The Guile Reference Manual
**************************
This manual documents Guile version 2.0.9.
Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2009,
2010, 2011, 2012, 2013 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.”
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 Representing Strings as Bytes
6.6.5.14 Conversion to/from C
6.6.5.15 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 Array 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 Arrays
6.7.5.1 Array Syntax
6.7.5.2 Array Procedures
6.7.5.3 Shared Arrays
6.7.5.4 Accessing Arrays from C
6.7.6 VLists
6.7.7 Record Overview
6.7.8 SRFI-9 Records
Non-toplevel Record Definitions
Custom Printers
Functional “Setters”
6.7.9 Records
6.7.10 Structures
6.7.10.1 Vtables
6.7.10.2 Structure Basics
6.7.10.3 Vtable Contents
6.7.10.4 Meta-Vtables
6.7.10.5 Vtable Example
6.7.10.6 Tail Arrays
6.7.11 Dictionary Types
6.7.12 Association Lists
6.7.12.1 Alist Key Equality
6.7.12.2 Adding or Setting Alist Entries
6.7.12.3 Retrieving Alist Entries
6.7.12.4 Removing Alist Entries
6.7.12.5 Sloppy Alist Functions
6.7.12.6 Alist Example
6.7.13 VList-Based Hash Lists or “VHashes”
6.7.14 Hash Tables
6.7.14.1 Hash Table Examples
6.7.14.2 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
6.14.12 Handling of Unicode byte order marks.
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
6.17.12 REPL Servers
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 provide and require
6.19.11 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 Transfer Codings
7.3.6 HTTP Requests
7.3.6.1 An Important Note on Character Sets
7.3.6.2 Request API
7.3.7 HTTP Responses
7.3.8 Web Client
7.3.9 Web Server
7.3.10 Web Examples
7.3.10.1 Hello, World!
7.3.10.2 Inspecting the Request
7.3.10.3 Higher-Level Interfaces
7.3.10.4 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
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-41 - Streams
7.5.27.1 SRFI-41 Stream Fundamentals
7.5.27.2 SRFI-41 Stream Primitives
7.5.27.3 SRFI-41 Stream Library
7.5.28 SRFI-42 - Eager Comprehensions
7.5.29 SRFI-45 - Primitives for Expressing Iterative Lazy Algorithms
7.5.30 SRFI-55 - Requiring Features
7.5.31 SRFI-60 - Integers as Bits
7.5.32 SRFI-61 - A more general ‘cond’ clause
7.5.33 SRFI-67 - Compare procedures
7.5.34 SRFI-69 - Basic hash tables
7.5.34.1 Creating hash tables
7.5.34.2 Accessing table items
7.5.34.3 Table properties
7.5.34.4 Hash table algorithms
7.5.35 SRFI-88 Keyword Objects
7.5.36 SRFI-98 Accessing environment variables.
7.5.37 SRFI-105 Curly-infix expressions.
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)
7.18 Curried Definitions
7.19 Statprof
7.20 Implementation notes
7.21 Usage
7.22 SXML
7.22.1 SXML Overview
7.22.2 Reading and Writing XML
7.22.3 SSAX: A Functional XML Parsing Toolkit
7.22.3.1 History
7.22.3.2 Implementation
7.22.3.3 Usage
7.22.4 Transforming SXML
7.22.4.1 Overview
7.22.4.2 Usage
7.22.5 SXML Tree Fold
7.22.5.1 Overview
7.22.5.2 Usage
7.22.6 SXPath
7.22.6.1 Overview
7.22.6.2 Usage
7.22.7 (sxml ssax input-parse)
7.22.7.1 Overview
7.22.7.2 Usage
7.22.8 (sxml apply-templates)
7.22.8.1 Overview
7.22.8.2 Usage
7.23 Texinfo Processing
7.23.1 (texinfo)
7.23.1.1 Overview
7.23.1.2 Usage
7.23.2 (texinfo docbook)
7.23.2.1 Overview
7.23.2.2 Usage
7.23.3 (texinfo html)
7.23.3.1 Overview
7.23.3.2 Usage
7.23.4 (texinfo indexing)
7.23.4.1 Overview
7.23.4.2 Usage
7.23.5 (texinfo string-utils)
7.23.5.1 Overview
7.23.5.2 Usage
7.23.6 (texinfo plain-text)
7.23.6.1 Overview
7.23.6.2 Usage
7.23.7 (texinfo serialize)
7.23.7.1 Overview
7.23.7.2 Usage
7.23.8 (texinfo reflection)
7.23.8.1 Overview
7.23.8.2 Usage
8 GOOPS
8.1 Copyright Notice
8.2 Class Definition
8.3 Instance Creation and Slot Access
8.4 Slot Options
8.5 Illustrating Slot Description
8.6 Methods and Generic Functions
8.6.1 Accessors
8.6.2 Extending Primitives
8.6.3 Merging Generics
8.6.4 Next-method
8.6.5 Generic Function and Method Examples
8.6.6 Handling Invocation Errors
8.7 Inheritance
8.7.1 Class Precedence List
8.7.2 Sorting Methods
8.8 Introspection
8.8.1 Classes
8.8.2 Instances
8.8.3 Slots
8.8.4 Generic Functions
8.8.5 Accessing Slots
8.9 Error Handling
8.10 GOOPS Object Miscellany
8.11 The Metaobject Protocol
8.11.1 Metaobjects and the Metaobject Protocol
8.11.2 Metaclasses
8.11.3 MOP Specification
8.11.4 Instance Creation Protocol
8.11.5 Class Definition Protocol
8.11.6 Customizing Class Definition
8.11.7 Method Definition
8.11.8 Method Definition Internals
8.11.9 Generic Function Internals
8.11.10 Generic Function Invocation
8.12 Redefining a Class
8.12.1 Default Class Redefinition Behaviour
8.12.2 Customizing Class Redefinition
8.13 Changing the Class of an Instance
9 Guile Implementation
9.1 A Brief History of Guile
9.1.1 The Emacs Thesis
9.1.2 Early Days
9.1.3 A Scheme of Many Maintainers
9.1.4 A Timeline of Selected Guile Releases
9.1.5 Status, or: Your Help Needed
9.2 Data Representation
9.2.1 A Simple Representation
9.2.2 Faster Integers
9.2.3 Cheaper Pairs
9.2.4 Conservative Garbage Collection
9.2.5 The SCM Type in Guile
9.2.5.1 Relationship between ‘SCM’ and ‘scm_t_bits’
9.2.5.2 Immediate objects
9.2.5.3 Non-immediate objects
9.2.5.4 Allocating Cells
9.2.5.5 Heap Cell Type Information
9.2.5.6 Accessing Cell Entries
9.3 A Virtual Machine for Guile
9.3.1 Why a VM?
9.3.2 VM Concepts
9.3.3 Stack Layout
9.3.4 Variables and the VM
9.3.5 Compiled Procedures are VM Programs
9.3.6 Instruction Set
9.3.6.1 Lexical Environment Instructions
9.3.6.2 Top-Level Environment Instructions
9.3.6.3 Procedure Call and Return Instructions
9.3.6.4 Function Prologue Instructions
9.3.6.5 Trampoline Instructions
9.3.6.6 Branch Instructions
9.3.6.7 Data Constructor Instructions
9.3.6.8 Loading Instructions
9.3.6.9 Dynamic Environment Instructions
9.3.6.10 Miscellaneous Instructions
9.3.6.11 Inlined Scheme Instructions
9.3.6.12 Inlined Mathematical Instructions
9.3.6.13 Inlined Bytevector Instructions
9.4 Compiling to the Virtual Machine
9.4.1 Compiler Tower
9.4.2 The Scheme Compiler
9.4.3 Tree-IL
9.4.4 GLIL
9.4.5 Assembly
9.4.6 Bytecode and Objcode
9.4.7 Writing New High-Level Languages
9.4.8 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
2.0.9.
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
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
() 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 ()
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
or any of its mirrors. The file will be named guile-VERSION.tar.gz.
The current version is 2.0.9, so the file you should grab is:
To unbundle Guile use the instruction
zcat guile-2.0.9.tar.gz | tar xvf -
which will create a directory called ‘guile-2.0.9’ 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-2.0.9
./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
=============================
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
#include
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
#include
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’. 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
.
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
.
$ 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::.
Prior to Guile 2.0, Guile provided an extension to ‘define’ syntax
that allowed you to nest the previous extension up to an arbitrary
depth. These are no longer provided by default, and instead have been
moved to *note Curried Definitions::
(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 ‘#’,
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 ()
…
…))
(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
⇒
#
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 is a good starting point for
all things Scheme.
• Dorai Sitaram’s online Scheme tutorial, "Teach Yourself Scheme in
Fixnum Days", at
.
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 . 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.
‘-C DIRECTORY’
Like ‘-L’, but adjusts the load path for _compiled_ files.
‘-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.
‘--language=LANG’
For the remainder of the command line arguments, assume that files
mentioned with ‘-l’ and expressions passed with ‘-c’ are written in
LANG. LANG must be the name of one of the languages supported by
the compiler (*note Compiler Tower::). When run interactively, set
the REPL’s language to LANG (*note Using Guile Interactively::).
The default language is ‘scheme’; other interesting values include
‘elisp’ (for Emacs Lisp), and ‘ecmascript’.
The example below shows the evaluation of expressions in Scheme,
Emacs Lisp, and ECMAScript:
guile -c "(apply + '(1 2))"
guile --language=elisp -c "(= (funcall (symbol-function '+) 1 2) 3)"
guile --language=ecmascript -c '(function (x) { return x * x; })(2);'
To load a file written in Scheme and one written in Emacs Lisp, and
then start a Scheme REPL, type:
guile -l foo.scm --language=elisp -l foo.el --language=scheme
‘-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_INSTALL_LOCALE’
This is a flag that can be used to tell Guile whether or not to
install the current locale at startup, via a call to ‘(setlocale
LC_ALL "")’. *Note Locales::, for more information on locales.
You may explicitly indicate that you do not want to install the
locale by setting ‘GUILE_INSTALL_LOCALE’ to ‘0’, or explicitly
enable it by setting the variable to ‘1’.
Usually, installing the current locale is the right thing to do.
It allows Guile to correctly parse and print strings with non-ASCII
characters. However, for compatibility with previous Guile 2.0
releases, this option is off by default. The next stable release
series of Guile (the 2.2 series) will install locales by default.
‘GUILE_STACK_SIZE’
Guile currently has a limited stack size for Scheme computations.
Attempting to call too many nested functions will signal an error.
This is good to detect infinite recursion, but sometimes the limit
is reached for normal computations. This environment variable, if
set to a positive integer, specifies the number of Scheme value
slots to allocate for the stack.
In the future we will implement stacks that can grow and shrink,
but for now this hack will have to do.
‘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. If it contains the
special path component ‘...’ (ellipsis), then the default path is
put in place of the ellipsis, otherwise the default path is placed
at the end. The result is stored in ‘%load-compiled-path’ (*note
Load Paths::).
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. If it contains the special path component
‘...’ (ellipsis), then the default path is put in place of the
ellipsis, otherwise the default path is placed at the end. The
result is stored in ‘%load-path’ (*note Load Paths::).
Here is an example using the Bash shell that prepends the current
directory to ‘%load-path’, and adds the relative directory
‘../srfi’ to the end:
$ env GUILE_LOAD_PATH=".:...:../srfi" \
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 \
../srfi)
(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 if value history is enabled, or false otherwise.
-- 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 arg …
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 [#:width w] [#:max-indent i]
Trace execution.
By default, the trace will limit its width to the width of your
terminal, or WIDTH if specified. Nested procedure invocations will
be printed farther to the right, though if the width of the
indentation passes the MAX-INDENT, the indentation is abbreviated.
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 [name] [exp]
With no arguments, lists all options. With one argument, shows the
current value of the NAME option. With two arguments, sets the
NAME option to the result of evaluating the Scheme expression EXP.
-- 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.
‘print’
A procedure of two arguments used to print the result of evaluating
each expression. The arguments are the current REPL and the value
to print. By default, ‘#f’, to use the default procedure.
‘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-default-option-set! 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 # ("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 # ...)
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 # ...)
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 = #
$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 = #
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 , 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 directory, whose value
is returned by invoking the ‘%site-ccache-dir’ procedure. As in the
previous example, if Guile 2.0 is installed on your system in ‘/usr/’,
then ‘(%site-ccache-dir)’ 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,
. *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 ‘’ 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, ‘’ 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
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
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
#include
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 , 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_c_vector_length (vector);
i = 0;
while (i < len && scm_is_pair (list))
{
scm_c_vector_set_x (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 and that it doesn’t overrun the list. Otherwise,
‘scm_c_vector_set_x’ would raise an error if the index is out of range,
and ‘scm_car’ and ‘scm_cdr’ would raise an error if the value is not a
pair.
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 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_c_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_c_vector_length (vector);
i = 0;
while (i < len)
{
scm_c_vector_set_x (vector, i, scm_car (list));
list = scm_cdr (list);
i++;
}
return vector;
}
This version relies on the error-checking behavior of ‘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-safety 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 ‘#’ 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
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_new_smob’, 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.
*/
smob = scm_new_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_new_smob’ 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_new_double_smob’. 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
#include
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.
*/
smob = scm_new_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 ("#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
#
guile> (define i (make-image "Whistler's Mother" 100 100))
guile> i
#
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
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 (). 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 ‘ :’, 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 (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_PKG [VERSIONS]
This macro runs the ‘pkg-config’ tool to find development files for
an available version of Guile.
By default, this macro will search for the latest stable version of
Guile (e.g. 2.0), falling back to the previous stable version
(e.g. 1.8) if it is available. If no guile-VERSION.pc file is
found, an error is signalled. The found version is stored in
GUILE_EFFECTIVE_VERSION.
If ‘GUILE_PROGS’ was already invoked, this macro ensures that the
development files have the same effective version as the Guile
program.
GUILE_EFFECTIVE_VERSION is marked for substitution, as by
‘AC_SUBST’.
-- Autoconf Macro: GUILE_FLAGS
This macro runs the ‘pkg-config’ tool to find out how to compile
and link programs against Guile. 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-VERSION’ for the Guile
library itself, and 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_PROGS
This macro looks for programs ‘guile’ and ‘guild’, setting
variables GUILE and GUILD to their paths, respectively. If ‘guile’
is not found, signal an error.
The effective version of the found ‘guile’ is set to
GUILE_EFFECTIVE_VERSION. This macro ensures that the effective
version is compatible with the result of a previous invocation of
‘GUILE_FLAGS’, if any.
As a legacy interface, it also looks for ‘guile-config’ and
‘guile-tools’, setting GUILE_CONFIG and GUILE_TOOLS.
The variables are 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
(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_ptrdiff scm_to_ptrdiff_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_ptrdiff_t (scm_t_ptrdiff 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 (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 Z 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 Z 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 count
-- C Function: scm_ash (n, count)
Return floor(n * 2^count). N and COUNT must be exact integers.
With N viewed as an infinite-precision twos-complement integer,
‘ash’ means a left shift introducing zero bits when COUNT is
positive, or a right shift dropping bits when COUNT is negative.
This is an “arithmetic” shift.
(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: round-ash n count
-- C Function: scm_round_ash (n, count)
Return round(n * 2^count). N and COUNT must be exact integers.
With N viewed as an infinite-precision twos-complement integer,
‘round-ash’ means a left shift introducing zero bits when COUNT is
positive, or a right shift rounding to the nearest integer (with
ties going to the nearest even integer) when COUNT is negative.
This is a rounded “arithmetic” shift.
(number->string (round-ash #b1 3) 2) ⇒ \"1000\"
(number->string (round-ash #b1010 -1) 2) ⇒ \"101\"
(number->string (round-ash #b1010 -2) 2) ⇒ \"10\"
(number->string (round-ash #b1011 -2) 2) ⇒ \"11\"
(number->string (round-ash #b1101 -2) 2) ⇒ \"11\"
(number->string (round-ash #b1110 -2) 2) ⇒ \"100\"
-- 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 Codepoint
Name
‘#\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’ if 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’ if code point of X is equal to the code point of Y,
else ‘#f’.
-- Scheme Procedure: char? x y
Return ‘#t’ if the code point of X is greater than the code point
of Y, else ‘#f’.
-- Scheme Procedure: char>=? x y
Return ‘#t’ if 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’ if 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’ if 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’ if 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’ if CHR is alphabetic, else ‘#f’.
-- Scheme Procedure: char-numeric? chr
-- C Function: scm_char_numeric_p (chr)
Return ‘#t’ if CHR is numeric, else ‘#f’.
-- Scheme Procedure: char-whitespace? chr
-- C Function: scm_char_whitespace_p (chr)
Return ‘#t’ if CHR is whitespace, else ‘#f’.
-- Scheme Procedure: char-upper-case? chr
-- C Function: scm_char_upper_case_p (chr)
Return ‘#t’ if CHR is uppercase, else ‘#f’.
-- Scheme Procedure: char-lower-case? chr
-- C Function: scm_char_lower_case_p (chr)
Return ‘#t’ if CHR is lowercase, else ‘#f’.
-- Scheme Procedure: char-is-both? chr
-- C Function: scm_char_is_both_p (chr)
Return ‘#t’ if 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_set …
-- C Function: scm_char_set_eq (char_sets)
Return ‘#t’ if all given character sets are equal.
-- Scheme Procedure: char-set<= char_set …
-- C Function: scm_char_set_leq (char_sets)
Return ‘#t’ if every character set CHAR_SETi is a subset of
character set CHAR_SETi+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 chr …
-- C Function: scm_char_set (chrs)
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’ if the character CH is contained in the character set
CS, or ‘#f’ otherwise.
-- 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 chr …
-- C Function: scm_char_set_adjoin (cs, chrs)
Add all character arguments to the first argument, which must be a
character set.
-- Scheme Procedure: char-set-delete cs chr …
-- C Function: scm_char_set_delete (cs, chrs)
Delete all character arguments from the first argument, which must
be a character set.
-- Scheme Procedure: char-set-adjoin! cs chr …
-- C Function: scm_char_set_adjoin_x (cs, chrs)
Add all character arguments to the first argument, which must be a
character set.
-- Scheme Procedure: char-set-delete! cs chr …
-- C Function: scm_char_set_delete_x (cs, chrs)
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 cs …
-- C Function: scm_char_set_union (char_sets)
Return the union of all argument character sets.
-- Scheme Procedure: char-set-intersection cs …
-- C Function: scm_char_set_intersection (char_sets)
Return the intersection of all argument character sets.
-- Scheme Procedure: char-set-difference cs1 cs …
-- C Function: scm_char_set_difference (cs1, char_sets)
Return the difference of all argument character sets.
-- Scheme Procedure: char-set-xor cs …
-- C Function: scm_char_set_xor (char_sets)
Return the exclusive-or of all argument character sets.
-- Scheme Procedure: char-set-diff+intersection cs1 cs …
-- C Function: scm_char_set_diff_plus_intersection (cs1, char_sets)
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 cs …
-- C Function: scm_char_set_union_x (cs1, char_sets)
Return the union of all argument character sets.
-- Scheme Procedure: char-set-intersection! cs1 cs …
-- C Function: scm_char_set_intersection_x (cs1, char_sets)
Return the intersection of all argument character sets.
-- Scheme Procedure: char-set-difference! cs1 cs …
-- C Function: scm_char_set_difference_x (cs1, char_sets)
Return the difference of all argument character sets.
-- Scheme Procedure: char-set-xor! cs1 cs …
-- C Function: scm_char_set_xor_x (cs1, char_sets)
Return the exclusive-or of all argument character sets.
-- Scheme Procedure: char-set-diff+intersection! cs1 cs2 cs …
-- C Function: scm_char_set_diff_plus_intersection_x (cs1, cs2,
char_sets)
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
100,000 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 DELIMITER
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.
‘strict-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 char_pred
-- C Function: scm_string_split (str, char_pred)
Split the string STR into a list of substrings delimited by
appearances of characters that
• equal CHAR_PRED, if it is a character,
• satisfy the predicate CHAR_PRED, if it is a procedure,
• are in the set CHAR_PRED, if it is a character set.
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
CHR 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 s3 …
Lexicographic equality predicate; return ‘#t’ if all 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 s3 …
Lexicographic ordering predicate; return ‘#t’ if, for every pair of
consecutive string arguments STR_I and STR_I+1, STR_I is
lexicographically greater than STR_I+1.
-- Scheme Procedure: string>=? s1 s2 s3 …
Lexicographic ordering predicate; return ‘#t’ if, for every pair of
consecutive string arguments STR_I and STR_I+1, STR_I is
lexicographically greater than or equal to STR_I+1.
-- Scheme Procedure: string-ci=? s1 s2 s3 …
Case-insensitive string equality predicate; return ‘#t’ if all
strings are the same length and their component characters match
(ignoring case) at each position; otherwise return ‘#f’.
-- Scheme Procedure: string-ci? s1 s2 s3 …
Case insensitive lexicographic ordering predicate; return ‘#t’ if,
for every pair of consecutive string arguments STR_I and STR_I+1,
STR_I is lexicographically greater than STR_I+1 regardless of case.
-- Scheme Procedure: string-ci>=? s1 s2 s3 …
Case insensitive lexicographic ordering predicate; return ‘#t’ if,
for every pair of consecutive string arguments STR_I and STR_I+1,
STR_I is lexicographically greater than or equal to STR_I+1
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 arg …
-- C Function: scm_string_append (args)
Return a newly allocated string whose characters form the
concatenation of the given strings, ARG ....
(let ((h "hello "))
(string-append h "world"))
⇒ "hello world"
-- Scheme Procedure: string-append/shared arg …
-- C Function: scm_string_append_shared (args)
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 (which must be strings) of LS 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 Representing Strings as Bytes
......................................
Out in the cold world outside of Guile, not all strings are treated in
the same way. Out there there are only bytes, and there are many ways
of representing a strings (sequences of characters) as binary data
(sequences of bytes).
As a user, usually you don’t have to think about this very much.
When you type on your keyboard, your system encodes your keystrokes as
bytes according to the locale that you have configured on your computer.
Guile uses the locale to decode those bytes back into characters –
hopefully the same characters that you typed in.
All is not so clear when dealing with a system with multiple users,
such as a web server. Your web server might get a request from one user
for data encoded in the ISO-8859-1 character set, and then another
request from a different user for UTF-8 data.
Guile provides an "iconv" module for converting between strings and
sequences of bytes. *Note Bytevectors::, for more on how Guile
represents raw byte sequences. This module gets its name from the
common UNIX command of the same name.
Note that often it is sufficient to just read and write strings from
ports instead of using these functions. To do this, specify the port
encoding using ‘set-port-encoding!’. *Note Ports::, for more on ports
and character encodings.
Unlike the rest of the procedures in this section, you have to load
the ‘iconv’ module before having access to these procedures:
(use-modules (ice-9 iconv))
-- Scheme Procedure: string->bytevector string encoding
[conversion-strategy]
Encode STRING as a sequence of bytes.
The string will be encoded in the character set specified by the
ENCODING string. If the string has characters that cannot be
represented in the encoding, by default this procedure raises an
‘encoding-error’. Pass a CONVERSION-STRATEGY argument to specify
other behaviors.
The return value is a bytevector. *Note Bytevectors::, for more on
bytevectors. *Note Ports::, for more on character encodings and
conversion strategies.
-- Scheme Procedure: bytevector->string bytevector encoding
[conversion-strategy]
Decode BYTEVECTOR into a string.
The bytes will be decoded from the character set by the ENCODING
string. If the bytes do not form a valid encoding, by default this
procedure raises an ‘decoding-error’. As with
‘string->bytevector’, pass the optional CONVERSION-STRATEGY
argument to modify this behavior. *Note Ports::, for more on
character encodings and conversion strategies.
-- Scheme Procedure: call-with-output-encoded-string encoding proc
[conversion-strategy]
Like ‘call-with-output-string’, but instead of returning a string,
returns a encoding of the string according to ENCODING, as a
bytevector. This procedure can be more efficient than collecting a
string and then converting it via ‘string->bytevector’.
6.6.5.14 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 usually
treat strings as character sequences, not byte sequences.
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 execution character set, used for string and
character constants. Most modern C compilers use UTF-8 by default,
so to convert C string constants we recommend
‘scm_from_utf8_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.
The Scheme interface to this function is ‘string->bytevector’, from
the ‘ice-9 iconv’ module. *Note Representing Strings as Bytes::.
-- 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 Scheme interface to this function is ‘bytevector->string’.
*Note Representing Strings as Bytes::.
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’.
6.6.5.15 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 (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. *Note
Representing Strings as Bytes::, for a more generic interface.
(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 Array API
................................................
As an extension to the R6RS, Guile allows bytevectors to be manipulated
with the "array" procedures (*note Arrays::). When using these APIs,
bytes are accessed one at a time as 8-bit unsigned integers:
(define bv #vu8(0 1 2 3))
(array? bv)
⇒ #t
(array-rank bv)
⇒ 1
(array-ref bv 2)
⇒ 2
;; Note the different argument order on array-set!.
(array-set! bv 77 2)
(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 (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 arg …
Return a newly allocated symbol whose characters form the
concatenation of the given symbols, ARG ....
(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 scm_from_latin1_symbol (const char *name)
-- C Function: SCM 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 scm_from_locale_symbol (const char *name)
-- C Function: SCM 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 execution character set, used for
string and character constants. Most modern C compilers use UTF-8
by default, so in such cases we recommend ‘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 (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 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::.
To handle keyword arguments from procedures implemented in C, use
‘scm_c_bind_keyword_arguments’ (*note Keyword Procedures::).
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 execution character set, used for
string and character constants. Most modern C compilers use UTF-8
by default, so in such cases we recommend ‘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.
-- C Function: void scm_c_bind_keyword_arguments (const char *subr, SCM
rest, scm_t_keyword_arguments_flags flags, SCM keyword1, SCM
*argp1, …, SCM keywordN, SCM *argpN, SCM_UNDEFINED)
Extract the specified keyword arguments from REST, which is not
modified. If the keyword argument KEYWORD1 is present in REST with
an associated value, that value is stored in the variable pointed
to by ARGP1, otherwise the variable is left unchanged. Similarly
for the other keywords and argument pointers up to KEYWORDN and
ARGPN. The argument list to ‘scm_c_bind_keyword_arguments’ must be
terminated by ‘SCM_UNDEFINED’.
Note that since the variables pointed to by ARGP1 through ARGPN are
left unchanged if the associated keyword argument is not present,
they should be initialized to their default values before calling
‘scm_c_bind_keyword_arguments’. Alternatively, you can initialize
them to ‘SCM_UNDEFINED’ before the call, and then use ‘SCM_UNBNDP’
after the call to see which ones were provided.
If an unrecognized keyword argument is present in REST and FLAGS
does not contain ‘SCM_ALLOW_OTHER_KEYS’, or if non-keyword
arguments are present and FLAGS does not contain
‘SCM_ALLOW_NON_KEYWORD_ARGUMENTS’, an exception is raised. SUBR
should be the name of the procedure receiving the keyword
arguments, for purposes of error reporting.
For example:
SCM k_delimiter;
SCM k_grammar;
SCM sym_infix;
SCM my_string_join (SCM strings, SCM rest)
{
SCM delimiter = SCM_UNDEFINED;
SCM grammar = sym_infix;
scm_c_bind_keyword_arguments ("my-string-join", rest, 0,
k_delimiter, &delimiter,
k_grammar, &grammar,
SCM_UNDEFINED);
if (SCM_UNBNDP (delimiter))
delimiter = scm_from_utf8_string (" ");
return scm_string_join (strings, delimiter, grammar);
}
void my_init ()
{
k_delimiter = scm_from_utf8_keyword ("delimiter");
k_grammar = scm_from_utf8_keyword ("grammar");
sym_infix = scm_from_utf8_symbol ("infix");
scm_c_define_gsubr ("my-string-join", 1, 0, 1, my_string_join);
}
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.
However, using these procedures is often detrimental to readability, and
error-prone. Thus, accessing the contents of a list is usually better
achieved using pattern matching techniques (*note Pattern Matching::).
-- 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’ if 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’ if 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 elem …
-- 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 ELEM ....
‘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 ELEM … 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 lst … obj
-- Scheme Procedure: append
-- Scheme Procedure: append! lst … obj
-- Scheme Procedure: append!
-- C Function: scm_append (lstlst)
-- C Function: scm_append_x (lstlst)
Return a list comprising all the elements of lists LST … OBJ. If
called with no arguments, return the empty list.
(append '(x) '(y)) ⇒ (x y)
(append '(a) '(b c d)) ⇒ (a b c d)
(append '(a (b)) '((c))) ⇒ (a (b) (c))
The last argument OBJ 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 OBJ. ‘append!’ modifies the given lists
to form its return.
For ‘scm_append’ and ‘scm_append_x’, LSTLST is a list of the list
operands LST … OBJ. 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 arg …
-- 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 vec)
Return the number of elements in VEC as a ‘size_t’.
-- Scheme Procedure: vector-ref vec k
-- C Function: scm_vector_ref (vec, k)
Return the contents of position K of VEC. K must be a valid index
of VEC.
(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 vec, size_t k)
Return the contents of position K (a ‘size_t’) of VEC.
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! vec k obj
-- C Function: scm_vector_set_x (vec, k, obj)
Store OBJ in position K of VEC. K must be a valid index of VEC.
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 vec, size_t k, SCM obj)
Store OBJ in position K (a ‘size_t’) of VEC.
-- Scheme Procedure: vector-fill! vec fill
-- C Function: scm_vector_fill_x (vec, fill)
Store FILL in every position of VEC. 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 the special case of one dimensional bit arrays, and
can thus be used with the array procedures, *Note 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 bit …
-- 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 vec, 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 vec, 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 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 vector of some kind
for the actual storage of its elements. Any kind of 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.
The array procedures are all polymorphic, treating strings, uniform
numeric vectors, bytevectors, bit vectors and ordinary vectors as one
dimensional arrays.
6.7.5.1 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
#<@lower><:len><@lower><:len>...
where ‘’ 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 ‘’ 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()’, where ‘’ 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.
6.7.5.2 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
-- C Function: scm_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 …
-- C Function: scm_array_ref (array, idxlist)
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 indices 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-length array
-- C Function: scm_array_length (array)
-- C Function: size_t scm_c_array_length (array)
Return the length of an array: its first dimension. It is an error
to ask for the length of an array of rank 0.
-- Scheme Procedure: array-rank array
-- C Function: scm_array_rank (array)
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? array …
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 src …
-- 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 src2 …
-- C Function: scm_array_for_each (proc, src1, srclist)
Apply PROC to each tuple of elements of SRC1 SRC2 …, 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 array RA, in lexicographic order,
as binary objects from PORT_OR_FD. If an end of file is
encountered, the objects up to that point are put into RA (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_FD may be omitted, in which case it defaults to the value
returned by ‘(current-input-port)’.
-- Scheme Procedure: uniform-array-write ra [port_or_fd [start [end]]]
-- C Function: scm_uniform_array_write (ra, port_or_fd, start, end)
Writes all elements of RA as binary objects to PORT_OR_FD.
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_FD may
be omitted, in which case it defaults to the value returned by
‘(current-output-port)’.
6.7.5.3 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 dim2 …
-- 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))
6.7.5.4 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__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__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.
6.7.6 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 vlist …
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.
6.7.7 Record Overview
---------------------
"Records", also called "structures", are Scheme’s primary mechanism to
define new disjoint types. A "record type" defines a list of "fields"
that instances of the type consist of. This is like C’s ‘struct’.
Historically, Guile has offered several different ways to define
record types and to create records, offering different features, and
making different trade-offs. Over the years, each “standard” has also
come with its own new record interface, leading to a maze of record
APIs.
At the highest level is SRFI-9, a high-level record interface
implemented by most Scheme implementations (*note SRFI-9 Records::). It
defines a simple and efficient syntactic abstraction of record types and
their associated type predicate, fields, and field accessors. SRFI-9 is
suitable for most uses, and this is the recommended way to create record
types in Guile. Similar high-level record APIs include SRFI-35 (*note
SRFI-35::) and R6RS records (*note rnrs records syntactic::).
Then comes Guile’s historical “records” API (*note Records::).
Record types defined this way are first-class objects. Introspection
facilities are available, allowing users to query the list of fields or
the value of a specific field at run-time, without prior knowledge of
the type.
Finally, the common denominator of these interfaces is Guile’s
"structure" API (*note Structures::). Guile’s structures are the
low-level building block for all other record APIs. Application writers
will normally not need to use it.
Records created with these APIs may all be pattern-matched using
Guile’s standard pattern matcher (*note Pattern Matching::).
6.7.8 SRFI-9 Records
--------------------
SRFI-9 standardizes a syntax for defining new record types and creating
predicate, constructor, and field getter and setter functions. In Guile
this is the recommended option to create new record types (*note Record
Overview::). It can be used with:
(use-modules (srfi srfi-9))
-- Scheme 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
(make-employee name age salary)
employee?
(name employee-name)
(age employee-age set-employee-age!)
(salary 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,
⇒ #>
(define fred (make-employee "Fred" 45 20000.00))
(employee? fred) ⇒ #t
(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 proc
Where TYPE corresponds to the first argument of
‘define-record-type’, and PROC 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!
(lambda (record port)
(write-char #\[ port)
(display (employee-name record) port)
(write-char #\] port)))
Functional “Setters”
....................
When writing code in a functional style, it is desirable to never alter
the contents of records. For such code, a simple way to return new
record instances based on existing ones is highly desirable.
The ‘(srfi srfi-9 gnu)’ module extends SRFI-9 with facilities to
return new record instances based on existing ones, only with one or
more field values changed—"functional setters". First, the
‘define-immutable-record-type’ works like ‘define-record-type’, except
that fields are immutable and setters are defined as functional setters.
-- Scheme Syntax: define-immutable-record-type type
(constructor fieldname …)
predicate
(fieldname accessor [modifier]) …
Define TYPE as a new record type, like ‘define-record-type’.
However, the record type is made _immutable_ (records may not be
mutated, even with ‘struct-set!’), and any MODIFIER is defined to
be a functional setter—a procedure that returns a new record
instance with the specified field changed, and leaves the original
unchanged (see example below.)
In addition, the generic ‘set-field’ and ‘set-fields’ macros may be
applied to any SRFI-9 record.
-- Scheme Syntax: set-field record (field sub-fields ...) value
Return a new record of RECORD’s type whose fields are equal to the
corresponding fields of RECORD except for the one specified by
FIELD.
FIELD must be the name of the getter corresponding to the field of
RECORD being “set”. Subsequent SUB-FIELDS must be record getters
designating sub-fields within that field value to be set (see
example below.)
-- Scheme Syntax: set-fields record ((field sub-fields ...) value) ...
Like ‘set-field’, but can be used to set more than one field at a
time. This expands to code that is more efficient than a series of
single ‘set-field’ calls.
To illustrate the use of functional setters, let’s assume these two
record type definitions:
(define-record-type
(address street city country)
address?
(street address-street)
(city address-city)
(country address-country))
(define-immutable-record-type
(person age email address)
person?
(age person-age set-person-age)
(email person-email set-person-email)
(address person-address set-person-address))
First, note that the ‘’ record type definition introduces named
functional setters. These may be used like this:
(define fsf-address
(address "Franklin Street" "Boston" "USA"))
(define rms
(person 30 "rms@gnu.org" fsf-address))
(and (equal? (set-person-age rms 60)
(person 60 "rms@gnu.org" fsf-address))
(= (person-age rms) 30))
⇒ #t
Here, the original ‘’ record, to which RMS is bound, is left
unchanged.
Now, suppose we want to change both the street and age of RMS. This
can be achieved using ‘set-fields’:
(set-fields rms
((person-age) 60)
((person-address address-street) "Temple Place"))
⇒ #< age: 60 email: "rms@gnu.org"
address: #< street: "Temple Place" city: "Boston" country: "USA">>
Notice how the above changed two fields of RMS, including the ‘street’
field of its ‘address’ field, in a concise way. Also note that
‘set-fields’ works equally well for types defined with just
‘define-record-type’.
6.7.9 Records
-------------
A "record type" is a first class object representing a user-defined data
type. A "record" is an instance of a record type.
Note that in many ways, this interface is too low-level for every-day
use. Most uses of records are better served by SRFI-9 records. *Note
SRFI-9 Records::.
-- 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.
6.7.10 Structures
-----------------
A "structure" is a first class data type which holds Scheme values or C
words in fields numbered 0 upwards. A "vtable" is a structure that
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::). Usually,
when you need to represent structured data, you just want to use
records. But sometimes you need to implement new kinds of structured
data abstractions, and for that purpose structures are useful. Indeed,
records in Guile are implemented with structures.
6.7.10.1 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.
Here are some examples. *Note Tail Arrays::, for information on
the legacy tail array facility.
(make-vtable "pw") ;; one writable field
(make-vtable "prpw") ;; one read-only and one writable
(make-vtable "pwuwuw") ;; one scheme and two uninterpreted
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 ‘#’ 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)
(format port "#<~a and ~a>"
(struct-ref struct 0)
(struct-ref struct 1))))
6.7.10.2 Structure Basics
.........................
This section describes the basic procedures for working with structures.
‘make-struct’ creates a structure, and ‘struct-ref’ and ‘struct-set!’
access its fields.
-- Scheme Procedure: make-struct vtable tail-size init …
-- Scheme Procedure: make-struct/no-tail vtable init …
Create a new structure, with layout per the given VTABLE (*note
Vtables::).
The optional INIT… arguments are initial values for the fields of
the structure. 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’).
Structures also have the ability to allocate a variable number of
additional cells at the end, at their tails. However, this legacy
"tail array" facilty is confusing and inefficient, and so we do not
recommend it. *Note Tail Arrays::, for more on the legacy tail
array interface.
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"
-- C Function: SCM scm_make_struct (SCM vtable, SCM tail_size, SCM
init_list)
-- C Function: SCM scm_c_make_struct (SCM vtable, SCM tail_size, SCM
init, ...)
-- C Function: SCM scm_c_make_structv (SCM vtable, SCM tail_size,
size_t n_inits, scm_t_bits init[])
There are a few ways to make structures from C. ‘scm_make_struct’
takes a list, ‘scm_c_make_struct’ takes variable arguments
terminated with SCM_UNDEFINED, and ‘scm_c_make_structv’ takes a
packed array.
-- 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 that describes STRUCT.
The vtable is effectively the type of the structure. See *note
Vtable Contents::, for more on vtables.
6.7.10.3 Vtable Contents
........................
A vtable is itself a structure. It has a specific set of fields
describing various aspects of its "instances": the structures created
from a vtable. Some of the fields are internal to Guile, some of them
are part of the public interface, and there may be additional fields
added on by the user.
Every vtable has a field for the layout of their instances, a field
for the procedure used to print its instances, and a field for the name
of the vtable itself. Access to the layout and printer is exposed
directly via field indexes. Access to the vtable name is exposed via
accessor procedures.
-- 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 Meta-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-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) ⊣ #
6.7.10.4 Meta-Vtables
.....................
As a structure, a vtable also has a vtable, which is also a structure.
Structures, their vtables, the vtables of the vtables, and so on form a
tree of structures. Making a new structure adds a leaf to the tree, and
if that structure is a vtable, it may be used to create other leaves.
If you traverse up the tree of vtables, via calling ‘struct-vtable’,
eventually you reach a root which is the vtable of itself:
scheme@(guile-user)> (current-module)
$1 = #
scheme@(guile-user)> (struct-vtable $1)
$2 = #
scheme@(guile-user)> (struct-vtable $2)
$3 = #< 12c30a0>
scheme@(guile-user)> (struct-vtable $3)
$4 = #< 12c3fa0>
scheme@(guile-user)> (struct-vtable $4)
$5 = #< 12c3fa0>
scheme@(guile-user)>
$6 = #< 12c3fa0>
In this example, we can say that ‘$1’ is an instance of ‘$2’, ‘$2’ is
an instance of ‘$3’, ‘$3’ is an instance of ‘$4’, and ‘$4’, strangely
enough, is an instance of itself. The value bound to ‘$4’ in this
console session also bound to ‘’ in the default
environment.
-- Scheme Variable:
A meta-vtable, useful for making new vtables.
All of these values are structures. All but ‘$1’ are vtables. As
‘$2’ is an instance of ‘$3’, and ‘$3’ is a vtable, we can say that ‘$3’
is a "meta-vtable": a vtable that can create vtables.
With this definition, we can specify more precisely what a vtable is:
a vtable is a structure made from a meta-vtable. Making a structure
from a meta-vtable runs some special checks to ensure that the first
field of the structure is a valid layout. Additionally, if these checks
see that the layout of the child vtable contains all the required fields
of a vtable, in the correct order, then the child vtable will also be a
meta-table, inheriting a magical bit from the parent.
-- Scheme Procedure: struct-vtable? obj
-- C Function: scm_struct_vtable_p (obj)
Return ‘#t’ if OBJ is a vtable structure: an instance of a
meta-vtable.
‘’ is a root of the vtable tree. (Normally there is
only one root in a given Guile process, but due to some legacy
interfaces there may be more than one.)
The set of required fields of a vtable is the set of fields in the
‘’, and is bound to ‘standard-vtable-fields’ in the
default environment. It is possible to create a meta-vtable that with
additional fields in its layout, which can be used to create vtables
with additional data:
scheme@(guile-user)> (struct-ref $3 vtable-index-layout)
$6 = pruhsruhpwphuhuhprprpw
scheme@(guile-user)> (struct-ref $4 vtable-index-layout)
$7 = pruhsruhpwphuhuh
scheme@(guile-user)> standard-vtable-fields
$8 = "pruhsruhpwphuhuh"
scheme@(guile-user)> (struct-ref $2 vtable-offset-user)
$9 = module
In this continuation of our earlier example, ‘$2’ is a vtable that
has extra fields, because its vtable, ‘$3’, was made from a meta-vtable
with an extended layout. ‘vtable-offset-user’ is a convenient
definition that indicates the number of fields in
‘standard-vtable-fields’.
-- Scheme Variable: standard-vtable-fields
A string containing the orderedq set of fields that a vtable must
have.
-- Scheme Variable: vtable-offset-user
The first index in a vtable that is available for a user.
-- 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.
With these definitions, one can define ‘make-vtable’ in this way:
(define* (make-vtable fields #:optional printer)
(make-struct/no-tail
(make-struct-layout fields)
printer))
6.7.10.5 Vtable Example
.......................
Let us bring these points together with an example. Consider a simple
object system with single inheritance. Objects will be normal
structures, and classes will be vtables with three extra class fields:
the name of the class, the parent class, and the list of fields.
So, first we need a meta-vtable that allocates instances with these
extra class fields.
(define
(make-vtable
(string-append standard-vtable-fields "pwpwpw")
(lambda (x port)
(format port "< ~a>" (class-name x)))))
(define (class? x)
(and (struct? x)
(eq? (struct-vtable x) )))
To make a structure with a specific meta-vtable, we will use
‘make-struct/no-tail’, passing it the computed instance layout and
printer, as with ‘make-vtable’, and additionally the extra three class
fields.
(define (make-class name parent fields)
(let* ((fields (compute-fields parent fields))
(layout (compute-layout fields)))
(make-struct/no-tail
layout
(lambda (x port)
(print-instance x port))
name
parent
fields)))
Instances will store their associated data in slots in the structure:
as many slots as there are fields. The ‘compute-layout’ procedure below
can compute a layout, and ‘field-index’ returns the slot corresponding
to a field.
(define-syntax-rule (define-accessor name n)
(define (name obj)
(struct-ref obj n)))
;; Accessors for classes
(define-accessor class-name (+ vtable-offset-user 0))
(define-accessor class-parent (+ vtable-offset-user 1))
(define-accessor class-fields (+ vtable-offset-user 2))
(define (compute-fields parent fields)
(if parent
(append (class-fields parent) fields)
fields))
(define (compute-layout fields)
(make-struct-layout
(string-concatenate (make-list (length fields) "pw"))))
(define (field-index class field)
(list-index (class-fields class) field))
(define (print-instance x port)
(format port "<~a" (class-name (struct-vtable x)))
(for-each (lambda (field idx)
(format port " ~a: ~a" field (struct-ref x idx)))
(class-fields (struct-vtable x))
(iota (length (class-fields (struct-vtable x)))))
(format port ">"))
So, at this point we can actually make a few classes:
(define-syntax-rule (define-class name parent field ...)
(define name (make-class 'name parent '(field ...))))
(define-class #f
width height)
(define-class
x y)
And finally, make an instance:
(make-struct/no-tail 400 300 10 20)
⇒ < width: 400 height: 300 x: 10 y: 20>
And that’s that. Note that there are many possible optimizations and
feature enhancements that can be made to this object system, and the
included GOOPS system does make most of them. For more simple use
cases, the records facility is usually sufficient. But sometimes you
need to make new kinds of data abstractions, and for that purpose,
structs are here.
6.7.10.6 Tail Arrays
....................
Guile’s structures have a facility whereby each instance of a vtable can
contain a variable-length tail array of values. The length of the tail
array is stored in the structure. This facility was originally intended
to allow C code to expose raw C structures with word-sized tail arrays
to Scheme.
However, the tail array facility is confusing and doesn’t work very
well. It is very rarely used, but it insinuates itself into all
invocations of ‘make-struct’. For this reason the clumsily-named
‘make-struct/no-tail’ procedure can actually be more elegant in actual
use, because it doesn’t have a random ‘0’ argument stuck in the middle.
Tail arrays also inhibit optimization by allowing instances to affect
their shapes. In the absence of tail arrays, all instances of a given
vtable have the same number and kinds of fields. This uniformity can be
exploited by the runtime and the optimizer. The presence of tail arrays
make some of these optimizations more difficult.
Finally, the tail array facility is ad-hoc and does not compose with
the rest of Guile. If a Guile user wants an array with user-specified
length, it’s best to use a vector. It is more clear in the code, and
the standard optimization techniques will do a good job with it.
That said, we should mention some details about the interface. A
vtable that has tail array has upper-case permission descriptors: ‘W’,
‘R’ or ‘O’, correspoding to tail arrays of writable, read-only, or
opaque elements. A tail array permission descriptor may only appear in
the last element of a vtable layout.
For exampple, ‘pW’ indicates a tail of writable Scheme-valued fields.
The ‘pW’ field itself holds the tail size, and the tail fields come
after it.
(define v (make-vtable "prpW")) ;; one fixed then a tail array
(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
6.7.11 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.
6.7.12 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.
6.7.12.1 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.
6.7.12.2 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.
6.7.12.3 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.
6.7.12.4 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.
6.7.12.5 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.
6.7.12.6 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"))
6.7.13 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’.
6.7.14 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.
6.7.14.1 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
⇒
#
;; 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)
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
Interesting results can be computed by using ‘hash-fold’ to work
through each element. This example will count the total number of
elements:
(hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
⇒
3
The same thing can be done with the procedure ‘hash-count’, which can
also count the number of elements matching a particular predicate. For
example, count the number of elements with string values:
(hash-count (lambda (key value) (string? value)) h)
⇒
2
Counting all the elements is a simple task using ‘const’:
(hash-count (const #t) h)
⇒
3
6.7.14.2 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.
When the number of entries in a 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.
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 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)
-- Scheme Procedure: hash-count pred table
-- C Function: scm_hash_count (pred, table)
Return the number of elements in the given hash TABLE that cause
‘(PRED KEY VALUE)’ to return true. To quickly determine the total
number of elements, use ‘(const #t)’ for PRED.
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 ‘#’.
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 obj2))
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 if EXP is a smob instance of the type indicated by TAG,
or false otherwise. The expression EXP can be evaluated more than
once, so it shouldn’t contain any side effects.
-- C Function: SCM scm_new_smob (scm_t_bits tag, void *data)
-- C Function: SCM scm_new_double_smob (scm_t_bits tag, void *data,
void *data2, void *data3)
Make a new smob 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.
-- 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’ if OBJ is a compiled procedure, or ‘#f’ otherwise.
-- 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.
-- Scheme Procedure: program-arguments-alist program [ip]
Return an association list describing the arguments that PROGRAM
accepts, or ‘#f’ if the information cannot be obtained.
The alist keys that are currently defined are ‘required’,
‘optional’, ‘keyword’, ‘allow-other-keys?’, and ‘rest’. For
example:
(program-arguments-alist
(lambda* (a b #:optional c #:key (d 1) #:rest e)
#t)) ⇒
((required . (a b))
(optional . (c))
(keyword . ((#:d . 4)))
(allow-other-keys? . #f)
(rest . d))
-- Scheme Procedure: program-lambda-list program [ip]
Return a representation of the arguments of PROGRAM as a lambda
list, or ‘#f’ if this information is not available.
For example:
(program-lambda-alist
(lambda* (a b #:optional c #:key (d 1) #:rest e)
#t)) ⇒
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])
body1 body2 …
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 …) body1 body2 …
-- library syntax: let-optional* rest-arg (binding …) body1 body2 …
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 BODY1 BODY2 … 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 …)
body1 body2 …
-- library syntax: let-keywords* args allow-other-keys? (binding …)
body1 body2 …
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 BODY1 BODY2 … 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 body1 body2 …
Like a mix of ‘define*’ and ‘define-public’.
-- library syntax: defmacro* name formals body1 body2 …
-- library syntax: defmacro*-public name formals body1 body2 …
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 *)
--> ( *)
--> (*)
| (* . )
|
Rest lists can be useful with ‘case-lambda’:
(define plus
(case-lambda
"Return the sum of all arguments."
(() 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*’ as well, but they
do not contribute to the “matching” behavior, and their interactions
with required, optional, and rest arguments can be surprising.
For the purposes of ‘case-lambda*’ (and of ‘case-lambda’, as a
special case), a clause "matches" if it has enough required arguments,
and not too many positional arguments. The required arguments are any
arguments before the ‘#:optional’, ‘#:key’, and ‘#:rest’ arguments.
"Positional" arguments are the required arguments, together with the
optional arguments.
In the absence of ‘#:key’ or ‘#:rest’ arguments, it’s easy to see how
there could be too many positional arguments: you pass 5 arguments to a
function that only takes 4 arguments, including optional arguments. If
there is a ‘#:rest’ argument, there can never be too many positional
arguments: any application with enough required arguments for a clause
will match that clause, even if there are also ‘#:key’ arguments.
Otherwise, for applications to a clause with ‘#:key’ arguments (and
without a ‘#:rest’ argument), a clause will match there only if there
are enough required arguments and if the next argument after binding
required and optional arguments, if any, is a keyword. For efficiency
reasons, Guile is currently unable to include keyword arguments in the
matching algorithm. Clauses match on positional arguments only, not by
comparing a given keyword to the available set of keyword arguments that
a function has.
Some examples follow.
(define f
(case-lambda*
((a #:optional b) 'clause-1)
((a #:optional b #:key c) 'clause-2)
((a #:key d) 'clause-3)
((#:key e #:rest f) 'clause-4)))
(f) ⇒ clause-4
(f 1) ⇒ clause-1
(f) ⇒ clause-4
(f #:e 10) clause-1
(f 1 #:foo) clause-1
(f 1 #:c 2) clause-2
(f #:a #:b #:c #:d #:e) clause-4
;; clause-2 will match anything that clause-3 would match.
(f 1 #:d 2) ⇒ error: bad keyword args in clause 2
Don’t forget that the clauses are matched in order, and the first
matching clause will be taken. This can result in a keyword being bound
to a required argument, as in the case of ‘f #:e 10’.
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 proc1 proc2 …
Compose PROC1 with the procedures PROC2 … such that the last PROC
argument is applied first and PROC1 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.
-- Scheme Procedure: and=> value proc
When VALUE is ‘#f’, return ‘#f’. Otherwise, return ‘(PROC VALUE)’.
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.
-- 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—a procedure that does not accept
arguments.
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 …) body1 body2 …
Define NAME as a procedure with parameters PARAMETERs and bodies
BODY1, BODY2, ....
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) …) exp1 exp2 …
Bind each KEYWORD to its corresponding TRANSFORMER while expanding
EXP1 EXP2 ....
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) …) exp1 exp2 …
Bind each KEYWORD to its corresponding TRANSFORMER while expanding
EXP1 EXP2 ....
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)
⇒ bar
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))
⇒ #
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))
⇒ #
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’ if SYNTAX-OBJECT is an identifier, or ‘#f’ otherwise.
;; 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 .
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’ if the syntax objects A and B refer to the same
lexically-bound identifier, or ‘#f’ otherwise.
-- Scheme Procedure: free-identifier=? a b
Return ‘#t’ if the syntax objects A and B refer to the same free
identifier, or ‘#f’ otherwise.
-- 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 #)
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))
;; if 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’ if OBJ is a syntax transformer, or ‘#f’ otherwise.
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’ if ITEMS is a list or vector such that, for each
element X and the next element Y of ITEMS, ‘(LESS Y X)’ returns
‘#f’. Otherwise return ‘#f’.
-- 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
⇒ #
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 arg …
-- C Function: scm_run_hook (hook, args)
Apply all procedures from the hook HOOK to the arguments ARG ....
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))) ;; Illegal access
(* a b))
;; The behavior of the expression above is unspecified
(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 expr …
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’. If 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, and the CLAUSEs must have the form
((DATUM1 …) EXPR1 EXPR2 …)
or
((DATUM1 …) => EXPRESSION)
and the last CLAUSE may have the form
(else EXPR1 EXPR2 …)
or
(else => EXPRESSION)
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.
For the ‘=>’ clause types, EXPRESSION is evaluated and the
resulting procedure is applied to the value of KEY. The result of
this procedure application is then the result of the
‘case’-expression.
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 val1 val2 …
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.
One example where this optimization matters is "escape
continuations". Escape continuations are delimited continuations whose
only use is to make a non-local exit—i.e., to escape from the current
continuation. Such continuations are invoked only once, and for this
reason they are sometimes called "one-shot continuations". A common use
of escape continuations is when throwing an exception (*note
Exceptions::).
The constructs below are syntactic sugar atop prompts to simplify the
use of escape continuations.
-- Scheme Procedure: call-with-escape-continuation proc
-- Scheme Procedure: call/ec proc
Call PROC with an escape continuation.
In the example below, the RETURN continuation is used to escape the
continuation of the call to ‘fold’.
(use-modules (ice-9 control)
(srfi srfi-1))
(define (prefix x lst)
;; Return all the elements before the first occurrence
;; of X in LST.
(call/ec
(lambda (return)
(fold (lambda (element prefix)
(if (equal? element x)
(return (reverse prefix)) ; escape `fold'
(cons element prefix)))
'()
lst))))
(prefix 'a '(0 1 2 a 3 4 5))
⇒ (0 1 2)
-- Scheme Syntax: let-escape-continuation k body …
-- Scheme Syntax: let/ec k body …
Bind K within BODY to an escape continuation.
This is equivalent to ‘(call/ec (lambda (K) BODY …))’.
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 val1 val2 …
Abort to the default prompt tag, passing VAL1 VAL2 … 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 body1 body2 …
Establish a prompt, and evaluate BODY1 BODY2 … within that prompt.
The prompt handler is designed to work with ‘shift’, described
below.
-- Scheme Syntax: shift cont body1 body2 …
Abort to the nearest ‘reset’, and evaluate BODY1 BODY2 … in a
context in which the captured continuation is bound to CONT.
As mentioned above, taken together, the BODY1 BODY2 … expressions
and the invocations of CONT implicitly establish a prompt.
Interested readers are invited to explore Oleg Kiselyov’s wonderful
web site at , 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 arg …
-- 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 scm_c_values (SCM *base, size_t n)
‘scm_c_values’ is an alternative to ‘scm_values’. It creates a new
values object, and copies into it the N values starting from BASE.
Currently this creates a list and passes it to ‘scm_values’, but we
expect that in the future we will be able to use more a efficient
representation.
-- C Function: size_t scm_c_nvalues (SCM obj)
If OBJ is a multiple-values object, returns the number of values it
contains. Otherwise returns 1.
-- C Function: SCM scm_c_value_ref (SCM obj, size_t idx)
Returns the value at the position specified by IDX in OBJ. Note
that OBJ 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.
The exception mechanism described in this section has connections
with "delimited continuations" (*note Prompts::). In particular,
throwing an exception is akin to invoking an "escape continuation"
(*note ‘call/ec’: Prompt 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 arg …
-- C Function: scm_throw (key, args)
Invoke the catch form matching KEY, passing ARG … 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)
:3:1: In procedure gsubr-apply …
: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 arg …
Raise an error with key ‘misc-error’ and a message constructed by
displaying MSG and writing ARG ....
-- 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)
-- C Function: void scm_misc_error (const char *SUBR, const char
*MESSAGE, SCM ARGS)
Throw an error with the various keys described above.
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 (port)
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.
-- Scheme Variable: %default-port-conversion-strategy
The fluid that defines the conversion strategy for newly created
ports, and for other conversion routines such as ‘scm_to_stringn’,
‘scm_from_stringn’, ‘string->pointer’, and ‘pointer->string’.
Its value must be one of the symbols described above, with the same
semantics: ‘'error’, ‘'substitute’, or ‘'escape’.
When Guile starts, its value is ‘'substitute’.
Note that ‘(set-port-conversion-strategy! #f SYM)’ is equivalent to
‘(fluid-set! %default-port-conversion-strategy SYM)’.
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 character COBJ 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])
In the past, Guile did not have a procedure that would just read out
all of the characters from a port. As a workaround, many people just
called ‘read-delimited’ with no delimiters, knowing that would produce
the behavior they wanted. This prompted Guile developers to add some
routines that would read all characters from a port. So it is that
‘(ice-9 rdelim)’ is also the home for procedures that can reading
undelimited text:
-- Scheme Procedure: read-string [port] [count]
Read all of the characters out of PORT and return them as a string.
If the COUNT is present, treat it as a limit to the number of
characters to read.
By default, read from the current input port, with no size limit on
the result. This procedure always returns a string, even if no
characters were read.
-- Scheme Procedure: read-string! buf [port] [start] [end]
Fill BUF with characters read from PORT, defaulting to the current
input port. Return the number of characters read.
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).
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
‘(# . #)’.
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 .
-- 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 [#:guess-encoding=#f]
[#:encoding=#f]
-- C Function: scm_open_file_with_encoding (filename, mode,
guess_encoding, encoding)
-- 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 the default
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.
Unless binary mode is requested, the character encoding of the new
port is determined as follows: First, if GUESS-ENCODING is true,
the ‘file-encoding’ procedure is used to guess the encoding of the
file (*note Character Encoding of Source Files::). If
GUESS-ENCODING is false or if ‘file-encoding’ fails, ENCODING is
used unless it is also false. As a last resort, the default port
encoding is used. *Note Ports::, for more information on port
encodings. It is an error to pass a non-false GUESS-ENCODING or
ENCODING if binary mode is requested.
If a file cannot be opened with the access requested, ‘open-file’
throws an exception.
When the file is opened, its encoding is set to the current
‘%default-port-encoding’, unless the ‘b’ flag was supplied.
Sometimes it is desirable to honor Emacs-style coding declarations
in files(1). When that is the case, the ‘file-encoding’ procedure
can be used as follows (*note ‘file-encoding’: Character Encoding
of Source Files.):
(let* ((port (open-input-file file))
(encoding (file-encoding port)))
(set-port-encoding! port (or encoding (port-encoding port))))
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 [#:guess-encoding=#f]
[#:encoding=#f] [#:binary=#f]
Open FILENAME for input. If BINARY is true, open the port in
binary mode, otherwise use text mode. ENCODING and GUESS-ENCODING
determine the character encoding as described above for
‘open-file’. Equivalent to
(open-file FILENAME
(if BINARY "rb" "r")
#:guess-encoding GUESS-ENCODING
#:encoding ENCODING)
-- Scheme Procedure: open-output-file filename [#:encoding=#f]
[#:binary=#f]
Open FILENAME for output. If BINARY is true, open the port in
binary mode, otherwise use text mode. ENCODING specifies the
character encoding as described above for ‘open-file’. Equivalent
to
(open-file FILENAME
(if BINARY "wb" "w")
#:encoding ENCODING)
-- Scheme Procedure: call-with-input-file filename proc
[#:guess-encoding=#f] [#:encoding=#f] [#:binary=#f]
-- Scheme Procedure: call-with-output-file filename proc
[#:encoding=#f] [#:binary=#f]
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
[#:guess-encoding=#f] [#:encoding=#f] [#:binary=#f]
-- Scheme Procedure: with-output-to-file filename thunk [#:encoding=#f]
[#:binary=#f]
-- Scheme Procedure: with-error-to-file filename thunk [#:encoding=#f]
[#:binary=#f]
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.
---------- Footnotes ----------
(1) Guile 2.0.0 to 2.0.7 would do this by default. This behavior was
deemed inappropriate and disabled starting from Guile 2.0.8.
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) ⇒ #
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, plus one non-standard
procedure ‘unget-bytevector’ (*note R6RS Binary Input::), 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-MODE-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 binary-port transcoder
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? input-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 bytes are available or
an end-of-file is reached. Return either the end-of-file object or
a new bytevector containing some of the available bytes (at least
one), and update the port position to point just past these bytes.
-- 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).
The ‘(ice-9 binary-ports)’ module provides the following procedure as
an extension to ‘(rnrs io ports)’:
-- Scheme Procedure: unget-bytevector port bv [start [count]]
-- C Function: scm_unget_bytevector (port, bv, start, count)
Place the contents of BV in PORT, optionally starting at index
START and limiting to COUNT octets, so that its bytes will be read
from left-to-right as the next bytes from PORT during subsequent
read operations. If called multiple times, the unread bytes will
be read again in last-in first-out order.
---------- 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 textual-input-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 textual-input-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 textual-input-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! textual-input-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 textual-input-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 textual-input-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 textual-input-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 textual-output-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:
‘#’. 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))
6.14.12 Handling of Unicode byte order marks.
---------------------------------------------
This section documents the finer points of Guile’s handling of Unicode
byte order marks (BOMs). A byte order mark (U+FEFF) is typically found
at the start of a UTF-16 or UTF-32 stream, to allow readers to reliably
determine the byte order. Occasionally, a BOM is found at the start of
a UTF-8 stream, but this is much less common and not generally
recommended.
Guile attempts to handle BOMs automatically, and in accordance with
the recommendations of the Unicode Standard, when the port encoding is
set to ‘UTF-8’, ‘UTF-16’, or ‘UTF-32’. In brief, Guile automatically
writes a BOM at the start of a UTF-16 or UTF-32 stream, and
automatically consumes one from the start of a UTF-8, UTF-16, or UTF-32
stream.
As specified in the Unicode Standard, a BOM is only handled specially
at the start of a stream, and only if the port encoding is set to
‘UTF-8’, ‘UTF-16’ or ‘UTF-32’. If the port encoding is set to
‘UTF-16BE’, ‘UTF-16LE’, ‘UTF-32BE’, or ‘UTF-32LE’, then BOMs are _not_
handled specially, and none of the special handling described in this
section applies.
• To ensure that Guile will properly detect the byte order of a
UTF-16 or UTF-32 stream, you must perform a textual read before any
writes, seeks, or binary I/O. Guile will not attempt to read a BOM
unless a read is explicitly requested at the start of the stream.
• If a textual write is performed before the first read, then an
arbitrary byte order will be chosen. Currently, big endian is the
default on all platforms, but that may change in the future. If
you wish to explicitly control the byte order of an output stream,
set the port encoding to ‘UTF-16BE’, ‘UTF-16LE’, ‘UTF-32BE’, or
‘UTF-32LE’, and explicitly write a BOM (‘#\xFEFF’) if desired.
• If ‘set-port-encoding!’ is called in the middle of a stream, Guile
treats this as a new logical “start of stream” for purposes of BOM
handling, and will forget about any BOMs that had previously been
seen. Therefore, it may choose a different byte order than had
been used previously. This is intended to support multiple logical
text streams embedded within a larger binary stream.
• Binary I/O operations are not guaranteed to update Guile’s notion
of whether the port is at the “start of the stream”, nor are they
guaranteed to produce or consume BOMs.
• For ports that support seeking (e.g. normal files), the input and
output streams are considered linked: if the user reads first, then
a BOM will be consumed (if appropriate), but later writes will
_not_ produce a BOM. Similarly, if the user writes first, then
later reads will _not_ consume a BOM.
• For ports that do not support seeking (e.g. pipes, sockets, and
terminals), the input and output streams are considered
_independent_ for purposes of BOM handling: the first read will
consume a BOM (if appropriate), and the first write will _also_
produce a BOM (if appropriate). However, the input and output
streams will always use the same byte order.
• Seeks to the beginning of a file will set the “start of stream”
flags. Therefore, a subsequent textual read or write will consume
or produce a BOM. However, unlike ‘set-port-encoding!’, if a byte
order had already been chosen for the port, it will remain in
effect after a seek, and cannot be changed by the presence of a
BOM. Seeks anywhere other than the beginning of a file clear the
“start of stream” flags.
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.
Internally, patterns and input strings are converted to the current
locale’s encoding, and then passed to the C library’s regular expression
routines (*note (libc)Regular Expressions::). The returned match
structures always point to characters in the strings, not to individual
bytes, even in the case of multi-byte encodings.
-- 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)
It is also possible to disable (or enable) case sensitivity within a
single file by placing the reader directives ‘#!fold-case’ (or
‘#!no-fold-case’) within the file itself.
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 global 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.
curly-infix no Support SRFI-105 curly infix expressions.
Note that Guile also includes a preliminary mechanism for setting
read options on a per-port basis. For instance, the ‘case-insensitive’
read option is set (or unset) on the port when the reader encounters the
‘#!fold-case’ or ‘#!no-fold-case’ reader directives. Similarly, the
‘#!curly-infix’ reader directive sets the ‘curly-infix’ read option on
the port, and ‘#!curly-infix-and-bracket-lists’ sets ‘curly-infix’ and
unsets ‘square-brackets’ on the port (*note SRFI-105::). There is
currently no other way to access or set the per-port read options.
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_OR_STATE. While EXP is
evaluated (using ‘primitive-eval’), MODULE_OR_STATE 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 arg … 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 ARG … and the elements of the ARGLST list.
‘scm_apply’ takes parameters corresponding to a Scheme level
‘(lambda (proc arg1 . rest) ...)’. So ARG1 and all but the last
element of the REST list make up ARG …, and the last element of
REST is the ARGLST list. Or if REST is the empty list ‘SCM_EOL’
then there’s no ARG …, and (ARG1) 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’,
‘format’, ‘duplicate-case-datum’, and ‘bad-case-datum’.
‘-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
(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. When Guile starts up, ‘%load-path’ is initialized to
the default load path ‘(list (%library-dir) (%site-dir)
(%global-site-dir) (%package-data-dir))’. The ‘GUILE_LOAD_PATH’
environment variable can be used to prepend or append additional
directories (*note Environment Variables::).
*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.
If FILENAME is a relative pathname and is not found in the list of
search paths, one of three things may happen, depending on the
optional second argument, EXCEPTION-ON-NOT-FOUND. If it is ‘#f’,
‘#f’ will be returned. If it is a procedure, it will be called
with no arguments. (This allows a distinction to be made between
exceptions raised by loading a file, and exceptions related to the
loader itself.) Otherwise an error is signalled.
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. The ‘GUILE_LOAD_COMPILED_PATH’ environment
variable can be used to prepend or append additional directories
(*note Environment Variables::).
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: parse-path-with-ellipsis path base
-- C Function: scm_parse_path_with_ellipsis (path, base)
Parse PATH, which is expected to be a colon-separated string, into
a list and return the resulting list with BASE (a list) spliced in
place of the ‘...’ path component, if present, or else BASE is
added to the end. If PATH is ‘#f’, BASE 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 or UTF-8, but the
built-in reader can interpret other character encodings as well. When
Guile loads Scheme source code, it uses the ‘file-encoding’ procedure
(described below) to try to guess the encoding of the file. In the
absence of any hints, UTF-8 is assumed. One way to provide a hint about
the encoding of a source file is to place a coding declaration in the
top 500 characters of the file.
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 the first block ‘#!’
comment in the file.
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.
Alternatively, one can use the ‘#:guess-encoding’ keyword argument of
‘open-file’ and related procedures. *Note File Ports::.
-- Scheme Procedure: file-encoding port
-- C Function: scm_file_encoding (port)
Attempt to scan the first few hundred bytes from the PORT for hints
about its character encoding. Return a string containing the
encoding name or ‘#f’ if the encoding cannot be determined. The
port is rewound.
Currently, the only supported method is to look for an Emacs-like
character coding declaration (*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.
Additional heuristics may be added in the future.
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. Also *note
SRFI-45::.
-- 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 FILE-NAME is a relative path, it is searched for relative to the
path that contains the file that the ‘include’ for appears in.
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.
6.17.12 REPL Servers
--------------------
The procedures in this section are provided by
(use-modules (system repl server))
When an application is written in Guile, it is often convenient to
allow the user to be able to interact with it by evaluating Scheme
expressions in a REPL.
The procedures of this module allow you to spawn a "REPL server",
which permits interaction over a local or TCP connection. Guile itself
uses them internally to implement the ‘--listen’ switch, *note
Command-line Options::.
-- Scheme Procedure: make-tcp-server-socket [#:host=#f] [#:addr]
[#:port=37146]
Return a stream socket bound to a given address ADDR and port
number PORT. If the HOST is given, and ADDR is not, then the HOST
string is converted to an address. If neither is given, we use the
loopback address.
-- Scheme Procedure: make-unix-domain-server-socket
[#:path="/tmp/guile-socket"]
Return a UNIX domain socket, bound to a given PATH.
-- Scheme Procedure: run-server [server-socket]
-- Scheme Procedure: spawn-server [server-socket]
Create and run a REPL, making it available over the given
SERVER-SOCKET. If SERVER-SOCKET is not provided, it defaults to
the socket created by calling ‘make-tcp-server-socket’ with no
arguments.
‘run-server’ runs the server in the current thread, whereas
‘spawn-server’ runs the server in a new thread.
-- Scheme Procedure: stop-server-and-clients!
Closes the connection on all running server sockets.
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 elem …
-- 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 Pipes::).
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 option …
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.
OPTION … are keyword/value pairs which specify more about the
defined module. The recognized options and their meaning are 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’ if VAR is bound to a value, or ‘#f’ otherwise. 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’ if 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='()]
[#:prefix=#f] [#:renamer=#f] [#: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.
static SCM eval_string_var;
/* NOTE: It is important that the call to 'my_init'
happens-before all calls to 'my_eval_string'. */
void my_init (void)
{
eval_string_var = scm_c_public_lookup ("ice-9 eval-string",
"eval-string");
}
SCM my_eval_string (SCM str)
{
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_NAME.
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.
In some rare cases, you may need to access the variable that
‘scm_module_define’ would have accessed, without changing the binding of
the existing variable, if one is present. In that case, use
‘scm_module_ensure_local_variable’:
-- C Function: SCM scm_module_ensure_local_variable (SCM MODULE, SCM
SYM)
Like ‘scm_module_define’, but if the SYM is already locally bound
in that module, the variable’s existing binding is not reset.
Returns a variable.
-- 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 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.
6.19.11 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-unlink libGL-obj)
libGL-obj
⇒ #
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)
‘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
#include
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 #
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 #
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 vers