
Tonder.

   Lazy evaluation is traditionally simulated in Scheme using `delay'
and `force'.  However, these primitives are not powerful enough to
express a large class of lazy algorithms that are iterative.  Indeed, it
is folklore in the Scheme community that typical iterative lazy
algorithms written using delay and force will often require unbounded
memory.

   This SRFI provides set of three operations: {`lazy', `delay',
`force'}, which allow the programmer to succinctly express lazy
algorithms while retaining bounded space behavior in cases that are
properly tail-recursive.  A general recipe for using these primitives is
provided. An additional procedure `eager' is provided for the
construction of eager promises in cases where efficiency is a concern.

   Although this SRFI redefines `delay' and `force', the extension is
conservative in the sense that the semantics of the subset {`delay',
`force'} in isolation (i.e., as long as the program does not use
`lazy') agrees with that in R5RS.  In other words, no program that uses
the R5RS definitions of delay and force will break if those definition
are replaced by the SRFI-45 definitions of delay and force.

 -- Scheme Syntax: delay expression
     Takes an expression of arbitrary type A and returns a promise of
     type `(Promise A)' which at some point in the future may be asked
     (by the `force' procedure) to evaluate the expression and deliver
     the resulting value.

 -- Scheme Syntax: lazy expression
     Takes an expression of type `(Promise A)' and returns a promise of
     type `(Promise A)' which at some point in the future may be asked
     (by the `force' procedure) to evaluate the expression and deliver
     the resulting promise.

 -- Scheme Procedure: force expression
     Takes an argument of type `(Promise A)' and returns a value of
     type A as follows: If a value of type A has been computed for the
     promise, this value is returned.  Otherwise, the promise is first
     evaluated, then overwritten by the obtained promise or value, and
     then force is again applied (iteratively) to the promise.

 -- Scheme Procedure: eager expression
     Takes an argument of type A and returns a value of type `(Promise
     A)'.  As opposed to `delay', the argument is evaluated eagerly.
     Semantically, writing `(eager expression)' is equivalent to writing

          (let ((value expression)) (delay value)).

     However, the former is more efficient since it does not require
     unnecessary creation and evaluation of thunks. We also have the
     equivalence

          (delay expression) = (lazy (eager expression))

   The following reduction rules may be helpful for reasoning about
these primitives.  However, they do not express the memoization and
memory usage semantics specified above:

     (force (delay expression)) -> expression
     (force (lazy  expression)) -> (force expression)
     (force (eager value))      -> value

Correct usage
.............

We now provide a general recipe for using the primitives {`lazy',
`delay', `force'} to express lazy algorithms in Scheme.  The
transformation is best described by way of an example: Consider the
stream-filter algorithm, expressed in a hypothetical lazy language as

     (define (stream-filter p? s)
       (if (null? s) '()
           (let ((h (car s))
                 (t (cdr s)))
             (if (p? h)
                 (cons h (stream-filter p? t))
                 (stream-filter p? t)))))

   This algorithm can be expressed as follows in Scheme:

     (define (stream-filter p? s)
       (lazy
          (if (null? (force s)) (delay '())
              (let ((h (car (force s)))
                    (t (cdr (force s))))
                (if (p? h)
                    (delay (cons h (stream-filter p? t)))
                    (stream-filter p? t))))))

   In other words, we

   * wrap all constructors (e.g., `'()', `cons') with `delay',

   * apply `force' to arguments of deconstructors (e.g., `car', `cdr'
     and `null?'),

   * wrap procedure bodies with `(lazy ...)'.

7.5.29 SRFI-55 - Requiring Features
-----------------------------------

SRFI-55 provides `require-extension' which is a portable mechanism to
load selected SRFI modules.  This is implemented in the Guile core,
there's no module needed to get SRFI-55 itself.

 -- library syntax: require-extension clause...
     Require each of the given CLAUSE features, throwing an error if
     any are unavailable.

     A CLAUSE is of the form `(IDENTIFIER arg...)'.  The only
     IDENTIFIER currently supported is `srfi' and the arguments are
     SRFI numbers.  For example to get SRFI-1 and SRFI-6,

          (require-extension (srfi 1 6))

     `require-extension' can only be used at the top-level.

     A Guile-specific program can simply `use-modules' to load SRFIs
     not already in the core, `require-extension' is for programs
     designed to be portable to other Scheme implementations.

7.5.30 SRFI-60 - Integers as Bits
---------------------------------

This SRFI provides various functions for treating integers as bits and
for bitwise manipulations.  These functions can be obtained with,

     (use-modules (srfi srfi-60))

   Integers are treated as infinite precision twos-complement, the same
as in the core logical functions (*note Bitwise Operations::).  And
likewise bit indexes start from 0 for the least significant bit.  The
following functions in this SRFI are already in the Guile core,

     `logand', `logior', `logxor', `lognot', `logtest', `logcount',
     `integer-length', `logbit?', `ash'


 -- Function: bitwise-and n1 ...
 -- Function: bitwise-ior n1 ...
 -- Function: bitwise-xor n1 ...
 -- Function: bitwise-not n
 -- Function: any-bits-set? j k
 -- Function: bit-set? index n
 -- Function: arithmetic-shift n count
 -- Function: bit-field n start end
 -- Function: bit-count n
     Aliases for `logand', `logior', `logxor', `lognot', `logtest',
     `logbit?', `ash', `bit-extract' and `logcount' respectively.

     Note that the name `bit-count' conflicts with `bit-count' in the
     core (*note Bit Vectors::).

 -- Function: bitwise-if mask n1 n0
 -- Function: bitwise-merge mask n1 n0
     Return an integer with bits selected from N1 and N0 according to
     MASK.  Those bits where MASK has 1s are taken from N1, and those
     where MASK has 0s are taken from N0.

          (bitwise-if 3 #b0101 #b1010) => 9

 -- Function: log2-binary-factors n
 -- Function: first-set-bit n
     Return a count of how many factors of 2 are present in N.  This is
     also the bit index of the lowest 1 bit in N.  If N is 0, the
     return is -1.

          (log2-binary-factors 6) => 1
          (log2-binary-factors -8) => 3

 -- Function: copy-bit index n newbit
     Return N with the bit at INDEX set according to NEWBIT.  NEWBIT
     should be `#t' to set the bit to 1, or `#f' to set it to 0.  Bits
     other than at INDEX are unchanged in the return.

          (copy-bit 1 #b0101 #t) => 7

 -- Function: copy-bit-field n newbits start end
     Return N with the bits from START (inclusive) to END (exclusive)
     changed to the value NEWBITS.

     The least significant bit in NEWBITS goes to START, the next to
     START+1, etc.  Anything in NEWBITS past the END given is ignored.

          (copy-bit-field #b10000 #b11 1 3) => #b10110

 -- Function: rotate-bit-field n count start end
     Return N with the bit field from START (inclusive) to END
     (exclusive) rotated upwards by COUNT bits.

     COUNT can be positive or negative, and it can be more than the
     field width (it'll be reduced modulo the width).

          (rotate-bit-field #b0110 2 1 4) => #b1010

 -- Function: reverse-bit-field n start end
     Return N with the bits from START (inclusive) to END (exclusive)
     reversed.

          (reverse-bit-field #b101001 2 4) => #b100101

 -- Function: integer->list n [len]
     Return bits from N in the form of a list of `#t' for 1 and `#f'
     for 0.  The least significant LEN bits are returned, and the first
     list element is the most significant of those bits.  If LEN is not
     given, the default is `(integer-length N)' (*note Bitwise
     Operations::).

          (integer->list 6)   => (#t #t #f)
          (integer->list 1 4) => (#f #f #f #t)

 -- Function: list->integer lst
 -- Function: booleans->integer bool...
     Return an integer formed bitwise from the given LST list of
     booleans, or for `booleans->integer' from the BOOL arguments.

     Each boolean is `#t' for a 1 and `#f' for a 0.  The first element
     becomes the most significant bit in the return.

          (list->integer '(#t #f #t #f)) => 10

7.5.31 SRFI-61 - A more general `cond' clause
---------------------------------------------

This SRFI extends RnRS `cond' to support test expressions that return
multiple values, as well as arbitrary definitions of test success.
SRFI 61 is implemented in the Guile core; there's no module needed to
get SRFI-61 itself.  Extended `cond' is documented in *note Simple
Conditional Evaluation: Conditionals.

7.5.32 SRFI-67 - Compare procedures
-----------------------------------

See the specification of SRFI-67
(http://srfi.schemers.org/srfi-67/srfi-67.html).

7.5.33 SRFI-69 - Basic hash tables
----------------------------------

This is a portable wrapper around Guile's built-in hash table and weak
table support.  *Note Hash Tables::, for information on that built-in
support.  Above that, this hash-table interface provides association of
equality and hash functions with tables at creation time, so variants
of each function are not required, as well as a procedure that takes
care of most uses for Guile hash table handles, which this SRFI does
not provide as such.

   Access it with:

     (use-modules (srfi srfi-69))

7.5.33.1 Creating hash tables
.............................

 -- Scheme Procedure: make-hash-table [equal-proc hash-proc #:weak
          weakness start-size]
     Create and answer a new hash table with EQUAL-PROC as the equality
     function and HASH-PROC as the hashing function.

     By default, EQUAL-PROC is `equal?'.  It can be any two-argument
     procedure, and should answer whether two keys are the same for
     this table's purposes.

     My default HASH-PROC assumes that `equal-proc' is no coarser than
     `equal?'  unless it is literally `string-ci=?'.  If provided,
     HASH-PROC should be a two-argument procedure that takes a key and
     the current table size, and answers a reasonably good hash integer
     between 0 (inclusive) and the size (exclusive).

     WEAKNESS should be `#f' or a symbol indicating how "weak" the hash
     table is:

    `#f'
          An ordinary non-weak hash table.  This is the default.

    `key'
          When the key has no more non-weak references at GC, remove
          that entry.

    `value'
          When the value has no more non-weak references at GC, remove
          that entry.

    `key-or-value'
          When either has no more non-weak references at GC, remove the
          association.

     As a legacy of the time when Guile couldn't grow hash tables,
     START-SIZE is an optional integer argument that specifies the
     approximate starting size for the hash table, which will be
     rounded to an algorithmically-sounder number.

   By "coarser" than `equal?', we mean that for all X and Y values
where `(EQUAL-PROC X Y)', `(equal? X Y)' as well.  If that does not
hold for your EQUAL-PROC, you must provide a HASH-PROC.

   In the case of weak tables, remember that "references" above always
refers to `eq?'-wise references.  Just because you have a reference to
some string `"foo"' doesn't mean that an association with key `"foo"'
in a weak-key table _won't_ be collected; it only counts as a reference
if the two `"foo"'s are `eq?', regardless of EQUAL-PROC.  As such, it
is usually only sensible to use `eq?' and `hashq' as the equivalence
and hash functions for a weak table.  *Note Weak References::, for more
information on Guile's built-in weak table support.

 -- Scheme Procedure: alist->hash-table alist [equal-proc hash-proc
          #:weak weakness start-size]
     As with `make-hash-table', but initialize it with the associations
     in ALIST.  Where keys are repeated in ALIST, the leftmost
     association takes precedence.

7.5.33.2 Accessing table items
..............................

 -- Scheme Procedure: hash-table-ref table key [default-thunk]
 -- Scheme Procedure: hash-table-ref/default table key default
     Answer the value associated with KEY in TABLE.  If KEY is not
     present, answer the result of invoking the thunk DEFAULT-THUNK,
     which signals an error instead by default.

     `hash-table-ref/default' is a variant that requires a third
     argument, DEFAULT, and answers DEFAULT itself instead of invoking
     it.

 -- Scheme Procedure: hash-table-set! table key new-value
     Set KEY to NEW-VALUE in TABLE.

 -- Scheme Procedure: hash-table-delete! table key
     Remove the association of KEY in TABLE, if present.  If absent, do
     nothing.

 -- Scheme Procedure: hash-table-exists? table key
     Answer whether KEY has an association in TABLE.

 -- Scheme Procedure: hash-table-update! table key modifier
          [default-thunk]
 -- Scheme Procedure: hash-table-update!/default table key modifier
          default
     Replace KEY's associated value in TABLE by invoking MODIFIER with
     one argument, the old value.

     If KEY is not present, and DEFAULT-THUNK is provided, invoke it
     with no arguments to get the "old value" to be passed to MODIFIER
     as above.  If DEFAULT-THUNK is not provided in such a case, signal
     an error.

     `hash-table-update!/default' is a variant that requires the fourth
     argument, which is used directly as the "old value" rather than as
     a thunk to be invoked to retrieve the "old value".

7.5.33.3 Table properties
.........................

 -- Scheme Procedure: hash-table-size table
     Answer the number of associations in TABLE.  This is guaranteed to
     run in constant time for non-weak tables.

 -- Scheme Procedure: hash-table-keys table
     Answer an unordered list of the keys in TABLE.

 -- Scheme Procedure: hash-table-values table
     Answer an unordered list of the values in TABLE.

 -- Scheme Procedure: hash-table-walk table proc
     Invoke PROC once for each association in TABLE, passing the key
     and value as arguments.

 -- Scheme Procedure: hash-table-fold table proc init
     Invoke `(PROC KEY VALUE PREVIOUS)' for each KEY and VALUE in
     TABLE, where PREVIOUS is the result of the previous invocation,
     using INIT as the first PREVIOUS value.  Answer the final PROC
     result.

 -- Scheme Procedure: hash-table->alist table
     Answer an alist where each association in TABLE is an association
     in the result.

7.5.33.4 Hash table algorithms
..............................

Each hash table carries an "equivalence function" and a "hash
function", used to implement key lookups.  Beginning users should
follow the rules for consistency of the default HASH-PROC specified
above.  Advanced users can use these to implement their own equivalence
and hash functions for specialized lookup semantics.

 -- Scheme Procedure: hash-table-equivalence-function hash-table
 -- Scheme Procedure: hash-table-hash-function hash-table
     Answer the equivalence and hash function of HASH-TABLE,
     respectively.

 -- Scheme Procedure: hash obj [size]
 -- Scheme Procedure: string-hash obj [size]
 -- Scheme Procedure: string-ci-hash obj [size]
 -- Scheme Procedure: hash-by-identity obj [size]
     Answer a hash value appropriate for equality predicate `equal?',
     `string=?', `string-ci=?', and `eq?', respectively.

   `hash' is a backwards-compatible replacement for Guile's built-in
`hash'.

7.5.34 SRFI-88 Keyword Objects
------------------------------

SRFI-88 (http://srfi.schemers.org/srfi-88/srfi-88.html) provides
"keyword objects", which are equivalent to Guile's keywords (*note
Keywords::).  SRFI-88 keywords can be entered using the "postfix
keyword syntax", which consists of an identifier followed by `:' (*note
`postfix' keyword syntax: Scheme Read.).  SRFI-88 can be made available
with:

     (use-modules (srfi srfi-88))

   Doing so installs the right reader option for keyword syntax, using
`(read-set! keywords 'postfix)'.  It also provides the procedures
described below.

 -- Scheme Procedure: keyword? obj
     Return `#t' if OBJ is a keyword.  This is the same procedure as
     the same-named built-in procedure (*note `keyword?': Keyword
     Procedures.).

          (keyword? foo:)         => #t
          (keyword? 'foo:)        => #t
          (keyword? "foo")        => #f

 -- Scheme Procedure: keyword->string kw
     Return the name of KW as a string, i.e., without the trailing
     colon.  The returned string may not be modified, e.g., with
     `string-set!'.

          (keyword->string foo:)  => "foo"

 -- Scheme Procedure: string->keyword str
     Return the keyword object whose name is STR.

          (keyword->string (string->keyword "a b c"))     => "a b c"

7.5.35 SRFI-98 Accessing environment variables.
-----------------------------------------------

This is a portable wrapper around Guile's built-in support for
interacting with the current environment, *Note Runtime Environment::.

 -- Scheme Procedure: get-environment-variable name
     Returns a string containing the value of the environment variable
     given by the string `name', or `#f' if the named environment
     variable is not found.  This is equivalent to `(getenv name)'.

 -- Scheme Procedure: get-environment-variables
     Returns the names and values of all the environment variables as an
     association list in which both the keys and the values are strings.

7.6 R6RS Support
================

*Note R6RS Libraries::, for more information on how to define R6RS
libraries, and their integration with Guile modules.

7.6.1 Incompatibilities with the R6RS
-------------------------------------

There are some incompatibilities between Guile and the R6RS.  Some of
them are intentional, some of them are bugs, and some are simply
unimplemented features.  Please let the Guile developers know if you
find one that is not on this list.

   * The R6RS specifies many situations in which a conforming
     implementation must signal a specific error.  Guile doesn't really
     care about that too much--if a correct R6RS program would not hit
     that error, we don't bother checking for it.

   * Multiple `library' forms in one file are not yet supported.  This
     is because the expansion of `library' sets the current module, but
     does not restore it.  This is a bug.

   * R6RS unicode escapes within strings are disabled by default,
     because they conflict with Guile's already-existing escapes. The
     same is the case for R6RS treatment of escaped newlines in strings.

     R6RS behavior can be turned on via a reader option. *Note String
     Syntax::, for more information.

   * A `set!' to a variable transformer may only expand to an
     expression, not a definition--even if the original `set!'
     expression was in definition context.

   * Instead of using the algorithm detailed in chapter 10 of the R6RS,
     expansion of toplevel forms happens sequentially.

     For example, while the expansion of the following set of toplevel
     definitions does the correct thing:

          (begin
           (define even?
             (lambda (x)
               (or (= x 0) (odd? (- x 1)))))
           (define-syntax odd?
             (syntax-rules ()
               ((odd? x) (not (even? x)))))
           (even? 10))
          => #t

     The same definitions outside of the `begin' wrapper do not:

          (define even?
            (lambda (x)
              (or (= x 0) (odd? (- x 1)))))
          (define-syntax odd?
            (syntax-rules ()
              ((odd? x) (not (even? x)))))
          (even? 10)
          <unnamed port>:4:18: In procedure even?:
          <unnamed port>:4:18: Wrong type to apply: #<syntax-transformer odd?>

     This is because when expanding the right-hand-side of `even?', the
     reference to `odd?' is not yet marked as a syntax transformer, so
     it is assumed to be a function.

     This bug will only affect top-level programs, not code in `library'
     forms.  Fixing it for toplevel forms seems doable, but tricky to
     implement in a backward-compatible way. Suggestions and/or patches
     would be appreciated.

   * The `(rnrs io ports)' module is incomplete.  Work is ongoing to
     fix this.

   * Guile does not prevent use of textual I/O procedures on binary
     ports.  More generally, it does not make a sharp distinction
     between binary and textual ports (*note binary-port?: R6RS Port
     Manipulation.).

7.6.2 R6RS Standard Libraries
-----------------------------

In contrast with earlier versions of the Revised Report, the R6RS
organizes the procedures and syntactic forms required of conforming
implementations into a set of "standard libraries" which can be
imported as necessary by user programs and libraries.  Here we briefly
list the libraries that have been implemented for Guile.

   We do not attempt to document these libraries fully here, as most of
their functionality is already available in Guile itself.  The
expectation is that most Guile users will use the well-known and
well-documented Guile modules.  These R6RS libraries are mostly useful
to users who want to port their code to other R6RS systems.

   The documentation in the following sections reproduces some of the
content of the library section of the Report, but is mostly intended to
provide supplementary information about Guile's implementation of the
R6RS standard libraries.  For complete documentation, design rationales
and further examples, we advise you to consult the "Standard Libraries"
section of the Report (*note R6RS Standard Libraries: (r6rs)Standard
Libraries.).

7.6.2.1 Library Usage
.....................

Guile implements the R6RS `library' form as a transformation to a native
Guile module definition.  As a consequence of this, all of the libraries
described in the following subsections, in addition to being available
for use by R6RS libraries and top-level programs, can also be imported
as if they were normal Guile modules--via a `use-modules' form, say.
For example, the R6RS "composite" library can be imported by:

       (import (rnrs (6)))

       (use-modules ((rnrs) :version (6)))

   For more information on Guile's library implementation, see (*note
R6RS Libraries::).

7.6.2.2 rnrs base
.................

The `(rnrs base (6))' library exports the procedures and syntactic
forms described in the main section of the Report (*note R6RS Base
library: (r6rs)Base library.).  They are grouped below by the existing
manual sections to which they correspond.

 -- Scheme Procedure: boolean? obj
 -- Scheme Procedure: not x
     *Note Booleans::, for documentation.

 -- Scheme Procedure: symbol? obj
 -- Scheme Procedure: symbol->string sym
 -- Scheme Procedure: string->symbol str
     *Note Symbol Primitives::, for documentation.

 -- Scheme Procedure: char? obj
 -- Scheme Procedure: char=?
 -- Scheme Procedure: char<?
 -- Scheme Procedure: char>?
 -- Scheme Procedure: char<=?
 -- Scheme Procedure: char>=?
 -- Scheme Procedure: integer->char n
 -- Scheme Procedure: char->integer chr
     *Note Characters::, for documentation.

 -- Scheme Procedure: list? x
 -- Scheme Procedure: null? x
     *Note List Predicates::, for documentation.

 -- Scheme Procedure: pair? x
 -- Scheme Procedure: cons x y
 -- Scheme Procedure: car pair
 -- Scheme Procedure: cdr pair
 -- Scheme Procedure: caar pair
 -- Scheme Procedure: cadr pair
 -- Scheme Procedure: cdar pair
 -- Scheme Procedure: cddr pair
 -- Scheme Procedure: caaar pair
 -- Scheme Procedure: caadr pair
 -- Scheme Procedure: cadar pair
 -- Scheme Procedure: cdaar pair
 -- Scheme Procedure: caddr pair
 -- Scheme Procedure: cdadr pair
 -- Scheme Procedure: cddar pair
 -- Scheme Procedure: cdddr pair
 -- Scheme Procedure: caaaar pair
 -- Scheme Procedure: caaadr pair
 -- Scheme Procedure: caadar pair
 -- Scheme Procedure: cadaar pair
 -- Scheme Procedure: cdaaar pair
 -- Scheme Procedure: cddaar pair
 -- Scheme Procedure: cdadar pair
 -- Scheme Procedure: cdaadr pair
 -- Scheme Procedure: cadadr pair
 -- Scheme Procedure: caaddr pair
 -- Scheme Procedure: caddar pair
 -- Scheme Procedure: cadddr pair
 -- Scheme Procedure: cdaddr pair
 -- Scheme Procedure: cddadr pair
 -- Scheme Procedure: cdddar pair
 -- Scheme Procedure: cddddr pair
     *Note Pairs::, for documentation.

 -- Scheme Procedure: number? obj
     *Note Numerical Tower::, for documentation.

 -- Scheme Procedure: string? obj
     *Note String Predicates::, for documentation.

 -- Scheme Procedure: procedure? obj
     *Note Procedure Properties::, for documentation.

 -- Scheme Syntax: define name value
 -- Scheme Syntax: set! variable-name value
     *Note Definition::, for documentation.

 -- Scheme Syntax: define-syntax keyword expression
 -- Scheme Syntax: let-syntax ((keyword transformer) ...) exp ...
 -- Scheme Syntax: letrec-syntax ((keyword transformer) ...) exp ...
     *Note Defining Macros::, for documentation.

 -- Scheme Syntax: identifier-syntax exp
     *Note Identifier Macros::, for documentation.

 -- Scheme Syntax: syntax-rules literals (pattern template) ...
     *Note Syntax Rules::, for documentation.

 -- Scheme Syntax: lambda formals body
     *Note Lambda::, for documentation.

 -- Scheme Syntax: let bindings body
 -- Scheme Syntax: let* bindings body
 -- Scheme Syntax: letrec bindings body
 -- Scheme Syntax: letrec* bindings body
     *Note Local Bindings::, for documentation.

 -- Scheme Syntax: let-values bindings body
 -- Scheme Syntax: let*-values bindings body
     *Note SRFI-11::, for documentation.

 -- Scheme Syntax: begin expr1 expr2 ...
     *Note begin::, for documentation.

 -- Scheme Syntax: quote expr
 -- Scheme Syntax: quasiquote expr
 -- Scheme Syntax: unquote expr
 -- Scheme Syntax: unquote-splicing expr
     *Note Expression Syntax::, for documentation.

 -- Scheme Syntax: if test consequence [alternate]
 -- Scheme Syntax: cond clause1 clause2 ...
 -- Scheme Syntax: case key clause1 clause2 ...
     *Note Conditionals::, for documentation.

 -- Scheme Syntax: and expr ...
 -- Scheme Syntax: or expr ...
     *Note and or::, for documentation.

 -- Scheme Procedure: eq? x y
 -- Scheme Procedure: eqv? x y
 -- Scheme Procedure: equal? x y
 -- Scheme Procedure: symbol=? symbol1 symbol2 ...
     *Note Equality::, for documentation.

     `symbol=?' is identical to `eq?'.

 -- Scheme Procedure: complex? z
     *Note Complex Numbers::, for documentation.

 -- Scheme Procedure: real-part z
 -- Scheme Procedure: imag-part z
 -- Scheme Procedure: make-rectangular real_part imaginary_part
 -- Scheme Procedure: make-polar x y
 -- Scheme Procedure: magnitude z
 -- Scheme Procedure: angle z
     *Note Complex::, for documentation.

 -- Scheme Procedure: sqrt z
 -- Scheme Procedure: exp z
 -- Scheme Procedure: expt z1 z2
 -- Scheme Procedure: log z
 -- Scheme Procedure: sin z
 -- Scheme Procedure: cos z
 -- Scheme Procedure: tan z
 -- Scheme Procedure: asin z
 -- Scheme Procedure: acos z
 -- Scheme Procedure: atan z
     *Note Scientific::, for documentation.

 -- Scheme Procedure: real? x
 -- Scheme Procedure: rational? x
 -- Scheme Procedure: numerator x
 -- Scheme Procedure: denominator x
 -- Scheme Procedure: rationalize x eps
     *Note Reals and Rationals::, for documentation.

 -- Scheme Procedure: exact? x
 -- Scheme Procedure: inexact? x
 -- Scheme Procedure: exact z
 -- Scheme Procedure: inexact z
     *Note Exactness::, for documentation.  The `exact' and `inexact'
     procedures are identical to the `inexact->exact' and
     `exact->inexact' procedures provided by Guile's code library.

 -- Scheme Procedure: integer? x
     *Note Integers::, for documentation.

 -- Scheme Procedure: odd? n
 -- Scheme Procedure: even? n
 -- Scheme Procedure: gcd x ...
 -- Scheme Procedure: lcm x ...
 -- Scheme Procedure: exact-integer-sqrt k
     *Note Integer Operations::, for documentation.

 -- Scheme Procedure: =
 -- Scheme Procedure: <
 -- Scheme Procedure: >
 -- Scheme Procedure: <=
 -- Scheme Procedure: >=
 -- Scheme Procedure: zero? x
 -- Scheme Procedure: positive? x
 -- Scheme Procedure: negative? x
     *Note Comparison::, for documentation.

 -- Scheme Procedure: for-each f lst1 lst2 ...
     *Note SRFI-1 Fold and Map::, for documentation.

 -- Scheme Procedure: list elem1 ... elemN
     *Note List Constructors::, for documentation.

 -- Scheme Procedure: length lst
 -- Scheme Procedure: list-ref lst k
 -- Scheme Procedure: list-tail lst k
     *Note List Selection::, for documentation.

 -- Scheme Procedure: append lst1 ... lstN
 -- Scheme Procedure: reverse lst
     *Note Append/Reverse::, for documentation.

 -- Scheme Procedure: number->string n [radix]
 -- Scheme Procedure: string->number str [radix]
     *Note Conversion::, for documentation.

 -- Scheme Procedure: string char ...
 -- Scheme Procedure: make-string k [chr]
 -- Scheme Procedure: list->string lst
     *Note String Constructors::, for documentation.

 -- Scheme Procedure: string->list str [start [end]]
     *Note List/String Conversion::, for documentation.

 -- Scheme Procedure: string-length str
 -- Scheme Procedure: string-ref str k
 -- Scheme Procedure: string-copy str [start [end]]
 -- Scheme Procedure: substring str start [end]
     *Note String Selection::, for documentation.

 -- Scheme Procedure: string=? [s1 [s2 . rest]]
 -- Scheme Procedure: string<? [s1 [s2 . rest]]
 -- Scheme Procedure: string>? [s1 [s2 . rest]]
 -- Scheme Procedure: string<=? [s1 [s2 . rest]]
 -- Scheme Procedure: string>=? [s1 [s2 . rest]]
     *Note String Comparison::, for documentation.

 -- Scheme Procedure: string-append . args
     *Note Reversing and Appending Strings::, for documentation.

 -- Scheme Procedure: string-for-each proc s [start [end]]
     *Note Mapping Folding and Unfolding::, for documentation.

 -- Scheme Procedure: + z1 ...
 -- Scheme Procedure: - z1 z2 ...
 -- Scheme Procedure: * z1 ...
 -- Scheme Procedure: / z1 z2 ...
 -- Scheme Procedure: max x1 x2 ...
 -- Scheme Procedure: min x1 x2 ...
 -- Scheme Procedure: abs x
 -- Scheme Procedure: truncate x
 -- Scheme Procedure: floor x
 -- Scheme Procedure: ceiling x
 -- Scheme Procedure: round x
     *Note Arithmetic::, for documentation.

 -- Scheme Procedure: div x y
 -- Scheme Procedure: mod x y
 -- Scheme Procedure: div-and-mod x y
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `div' returns the integer Q and `mod'
     returns the real number R such that X = Q*Y + R and 0 <= R <
     abs(Y).  `div-and-mod' returns both Q and R, and is more efficient
     than computing each separately.  Note that when Y > 0, `div'
     returns floor(X/Y), otherwise it returns ceiling(X/Y).

          (div 123 10) => 12
          (mod 123 10) => 3
          (div-and-mod 123 10) => 12 and 3
          (div-and-mod 123 -10) => -12 and 3
          (div-and-mod -123 10) => -13 and 7
          (div-and-mod -123 -10) => 13 and 7
          (div-and-mod -123.2 -63.5) => 2.0 and 3.8
          (div-and-mod 16/3 -10/7) => -3 and 22/21

 -- Scheme Procedure: div0 x y
 -- Scheme Procedure: mod0 x y
 -- Scheme Procedure: div0-and-mod0 x y
     These procedures accept two real numbers X and Y, where the
     divisor Y must be non-zero.  `div0' returns the integer Q and
     `mod0' returns the real number R such that X = Q*Y + R and
     -abs(Y/2) <= R < abs(Y/2).  `div0-and-mod0' returns both Q and R,
     and is more efficient than computing each separately.

     Note that `div0' returns X/Y rounded to the nearest integer.  When
     X/Y lies exactly half-way between two integers, the tie is broken
     according to the sign of Y.  If Y > 0, ties are rounded toward
     positive infinity, otherwise they are rounded toward negative
     infinity.  This is a consequence of the requirement that -abs(Y/2)
     <= R < abs(Y/2).

          (div0 123 10) => 12
          (mod0 123 10) => 3
          (div0-and-mod0 123 10) => 12 and 3
          (div0-and-mod0 123 -10) => -12 and 3
          (div0-and-mod0 -123 10) => -12 and -3
          (div0-and-mod0 -123 -10) => 12 and -3
          (div0-and-mod0 -123.2 -63.5) => 2.0 and 3.8
          (div0-and-mod0 16/3 -10/7) => -4 and -8/21

 -- Scheme Procedure: real-valued? obj
 -- Scheme Procedure: rational-valued? obj
 -- Scheme Procedure: integer-valued? obj
     These procedures return `#t' if and only if their arguments can,
     respectively, be coerced to a real, rational, or integer value
     without a loss of numerical precision.

     `real-valued?' will return `#t' for complex numbers whose
     imaginary parts are zero.

 -- Scheme Procedure: nan? x
 -- Scheme Procedure: infinite? x
 -- Scheme Procedure: finite? x
     `nan?' returns `#t' if X is a NaN value, `#f' otherwise.
     `infinite?' returns `#t' if X is an infinite value, `#f'
     otherwise.  `finite?' returns `#t' if X is neither infinite nor a
     NaN value, otherwise it returns `#f'.  Every real number satisfies
     exactly one of these predicates.  An exception is raised if X is
     not real.

 -- Scheme Syntax: assert expr
     Raises an `&assertion' condition if EXPR evaluates to `#f';
     otherwise evaluates to the value of EXPR.

 -- Scheme Procedure: error who message irritant1 ...
 -- Scheme Procedure: assertion-violation who message irritant1 ...
     These procedures raise compound conditions based on their
     arguments: If WHO is not `#f', the condition will include a `&who'
     condition whose `who' field is set to WHO; a `&message' condition
     will be included with a `message' field equal to MESSAGE; an
     `&irritants' condition will be included with its `irritants' list
     given by `irritant1 ...'.

     `error' produces a compound condition with the simple conditions
     described above, as well as an `&error' condition;
     `assertion-violation' produces one that includes an `&assertion'
     condition.

 -- Scheme Procedure: vector-map proc v
 -- Scheme Procedure: vector-for-each proc v
     These procedures implement the `map' and `for-each' contracts over
     vectors.

 -- Scheme Procedure: vector . l
 -- Scheme Procedure: vector? obj
 -- Scheme Procedure: make-vector len
 -- Scheme Procedure: make-vector len fill
 -- Scheme Procedure: list->vector l
 -- Scheme Procedure: vector->list v
     *Note Vector Creation::, for documentation.

 -- Scheme Procedure: vector-length vector
 -- Scheme Procedure: vector-ref vector k
 -- Scheme Procedure: vector-set! vector k obj
 -- Scheme Procedure: vector-fill! v fill
     *Note Vector Accessors::, for documentation.

 -- Scheme Procedure: call-with-current-continuation proc
 -- Scheme Procedure: call/cc proc
     *Note Continuations::, for documentation.

 -- Scheme Procedure: values arg1 ... argN
 -- Scheme Procedure: call-with-values producer consumer
     *Note Multiple Values::, for documentation.

 -- Scheme Procedure: dynamic-wind in_guard thunk out_guard
     *Note Dynamic Wind::, for documentation.

 -- Scheme Procedure: apply proc arg1 ... argN arglst
     *Note Fly Evaluation::, for documentation.

7.6.2.3 rnrs unicode
....................

The `(rnrs unicode (6))' library provides procedures for manipulating
Unicode characters and strings.

 -- Scheme Procedure: char-upcase char
 -- Scheme Procedure: char-downcase char
 -- Scheme Procedure: char-titlecase char
 -- Scheme Procedure: char-foldcase char
     These procedures translate their arguments from one Unicode
     character set to another.  `char-upcase', `char-downcase', and
     `char-titlecase' are identical to their counterparts in the Guile
     core library; *Note Characters::, for documentation.

     `char-foldcase' returns the result of applying `char-upcase' to
     its argument, followed by `char-downcase'--except in the case of
     the Turkic characters `U+0130' and `U+0131', for which the
     procedure acts as the identity function.

 -- Scheme Procedure: char-ci=? char1 char2 char3 ...
 -- Scheme Procedure: char-ci<? char1 char2 char3 ...
 -- Scheme Procedure: char-ci>? char1 char2 char3 ...
 -- Scheme Procedure: char-ci<=? char1 char2 char3 ...
 -- Scheme Procedure: char-ci>=? char1 char2 char3 ...
     These procedures facilitate case-insensitive comparison of Unicode
     characters.  They are identical to the procedures provided by
     Guile's core library.  *Note Characters::, for documentation.

 -- Scheme Procedure: char-alphabetic? char
 -- Scheme Procedure: char-numeric? char
 -- Scheme Procedure: char-whitespace? char
 -- Scheme Procedure: char-upper-case? char
 -- Scheme Procedure: char-lower-case? char
 -- Scheme Procedure: char-title-case? char
     These procedures implement various Unicode character set
     predicates.  They are identical to the procedures provided by
     Guile's core library.  *Note Characters::, for documentation.

 -- Scheme Procedure: char-general-category char
     *Note Characters::, for documentation.

 -- Scheme Procedure: string-upcase string
 -- Scheme Procedure: string-downcase string
 -- Scheme Procedure: string-titlecase string
 -- Scheme Procedure: string-foldcase string
     These procedures perform Unicode case folding operations on their
     input.  *Note Alphabetic Case Mapping::, for documentation.

 -- Scheme Procedure: string-ci=? string1 string2 string3 ...
 -- Scheme Procedure: string-ci<? string1 string2 string3 ...
 -- Scheme Procedure: string-ci>? string1 string2 string3 ...
 -- Scheme Procedure: string-ci<=? string1 string2 string3 ...
 -- Scheme Procedure: string-ci>=? string1 string2 string3 ...
     These procedures perform case-insensitive comparison on their
     input.  *Note String Comparison::, for documentation.

 -- Scheme Procedure: string-normalize-nfd string
 -- Scheme Procedure: string-normalize-nfkd string
 -- Scheme Procedure: string-normalize-nfc string
 -- Scheme Procedure: string-normalize-nfkc string
     These procedures perform Unicode string normalization operations on
     their input.  *Note String Comparison::, for documentation.

7.6.2.4 rnrs bytevectors
........................

The `(rnrs bytevectors (6))' library provides procedures for working
with blocks of binary data.  This functionality is documented in its
own section of the manual; *Note Bytevectors::.

7.6.2.5 rnrs lists
..................

The `(rnrs lists (6))' library provides procedures additional
procedures for working with lists.

 -- Scheme Procedure: find proc list
     This procedure is identical to the one defined in Guile's SRFI-1
     implementation.  *Note SRFI-1 Searching::, for documentation.

 -- Scheme Procedure: for-all proc list1 list2 ...
 -- Scheme Procedure: exists proc list1 list2 ...
     The `for-all' procedure is identical to the `every' procedure
     defined by SRFI-1; the `exists' procedure is identical to SRFI-1's
     `any'.  *Note SRFI-1 Searching::, for documentation.

 -- Scheme Procedure: filter proc list
 -- Scheme Procedure: partition proc list
     These procedures are identical to the ones provided by SRFI-1.
     *Note List Modification::, for a description of `filter'; *Note
     SRFI-1 Filtering and Partitioning::, for `partition'.

 -- Scheme Procedure: fold-left combine nil list1 list2 ... listn
 -- Scheme Procedure: fold-right combine nil list1 list2 ... listn
     These procedures are identical to the `fold' and `fold-right'
     procedures provided by SRFI-1.  *Note SRFI-1 Fold and Map::, for
     documentation.

 -- Scheme Procedure: remp proc list
 -- Scheme Procedure: remove obj list
 -- Scheme Procedure: remv obj list
 -- Scheme Procedure: remq obj list
     `remove', `remv', and `remq' are identical to the `delete',
     `delv', and `delq' procedures provided by Guile's core library,
     (*note List Modification::).  `remp' is identical to the alternate
     `remove' procedure provided by SRFI-1; *Note SRFI-1 Deleting::.

 -- Scheme Procedure: memp proc list
 -- Scheme Procedure: member obj list
 -- Scheme Procedure: memv obj list
 -- Scheme Procedure: memq obj list
     `member', `memv', and `memq' are identical to the procedures
     provided by Guile's core library; *Note List Searching::, for
     their documentation.  `memp' uses the specified predicate function
     `proc' to test elements of the list LIST--it behaves similarly to
     `find', except that it returns the first sublist of LIST whose
     `car' satisfies PROC.

 -- Scheme Procedure: assp proc alist
 -- Scheme Procedure: assoc obj alist
 -- Scheme Procedure: assv obj alist
 -- Scheme Procedure: assq obj alist
     `assoc', `assv', and `assq' are identical to the procedures
     provided by Guile's core library; *Note Alist Key Equality::, for
     their documentation.  `assp' uses the specified predicate function
     `proc' to test keys in the association list ALIST.

 -- Scheme Procedure: cons* obj1 ... obj
 -- Scheme Procedure: cons* obj
     This procedure is identical to the one exported by Guile's core
     library.  *Note List Constructors::, for documentation.

7.6.2.6 rnrs sorting
....................

The `(rnrs sorting (6))' library provides procedures for sorting lists
and vectors.

 -- Scheme Procedure: list-sort proc list
 -- Scheme Procedure: vector-sort proc vector
     These procedures return their input sorted in ascending order,
     without modifying the original data.  PROC must be a procedure
     that takes two elements from the input list or vector as
     arguments, and returns a true value if the first is "less" than
     the second, `#f' otherwise.  `list-sort' returns a list;
     `vector-sort' returns a vector.

     Both `list-sort' and `vector-sort' are implemented in terms of the
     `stable-sort' procedure from Guile's core library.  *Note
     Sorting::, for a discussion of the behavior of that procedure.

 -- Scheme Procedure: vector-sort! proc vector
     Performs a destructive, "in-place" sort of VECTOR, using PROC as
     described above to determine an ascending ordering of elements.
     `vector-sort!' returns an unspecified value.

     This procedure is implemented in terms of the `sort!' procedure
     from Guile's core library.  *Note Sorting::, for more information.

7.6.2.7 rnrs control
....................

The `(rnrs control (6))' library provides syntactic forms useful for
constructing conditional expressions and controlling the flow of
execution.

 -- Scheme Syntax: when test expression1 expression2 ...
 -- Scheme Syntax: unless test expression1 expression2 ...
     The `when' form is evaluated by evaluating the specified TEST
     expression; if the result is a true value, the EXPRESSIONs that
     follow it are evaluated in order, and the value of the final
     EXPRESSION becomes the value of the entire `when' expression.

     The `unless' form behaves similarly, with the exception that the
     specified EXPRESSIONs are only evaluated if the value of TEST is
     false.

 -- Scheme Syntax: do ((variable init step) ...) (test expression ...)
          command ...
     This form is identical to the one provided by Guile's core library.
     *Note while do::, for documentation.

 -- Scheme Syntax: case-lambda clause ...
     This form is identical to the one provided by Guile's core library.
     *Note Case-lambda::, for documentation.

7.6.2.8 R6RS Records
....................

The manual sections below describe Guile's implementation of R6RS
records, which provide support for user-defined data types.  The R6RS
records API provides a superset of the features provided by Guile's
"native" records, as well as those of the SRFI-9 records API; *Note
Records::, and *note SRFI-9::, for a description of those interfaces.

   As with SRFI-9 and Guile's native records, R6RS records are
constructed using a record-type descriptor that specifies attributes
like the record's name, its fields, and the mutability of those fields.

   R6RS records extend this framework to support single inheritance via
the specification of a "parent" type for a record type at definition
time.  Accessors and mutator procedures for the fields of a parent type
may be applied to records of a subtype of this parent.  A record type
may be "sealed", in which case it cannot be used as the parent of
another record type.

   The inheritance mechanism for record types also informs the process
of initializing the fields of a record and its parents.  Constructor
procedures that generate new instances of a record type are obtained
from a record constructor descriptor, which encapsulates the record-type
descriptor of the record to be constructed along with a "protocol"
procedure that defines how constructors for record subtypes delegate to
the constructors of their parent types.

   A protocol is a procedure used by the record system at construction
time to bind arguments to the fields of the record being constructed.
The protocol procedure is passed a procedure N that accepts the
arguments required to construct the record's parent type; this
procedure, when invoked, will return a procedure P that accepts the
arguments required to construct a new instance of the record type
itself and returns a new instance of the record type.

   The protocol should in turn return a procedure that uses N and P to
initialize the fields of the record type and its parent type(s).  This
procedure will be the constructor returned by

   As a trivial example, consider the hypothetical record type `pixel',
which encapsulates an x-y location on a screen, and `voxel', which has
`pixel' as its parent type and stores an additional coordinate.  The
following protocol produces a constructor procedure that accepts all
three coordinates, uses the first two to initialize the fields of
`pixel', and binds the third to the single field of `voxel'.

       (lambda (n)
         (lambda (x y z)
           (let ((p (n x y)))
             (p z))))

   It may be helpful to think of protocols as "constructor factories"
that produce chains of delegating constructors glued together by the
helper procedure N.

   An R6RS record type may be declared to be "nongenerative" via the
use of a unique generated or user-supplied symbol--or "uid"--such that
subsequent record type declarations with the same uid and attributes
will return the previously-declared record-type descriptor.

   R6RS record types may also be declared to be "opaque", in which case
the various predicates and introspection procedures defined in `(rnrs
records introspection)' will behave as if records of this type are not
records at all.

   Note that while the R6RS records API shares much of its namespace
with both the SRFI-9 and native Guile records APIs, it is not currently
compatible with either.

7.6.2.9 rnrs records syntactic
..............................

The `(rnrs records syntactic (6))' library exports the syntactic API
for working with R6RS records.

 -- Scheme Syntax: define-record-type name-spec record-clause*
     Defines a new record type, introducing bindings for a record-type
     descriptor, a record constructor descriptor, a constructor
     procedure, a record predicate, and accessor and mutator procedures
     for the new record type's fields.

     NAME-SPEC must either be an identifier or must take the form
     `(record-name constructor-name predicate-name)', where
     RECORD-NAME, CONSTRUCTOR-NAME, and PREDICATE-NAME are all
     identifiers and specify the names to which, respectively, the
     record-type descriptor, constructor, and predicate procedures will
     be bound.  If NAME-SPEC is only an identifier, it specifies the
     name to which the generated record-type descriptor will be bound.

     Each RECORD-CLAUSE must be one of the following:

        * `(fields field-spec*)', where each FIELD-SPEC specifies a
          field of the new record type and takes one of the following
          forms:
             * `(immutable field-name accessor-name)', which specifies
               an immutable field with the name FIELD-NAME and binds an
               accessor procedure for it to the name given by
               ACCESSOR-NAME

             * `(mutable field-name accessor-name mutator-name)', which
               specifies a mutable field with the name FIELD-NAME and
               binds accessor and mutator procedures to ACCESSOR-NAME
               and MUTATOR-NAME, respectively

             * `(immutable field-name)', which specifies an immutable
               field with the name FIELD-NAME; an accessor procedure
               for it will be created and named by appending record
               name and FIELD-NAME with a hyphen separator

             * `(mutable field-name'), which specifies a mutable field
               with the name FIELD-NAME; an accessor procedure for it
               will be created and named as described above; a mutator
               procedure will also be created and named by appending
               `-set!' to the accessor name

             * `field-name', which specifies an immutable field with
               the name FIELD-NAME; an access procedure for it will be
               created and named as described above

        * `(parent parent-name)', where PARENT-NAME is a symbol giving
          the name of the record type to be used as the parent of the
          new record type

        * `(protocol expression)', where EXPRESSION evaluates to a
          protocol procedure which behaves as described above, and is
          used to create a record constructor descriptor for the new
          record type

        * `(sealed sealed?)', where SEALED? is a boolean value that
          specifies whether or not the new record type is sealed

        * `(opaque opaque?)', where OPAQUE? is a boolean value that
          specifies whether or not the new record type is opaque

        * `(nongenerative [uid])', which specifies that the record type
          is nongenerative via the optional uid UID.  If UID is not
          specified, a unique uid will be generated at expansion time

        * `(parent-rtd parent-rtd parent-cd)', a more explicit form of
          the `parent' form above; PARENT-RTD and PARENT-CD should
          evaluate to a record-type descriptor and a record constructor
          descriptor, respectively

 -- Scheme Syntax: record-type-descriptor record-name
     Evaluates to the record-type descriptor associated with the type
     specified by RECORD-NAME.

 -- Scheme Syntax: record-constructor-descriptor record-name
     Evaluates to the record-constructor descriptor associated with the
     type specified by RECORD-NAME.

7.6.2.10 rnrs records procedural
................................

The `(rnrs records procedural (6))' library exports the procedural API
for working with R6RS records.

 -- Scheme Procedure: make-record-type-descriptor name parent uid
          sealed? opaque? fields
     Returns a new record-type descriptor with the specified
     characteristics: NAME must be a symbol giving the name of the new
     record type; PARENT must be either `#f' or a non-sealed record-type
     descriptor for the returned record type to extend; UID must be
     either `#f', indicating that the record type is generative, or a
     symbol giving the type's nongenerative uid; SEALED? and OPAQUE?
     must be boolean values that specify the sealedness and opaqueness
     of the record type; FIELDS must be a vector of zero or more field
     specifiers of the form `(mutable name)' or `(immutable name)',
     where name is a symbol giving a name for the field.

     If UID is not `#f', it must be a symbol

 -- Scheme Procedure: record-type-descriptor? obj
     Returns `#t' if OBJ is a record-type descriptor, `#f' otherwise.

 -- Scheme Procedure: make-record-constructor-descriptor rtd
          parent-constructor-descriptor protocol
     Returns a new record constructor descriptor that can be used to
     produce constructors for the record type specified by the
     record-type descriptor RTD and whose delegation and binding
     behavior are specified by the protocol procedure PROTOCOL.

     PARENT-CONSTRUCTOR-DESCRIPTOR specifies a record constructor
     descriptor for the parent type of RTD, if one exists.  If RTD
     represents a base type, then PARENT-CONSTRUCTOR-DESCRIPTOR must be
     `#f'.  If RTD is an extension of another type,
     PARENT-CONSTRUCTOR-DESCRIPTOR may still be `#f', but protocol must
     also be `#f' in this case.

 -- Scheme Procedure: record-constructor rcd
     Returns a record constructor procedure by invoking the protocol
     defined by the record-constructor descriptor RCD.

 -- Scheme Procedure: record-predicate rtd
     Returns the record predicate procedure for the record-type
     descriptor RTD.

 -- Scheme Procedure: record-accessor rtd k
     Returns the record field accessor procedure for the Kth field of
     the record-type descriptor RTD.

 -- Scheme Procedure: record-mutator rtd k
     Returns the record field mutator procedure for the Kth field of
     the record-type descriptor RTD.  An `&assertion' condition will be
     raised if this field is not mutable.

7.6.2.11 rnrs records inspection
................................

The `(rnrs records inspection (6))' library provides procedures useful
for accessing metadata about R6RS records.

 -- Scheme Procedure: record? obj
     Return `#t' if the specified object is a non-opaque R6RS record,
     `#f' otherwise.

 -- Scheme Procedure: record-rtd record
     Returns the record-type descriptor for RECORD.  An `&assertion' is
     raised if RECORD is opaque.

 -- Scheme Procedure: record-type-name rtd
     Returns the name of the record-type descriptor RTD.

 -- Scheme Procedure: record-type-parent rtd
     Returns the parent of the record-type descriptor RTD, or `#f' if
     it has none.

 -- Scheme Procedure: record-type-uid rtd
     Returns the uid of the record-type descriptor RTD, or `#f' if it
     has none.

 -- Scheme Procedure: record-type-generative? rtd
     Returns `#t' if the record-type descriptor RTD is generative, `#f'
     otherwise.

 -- Scheme Procedure: record-type-sealed? rtd
     Returns `#t' if the record-type descriptor RTD is sealed, `#f'
     otherwise.

 -- Scheme Procedure: record-type-opaque? rtd
     Returns `#t' if the record-type descriptor RTD is opaque, `#f'
     otherwise.

 -- Scheme Procedure: record-type-field-names rtd
     Returns a vector of symbols giving the names of the fields defined
     by the record-type descriptor RTD (and not any of its sub- or
     supertypes).

 -- Scheme Procedure: record-field-mutable? rtd k
     Returns `#t' if the field at index K of the record-type descriptor
     RTD (and not any of its sub- or supertypes) is mutable.

7.6.2.12 rnrs exceptions
........................

The `(rnrs exceptions (6))' library provides functionality related to
signaling and handling exceptional situations.  This functionality is
similar to the exception handling systems provided by Guile's core
library *Note Exceptions::, and by the SRFI-18 and SRFI-34
modules--*Note SRFI-18 Exceptions::, and *note SRFI-34::,
respectively--but there are some key differences in concepts and
behavior.

   A raised exception may be "continuable" or "non-continuable".  When
an exception is raised non-continuably, another exception, with the
condition type `&non-continuable', will be raised when the exception
handler returns locally.  Raising an exception continuably captures the
current continuation and invokes it after a local return from the
exception handler.

   Like SRFI-18 and SRFI-34, R6RS exceptions are implemented on top of
Guile's native `throw' and `catch' forms, and use custom "throw keys"
to identify their exception types.  As a consequence, Guile's `catch'
form can handle exceptions thrown by these APIs, but the reverse is not
true: Handlers registered by the `with-exception-handler' procedure
described below will only be called on exceptions thrown by the
corresponding `raise' procedure.

 -- Scheme Procedure: with-exception-handler handler thunk
     Installs HANDLER, which must be a procedure taking one argument,
     as the current exception handler during the invocation of THUNK, a
     procedure taking zero arguments.  The handler in place at the time
     `with-exception-handler' is called is made current again once
     either THUNK returns or HANDLER is invoked after an exception is
     thrown from within THUNK.

     This procedure is similar to the `with-throw-handler' procedure
     provided by Guile's code library; (*note Throw Handlers::).

 -- Scheme Syntax: guard (variable clause1 clause2 ...) body
     Evaluates the expression given by BODY, first creating an ad hoc
     exception handler that binds a raised exception to VARIABLE and
     then evaluates the specified CLAUSEs as if they were part of a
     `cond' expression, with the value of the first matching clause
     becoming the value of the `guard' expression (*note
     Conditionals::).  If none of the clause's test expressions
     evaluates to `#t', the exception is re-raised, with the exception
     handler that was current before the evaluation of the `guard' form.

     For example, the expression

          (guard (ex ((eq? ex 'foo) 'bar) ((eq? ex 'bar) 'baz))
            (raise 'bar))

     evaluates to `baz'.

 -- Scheme Procedure: raise obj
     Raises a non-continuable exception by invoking the
     currently-installed exception handler on OBJ.  If the handler
     returns, a `&non-continuable' exception will be raised in the
     dynamic context in which the handler was installed.

 -- Scheme Procedure: raise-continuable obj
     Raises a continuable exception by invoking currently-installed
     exception handler on OBJ.

7.6.2.13 rnrs conditions
........................

The `(rnrs condition (6))' library provides forms and procedures for
constructing new condition types, as well as a library of pre-defined
condition types that represent a variety of common exceptional
situations.  Conditions are records of a subtype of the `&condition'
record type, which is neither sealed nor opaque.  *Note R6RS Records::.

   Conditions may be manipulated singly, as "simple conditions", or
when composed with other conditions to form "compound conditions".
Compound conditions do not "nest"--constructing a new compound
condition out of existing compound conditions will "flatten" them into
their component simple conditions.  For example, making a new condition
out of a `&message' condition and a compound condition that contains an
`&assertion' condition and another `&message' condition will produce a
compound condition that contains two `&message' conditions and one
`&assertion' condition.

   The record type predicates and field accessors described below can
operate on either simple or compound conditions.  In the latter case,
the predicate returns `#t' if the compound condition contains a
component simple condition of the appropriate type; the field accessors
return the requisite fields from the first component simple condition
found to be of the appropriate type.

   This library is quite similar to the SRFI-35 conditions module
(*note SRFI-35::).  Among other minor differences, the `(rnrs
conditions)' library features slightly different semantics around
condition field accessors, and comes with a larger number of
pre-defined condition types.  The two APIs are not currently compatible,
however; the `condition?' predicate from one API will return `#f' when
applied to a condition object created in the other.

 -- Condition Type: &condition
 -- Scheme Procedure: condition? obj
     The base record type for conditions.

 -- Scheme Procedure: condition condition1 ...
 -- Scheme Procedure: simple-conditions condition
     The `condition' procedure creates a new compound condition out of
     its condition arguments, flattening any specified compound
     conditions into their component simple conditions as described
     above.

     `simple-conditions' returns a list of the component simple
     conditions of the compound condition `condition', in the order in
     which they were specified at construction time.

 -- Scheme Procedure: condition-predicate rtd
 -- Scheme Procedure: condition-accessor rtd proc
     These procedures return condition predicate and accessor
     procedures for the specified condition record type RTD.

 -- Scheme Syntax: define-condition-type condition-type supertype
          constructor predicate field-spec ...
     Evaluates to a new record type definition for a condition type
     with the name CONDITION-TYPE that has the condition type SUPERTYPE
     as its parent.  A default constructor, which binds its arguments
     to the fields of this type and its parent types, will be bound to
     the identifier CONSTRUCTOR; a condition predicate will be bound to
     PREDICATE.  The fields of the new type, which are immutable, are
     specified by the FIELD-SPECs, each of which must be of the form:
          (field accessor)
     where FIELD gives the name of the field and ACCESSOR gives the
     name for a binding to an accessor procedure created for this field.

 -- Condition Type: &message
 -- Scheme Procedure: make-message-condition message
 -- Scheme Procedure: message-condition? obj
 -- Scheme Procedure: condition-message condition
     A type that includes a message describing the condition that
     occurred.

 -- Condition Type: &warning
 -- Scheme Procedure: make-warning
 -- Scheme Procedure: warning? obj
     A base type for representing non-fatal conditions during execution.

 -- Condition Type: &serious
 -- Scheme Procedure: make-serious-condition
 -- Scheme Procedure: serious-condition? obj
     A base type for conditions representing errors serious enough that
     cannot be ignored.

 -- Condition Type: &error
 -- Scheme Procedure: make-error
 -- Scheme Procedure: error? obj
     A base type for conditions representing errors.

 -- Condition Type: &violation
 -- Scheme Procedure: make-violation
 -- Scheme Procedure: violation?
     A subtype of `&serious' that can be used to represent violations
     of a language or library standard.

 -- Condition Type: &assertion
 -- Scheme Procedure: make-assertion-violation
 -- Scheme Procedure: assertion-violation? obj
     A subtype of `&violation' that indicates an invalid call to a
     procedure.

 -- Condition Type: &irritants
 -- Scheme Procedure: make-irritants-condition irritants
 -- Scheme Procedure: irritants-condition? obj
 -- Scheme Procedure: condition-irritants condition
     A base type used for storing information about the causes of
     another condition in a compound condition.

 -- Condition Type: &who
 -- Scheme Procedure: make-who-condition who
 -- Scheme Procedure: who-condition? obj
 -- Scheme Procedure: condition-who condition
     A base type used for storing the identity, a string or symbol, of
     the entity responsible for another condition in a compound
     condition.

 -- Condition Type: &non-continuable
 -- Scheme Procedure: make-non-continuable-violation
 -- Scheme Procedure: non-continuable-violation? obj
     A subtype of `&violation' used to indicate that an exception
     handler invoked by `raise' has returned locally.

 -- Condition Type: &implementation-restriction
 -- Scheme Procedure: make-implementation-restriction-violation
 -- Scheme Procedure: implementation-restriction-violation? obj
     A subtype of `&violation' used to indicate a violation of an
     implementation restriction.

 -- Condition Type: &lexical
 -- Scheme Procedure: make-lexical-violation
 -- Scheme Procedure: lexical-violation? obj
     A subtype of `&violation' used to indicate a syntax violation at
     the level of the datum syntax.

 -- Condition Type: &syntax
 -- Scheme Procedure: make-syntax-violation form subform
 -- Scheme Procedure: syntax-violation? obj
 -- Scheme Procedure: syntax-violation-form condition
 -- Scheme Procedure: syntax-violation-subform condition
     A subtype of `&violation' that indicates a syntax violation.  The
     FORM and SUBFORM fields, which must be datum values, indicate the
     syntactic form responsible for the condition.

 -- Condition Type: &undefined
 -- Scheme Procedure: make-undefined-violation
 -- Scheme Procedure: undefined-violation? obj
     A subtype of `&violation' that indicates a reference to an unbound
     identifier.

7.6.2.14 I/O Conditions
.......................

These condition types are exported by both the `(rnrs io ports (6))'
and `(rnrs io simple (6))' libraries.

 -- Condition Type: &i/o
 -- Scheme Procedure: make-i/o-error
 -- Scheme Procedure: i/o-error? obj
     A condition supertype for more specific I/O errors.

 -- Condition Type: &i/o-read
 -- Scheme Procedure: make-i/o-read-error
 -- Scheme Procedure: i/o-read-error? obj
     A subtype of `&i/o'; represents read-related I/O errors.

 -- Condition Type: &i/o-write
 -- Scheme Procedure: make-i/o-write-error
 -- Scheme Procedure: i/o-write-error? obj
     A subtype of `&i/o'; represents write-related I/O errors.

 -- Condition Type: &i/o-invalid-position
 -- Scheme Procedure: make-i/o-invalid-position-error position
 -- Scheme Procedure: i/o-invalid-position-error? obj
 -- Scheme Procedure: i/o-error-position condition
     A subtype of `&i/o'; represents an error related to an attempt to
     set the file position to an invalid position.

 -- Condition Type: &i/o-filename
 -- Scheme Procedure: make-io-filename-error filename
 -- Scheme Procedure: i/o-filename-error? obj
 -- Scheme Procedure: i/o-error-filename condition
     A subtype of `&i/o'; represents an error related to an operation on
     a named file.

 -- Condition Type: &i/o-file-protection
 -- Scheme Procedure: make-i/o-file-protection-error filename
 -- Scheme Procedure: i/o-file-protection-error? obj
     A subtype of `&i/o-filename'; represents an error resulting from an
     attempt to access a named file for which the caller had
     insufficient permissions.

 -- Condition Type: &i/o-file-is-read-only
 -- Scheme Procedure: make-i/o-file-is-read-only-error filename
 -- Scheme Procedure: i/o-file-is-read-only-error? obj
     A subtype of `&i/o-file-protection'; represents an error related to
     an attempt to write to a read-only file.

 -- Condition Type: &i/o-file-already-exists
 -- Scheme Procedure: make-i/o-file-already-exists-error filename
 -- Scheme Procedure: i/o-file-already-exists-error? obj
     A subtype of `&i/o-filename'; represents an error related to an
     operation on an existing file that was assumed not to exist.

 -- Condition Type: &i/o-file-does-not-exist
 -- Scheme Procedure: make-i/o-file-does-not-exist-error
 -- Scheme Procedure: i/o-file-does-not-exist-error? obj
     A subtype of `&i/o-filename'; represents an error related to an
     operation on a non-existent file that was assumed to exist.

 -- Condition Type: &i/o-port
 -- Scheme Procedure: make-i/o-port-error port
 -- Scheme Procedure: i/o-port-error? obj
 -- Scheme Procedure: i/o-error-port condition
     A subtype of `&i/o'; represents an error related to an operation on
     the port PORT.

7.6.2.15 rnrs io ports
......................

The `(rnrs io ports (6))' library provides various procedures and
syntactic forms for use in writing to and reading from ports.  This
functionality is documented in its own section of the manual; (*note
R6RS I/O Ports::).

7.6.2.16 rnrs io simple
.......................

The `(rnrs io simple (6))' library provides convenience functions for
performing textual I/O on ports.  This library also exports all of the
condition types and associated procedures described in (*note I/O
Conditions::).  In the context of this section, when stating that a
procedure behaves "identically" to the corresponding procedure in
Guile's core library, this is modulo the behavior wrt. conditions: such
procedures raise the appropriate R6RS conditions in case of error, but
otherwise behave identically.

     Note: There are still known issues regarding
     condition-correctness; some errors may still be thrown as native
     Guile exceptions instead of the appropriate R6RS conditions.

 -- Scheme Procedure: eof-object
 -- Scheme Procedure: eof-object? obj
     These procedures are identical to the ones provided by the `(rnrs
     io ports (6))' library.  *Note R6RS I/O Ports::, for documentation.

 -- Scheme Procedure: input-port? obj
 -- Scheme Procedure: output-port? obj
     These procedures are identical to the ones provided by Guile's core
     library.  *Note Ports::, for documentation.

 -- Scheme Procedure: call-with-input-file filename proc
 -- Scheme Procedure: call-with-output-file filename proc
 -- Scheme Procedure: open-input-file filename
 -- Scheme Procedure: open-output-file filename
 -- Scheme Procedure: with-input-from-file filename thunk
 -- Scheme Procedure: with-output-to-file filename thunk
     These procedures are identical to the ones provided by Guile's core
     library.  *Note File Ports::, for documentation.

 -- Scheme Procedure: close-input-port input-port
 -- Scheme Procedure: close-output-port output-port
     These procedures are identical to the ones provided by Guile's core
     library.  *Note Closing::, for documentation.

 -- Scheme Procedure: peek-char
 -- Scheme Procedure: peek-char textual-input-port
 -- Scheme Procedure: read-char
 -- Scheme Procedure: read-char textual-input-port
     These procedures are identical to the ones provided by Guile's core
     library.  *Note Reading::, for documentation.

 -- Scheme Procedure: read
 -- Scheme Procedure: read textual-input-port
     This procedure is identical to the one provided by Guile's core
     library.  *Note Scheme Read::, for documentation.

 -- Scheme Procedure: display obj
 -- Scheme Procedure: display obj textual-output-port
 -- Scheme Procedure: newline
 -- Scheme Procedure: newline textual-output-port
 -- Scheme Procedure: write obj
 -- Scheme Procedure: write obj textual-output-port
 -- Scheme Procedure: write-char char
 -- Scheme Procedure: write-char char textual-output-port
     These procedures are identical to the ones provided by Guile's core
     library.  *Note Writing::, for documentation.

7.6.2.17 rnrs files
...................

The `(rnrs files (6))' library provides the `file-exists?' and
`delete-file' procedures, which test for the existence of a file and
allow the deletion of files from the file system, respectively.

   These procedures are identical to the ones provided by Guile's core
library.  *Note File System::, for documentation.

7.6.2.18 rnrs programs
......................

The `(rnrs programs (6))' library provides procedures for process
management and introspection.

 -- Scheme Procedure: command-line
     This procedure is identical to the one provided by Guile's core
     library.  *Note Runtime Environment::, for documentation.

 -- Scheme Procedure: exit
 -- Scheme Procedure: exit obj
     This procedure is identical to the one provided by Guile's core
     library.

7.6.2.19 rnrs arithmetic fixnums
................................

The `(rnrs arithmetic fixnums (6))' library provides procedures for
performing arithmetic operations on an implementation-dependent range of
exact integer values, which R6RS refers to as "fixnums".  In Guile, the
size of a fixnum is determined by the size of the `SCM' type; a single
SCM struct is guaranteed to be able to hold an entire fixnum, making
fixnum computations particularly efficient--(*note The SCM Type::).  On
32-bit systems, the most negative and most positive fixnum values are,
respectively, -536870912 and 536870911.

   Unless otherwise specified, all of the procedures below take fixnums
as arguments, and will raise an `&assertion' condition if passed a
non-fixnum argument or an `&implementation-restriction' condition if
their result is not itself a fixnum.

 -- Scheme Procedure: fixnum? obj
     Returns `#t' if OBJ is a fixnum, `#f' otherwise.

 -- Scheme Procedure: fixnum-width
 -- Scheme Procedure: least-fixnum
 -- Scheme Procedure: greatest-fixnum
     These procedures return, respectively, the maximum number of bits
     necessary to represent a fixnum value in Guile, the minimum fixnum
     value, and the maximum fixnum value.

 -- Scheme Procedure: fx=? fx1 fx2 fx3 ...
 -- Scheme Procedure: fx>? fx1 fx2 fx3 ...
 -- Scheme Procedure: fx<? fx1 fx2 fx3 ...
 -- Scheme Procedure: fx>=? fx1 fx2 fx3 ...
 -- Scheme Procedure: fx<=? fx1 fx2 fx3 ...
     These procedures return `#t' if their fixnum arguments are
     (respectively): equal, monotonically increasing, monotonically
     decreasing, monotonically nondecreasing, or monotonically
     nonincreasing; `#f' otherwise.

 -- Scheme Procedure: fxzero? fx
 -- Scheme Procedure: fxpositive? fx
 -- Scheme Procedure: fxnegative? fx
 -- Scheme Procedure: fxodd? fx
 -- Scheme Procedure: fxeven? fx
     These numerical predicates return `#t' if FX is, respectively,
     zero, greater than zero, less than zero, odd, or even; `#f'
     otherwise.

 -- Scheme Procedure: fxmax fx1 fx2 ...
 -- Scheme Procedure: fxmin fx1 fx2 ...
     These procedures return the maximum or minimum of their arguments.

 -- Scheme Procedure: fx+ fx1 fx2
 -- Scheme Procedure: fx* fx1 fx2
     These procedures return the sum or product of their arguments.

 -- Scheme Procedure: fx- fx1 fx2
 -- Scheme Procedure: fx- fx
     Returns the difference of FX1 and FX2, or the negation of FX, if
     called with a single argument.

     An `&assertion' condition is raised if the result is not itself a
     fixnum.

 -- Scheme Procedure: fxdiv-and-mod fx1 fx2
 -- Scheme Procedure: fxdiv fx1 fx2
 -- Scheme Procedure: fxmod fx1 fx2
 -- Scheme Procedure: fxdiv0-and-mod0 fx1 fx2
 -- Scheme Procedure: fxdiv0 fx1 fx2
 -- Scheme Procedure: fxmod0 fx1 fx2
     These procedures implement number-theoretic division on fixnums;
     *Note (rnrs base)::, for a description of their semantics.

 -- Scheme Procedure: fx+/carry fx1 fx2 fx3
     Returns the two fixnum results of the following computation:
          (let* ((s (+ fx1 fx2 fx3))
                 (s0 (mod0 s (expt 2 (fixnum-width))))
                 (s1 (div0 s (expt 2 (fixnum-width)))))
            (values s0 s1))

 -- Scheme Procedure: fx-/carry fx1 fx2 fx3
     Returns the two fixnum results of the following computation:
          (let* ((d (- fx1 fx2 fx3))
                 (d0 (mod0 d (expt 2 (fixnum-width))))
                 (d1 (div0 d (expt 2 (fixnum-width)))))
            (values d0 d1))

 -- Scheme Procedure: fx*/carry fx1 fx2 fx3
          Returns the two fixnum results of the following computation:
          (let* ((s (+ (* fx1 fx2) fx3))
                 (s0 (mod0 s (expt 2 (fixnum-width))))
                 (s1 (div0 s (expt 2 (fixnum-width)))))
            (values s0 s1))

 -- Scheme Procedure: fxnot fx
 -- Scheme Procedure: fxand fx1 ...
 -- Scheme Procedure: fxior fx1 ...
 -- Scheme Procedure: fxxor fx1 ...
     These procedures are identical to the `lognot', `logand',
     `logior', and `logxor' procedures provided by Guile's core
     library.  *Note Bitwise Operations::, for documentation.

 -- Scheme Procedure: fxif fx1 fx2 fx3
     Returns the bitwise "if" of its fixnum arguments.  The bit at
     position `i' in the return value will be the `i'th bit from FX2 if
     the `i'th bit of FX1 is 1, the `i'th bit from FX3.

 -- Scheme Procedure: fxbit-count fx
     Returns the number of 1 bits in the two's complement
     representation of FX.

 -- Scheme Procedure: fxlength fx
     Returns the number of bits necessary to represent FX.

 -- Scheme Procedure: fxfirst-bit-set fx
     Returns the index of the least significant 1 bit in the two's
     complement representation of FX.

 -- Scheme Procedure: fxbit-set? fx1 fx2
     Returns `#t' if the FX2th bit in the two's complement
     representation of FX1 is 1, `#f' otherwise.

 -- Scheme Procedure: fxcopy-bit fx1 fx2 fx3
     Returns the result of setting the FX2th bit of FX1 to the FX2th
     bit of FX3.

 -- Scheme Procedure: fxbit-field fx1 fx2 fx3
     Returns the integer representation of the contiguous sequence of
     bits in FX1 that starts at position FX2 (inclusive) and ends at
     position FX3 (exclusive).

 -- Scheme Procedure: fxcopy-bit-field fx1 fx2 fx3 fx4
     Returns the result of replacing the bit field in FX1 with start
     and end positions FX2 and FX3 with the corresponding bit field
     from FX4.

 -- Scheme Procedure: fxarithmetic-shift fx1 fx2
 -- Scheme Procedure: fxarithmetic-shift-left fx1 fx2
 -- Scheme Procedure: fxarithmetic-shift-right fx1 fx2
     Returns the result of shifting the bits of FX1 right or left by
     the FX2 positions.  `fxarithmetic-shift' is identical to
     `fxarithmetic-shift-left'.

 -- Scheme Procedure: fxrotate-bit-field fx1 fx2 fx3 fx4
     Returns the result of cyclically permuting the bit field in FX1
     with start and end positions FX2 and FX3 by FX4 bits in the
     direction of more significant bits.

 -- Scheme Procedure: fxreverse-bit-field fx1 fx2 fx3
     Returns the result of reversing the order of the bits of FX1
     between position FX2 (inclusive) and position FX3 (exclusive).

7.6.2.20 rnrs arithmetic flonums
................................

The `(rnrs arithmetic flonums (6))' library provides procedures for
performing arithmetic operations on inexact representations of real
numbers, which R6RS refers to as "flonums".

   Unless otherwise specified, all of the procedures below take flonums
as arguments, and will raise an `&assertion' condition if passed a
non-flonum argument.

 -- Scheme Procedure: flonum? obj
     Returns `#t' if OBJ is a flonum, `#f' otherwise.

 -- Scheme Procedure: real->flonum x
     Returns the flonum that is numerically closest to the real number
     X.

 -- Scheme Procedure: fl=? fl1 fl2 fl3 ...
 -- Scheme Procedure: fl<? fl1 fl2 fl3 ...
 -- Scheme Procedure: fl<=? fl1 fl2 fl3 ...
 -- Scheme Procedure: fl>? fl1 fl2 fl3 ...
 -- Scheme Procedure: fl>=? fl1 fl2 fl3 ...
     These procedures return `#t' if their flonum arguments are
     (respectively): equal, monotonically increasing, monotonically
     decreasing, monotonically nondecreasing, or monotonically
     nonincreasing; `#f' otherwise.

 -- Scheme Procedure: flinteger? fl
 -- Scheme Procedure: flzero? fl
 -- Scheme Procedure: flpositive? fl
 -- Scheme Procedure: flnegative? fl
 -- Scheme Procedure: flodd? fl
 -- Scheme Procedure: fleven? fl
     These numerical predicates return `#t' if FL is, respectively, an
     integer, zero, greater than zero, less than zero, odd, even, `#f'
     otherwise.  In the case of `flodd?' and `fleven?', FL must be an
     integer-valued flonum.

 -- Scheme Procedure: flfinite? fl
 -- Scheme Procedure: flinfinite? fl
 -- Scheme Procedure: flnan? fl
     These numerical predicates return `#t' if FL is, respectively, not
     infinite, infinite, or a `NaN' value.

 -- Scheme Procedure: flmax fl1 fl2 ...
 -- Scheme Procedure: flmin fl1 fl2 ...
     These procedures return the maximum or minimum of their arguments.

 -- Scheme Procedure: fl+ fl1 ...
 -- Scheme Procedure: fl* fl ...
     These procedures return the sum or product of their arguments.

 -- Scheme Procedure: fl- fl1 fl2 ...
 -- Scheme Procedure: fl- fl
 -- Scheme Procedure: fl/ fl1 fl2 ...
 -- Scheme Procedure: fl/ fl
     These procedures return, respectively, the difference or quotient
     of their arguments when called with two arguments; when called
     with a single argument, they return the additive or multiplicative
     inverse of FL.

 -- Scheme Procedure: flabs fl
     Returns the absolute value of FL.

 -- Scheme Procedure: fldiv-and-mod fl1 fl2
 -- Scheme Procedure: fldiv fl1 fl2
 -- Scheme Procedure: fldmod fl1 fl2
 -- Scheme Procedure: fldiv0-and-mod0 fl1 fl2
 -- Scheme Procedure: fldiv0 fl1 fl2
 -- Scheme Procedure: flmod0 fl1 fl2
     These procedures implement number-theoretic division on flonums;
     *Note (rnrs base)::, for a description for their semantics.

 -- Scheme Procedure: flnumerator fl
 -- Scheme Procedure: fldenominator fl
     These procedures return the numerator or denominator of FL as a
     flonum.

 -- Scheme Procedure: flfloor fl1
 -- Scheme Procedure: flceiling fl
 -- Scheme Procedure: fltruncate fl
 -- Scheme Procedure: flround fl
     These procedures are identical to the `floor', `ceiling',
     `truncate', and `round' procedures provided by Guile's core
     library.  *Note Arithmetic::, for documentation.

 -- Scheme Procedure: flexp fl
 -- Scheme Procedure: fllog fl
 -- Scheme Procedure: fllog fl1 fl2
 -- Scheme Procedure: flsin fl
 -- Scheme Procedure: flcos fl
 -- Scheme Procedure: fltan fl
 -- Scheme Procedure: flasin fl
 -- Scheme Procedure: flacos fl
 -- Scheme Procedure: flatan fl
 -- Scheme Procedure: flatan fl1 fl2
     These procedures, which compute the usual transcendental
     functions, are the flonum variants of the procedures provided by
     the R6RS base library (*note (rnrs base)::).

 -- Scheme Procedure: flsqrt fl
     Returns the square root of FL.  If FL is `-0.0', -0.0 is returned;
     for other negative values, a `NaN' value is returned.

 -- Scheme Procedure: flexpt fl1 fl2
     Returns the value of FL1 raised to the power of FL2.

   The following condition types are provided to allow Scheme
implementations that do not support infinities or `NaN' values to
indicate that a computation resulted in such a value.  Guile supports
both of these, so these conditions will never be raised by Guile's
standard libraries implementation.

 -- Condition Type: &no-infinities
 -- Scheme Procedure: make-no-infinities-violation obj
 -- Scheme Procedure: no-infinities-violation?
     A condition type indicating that a computation resulted in an
     infinite value on a Scheme implementation incapable of
     representing infinities.

 -- Condition Type: &no-nans
 -- Scheme Procedure: make-no-nans-violation obj
 -- Scheme Procedure: no-nans-violation? obj
     A condition type indicating that a computation resulted in a `NaN'
     value on a Scheme implementation incapable of representing `NaN's.

 -- Scheme Procedure: fixnum->flonum fx
     Returns the flonum that is numerically closest to the fixnum FX.

7.6.2.21 rnrs arithmetic bitwise
................................

The `(rnrs arithmetic bitwise (6))' library provides procedures for
performing bitwise arithmetic operations on the two's complement
representations of fixnums.

   This library and the procedures it exports share functionality with
SRFI-60, which provides support for bitwise manipulation of integers
(*note SRFI-60::).

 -- Scheme Procedure: bitwise-not ei
 -- Scheme Procedure: bitwise-and ei1 ...
 -- Scheme Procedure: bitwise-ior ei1 ...
 -- Scheme Procedure: bitwise-xor ei1 ...
     These procedures are identical to the `lognot', `logand',
     `logior', and `logxor' procedures provided by Guile's core
     library.  *Note Bitwise Operations::, for documentation.

 -- Scheme Procedure: bitwise-if ei1 ei2 ei3
     Returns the bitwise "if" of its arguments.  The bit at position
     `i' in the return value will be the `i'th bit from EI2 if the
     `i'th bit of EI1 is 1, the `i'th bit from EI3.

 -- Scheme Procedure: bitwise-bit-count ei
     Returns the number of 1 bits in the two's complement
     representation of EI.

 -- Scheme Procedure: bitwise-length ei
     Returns the number of bits necessary to represent EI.

 -- Scheme Procedure: bitwise-first-bit-set ei
     Returns the index of the least significant 1 bit in the two's
     complement representation of EI.

 -- Scheme Procedure: bitwise-bit-set? ei1 ei2
     Returns `#t' if the EI2th bit in the two's complement
     representation of EI1 is 1, `#f' otherwise.

 -- Scheme Procedure: bitwise-copy-bit ei1 ei2 ei3
     Returns the result of setting the EI2th bit of EI1 to the EI2th
     bit of EI3.

 -- Scheme Procedure: bitwise-bit-field ei1 ei2 ei3
     Returns the integer representation of the contiguous sequence of
     bits in EI1 that starts at position EI2 (inclusive) and ends at
     position EI3 (exclusive).

 -- Scheme Procedure: bitwise-copy-bit-field ei1 ei2 ei3 ei4
     Returns the result of replacing the bit field in EI1 with start
     and end positions EI2 and EI3 with the corresponding bit field
     from EI4.

 -- Scheme Procedure: bitwise-arithmetic-shift ei1 ei2
 -- Scheme Procedure: bitwise-arithmetic-shift-left ei1 ei2
 -- Scheme Procedure: bitwise-arithmetic-shift-right ei1 ei2
     Returns the result of shifting the bits of EI1 right or left by
     the EI2 positions.  `bitwise-arithmetic-shift' is identical to
     `bitwise-arithmetic-shift-left'.

 -- Scheme Procedure: bitwise-rotate-bit-field ei1 ei2 ei3 ei4
     Returns the result of cyclically permuting the bit field in EI1
     with start and end positions EI2 and EI3 by EI4 bits in the
     direction of more significant bits.

 -- Scheme Procedure: bitwise-reverse-bit-field ei1 ei2 ei3
     Returns the result of reversing the order of the bits of EI1
     between position EI2 (inclusive) and position EI3 (exclusive).

7.6.2.22 rnrs syntax-case
.........................

The `(rnrs syntax-case (6))' library provides access to the
`syntax-case' system for writing hygienic macros.  With one exception,
all of the forms and procedures exported by this library are
"re-exports" of Guile's native support for `syntax-case'; *Note Syntax
Case::, for documentation, examples, and rationale.

 -- Scheme Procedure: make-variable-transformer proc
     Creates a new variable transformer out of PROC, a procedure that
     takes a syntax object as input and returns a syntax object.  If an
     identifier to which the result of this procedure is bound appears
     on the left-hand side of a `set!' expression, PROC will be called
     with a syntax object representing the entire `set!' expression,
     and its return value will replace that `set!' expression.

 -- Scheme Syntax: syntax-case expression (literal ...) clause ...
     The `syntax-case' pattern matching form.

 -- Scheme Syntax: syntax template
 -- Scheme Syntax: quasisyntax template
 -- Scheme Syntax: unsyntax template
 -- Scheme Syntax: unsyntax-splicing template
     These forms allow references to be made in the body of a
     syntax-case output expression subform to datum and non-datum
     values.  They are identical to the forms provided by Guile's core
     library; *Note Syntax Case::, for documentation.

 -- Scheme Procedure: identifier? obj
 -- Scheme Procedure: bound-identifier=? id1 id2
 -- Scheme Procedure: free-identifier=? id1 id2
     These predicate procedures operate on syntax objects representing
     Scheme identifiers.  `identifier?' returns `#t' if OBJ represents
     an identifier, `#f' otherwise.  `bound-identifier=?' returns `#t'
     if and only if a binding for ID1 would capture a reference to ID2
     in the transformer's output, or vice-versa.  `free-identifier=?'
     returns `#t' if and only ID1 and ID2 would refer to the same
     binding in the output of the transformer, independent of any
     bindings introduced by the transformer.

 -- Scheme Procedure: generate-temporaries l
     Returns a list, of the same length as L, which must be a list or a
     syntax object representing a list, of globally unique symbols.

 -- Scheme Procedure: syntax->datum syntax-object
 -- Scheme Procedure: datum->syntax template-id datum
     These procedures convert wrapped syntax objects to and from Scheme
     datum values.  The syntax object returned by `datum->syntax' shares
     contextual information with the syntax object TEMPLATE-ID.

 -- Scheme Procedure: syntax-violation whom message form
 -- Scheme Procedure: syntax-violation whom message form subform
     Constructs a new compound condition that includes the following
     simple conditions:
        * If WHOM is not `#f', a `&who' condition with the WHOM as its
          field

        * A `&message' condition with the specified MESSAGE

        * A `&syntax' condition with the specified FORM and optional
          SUBFORM fields

7.6.2.23 rnrs hashtables
........................

The `(rnrs hashtables (6))' library provides structures and procedures
for creating and accessing hash tables.  The hash tables API defined by
R6RS is substantially similar to both Guile's native hash tables
implementation as well as the one provided by SRFI-69; *Note Hash
Tables::, and *note SRFI-69::, respectively.  Note that you can write
portable R6RS library code that manipulates SRFI-69 hash tables (by
importing the `(srfi :69)' library); however, hash tables created by
one API cannot be used by another.

   Like SRFI-69 hash tables--and unlike Guile's native ones--R6RS hash
tables associate hash and equality functions with a hash table at the
time of its creation.  Additionally, R6RS allows for the creation (via
`hashtable-copy'; see below) of immutable hash tables.

 -- Scheme Procedure: make-eq-hashtable
 -- Scheme Procedure: make-eq-hashtable k
     Returns a new hash table that uses `eq?' to compare keys and
     Guile's `hashq' procedure as a hash function.  If K is given, it
     specifies the initial capacity of the hash table.

 -- Scheme Procedure: make-eqv-hashtable
 -- Scheme Procedure: make-eqv-hashtable k
     Returns a new hash table that uses `eqv?' to compare keys and
     Guile's `hashv' procedure as a hash function.  If K is given, it
     specifies the initial capacity of the hash table.

 -- Scheme Procedure: make-hashtable hash-function equiv
 -- Scheme Procedure: make-hashtable hash-function equiv k
     Returns a new hash table that uses EQUIV to compare keys and
     HASH-FUNCTION as a hash function.  EQUIV must be a procedure that
     accepts two arguments and returns a true value if they are
     equivalent, `#f' otherwise; HASH-FUNCTION must be a procedure that
     accepts one argument and returns a non-negative integer.

     If K is given, it specifies the initial capacity of the hash table.

 -- Scheme Procedure: hashtable? obj
     Returns `#t' if OBJ is an R6RS hash table, `#f' otherwise.

 -- Scheme Procedure: hashtable-size hashtable
     Returns the number of keys currently in the hash table HASHTABLE.

 -- Scheme Procedure: hashtable-ref hashtable key default
     Returns the value associated with KEY in the hash table HASHTABLE,
     or DEFAULT if none is found.

 -- Scheme Procedure: hashtable-set! hashtable key obj
     Associates the key KEY with the value OBJ in the hash table
     HASHTABLE, and returns an unspecified value.  An `&assertion'
     condition is raised if HASHTABLE is immutable.

 -- Scheme Procedure: hashtable-delete! hashtable key
     Removes any association found for the key KEY in the hash table
     HASHTABLE, and returns an unspecified value.  An `&assertion'
     condition is raised if HASHTABLE is immutable.

 -- Scheme Procedure: hashtable-contains? hashtable key
     Returns `#t' if the hash table HASHTABLE contains an association
     for the key KEY, `#f' otherwise.

 -- Scheme Procedure: hashtable-update! hashtable key proc default
     Associates with KEY in the hash table HASHTABLE the result of
     calling PROC, which must be a procedure that takes one argument,
     on the value currently associated KEY in HASHTABLE--or on DEFAULT
     if no such association exists.  An `&assertion' condition is
     raised if HASHTABLE is immutable.

 -- Scheme Procedure: hashtable-copy hashtable
 -- Scheme Procedure: hashtable-copy hashtable mutable
     Returns a copy of the hash table HASHTABLE.  If the optional
     argument MUTABLE is a true value, the new hash table will be
     immutable.

 -- Scheme Procedure: hashtable-clear! hashtable
 -- Scheme Procedure: hashtable-clear! hashtable k
     Removes all of the associations from the hash table HASHTABLE.
     The optional argument K, which specifies a new capacity for the
     hash table, is accepted by Guile's `(rnrs hashtables)'
     implementation, but is ignored.

 -- Scheme Procedure: hashtable-keys hashtable
     Returns a vector of the keys with associations in the hash table
     HASHTABLE, in an unspecified order.

 -- Scheme Procedure: hashtable-entries hashtable
     Return two values--a vector of the keys with associations in the
     hash table HASHTABLE, and a vector of the values to which these
     keys are mapped, in corresponding but unspecified order.

 -- Scheme Procedure: hashtable-equivalence-function hashtable
     Returns the equivalence predicated use by HASHTABLE.  This
     procedure returns `eq?' and `eqv?', respectively, for hash tables
     created by `make-eq-hashtable' and `make-eqv-hashtable'.

 -- Scheme Procedure: hashtable-hash-function hashtable
     Returns the hash function used by HASHTABLE.  For hash tables
     created by `make-eq-hashtable' or `make-eqv-hashtable', `#f' is
     returned.

 -- Scheme Procedure: hashtable-mutable? hashtable
     Returns `#t' if HASHTABLE is mutable, `#f' otherwise.

   A number of hash functions are provided for convenience:

 -- Scheme Procedure: equal-hash obj
     Returns an integer hash value for OBJ, based on its structure and
     current contents. This hash function is suitable for use with
     `equal?' as an equivalence function.

 -- Scheme Procedure: string-hash string
 -- Scheme Procedure: symbol-hash symbol
     These procedures are identical to the ones provided by Guile's core
     library.  *Note Hash Table Reference::, for documentation.

 -- Scheme Procedure: string-ci-hash string
     Returns an integer hash value for STRING based on its contents,
     ignoring case.  This hash function is suitable for use with
     `string-ci=?' as an equivalence function.

7.6.2.24 rnrs enums
...................

The `(rnrs enums (6))' library provides structures and procedures for
working with enumerable sets of symbols.  Guile's implementation
defines an "enum-set" record type that encapsulates a finite set of
distinct symbols, the "universe", and a subset of these symbols, which
define the enumeration set.

   The SRFI-1 list library provides a number of procedures for
performing set operations on lists; Guile's `(rnrs enums)'
implementation makes use of several of them.  *Note SRFI-1 Set
Operations::, for more information.

 -- Scheme Procedure: make-enumeration symbol-list
     Returns a new enum-set whose universe and enumeration set are both
     equal to SYMBOL-LIST, a list of symbols.

 -- Scheme Procedure: enum-set-universe enum-set
     Returns an enum-set representing the universe of ENUM-SET, an
     enum-set.

 -- Scheme Procedure: enum-set-indexer enum-set
     Returns a procedure that takes a single argument and returns the
     zero-indexed position of that argument in the universe of
     ENUM-SET, or `#f' if its argument is not a member of that universe.

 -- Scheme Procedure: enum-set-constructor enum-set
     Returns a procedure that takes a single argument, a list of symbols
     from the universe of ENUM-SET, an enum-set, and returns a new
     enum-set with the same universe that represents a subset
     containing the specified symbols.

 -- Scheme Procedure: enum-set->list enum-set
     Returns a list containing the symbols of the set represented by
     ENUM-SET, an enum-set, in the order that they appear in the
     universe of ENUM-SET.

 -- Scheme Procedure: enum-set-member? symbol enum-set
 -- Scheme Procedure: enum-set-subset? enum-set1 enum-set2
 -- Scheme Procedure: enum-set=? enum-set1 enum-set2
     These procedures test for membership of symbols and enum-sets in
     other enum-sets.  `enum-set-member?' returns `#t' if and only if
     SYMBOL is a member of the subset specified by ENUM-SET.
     `enum-set-subset?' returns `#t' if and only if the universe of
     ENUM-SET1 is a subset of the universe of ENUM-SET2 and every
     symbol in ENUM-SET1 is present in ENUM-SET2.  `enum-set=?' returns
     `#t' if and only if ENUM-SET1 is a subset, as per
     `enum-set-subset?' of ENUM-SET2 and vice versa.

 -- Scheme Procedure: enum-set-union enum-set1 enum-set2
 -- Scheme Procedure: enum-set-intersection enum-set1 enum-set2
 -- Scheme Procedure: enum-set-difference enum-set1 enum-set2
     These procedures return, respectively, the union, intersection, and
     difference of their enum-set arguments.

 -- Scheme Procedure: enum-set-complement enum-set
     Returns ENUM-SET's complement (an enum-set), with regard to its
     universe.

 -- Scheme Procedure: enum-set-projection enum-set1 enum-set2
     Returns the projection of the enum-set ENUM-SET1 onto the universe
     of the enum-set ENUM-SET2.

 -- Scheme Syntax: define-enumeration type-name (symbol ...)
          constructor-syntax
     Evaluates to two new definitions: A constructor bound to
     CONSTRUCTOR-SYNTAX that behaves similarly to constructors created
     by `enum-set-constructor', above, and creates new ENUM-SETs in the
     universe specified by `(symbol ...)'; and a "predicate macro"
     bound to TYPE-NAME, which has the following form:

          (TYPE-NAME sym)

     If SYM is a member of the universe specified by the SYMBOLs above,
     this form evaluates to SYM.  Otherwise, a `&syntax' condition is
     raised.

7.6.2.25 rnrs
.............

The `(rnrs (6))' library is a composite of all of the other R6RS
standard libraries--it imports and re-exports all of their exported
procedures and syntactic forms--with the exception of the following
libraries:

   * `(rnrs eval (6))'

   * `(rnrs mutable-pairs (6))'

   * `(rnrs mutable-strings (6))'

   * `(rnrs r5rs (6))'

7.6.2.26 rnrs eval
..................

The `(rnrs eval (6)' library provides procedures for performing
"on-the-fly" evaluation of expressions.

 -- Scheme Procedure: eval expression environment
     Evaluates EXPRESSION, which must be a datum representation of a
     valid Scheme expression, in the environment specified by
     ENVIRONMENT.  This procedure is identical to the one provided by
     Guile's code library; *Note Fly Evaluation::, for documentation.

 -- Scheme Procedure: environment import-spec ...
     Constructs and returns a new environment based on the specified
     IMPORT-SPECs, which must be datum representations of the import
     specifications used with the `import' form.  *Note R6RS
     Libraries::, for documentation.

7.6.2.27 rnrs mutable-pairs
...........................

The `(rnrs mutable-pairs (6))' library provides the `set-car!' and
`set-cdr!' procedures, which allow the `car' and `cdr' fields of a pair
to be modified.

   These procedures are identical to the ones provide by Guile's core
library.  *Note Pairs::, for documentation.  All pairs in Guile are
mutable; consequently, these procedures will never throw the
`&assertion' condition described in the R6RS libraries specification.

7.6.2.28 rnrs mutable-strings
.............................

The `(rnrs mutable-strings (6))' library provides the `string-set!' and
`string-fill!' procedures, which allow the content of strings to be
modified "in-place."

   These procedures are identical to the ones provided by Guile's core
library.  *Note String Modification::, for documentation.  All strings
in Guile are mutable; consequently, these procedures will never throw
the `&assertion' condition described in the R6RS libraries
specification.

7.6.2.29 rnrs r5rs
..................

The `(rnrs r5rs (6))' library exports bindings for some procedures
present in R5RS but omitted from the R6RS base library specification.

 -- Scheme Procedure: exact->inexact z
 -- Scheme Procedure: inexact->exact z
     These procedures are identical to the ones provided by Guile's core
     library.  *Note Exactness::, for documentation.

 -- Scheme Procedure: quotient n1 n2
 -- Scheme Procedure: remainder n1 n2
 -- Scheme Procedure: modulo n1 n2
     These procedures are identical to the ones provided by Guile's core
     library.  *Note Integer Operations::, for documentation.

 -- Scheme Syntax: delay expr
 -- Scheme Procedure: force promise
     The `delay' form and the `force' procedure are identical to their
     counterparts in Guile's core library.  *Note Delayed Evaluation::,
     for documentation.

 -- Scheme Procedure: null-environment n
 -- Scheme Procedure: scheme-report-environment n
     These procedures are identical to the ones provided by the `(ice-9
     r5rs)' Guile module.  *Note Environments::, for documentation.

7.7 Pattern Matching
====================

The `(ice-9 match)' module provides a "pattern matcher", written by
Alex Shinn, and compatible with Andrew K. Wright's pattern matcher
found in many Scheme implementations.

   A pattern matcher can match an object against several patterns and
extract the elements that make it up.  Patterns can represent any Scheme
object: lists, strings, symbols, records, etc.  They can optionally
contain "pattern variables".  When a matching pattern is found, an
expression associated with the pattern is evaluated, optionally with all
pattern variables bound to the corresponding elements of the object:

     (let ((l '(hello (world))))
       (match l           ;; <- the input object
         (('hello (who))  ;; <- the pattern
          who)))          ;; <- the expression evaluated upon matching
     => world

   In this example, list L matches the pattern `('hello (who))',
because it is a two-element list whose first element is the symbol
`hello' and whose second element is a one-element list.  Here WHO is a
pattern variable.  `match', the pattern matcher, locally binds WHO to
the value contained in this one-element list--i.e., the symbol `world'.

   The same object can be matched against a simpler pattern:

     (let ((l '(hello (world))))
       (match l
         ((x y)
          (values x y))))
     => hello
     => (world)

   Here pattern `(x y)' matches any two-element list, regardless of the
types of these elements.  Pattern variables X and Y are bound to,
respectively, the first and second element of L.

   The pattern matcher is defined as follows:

 -- Scheme Syntax: match exp clause ...
     Match object EXP against the patterns in the given CLAUSEs, in the
     order in which they appear.  Return the value produced by the
     first matching clause.  If no CLAUSE matches, throw an exception
     with key `match-error'.

     Each CLAUSE has the form `(pattern body)'.  Each PATTERN must
     follow the syntax described below.  Each BODY is an arbitrary
     Scheme expression, possibly referring to pattern variables of
     PATTERN.

   The syntax and interpretation of patterns is as follows:

        patterns:                       matches:

pat ::= identifier                      anything, and binds identifier
      | _                               anything
      | ()                              the empty list
      | #t                              #t
      | #f                              #f
      | string                          a string
      | number                          a number
      | character                       a character
      | 'sexp                           an s-expression
      | 'symbol                         a symbol (special case of s-expr)
      | (pat_1 ... pat_n)               list of n elements
      | (pat_1 ... pat_n . pat_{n+1})   list of n or more
      | (pat_1 ... pat_n pat_n+1 ooo)   list of n or more, each element
                                          of remainder must match pat_n+1
      | #(pat_1 ... pat_n)              vector of n elements
      | #(pat_1 ... pat_n pat_n+1 ooo)  vector of n or more, each element
                                          of remainder must match pat_n+1
      | #&pat                           box
      | ($ record-name pat_1 ... pat_n) a record
      | (= field pat)                   a ``field'' of an object
      | (and pat_1 ... pat_n)           if all of pat_1 thru pat_n match
      | (or pat_1 ... pat_n)            if any of pat_1 thru pat_n match
      | (not pat_1 ... pat_n)           if all pat_1 thru pat_n don't match
      | (? predicate pat_1 ... pat_n)   if predicate true and all of
                                          pat_1 thru pat_n match
      | (set! identifier)               anything, and binds setter
      | (get! identifier)               anything, and binds getter
      | `qp                             a quasi-pattern
      | (identifier *** pat)            matches pat in a tree and binds
                                        identifier to the path leading
                                        to the object that matches pat

ooo ::= ...                             zero or more
      | ___                             zero or more
      | ..1                             1 or more

        quasi-patterns:                 matches:

qp  ::= ()                              the empty list
      | #t                              #t
      | #f                              #f
      | string                          a string
      | number                          a number
      | character                       a character
      | identifier                      a symbol
      | (qp_1 ... qp_n)                 list of n elements
      | (qp_1 ... qp_n . qp_{n+1})      list of n or more
      | (qp_1 ... qp_n qp_n+1 ooo)      list of n or more, each element
                                          of remainder must match qp_n+1
      | #(qp_1 ... qp_n)                vector of n elements
      | #(qp_1 ... qp_n qp_n+1 ooo)     vector of n or more, each element
                                          of remainder must match qp_n+1
      | #&qp                            box
      | ,pat                            a pattern
      | ,@pat                           a pattern

   The names `quote', `quasiquote', `unquote', `unquote-splicing', `?',
`_', `$', `and', `or', `not', `set!', `get!', `...', and `___' cannot
be used as pattern variables.

   Here is a more complex example:

     (use-modules (srfi srfi-9))

     (let ()
       (define-record-type person
         (make-person name friends)
         person?
         (name    person-name)
         (friends person-friends))

       (letrec ((alice (make-person "Alice" (delay (list bob))))
                (bob   (make-person "Bob" (delay (list alice)))))
         (match alice
           (($ person name (= force (($ person "Bob"))))
            (list 'friend-of-bob name))
           (_ #f))))

     => (friend-of-bob "Alice")

Here the `$' pattern is used to match a SRFI-9 record of type PERSON
containing two or more slots.  The value of the first slot is bound to
NAME.  The `=' pattern is used to apply `force' on the second slot, and
then checking that the result matches the given pattern.  In other
words, the complete pattern matches any PERSON whose second slot is a
promise that evaluates to a one-element list containing a PERSON whose
first slot is `"Bob"'.

   Please refer to the `ice-9/match.upstream.scm' file in your Guile
installation for more details.

   Guile also comes with a pattern matcher specifically tailored to SXML
trees, *Note sxml-match::.

7.8 Readline Support
====================

Guile comes with an interface module to the readline library (*note
Top: (readline)Top.).  This makes interactive use much more convenient,
because of the command-line editing features of readline.  Using
`(ice-9 readline)', you can navigate through the current input line
with the cursor keys, retrieve older command lines from the input
history and even search through the history entries.

7.8.1 Loading Readline Support
------------------------------

The module is not loaded by default and so has to be loaded and
activated explicitly.  This is done with two simple lines of code:

     (use-modules (ice-9 readline))
     (activate-readline)

   The first line will load the necessary code, and the second will
activate readline's features for the REPL.  If you plan to use this
module often, you should save these to lines to your `.guile' personal
startup file.

   You will notice that the REPL's behaviour changes a bit when you have
loaded the readline module.  For example, when you press Enter before
typing in the closing parentheses of a list, you will see the
"continuation" prompt, three dots: `...'  This gives you a nice visual
feedback when trying to match parentheses.  To make this even easier,
"bouncing parentheses" are implemented.  That means that when you type
in a closing parentheses, the cursor will jump to the corresponding
opening parenthesis for a short time, making it trivial to make them
match.

   Once the readline module is activated, all lines entered
interactively will be stored in a history and can be recalled later
using the cursor-up and -down keys.  Readline also understands the
Emacs keys for navigating through the command line and history.

   When you quit your Guile session by evaluating `(quit)' or pressing
Ctrl-D, the history will be saved to the file `.guile_history' and read
in when you start Guile for the next time.  Thus you can start a new
Guile session and still have the (probably long-winded) definition
expressions available.

   You can specify a different history file by setting the environment
variable `GUILE_HISTORY'.  And you can make Guile specific
customizations to your `.inputrc' by testing for application `Guile'
(*note Conditional Init Constructs: (readline)Conditional Init
Constructs.).  For instance to define a key inserting a matched pair of
parentheses,

     $if Guile
       "\C-o": "()\C-b"
     $endif

7.8.2 Readline Options
----------------------

The readline interface module can be tweaked in a few ways to better
suit the user's needs.  Configuration is done via the readline module's
options interface, in a similar way to the evaluator and debugging
options (*note Runtime Options::).

 -- Scheme Procedure: readline-options
 -- Scheme Procedure: readline-enable option-name
 -- Scheme Procedure: readline-disable option-name
 -- Scheme Syntax: readline-set! option-name value
     Accessors for the readline options.  Note that unlike the
     enable/disable procedures, `readline-set!' is syntax, which
     expects an unquoted option name.

   Here is the list of readline options generated by typing
`(readline-options 'help)' in Guile.  You can also see the default
values.

     history-file    yes     Use history file.
     history-length  200     History length.
     bounce-parens   500     Time (ms) to show matching opening parenthesis
                             (0 = off).

   The readline options interface can only be used _after_ loading the
readline module, because it is defined in that module.

7.8.3 Readline Functions
------------------------

The following functions are provided by

     (use-modules (ice-9 readline))

   There are two ways to use readline from Scheme code, either make
calls to `readline' directly to get line by line input, or use the
readline port below with all the usual reading functions.

 -- Function: readline [prompt]
     Read a line of input from the user and return it as a string
     (without a newline at the end).  PROMPT is the prompt to show, or
     the default is the string set in `set-readline-prompt!' below.

          (readline "Type something: ") => "hello"

 -- Function: set-readline-input-port! port
 -- Function: set-readline-output-port! port
     Set the input and output port the readline function should read
     from and write to.  PORT must be a file port (*note File Ports::),
     and should usually be a terminal.

     The default is the `current-input-port' and `current-output-port'
     (*note Default Ports::) when `(ice-9 readline)' loads, which in an
     interactive user session means the Unix "standard input" and
     "standard output".

7.8.3.1 Readline Port
.....................

 -- Function: readline-port
     Return a buffered input port (*note Buffered Input::) which calls
     the `readline' function above to get input.  This port can be used
     with all the usual reading functions (`read', `read-char', etc),
     and the user gets the interactive editing features of readline.

     There's only a single readline port created.  `readline-port'
     creates it when first called, and on subsequent calls just returns
     what it previously made.

 -- Function: activate-readline
     If the `current-input-port' is a terminal (*note `isatty?':
     Terminals and Ptys.) then enable readline for all reading from
     `current-input-port' (*note Default Ports::) and enable readline
     features in the interactive REPL (*note The REPL::).

          (activate-readline)
          (read-char)

     `activate-readline' enables readline on `current-input-port'
     simply by a `set-current-input-port' to the `readline-port' above.
     An application can do that directly if the extra REPL features
     that `activate-readline' adds are not wanted.

 -- Function: set-readline-prompt! prompt1 [prompt2]
     Set the prompt string to print when reading input.  This is used
     when reading through `readline-port', and is also the default
     prompt for the `readline' function above.

     PROMPT1 is the initial prompt shown.  If a user might enter an
     expression across multiple lines, then PROMPT2 is a different
     prompt to show further input required.  In the Guile REPL for
     instance this is an ellipsis (`...').

     See `set-buffered-input-continuation?!' (*note Buffered Input::)
     for an application to indicate the boundaries of logical
     expressions (assuming of course an application has such a notion).

7.8.3.2 Completion
..................

 -- Function: with-readline-completion-function completer thunk
     Call `(THUNK)' with COMPLETER as the readline tab completion
     function to be used in any readline calls within that THUNK.
     COMPLETER can be `#f' for no completion.

     COMPLETER will be called as `(COMPLETER text state)', as described
     in (*note How Completing Works: (readline)How Completing Works.).
     TEXT is a partial word to be completed, and each COMPLETER call
     should return a possible completion string or `#f' when no more.
     STATE is `#f' for the first call asking about a new TEXT then `#t'
     while getting further completions of that TEXT.

     Here's an example COMPLETER for user login names from the password
     file (*note User Information::), much like readline's own
     `rl_username_completion_function',

          (define (username-completer-function text state)
            (if (not state)
                (setpwent))  ;; new, go to start of database
            (let more ((pw (getpwent)))
              (if pw
                  (if (string-prefix? text (passwd:name pw))
                      (passwd:name pw)     ;; this name matches, return it
                      (more (getpwent)))   ;; doesn't match, look at next
                  (begin
                    ;; end of database, close it and return #f
                    (endpwent)
                    #f))))

 -- Function: apropos-completion-function text state
     A completion function offering completions for Guile functions and
     variables (all `define's).  This is the default completion
     function.

 -- Function: filename-completion-function text state
     A completion function offering filename completions.  This is
     readline's `rl_filename_completion_function' (*note Completion
     Functions: (readline)Completion Functions.).

 -- Function: make-completion-function string-list
     Return a completion function which offers completions from the
     possibilities in STRING-LIST.  Matching is case-sensitive.

7.9 Pretty Printing
===================

The module `(ice-9 pretty-print)' provides the procedure
`pretty-print', which provides nicely formatted output of Scheme
objects.  This is especially useful for deeply nested or complex data
structures, such as lists and vectors.

   The module is loaded by entering the following:

     (use-modules (ice-9 pretty-print))

   This makes the procedure `pretty-print' available.  As an example
how `pretty-print' will format the output, see the following:

     (pretty-print '(define (foo) (lambda (x)
     (cond ((zero? x) #t) ((negative? x) -x) (else
     (if (= x 1) 2 (* x x x)))))))
     -|
     (define (foo)
       (lambda (x)
         (cond ((zero? x) #t)
               ((negative? x) -x)
               (else (if (= x 1) 2 (* x x x))))))

 -- Scheme Procedure: pretty-print obj [port] [keyword-options]
     Print the textual representation of the Scheme object OBJ to PORT.
     PORT defaults to the current output port, if not given.

     The further KEYWORD-OPTIONS are keywords and parameters as follows,

    #:display? FLAG
          If FLAG is true then print using `display'.  The default is
          `#f' which means use `write' style.  (*note Writing::)

    #:per-line-prefix STRING
          Print the given STRING as a prefix on each line.  The default
          is no prefix.

    #:width COLUMNS
          Print within the given COLUMNS.  The default is 79.

   Also exported by the `(ice-9 pretty-print)' module is
`truncated-print', a procedure to print Scheme datums, truncating the
output to a certain number of characters. This is useful when you need
to present an arbitrary datum to the user, but you only have one line
in which to do so.

     (define exp '(a b #(c d e) f . g))
     (truncated-print exp #:width 10) (newline)
     -| (a b . #)
     (truncated-print exp #:width 15) (newline)
     -| (a b # f . g)
     (truncated-print exp #:width 18) (newline)
     -| (a b #(c ...) . #)
     (truncated-print exp #:width 20) (newline)
     -| (a b #(c d e) f . g)
     (truncated-print "The quick brown fox" #:width 20) (newline)
     -| "The quick brown..."
     (truncated-print (current-module) #:width 20) (newline)
     -| #<directory (gui...>

   `truncated-print' will not output a trailing newline. If an
expression does not fit in the given width, it will be truncated -
possibly ellipsized(1), or in the worst case, displayed as #.

 -- Scheme Procedure: truncated-print obj [port] [keyword-options]
     Print OBJ, truncating the output, if necessary, to make it fit
     into WIDTH characters. By default, X will be printed using
     `write', though that behavior can be overridden via the DISPLAY?
     keyword argument.

     The default behaviour is to print depth-first, meaning that the
     entire remaining width will be available to each sub-expression of
     X - e.g., if X is a vector, each member of X. One can attempt to
     "ration" the available width, trying to allocate it equally to each
     sub-expression, via the BREADTH-FIRST? keyword argument.

     The further KEYWORD-OPTIONS are keywords and parameters as follows,

    #:display? FLAG
          If FLAG is true then print using `display'.  The default is
          `#f' which means use `write' style.  (*note Writing::)

    #:width COLUMNS
          Print within the given COLUMNS.  The default is 79.

    #:breadth-first? FLAG
          If FLAG is true, then allocate the available width
          breadth-first among elements of a compound data structure
          (list, vector, pair, etc.). The default is `#f' which means
          that any element is allowed to consume all of the available
          width.

   ---------- Footnotes ----------

   (1) On Unicode-capable ports, the ellipsis is represented by
character `HORIZONTAL ELLIPSIS' (U+2026), otherwise it is represented
by three dots.

7.10 Formatted Output
=====================

The `format' function is a powerful way to print numbers, strings and
other objects together with literal text under the control of a format
string.  This function is available from

     (use-modules (ice-9 format))

   A format string is generally more compact and easier than using just
the standard procedures like `display', `write' and `newline'.
Parameters in the output string allow various output styles, and
parameters can be taken from the arguments for runtime flexibility.

   `format' is similar to the Common Lisp procedure of the same name,
but it's not identical and doesn't have quite all the features found in
Common Lisp.

   C programmers will note the similarity between `format' and
`printf', though escape sequences are marked with ~ instead of %, and
are more powerful.


 -- Scheme Procedure: format dest fmt [args...]
     Write output specified by the FMT string to DEST.  DEST can be an
     output port, `#t' for `current-output-port' (*note Default
     Ports::), or `#f' to return the output as a string.

     FMT can contain literal text to be output, and ~ escapes.  Each
     escape has the form

          ~ [param [, param...] [:] [@] code

     code is a character determining the escape sequence.  The : and @
     characters are optional modifiers, one or both of which change the
     way various codes operate.  Optional parameters are accepted by
     some codes too.  Parameters have the following forms,

    [+/-]number
          An integer, with optional + or -.

    '  (apostrophe)
          The following character in the format string, for instance 'z
          for z.

    v
          The next function argument as the parameter.  v stands for
          "variable", a parameter can be calculated at runtime and
          included in the arguments.  Upper case V can be used too.

    #
          The number of arguments remaining.  (See ~* below for some
          usages.)

     Parameters are separated by commas (,).  A parameter can be left
     empty to keep its default value when supplying later parameters.


     The following escapes are available.  The code letters are not
     case-sensitive, upper and lower case are the same.

    ~a
    ~s
          Object output.  Parameters: MINWIDTH, PADINC, MINPAD, PADCHAR.

          ~a outputs an argument like `display', ~s outputs an argument
          like `write' (*note Writing::).

               (format #t "~a" "foo") -| foo
               (format #t "~s" "foo") -| "foo"

          ~:a and ~:s put objects that don't have an external
          representation in quotes like a string.

               (format #t "~:a" car) -| "#<primitive-procedure car>"

          If the output is less than MINWIDTH characters (default 0),
          it's padded on the right with PADCHAR (default space).  ~@a
          and ~@s put the padding on the left instead.

               (format #f "~5a" 'abc)       => "abc  "
               (format #f "~5,,,'-@a" 'abc) => "--abc"

          MINPAD is a minimum for the padding then plus a multiple of
          PADINC.  Ie. the padding is MINPAD + N * PADINC, where N is
          the smallest integer making the total object plus padding
          greater than or equal to MINWIDTH.  The default MINPAD is 0
          and the default PADINC is 1 (imposing no minimum or multiple).

               (format #f "~5,1,4a" 'abc) => "abc    "

    ~c
          Character.  Parameter: CHARNUM.

          Output a character.  The default is to simply output, as per
          `write-char' (*note Writing::).  ~@c prints in `write' style.
          ~:c prints control characters (ASCII 0 to 31) in ^X form.

               (format #t "~c" #\z)        -| z
               (format #t "~@c" #\z)       -| #\z
               (format #t "~:c" #\newline) -| ^J

          If the CHARNUM parameter is given then an argument is not
          taken but instead the character is `(integer->char CHARNUM)'
          (*note Characters::).  This can be used for instance to output
          characters given by their ASCII code.

               (format #t "~65c")  -| A

    ~d
    ~x
    ~o
    ~b
          Integer.  Parameters: MINWIDTH, PADCHAR, COMMACHAR,
          COMMAWIDTH.

          Output an integer argument as a decimal, hexadecimal, octal
          or binary integer (respectively).

               (format #t "~d" 123) -| 123

          ~@d etc shows a + sign is shown on positive numbers.

               (format #t "~@b" 12) -| +1100

          If the output is less than the MINWIDTH parameter (default no
          minimum), it's padded on the left with the PADCHAR parameter
          (default space).

               (format #t "~5,'*d" 12)   -| ***12
               (format #t "~5,'0d" 12)   -| 00012
               (format #t "~3d"    1234) -| 1234

          ~:d adds commas (or the COMMACHAR parameter) every three
          digits (or the COMMAWIDTH parameter many).

               (format #t "~:d" 1234567)         -| 1,234,567
               (format #t "~10,'*,'/,2:d" 12345) -| ***1/23/45

          Hexadecimal ~x output is in lower case, but the ~( and ~)
          case conversion directives described below can be used to get
          upper case.

               (format #t "~x"       65261) -| feed
               (format #t "~:@(~x~)" 65261) -| FEED

    ~r
          Integer in words, roman numerals, or a specified radix.
          Parameters: RADIX, MINWIDTH, PADCHAR, COMMACHAR, COMMAWIDTH.

          With no parameters output is in words as a cardinal like
          "ten", or ~:r prints an ordinal like "tenth".

               (format #t "~r" 9)  -| nine        ;; cardinal
               (format #t "~r" -9) -| minus nine  ;; cardinal
               (format #t "~:r" 9) -| ninth       ;; ordinal

          And also with no parameters, ~@r gives roman numerals and
          ~:@r gives old roman numerals.  In old roman numerals there's
          no "subtraction", so 9 is VIIII instead of IX.  In both cases
          only positive numbers can be output.

               (format #t "~@r" 89)  -| LXXXIX     ;; roman
               (format #t "~:@r" 89) -| LXXXVIIII  ;; old roman

          When a parameter is given it means numeric output in the
          specified RADIX.  The modifiers and parameters following the
          radix are the same as described for ~d etc above.

               (format #f "~3r" 27)   => "1000"    ;; base 3
               (format #f "~3,5r" 26) => "  222"   ;; base 3 width 5

    ~f
          Fixed-point float.  Parameters: WIDTH, DECIMALS, SCALE,
          OVERFLOWCHAR, PADCHAR.

          Output a number or number string in fixed-point format, ie.
          with a decimal point.

               (format #t "~f" 5)      -| 5.0
               (format #t "~f" "123")  -| 123.0
               (format #t "~f" "1e-1") -| 0.1

          ~@f prints a + sign on positive numbers (including zero).

               (format #t "~@f" 0) -| +0.0

          If the output is less than WIDTH characters it's padded on the
          left with PADCHAR (space by default).  If the output equals or
          exceeds WIDTH then there's no padding.  The default for WIDTH
          is no padding.

               (format #f "~6f" -1.5)      => "  -1.5"
               (format #f "~6,,,,'*f" 23)  => "**23.0"
               (format #f "~6f" 1234567.0) => "1234567.0"

          DECIMALS is how many digits to print after the decimal point,
          with the value rounded or padded with zeros as necessary.
          (The default is to output as many decimals as required.)

               (format #t "~1,2f" 3.125) -| 3.13
               (format #t "~1,2f" 1.5)   -| 1.50

          SCALE is a power of 10 applied to the value, moving the
          decimal point that many places.  A positive SCALE increases
          the value shown, a negative decreases it.

               (format #t "~,,2f" 1234)  -| 123400.0
               (format #t "~,,-2f" 1234) -| 12.34

          If OVERFLOWCHAR and WIDTH are both given and if the output
          would exceed WIDTH, then that many OVERFLOWCHARs are printed
          instead of the value.

               (format #t "~6,,,'xf" 12345) -| 12345.
               (format #t "~5,,,'xf" 12345) -| xxxxx

    ~e
          Exponential float.  Parameters: WIDTH, MANTDIGITS, EXPDIGITS,
          INTDIGITS, OVERFLOWCHAR, PADCHAR, EXPCHAR.

          Output a number or number string in exponential notation.

               (format #t "~e" 5000.25) -| 5.00025E+3
               (format #t "~e" "123.4") -| 1.234E+2
               (format #t "~e" "1e4")   -| 1.0E+4

          ~@e prints a + sign on positive numbers (including zero).
          (This is for the mantissa, a + or - sign is always shown on
          the exponent.)

               (format #t "~@e" 5000.0) -| +5.0E+3

          If the output is less than WIDTH characters it's padded on the
          left with PADCHAR (space by default).  The default for WIDTH
          is to output with no padding.

               (format #f "~10e" 1234.0)     => "  1.234E+3"
               (format #f "~10,,,,,'*e" 0.5) => "****5.0E-1"

          MANTDIGITS is the number of digits shown in the mantissa after
          the decimal point.  The value is rounded or trailing zeros
          are added as necessary.  The default MANTDIGITS is to show as
          much as needed by the value.

               (format #f "~,3e" 11111.0) => "1.111E+4"
               (format #f "~,8e" 123.0)   => "1.23000000E+2"

          EXPDIGITS is the minimum number of digits shown for the
          exponent, with leading zeros added if necessary.  The default
          for EXPDIGITS is to show only as many digits as required.  At
          least 1 digit is always shown.

               (format #f "~,,1e" 1.0e99) => "1.0E+99"
               (format #f "~,,6e" 1.0e99) => "1.0E+000099"

          INTDIGITS (default 1) is the number of digits to show before
          the decimal point in the mantissa.  INTDIGITS can be zero, in
          which case the integer part is a single 0, or it can be
          negative, in which case leading zeros are shown after the
          decimal point.

               (format #t "~,,,3e" 12345.0)  -| 123.45E+2
               (format #t "~,,,0e" 12345.0)  -| 0.12345E+5
               (format #t "~,,,-3e" 12345.0) -| 0.00012345E+8

          If OVERFLOWCHAR is given then WIDTH is a hard limit.  If the
          output would exceed WIDTH then instead that many
          OVERFLOWCHARs are printed.

               (format #f "~6,,,,'xe" 100.0) => "1.0E+2"
               (format #f "~3,,,,'xe" 100.0) => "xxx"

          EXPCHAR is the exponent marker character (default E).

               (format #t "~,,,,,,'ee" 100.0) -| 1.0e+2

    ~g
          General float.  Parameters: WIDTH, MANTDIGITS, EXPDIGITS,
          INTDIGITS, OVERFLOWCHAR, PADCHAR, EXPCHAR.

          Output a number or number string in either exponential format
          the same as ~e, or fixed-point format like ~f but aligned
          where the mantissa would have been and followed by padding
          where the exponent would have been.

          Fixed-point is used when the absolute value is 0.1 or more
          and it takes no more space than the mantissa in exponential
          format, ie.  basically up to MANTDIGITS digits.

               (format #f "~12,4,2g" 999.0)    => "   999.0    "
               (format #f "~12,4,2g" "100000") => "  1.0000E+05"

          The parameters are interpreted as per ~e above.  When
          fixed-point is used, the DECIMALS parameter to ~f is
          established from MANTDIGITS, so as to give a total
          MANTDIGITS+1 figures.

    ~$
          Monetary style fixed-point float.  Parameters: DECIMALS,
          INTDIGITS, WIDTH, PADCHAR.

          Output a number or number string in fixed-point format, ie.
          with a decimal point.  DECIMALS is the number of decimal
          places to show, default 2.

               (format #t "~$" 5)       -| 5.00
               (format #t "~4$" "2.25") -| 2.2500
               (format #t "~4$" "1e-2") -| 0.0100

          ~@$ prints a + sign on positive numbers (including zero).

               (format #t "~@$" 0) -| +0.00

          INTDIGITS is a minimum number of digits to show in the integer
          part of the value (default 1).

               (format #t "~,3$" 9.5)   -| 009.50
               (format #t "~,0$" 0.125) -| .13

          If the output is less than WIDTH characters (default 0), it's
          padded on the left with PADCHAR (default space).  ~:$ puts
          the padding after the sign.

               (format #f "~,,8$" -1.5)   => "   -1.50"
               (format #f "~,,8:$" -1.5)  => "-   1.50"
               (format #f "~,,8,'.:@$" 3) => "+...3.00"

          Note that floating point for dollar amounts is generally not
          a good idea, because a cent 0.01 cannot be represented
          exactly in the binary floating point Guile uses, which leads
          to slowly accumulating rounding errors.  Keeping values as
          cents (or fractions of a cent) in integers then printing with
          the scale option in ~f may be a better approach.

    ~i
          Complex fixed-point float.  Parameters: WIDTH, DECIMALS,
          SCALE, OVERFLOWCHAR, PADCHAR.

          Output the argument as a complex number, with both real and
          imaginary part shown (even if one or both are zero).

          The parameters and modifiers are the same as for fixed-point
          ~f described above.  The real and imaginary parts are both
          output with the same given parameters and modifiers, except
          that for the imaginary part the @ modifier is always enabled,
          so as to print a + sign between the real and imaginary parts.

               (format #t "~i" 1)  -| 1.0+0.0i

    ~p
          Plural.  No parameters.

          Output nothing if the argument is 1, or `s' for any other
          value.

               (format #t "enter name~p" 1) -| enter name
               (format #t "enter name~p" 2) -| enter names

          ~@p prints `y' for 1 or `ies' otherwise.

               (format #t "pupp~@p" 1) -| puppy
               (format #t "pupp~@p" 2) -| puppies

          ~:p re-uses the preceding argument instead of taking a new
          one, which can be convenient when printing some sort of count.

               (format #t "~d cat~:p" 9)   -| 9 cats
               (format #t "~d pupp~:@p" 5) -| 5 puppies

          ~p is designed for English plurals and there's no attempt to
          support other languages.  ~[ conditionals (below) may be able
          to help.  When using `gettext' to translate messages
          `ngettext' is probably best though (*note
          Internationalization::).

    ~y
          Structured printing.  Parameters: WIDTH.

          ~y outputs an argument using `pretty-print' (*note Pretty
          Printing::). The result will be formatted to fit within WIDTH
          columns (79 by default), consuming multiple lines if
          necessary.

          ~@y outputs an argument using `truncated-print' (*note Pretty
          Printing::). The resulting code will be formatted to fit
          within WIDTH columns (79 by default), on a single line. The
          output will be truncated if necessary.

          ~:@y is like ~@y, except the WIDTH parameter is interpreted
          to be the maximum column to which to output. That is to say,
          if you are at column 10, and ~60:@y is seen, the datum will
          be truncated to 50 columns.

    ~?
    ~k
          Sub-format.  No parameters.

          Take a format string argument and a second argument which is
          a list of arguments for that string, and output the result.

               (format #t "~?" "~d ~d" '(1 2))    -| 1 2

          ~@? takes arguments for the sub-format directly rather than
          in a list.

               (format #t "~@? ~s" "~d ~d" 1 2 "foo") -| 1 2 "foo"

          ~? and ~k are the same, ~k is provided for T-Scheme
          compatibility.

    ~*
          Argument jumping.  Parameter: N.

          Move forward N arguments (default 1) in the argument list.
          ~:* moves backwards.  (N cannot be negative.)

               (format #f "~d ~2*~d" 1 2 3 4) => "1 4"
               (format #f "~d ~:*~d" 6)       => "6 6"

          ~@* moves to argument number N.  The first argument is number
          0 (and that's the default for N).

               (format #f "~d~d again ~@*~d~d" 1 2) => "12 again 12"
               (format #f "~d~d~d ~1@*~d~d" 1 2 3)  => "123 23"

          A # move to the end followed by a : modifier move back can be
          used for an absolute position relative to the end of the
          argument list, a reverse of what the @ modifier does.

               (format #t "~#*~2:*~a" 'a 'b 'c 'd)   -| c

          At the end of the format string the current argument position
          doesn't matter, any further arguments are ignored.

    ~t
          Advance to a column position.  Parameters: COLNUM, COLINC,
          PADCHAR.

          Output PADCHAR (space by default) to move to the given COLNUM
          column.  The start of the line is column 0, the default for
          COLNUM is 1.

               (format #f "~tX")  => " X"
               (format #f "~3tX") => "   X"

          If the current column is already past COLNUM, then the move is
          to there plus a multiple of COLINC, ie. column COLNUM + N *
          COLINC for the smallest N which makes that value greater than
          or equal to the current column.  The default COLINC is 1
          (which means no further move).

               (format #f "abcd~2,5,'.tx") => "abcd...x"

          ~@t takes COLNUM as an offset from the current column.
          COLNUM many pad characters are output, then further padding to
          make the current column a multiple of COLINC, if it isn't
          already so.

               (format #f "a~3,5'*@tx") => "a****x"

          ~t is implemented using `port-column' (*note Reading::), so
          it works even there has been other output before `format'.

    ~~
          Tilde character.  Parameter: N.

          Output a tilde character ~, or N many if a parameter is
          given.  Normally ~ introduces an escape sequence, ~~ is the
          way to output a literal tilde.

    ~%
          Newline.  Parameter: N.

          Output a newline character, or N many if a parameter is given.
          A newline (or a few newlines) can of course be output just by
          including them in the format string.

    ~&
          Start a new line.  Parameter: N.

          Output a newline if not already at the start of a line.  With
          a parameter, output that many newlines, but with the first
          only if not already at the start of a line.  So for instance
          3 would be a newline if not already at the start of a line,
          and 2 further newlines.

    ~_
          Space character.  Parameter: N.

          Output a space character, or N many if a parameter is given.

          With a variable parameter this is one way to insert runtime
          calculated padding (~t or the various field widths can do
          similar things).

               (format #f "~v_foo" 4) => "    foo"

    ~/
          Tab character.  Parameter: N.

          Output a tab character, or N many if a parameter is given.

    ~|
          Formfeed character.  Parameter: N.

          Output a formfeed character, or N many if a parameter is
          given.

    ~!
          Force output.  No parameters.

          At the end of output, call `force-output' to flush any
          buffers on the destination (*note Writing::).  ~! can occur
          anywhere in the format string, but the force is done at the
          end of output.

          When output is to a string (destination `#f'), ~! does
          nothing.

    ~newline  (ie. newline character)
          Continuation line.  No parameters.

          Skip this newline and any following whitespace in the format
          string, ie. don't send it to the output.  This can be used to
          break up a long format string for readability, but not print
          the extra whitespace.

               (format #f "abc~
                           ~d def~
                           ~d" 1 2) => "abc1 def2"

          ~:newline skips the newline but leaves any further whitespace
          to be printed normally.

          ~@newline prints the newline then skips following whitespace.

    ~( ~)
          Case conversion.  No parameters.

          Between ~( and ~) the case of all output is changed.  The
          modifiers on ~( control the conversion.

               ~( -- lower case.

               ~:@( -- upper case.

          For example,

               (format #t "~(Hello~)")   -| hello
               (format #t "~:@(Hello~)") -| HELLO

          In the future it's intended the modifiers : and @ alone will
          capitalize the first letters of words, as per Common Lisp
          `format', but the current implementation of this is flawed and
          not recommended for use.

          Case conversions do not nest, currently.  This might change
          in the future, but if it does then it will be to Common Lisp
          style where the outermost conversion has priority, overriding
          inner ones (making those fairly pointless).

    ~{ ~}
          Iteration.  Parameter: MAXREPS (for ~{).

          The format between ~{ and ~} is iterated.  The modifiers to
          ~{ determine how arguments are taken.  The default is a list
          argument with each iteration successively consuming elements
          from it.  This is a convenient way to output a whole list.

               (format #t "~{~d~}"     '(1 2 3))       -| 123
               (format #t "~{~s=~d ~}" '("x" 1 "y" 2)) -| "x"=1 "y"=2

          ~:{ takes a single argument which is a list of lists, each of
          those contained lists gives the arguments for the iterated
          format.

               (format #t "~:{~dx~d ~}" '((1 2) (3 4) (5 6)))
               -| 1x2 3x4 5x6

          ~@{ takes arguments directly, with each iteration
          successively consuming arguments.

               (format #t "~@{~d~}"     1 2 3)       -| 123
               (format #t "~@{~s=~d ~}" "x" 1 "y" 2) -| "x"=1 "y"=2

          ~:@{ takes list arguments, one argument for each iteration,
          using that list for the format.

               (format #t "~:@{~dx~d ~}" '(1 2) '(3 4) '(5 6))
               -| 1x2 3x4 5x6

          Iterating stops when there are no more arguments or when the
          MAXREPS parameter to ~{ is reached (default no maximum).

               (format #t "~2{~d~}" '(1 2 3 4)) -| 12

          If the format between ~{ and ~} is empty, then a format
          string argument is taken (before iteration argument(s)) and
          used instead.  This allows a sub-format (like ~? above) to be
          iterated.

               (format #t "~{~}" "~d" '(1 2 3)) -| 123

          Iterations can be nested, an inner iteration operates in the
          same way as described, but of course on the arguments the
          outer iteration provides it.  This can be used to work into
          nested list structures.  For example in the following the
          inner ~{~d~}x is applied to `(1 2)' then `(3 4 5)' etc.

               (format #t "~{~{~d~}x~}" '((1 2) (3 4 5))) -| 12x345x

          See also ~^ below for escaping from iteration.

    ~[ ~; ~]
          Conditional.  Parameter: SELECTOR.

          A conditional block is delimited by ~[ and ~], and ~;
          separates clauses within the block.  ~[ takes an integer
          argument and that number clause is used.  The first clause is
          number 0.

               (format #f "~[peach~;banana~;mango~]" 1)  => "banana"

          The SELECTOR parameter can be used for the clause number,
          instead of taking an argument.

               (format #f "~2[peach~;banana~;mango~]") => "mango"

          If the clause number is out of range then nothing is output.
          Or the last clause can be ~:; to use that for a number out of
          range.

               (format #f "~[banana~;mango~]"         99) => ""
               (format #f "~[banana~;mango~:;fruit~]" 99) => "fruit"

          ~:[ treats the argument as a flag, and expects two clauses.
          The first is used if the argument is `#f' or the second
          otherwise.

               (format #f "~:[false~;not false~]" #f)   => "false"
               (format #f "~:[false~;not false~]" 'abc) => "not false"

               (let ((n 3))
                 (format #t "~d gnu~:[s are~; is~] here" n (= 1 n)))
               -| 3 gnus are here

          ~@[ also treats the argument as a flag, and expects one
          clause.  If the argument is `#f' then no output is produced
          and the argument is consumed, otherwise the clause is used
          and the argument is not consumed, it's left for the clause.
          This can be used for instance to suppress output if `#f'
          means something not available.

               (format #f "~@[temperature=~d~]" 27) => "temperature=27"
               (format #f "~@[temperature=~d~]" #f) => ""

    ~^
          Escape.  Parameters: VAL1, VAL2, VAL3.

          Stop formatting if there are no more arguments.  This can be
          used for instance to have a format string adapt to a variable
          number of arguments.

               (format #t "~d~^ ~d" 1)   -| 1
               (format #t "~d~^ ~d" 1 2) -| 1 2

          Within a ~{ ~} iteration, ~^ stops the current iteration step
          if there are no more arguments to that step, but continuing
          with possible further steps and the rest of the format.  This
          can be used for instance to avoid a separator on the last
          iteration, or to adapt to variable length argument lists.

               (format #f "~{~d~^/~} go"    '(1 2 3))     => "1/2/3 go"
               (format #f "~:{ ~d~^~d~} go" '((1) (2 3))) => " 1 23 go"

          Within a ~? sub-format, ~^ operates just on that sub-format.
          If it terminates the sub-format then the originating format
          will still continue.

               (format #t "~? items" "~d~^ ~d" '(1))   -| 1 items
               (format #t "~? items" "~d~^ ~d" '(1 2)) -| 1 2 items

          The parameters to ~^ (which are numbers) change the condition
          used to terminate.  For a single parameter, termination is
          when that value is zero (notice this makes plain ~^
          equivalent to ~#^).  For two parameters, termination is when
          those two are equal.  For three parameters, termination is
          when VAL1 <= VAL2 and VAL2 <= VAL3.

    ~q
          Inquiry message.  Insert a copyright message into the output.

          ~:q inserts the format implementation version.


     It's an error if there are not enough arguments for the escapes in
     the format string, but any excess arguments are ignored.

     Iterations ~{ ~} and conditionals ~[ ~; ~] can be nested, but must
     be properly nested, meaning the inner form must be entirely within
     the outer form.  So it's not possible, for instance, to try to
     conditionalize the endpoint of an iteration.

          (format #t "~{ ~[ ... ~] ~}" ...)       ;; good
          (format #t "~{ ~[ ... ~} ... ~]" ...)   ;; bad

     The same applies to case conversions ~( ~), they must properly
     nest with respect to iterations and conditionals (though currently
     a case conversion cannot nest within another case conversion).

     When a sub-format (~?) is used, that sub-format string must be
     self-contained.  It cannot for instance give a ~{ to begin an
     iteration form and have the ~} up in the originating format, or
     similar.


   Guile contains a `format' procedure even when the module `(ice-9
format)' is not loaded.  The default `format' is `simple-format' (*note
Writing::), it doesn't support all escape sequences documented in this
section, and will signal an error if you try to use one of them.  The
reason for two versions is that the full `format' is fairly large and
requires some time to load.  `simple-format' is often adequate too.

7.11 File Tree Walk
===================

The functions in this section traverse a tree of files and directories.
They come in two flavors: the first one is a high-level functional
interface, and the second one is similar to the C `ftw' and `nftw'
routines (*note Working with Directory Trees: (libc)Working with
Directory Trees.).

     (use-modules (ice-9 ftw))


 -- Scheme Procedure: file-system-tree file-name [enter? [stat]]
     Return a tree of the form `(FILE-NAME STAT CHILDREN ...)' where
     STAT is the result of `(STAT FILE-NAME)' and CHILDREN are similar
     structures for each file contained in FILE-NAME when it designates
     a directory.

     The optional ENTER? predicate is invoked as `(ENTER?  NAME STAT)'
     and should return true to allow recursion into directory NAME; the
     default value is a procedure that always returns `#t'.  When a
     directory does not match ENTER?, it nonetheless appears in the
     resulting tree, only with zero children.

     The STAT argument is optional and defaults to `lstat', as for
     `file-system-fold' (see below.)

     The example below shows how to obtain a hierarchical listing of the
     files under the `module/language' directory in the Guile source
     tree, discarding their `stat' info:

          (use-modules (ice-9 match))

          (define remove-stat
            ;; Remove the `stat' object the `file-system-tree' provides
            ;; for each file in the tree.
            (match-lambda
              ((name stat)              ; flat file
               name)
              ((name stat children ...) ; directory
               (list name (map remove-stat children)))))

          (let ((dir (string-append (assq-ref %guile-build-info 'top_srcdir)
                                    "/module/language")))
            (remove-stat (file-system-tree dir)))

          =>
          ("language"
           (("value" ("spec.go" "spec.scm"))
            ("scheme"
             ("spec.go"
              "spec.scm"
              "compile-tree-il.scm"
              "decompile-tree-il.scm"
              "decompile-tree-il.go"
              "compile-tree-il.go"))
            ("tree-il"
             ("spec.go"
              "fix-letrec.go"
              "inline.go"
              "fix-letrec.scm"
              "compile-glil.go"
              "spec.scm"
              "optimize.scm"
              "primitives.scm"
              ...))
            ...))

   It is often desirable to process directories entries directly, rather
than building up a tree of entries in memory, like `file-system-tree'
does.  The following procedure, a "combinator", is designed to allow
directory entries to be processed directly as a directory tree is
traversed; in fact, `file-system-tree' is implemented in terms of it.

 -- Scheme Procedure: file-system-fold enter? leaf down up skip error
          init file-name [stat]
     Traverse the directory at FILE-NAME, recursively, and return the
     result of the successive applications of the LEAF, DOWN, UP, and
     SKIP procedures as described below.

     Enter sub-directories only when `(ENTER? PATH STAT RESULT)'
     returns true.  When a sub-directory is entered, call `(DOWN PATH
     STAT RESULT)', where PATH is the path of the sub-directory and
     STAT the result of `(false-if-exception (STAT PATH))'; when it is
     left, call `(UP PATH STAT RESULT)'.

     For each file in a directory, call `(LEAF PATH STAT RESULT)'.

     When ENTER? returns `#f', or when an unreadable directory is
     encountered, call `(SKIP PATH STAT RESULT)'.

     When FILE-NAME names a flat file, `(LEAF PATH STAT INIT)' is
     returned.

     When an `opendir' or STAT call fails, call `(ERROR PATH STAT ERRNO
     RESULT)', with ERRNO being the operating system error number that
     was raised--e.g., `EACCES'--and STAT either `#f' or the result of
     the STAT call for that entry, when available.

     The special `.' and `..' entries are not passed to these
     procedures.  The PATH argument to the procedures is a full file
     name--e.g., `"../foo/bar/gnu"'; if FILE-NAME is an absolute file
     name, then PATH is also an absolute file name.  Files and
     directories, as identified by their device/inode number pair, are
     traversed only once.

     The optional STAT argument defaults to `lstat', which means that
     symbolic links are not followed; the `stat' procedure can be used
     instead when symbolic links are to be followed (*note stat: File
     System.).

     The example below illustrates the use of `file-system-fold':

          (define (total-file-size file-name)
            "Return the size in bytes of the files under FILE-NAME (similar
          to `du --apparent-size' with GNU Coreutils.)"

            (define (enter? name stat result)
              ;; Skip version control directories.
              (not (member (basename name) '(".git" ".svn" "CVS"))))
            (define (leaf name stat result)
              ;; Return RESULT plus the size of the file at NAME.
              (+ result (stat:size stat)))

            ;; Count zero bytes for directories.
            (define (down name stat result) result)
            (define (up name stat result) result)

            ;; Likewise for skipped directories.
            (define (skip name stat result) result)

            ;; Ignore unreadable files/directories but warn the user.
            (define (error name stat errno result)
              (format (current-error-port) "warning: ~a: ~a~%"
                      name (strerror errno))
              result)

            (file-system-fold enter? leaf down up skip error
                                     0  ; initial counter is zero bytes
                                     file-name))

          (total-file-size ".")
          => 8217554

          (total-file-size "/dev/null")
          => 0

   The alternative C-like functions are described below.

 -- Scheme Procedure: scandir name [select? [entry<?]]
     Return the list of the names of files contained in directory NAME
     that match predicate SELECT? (by default, all files).  The
     returned list of file names is sorted according to ENTRY<?, which
     defaults to `string-locale<?' such that file names are sorted in
     the locale's alphabetical order (*note Text Collation::).  Return
     `#f' when NAME is unreadable or is not a directory.

     This procedure is modeled after the C library function of the same
     name (*note Scanning Directory Content: (libc)Scanning Directory
     Content.).

 -- Scheme Procedure: ftw startname proc ['hash-size n]
     Walk the file system tree descending from STARTNAME, calling PROC
     for each file and directory.

     Hard links and symbolic links are followed.  A file or directory is
     reported to PROC only once, and skipped if seen again in another
     place.  One consequence of this is that `ftw' is safe against
     circularly linked directory structures.

     Each PROC call is `(PROC filename statinfo flag)' and it should
     return `#t' to continue, or any other value to stop.

     FILENAME is the item visited, being STARTNAME plus a further path
     and the name of the item.  STATINFO is the return from `stat'
     (*note File System::) on FILENAME.  FLAG is one of the following
     symbols,

    `regular'
          FILENAME is a file, this includes special files like devices,
          named pipes, etc.

    `directory'
          FILENAME is a directory.

    `invalid-stat'
          An error occurred when calling `stat', so nothing is known.
          STATINFO is `#f' in this case.

    `directory-not-readable'
          FILENAME is a directory, but one which cannot be read and
          hence won't be recursed into.

    `symlink'
          FILENAME is a dangling symbolic link.  Symbolic links are
          normally followed and their target reported, the link itself
          is reported if the target does not exist.

     The return value from `ftw' is `#t' if it ran to completion, or
     otherwise the non-`#t' value from PROC which caused the stop.

     Optional argument symbol `hash-size' and an integer can be given
     to set the size of the hash table used to track items already
     visited.  (*note Hash Table Reference::)

     In the current implementation, returning non-`#t' from PROC is the
     only valid way to terminate `ftw'.  PROC must not use `throw' or
     similar to escape.

 -- Scheme Procedure: nftw startname proc ['chdir] ['depth] ['hash-size
          n] ['mount] ['physical]
     Walk the file system tree starting at STARTNAME, calling PROC for
     each file and directory.  `nftw' has extra features over the basic
     `ftw' described above.

     Like `ftw', hard links and symbolic links are followed.  A file or
     directory is reported to PROC only once, and skipped if seen again
     in another place.  One consequence of this is that `nftw' is safe
     against circular linked directory structures.

     Each PROC call is `(PROC filename statinfo flag base level)' and
     it should return `#t' to continue, or any other value to stop.

     FILENAME is the item visited, being STARTNAME plus a further path
     and the name of the item.  STATINFO is the return from `stat' on
     FILENAME (*note File System::).  BASE is an integer offset into
     FILENAME which is where the basename for this item begins.  LEVEL
     is an integer giving the directory nesting level, starting from 0
     for the contents of STARTNAME (or that item itself if it's a
     file).  FLAG is one of the following symbols,

    `regular'
          FILENAME is a file, including special files like devices,
          named pipes, etc.

    `directory'
          FILENAME is a directory.

    `directory-processed'
          FILENAME is a directory, and its contents have all been
          visited.  This flag is given instead of `directory' when the
          `depth' option below is used.

    `invalid-stat'
          An error occurred when applying `stat' to FILENAME, so
          nothing is known about it.  STATINFO is `#f' in this case.

    `directory-not-readable'
          FILENAME is a directory, but one which cannot be read and
          hence won't be recursed into.

    `stale-symlink'
          FILENAME is a dangling symbolic link.  Links are normally
          followed and their target reported, the link itself is
          reported if its target does not exist.

    `symlink'
          When the `physical' option described below is used, this
          indicates FILENAME is a symbolic link whose target exists (and
          is not being followed).

     The following optional arguments can be given to modify the way
     `nftw' works.  Each is passed as a symbol (and `hash-size' takes a
     following integer value).

    `chdir'
          Change to the directory containing the item before calling
          PROC.  When `nftw' returns the original current directory is
          restored.

          Under this option, generally the BASE parameter to each PROC
          call should be used to pick out the base part of the
          FILENAME.  The FILENAME is still a path but with a changed
          directory it won't be valid (unless the STARTNAME directory
          was absolute).

    `depth'
          Visit files "depth first", meaning PROC is called for the
          contents of each directory before it's called for the
          directory itself.  Normally a directory is reported first,
          then its contents.

          Under this option, the FLAG to PROC for a directory is
          `directory-processed' instead of `directory'.

    `hash-size N'
          Set the size of the hash table used to track items already
          visited.  (*note Hash Table Reference::)

    `mount'
          Don't cross a mount point, meaning only visit items on the
          same file system as STARTNAME (ie. the same `stat:dev').

    `physical'
          Don't follow symbolic links, instead report them to PROC as
          `symlink'.  Dangling links (those whose target doesn't exist)
          are still reported as `stale-symlink'.

     The return value from `nftw' is `#t' if it ran to completion, or
     otherwise the non-`#t' value from PROC which caused the stop.

     In the current implementation, returning non-`#t' from PROC is the
     only valid way to terminate `ftw'.  PROC must not use `throw' or
     similar to escape.

7.12 Queues
===========

The functions in this section are provided by

     (use-modules (ice-9 q))

   This module implements queues holding arbitrary scheme objects and
designed for efficient first-in / first-out operations.

   `make-q' creates a queue, and objects are entered and removed with
`enq!' and `deq!'.  `q-push!' and `q-pop!' can be used too, treating
the front of the queue like a stack.


 -- Scheme Procedure: make-q
     Return a new queue.

 -- Scheme Procedure: q? obj
     Return `#t' if OBJ is a queue, or `#f' if not.

     Note that queues are not a distinct class of objects but are
     implemented with cons cells.  For that reason certain list
     structures can get `#t' from `q?'.

 -- Scheme Procedure: enq! q obj
     Add OBJ to the rear of Q, and return Q.

 -- Scheme Procedure: deq! q
 -- Scheme Procedure: q-pop! q
     Remove and return the front element from Q.  If Q is empty, a
     `q-empty' exception is thrown.

     `deq!' and `q-pop!' are the same operation, the two names just let
     an application match `enq!' with `deq!', or `q-push!' with
     `q-pop!'.

 -- Scheme Procedure: q-push! q obj
     Add OBJ to the front of Q, and return Q.

 -- Scheme Procedure: q-length q
     Return the number of elements in Q.

 -- Scheme Procedure: q-empty? q
     Return true if Q is empty.

 -- Scheme Procedure: q-empty-check q
     Throw a `q-empty' exception if Q is empty.

 -- Scheme Procedure: q-front q
     Return the first element of Q (without removing it).  If Q is
     empty, a `q-empty' exception is thrown.

 -- Scheme Procedure: q-rear q
     Return the last element of Q (without removing it).  If Q is
     empty, a `q-empty' exception is thrown.

 -- Scheme Procedure: q-remove! q obj
     Remove all occurrences of OBJ from Q, and return Q.  OBJ is
     compared to queue elements using `eq?'.


   The `q-empty' exceptions described above are thrown just as `(throw
'q-empty)', there's no message etc like an error throw.

   A queue is implemented as a cons cell, the `car' containing a list
of queued elements, and the `cdr' being the last cell in that list (for
ease of enqueuing).

     (LIST . LAST-CELL)

If the queue is empty, LIST is the empty list and LAST-CELL is `#f'.

   An application can directly access the queue list if desired, for
instance to search the elements or to insert at a specific point.

 -- Scheme Procedure: sync-q! q
     Recompute the LAST-CELL field in Q.

     All the operations above maintain LAST-CELL as described, so
     normally there's no need for `sync-q!'.  But if an application
     modifies the queue LIST then it must either maintain LAST-CELL
     similarly, or call `sync-q!' to recompute it.

7.13 Streams
============

A stream represents a sequence of values, each of which is calculated
only when required.  This allows large or even infinite sequences to be
represented and manipulated with familiar operations like "car", "cdr",
"map" or "fold".  In such manipulations only as much as needed is
actually held in memory at any one time.  The functions in this section
are available from

     (use-modules (ice-9 streams))

   Streams are implemented using promises (*note Delayed Evaluation::),
which is how the underlying calculation of values is made only when
needed, and the values then retained so the calculation is not repeated.

Here is a simple example producing a stream of all odd numbers,

     (define odds (make-stream (lambda (state)
                                 (cons state (+ state 2)))
                               1))
     (stream-car odds)              => 1
     (stream-car (stream-cdr odds)) => 3

`stream-map' could be used to derive a stream of odd squares,

     (define (square n) (* n n))
     (define oddsquares (stream-map square odds))

   These are infinite sequences, so it's not possible to convert them to
a list, but they could be printed (infinitely) with for example

     (stream-for-each (lambda (n sq)
                        (format #t "~a squared is ~a\n" n sq))
                      odds oddsquares)
     -|
     1 squared is 1
     3 squared is 9
     5 squared is 25
     7 squared is 49
     ...


 -- Scheme Procedure: make-stream proc initial-state
     Return a new stream, formed by calling PROC successively.

     Each call is `(PROC STATE)', it should return a pair, the `car'
     being the value for the stream, and the `cdr' being the new STATE
     for the next call.  For the first call STATE is the given
     INITIAL-STATE.  At the end of the stream, PROC should return some
     non-pair object.

 -- Scheme Procedure: stream-car stream
     Return the first element from STREAM.  STREAM must not be empty.

 -- Scheme Procedure: stream-cdr stream
     Return a stream which is the second and subsequent elements of
     STREAM.  STREAM must not be empty.

 -- Scheme Procedure: stream-null? stream
     Return true if STREAM is empty.

 -- Scheme Procedure: list->stream list
 -- Scheme Procedure: vector->stream vector
     Return a stream with the contents of LIST or VECTOR.

     LIST or VECTOR should not be modified subsequently, since it's
     unspecified whether changes there will be reflected in the stream
     returned.

 -- Scheme Procedure: port->stream port readproc
     Return a stream which is the values obtained by reading from PORT
     using READPROC.  Each read call is `(READPROC PORT)', and it
     should return an EOF object (*note Reading::) at the end of input.

     For example a stream of characters from a file,

          (port->stream (open-input-file "/foo/bar.txt") read-char)

 -- Scheme Procedure: stream->list stream
     Return a list which is the entire contents of STREAM.

 -- Scheme Procedure: stream->reversed-list stream
     Return a list which is the entire contents of STREAM, but in
     reverse order.

 -- Scheme Procedure: stream->list&length stream
     Return two values (*note Multiple Values::), being firstly a list
     which is the entire contents of STREAM, and secondly the number of
     elements in that list.

 -- Scheme Procedure: stream->reversed-list&length stream
     Return two values (*note Multiple Values::) being firstly a list
     which is the entire contents of STREAM, but in reverse order, and
     secondly the number of elements in that list.

 -- Scheme Procedure: stream->vector stream
     Return a vector which is the entire contents of STREAM.

 -- Scheme Procedure: stream-fold proc init stream0 ... streamN
     Apply PROC successively over the elements of the given streams,
     from first to last until the end of the shortest stream is reached.
     Return the result from the last PROC call.

     Each call is `(PROC elem0 ... elemN prev)', where each ELEM is
     from the corresponding STREAM.  PREV is the return from the
     previous PROC call, or the given INIT for the first call.

 -- Scheme Procedure: stream-for-each proc stream0 ... streamN
     Call PROC on the elements from the given STREAMs.  The return
     value is unspecified.

     Each call is `(PROC elem0 ... elemN)', where each ELEM is from the
     corresponding STREAM.  `stream-for-each' stops when it reaches the
     end of the shortest STREAM.

 -- Scheme Procedure: stream-map proc stream0 ... streamN
     Return a new stream which is the results of applying PROC to the
     elements of the given STREAMs.

     Each call is `(PROC elem0 ... elemN)', where each ELEM is from the
     corresponding STREAM.  The new stream ends when the end of the
     shortest given STREAM is reached.

7.14 Buffered Input
===================

The following functions are provided by

     (use-modules (ice-9 buffered-input))

   A buffered input port allows a reader function to return chunks of
characters which are to be handed out on reading the port.  A notion of
further input for an application level logical expression is maintained
too, and passed through to the reader.

 -- Scheme Procedure: make-buffered-input-port reader
     Create an input port which returns characters obtained from the
     given READER function.  READER is called (READER cont), and should
     return a string or an EOF object.

     The new port gives precisely the characters returned by READER,
     nothing is added, so if any newline characters or other separators
     are desired they must come from the reader function.

     The CONT parameter to READER is `#f' for initial input, or `#t'
     when continuing an expression.  This is an application level
     notion, set with `set-buffered-input-continuation?!' below.  If
     the user has entered a partial expression then it allows READER
     for instance to give a different prompt to show more is required.

 -- Scheme Procedure: make-line-buffered-input-port reader
     Create an input port which returns characters obtained from the
     specified READER function, similar to `make-buffered-input-port'
     above, but where READER is expected to be a line-oriented.

     READER is called (READER cont), and should return a string or an
     EOF object as above.  Each string is a line of input without a
     newline character, the port code inserts a newline after each
     string.

 -- Scheme Procedure: set-buffered-input-continuation?! port cont
     Set the input continuation flag for a given buffered input PORT.

     An application uses this by calling with a CONT flag of `#f' when
     beginning to read a new logical expression.  For example with the
     Scheme `read' function (*note Scheme Read::),

          (define my-port (make-buffered-input-port my-reader))

          (set-buffered-input-continuation?! my-port #f)
          (let ((obj (read my-port)))
            ...

7.15 Expect
===========

The macros in this section are made available with:

     (use-modules (ice-9 expect))

   `expect' is a macro for selecting actions based on the output from a
port.  The name comes from a tool of similar functionality by Don Libes.
Actions can be taken when a particular string is matched, when a timeout
occurs, or when end-of-file is seen on the port.  The `expect' macro is
described below; `expect-strings' is a front-end to `expect' based on
regexec (see the regular expression documentation).

 -- Macro: expect-strings clause ...
     By default, `expect-strings' will read from the current input port.
     The first term in each clause consists of an expression evaluating
     to a string pattern (regular expression).  As characters are read
     one-by-one from the port, they are accumulated in a buffer string
     which is matched against each of the patterns.  When a pattern
     matches, the remaining expression(s) in the clause are evaluated
     and the value of the last is returned.  For example:

          (with-input-from-file "/etc/passwd"
            (lambda ()
              (expect-strings
                ("^nobody" (display "Got a nobody user.\n")
                           (display "That's no problem.\n"))
                ("^daemon" (display "Got a daemon user.\n")))))

     The regular expression is compiled with the `REG_NEWLINE' flag, so
     that the ^ and $ anchors will match at any newline, not just at
     the start and end of the string.

     There are two other ways to write a clause:

     The expression(s) to evaluate can be omitted, in which case the
     result of the regular expression match (converted to strings, as
     obtained from regexec with match-pick set to "") will be returned
     if the pattern matches.

     The symbol `=>' can be used to indicate that the expression is a
     procedure which will accept the result of a successful regular
     expression match.  E.g.,

          ("^daemon" => write)
          ("^d(aemon)" => (lambda args (for-each write args)))
          ("^da(em)on" => (lambda (all sub)
                            (write all) (newline)
                            (write sub) (newline)))

     The order of the substrings corresponds to the order in which the
     opening brackets occur.

     A number of variables can be used to control the behaviour of
     `expect' (and `expect-strings').  Most have default top-level
     bindings to the value `#f', which produces the default behaviour.
     They can be redefined at the top level or locally bound in a form
     enclosing the expect expression.

    `expect-port'
          A port to read characters from, instead of the current input
          port.

    `expect-timeout'
          `expect' will terminate after this number of seconds,
          returning `#f' or the value returned by expect-timeout-proc.

    `expect-timeout-proc'
          A procedure called if timeout occurs.  The procedure takes a
          single argument: the accumulated string.

    `expect-eof-proc'
          A procedure called if end-of-file is detected on the input
          port.  The procedure takes a single argument: the accumulated
          string.

    `expect-char-proc'
          A procedure to be called every time a character is read from
          the port.  The procedure takes a single argument: the
          character which was read.

    `expect-strings-compile-flags'
          Flags to be used when compiling a regular expression, which
          are passed to `make-regexp' *Note Regexp Functions::.  The
          default value is `regexp/newline'.

    `expect-strings-exec-flags'
          Flags to be used when executing a regular expression, which
          are passed to regexp-exec *Note Regexp Functions::.  The
          default value is `regexp/noteol', which prevents `$' from
          matching the end of the string while it is still accumulating,
          but still allows it to match after a line break or at the end
          of file.

     Here's an example using all of the variables:

          (let ((expect-port (open-input-file "/etc/passwd"))
                (expect-timeout 1)
                (expect-timeout-proc
                  (lambda (s) (display "Times up!\n")))
                (expect-eof-proc
                  (lambda (s) (display "Reached the end of the file!\n")))
                (expect-char-proc display)
                (expect-strings-compile-flags (logior regexp/newline regexp/icase))
                (expect-strings-exec-flags 0))
             (expect-strings
               ("^nobody"  (display "Got a nobody user\n"))))

 -- Macro: expect clause ...
     `expect' is used in the same way as `expect-strings', but tests
     are specified not as patterns, but as procedures.  The procedures
     are called in turn after each character is read from the port,
     with two arguments: the value of the accumulated string and a flag
     to indicate whether end-of-file has been reached.  The flag will
     usually be `#f', but if end-of-file is reached, the procedures are
     called an additional time with the final accumulated string and
     `#t'.

     The test is successful if the procedure returns a non-false value.

     If the `=>' syntax is used, then if the test succeeds it must
     return a list containing the arguments to be provided to the
     corresponding expression.

     In the following example, a string will only be matched at the
     beginning of the file:

          (let ((expect-port (open-input-file "/etc/passwd")))
            (expect
               ((lambda (s eof?) (string=? s "fnord!"))
                  (display "Got a nobody user!\n"))))

     The control variables described for `expect-strings' also
     influence the behaviour of `expect', with the exception of
     variables whose names begin with `expect-strings-'.

7.16 `sxml-match': Pattern Matching of SXML
===========================================

The `(sxml match)' module provides syntactic forms for pattern matching
of SXML trees, in a "by example" style reminiscent of the pattern
matching of the `syntax-rules' and `syntax-case' macro systems.  *Note
the `(sxml simple)' module: sxml simple, for more information on SXML.

   The following example(1) provides a brief illustration, transforming
a music album catalog language into HTML.

     (define (album->html x)
       (sxml-match x
         [(album (@ (title ,t)) (catalog (num ,n) (fmt ,f)) ...)
          `(ul (li ,t)
               (li (b ,n) (i ,f)) ...)]))

   Three macros are provided: `sxml-match', `sxml-match-let', and
`sxml-match-let*'.

   Compared to a standard s-expression pattern matcher (*note Pattern
Matching::), `sxml-match' provides the following benefits:

   * matching of SXML elements does not depend on any degree of
     normalization of the SXML;

   * matching of SXML attributes (within an element) is under-ordered;
     the order of the attributes specified within the pattern need not
     match the ordering with the element being matched;

   * all attributes specified in the pattern must be present in the
     element being matched; in the spirit that XML is 'extensible', the
     element being matched may include additional attributes not
     specified in the pattern.

   The present module is a descendant of WebIt!, and was inspired by an
s-expression pattern matcher developed by Erik Hilsdale, Dan Friedman,
and Kent Dybvig at Indiana University.

Syntax
------

`sxml-match' provides `case'-like form for pattern matching of XML
nodes.

 -- Scheme Syntax: sxml-match input-expression clause ...
     Match INPUT-EXPRESSION, an SXML tree, according to the given
     CLAUSEs (one or more), each consisting of a pattern and one or
     more expressions to be evaluated if the pattern match succeeds.
     Optionally, each CLAUSE within `sxml-match' may include a "guard
     expression".

   The pattern notation is based on that of Scheme's `syntax-rules' and
`syntax-case' macro systems.  The grammar for the `sxml-match' syntax
is given below:

match-form ::= (sxml-match input-expression
                 clause+)

clause ::= [node-pattern action-expression+]
         | [node-pattern (guard expression*) action-expression+]

node-pattern ::= literal-pattern
               | pat-var-or-cata
               | element-pattern
               | list-pattern

literal-pattern ::= string
                  | character
                  | number
                  | #t
                  | #f

attr-list-pattern ::= (@ attribute-pattern*)
                    | (@ attribute-pattern* . pat-var-or-cata)

attribute-pattern ::= (tag-symbol attr-val-pattern)

attr-val-pattern ::= literal-pattern
                   | pat-var-or-cata
                   | (pat-var-or-cata default-value-expr)

element-pattern ::= (tag-symbol attr-list-pattern?)
                  | (tag-symbol attr-list-pattern? nodeset-pattern)
                  | (tag-symbol attr-list-pattern?
                                nodeset-pattern? . pat-var-or-cata)

list-pattern ::= (list nodeset-pattern)
               | (list nodeset-pattern? . pat-var-or-cata)
               | (list)

nodeset-pattern ::= node-pattern
                  | node-pattern ...
                  | node-pattern nodeset-pattern
                  | node-pattern ... nodeset-pattern

pat-var-or-cata ::= (unquote var-symbol)
                  | (unquote [var-symbol*])
                  | (unquote [cata-expression -> var-symbol*])

   Within a list or element body pattern, ellipses may appear only
once, but may be followed by zero or more node patterns.

   Guard expressions cannot refer to the return values of catamorphisms.

   Ellipses in the output expressions must appear only in an expression
context; ellipses are not allowed in a syntactic form.

   The sections below illustrate specific aspects of the `sxml-match'
pattern matcher.

Matching XML Elements
---------------------

The example below illustrates the pattern matching of an XML element:

     (sxml-match '(e (@ (i 1)) 3 4 5)
       [(e (@ (i ,d)) ,a ,b ,c) (list d a b c)]
       [,otherwise #f])

   Each clause in `sxml-match' contains two parts: a pattern and one or
more expressions which are evaluated if the pattern is successfully
match.  The example above matches an element `e' with an attribute `i'
and three children.

   Pattern variables are must be "unquoted" in the pattern.  The above
expression binds D to `1', A to `3', B to `4', and C to `5'.

Ellipses in Patterns
--------------------

As in `syntax-rules', ellipses may be used to specify a repeated
pattern.  Note that the pattern `item ...' specifies zero-or-more
matches of the pattern `item'.

   The use of ellipses in a pattern is illustrated in the code fragment
below, where nested ellipses are used to match the children of repeated
instances of an `a' element, within an element `d'.

     (define x '(d (a 1 2 3) (a 4 5) (a 6 7 8) (a 9 10)))

     (sxml-match x
       [(d (a ,b ...) ...)
        (list (list b ...) ...)])

   The above expression returns a value of `((1 2 3) (4 5) (6 7 8) (9
10))'.

Ellipses in Quasiquote'd Output
-------------------------------

Within the body of an `sxml-match' form, a slightly extended version of
quasiquote is provided, which allows the use of ellipses.  This is
illustrated in the example below.

     (sxml-match '(e 3 4 5 6 7)
       [(e ,i ... 6 7) `("start" ,(list 'wrap i) ... "end")]
       [,otherwise #f])

   The general pattern is that ``(something ,i ...)' is rewritten as
``(something ,@i)'.

Matching Nodesets
-----------------

A nodeset pattern is designated by a list in the pattern, beginning the
identifier list.  The example below illustrates matching a nodeset.

     (sxml-match '("i" "j" "k" "l" "m")
       [(list ,a ,b ,c ,d ,e)
        `((p ,a) (p ,b) (p ,c) (p ,d) (p ,e))])

   This example wraps each nodeset item in an HTML paragraph element.
This example can be rewritten and simplified through using ellipsis:

     (sxml-match '("i" "j" "k" "l" "m")
       [(list ,i ...)
        `((p ,i) ...)])

   This version will match nodesets of any length, and wrap each item
in the nodeset in an HTML paragraph element.

Matching the "Rest" of a Nodeset
--------------------------------

Matching the "rest" of a nodeset is achieved by using a `. rest)'
pattern at the end of an element or nodeset pattern.

   This is illustrated in the example below:

     (sxml-match '(e 3 (f 4 5 6) 7)
       [(e ,a (f . ,y) ,d)
        (list a y d)])

   The above expression returns `(3 (4 5 6) 7)'.

Matching the Unmatched Attributes
---------------------------------

Sometimes it is useful to bind a list of attributes present in the
element being matched, but which do not appear in the pattern.  This is
achieved by using a `. rest)' pattern at the end of the attribute list
pattern.  This is illustrated in the example below:

     (sxml-match '(a (@ (z 1) (y 2) (x 3)) 4 5 6)
       [(a (@ (y ,www) . ,qqq) ,t ,u ,v)
        (list www qqq t u v)])

   The above expression matches the attribute `y' and binds a list of
the remaining attributes to the variable QQQ.  The result of the above
expression is `(2 ((z 1) (x 3)) 4 5 6)'.

   This type of pattern also allows the binding of all attributes:

     (sxml-match '(a (@ (z 1) (y 2) (x 3)))
       [(a (@ . ,qqq))
        qqq])

Default Values in Attribute Patterns
------------------------------------

It is possible to specify a default value for an attribute which is
used if the attribute is not present in the element being matched.
This is illustrated in the following example:

     (sxml-match '(e 3 4 5)
       [(e (@ (z (,d 1))) ,a ,b ,c) (list d a b c)])

   The value `1' is used when the attribute `z' is absent from the
element `e'.

Guards in Patterns
------------------

Guards may be added to a pattern clause via the `guard' keyword.  A
guard expression may include zero or more expressions which are
evaluated only if the pattern is matched.  The body of the clause is
only evaluated if the guard expressions evaluate to `#t'.

   The use of guard expressions is illustrated below:

     (sxml-match '(a 2 3)
       ((a ,n) (guard (number? n)) n)
       ((a ,m ,n) (guard (number? m) (number? n)) (+ m n)))

Catamorphisms
-------------

The example below illustrates the use of explicit recursion within an
`sxml-match' form.  This example implements a simple calculator for the
basic arithmetic operations, which are represented by the XML elements
`plus', `minus', `times', and `div'.

     (define simple-eval
       (lambda (x)
         (sxml-match x
           [,i (guard (integer? i)) i]
           [(plus ,x ,y) (+ (simple-eval x) (simple-eval y))]
           [(times ,x ,y) (* (simple-eval x) (simple-eval y))]
           [(minus ,x ,y) (- (simple-eval x) (simple-eval y))]
           [(div ,x ,y) (/ (simple-eval x) (simple-eval y))]
           [,otherwise (error "simple-eval: invalid expression" x)])))

   Using the catamorphism feature of `sxml-match', a more concise
version of `simple-eval' can be written.  The pattern `,[x]'
recursively invokes the pattern matcher on the value bound in this
position.

     (define simple-eval
       (lambda (x)
         (sxml-match x
           [,i (guard (integer? i)) i]
           [(plus ,[x] ,[y]) (+ x y)]
           [(times ,[x] ,[y]) (* x y)]
           [(minus ,[x] ,[y]) (- x y)]
           [(div ,[x] ,[y]) (/ x y)]
           [,otherwise (error "simple-eval: invalid expression" x)])))

Named-Catamorphisms
-------------------

It is also possible to explicitly name the operator in the "cata"
position.  Where `,[id*]' recurs to the top of the current `sxml-match',
`,[cata -> id*]' recurs to `cata'.  `cata' must evaluate to a procedure
which takes one argument, and returns as many values as there are
identifiers following `->'.

   Named catamorphism patterns allow processing to be split into
multiple, mutually recursive procedures.  This is illustrated in the
example below: a transformation that formats a "TV Guide" into HTML.

     (define (tv-guide->html g)
       (define (cast-list cl)
         (sxml-match cl
           [(CastList (CastMember (Character (Name ,ch)) (Actor (Name ,a))) ...)
            `(div (ul (li ,ch ": " ,a) ...))]))
       (define (prog p)
         (sxml-match p
           [(Program (Start ,start-time) (Duration ,dur) (Series ,series-title)
                     (Description ,desc ...))
            `(div (p ,start-time
                     (br) ,series-title
                     (br) ,desc ...))]
           [(Program (Start ,start-time) (Duration ,dur) (Series ,series-title)
                     (Description ,desc ...)
                     ,[cast-list -> cl])
            `(div (p ,start-time
                     (br) ,series-title
                     (br) ,desc ...)
                  ,cl)]))
       (sxml-match g
         [(TVGuide (@ (start ,start-date)
                      (end ,end-date))
                   (Channel (Name ,nm) ,[prog -> p] ...) ...)
          `(html (head (title "TV Guide"))
                 (body (h1 "TV Guide")
                       (div (h2 ,nm) ,p ...) ...))]))

`sxml-match-let' and `sxml-match-let*'
--------------------------------------

 -- Scheme Syntax: sxml-match-let ((pat expr) ...) expression0
          expression ...)
 -- Scheme Syntax: sxml-match-let* ((pat expr) ...) expression0
          expression ...)
     These forms generalize the `let' and `let*' forms of Scheme to
     allow an XML pattern in the binding position, rather than a simple
     variable.

   For example, the expression below:

     (sxml-match-let ([(a ,i ,j) '(a 1 2)])
       (+ i j))

   binds the variables I and J to `1' and `2' in the XML value given.

   ---------- Footnotes ----------

   (1) This example is taken from a paper by Krishnamurthi et al.
Their paper was the first to show the usefulness of the `syntax-rules'
style of pattern matching for transformation of XML, though the
language described, XT3D, is an XML language.

7.17 The Scheme shell (scsh)
============================

An incomplete port of the Scheme shell (scsh) was once available for
Guile as a separate package.  However this code has bitrotten somewhat.
The pieces are available in Guile's legacy CVS repository, which may be
browsed at
`http://cvs.savannah.gnu.org/viewvc/guile/guile-scsh/?root=guile'.

   For information about scsh see `http://www.scsh.net/'.

   This bitrotting is a bit of a shame, as there is a good deal of
well-written Scheme code in scsh.  Adopting this code and porting it to
current Guile should be an educational experience, in addition to
providing something of value to Guile folks.

8 Standard Library
******************

8.1 (statprof)
==============

8.1.1 Overview
--------------

`(statprof)' is intended to be a fairly simple statistical profiler for
guile. It is in the early stages yet, so consider its output still
suspect, and please report any bugs to <guile-devel at gnu.org>, or to
me directly at <rlb at defaultvalue.org>.

   A simple use of statprof would look like this:

       (statprof-reset 0 50000 #t)
       (statprof-start)
       (do-something)
       (statprof-stop)
       (statprof-display)

   This would reset statprof, clearing all accumulated statistics, then
start profiling, run some code, stop profiling, and finally display a
gprof flat-style table of statistics which will look something like
this:

       %   cumulative      self              self    total
      time    seconds   seconds    calls  ms/call  ms/call  name
      35.29      0.23      0.23     2002     0.11     0.11  -
      23.53      0.15      0.15     2001     0.08     0.08  positive?
      23.53      0.15      0.15     2000     0.08     0.08  +
      11.76      0.23      0.08     2000     0.04     0.11  do-nothing
       5.88      0.64      0.04     2001     0.02     0.32  loop
       0.00      0.15      0.00        1     0.00   150.59  do-something
      ...

   All of the numerical data with the exception of the calls column is
statistically approximate. In the following column descriptions, and in
all of statprof, "time" refers to execution time (both user and system),
not wall clock time.

% time
     The percent of the time spent inside the procedure itself (not
     counting children).

cumulative seconds
     The total number of seconds spent in the procedure, including
     children.

self seconds
     The total number of seconds spent in the procedure itself (not
     counting children).

calls
     The total number of times the procedure was called.

self ms/call
     The average time taken by the procedure itself on each call, in ms.

total ms/call
     The average time taken by each call to the procedure, including
     time spent in child functions.

name
     The name of the procedure.


   The profiler uses `eq?' and the procedure object itself to identify
the procedures, so it won't confuse different procedures with the same
name. They will show up as two different rows in the output.

   Right now the profiler is quite simplistic. I cannot provide
call-graphs or other higher level information. What you see in the
table is pretty much all there is. Patches are welcome :-)

8.1.2 Implementation notes
--------------------------

The profiler works by setting the unix profiling signal `ITIMER_PROF'
to go off after the interval you define in the call to
`statprof-reset'. When the signal fires, a sampling routine is run
which looks at the current procedure that's executing, and then crawls
up the stack, and for each procedure encountered, increments that
procedure's sample count. Note that if a procedure is encountered
multiple times on a given stack, it is only counted once. After the
sampling is complete, the profiler resets profiling timer to fire again
after the appropriate interval.

   Meanwhile, the profiler keeps track, via `get-internal-run-time',
how much CPU time (system and user - which is also what `ITIMER_PROF'
tracks), has elapsed while code has been executing within a
statprof-start/stop block.

   The profiler also tries to avoid counting or timing its own code as
much as possible.

8.1.3 Usage
-----------

 -- Function: statprof-active?
     Returns `#t' if `statprof-start' has been called more times than
     `statprof-stop', `#f' otherwise.


 -- Function: statprof-start
     Start the profiler.`'


 -- Function: statprof-stop
     Stop the profiler.`'


 -- Function: statprof-reset sample-seconds sample-microseconds
          count-calls? [full-stacks?]
     Reset the statprof sampler interval to SAMPLE-SECONDS and
     SAMPLE-MICROSECONDS. If COUNT-CALLS? is true, arrange to
     instrument procedure calls as well as collecting statistical
     profiling data. If FULL-STACKS? is true, collect all sampled
     stacks into a list for later analysis.

     Enables traps and debugging as necessary.


 -- Function: statprof-accumulated-time
     Returns the time accumulated during the last statprof run.`'


 -- Function: statprof-sample-count
     Returns the number of samples taken during the last statprof run.`'


 -- Function: statprof-fold-call-data proc init
     Fold PROC over the call-data accumulated by statprof. Cannot be
     called while statprof is active. PROC should take two arguments,
     `(CALL-DATA PRIOR-RESULT)'.

     Note that a given proc-name may appear multiple times, but if it
     does, it represents different functions with the same name.


 -- Function: statprof-proc-call-data proc
     Returns the call-data associated with PROC, or `#f' if none is
     available.


 -- Function: statprof-call-data-name cd

 -- Function: statprof-call-data-calls cd

 -- Function: statprof-call-data-cum-samples cd

 -- Function: statprof-call-data-self-samples cd

 -- Function: statprof-call-data->stats call-data
     Returns an object of type `statprof-stats'.


 -- Function: statprof-stats-proc-name stats

 -- Function: statprof-stats-%-time-in-proc stats

 -- Function: statprof-stats-cum-secs-in-proc stats

 -- Function: statprof-stats-self-secs-in-proc stats

 -- Function: statprof-stats-calls stats

 -- Function: statprof-stats-self-secs-per-call stats

 -- Function: statprof-stats-cum-secs-per-call stats

 -- Function: statprof-display . _
     Displays a gprof-like summary of the statistics collected. Unless
     an optional PORT argument is passed, uses the current output port.


 -- Function: statprof-display-anomolies
     A sanity check that attempts to detect anomolies in statprof's
     statistics.`'


 -- Function: statprof-fetch-stacks
     Returns a list of stacks, as they were captured since the last
     call to `statprof-reset'.

     Note that stacks are only collected if the FULL-STACKS? argument
     to `statprof-reset' is true.


 -- Function: statprof-fetch-call-tree
     Return a call tree for the previous statprof run.

     The return value is a list of nodes, each of which is of the type:
     @@code
      node ::= (@@var@{proc@} @@var@{count@} . @@var@{nodes@})
     @@end code


 -- Function: statprof thunk [#:loop] [#:hz] [#:count-calls?]
          [#:full-stacks?]
     Profiles the execution of THUNK.

     The stack will be sampled HZ times per second, and the thunk
     itself will be called LOOP times.

     If COUNT-CALLS? is true, all procedure calls will be recorded.
     This operation is somewhat expensive.

     If FULL-STACKS? is true, at each sample, statprof will store away
     the whole call tree, for later analysis. Use
     `statprof-fetch-stacks' or `statprof-fetch-call-tree' to retrieve
     the last-stored stacks.


 -- Special Form: with-statprof args
     Profiles the expressions in its body.

     Keyword arguments:

    `#:loop'
          Execute the body LOOP number of times, or `#f' for no looping

          default: `#f'

    `#:hz'
          Sampling rate

          default: `20'

    `#:count-calls?'
          Whether to instrument each function call (expensive)

          default: `#f'

    `#:full-stacks?'
          Whether to collect away all sampled stacks into a list

          default: `#f'



 -- Function: gcprof thunk [#:loop] [#:full-stacks?]
     Do an allocation profile of the execution of THUNK.

     The stack will be sampled soon after every garbage collection,
     yielding an approximate idea of what is causing allocation in your
     program.

     Since GC does not occur very frequently, you may need to use the
     LOOP parameter, to cause THUNK to be called LOOP times.

     If FULL-STACKS? is true, at each sample, statprof will store away
     the whole call tree, for later analysis. Use
     `statprof-fetch-stacks' or `statprof-fetch-call-tree' to retrieve
     the last-stored stacks.


8.2 (sxml apply-templates)
==========================

8.2.1 Overview
--------------

Pre-order traversal of a tree and creation of a new tree:

     	apply-templates:: tree x <templates> -> <new-tree>

   where

      <templates> ::= (<template> ...)
      <template>  ::= (<node-test> <node-test> ... <node-test> . <handler>)
      <node-test> ::= an argument to node-typeof? above
      <handler>   ::= <tree> -> <new-tree>

   This procedure does a _normal_, pre-order traversal of an SXML tree.
It walks the tree, checking at each node against the list of matching
templates.

   If the match is found (which must be unique, i.e., unambiguous), the
corresponding handler is invoked and given the current node as an
argument. The result from the handler, which must be a `<tree>', takes
place of the current node in the resulting tree. The name of the
function is not accidental: it resembles rather closely an
`apply-templates' function of XSLT.

8.2.2 Usage
-----------

 -- Function: apply-templates tree templates

8.3 (sxml fold)
===============

8.3.1 Overview
--------------

`(sxml fold)' defines a number of variants of the "fold" algorithm for
use in transforming SXML trees. Additionally it defines the layout
operator, `fold-layout', which might be described as a context-passing
variant of SSAX's `pre-post-order'.

8.3.2 Usage
-----------

 -- Function: foldt fup fhere tree
     The standard multithreaded tree fold.

     FUP is of type [a] -> a. FHERE is of type object -> a.


 -- Function: foldts fdown fup fhere seed tree
     The single-threaded tree fold originally defined in SSAX. *Note
     (sxml ssax): sxml ssax, for more information.


 -- Function: foldts* fdown fup fhere seed tree
     A variant of *note foldts: sxml fold foldts. that allows pre-order
     tree rewrites. Originally defined in Andy Wingo's 2007 paper,
     _Applications of fold to XML transformation_.


 -- Function: fold-values proc list . seeds
     A variant of *note fold: SRFI-1 Fold and Map. that allows
     multi-valued seeds. Note that the order of the arguments differs
     from that of `fold'.


 -- Function: foldts*-values fdown fup fhere tree . seeds
     A variant of *note foldts*: sxml fold foldts*. that allows
     multi-valued seeds. Originally defined in Andy Wingo's 2007 paper,
     _Applications of fold to XML transformation_.


 -- Function: fold-layout tree bindings params layout stylesheet
     A traversal combinator in the spirit of SSAX's *note
     pre-post-order: sxml transform pre-post-order.

     `fold-layout' was originally presented in Andy Wingo's 2007 paper,
     _Applications of fold to XML transformation_.

          bindings := (<binding>...)
          binding  := (<tag> <bandler-pair>...)
                    | (*default* . <post-handler>)
                    | (*text* . <text-handler>)
          tag      := <symbol>
          handler-pair := (pre-layout . <pre-layout-handler>)
                    | (post . <post-handler>)
                    | (bindings . <bindings>)
                    | (pre . <pre-handler>)
                    | (macro . <macro-handler>)

    PRE-LAYOUT-HANDLER
          A function of three arguments:

         KIDS
               the kids of the current node, before traversal

         PARAMS
               the params of the current node

         LAYOUT
               the layout coming into this node


          PRE-LAYOUT-HANDLER is expected to use this information to
          return a layout to pass to the kids. The default
          implementation returns the layout given in the arguments.

    POST-HANDLER
          A function of five arguments:

         TAG
               the current tag being processed

         PARAMS
               the params of the current node

         LAYOUT
               the layout coming into the current node, before any kids
               were processed

         KLAYOUT
               the layout after processing all of the children

         KIDS
               the already-processed child nodes


          POST-HANDLER should return two values, the layout to pass to
          the next node and the final tree.

    TEXT-HANDLER
          TEXT-HANDLER is a function of three arguments:

         TEXT
               the string

         PARAMS
               the current params

         LAYOUT
               the current layout


          TEXT-HANDLER should return two values, the layout to pass to
          the next node and the value to which the string should
          transform.



8.4 (sxml simple)
=================

8.4.1 Overview
--------------

A simple interface to XML parsing and serialization.

8.4.2 Usage
-----------

 -- Function: xml->sxml [port]
     Use SSAX to parse an XML document into SXML. Takes one optional
     argument, PORT, which defaults to the current input port.


 -- Function: sxml->xml tree [port]
     Serialize the sxml tree TREE as XML. The output will be written to
     the current output port, unless the optional argument PORT is
     present.


 -- Function: sxml->string sxml
     Detag an sxml tree SXML into a string. Does not perform any
     formatting.


8.5 (sxml ssax)
===============

8.5.1 Overview
--------------

Functional XML parsing framework
................................

SAX/DOM and SXML parsers with support for XML Namespaces and validation
.......................................................................

This is a package of low-to-high level lexing and parsing procedures
that can be combined to yield a SAX, a DOM, a validating parser, or a
parser intended for a particular document type. The procedures in the
package can be used separately to tokenize or parse various pieces of
XML documents. The package supports XML Namespaces, internal and
external parsed entities, user-controlled handling of whitespace, and
validation. This module therefore is intended to be a framework, a set
of "Lego blocks" you can use to build a parser following any discipline
and performing validation to any degree. As an example of the parser
construction, this file includes a semi-validating SXML parser.

   The present XML framework has a "sequential" feel of SAX yet a
"functional style" of DOM. Like a SAX parser, the framework scans the
document only once and permits incremental processing. An application
that handles document elements in order can run as efficiently as
possible. _Unlike_ a SAX parser, the framework does not require an
application register stateful callbacks and surrender control to the
parser. Rather, it is the application that can drive the framework -
calling its functions to get the current lexical or syntax element.
These functions do not maintain or mutate any state save the input port.
Therefore, the framework permits parsing of XML in a pure functional
style, with the input port being a monad (or a linear, read-once
parameter).

   Besides the PORT, there is another monad - SEED. Most of the middle-
and high-level parsers are single-threaded through the SEED. The
functions of this framework do not process or affect the SEED in any
way: they simply pass it around as an instance of an opaque datatype.
User functions, on the other hand, can use the seed to maintain user's
state, to accumulate parsing results, etc. A user can freely mix his
own functions with those of the framework. On the other hand, the user
may wish to instantiate a high-level parser: `SSAX:make-elem-parser' or
`SSAX:make-parser'. In the latter case, the user must provide functions
of specific signatures, which are called at predictable moments during
the parsing: to handle character data, element data, or processing
instructions (PI). The functions are always given the SEED, among other
parameters, and must return the new SEED.

   From a functional point of view, XML parsing is a combined
pre-post-order traversal of a "tree" that is the XML document itself.
This down-and-up traversal tells the user about an element when its
start tag is encountered. The user is notified about the element once
more, after all element's children have been handled. The process of XML
parsing therefore is a fold over the raw XML document. Unlike a fold
over trees defined in [1], the parser is necessarily single-threaded -
obviously as elements in a text XML document are laid down sequentially.
The parser therefore is a tree fold that has been transformed to accept
an accumulating parameter [1,2].

   Formally, the denotational semantics of the parser can be expressed
as

      parser:: (Start-tag -> Seed -> Seed) ->
     	   (Start-tag -> Seed -> Seed -> Seed) ->
     	   (Char-Data -> Seed -> Seed) ->
     	   XML-text-fragment -> Seed -> Seed
      parser fdown fup fchar "<elem attrs> content </elem>" seed
       = fup "<elem attrs>" seed
     	(parser fdown fup fchar "content" (fdown "<elem attrs>" seed))

      parser fdown fup fchar "char-data content" seed
       = parser fdown fup fchar "content" (fchar "char-data" seed)

      parser fdown fup fchar "elem-content content" seed
       = parser fdown fup fchar "content" (
     	parser fdown fup fchar "elem-content" seed)

   Compare the last two equations with the left fold

      fold-left kons elem:list seed = fold-left kons list (kons elem seed)

   The real parser created by `SSAX:make-parser' is slightly more
complicated, to account for processing instructions, entity references,
namespaces, processing of document type declaration, etc.

   The XML standard document referred to in this module
is`http://www.w3.org/TR/1998/REC-xml-19980210.html'

   The present file also defines a procedure that parses the text of an
XML document or of a separate element into SXML, an S-expression-based
model of an XML Information Set. SXML is also an Abstract Syntax Tree
of an XML document. SXML is similar but not identical to DOM; SXML is
particularly suitable for Scheme-based XML/HTML authoring, SXPath
queries, and tree transformations. See SXML.html for more details. SXML
is a term implementation of evaluation of the XML document [3]. The
other implementation is context-passing.

   The present frameworks fully supports the XML Namespaces
Recommendation:`http://www.w3.org/TR/REC-xml-names/' Other links:

[1]
     Jeremy Gibbons, Geraint Jones, "The Under-appreciated Unfold,"
     Proc.  ICFP'98, 1998, pp. 273-279.

[2]
     Richard S. Bird, The promotion and accumulation strategies in
     transformational programming, ACM Trans. Progr. Lang. Systems,
     6(4):487-504, October 1984.

[3]
     Ralf Hinze, "Deriving Backtracking Monad Transformers," Functional
     Pearl. Proc ICFP'00, pp. 186-197.


8.5.2 Usage
-----------

 -- Function: current-ssax-error-port

 -- Function: with-ssax-error-to-port port thunk

 -- Function: xml-token? _
      -- Scheme Procedure: pair? x
          Return `#t' if X is a pair; otherwise return `#f'.


 -- Special Form: xml-token-kind token

 -- Special Form: xml-token-head token

 -- Function: make-empty-attlist

 -- Function: attlist-add attlist name-value

 -- Function: attlist-null? _
      -- Scheme Procedure: null? x
          Return `#t' iff X is the empty list, else `#f'.


 -- Function: attlist-remove-top attlist

 -- Function: attlist->alist attlist

 -- Function: attlist-fold kons knil lis1

 -- Function: define-parsed-entity! entity str
     Define a new parsed entity. ENTITY should be a symbol.

     Instances of &ENTITY; in XML text will be replaced with the string
     STR, which will then be parsed.


 -- Function: reset-parsed-entity-definitions!
     Restore the set of parsed entity definitions to its initial state.


 -- Function: ssax:uri-string->symbol uri-str

 -- Function: ssax:skip-internal-dtd port

 -- Function: ssax:read-pi-body-as-string port

 -- Function: ssax:reverse-collect-str-drop-ws fragments

 -- Function: ssax:read-markup-token port

 -- Function: ssax:read-cdata-body port str-handler seed

 -- Function: ssax:read-char-ref port

 -- Function: ssax:read-attributes port entities

 -- Function: ssax:complete-start-tag tag-head port elems entities
          namespaces

 -- Function: ssax:read-external-id port

 -- Function: ssax:read-char-data port expect-eof? str-handler seed

 -- Function: ssax:xml->sxml port namespace-prefix-assig

 -- Special Form: ssax:make-parser . kw-val-pairs

 -- Special Form: ssax:make-pi-parser orig-handlers

 -- Special Form: ssax:make-elem-parser my-new-level-seed
          my-finish-element my-char-data-handler my-pi-handlers

8.6 (sxml ssax input-parse)
===========================

8.6.1 Overview
--------------

A simple lexer.

   The procedures in this module surprisingly often suffice to parse an
input stream. They either skip, or build and return tokens, according to
inclusion or delimiting semantics. The list of characters to expect,
include, or to break at may vary from one invocation of a function to
another. This allows the functions to easily parse even
context-sensitive languages.

   EOF is generally frowned on, and thrown up upon if encountered.
Exceptions are mentioned specifically. The list of expected characters
(characters to skip until, or break-characters) may include an EOF
"character", which is to be coded as the symbol, `*eof*'.

   The input stream to parse is specified as a "port", which is usually
the last (and optional) argument. It defaults to the current input port
if omitted.

   If the parser encounters an error, it will throw an exception to the
key `parser-error'. The arguments will be of the form `(PORT MESSAGE
SPECIALISING-MSG*)'.

   The first argument is a port, which typically points to the offending
character or its neighborhood. You can then use `port-column' and
`port-line' to query the current position. MESSAGE is the description
of the error. Other arguments supply more details about the problem.

8.6.2 Usage
-----------

 -- Function: peek-next-char [port]

 -- Function: assert-curr-char expected-chars comment [port]

 -- Function: skip-until arg [port]

 -- Function: skip-while skip-chars [port]

 -- Function: next-token prefix-skipped-chars break-chars [comment]
          [port]

 -- Function: next-token-of incl-list/pred [port]

 -- Function: read-text-line [port]

 -- Function: read-string n [port]

 -- Function: find-string-from-port? _ _ . _
     Looks for STR in <INPUT-PORT>, optionally within the first
     MAX-NO-CHAR characters.


8.7 (sxml transform)
====================

8.7.1 Overview
--------------

SXML expression tree transformers
---------------------------------

Pre-Post-order traversal of a tree and creation of a new tree
.............................................................

     pre-post-order:: <tree> x <bindings> -> <new-tree>

   where

      <bindings> ::= (<binding> ...)
      <binding> ::= (<trigger-symbol> *preorder* . <handler>) |
                    (<trigger-symbol> *macro* . <handler>) |
     		(<trigger-symbol> <new-bindings> . <handler>) |
     		(<trigger-symbol> . <handler>)
      <trigger-symbol> ::= XMLname | *text* | *default*
      <handler> :: <trigger-symbol> x [<tree>] -> <new-tree>

   The pre-post-order function visits the nodes and nodelists
pre-post-order (depth-first). For each `<Node>' of the form `(NAME
<Node> ...)', it looks up an association with the given NAME among its
<BINDINGS>. If failed, `pre-post-order' tries to locate a `*default*'
binding. It's an error if the latter attempt fails as well. Having
found a binding, the `pre-post-order' function first checks to see if
the binding is of the form

     	(<trigger-symbol> *preorder* . <handler>)

   If it is, the handler is 'applied' to the current node. Otherwise,
the pre-post-order function first calls itself recursively for each
child of the current node, with <NEW-BINDINGS> prepended to the
<BINDINGS> in effect. The result of these calls is passed to the
<HANDLER> (along with the head of the current <NODE>). To be more
precise, the handler is _applied_ to the head of the current node and
its processed children. The result of the handler, which should also be
a `<tree>', replaces the current <NODE>. If the current <NODE> is a
text string or other atom, a special binding with a symbol `*text*' is
looked up.

   A binding can also be of a form

     	(<trigger-symbol> *macro* . <handler>)

   This is equivalent to `*preorder*' described above. However, the
result is re-processed again, with the current stylesheet.

8.7.2 Usage
-----------

 -- Function: SRV:send-reply . fragments
     Output the FRAGMENTS to the current output port.

     The fragments are a list of strings, characters, numbers, thunks,
     `#f', `#t' - and other fragments. The function traverses the tree
     depth-first, writes out strings and characters, executes thunks,
     and ignores `#f' and `'()'. The function returns `#t' if anything
     was written at all; otherwise the result is `#f' If `#t' occurs
     among the fragments, it is not written out but causes the result
     of `SRV:send-reply' to be `#t'.


 -- Function: foldts fdown fup fhere seed tree

 -- Function: post-order tree bindings

 -- Function: pre-post-order tree bindings

 -- Function: replace-range beg-pred end-pred forest

8.8 (sxml xpath)
================

8.8.1 Overview
--------------

SXPath: SXML Query Language
---------------------------

SXPath is a query language for SXML, an instance of XML Information set
(Infoset) in the form of s-expressions. See `(sxml ssax)' for the
definition of SXML and more details. SXPath is also a translation into
Scheme of an XML Path Language, XPath (http://www.w3.org/TR/xpath).
XPath and SXPath describe means of selecting a set of Infoset's items or
their properties.

   To facilitate queries, XPath maps the XML Infoset into an explicit
tree, and introduces important notions of a location path and a current,
context node. A location path denotes a selection of a set of nodes
relative to a context node. Any XPath tree has a distinguished, root
node - which serves as the context node for absolute location paths.
Location path is recursively defined as a location step joined with a
location path. A location step is a simple query of the database
relative to a context node. A step may include expressions that further
filter the selected set. Each node in the resulting set is used as a
context node for the adjoining location path. The result of the step is
a union of the sets returned by the latter location paths.

   The SXML representation of the XML Infoset (see SSAX.scm) is rather
suitable for querying as it is. Bowing to the XPath specification, we
will refer to SXML information items as 'Nodes':

      	<Node> ::= <Element> | <attributes-coll> | <attrib>
      		   | "text string" | <PI>

   This production can also be described as

     	<Node> ::= (name . <Nodeset>) | "text string"

   An (ordered) set of nodes is just a list of the constituent nodes:

      	<Nodeset> ::= (<Node> ...)

   Nodesets, and Nodes other than text strings are both lists. A
<Nodeset> however is either an empty list, or a list whose head is not
a symbol. A symbol at the head of a node is either an XML name (in
which case it's a tag of an XML element), or an administrative name
such as '@'. This uniform list representation makes processing rather
simple and elegant, while avoiding confusion. The multi-branch tree
structure formed by the mutually-recursive datatypes <Node> and
<Nodeset> lends itself well to processing by functional languages.

   A location path is in fact a composite query over an XPath tree or
its branch. A singe step is a combination of a projection, selection or
a transitive closure. Multiple steps are combined via join and union
operations. This insight allows us to _elegantly_ implement XPath as a
sequence of projection and filtering primitives - converters - joined
by "combinators". Each converter takes a node and returns a nodeset
which is the result of the corresponding query relative to that node. A
converter can also be called on a set of nodes. In that case it returns
a union of the corresponding queries over each node in the set.  The
union is easily implemented as a list append operation as all nodes in
a SXML tree are considered distinct, by XPath conventions. We also
preserve the order of the members in the union. Query combinators are
high-order functions: they take converter(s) (which is a Node|Nodeset ->
Nodeset function) and compose or otherwise combine them. We will be
concerned with only relative location paths [XPath]: an absolute
location path is a relative path applied to the root node.

   Similarly to XPath, SXPath defines full and abbreviated notations for
location paths. In both cases, the abbreviated notation can be
mechanically expanded into the full form by simple rewriting rules. In
case of SXPath the corresponding rules are given as comments to a sxpath
function, below. The regression test suite at the end of this file shows
a representative sample of SXPaths in both notations, juxtaposed with
the corresponding XPath expressions. Most of the samples are borrowed
literally from the XPath specification, while the others are adjusted
for our running example, tree1.

8.8.2 Usage
-----------

 -- Function: nodeset? x

 -- Function: node-typeof? crit

 -- Function: node-eq? other

 -- Function: node-equal? other

 -- Function: node-pos n

 -- Function: filter pred?
      -- Scheme Procedure: filter pred list
          Return all the elements of 2nd arg LIST that satisfy predicate
          PRED.  The list is not disordered - elements that appear in the
          result list occur in the same order as they occur in the argument
          list. The returned list may share a common tail with the argument
          list. The dynamic order in which the various applications of pred
          are made is not specified.

               (filter even? '(0 7 8 8 43 -4)) => (0 8 8 -4)


 -- Function: take-until pred?

 -- Function: take-after pred?

 -- Function: map-union proc lst

 -- Function: node-reverse node-or-nodeset

 -- Function: node-trace title

 -- Function: select-kids test-pred?

 -- Function: node-self pred?
      -- Scheme Procedure: filter pred list
          Return all the elements of 2nd arg LIST that satisfy predicate
          PRED.  The list is not disordered - elements that appear in the
          result list occur in the same order as they occur in the argument
          list. The returned list may share a common tail with the argument
          list. The dynamic order in which the various applications of pred
          are made is not specified.

               (filter even? '(0 7 8 8 43 -4)) => (0 8 8 -4)


 -- Function: node-join . selectors

 -- Function: node-reduce . converters

 -- Function: node-or . converters

 -- Function: node-closure test-pred?

 -- Function: node-parent rootnode

 -- Function: sxpath path

8.9 (texinfo)
=============

8.9.1 Overview
--------------

Texinfo processing in scheme
............................

This module parses texinfo into SXML. TeX will always be the processor
of choice for print output, of course. However, although `makeinfo'
works well for info, its output in other formats is not very
customizable, and the program is not extensible as a whole. This module
aims to provide an extensible framework for texinfo processing that
integrates texinfo into the constellation of SXML processing tools.

Notes on the SXML vocabulary
............................

Consider the following texinfo fragment:

      @deffn Primitive set-car! pair value
      This function...
      @end deffn

   Logically, the category (Primitive), name (set-car!), and arguments
(pair value) are "attributes" of the deffn, with the description as the
content. However, texinfo allows for @-commands within the arguments to
an environment, like `@deffn', which means that texinfo "attributes"
are PCDATA. XML attributes, on the other hand, are CDATA. For this
reason, "attributes" of texinfo @-commands are called "arguments", and
are grouped under the special element, `%'.

   Because `%' is not a valid NCName, stexinfo is a superset of SXML. In
the interests of interoperability, this module provides a conversion
function to replace the `%' with `texinfo-arguments'.

8.9.2 Usage
-----------

 -- Function: call-with-file-and-dir filename proc
     Call the one-argument procedure PROC with an input port that reads
     from FILENAME. During the dynamic extent of PROC's execution, the
     current directory will be `(dirname FILENAME)'. This is useful for
     parsing documents that can include files by relative path name.


 -- Variable: texi-command-specs

 -- Function: texi-command-depth command max-depth
     Given the texinfo command COMMAND, return its nesting level, or
     `#f' if it nests too deep for MAX-DEPTH.

     Examples:

           (texi-command-depth 'chapter 4)        => 1
           (texi-command-depth 'top 4)            => 0
           (texi-command-depth 'subsection 4)     => 3
           (texi-command-depth 'appendixsubsec 4) => 3
           (texi-command-depth 'subsection 2)     => #f


 -- Function: texi-fragment->stexi string-or-port
     Parse the texinfo commands in STRING-OR-PORT, and return the
     resultant stexi tree. The head of the tree will be the special
     command, `*fragment*'.


 -- Function: texi->stexi port
     Read a full texinfo document from PORT and return the parsed stexi
     tree. The parsing will start at the `@settitle' and end at `@bye'
     or EOF.


 -- Function: stexi->sxml tree
     Transform the stexi tree TREE into sxml. This involves replacing
     the `%' element that keeps the texinfo arguments with an element
     for each argument.

     FIXME: right now it just changes % to `texinfo-arguments' - that
     doesn't hang with the idea of making a dtd at some point


8.10 (texinfo docbook)
======================

8.10.1 Overview
---------------

This module exports procedures for transforming a limited subset of the
SXML representation of docbook into stexi. It is not complete by any
means. The intention is to gather a number of routines and stylesheets
so that external modules can parse specific subsets of docbook, for
example that set generated by certain tools.

8.10.2 Usage
------------

 -- Variable: *sdocbook->stexi-rules*

 -- Variable: *sdocbook-block-commands*

 -- Function: sdocbook-flatten sdocbook
     "Flatten" a fragment of sdocbook so that block elements do not nest
     inside each other.

     Docbook is a nested format, where e.g. a `refsect2' normally
     appears inside a `refsect1'. Logical divisions in the document are
     represented via the tree topology; a `refsect2' element _contains_
     all of the elements in its section.

     On the contrary, texinfo is a flat format, in which sections are
     marked off by standalone section headers like `@chapter', and block
     elements do not nest inside each other.

     This function takes a nested sdocbook fragment SDOCBOOK and
     flattens all of the sections, such that e.g.

           (refsect1 (refsect2 (para "Hello")))

     becomes

           ((refsect1) (refsect2) (para "Hello"))

     Oftentimes (always?) sectioning elements have `<title>' as their
     first element child; users interested in processing the `refsect*'
     elements into proper sectioning elements like `chapter' might be
     interested in `replace-titles' and `filter-empty-elements'.  *Note
     replace-titles: texinfo docbook replace-titles, and *note
     filter-empty-elements: texinfo docbook filter-empty-elements.

     Returns a nodeset, as described in *note sxml xpath::. That is to
     say, this function returns an untagged list of stexi elements.


 -- Function: filter-empty-elements sdocbook
     Filters out empty elements in an sdocbook nodeset. Mostly useful
     after running `sdocbook-flatten'.


 -- Function: replace-titles sdocbook-fragment
     Iterate over the sdocbook nodeset SDOCBOOK-FRAGMENT, transforming
     contiguous `refsect' and `title' elements into the appropriate
     texinfo sectioning command. Most useful after having run
     `sdocbook-flatten'.

     For example:

           (replace-titles '((refsect1) (title "Foo") (para "Bar.")))
              => '((chapter "Foo") (para "Bar."))


8.11 (texinfo html)
===================

8.11.1 Overview
---------------

This module implements transformation from `stexi' to HTML. Note that
the output of `stexi->shtml' is actually SXML with the HTML vocabulary.
This means that the output can be further processed, and that it must
eventually be serialized by *note sxml->xml: sxml simple sxml->xml.
References (i.e., the `@ref' family of commands) are resolved by a
"ref-resolver". *Note add-ref-resolver!: texinfo html
add-ref-resolver!, for more information.

8.11.2 Usage
------------

 -- Function: add-ref-resolver! proc
     Add PROC to the head of the list of ref-resolvers. PROC will be
     expected to take the name of a node and the name of a manual and
     return the URL of the referent, or `#f' to pass control to the next
     ref-resolver in the list.

     The default ref-resolver will return the concatenation of the
     manual name, `#', and the node name.


 -- Function: stexi->shtml tree
     Transform the stexi TREE into shtml, resolving references via
     ref-resolvers. See the module commentary for more details.


 -- Function: urlify str

8.12 (texinfo indexing)
=======================

8.12.1 Overview
---------------

Given a piece of stexi, return an index of a specified variety.

   Note that currently, `stexi-extract-index' doesn't differentiate
between different kinds of index entries. That's a bug ;)

8.12.2 Usage
------------

 -- Function: stexi-extract-index tree manual-name kind
     Given an stexi tree TREE, index all of the entries of type KIND.
     KIND can be one of the predefined texinfo indices (`concept',
     `variable', `function', `key', `program', `type') or one of the
     special symbols `auto' or `all'. `auto' will scan the stext for a
     `(printindex)' statement, and `all' will generate an index from
     all entries, regardless of type.

     The returned index is a list of pairs, the CAR of which is the
     entry (a string) and the CDR of which is a node name (a string).


8.13 (texinfo string-utils)
===========================

8.13.1 Overview
---------------

Module `(texinfo string-utils)' provides various string-related
functions useful to Guile's texinfo support.

8.13.2 Usage
------------

 -- Function: escape-special-chars str special-chars escape-char
     Returns a copy of STR with all given special characters preceded
     by the given ESCAPE-CHAR.

     SPECIAL-CHARS can either be a single character, or a string
     consisting of all the special characters.

          ;; make a string regexp-safe...
           (escape-special-chars "***(Example String)***"
                                "[]()/*."
                                #\\)
          => "\\*\\*\\*\\(Example String\\)\\*\\*\\*"

          ;; also can escape a singe char...
           (escape-special-chars "richardt@vzavenue.net"
                                #\@
                                #\@)
          => "richardt@@vzavenue.net"


 -- Function: transform-string str match? replace [start] [end]
     Uses MATCH? against each character in STR, and performs a
     replacement on each character for which matches are found.

     MATCH? may either be a function, a character, a string, or `#t'.
     If MATCH? is a function, then it takes a single character as
     input, and should return `#t' for matches.  MATCH? is a character,
     it is compared to each string character using `char=?'. If MATCH?
     is a string, then any character in that string will be considered
     a match. `#t' will cause every character to be a match.

     If REPLACE is a function, it is called with the matched character
     as an argument, and the returned value is sent to the output
     string via `display'. If REPLACE is anything else, it is sent
     through the output string via `display'.

     Note that te replacement for the matched characters does not need
     to be a single character. That is what differentiates this
     function from `string-map', and what makes it useful for
     applications such as converting `#\&' to `"&amp;"' in web page
     text. Some other functions in this module are just wrappers around
     common uses of `transform-string'. Transformations not possible
     with this function should probably be done with regular
     expressions.

     If START and END are given, they control which portion of the
     string undergoes transformation. The entire input string is still
     output, though. So, if START is `5', then the first five
     characters of STR will still appear in the returned string.

          ; these two are equivalent...
           (transform-string str #\space #\-) ; change all spaces to -'s
           (transform-string str (lambda (c) (char=? #\space c)) #\-)


 -- Function: expand-tabs str [tab-size]
     Returns a copy of STR with all tabs expanded to spaces.  TAB-SIZE
     defaults to 8.

     Assuming tab size of 8, this is equivalent to:

           (transform-string str #\tab "        ")


 -- Function: center-string str [width] [chr] [rchr]
     Returns a copy of STR centered in a field of WIDTH characters. Any
     needed padding is done by character CHR, which defaults to
     `#\space'. If RCHR is provided, then the padding to the right will
     use it instead. See the examples below. left and RCHR on the
     right. The default WIDTH is 80. The default LCHR and RCHR is
     `#\space'. The string is never truncated.

           (center-string "Richard Todd" 24)
          => "      Richard Todd      "

           (center-string " Richard Todd " 24 #\=)
          => "===== Richard Todd ====="

           (center-string " Richard Todd " 24 #\< #\>)
          => "<<<<< Richard Todd >>>>>"


 -- Function: left-justify-string str [width] [chr]
     `left-justify-string str [width chr]'. Returns a copy of STR
     padded with CHR such that it is left justified in a field of WIDTH
     characters. The default WIDTH is 80. Unlike `string-pad' from
     srfi-13, the string is never truncated.


 -- Function: right-justify-string str [width] [chr]
     Returns a copy of STR padded with CHR such that it is right
     justified in a field of WIDTH characters. The default WIDTH is 80.
     The default CHR is `#\space'. Unlike `string-pad' from srfi-13,
     the string is never truncated.


 -- Function: collapse-repeated-chars str [chr] [num]
     Returns a copy of STR with all repeated instances of CHR collapsed
     down to at most NUM instances. The default value for CHR is
     `#\space', and the default value for NUM is 1.

           (collapse-repeated-chars "H  e  l  l  o")
          => "H e l l o"
           (collapse-repeated-chars "H--e--l--l--o" #\-)
          => "H-e-l-l-o"
           (collapse-repeated-chars "H-e--l---l----o" #\- 2)
          => "H-e--l--l--o"


 -- Function: make-text-wrapper [#:line-width] [#:expand-tabs?]
          [#:tab-width] [#:collapse-whitespace?] [#:subsequent-indent]
          [#:initial-indent] [#:break-long-words?]
     Returns a procedure that will split a string into lines according
     to the given parameters.

    `#:line-width'
          This is the target length used when deciding where to wrap
          lines.  Default is 80.

    `#:expand-tabs?'
          Boolean describing whether tabs in the input should be
          expanded. Default is #t.

    `#:tab-width'
          If tabs are expanded, this will be the number of spaces to
          which they expand. Default is 8.

    `#:collapse-whitespace?'
          Boolean describing whether the whitespace inside the existing
          text should be removed or not. Default is #t.

          If text is already well-formatted, and is just being wrapped
          to fit in a different width, then set this to `#f'. This way,
          many common text conventions (such as two spaces between
          sentences) can be preserved if in the original text. If the
          input text spacing cannot be trusted, then leave this setting
          at the default, and all repeated whitespace will be collapsed
          down to a single space.

    `#:initial-indent'
          Defines a string that will be put in front of the first line
          of wrapped text. Default is the empty string, "".

    `#:subsequent-indent'
          Defines a string that will be put in front of all lines of
          wrapped text, except the first one. Default is the empty
          string, "".

    `#:break-long-words?'
          If a single word is too big to fit on a line, this setting
          tells the wrapper what to do. Defaults to #t, which will
          break up long words. When set to #f, the line will be
          allowed, even though it is longer than the defined
          `#:line-width'.


     The return value is a procedure of one argument, the input string,
     which returns a list of strings, where each element of the list is
     one line.


 -- Function: fill-string str . kwargs
     Wraps the text given in string STR according to the parameters
     provided in KEYWDS, or the default setting if they are not given.
     Returns a single string with the wrapped text. Valid keyword
     arguments are discussed in `make-text-wrapper'.


 -- Function: string->wrapped-lines str . kwargs
     `string->wrapped-lines str keywds ...'. Wraps the text given in
     string STR according to the parameters provided in KEYWDS, or the
     default setting if they are not given. Returns a list of strings
     representing the formatted lines. Valid keyword arguments are
     discussed in `make-text-wrapper'.


8.14 (texinfo plain-text)
=========================

8.14.1 Overview
---------------

Transformation from stexi to plain-text. Strives to re-create the output
from `info'; comes pretty damn close.

8.14.2 Usage
------------

 -- Function: stexi->plain-text tree
     Transform TREE into plain text. Returns a string.


8.15 (texinfo serialize)
========================

8.15.1 Overview
---------------

Serialization of `stexi' to plain texinfo.

8.15.2 Usage
------------

 -- Function: stexi->texi tree
     Serialize the stexi TREE into plain texinfo.


8.16 (texinfo reflection)
=========================

8.16.1 Overview
---------------

Routines to generare `stexi' documentation for objects and modules.

   Note that in this context, an "object" is just a value associated
with a location. It has nothing to do with GOOPS.

8.16.2 Usage
------------

 -- Function: module-stexi-documentation sym-name [%docs-resolver]
          [#:docs-resolver]
     Return documentation for the module named SYM-NAME. The
     documentation will be formatted as `stexi' (*note texinfo:
     texinfo.).


 -- Function: script-stexi-documentation scriptpath
     Return documentation for given script. The documentation will be
     taken from the script's commentary, and will be returned in the
     `stexi' format (*note texinfo: texinfo.).


 -- Function: object-stexi-documentation _ [_] [#:force]

 -- Function: package-stexi-standard-copying name version updated years
          copyright-holder permissions
     Create a standard texinfo `copying' section.

     YEARS is a list of years (as integers) in which the modules being
     documented were released. All other arguments are strings.


 -- Function: package-stexi-standard-titlepage name version updated
          authors
     Create a standard GNU title page.

     AUTHORS is a list of `(NAME . EMAIL)' pairs. All other arguments
     are strings.

     Here is an example of the usage of this procedure:

           (package-stexi-standard-titlepage
            "Foolib"
            "3.2"
            "26 September 2006"
            '(("Alyssa P Hacker" . "alyssa@example.com"))
            '(2004 2005 2006)
            "Free Software Foundation, Inc."
            "Standard GPL permissions blurb goes here")


 -- Function: package-stexi-generic-menu name entries
     Create a menu from a generic alist of entries, the car of which
     should be the node name, and the cdr the description. As an
     exception, an entry of `#f' will produce a separator.


 -- Function: package-stexi-standard-menu name modules
          module-descriptions extra-entries
     Create a standard top node and menu, suitable for processing by
     makeinfo.


 -- Function: package-stexi-extended-menu name module-pairs
          script-pairs extra-entries
     Create an "extended" menu, like the standard menu but with a
     section for scripts.


 -- Function: package-stexi-standard-prologue name filename category
          description copying titlepage menu
     Create a standard prologue, suitable for later serialization to
     texinfo and .info creation with makeinfo.

     Returns a list of stexinfo forms suitable for passing to
     `package-stexi-documentation' as the prologue. *Note texinfo
     reflection package-stexi-documentation::, *note
     package-stexi-standard-titlepage: texinfo reflection
     package-stexi-standard-titlepage, *note
     package-stexi-standard-copying: texinfo reflection
     package-stexi-standard-copying, and *note
     package-stexi-standard-menu: texinfo reflection
     package-stexi-standard-menu.


 -- Function: package-stexi-documentation modules name filename
          prologue epilogue [#:module-stexi-documentation-args]
          [#:scripts]
     Create stexi documentation for a "package", where a package is a
     set of modules that is released together.

     MODULES is expected to be a list of module names, where a module
     name is a list of symbols. The stexi that is returned will be
     titled NAME and a texinfo filename of FILENAME.

     PROLOGUE and EPILOGUE are lists of stexi forms that will be
     spliced into the output document before and after the generated
     modules documentation, respectively. *Note texinfo reflection
     package-stexi-standard-prologue::, to create a conventional GNU
     texinfo prologue.

     MODULE-STEXI-DOCUMENTATION-ARGS is an optional argument that, if
     given, will be added to the argument list when
     `module-texi-documentation' is called. For example, it might be
     useful to define a `#:docs-resolver' argument.


 -- Function: package-stexi-documentation-for-include modules
          module-descriptions [#:module-stexi-documentation-args]
     Create stexi documentation for a "package", where a package is a
     set of modules that is released together.

     MODULES is expected to be a list of module names, where a module
     name is a list of symbols. Returns an stexinfo fragment.

     Unlike `package-stexi-documentation', this function simply produces
     a menu and the module documentations instead of producing a full
     texinfo document. This can be useful if you write part of your
     manual by hand, and just use `@include' to pull in the
     automatically generated parts.

     MODULE-STEXI-DOCUMENTATION-ARGS is an optional argument that, if
     given, will be added to the argument list when
     `module-texi-documentation' is called. For example, it might be
     useful to define a `#:docs-resolver' argument.


9 GOOPS
*******

GOOPS is the object oriented extension to Guile. Its implementation is
derived from STk-3.99.3 by Erick Gallesio and version 1.3 of Gregor
Kiczales' `Tiny-Clos'.  It is very close in spirit to CLOS, the Common
Lisp Object System, but is adapted for the Scheme language.

   GOOPS is a full object oriented system, with classes, objects,
multiple inheritance, and generic functions with multi-method dispatch.
Furthermore its implementation relies on a meta object protocol --
which means that GOOPS's core operations are themselves defined as
methods on relevant classes, and can be customised by overriding or
redefining those methods.

   To start using GOOPS you first need to import the `(oop goops)'
module.  You can do this at the Guile REPL by evaluating:

     (use-modules (oop goops))
   
9.1 Copyright Notice
====================

The material in this chapter is partly derived from the STk Reference
Manual written by Erick Gallesio, whose copyright notice is as follows.

   Copyright © 1993-1999 Erick Gallesio - I3S-CNRS/ESSI <eg@unice.fr>
Permission to use, copy, modify, distribute,and license this software
and its documentation for any purpose is hereby granted, provided that
existing copyright notices are retained in all copies and that this
notice is included verbatim in any distributions.  No written
agreement, license, or royalty fee is required for any of the
authorized uses.  This software is provided "AS IS" without express or
implied warranty.

   The material has been adapted for use in Guile, with the author's
permission.

9.2 Class Definition
====================

A new class is defined with the `define-class' syntax:

     (define-class CLASS (SUPERCLASS ...)
        SLOT-DESCRIPTION ...
        CLASS-OPTION ...)

   CLASS is the class being defined.  The list of SUPERCLASSes
specifies which existing classes, if any, to inherit slots and
properties from.  "Slots" hold per-instance(1) data, for instances of
that class -- like "fields" or "member variables" in other object
oriented systems.  Each SLOT-DESCRIPTION gives the name of a slot and
optionally some "properties" of this slot; for example its initial
value, the name of a function which will access its value, and so on.
Class options, slot descriptions and inheritance are discussed more
below.  

 -- syntax: define-class name (super ...) slot-definition ... . options
     Define a class called NAME that inherits from SUPERs, with direct
     slots defined by SLOT-DEFINITIONs and class options OPTIONS.  The
     newly created class is bound to the variable name NAME in the
     current environment.

     Each SLOT-DEFINITION is either a symbol that names the slot or a
     list,

          (SLOT-NAME-SYMBOL . SLOT-OPTIONS)

     where SLOT-NAME-SYMBOL is a symbol and SLOT-OPTIONS is a list with
     an even number of elements.  The even-numbered elements of
     SLOT-OPTIONS (counting from zero) are slot option keywords; the
     odd-numbered elements are the corresponding values for those
     keywords.

     OPTIONS is a similarly structured list containing class option
     keywords and corresponding values.

   As an example, let us define a type for representing a complex number
in terms of two real numbers.(2)  This can be done with the following
class definition:

     (define-class <my-complex> (<number>)
        r i)

   This binds the variable `<my-complex>' to a new class whose
instances will contain two slots.  These slots are called `r' and `i'
and will hold the real and imaginary parts of a complex number. Note
that this class inherits from `<number>', which is a predefined
class.(3)

   Slot options are described in the next section.  The possible class
options are as follows.

 -- class option: #:metaclass metaclass
     The `#:metaclass' class option specifies the metaclass of the class
     being defined.  METACLASS must be a class that inherits from
     `<class>'.  For the use of metaclasses, see *note Metaobjects and
     the Metaobject Protocol:: and *note Metaclasses::.

     If the `#:metaclass' option is absent, GOOPS reuses or constructs a
     metaclass for the new class by calling `ensure-metaclass' (*note
     ensure-metaclass: Class Definition Protocol.).

 -- class option: #:name name
     The `#:name' class option specifies the new class's name.  This
     name is used to identify the class whenever related objects - the
     class itself, its instances and its subclasses - are printed.

     If the `#:name' option is absent, GOOPS uses the first argument to
     `define-class' as the class name.

   ---------- Footnotes ----------

   (1) Usually -- but see also the `#:allocation' slot option.

   (2) Of course Guile already provides complex numbers, and
`<complex>' is in fact a predefined class in GOOPS; but the definition
here is still useful as an example.

   (3) `<number>' is the direct superclass of the predefined class
`<complex>'; `<complex>' is the superclass of `<real>', and `<real>' is
the superclass of `<integer>'.

9.3 Instance Creation and Slot Access
=====================================

An instance (or object) of a defined class can be created with `make'.
`make' takes one mandatory parameter, which is the class of the
instance to create, and a list of optional arguments that will be used
to initialize the slots of the new instance.  For instance the
following form

     (define c (make <my-complex>))

creates a new `<my-complex>' object and binds it to the Scheme variable
`c'.

 -- generic: make
 -- method: make (class <class>) . initargs
     Create and return a new instance of class CLASS, initialized using
     INITARGS.

     In theory, INITARGS can have any structure that is understood by
     whatever methods get applied when the `initialize' generic function
     is applied to the newly allocated instance.

     In practice, specialized `initialize' methods would normally call
     `(next-method)', and so eventually the standard GOOPS `initialize'
     methods are applied.  These methods expect INITARGS to be a list
     with an even number of elements, where even-numbered elements
     (counting from zero) are keywords and odd-numbered elements are
     the corresponding values.

     GOOPS processes initialization argument keywords automatically for
     slots whose definition includes the `#:init-keyword' option (*note
     init-keyword: Slot Options.).  Other keyword value pairs can only
     be processed by an `initialize' method that is specialized for the
     new instance's class.  Any unprocessed keyword value pairs are
     ignored.

 -- generic: make-instance
 -- method: make-instance (class <class>) . initargs
     `make-instance' is an alias for `make'.

   The slots of the new complex number can be accessed using `slot-ref'
and `slot-set!'.  `slot-set!'  sets the value of an object slot and
`slot-ref' retrieves it.

     (slot-set! c 'r 10)
     (slot-set! c 'i 3)
     (slot-ref c 'r) => 10
     (slot-ref c 'i) => 3

   The `(oop goops describe)' module provides a `describe' function
that is useful for seeing all the slots of an object; it prints the
slots and their values to standard output.

     (describe c)
     -|
     #<<my-complex> 401d8638> is an instance of class <my-complex>
     Slots are:
          r = 10
          i = 3

9.4 Slot Options
================

When specifying a slot (in a `(define-class ...)' form), various
options can be specified in addition to the slot's name.  Each option
is specified by a keyword.  The list of possible keywords is as follows.

 -- slot option: #:init-value init-value
 -- slot option: #:init-form init-form
 -- slot option: #:init-thunk init-thunk
 -- slot option: #:init-keyword init-keyword
     These options provide various ways to specify how to initialize the
     slot's value at instance creation time.  

     INIT-VALUE specifies a fixed initial slot value (shared across all
     new instances of the class).

     INIT-THUNK specifies a thunk that will provide a default value for
     the slot.  The thunk is called when a new instance is created and
     should return the desired initial slot value.

     INIT-FORM specifies a form that, when evaluated, will return an
     initial value for the slot.  The form is evaluated each time that
     an instance of the class is created, in the lexical environment of
     the containing `define-class' expression.

     INIT-KEYWORD specifies a keyword that can be used to pass an
     initial slot value to `make' when creating a new instance.

     Note that, since an `init-value' value is shared across all
     instances of a class, you should only use it when the initial
     value is an immutable value, like a constant.  If you want to
     initialize a slot with a fresh, independently mutable value, you
     should use `init-thunk' or `init-form' instead.  Consider the
     following example.

          (define-class <chbouib> ()
            (hashtab #:init-value (make-hash-table)))

     Here only one hash table is created and all instances of
     `<chbouib>' have their `hashtab' slot refer to it.  In order to
     have each instance of `<chbouib>' refer to a new hash table, you
     should instead write:

          (define-class <chbouib> ()
            (hashtab #:init-thunk make-hash-table))

     or:

          (define-class <chbouib> ()
            (hashtab #:init-form (make-hash-table)))

     If more than one of these options is specified for the same slot,
     the order of precedence, highest first is

        * `#:init-keyword', if INIT-KEYWORD is present in the options
          passed to `make'

        * `#:init-thunk', `#:init-form' or `#:init-value'.

     If the slot definition contains more than one initialization
     option of the same precedence, the later ones are ignored.  If a
     slot is not initialized at all, its value is unbound.

     In general, slots that are shared between more than one instance
     are only initialized at new instance creation time if the slot
     value is unbound at that time.  However, if the new instance
     creation specifies a valid init keyword and value for a shared
     slot, the slot is re-initialized regardless of its previous value.

     Note, however, that the power of GOOPS' metaobject protocol means
     that everything written here may be customized or overridden for
     particular classes!  The slot initializations described here are
     performed by the least specialized method of the generic function
     `initialize', whose signature is

          (define-method (initialize (object <object>) initargs) ...)

     The initialization of instances of any given class can be
     customized by defining a `initialize' method that is specialized
     for that class, and the author of the specialized method may
     decide to call `next-method' - which will result in a call to the
     next less specialized `initialize' method - at any point within the
     specialized code, or maybe not at all.  In general, therefore, the
     initialization mechanisms described here may be modified or
     overridden by more specialized code, or may not be supported at
     all for particular classes.

 -- slot option: #:getter getter
 -- slot option: #:setter setter
 -- slot option: #:accessor accessor
     Given an object OBJ with slots named `foo' and `bar', it is always
     possible to read and write those slots by calling `slot-ref' and
     `slot-set!' with the relevant slot name; for example:

          (slot-ref OBJ 'foo)
          (slot-set! OBJ 'bar 25)

     The `#:getter', `#:setter' and `#:accessor' options, if present,
     tell GOOPS to create generic function and method definitions that
     can be used to get and set the slot value more conveniently.
     GETTER specifies a generic function to which GOOPS will add a
     method for getting the slot value.  SETTER specifies a generic
     function to which GOOPS will add a method for setting the slot
     value.  ACCESSOR specifies an accessor to which GOOPS will add
     methods for both getting and setting the slot value.

     So if a class includes a slot definition like this:

          (c #:getter get-count #:setter set-count #:accessor count)

     GOOPS defines generic function methods such that the slot value
     can be referenced using either the getter or the accessor -

          (let ((current-count (get-count obj))) ...)
          (let ((current-count (count obj))) ...)

     - and set using either the setter or the accessor -

          (set-count obj (+ 1 current-count))
          (set! (count obj) (+ 1 current-count))

     Note that

        * with an accessor, the slot value is set using the generalized
          `set!' syntax

        * in practice, it is unusual for a slot to use all three of
          these options: read-only, write-only and read-write slots
          would typically use only `#:getter', `#:setter' and
          `#:accessor' options respectively.

     The binding of the specified names is done in the environment of
     the `define-class' expression.  If the names are already bound (in
     that environment) to values that cannot be upgraded to generic
     functions, those values are overwritten when the `define-class'
     expression is evaluated.  For more detail, see *note
     ensure-generic: Generic Function Internals.

 -- slot option: #:allocation allocation
     The `#:allocation' option tells GOOPS how to allocate storage for
     the slot.  Possible values for ALLOCATION are

        * `#:instance'

          Indicates that GOOPS should create separate storage for this
          slot in each new instance of the containing class (and its
          subclasses).  This is the default.

        * `#:class'

          Indicates that GOOPS should create storage for this slot that
          is shared by all instances of the containing class (and its
          subclasses).  In other words, a slot in class C with
          allocation `#:class' is shared by all INSTANCEs for which
          `(is-a? INSTANCE C)'.  This permits defining a kind of global
          variable which can be accessed only by (in)direct instances
          of the class which defines the slot.

        * `#:each-subclass'

          Indicates that GOOPS should create storage for this slot that
          is shared by all _direct_ instances of the containing class,
          and that whenever a subclass of the containing class is
          defined, GOOPS should create a new storage for the slot that
          is shared by all _direct_ instances of the subclass.  In
          other words, a slot with allocation `#:each-subclass' is
          shared by all instances with the same `class-of'.

        * `#:virtual'

          Indicates that GOOPS should not allocate storage for this
          slot.  The slot definition must also include the `#:slot-ref'
          and `#:slot-set!' options to specify how to reference and set
          the value for this slot.  See the example below.

     Slot allocation options are processed when defining a new class by
     the generic function `compute-get-n-set', which is specialized by
     the class's metaclass.  Hence new types of slot allocation can be
     implemented by defining a new metaclass and a method for
     `compute-get-n-set' that is specialized for the new metaclass.  For
     an example of how to do this, see *note Customizing Class
     Definition::.

 -- slot option: #:slot-ref getter
 -- slot option: #:slot-set! setter
     The `#:slot-ref' and `#:slot-set!' options must be specified if
     the slot allocation is `#:virtual', and are ignored otherwise.

     GETTER should be a closure taking a single INSTANCE parameter that
     returns the current slot value.  SETTER should be a closure taking
     two parameters - INSTANCE and NEW-VAL - that sets the slot value
     to NEW-VAL.

9.5 Illustrating Slot Description
=================================

To illustrate slot description, we can redefine the `<my-complex>'
class seen before. A definition could be:

     (define-class <my-complex> (<number>)
        (r #:init-value 0 #:getter get-r #:setter set-r! #:init-keyword #:r)
        (i #:init-value 0 #:getter get-i #:setter set-i! #:init-keyword #:i))

With this definition, the `r' and `i' slots are set to 0 by default,
and can be initialised to other values by calling `make' with the `#:r'
and `#:i' keywords.  Also the generic functions `get-r', `set-r!',
`get-i' and `set-i!'  are automatically defined to read and write the
slots.

     (define c1 (make <my-complex> #:r 1 #:i 2))
     (get-r c1) => 1
     (set-r! c1 12)
     (get-r c1) => 12
     (define c2 (make <my-complex> #:r 2))
     (get-r c2) => 2
     (get-i c2) => 0

   Accessors can both read and write a slot.  So, another definition of
the `<my-complex>' class, using the `#:accessor' option, could be:

     (define-class <my-complex> (<number>)
        (r #:init-value 0 #:accessor real-part #:init-keyword #:r)
        (i #:init-value 0 #:accessor imag-part #:init-keyword #:i))

With this definition, the `r' slot can be read with:
     (real-part c)
   and set with:
     (set! (real-part c) new-value)

   Suppose now that we want to manipulate complex numbers with both
rectangular and polar coordinates.  One solution could be to have a
definition of complex numbers which uses one particular representation
and some conversion functions to pass from one representation to the
other.  A better solution is to use virtual slots, like this:

     (define-class <my-complex> (<number>)
        ;; True slots use rectangular coordinates
        (r #:init-value 0 #:accessor real-part #:init-keyword #:r)
        (i #:init-value 0 #:accessor imag-part #:init-keyword #:i)
        ;; Virtual slots access do the conversion
        (m #:accessor magnitude #:init-keyword #:magn
           #:allocation #:virtual
           #:slot-ref (lambda (o)
                       (let ((r (slot-ref o 'r)) (i (slot-ref o 'i)))
                         (sqrt (+ (* r r) (* i i)))))
           #:slot-set! (lambda (o m)
                         (let ((a (slot-ref o 'a)))
                           (slot-set! o 'r (* m (cos a)))
                           (slot-set! o 'i (* m (sin a))))))
        (a #:accessor angle #:init-keyword #:angle
           #:allocation #:virtual
           #:slot-ref (lambda (o)
                       (atan (slot-ref o 'i) (slot-ref o 'r)))
           #:slot-set! (lambda(o a)
                        (let ((m (slot-ref o 'm)))
                           (slot-set! o 'r (* m (cos a)))
                           (slot-set! o 'i (* m (sin a)))))))

   In this class definition, the magnitude `m' and angle `a' slots are
virtual, and are calculated, when referenced, from the normal (i.e.
`#:allocation #:instance') slots `r' and `i', by calling the function
defined in the relevant `#:slot-ref' option.  Correspondingly, writing
`m' or `a' leads to calling the function defined in the `#:slot-set!'
option.  Thus the following expression

     (slot-set! c 'a 3)

permits to set the angle of the `c' complex number.

     (define c (make <my-complex> #:r 12 #:i 20))
     (real-part c) => 12
     (angle c) => 1.03037682652431
     (slot-set! c 'i 10)
     (set! (real-part c) 1)
     (describe c)
     -|
     #<<my-complex> 401e9b58> is an instance of class <my-complex>
     Slots are:
          r = 1
          i = 10
          m = 10.0498756211209
          a = 1.47112767430373

   Since initialization keywords have been defined for the four slots,
we can now define the standard Scheme primitives `make-rectangular' and
`make-polar'.

     (define make-rectangular
        (lambda (x y) (make <my-complex> #:r x #:i y)))

     (define make-polar
        (lambda (x y) (make <my-complex> #:magn x #:angle y)))

9.6 Methods and Generic Functions
=================================

A GOOPS method is like a Scheme procedure except that it is specialized
for a particular set of argument classes, and will only be used when the
actual arguments in a call match the classes in the method definition.

     (define-method (+ (x <string>) (y <string>))
       (string-append x y))

     (+ "abc" "de") => "abcde"

   A method is not formally associated with any single class (as it is
in many other object oriented languages), because a method can be
specialized for a combination of several classes.  If you've studied
object orientation in non-Lispy languages, you may remember discussions
such as whether a method to stretch a graphical image around a surface
should be a method of the image class, with a surface as a parameter, or
a method of the surface class, with an image as a parameter.  In GOOPS
you'd just write

     (define-method (stretch (im <image>) (sf <surface>))
       ...)

and the question of which class the method is more associated with does
not need answering.

   There can simultaneously be several methods with the same name but
different sets of specializing argument classes; for example:

     (define-method (+ (x <string>) (y <string)) ...)
     (define-method (+ (x <matrix>) (y <matrix>)) ...)
     (define-method (+ (f <fish>) (b <bicycle>)) ...)
     (define-method (+ (a <foo>) (b <bar>) (c <baz>)) ...)

A generic function is a container for the set of such methods that a
program intends to use.

   If you look at a program's source code, and see `(+ x y)' somewhere
in it, conceptually what is happening is that the program at that point
calls a generic function (in this case, the generic function bound to
the identifier `+').  When that happens, Guile works out which of the
generic function's methods is the most appropriate for the arguments
that the function is being called with; then it evaluates the method's
code with the arguments as formal parameters.  This happens every time
that a generic function call is evaluated -- it isn't assumed that a
given source code call will end up invoking the same method every time.

   Defining an identifier as a generic function is done with the
`define-generic' macro.  Definition of a new method is done with the
`define-method' macro.  Note that `define-method' automatically does a
`define-generic' if the identifier concerned is not already a generic
function, so often an explicit `define-generic' call is not needed.  

 -- syntax: define-generic symbol
     Create a generic function with name SYMBOL and bind it to the
     variable SYMBOL.  If SYMBOL was previously bound to a Scheme
     procedure (or procedure-with-setter), the old procedure (and
     setter) is incorporated into the new generic function as its
     default procedure (and setter).  Any other previous value,
     including an existing generic function, is discarded and replaced
     by a new, empty generic function.

 -- syntax: define-method (generic parameter ...) . body
     Define a method for the generic function or accessor GENERIC with
     parameters PARAMETERs and body BODY.

     GENERIC is a generic function.  If GENERIC is a variable which is
     not yet bound to a generic function object, the expansion of
     `define-method' will include a call to `define-generic'.  If
     GENERIC is `(setter GENERIC-WITH-SETTER)', where
     GENERIC-WITH-SETTER is a variable which is not yet bound to a
     generic-with-setter object, the expansion will include a call to
     `define-accessor'.

     Each PARAMETER must be either a symbol or a two-element list
     `(SYMBOL CLASS)'.  The symbols refer to variables in the BODY that
     will be bound to the parameters supplied by the caller when
     calling this method.  The CLASSes, if present, specify the
     possible combinations of parameters to which this method can be
     applied.

     BODY is the body of the method definition.

   `define-method' expressions look a little like Scheme procedure
definitions of the form

     (define (name formals ...) . body)

   The important difference is that each formal parameter, apart from
the possible "rest" argument, can be qualified by a class name:
`FORMAL' becomes `(FORMAL CLASS)'.  The meaning of this qualification
is that the method being defined will only be applicable in a
particular generic function invocation if the corresponding argument is
an instance of `CLASS' (or one of its subclasses).  If more than one of
the formal parameters is qualified in this way, then the method will
only be applicable if each of the corresponding arguments is an
instance of its respective qualifying class.

   Note that unqualified formal parameters act as though they are
qualified by the class `<top>', which GOOPS uses to mean the superclass
of all valid Scheme types, including both primitive types and GOOPS
classes.

   For example, if a generic function method is defined with PARAMETERs
`(s1 <square>)' and `(n <number>)', that method is only applicable to
invocations of its generic function that have two parameters where the
first parameter is an instance of the `<square>' class and the second
parameter is a number.

9.6.1 Accessors
---------------

An accessor is a generic function that can also be used with the
generalized `set!' syntax (*note Procedures with Setters::).  Guile
will handle a call like

     (set! (`accessor' `args'...) `value')

by calling the most specialized method of `accessor' that matches the
classes of `args' and `value'.  `define-accessor' is used to bind an
identifier to an accessor.

 -- syntax: define-accessor symbol
     Create an accessor with name SYMBOL and bind it to the variable
     SYMBOL.  If SYMBOL was previously bound to a Scheme procedure (or
     procedure-with-setter), the old procedure (and setter) is
     incorporated into the new accessor as its default procedure (and
     setter).  Any other previous value, including an existing generic
     function or accessor, is discarded and replaced by a new, empty
     accessor.

9.6.2 Extending Primitives
--------------------------

Many of Guile's primitive procedures can be extended by giving them a
generic function definition that operates in conjunction with their
normal C-coded implementation.  When a primitive is extended in this
way, it behaves like a generic function with the C-coded implementation
as its default method.

   This extension happens automatically if a method is defined (by a
`define-method' call) for a variable whose current value is a
primitive.  But it can also be forced by calling
`enable-primitive-generic!'.

 -- primitive procedure: enable-primitive-generic! primitive
     Force the creation of a generic function definition for PRIMITIVE.

   Once the generic function definition for a primitive has been
created, it can be retrieved using `primitive-generic-generic'.

 -- primitive procedure: primitive-generic-generic primitive
     Return the generic function definition of PRIMITIVE.

     `primitive-generic-generic' raises an error if PRIMITIVE is not a
     primitive with generic capability.

9.6.3 Merging Generics
----------------------

GOOPS generic functions and accessors often have short, generic names.
For example, if a vector package provides an accessor for the X
coordinate of a vector, that accessor may just be called `x'.  It
doesn't need to be called, for example, `vector:x', because GOOPS will
work out, when it sees code like `(x OBJ)', that the vector-specific
method of `x' should be called if OBJ is a vector.

   That raises the question, though, of what happens when different
packages define a generic function with the same name.  Suppose we work
with a graphical package which needs to use two independent vector
packages for 2D and 3D vectors respectively.  If both packages export
`x', what does the code using those packages end up with?

   *note duplicate binding handlers: Creating Guile Modules. explains
how this is resolved for conflicting bindings in general.  For generics,
there is a special duplicates handler, `merge-generics', which tells
the module system to merge generic functions with the same name.  Here
is an example:

     (define-module (math 2D-vectors)
       #:use-module (oop goops)
       #:export (x y ...))

     (define-module (math 3D-vectors)
       #:use-module (oop goops)
       #:export (x y z ...))

     (define-module (my-module)
       #:use-module (oop goops)
       #:use-module (math 2D-vectors)
       #:use-module (math 3D-vectors)
       #:duplicates (merge-generics))

   The generic function `x' in `(my-module)' will now incorporate all
of the methods of `x' from both imported modules.

   To be precise, there will now be three distinct generic functions
named `x': `x' in `(math 2D-vectors)', `x' in `(math 3D-vectors)', and
`x' in `(my-module)'; and these functions share their methods in an
interesting and dynamic way.

   To explain, let's call the imported generic functions (in `(math
2D-vectors)' and `(math 3D-vectors)') the "ancestors", and the merged
generic function (in `(my-module)'), the "descendant".  The general
rule is that for any generic function G, the applicable methods are
selected from the union of the methods of G's descendant functions, the
methods of G itself and the methods of G's ancestor functions.

   Thus ancestor functions effectively share methods with their
descendants, and vice versa.  In the example above, `x' in `(math
2D-vectors)' will share the methods of `x' in `(my-module)' and vice
versa.(1)  Sharing is dynamic, so adding another new method to a
descendant implies adding it to that descendant's ancestors too.

   ---------- Footnotes ----------

   (1) But note that `x' in `(math 2D-vectors)' doesn't share methods
with `x' in `(math 3D-vectors)', so modularity is still preserved.

9.6.4 Next-method
-----------------

When you call a generic function, with a particular set of arguments,
GOOPS builds a list of all the methods that are applicable to those
arguments and orders them by how closely the method definitions match
the actual argument types.  It then calls the method at the top of this
list.  If the selected method's code wants to call on to the next method
in this list, it can do so by using `next-method'.

     (define-method (Test (a <integer>)) (cons 'integer (next-method)))
     (define-method (Test (a <number>))  (cons 'number  (next-method)))
     (define-method (Test a)             (list 'top))

   With these definitions,

     (Test 1)   => (integer number top)
     (Test 1.0) => (number top)
     (Test #t)  => (top)

   `next-method' is always called as just `(next-method)'.  The
arguments for the next method call are always implicit, and always the
same as for the original method call.

   If you want to call on to a method with the same name but with a
different set of arguments (as you might with overloaded methods in C++,
for example), you do not use `next-method', but instead simply write
the new call as usual:

     (define-method (Test (a <number>) min max)
       (if (and (>= a min) (<= a max))
           (display "Number is in range\n"))
       (Test a))

     (Test 2 1 10)
     -|
     Number is in range
     =>
     (integer number top)

   (You should be careful in this case that the `Test' calls do not
lead to an infinite recursion, but this consideration is just the same
as in Scheme code in general.)

9.6.5 Generic Function and Method Examples
------------------------------------------

Consider the following definitions:

     (define-generic G)
     (define-method (G (a <integer>) b) 'integer)
     (define-method (G (a <real>) b) 'real)
     (define-method (G a b) 'top)

   The `define-generic' call defines G as a generic function.  The
three next lines define methods for G.  Each method uses a sequence of
"parameter specializers" that specify when the given method is
applicable.  A specializer permits to indicate the class a parameter
must belong to (directly or indirectly) to be applicable.  If no
specializer is given, the system defaults it to `<top>'.  Thus, the
first method definition is equivalent to

     (define-method (G (a <integer>) (b <top>)) 'integer)

   Now, let's look at some possible calls to the generic function G:

     (G 2 3)    => integer
     (G 2 #t)   => integer
     (G 1.2 'a) => real
     (G #t #f)  => top
     (G 1 2 3)  => error (since no method exists for 3 parameters)

   The methods above use only one specializer per parameter list.  But
in general, any or all of a method's parameters may be specialized.
Suppose we define now:

     (define-method (G (a <integer>) (b <number>))  'integer-number)
     (define-method (G (a <integer>) (b <real>))    'integer-real)
     (define-method (G (a <integer>) (b <integer>)) 'integer-integer)
     (define-method (G a (b <number>))              'top-number)

With these definitions:

     (G 1 2)   => integer-integer
     (G 1 1.0) => integer-real
     (G 1 #t)  => integer
     (G 'a 1)  => top-number

   As a further example we shall continue to define operations on the
`<my-complex>' class.  Suppose that we want to use it to implement
complex numbers completely.  For instance a definition for the addition
of two complex numbers could be

     (define-method (new-+ (a <my-complex>) (b <my-complex>))
       (make-rectangular (+ (real-part a) (real-part b))
                         (+ (imag-part a) (imag-part b))))

   To be sure that the `+' used in the method `new-+' is the standard
addition we can do:

     (define-generic new-+)

     (let ((+ +))
       (define-method (new-+ (a <my-complex>) (b <my-complex>))
         (make-rectangular (+ (real-part a) (real-part b))
                           (+ (imag-part a) (imag-part b)))))

   The `define-generic' ensures here that `new-+' will be defined in
the global environment. Once this is done, we can add methods to the
generic function `new-+' which make a closure on the `+' symbol.  A
complete writing of the `new-+' methods is shown in *note fig:newplus::.

     (define-generic new-+)

     (let ((+ +))

       (define-method (new-+ (a <real>) (b <real>)) (+ a b))

       (define-method (new-+ (a <real>) (b <my-complex>))
         (make-rectangular (+ a (real-part b)) (imag-part b)))

       (define-method (new-+ (a <my-complex>) (b <real>))
         (make-rectangular (+ (real-part a) b) (imag-part a)))

       (define-method (new-+ (a <my-complex>) (b <my-complex>))
         (make-rectangular (+ (real-part a) (real-part b))
                           (+ (imag-part a) (imag-part b))))

       (define-method (new-+ (a <number>))  a)

       (define-method (new-+) 0)

       (define-method (new-+ . args)
         (new-+ (car args)
           (apply new-+ (cdr args)))))

     (set! + new-+)

Figure 9.1: Extending `+' to handle complex numbers

   We take advantage here of the fact that generic function are not
obliged to have a fixed number of parameters.  The four first methods
implement dyadic addition.  The fifth method says that the addition of
a single element is this element itself.  The sixth method says that
using the addition with no parameter always return 0 (as is also true
for the primitive `+').  The last method takes an arbitrary number of
parameters(1).  This method acts as a kind of `reduce': it calls the
dyadic addition on the _car_ of the list and on the result of applying
it on its rest.  To finish, the `set!' permits to redefine the `+'
symbol to our extended addition.

   To conclude our implementation (integration?) of complex numbers, we
could redefine standard Scheme predicates in the following manner:

     (define-method (complex? c <my-complex>) #t)
     (define-method (complex? c)           #f)

     (define-method (number? n <number>) #t)
     (define-method (number? n)          #f)
     ...

   Standard primitives in which complex numbers are involved could also
be redefined in the same manner.

   ---------- Footnotes ----------

   (1) The parameter list for a `define-method' follows the conventions
used for Scheme procedures. In particular it can use the dot notation
or a symbol to denote an arbitrary number of parameters

9.6.6 Handling Invocation Errors
--------------------------------

If a generic function is invoked with a combination of parameters for
which there is no applicable method, GOOPS raises an error.

 -- generic: no-method
 -- method: no-method (gf <generic>) args
     When an application invokes a generic function, and no methods at
     all have been defined for that generic function, GOOPS calls the
     `no-method' generic function.  The default method calls
     `goops-error' with an appropriate message.

 -- generic: no-applicable-method
 -- method: no-applicable-method (gf <generic>) args
     When an application applies a generic function to a set of
     arguments, and no methods have been defined for those argument
     types, GOOPS calls the `no-applicable-method' generic function.
     The default method calls `goops-error' with an appropriate message.

 -- generic: no-next-method
 -- method: no-next-method (gf <generic>) args
     When a generic function method calls `(next-method)' to invoke the
     next less specialized method for that generic function, and no less
     specialized methods have been defined for the current generic
     function arguments, GOOPS calls the `no-next-method' generic
     function.  The default method calls `goops-error' with an
     appropriate message.

9.7 Inheritance
===============

Here are some class definitions to help illustrate inheritance:

     (define-class A () a)
     (define-class B () b)
     (define-class C () c)
     (define-class D (A B) d a)
     (define-class E (A C) e c)
     (define-class F (D E) f)

   `A', `B', `C' have a null list of superclasses.  In this case, the
system will replace the null list by a list which only contains
`<object>', the root of all the classes defined by `define-class'.
`D', `E', `F' use multiple inheritance: each class inherits from two
previously defined classes.  Those class definitions define a hierarchy
which is shown in *note fig:hier::.  In this figure, the class `<top>'
is also shown; this class is the superclass of all Scheme objects.  In
particular, `<top>' is the superclass of all standard Scheme types.

          <top>
          / \\\_____________________
         /   \\___________          \
        /     \           \          \
    <object>  <pair>  <procedure>  <number>
    /  |  \                           |
   /   |   \                          |
  A    B    C                      <complex>
  |\__/__   |                         |
   \ /   \ /                          |
    D     E                         <real>
     \   /                            |
       F                              |
                                   <integer>

Figure 9.2: A class hierarchy.

   When a class has superclasses, its set of slots is calculated by
taking the union of its own slots and those of all its superclasses.
Thus each instance of D will have three slots, `a', `b' and `d'). The
slots of a class can be discovered using the `class-slots' primitive.
For instance,

     (class-slots A) => ((a))
     (class-slots E) => ((a) (e) (c))
     (class-slots F) => ((e) (c) (b) (d) (a) (f))

The ordering of the returned slots is not significant.

9.7.1 Class Precedence List
---------------------------

What happens when a class inherits from two or more superclasses that
have a slot with the same name but incompatible definitions -- for
example, different init values or slot allocations?  We need a rule for
deciding which slot definition the derived class ends up with, and this
rule is provided by the class's "Class Precedence List".(1)

   Another problem arises when invoking a generic function, and there is
more than one method that could apply to the call arguments.  Here we
need a way of ordering the applicable methods, so that Guile knows which
method to use first, which to use next if that method calls
`next-method', and so on.  One of the ingredients for this ordering is
determining, for each given call argument, which of the specializing
classes, from each applicable method's definition, is the most specific
for that argument; and here again the class precedence list helps.

   If inheritance was restricted such that each class could only have
one superclass -- which is known as "single" inheritance -- class
ordering would be easy.  The rule would be simply that a subclass is
considered more specific than its superclass.

   With multiple inheritance, ordering is less obvious, and we have to
impose an arbitrary rule to determine precedence. Suppose we have

     (define-class X ()
        (x #:init-value 1))

     (define-class Y ()
        (x #:init-value 2))

     (define-class Z (X Y)
        (...))

Clearly the `Z' class is more specific than `X' or `Y', for instances
of `Z'.  But which is more specific out of `X' and `Y' -- and hence,
for the definitions above, which `#:init-value' will take effect when
creating an instance of `Z'?  The rule in GOOPS is that the
superclasses listed earlier are more specific than those listed later.
Hence `X' is more specific than `Y', and the `#:init-value' for slot
`x' in instances of `Z' will be 1.

   Hence there is a linear ordering for a class and all its
superclasses, from most specific to least specific, and this ordering is
called the Class Precedence List of the class.

   In fact the rules above are not quite enough to always determine a
unique order, but they give an idea of how things work.  For example,
for the `F' class shown in *note Figure 9.2: fig:hier, the class
precedence list is

     (f d e a c b <object> <top>)

In cases where there is any ambiguity (like this one), it is a bad idea
for programmers to rely on exactly what the order is.  If the order for
some superclasses is important, it can be expressed directly in the
class definition.

   The precedence list of a class can be obtained by calling
`class-precedence-list'.  This function returns a ordered list whose
first element is the most specific class.  For instance:

     (class-precedence-list B) => (#<<class> B 401b97c8>
                                          #<<class> <object> 401e4a10>
                                          #<<class> <top> 4026a9d8>)

Or for a more immediately readable result:

     (map class-name (class-precedence-list B)) => (B <object> <top>)

   ---------- Footnotes ----------

   (1) This section is an adaptation of material from Jeff Dalton's
(J.Dalton@ed.ac.uk) `Brief introduction to CLOS'

9.7.2 Sorting Methods
---------------------

Now, with the idea of the class precedence list, we can state precisely
how the possible methods are sorted when more than one of the methods of
a generic function are applicable to the call arguments.

   The rules are that
   * the applicable methods are sorted in order of specificity, and the
     most specific method is used first, then the next if that method
     calls `next-method', and so on

   * a method M1 is more specific than another method M2 if the first
     specializing class that differs, between the definitions of M1 and
     M2, is more specific, in M1's definition, for the corresponding
     actual call argument, than the specializing class in M2's
     definition

   * a class C1 is more specific than another class C2, for an object of
     actual class C, if C1 comes before C2 in C's class precedence list.

9.8 Introspection
=================

"Introspection", or "reflection", means being able to obtain
information dynamically about GOOPS objects.  It is perhaps best
illustrated by considering an object oriented language that does not
provide any introspection, namely C++.

   Nothing in C++ allows a running program to obtain answers to the
following types of question:

   * What are the data members of this object or class?

   * What classes does this class inherit from?

   * Is this method call virtual or non-virtual?

   * If I invoke `Employee::adjustHoliday()', what class contains the
     `adjustHoliday()' method that will be applied?

   In C++, answers to such questions can only be determined by looking
at the source code, if you have access to it.  GOOPS, on the other hand,
includes procedures that allow answers to these questions -- or their
GOOPS equivalents -- to be obtained dynamically, at run time.

9.8.1 Classes
-------------

A GOOPS class is itself an instance of the `<class>' class, or of a
subclass of `<class>'.  The definition of the `<class>' class has slots
that are used to describe the properties of a class, including the
following.

 -- primitive procedure: class-name class
     Return the name of class CLASS.  This is the value of CLASS's
     `name' slot.

 -- primitive procedure: class-direct-supers class
     Return a list containing the direct superclasses of CLASS.  This
     is the value of CLASS's `direct-supers' slot.

 -- primitive procedure: class-direct-slots class
     Return a list containing the slot definitions of the direct slots
     of CLASS.  This is the value of CLASS's `direct-slots' slot.

 -- primitive procedure: class-direct-subclasses class
     Return a list containing the direct subclasses of CLASS.  This is
     the value of CLASS's `direct-subclasses' slot.

 -- primitive procedure: class-direct-methods class
     Return a list of all the generic function methods that use CLASS
     as a formal parameter specializer.  This is the value of CLASS's
     `direct-methods' slot.

 -- primitive procedure: class-precedence-list class
     Return the class precedence list for class CLASS (*note Class
     Precedence List::).  This is the value of CLASS's `cpl' slot.

 -- primitive procedure: class-slots class
     Return a list containing the slot definitions for all CLASS's
     slots, including any slots that are inherited from superclasses.
     This is the value of CLASS's `slots' slot.

 -- procedure: class-subclasses class
     Return a list of all subclasses of CLASS.

 -- procedure: class-methods class
     Return a list of all methods that use CLASS or a subclass of CLASS
     as one of its formal parameter specializers.

9.8.2 Instances
---------------

 -- primitive procedure: class-of value
     Return the GOOPS class of any Scheme VALUE.

 -- primitive procedure: instance? object
     Return `#t' if OBJECT is any GOOPS instance, otherwise `#f'.

 -- procedure: is-a? object class
     Return `#t' if OBJECT is an instance of CLASS or one of its
     subclasses.

   You can use the `is-a?' predicate to ask whether any given value
belongs to a given class, or `class-of' to discover the class of a
given value.  Note that when GOOPS is loaded (by code using the `(oop
goops)' module) built-in classes like `<string>', `<list>' and
`<number>' are automatically set up, corresponding to all Guile Scheme
types.

     (is-a? 2.3 <number>) => #t
     (is-a? 2.3 <real>) => #t
     (is-a? 2.3 <string>) => #f
     (is-a? '("a" "b") <string>) => #f
     (is-a? '("a" "b") <list>) => #t
     (is-a? (car '("a" "b")) <string>) => #t
     (is-a? <string> <class>) => #t
     (is-a? <class> <string>) => #f

     (class-of 2.3) => #<<class> <real> 908c708>
     (class-of #(1 2 3)) => #<<class> <vector> 908cd20>
     (class-of <string>) => #<<class> <class> 8bd3e10>
     (class-of <class>) => #<<class> <class> 8bd3e10>

9.8.3 Slots
-----------

 -- procedure: class-slot-definition class slot-name
     Return the slot definition for the slot named SLOT-NAME in class
     CLASS.  SLOT-NAME should be a symbol.

 -- procedure: slot-definition-name slot-def
     Extract and return the slot name from SLOT-DEF.

 -- procedure: slot-definition-options slot-def
     Extract and return the slot options from SLOT-DEF.

 -- procedure: slot-definition-allocation slot-def
     Extract and return the slot allocation option from SLOT-DEF.  This
     is the value of the `#:allocation' keyword (*note allocation: Slot
     Options.), or `#:instance' if the `#:allocation' keyword is absent.

 -- procedure: slot-definition-getter slot-def
     Extract and return the slot getter option from SLOT-DEF.  This is
     the value of the `#:getter' keyword (*note getter: Slot Options.),
     or `#f' if the `#:getter' keyword is absent.

 -- procedure: slot-definition-setter slot-def
     Extract and return the slot setter option from SLOT-DEF.  This is
     the value of the `#:setter' keyword (*note setter: Slot Options.),
     or `#f' if the `#:setter' keyword is absent.

 -- procedure: slot-definition-accessor slot-def
     Extract and return the slot accessor option from SLOT-DEF.  This
     is the value of the `#:accessor' keyword (*note accessor: Slot
     Options.), or `#f' if the `#:accessor' keyword is absent.

 -- procedure: slot-definition-init-value slot-def
     Extract and return the slot init-value option from SLOT-DEF.  This
     is the value of the `#:init-value' keyword (*note init-value: Slot
     Options.), or the unbound value if the `#:init-value' keyword is
     absent.

 -- procedure: slot-definition-init-form slot-def
     Extract and return the slot init-form option from SLOT-DEF.  This
     is the value of the `#:init-form' keyword (*note init-form: Slot
     Options.), or the unbound value if the `#:init-form' keyword is
     absent.

 -- procedure: slot-definition-init-thunk slot-def
     Extract and return the slot init-thunk option from SLOT-DEF.  This
     is the value of the `#:init-thunk' keyword (*note init-thunk: Slot
     Options.), or `#f' if the `#:init-thunk' keyword is absent.

 -- procedure: slot-definition-init-keyword slot-def
     Extract and return the slot init-keyword option from SLOT-DEF.
     This is the value of the `#:init-keyword' keyword (*note
     init-keyword: Slot Options.), or `#f' if the `#:init-keyword'
     keyword is absent.

 -- procedure: slot-init-function class slot-name
     Return the initialization function for the slot named SLOT-NAME in
     class CLASS.  SLOT-NAME should be a symbol.

     The returned initialization function incorporates the effects of
     the standard `#:init-thunk', `#:init-form' and `#:init-value' slot
     options.  These initializations can be overridden by the
     `#:init-keyword' slot option or by a specialized `initialize'
     method, so, in general, the function returned by
     `slot-init-function' may be irrelevant.  For a fuller discussion,
     see *note init-value: Slot Options.

9.8.4 Generic Functions
-----------------------

A generic function is an instance of the `<generic>' class, or of a
subclass of `<generic>'.  The definition of the `<generic>' class has
slots that are used to describe the properties of a generic function.

 -- primitive procedure: generic-function-name gf
     Return the name of generic function GF.

 -- primitive procedure: generic-function-methods gf
     Return a list of the methods of generic function GF.  This is the
     value of GF's `methods' slot.

   Similarly, a method is an instance of the `<method>' class, or of a
subclass of `<method>'; and the definition of the `<method>' class has
slots that are used to describe the properties of a method.

 -- primitive procedure: method-generic-function method
     Return the generic function that METHOD belongs to.  This is the
     value of METHOD's `generic-function' slot.

 -- primitive procedure: method-specializers method
     Return a list of METHOD's formal parameter specializers .  This is
     the value of METHOD's `specializers' slot.

 -- primitive procedure: method-procedure method
     Return the procedure that implements METHOD.  This is the value of
     METHOD's `procedure' slot.

 -- generic: method-source
 -- method: method-source (m <method>)
     Return an expression that prints to show the definition of method
     M.

          (define-generic cube)

          (define-method (cube (n <number>))
            (* n n n))

          (map method-source (generic-function-methods cube))
          =>
          ((method ((n <number>)) (* n n n)))

9.8.5 Accessing Slots
---------------------

Any slot, regardless of its allocation, can be queried, referenced and
set using the following four primitive procedures.

 -- primitive procedure: slot-exists? obj slot-name
     Return `#t' if OBJ has a slot with name SLOT-NAME, otherwise `#f'.

 -- primitive procedure: slot-bound? obj slot-name
     Return `#t' if the slot named SLOT-NAME in OBJ has a value,
     otherwise `#f'.

     `slot-bound?' calls the generic function `slot-missing' if OBJ
     does not have a slot called SLOT-NAME (*note slot-missing:
     Accessing Slots.).

 -- primitive procedure: slot-ref obj slot-name
     Return the value of the slot named SLOT-NAME in OBJ.

     `slot-ref' calls the generic function `slot-missing' if OBJ does
     not have a slot called SLOT-NAME (*note slot-missing: Accessing
     Slots.).

     `slot-ref' calls the generic function `slot-unbound' if the named
     slot in OBJ does not have a value (*note slot-unbound: Accessing
     Slots.).

 -- primitive procedure: slot-set! obj slot-name value
     Set the value of the slot named SLOT-NAME in OBJ to VALUE.

     `slot-set!' calls the generic function `slot-missing' if OBJ does
     not have a slot called SLOT-NAME (*note slot-missing: Accessing
     Slots.).

   GOOPS stores information about slots in classes.  Internally, all of
these procedures work by looking up the slot definition for the slot
named SLOT-NAME in the class `(class-of OBJ)', and then using the slot
definition's "getter" and "setter" closures to get and set the slot
value.

   The next four procedures differ from the previous ones in that they
take the class as an explicit argument, rather than assuming `(class-of
OBJ)'.  Therefore they allow you to apply the "getter" and "setter"
closures of a slot definition in one class to an instance of a
different class.

 -- primitive procedure: slot-exists-using-class? class obj slot-name
     Return `#t' if CLASS has a slot definition for a slot with name
     SLOT-NAME, otherwise `#f'.

 -- primitive procedure: slot-bound-using-class? class obj slot-name
     Return `#t' if applying `slot-ref-using-class' to the same
     arguments would call the generic function `slot-unbound', otherwise
     `#f'.

     `slot-bound-using-class?' calls the generic function
     `slot-missing' if CLASS does not have a slot definition for a slot
     called SLOT-NAME (*note slot-missing: Accessing Slots.).

 -- primitive procedure: slot-ref-using-class class obj slot-name
     Apply the "getter" closure for the slot named SLOT-NAME in CLASS
     to OBJ, and return its result.

     `slot-ref-using-class' calls the generic function `slot-missing'
     if CLASS does not have a slot definition for a slot called
     SLOT-NAME (*note slot-missing: Accessing Slots.).

     `slot-ref-using-class' calls the generic function `slot-unbound'
     if the application of the "getter" closure to OBJ returns an
     unbound value (*note slot-unbound: Accessing Slots.).

 -- primitive procedure: slot-set-using-class! class obj slot-name value
     Apply the "setter" closure for the slot named SLOT-NAME in CLASS
     to OBJ and VALUE.

     `slot-set-using-class!' calls the generic function `slot-missing'
     if CLASS does not have a slot definition for a slot called
     SLOT-NAME (*note slot-missing: Accessing Slots.).

   Slots whose allocation is per-class rather than per-instance can be
referenced and set without needing to specify any particular instance.

 -- procedure: class-slot-ref class slot-name
     Return the value of the slot named SLOT-NAME in class CLASS.  The
     named slot must have `#:class' or `#:each-subclass' allocation
     (*note allocation: Slot Options.).

     If there is no such slot with `#:class' or `#:each-subclass'
     allocation, `class-slot-ref' calls the `slot-missing' generic
     function with arguments CLASS and SLOT-NAME.  Otherwise, if the
     slot value is unbound, `class-slot-ref' calls the `slot-unbound'
     generic function, with the same arguments.

 -- procedure: class-slot-set! class slot-name value
     Set the value of the slot named SLOT-NAME in class CLASS to VALUE.
     The named slot must have `#:class' or `#:each-subclass' allocation
     (*note allocation: Slot Options.).

     If there is no such slot with `#:class' or `#:each-subclass'
     allocation, `class-slot-ref' calls the `slot-missing' generic
     function with arguments CLASS and SLOT-NAME.

   When a `slot-ref' or `slot-set!' call specifies a non-existent slot
name, or tries to reference a slot whose value is unbound, GOOPS calls
one of the following generic functions.

 -- generic: slot-missing
 -- method: slot-missing (class <class>) slot-name
 -- method: slot-missing (class <class>) (object <object>) slot-name
 -- method: slot-missing (class <class>) (object <object>) slot-name
          value
     When an application attempts to reference or set a class or
     instance slot by name, and the slot name is invalid for the
     specified CLASS or OBJECT, GOOPS calls the `slot-missing' generic
     function.

     The default methods all call `goops-error' with an appropriate
     message.

 -- generic: slot-unbound
 -- method: slot-unbound (object <object>)
 -- method: slot-unbound (class <class>) slot-name
 -- method: slot-unbound (class <class>) (object <object>) slot-name
     When an application attempts to reference a class or instance
     slot, and the slot's value is unbound, GOOPS calls the
     `slot-unbound' generic function.

     The default methods all call `goops-error' with an appropriate
     message.

9.9 Error Handling
==================

The procedure `goops-error' is called to raise an appropriate error by
the default methods of the following generic functions:

   * `slot-missing' (*note slot-missing: Accessing Slots.)

   * `slot-unbound' (*note slot-unbound: Accessing Slots.)

   * `no-method' (*note no-method: Handling Invocation Errors.)

   * `no-applicable-method' (*note no-applicable-method: Handling
     Invocation Errors.)

   * `no-next-method' (*note no-next-method: Handling Invocation
     Errors.)

   If you customize these functions for particular classes or
metaclasses, you may still want to use `goops-error' to signal any error
conditions that you detect.

 -- procedure: goops-error format-string . args
     Raise an error with key `goops-error' and error message constructed
     from FORMAT-STRING and ARGS.  Error message formatting is as done
     by `scm-error'.

9.10 GOOPS Object Miscellany
============================

Here we cover some points about GOOPS objects that aren't substantial
enough to merit sections on their own.

Object Equality
---------------

When GOOPS is loaded, `eqv?', `equal?' and `=' become generic
functions, and you can define methods for them, specialized for your
own classes, so as to control what the various kinds of equality mean
for your classes.

   For example, the `assoc' procedure, for looking up an entry in an
alist, is specified as using `equal?' to determine when the car of an
entry in the alist is the same as the key parameter that `assoc' is
called with.  Hence, if you had defined a new class, and wanted to use
instances of that class as the keys in an alist, you could define a
method for `equal?', for your class, to control `assoc''s lookup
precisely.

Cloning Objects
---------------

 -- generic: shallow-clone
 -- method: shallow-clone (self <object>)
     Return a "shallow" clone of SELF.  The default method makes a
     shallow clone by allocating a new instance and copying slot values
     from self to the new instance.  Each slot value is copied either
     as an immediate value or by reference.

 -- generic: deep-clone
 -- method: deep-clone (self <object>)
     Return a "deep" clone of SELF.  The default method makes a deep
     clone by allocating a new instance and copying or cloning slot
     values from self to the new instance.  If a slot value is an
     instance (satisfies `instance?'), it is cloned by calling
     `deep-clone' on that value.  Other slot values are copied either
     as immediate values or by reference.

Write and Display
-----------------

 -- primitive generic: write object port
 -- primitive generic: display object port
     When GOOPS is loaded, `write' and `display' become generic
     functions with special methods for printing

        * objects - instances of the class `<object>'

        * foreign objects - instances of the class `<foreign-object>'

        * classes - instances of the class `<class>'

        * generic functions - instances of the class `<generic>'

        * methods - instances of the class `<method>'.

     `write' and `display' print non-GOOPS values in the same way as
     the Guile primitive `write' and `display' functions.

   In addition to the cases mentioned, you can of course define `write'
and `display' methods for your own classes, to customize how instances
of those classes are printed.

9.11 The Metaobject Protocol
============================

At this point, we've said about as much as can be said about GOOPS
without having to confront the idea of the metaobject protocol.  There
are a couple more topics that could be discussed in isolation first --
class redefinition, and changing the class of existing instances -- but
in practice developers using them will be advanced enough to want to
understand the metaobject protocol too, and will probably be using the
protocol to customize exactly what happens during these events.

   So let's plunge in.  GOOPS is based on a "metaobject protocol" (aka
"MOP") derived from the ones used in CLOS (the Common Lisp Object
System), tiny-clos (a small Scheme implementation of a subset of CLOS
functionality) and STKlos.

   The MOP underlies many possible GOOPS customizations -- such as
defining an `initialize' method to customize the initialization of
instances of an application-defined class -- and an understanding of
the MOP makes it much easier to explain such customizations in a precise
way.  And at a deeper level, understanding the MOP is a key part of
understanding GOOPS, and of taking full advantage of GOOPS' power, by
customizing the behaviour of GOOPS itself.

9.11.1 Metaobjects and the Metaobject Protocol
----------------------------------------------

The building blocks of GOOPS are classes, slot definitions, instances,
generic functions and methods.  A class is a grouping of inheritance
relations and slot definitions.  An instance is an object with slots
that are allocated following the rules implied by its class's
superclasses and slot definitions.  A generic function is a collection
of methods and rules for determining which of those methods to apply
when the generic function is invoked.  A method is a procedure and a set
of specializers that specify the type of arguments to which the
procedure is applicable.

   Of these entities, GOOPS represents classes, generic functions and
methods as "metaobjects".  In other words, the values in a GOOPS
program that describe classes, generic functions and methods, are
themselves instances (or "objects") of special GOOPS classes that
encapsulate the behaviour, respectively, of classes, generic functions,
and methods.

   (The other two entities are slot definitions and instances.  Slot
definitions are not strictly instances, but every slot definition is
associated with a GOOPS class that specifies the behaviour of the slot
as regards accessibility and protection from garbage collection.
Instances are of course objects in the usual sense, and there is no
benefit from thinking of them as metaobjects.)

   The "metaobject protocol" (or "MOP") is the specification of the
generic functions which determine the behaviour of these metaobjects and
the circumstances in which these generic functions are invoked.

   For a concrete example of what this means, consider how GOOPS
calculates the set of slots for a class that is being defined using
`define-class'.  The desired set of slots is the union of the new
class's direct slots and the slots of all its superclasses.  But
`define-class' itself does not perform this calculation.  Instead,
there is a method of the `initialize' generic function that is
specialized for instances of type `<class>', and it is this method that
performs the slot calculation.

   `initialize' is a generic function which GOOPS calls whenever a new
instance is created, immediately after allocating memory for a new
instance, in order to initialize the new instance's slots.  The sequence
of steps is as follows.

   * `define-class' uses `make' to make a new instance of the `<class>'
     class, passing as initialization arguments the superclasses, slot
     definitions and class options that were specified in the
     `define-class' form.

   * `make' allocates memory for the new instance, and invokes the
     `initialize' generic function to initialize the new instance's
     slots.

   * The `initialize' generic function applies the method that is
     specialized for instances of type `<class>', and this method
     performs the slot calculation.

   In other words, rather than being hardcoded in `define-class', the
default behaviour of class definition is encapsulated by generic
function methods that are specialized for the class `<class>'.

   It is possible to create a new class that inherits from `<class>',
which is called a "metaclass", and to write a new `initialize' method
that is specialized for instances of the new metaclass.  Then, if the
`define-class' form includes a `#:metaclass' class option whose value
is the new metaclass, the class that is defined by the `define-class'
form will be an instance of the new metaclass rather than of the
default `<class>', and will be defined in accordance with the new
`initialize' method.  Thus the default slot calculation, as well as any
other aspect of the new class's relationship with its superclasses, can
be modified or overridden.

   In a similar way, the behaviour of generic functions can be modified
or overridden by creating a new class that inherits from the standard
generic function class `<generic>', writing appropriate methods that
are specialized to the new class, and creating new generic functions
that are instances of the new class.

   The same is true for method metaobjects.  And the same basic
mechanism allows the application class author to write an `initialize'
method that is specialized to their application class, to initialize
instances of that class.

   Such is the power of the MOP.  Note that `initialize' is just one of
a large number of generic functions that can be customized to modify
the behaviour of application objects and classes and of GOOPS itself.
Each following section covers a particular area of GOOPS functionality,
and describes the generic functions that are relevant for customization
of that area.

9.11.2 Metaclasses
------------------

A "metaclass" is the class of an object which represents a GOOPS class.
Put more succinctly, a metaclass is a class's class.

   Most GOOPS classes have the metaclass `<class>' and, by default, any
new class that is created using `define-class' has the metaclass
`<class>'.

   But what does this really mean?  To find out, let's look in more
detail at what happens when a new class is created using `define-class':

     (define-class <my-class> (<object>) . slots)

Guile expands this to something like:

     (define <my-class> (class (<object>) . slots))

which in turn expands to:

     (define <my-class>
       (make <class> #:supers (list <object>) #:slots slots))

   As this expansion makes clear, the resulting value of `<my-class>'
is an instance of the class `<class>' with slot values specifying the
superclasses and slot definitions for the class `<my-class>'.
(`#:supers' and `#:slots' are initialization keywords for the `dsupers'
and `dslots' slots of the `<class>' class.)

   Now suppose that you want to define a new class with a metaclass
other than the default `<class>'.  This is done by writing:

     (define-class <my-class2> (<object>)
        slot ...
        #:metaclass <my-metaclass>)

and Guile expands _this_ to something like:

     (define <my-class2>
       (make <my-metaclass> #:supers (list <object>) #:slots slots))

   In this case, the value of `<my-class2>' is an instance of the more
specialized class `<my-metaclass>'.  Note that `<my-metaclass>' itself
must previously have been defined as a subclass of `<class>'.  For a
full discussion of when and how it is useful to define new metaclasses,
see *note MOP Specification::.

   Now let's make an instance of `<my-class2>':

     (define my-object (make <my-class2> ...))

   All of the following statements are correct expressions of the
relationships between `my-object', `<my-class2>', `<my-metaclass>' and
`<class>'.

   * `my-object' is an instance of the class `<my-class2>'.

   * `<my-class2>' is an instance of the class `<my-metaclass>'.

   * `<my-metaclass>' is an instance of the class `<class>'.

   * The class of `my-object' is `<my-class2>'.

   * The class of `<my-class2>' is `<my-metaclass>'.

   * The class of `<my-metaclass>' is `<class>'.

9.11.3 MOP Specification
------------------------

The aim of the MOP specification in this chapter is to specify all the
customizable generic function invocations that can be made by the
standard GOOPS syntax, procedures and methods, and to explain the
protocol for customizing such invocations.

   A generic function invocation is customizable if the types of the
arguments to which it is applied are not completely determined by the
lexical context in which the invocation appears.  For example, the
`(initialize INSTANCE INITARGS)' invocation in the default
`make-instance' method is customizable, because the type of the
`INSTANCE' argument is determined by the class that was passed to
`make-instance'.

   (Whereas -- to give a counter-example -- the `(make <generic> #:name
',name)' invocation in `define-generic' is not customizable, because
all of its arguments have lexically determined types.)

   When using this rule to decide whether a given generic function
invocation is customizable, we ignore arguments that are expected to be
handled in method definitions as a single "rest" list argument.

   For each customizable generic function invocation, the "invocation
protocol" is explained by specifying

   * what, conceptually, the applied method is intended to do

   * what assumptions, if any, the caller makes about the applied
     method's side effects

   * what the caller expects to get as the applied method's return
     value.

9.11.4 Instance Creation Protocol
---------------------------------

`make <class> . INITARGS' (method)

   * `allocate-instance CLASS INITARGS' (generic)

     The applied `allocate-instance' method should allocate storage for
     a new instance of class CLASS and return the uninitialized
     instance.

   * `initialize INSTANCE INITARGS' (generic)

     INSTANCE is the uninitialized instance returned by
     `allocate-instance'.  The applied method should initialize the new
     instance in whatever sense is appropriate for its class.  The
     method's return value is ignored.

   `make' itself is a generic function.  Hence the `make' invocation
itself can be customized in the case where the new instance's metaclass
is more specialized than the default `<class>', by defining a `make'
method that is specialized to that metaclass.

   Normally, however, the method for classes with metaclass `<class>'
will be applied.  This method calls two generic functions:

   * (allocate-instance CLASS . INITARGS)

   * (initialize INSTANCE . INITARGS)

   `allocate-instance' allocates storage for and returns the new
instance, uninitialized.  You might customize `allocate-instance', for
example, if you wanted to provide a GOOPS wrapper around some other
object programming system.

   To do this, you would create a specialized metaclass, which would
act as the metaclass for all classes and instances from the other
system.  Then define an `allocate-instance' method, specialized to that
metaclass, which calls a Guile primitive C function (or FFI code), which
in turn allocates the new instance using the interface of the other
object system.

   In this case, for a complete system, you would also need to
customize a number of other generic functions like `make' and
`initialize', so that GOOPS knows how to make classes from the other
system, access instance slots, and so on.

   `initialize' initializes the instance that is returned by
`allocate-instance'.  The standard GOOPS methods perform
initializations appropriate to the instance class.

   * At the least specialized level, the method for instances of type
     `<object>' performs internal GOOPS instance initialization, and
     initializes the instance's slots according to the slot definitions
     and any slot initialization keywords that appear in INITARGS.

   * The method for instances of type `<class>' calls `(next-method)',
     then performs the class initializations described in *note Class
     Definition Protocol::.

   * and so on for generic functions, methods, operator classes ...

   Similarly, you can customize the initialization of instances of any
application-defined class by defining an `initialize' method
specialized to that class.

   Imagine a class whose instances' slots need to be initialized at
instance creation time by querying a database.  Although it might be
possible to achieve this a combination of `#:init-thunk' keywords and
closures in the slot definitions, it may be neater to write an
`initialize' method for the class that queries the database once and
initializes all the dependent slot values according to the results.

9.11.5 Class Definition Protocol
--------------------------------

Here is a summary diagram of the syntax, procedures and generic
functions that may be involved in class definition.

`define-class' (syntax)

   * `class' (syntax)

        * `make-class' (procedure)

             * `ensure-metaclass' (procedure)

             * `make METACLASS ...' (generic)

                  * `allocate-instance' (generic)

                  * `initialize' (generic)

                       * `compute-cpl' (generic)

                            * `compute-std-cpl' (procedure)

                       * `compute-slots' (generic)

                       * `compute-get-n-set' (generic)

                       * `compute-getter-method' (generic)

                       * `compute-setter-method' (generic)

   * `class-redefinition' (generic)

        * `remove-class-accessors' (generic)

        * `update-direct-method!' (generic)

        * `update-direct-subclass!' (generic)

   Wherever a step above is marked as "generic", it can be customized,
and the detail shown below it is only "correct" insofar as it describes
what the default method of that generic function does.  For example, if
you write an `initialize' method, for some metaclass, that does not
call `next-method' and does not call `compute-cpl', then `compute-cpl'
will not be called when a class is defined with that metaclass.

   A `(define-class ...)' form (*note Class Definition::) expands to an
expression which

   * checks that it is being evaluated only at top level

   * defines any accessors that are implied by the SLOT-DEFINITIONs

   * uses `class' to create the new class

   * checks for a previous class definition for NAME and, if found,
     handles the redefinition by invoking `class-redefinition' (*note
     Redefining a Class::).

 -- syntax: class name (super ...) slot-definition ... . options
     Return a newly created class that inherits from SUPERs, with
     direct slots defined by SLOT-DEFINITIONs and class options
     OPTIONS.  For the format of SLOT-DEFINITIONs and OPTIONS, see
     *note define-class: Class Definition.

`class' expands to an expression which

   * processes the class and slot definition options to check that they
     are well-formed, to convert the `#:init-form' option to an
     `#:init-thunk' option, to supply a default environment parameter
     (the current top-level environment) and to evaluate all the bits
     that need to be evaluated

   * calls `make-class' to create the class with the processed and
     evaluated parameters.

 -- procedure: make-class supers slots . options
     Return a newly created class that inherits from SUPERS, with
     direct slots defined by SLOTS and class options OPTIONS.  For the
     format of SLOTS and OPTIONS, see *note define-class: Class
     Definition, except note that for `make-class', SLOTS and OPTIONS
     are separate list parameters: SLOTS here is a list of slot
     definitions.

`make-class'

   * adds `<object>' to the SUPERS list if SUPERS is empty or if none
     of the classes in SUPERS have `<object>' in their class precedence
     list

   * defaults the `#:environment', `#:name' and `#:metaclass' options,
     if they are not specified by OPTIONS, to the current top-level
     environment, the unbound value, and `(ensure-metaclass SUPERS)'
     respectively

   * checks for duplicate classes in SUPERS and duplicate slot names in
     SLOTS, and signals an error if there are any duplicates

   * calls `make', passing the metaclass as the first parameter and all
     other parameters as option keywords with values.

 -- procedure: ensure-metaclass supers env
     Return a metaclass suitable for a class that inherits from the
     list of classes in SUPERS.  The returned metaclass is the union by
     inheritance of the metaclasses of the classes in SUPERS.

     In the simplest case, where all the SUPERS are straightforward
     classes with metaclass `<class>', the returned metaclass is just
     `<class>'.

     For a more complex example, suppose that SUPERS contained one
     class with metaclass `<operator-class>' and one with metaclass
     `<foreign-object-class>'.  Then the returned metaclass would be a
     class that inherits from both `<operator-class>' and
     `<foreign-object-class>'.

     If SUPERS is the empty list, `ensure-metaclass' returns the
     default GOOPS metaclass `<class>'.

     GOOPS keeps a list of the metaclasses created by
     `ensure-metaclass', so that each required type of metaclass only
     has to be created once.

     The `env' parameter is ignored.

 -- generic: make metaclass ...
     METACLASS is the metaclass of the class being defined, either
     taken from the `#:metaclass' class option or computed by
     `ensure-metaclass'.  The applied method must create and return the
     fully initialized class metaobject for the new class definition.

   The `(make METACLASS ...)' invocation is a particular case of the
instance creation protocol covered in the previous section.  It will
create an class metaobject with metaclass METACLASS.  By default, this
metaobject will be initialized by the `initialize' method that is
specialized for instances of type `<class>'.

   The `initialize' method for classes (signature `(initialize <class>
initargs)') calls the following generic functions.

   * `compute-cpl CLASS' (generic)

     The applied method should compute and return the class precedence
     list for CLASS as a list of class metaobjects.  When `compute-cpl'
     is called, the following CLASS metaobject slots have all been
     initialized: `name', `direct-supers', `direct-slots',
     `direct-subclasses' (empty), `direct-methods'.  The value returned
     by `compute-cpl' will be stored in the `cpl' slot.

   * `compute-slots CLASS' (generic)

     The applied method should compute and return the slots (union of
     direct and inherited) for CLASS as a list of slot definitions.
     When `compute-slots' is called, all the CLASS metaobject slots
     mentioned for `compute-cpl' have been initialized, plus the
     following: `cpl', `redefined' (`#f'), `environment'.  The value
     returned by `compute-slots' will be stored in the `slots' slot.

   * `compute-get-n-set CLASS SLOT-DEF' (generic)

     `initialize' calls `compute-get-n-set' for each slot computed by
     `compute-slots'.  The applied method should compute and return a
     pair of closures that, respectively, get and set the value of the
     specified slot.  The get closure should have arity 1 and expect a
     single argument that is the instance whose slot value is to be
     retrieved.  The set closure should have arity 2 and expect two
     arguments, where the first argument is the instance whose slot
     value is to be set and the second argument is the new value for
     that slot.  The closures should be returned in a two element list:
     `(list GET SET)'.

     The closures returned by `compute-get-n-set' are stored as part of
     the value of the CLASS metaobject's `getters-n-setters' slot.
     Specifically, the value of this slot is a list with the same
     number of elements as there are slots in the class, and each
     element looks either like

          `(SLOT-NAME-SYMBOL INIT-FUNCTION . INDEX)'

     or like

          `(SLOT-NAME-SYMBOL INIT-FUNCTION GET SET)'

     Where the get and set closures are replaced by INDEX, the slot is
     an instance slot and INDEX is the slot's index in the underlying
     structure: GOOPS knows how to get and set the value of such slots
     and so does not need specially constructed get and set closures.
     Otherwise, GET and SET are the closures returned by
     `compute-get-n-set'.

     The structure of the `getters-n-setters' slot value is important
     when understanding the next customizable generic functions that
     `initialize' calls...

   * `compute-getter-method CLASS GNS' (generic)

     `initialize' calls `compute-getter-method' for each of the class's
     slots (as determined by `compute-slots') that includes a
     `#:getter' or `#:accessor' slot option.  GNS is the element of the
     CLASS metaobject's `getters-n-setters' slot that specifies how the
     slot in question is referenced and set, as described above under
     `compute-get-n-set'.  The applied method should create and return
     a method that is specialized for instances of type CLASS and uses
     the get closure to retrieve the slot's value.  `initialize' uses
     `add-method!' to add the returned method to the generic function
     named by the slot definition's `#:getter' or `#:accessor' option.

   * `compute-setter-method CLASS GNS' (generic)

     `compute-setter-method' is invoked with the same arguments as
     `compute-getter-method', for each of the class's slots that
     includes a `#:setter' or `#:accessor' slot option.  The applied
     method should create and return a method that is specialized for
     instances of type CLASS and uses the set closure to set the slot's
     value.  `initialize' then uses `add-method!' to add the returned
     method to the generic function named by the slot definition's
     `#:setter' or `#:accessor' option.

9.11.6 Customizing Class Definition
-----------------------------------

If the metaclass of the new class is something more specialized than the
default `<class>', then the type of CLASS in the calls above is more
specialized than `<class>', and hence it becomes possible to define
generic function methods, specialized for the new class's metaclass,
that can modify or override the default behaviour of `initialize',
`compute-cpl' or `compute-get-n-set'.

   `compute-cpl' computes the class precedence list ("CPL") for the new
class (*note Class Precedence List::), and returns it as a list of
class objects.  The CPL is important because it defines a superclass
ordering that is used, when a generic function is invoked upon an
instance of the class, to decide which of the available generic function
methods is the most specific.  Hence `compute-cpl' could be customized
in order to modify the CPL ordering algorithm for all classes with a
special metaclass.

   The default CPL algorithm is encapsulated by the `compute-std-cpl'
procedure, which is called by the default `compute-cpl' method.

 -- procedure: compute-std-cpl class
     Compute and return the class precedence list for CLASS according
     to the algorithm described in *note Class Precedence List::.

   `compute-slots' computes and returns a list of all slot definitions
for the new class.  By default, this list includes the direct slot
definitions from the `define-class' form, plus the slot definitions
that are inherited from the new class's superclasses.  The default
`compute-slots' method uses the CPL computed by `compute-cpl' to
calculate this union of slot definitions, with the rule that slots
inherited from superclasses are shadowed by direct slots with the same
name.  One possible reason for customizing `compute-slots' would be to
implement an alternative resolution strategy for slot name conflicts.

   `compute-get-n-set' computes the low-level closures that will be
used to get and set the value of a particular slot, and returns them in
a list with two elements.

   The closures returned depend on how storage for that slot is
allocated.  The standard `compute-get-n-set' method, specialized for
classes of type `<class>', handles the standard GOOPS values for the
`#:allocation' slot option (*note allocation: Slot Options.).  By
defining a new `compute-get-n-set' method for a more specialized
metaclass, it is possible to support new types of slot allocation.

   Suppose you wanted to create a large number of instances of some
class with a slot that should be shared between some but not all
instances of that class - say every 10 instances should share the same
slot storage.  The following example shows how to implement and use a
new type of slot allocation to do this.

     (define-class <batched-allocation-metaclass> (<class>))

     (let ((batch-allocation-count 0)
           (batch-get-n-set #f))
       (define-method (compute-get-n-set
                          (class <batched-allocation-metaclass>) s)
         (case (slot-definition-allocation s)
           ((#:batched)
            ;; If we've already used the same slot storage for 10 instances,
            ;; reset variables.
            (if (= batch-allocation-count 10)
                (begin
                  (set! batch-allocation-count 0)
                  (set! batch-get-n-set #f)))
            ;; If we don't have a current pair of get and set closures,
            ;; create one.  make-closure-variable returns a pair of closures
            ;; around a single Scheme variable - see goops.scm for details.
            (or batch-get-n-set
                (set! batch-get-n-set (make-closure-variable)))
            ;; Increment the batch allocation count.
            (set! batch-allocation-count (+ batch-allocation-count 1))
            batch-get-n-set)

           ;; Call next-method to handle standard allocation types.
           (else (next-method)))))

     (define-class <class-using-batched-slot> ()
       ...
       (c #:allocation #:batched)
       ...
       #:metaclass <batched-allocation-metaclass>)

   The usage of `compute-getter-method' and `compute-setter-method' is
described in *note Class Definition Protocol::.

   `compute-cpl' and `compute-get-n-set' are called by the standard
`initialize' method for classes whose metaclass is `<class>'.  But
`initialize' itself can also be modified, by defining an `initialize'
method specialized to the new class's metaclass.  Such a method could
complete override the standard behaviour, by not calling
`(next-method)' at all, but more typically it would perform additional
class initialization steps before and/or after calling `(next-method)'
for the standard behaviour.

9.11.7 Method Definition
------------------------

`define-method' (syntax)

   * `add-method! TARGET METHOD' (generic)

`define-method' invokes the `add-method!' generic function to handle
adding the new method to a variety of possible targets.  GOOPS includes
methods to handle TARGET as

   * a generic function (the most common case)

   * a procedure

   * a primitive generic (*note Extending Primitives::)

   By defining further methods for `add-method!', you can theoretically
handle adding methods to further types of target.

9.11.8 Method Definition Internals
----------------------------------

`define-method':

   * checks the form of the first parameter, and applies the following
     steps to the accessor's setter if it has the `(setter ...)' form

   * interpolates a call to `define-generic' or `define-accessor' if a
     generic function is not already defined with the supplied name

   * calls `method' with the PARAMETERs and BODY, to make a new method
     instance

   * calls `add-method!' to add this method to the relevant generic
     function.

 -- syntax: method (parameter ...) . body
     Make a method whose specializers are defined by the classes in
     PARAMETERs and whose procedure definition is constructed from the
     PARAMETER symbols and BODY forms.

     The PARAMETER and BODY parameters should be as for `define-method'
     (*note define-method: Methods and Generic Functions.).

`method':

   * extracts formals and specializing classes from the PARAMETERs,
     defaulting the class for unspecialized parameters to `<top>'

   * creates a closure using the formals and the BODY forms

   * calls `make' with metaclass `<method>' and the specializers and
     closure using the `#:specializers' and `#:procedure' keywords.

 -- procedure: make-method specializers procedure
     Make a method using SPECIALIZERS and PROCEDURE.

     SPECIALIZERS should be a list of classes that specifies the
     parameter combinations to which this method will be applicable.

     PROCEDURE should be the closure that will applied to the generic
     function parameters when this method is invoked.

`make-method' is a simple wrapper around `make' with metaclass
`<method>'.

 -- generic: add-method! target method
     Generic function for adding method METHOD to TARGET.

 -- method: add-method! (generic <generic>) (method <method>)
     Add method METHOD to the generic function GENERIC.

 -- method: add-method! (proc <procedure>) (method <method>)
     If PROC is a procedure with generic capability (*note
     generic-capability?: Extending Primitives.), upgrade it to a
     primitive generic and add METHOD to its generic function
     definition.

 -- method: add-method! (pg <primitive-generic>) (method <method>)
     Add method METHOD to the generic function definition of PG.

     Implementation: `(add-method! (primitive-generic-generic pg)
     method)'.

 -- method: add-method! (whatever <top>) (method <method>)
     Raise an error indicating that WHATEVER is not a valid generic
     function.

9.11.9 Generic Function Internals
---------------------------------

`define-generic' calls `ensure-generic' to upgrade a pre-existing
procedure value, or `make' with metaclass `<generic>' to create a new
generic function.

   `define-accessor' calls `ensure-accessor' to upgrade a pre-existing
procedure value, or `make-accessor' to create a new accessor.

 -- procedure: ensure-generic old-definition [name]
     Return a generic function with name NAME, if possible by using or
     upgrading OLD-DEFINITION.  If unspecified, NAME defaults to `#f'.

     If OLD-DEFINITION is already a generic function, it is returned
     unchanged.

     If OLD-DEFINITION is a Scheme procedure or procedure-with-setter,
     `ensure-generic' returns a new generic function that uses
     OLD-DEFINITION for its default procedure and setter.

     Otherwise `ensure-generic' returns a new generic function with no
     defaults and no methods.

 -- procedure: make-generic [name]
     Return a new generic function with name `(car NAME)'.  If
     unspecified, NAME defaults to `#f'.

   `ensure-generic' calls `make' with metaclasses `<generic>' and
`<generic-with-setter>', depending on the previous value of the
variable that it is trying to upgrade.

   `make-generic' is a simple wrapper for `make' with metaclass
`<generic>'.

 -- procedure: ensure-accessor proc [name]
     Return an accessor with name NAME, if possible by using or
     upgrading PROC.  If unspecified, NAME defaults to `#f'.

     If PROC is already an accessor, it is returned unchanged.

     If PROC is a Scheme procedure, procedure-with-setter or generic
     function, `ensure-accessor' returns an accessor that reuses the
     reusable elements of PROC.

     Otherwise `ensure-accessor' returns a new accessor with no defaults
     and no methods.

 -- procedure: make-accessor [name]
     Return a new accessor with name `(car NAME)'.  If unspecified,
     NAME defaults to `#f'.

   `ensure-accessor' calls `make' with metaclass
`<generic-with-setter>', as well as calls to `ensure-generic',
`make-accessor' and (tail recursively) `ensure-accessor'.

   `make-accessor' calls `make' twice, first with metaclass `<generic>'
to create a generic function for the setter, then with metaclass
`<generic-with-setter>' to create the accessor, passing the setter
generic function as the value of the `#:setter' keyword.

9.11.10 Generic Function Invocation
-----------------------------------

There is a detailed and customizable protocol involved in the process of
invoking a generic function -- i.e., in the process of deciding which
of the generic function's methods are applicable to the current
arguments, and which one of those to apply.  Here is a summary diagram
of the generic functions involved.

`apply-generic' (generic)

   * `no-method' (generic)

   * `compute-applicable-methods' (generic)

   * `sort-applicable-methods' (generic)

        * `method-more-specific?' (generic)

   * `apply-methods' (generic)

        * `apply-method' (generic)

        * `no-next-method' (generic)

   * `no-applicable-method'

   We do not yet have full documentation for these.  Please refer to the
code (`oop/goops.scm') for details.

9.12 Redefining a Class
=======================

Suppose that a class `<my-class>' is defined using `define-class'
(*note define-class: Class Definition.), with slots that have accessor
functions, and that an application has created several instances of
`<my-class>' using `make' (*note make: Instance Creation.).  What then
happens if `<my-class>' is redefined by calling `define-class' again?

9.12.1 Default Class Redefinition Behaviour
-------------------------------------------

GOOPS' default answer to this question is as follows.

   * All existing direct instances of `<my-class>' are converted to be
     instances of the new class.  This is achieved by preserving the
     values of slots that exist in both the old and new definitions, and
     initializing the values of new slots in the usual way (*note make:
     Instance Creation.).

   * All existing subclasses of `<my-class>' are redefined, as though
     the `define-class' expressions that defined them were re-evaluated
     following the redefinition of `<my-class>', and the class
     redefinition process described here is applied recursively to the
     redefined subclasses.

   * Once all of its instances and subclasses have been updated, the
     class metaobject previously bound to the variable `<my-class>' is
     no longer needed and so can be allowed to be garbage collected.

   To keep things tidy, GOOPS also needs to do a little housekeeping on
methods that are associated with the redefined class.

   * Slot accessor methods for slots in the old definition should be
     removed from their generic functions.  They will be replaced by
     accessor methods for the slots of the new class definition.

   * Any generic function method that uses the old `<my-class>'
     metaobject as one of its formal parameter specializers must be
     updated to refer to the new `<my-class>' metaobject.  (Whenever a
     new generic function method is defined, `define-method' adds the
     method to a list stored in the class metaobject for each class
     used as a formal parameter specializer, so it is easy to identify
     all the methods that must be updated when a class is redefined.)

   If this class redefinition strategy strikes you as rather
counter-intuitive, bear in mind that it is derived from similar
behaviour in other object systems such as CLOS, and that experience in
those systems has shown it to be very useful in practice.

   Also bear in mind that, like most of GOOPS' default behaviour, it can
be customized...

9.12.2 Customizing Class Redefinition
-------------------------------------

When `define-class' notices that a class is being redefined, it
constructs the new class metaobject as usual, then invokes the
`class-redefinition' generic function with the old and new classes as
arguments.  Therefore, if the old or new classes have metaclasses other
than the default `<class>', class redefinition behaviour can be
customized by defining a `class-redefinition' method that is
specialized for the relevant metaclasses.

 -- generic: class-redefinition
     Handle the class redefinition from OLD-CLASS to NEW-CLASS, and
     return the new class metaobject that should be bound to the
     variable specified by `define-class''s first argument.

 -- method: class-redefinition (old-class <class>) (new-class <class>)
     Implements GOOPS' default class redefinition behaviour, as
     described in *note Default Class Redefinition Behaviour::.
     Returns the metaobject for the new class definition.

   The default `class-redefinition' method, for classes with the
default metaclass `<class>', calls the following generic functions,
which could of course be individually customized.

 -- generic: remove-class-accessors! old
     The default `remove-class-accessors!' method removes the accessor
     methods of the old class from all classes which they specialize.

 -- generic: update-direct-method! method old new
     The default `update-direct-method!' method substitutes the new
     class for the old in all methods specialized to the old class.

 -- generic: update-direct-subclass! subclass old new
     The default `update-direct-subclass!' method invokes
     `class-redefinition' recursively to handle the redefinition of
     subclasses.

   An alternative class redefinition strategy could be to leave all
existing instances as instances of the old class, but accepting that the
old class is now "nameless", since its name has been taken over by the
new definition.  In this strategy, any existing subclasses could also
be left as they are, on the understanding that they inherit from a
nameless superclass.

   This strategy is easily implemented in GOOPS, by defining a new
metaclass, that will be used as the metaclass for all classes to which
the strategy should apply, and then defining a `class-redefinition'
method that is specialized for this metaclass:

     (define-class <can-be-nameless> (<class>))

     (define-method (class-redefinition (old <can-be-nameless>)
                                        (new <class>))
       new)

   When customization can be as easy as this, aren't you glad that GOOPS
implements the far more difficult strategy as its default!

9.13 Changing the Class of an Instance
======================================

When a class is redefined, any existing instance of the redefined class
will be modified for the new class definition before the next time that
any of the instance's slots is referenced or set.  GOOPS modifies each
instance by calling the generic function `change-class'.

   More generally, you can change the class of an existing instance at
any time by invoking the generic function `change-class' with two
arguments: the instance and the new class.

   The default method for `change-class' decides how to implement the
change of class by looking at the slot definitions for the instance's
existing class and for the new class.  If the new class has slots with
the same name as slots in the existing class, the values for those slots
are preserved.  Slots that are present only in the existing class are
discarded.  Slots that are present only in the new class are initialized
using the corresponding slot definition's init function (*note
slot-init-function: Classes.).

 -- generic: change-class instance new-class

 -- method: change-class (obj <object>) (new <class>)
     Modify instance OBJ to make it an instance of class NEW.

     The value of each of OBJ's slots is preserved only if a similarly
     named slot exists in NEW; any other slot values are discarded.

     The slots in NEW that do not correspond to any of OBJ's
     pre-existing slots are initialized according to NEW's slot
     definitions' init functions.

   The default `change-class' method also invokes another generic
function, `update-instance-for-different-class', as the last thing that
it does before returning.  The applied
`update-instance-for-different-class' method can make any further
adjustments to NEW-INSTANCE that are required to complete or modify the
change of class.  The return value from the applied method is ignored.

 -- generic: update-instance-for-different-class old-instance
          new-instance
     A generic function that can be customized to put finishing touches
     to an instance whose class has just been changed.  The default
     `update-instance-for-different-class' method does nothing.

   Customized change of class behaviour can be implemented by defining
`change-class' methods that are specialized either by the class of the
instances to be modified or by the metaclass of the new class.

10 Guile Implementation
***********************

At some point, after one has been programming in Scheme for some time,
another level of Scheme comes into view: its implementation. Knowledge
of how Scheme can be implemented turns out to be necessary to become an
expert hacker. As Peter Norvig notes in his retrospective on PAIP(1),
"The expert Lisp programmer eventually develops a good `efficiency
model'."

   By this Norvig means that over time, the Lisp hacker eventually
develops an understanding of how much her code "costs" in terms of
space and time.

   This chapter describes Guile as an implementation of Scheme: its
history, how it represents and evaluates its data, and its compiler.
This knowledge can help you to make that step from being one who is
merely familiar with Scheme to being a real hacker.

   ---------- Footnotes ----------

   (1) PAIP is the common abbreviation for `Paradigms of Artificial
Intelligence Programming', an old but still useful text on Lisp.
Norvig's retrospective sums up the lessons of PAIP, and can be found at
`http://norvig.com/Lisp-retro.html'.

10.1 A Brief History of Guile
=============================

Guile is an artifact of historical processes, both as code and as a
community of hackers. It is sometimes useful to know this history when
hacking the source code, to know about past decisions and future
directions.

   Of course, the real history of Guile is written by the hackers
hacking and not the writers writing, so we round up the section with a
note on current status and future directions.

10.1.1 The Emacs Thesis
-----------------------

The story of Guile is the story of bringing the development experience
of Emacs to the mass of programs on a GNU system.

   Emacs, when it was first created in its GNU form in 1984, was a new
take on the problem of "how to make a program". The Emacs thesis is
that it is delightful to create composite programs based on an
orthogonal kernel written in a low-level language together with a
powerful, high-level extension language.

   Extension languages foster extensible programs, programs which adapt
readily to different users and to changing times. Proof of this can be
seen in Emacs' current and continued existence, spanning more than a
quarter-century.

   Besides providing for modification of a program by others, extension
languages are good for _intension_ as well. Programs built in "the
Emacs way" are pleasurable and easy for their authors to flesh out with
the features that they need.

   After the Emacs experience was appreciated more widely, a number of
hackers started to consider how to spread this experience to the rest
of the GNU system. It was clear that the easiest way to Emacsify a
program would be to embed a shared language implementation into it.

10.1.2 Early Days
-----------------

Tom Lord was the first to fully concentrate his efforts on an
embeddable language runtime, which he named "GEL", the GNU Extension
Language.

   GEL was the product of converting SCM, Aubrey Jaffer's implementation
of Scheme, into something more appropriate to embedding as a library.
(SCM was itself based on an implementation by George Carrette, SIOD.)

   Lord managed to convince Richard Stallman to dub GEL the official
extension language for the GNU project. It was a natural fit, given
that Scheme was a cleaner, more modern Lisp than Emacs Lisp. Part of
the argument was that eventually when GEL became more capable, it could
gain the ability to execute other languages, especially Emacs Lisp.

   Due to a naming conflict with another programming language, Jim
Blandy suggested a new name for GEL: "Guile". Besides being a recursive
acronym, "Guile" craftily follows the naming of its ancestors,
"Planner", "Conniver", and "Schemer". (The latter was truncated to
"Scheme" due to a 6-character file name limit on an old operating
system.) Finally, "Guile" suggests "guy-ell", or "Guy L.  Steele", who,
together with Gerald Sussman, originally discovered Scheme.

   Around the same time that Guile (then GEL) was readying itself for
public release, another extension language was gaining in popularity,
Tcl. Many developers found advantages in Tcl because of its shell-like
syntax and its well-developed graphical widgets library, Tk. Also, at
the time there was a large marketing push promoting Tcl as a "universal
extension language".

   Richard Stallman, as the primary author of GNU Emacs, had a
particular vision of what extension languages should be, and Tcl did
not seem to him to be as capable as Emacs Lisp. He posted a criticism
to the comp.lang.tcl newsgroup, sparking one of the internet's legendary
flamewars. As part of these discussions, retrospectively dubbed the
"Tcl Wars", he announced the Free Software Foundation's intent to
promote Guile as the extension language for the GNU project.

   It is a common misconception that Guile was created as a reaction to
Tcl. While it is true that the public announcement of Guile happened at
the same time as the "Tcl wars", Guile was created out of a condition
that existed outside the polemic. Indeed, the need for a powerful
language to bridge the gap between extension of existing applications
and a more fully dynamic programming environment is still with us today.

10.1.3 A Scheme of Many Maintainers
-----------------------------------

Surveying the field, it seems that Scheme implementations correspond
with their maintainers on an N-to-1 relationship. That is to say, that
those people that implement Schemes might do so on a number of
occasions, but that the lifetime of a given Scheme is tied to the
maintainership of one individual.

   Guile is atypical in this regard.

   Tom Lord maintained Guile for its first year and a half or so,
corresponding to the end of 1994 through the middle of 1996. The
releases made in this time constitute an arc from SCM as a standalone
program to Guile as a reusable, embeddable library, but passing through
a explosion of features: embedded Tcl and Tk, a toolchain for compiling
and disassembling Java, addition of a C-like syntax, creation of a
module system, and a start at a rich POSIX interface.

   Only some of those features remain in Guile. There were ongoing
tensions between providing a small, embeddable language, and one which
had all of the features (e.g. a graphical toolkit) that a modern Emacs
might need. In the end, as Guile gained in uptake, the development team
decided to focus on depth, documentation and orthogonality rather than
on breadth. This has been the focus of Guile ever since, although there
is a wide range of third-party libraries for Guile.

   Jim Blandy presided over that period of stabilization, in the three
years until the end of 1999, when he too moved on to other projects.
Since then, Guile has had a group maintainership. The first group was
Maciej Stachowiak, Mikael Djurfeldt, and Marius Vollmer, with Vollmer
staying on the longest. By late 2007, Vollmer had mostly moved on to
other things, so Neil Jerram and Ludovic Courtès stepped up to take on
the primary maintenance responsibility. Jerram and Courtès were joined
by Andy Wingo in late 2009.

   Of course, a large part of the actual work on Guile has come from
other contributors too numerous to mention, but without whom the world
would be a poorer place.

10.1.4 A Timeline of Selected Guile Releases
--------------------------------------------

guile-i -- 4 February 1995
     SCM, turned into a library.

guile-ii -- 6 April 1995
     A low-level module system was added. Tcl/Tk support was added,
     allowing extension of Scheme by Tcl or vice versa. POSIX support
     was improved, and there was an experimental stab at Java
     integration.

guile-iii -- 18 August 1995
     The C-like syntax, ctax, was improved, but mostly this release
     featured a start at the task of breaking Guile into pieces.

1.0 -- 5 January 1997
     `#f' was distinguished from `'()'. User-level, cooperative
     multi-threading was added. Source-level debugging became more
     useful, and programmer's and user's manuals were begun. The module
     system gained a high-level interface, which is still used today in
     more or less the same form.

1.1 -- 16 May 1997
1.2 -- 24 June 1997
     Support for Tcl/Tk and ctax were split off as separate packages,
     and have remained there since. Guile became more compatible with
     SCSH, and more useful as a UNIX scripting language. Libguile could
     now be built as a shared library, and third-party extensions
     written in C became loadable via dynamic linking.

1.3.0 -- 19 October 1998
     Command-line editing became much more pleasant through the use of
     the readline library. The initial support for internationalization
     via multi-byte strings was removed; 10 years were to pass before
     proper internationalization would land again. Initial Emacs Lisp
     support landed, ports gained better support for file descriptors,
     and fluids were added.

1.3.2 -- 20 August 1999
1.3.4 -- 25 September 1999
1.4 -- 21 June 2000
     A long list of lispy features were added: hooks, Common Lisp's
     `format', optional and keyword procedure arguments, `getopt-long',
     sorting, random numbers, and many other fixes and enhancements.
     Guile also gained an interactive debugger, interactive help, and
     better backtraces.

1.6 -- 6 September 2002
     Guile gained support for the R5RS standard, and added a number of
     SRFI modules. The module system was expanded with programmatic
     support for identifier selection and renaming. The GOOPS object
     system was merged into Guile core.

1.8 -- 20 February 2006
     Guile's arbitrary-precision arithmetic switched to use the GMP
     library, and added support for exact rationals. Guile's embedded
     user-space threading was removed in favor of POSIX pre-emptive
     threads, providing true multiprocessing. Gettext support was added,
     and Guile's C API was cleaned up and orthogonalized in a massive
     way.

2.0 -- 16 February 2010
     A virtual machine was added to Guile, along with the associated
     compiler and toolchain. Support for internationalization was
     finally reimplemented, in terms of unicode, locales, and
     libunistring. Running Guile instances became controllable and
     debuggable from within Emacs, via Geiser. Guile caught up to
     features found in a number of other Schemes: SRFI-18 threads,
     module-hygienic macros, a profiler, tracer, and debugger, SSAX XML
     integration, bytevectors, a dynamic FFI, delimited continuations,
     module versions, and partial support for R6RS.

10.1.5 Status, or: Your Help Needed
-----------------------------------

Guile has achieved much of what it set out to achieve, but there is
much remaining to do.

   There is still the old problem of bringing existing applications into
a more Emacs-like experience. Guile has had some successes in this
respect, but still most applications in the GNU system are without
Guile integration.

   Getting Guile to those applications takes an investment, the
"hacktivation energy" needed to wire Guile into a program that only
pays off once it is good enough to enable new kinds of behavior. This
would be a great way for new hackers to contribute: take an application
that you use and that you know well, think of something that it can't
yet do, and figure out a way to integrate Guile and implement that task
in Guile.

   With time, perhaps this exposure can reverse itself, whereby programs
can run under Guile instead of vice versa, eventually resulting in the
Emacsification of the entire GNU system. Indeed, this is the reason for
the naming of the many Guile modules that live in the `ice-9'
namespace, a nod to the fictional substance in Kurt Vonnegut's novel,
Cat's Cradle, capable of acting as a seed crystal to crystallize the
mass of software.

   Implicit to this whole discussion is the idea that dynamic languages
are somehow better than languages like C. While languages like C have
their place, Guile's take on this question is that yes, Scheme is more
expressive than C, and more fun to write. This realization carries an
imperative with it to write as much code in Scheme as possible rather
than in other languages.

   These days it is possible to write extensible applications almost
entirely from high-level languages, through byte-code and native
compilation, speed gains in the underlying hardware, and foreign call
interfaces in the high-level language. Smalltalk systems are like this,
as are Common Lisp-based systems. While there already are a number of
pure-Guile applications out there, users still need to drop down to C
for some tasks: interfacing to system libraries that don't have
prebuilt Guile interfaces, and for some tasks requiring high
performance.

   The addition of the virtual machine in Guile 2.0, together with the
compiler infrastructure, should go a long way to addressing the speed
issues. But there is much optimization to be done. Interested
contributors will find lots of delightful low-hanging fruit, from
simple profile-driven optimization to hacking a just-in-time compiler
from VM bytecode to native code.

   Still, even with an all-Guile application, sometimes you want to
provide an opportunity for users to extend your program from a language
with a syntax that is closer to C, or to Python. Another interesting
idea to consider is compiling e.g. Python to Guile. It's not that
far-fetched of an idea: see for example IronPython or JRuby.

   And then there's Emacs itself. Though there is a somewhat-working
Emacs Lisp language frontend for Guile, it cannot yet execute all of
Emacs Lisp. A serious integration of Guile with Emacs would replace the
Elisp virtual machine with Guile, and provide the necessary C shims so
that Guile could emulate Emacs' C API. This would give lots of exciting
things to Emacs: native threads, a real object system, more
sophisticated types, cleaner syntax, and access to all of the Guile
extensions.

   Finally, there is another axis of crystallization, the axis between
different Scheme implementations. Guile does not yet support the latest
Scheme standard, R6RS, and should do so. Like all standards, R6RS is
imperfect, but supporting it will allow more code to run on Guile
without modification, and will allow Guile hackers to produce code
compatible with other schemes. Help in this regard would be much
appreciated.

10.2 Data Representation
========================

Scheme is a latently-typed language; this means that the system cannot,
in general, determine the type of a given expression at compile time.
Types only become apparent at run time.  Variables do not have fixed
types; a variable may hold a pair at one point, an integer at the next,
and a thousand-element vector later.  Instead, values, not variables,
have fixed types.

   In order to implement standard Scheme functions like `pair?' and
`string?' and provide garbage collection, the representation of every
value must contain enough information to accurately determine its type
at run time.  Often, Scheme systems also use this information to
determine whether a program has attempted to apply an operation to an
inappropriately typed value (such as taking the `car' of a string).

   Because variables, pairs, and vectors may hold values of any type,
Scheme implementations use a uniform representation for values -- a
single type large enough to hold either a complete value or a pointer
to a complete value, along with the necessary typing information.

   The following sections will present a simple typing system, and then
make some refinements to correct its major weaknesses. We then conclude
with a discussion of specific choices that Guile has made regarding
garbage collection and data representation.

10.2.1 A Simple Representation
------------------------------

The simplest way to represent Scheme values in C would be to represent
each value as a pointer to a structure containing a type indicator,
followed by a union carrying the real value. Assuming that `SCM' is the
name of our universal type, we can write:

     enum type { integer, pair, string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         int integer;
         struct { SCM car, cdr; } pair;
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };
   with the ellipses replaced with code for the remaining Scheme types.

   This representation is sufficient to implement all of Scheme's
semantics.  If X is an `SCM' value:
   *   To test if X is an integer, we can write `X->type == integer'.

   *   To find its value, we can write `X->value.integer'.

   *   To test if X is a vector, we can write `X->type == vector'.

   *   If we know X is a vector, we can write
     `X->value.vector.elts[0]' to refer to its first element.

   *   If we know X is a pair, we can write   `X->value.pair.car' to
     extract its car.

10.2.2 Faster Integers
----------------------

Unfortunately, the above representation has a serious disadvantage.  In
order to return an integer, an expression must allocate a `struct
value', initialize it to represent that integer, and return a pointer to
it.  Furthermore, fetching an integer's value requires a memory
reference, which is much slower than a register reference on most
processors.  Since integers are extremely common, this representation is
too costly, in both time and space.  Integers should be very cheap to
create and manipulate.

   One possible solution comes from the observation that, on many
architectures, heap-allocated data (i.e., what you get when you call
`malloc') must be aligned on an eight-byte boundary. (Whether or not
the machine actually requires it, we can write our own allocator for
`struct value' objects that assures this is true.) In this case, the
lower three bits of the structure's address are known to be zero.

   This gives us the room we need to provide an improved representation
for integers.  We make the following rules:
   * If the lower three bits of an `SCM' value are zero, then the SCM
     value is a pointer to a `struct value', and everything proceeds as
     before.

   * Otherwise, the `SCM' value represents an integer, whose value
     appears in its upper bits.

   Here is C code implementing this convention:
     enum type { pair, string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         struct { SCM car, cdr; } pair;
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

     #define POINTER_P(x) (((int) (x) & 7) == 0)
     #define INTEGER_P(x) (! POINTER_P (x))

     #define GET_INTEGER(x)  ((int) (x) >> 3)
     #define MAKE_INTEGER(x) ((SCM) (((x) << 3) | 1))

   Notice that `integer' no longer appears as an element of `enum
type', and the union has lost its `integer' member.  Instead, we use
the `POINTER_P' and `INTEGER_P' macros to make a coarse classification
of values into integers and non-integers, and do further type testing
as before.

   Here's how we would answer the questions posed above (again, assume
X is an `SCM' value):
   *   To test if X is an integer, we can write `INTEGER_P (X)'.

   *   To find its value, we can write `GET_INTEGER (X)'.

   *   To test if X is a vector, we can write:
            `POINTER_P (X) && X->type == vector'
       Given the new representation, we must make sure X is truly a
     pointer before we dereference it to determine its complete type.

   *   If we know X is a vector, we can write
     `X->value.vector.elts[0]' to refer to its first element, as
     before.

   *   If we know X is a pair, we can write   `X->value.pair.car' to
     extract its car, just as before.

   This representation allows us to operate more efficiently on integers
than the first.  For example, if X and Y are known to be integers, we
can compute their sum as follows:
     MAKE_INTEGER (GET_INTEGER (X) + GET_INTEGER (Y))
   Now, integer math requires no allocation or memory references. Most
real Scheme systems actually implement addition and other operations
using an even more efficient algorithm, but this essay isn't about
bit-twiddling. (Hint: how do you decide when to overflow to a bignum?
How would you do it in assembly?)

10.2.3 Cheaper Pairs
--------------------

However, there is yet another issue to confront. Most Scheme heaps
contain more pairs than any other type of object; Jonathan Rees said at
one point that pairs occupy 45% of the heap in his Scheme
implementation, Scheme 48. However, our representation above spends
three `SCM'-sized words per pair -- one for the type, and two for the
CAR and CDR. Is there any way to represent pairs using only two words?

   Let us refine the convention we established earlier.  Let us assert
that:
   *   If the bottom three bits of an `SCM' value are `#b000', then
     it is a pointer, as before.

   *   If the bottom three bits are `#b001', then the upper bits are an
      integer.  This is a bit more restrictive than before.

   *   If the bottom two bits are `#b010', then the value, with the
     bottom   three bits masked out, is the address of a pair.

   Here is the new C code:
     enum type { string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

     struct pair {
       SCM car, cdr;
     };

     #define POINTER_P(x) (((int) (x) & 7) == 0)

     #define INTEGER_P(x)  (((int) (x) & 7) == 1)
     #define GET_INTEGER(x)  ((int) (x) >> 3)
     #define MAKE_INTEGER(x) ((SCM) (((x) << 3) | 1))

     #define PAIR_P(x) (((int) (x) & 7) == 2)
     #define GET_PAIR(x) ((struct pair *) ((int) (x) & ~7))

   Notice that `enum type' and `struct value' now only contain
provisions for vectors and strings; both integers and pairs have become
special cases.  The code above also assumes that an `int' is large
enough to hold a pointer, which isn't generally true.

   Our list of examples is now as follows:
   *   To test if X is an integer, we can write `INTEGER_P   (X)'; this
     is as before.

   *   To find its value, we can write `GET_INTEGER (X)', as   before.

   *   To test if X is a vector, we can write:
            `POINTER_P (X) && X->type == vector'
       We must still make sure that X is a pointer to a `struct
     value' before dereferencing it to find its type.

   *   If we know X is a vector, we can write
     `X->value.vector.elts[0]' to refer to its first element, as
     before.

   *   We can write `PAIR_P (X)' to determine if X is a   pair, and
     then write `GET_PAIR (X)->car' to refer to its   car.

   This change in representation reduces our heap size by 15%.  It also
makes it cheaper to decide if a value is a pair, because no memory
references are necessary; it suffices to check the bottom two bits of
the `SCM' value.  This may be significant when traversing lists, a
common activity in a Scheme system.

   Again, most real Scheme systems use a slightly different
implementation; for example, if GET_PAIR subtracts off the low bits of
`x', instead of masking them off, the optimizer will often be able to
combine that subtraction with the addition of the offset of the
structure member we are referencing, making a modified pointer as fast
to use as an unmodified pointer.

10.2.4 Conservative Garbage Collection
--------------------------------------

Aside from the latent typing, the major source of constraints on a
Scheme implementation's data representation is the garbage collector.
The collector must be able to traverse every live object in the heap, to
determine which objects are not live, and thus collectable.

   There are many ways to implement this. Guile's garbage collection is
built on a library, the Boehm-Demers-Weiser conservative garbage
collector (BDW-GC). The BDW-GC "just works", for the most part. But
since it is interesting to know how these things work, we include here a
high-level description of what the BDW-GC does.

   Garbage collection has two logical phases: a "mark" phase, in which
the set of live objects is enumerated, and a "sweep" phase, in which
objects not traversed in the mark phase are collected. Correct
functioning of the collector depends on being able to traverse the
entire set of live objects.

   In the mark phase, the collector scans the system's global variables
and the local variables on the stack to determine which objects are
immediately accessible by the C code. It then scans those objects to
find the objects they point to, and so on. The collector logically sets
a "mark bit" on each object it finds, so each object is traversed only
once.

   When the collector can find no unmarked objects pointed to by marked
objects, it assumes that any objects that are still unmarked will never
be used by the program (since there is no path of dereferences from any
global or local variable that reaches them) and deallocates them.

   In the above paragraphs, we did not specify how the garbage collector
finds the g