Guile Reference Manual

This reference manual documents Guile, GNU's Ubiquitous Intelligent Language for Extensions. This is edition 1.1 corresponding to Guile 1.8.7.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with the no Invariant Sections, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Text “You are free to copy and modify this GNU Manual.”. A copy of the license is included in the section entitled “GNU Free Documentation License”.

1 Preface

This reference manual documents Guile, GNU's Ubiquitous Intelligent Language for Extensions. It describes how to use Guile in many useful and interesting ways.

This is edition 1.1 of the reference manual, and corresponds to Guile version 1.8.7.

1.1 Layout of this Manual

1.2 Conventions used in this Manual

1.3 Contributors to this Manual

The Guile reference and tutorial manuals were written and edited largely 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 contributed by Gary Houston (contributions to POSIX system calls and networking, expect, I/O internals and extensions, slib installation, error handling) and Tim Pierce (sections on script interpreter triggers, alists, function tracing).

Tom Lord contributed a great deal of material with early Guile snapshots; although most of this text has been rewritten, all of it was important, and some of the structure remains.

Aubrey Jaffer wrote the SCM Scheme implementation and manual upon which the Guile program and manual are based. Some portions of the SCM and SLIB manuals have been included here verbatim.

Since Guile 1.4, Neil Jerram has been maintaining and improving the reference manual. Among other contributions, he wrote the Basic Ideas chapter, developed the tools for keeping the manual in sync with snarfed libguile docstrings, and reorganized the structure so as to accommodate docstrings for all Guile's primitives.

Martin Grabmueller has made substantial contributions throughout the reference 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.

1.4 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.

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 resticted 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.

2 Introduction to Guile

2.1 What is Guile?

Guile is an interpreter for the Scheme programming language, packaged for use in a wide variety of environments. 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.

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. However, Guile is also packaged as an object library, allowing 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.

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.

In the future, we hope to expand Guile to support other languages like Tcl and Perl by translating them to Scheme code. 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.

2.2 Obtaining and Installing Guile

Guile can be obtained from the main GNU archive site ftp://ftp.gnu.org or any of its mirrors. The file will be named guile-version.tar.gz. The current version is 1.8.7, so the file you should grab is:

which will create a directory called guile-1.8.7 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

This will install the Guile executable guile, the Guile library -lguile and various associated header files and support libraries. It will also install the Guile tutorial and 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 Introduction. This will also be installed in your info directory.

2.3 A Whirlwind Tour

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.

2.3.1 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 guile> prompts:

2.3.2 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.

2.3.3 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.

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

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

2.3.4 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 extensions, 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.

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

For creating shared libraries portably, we recommend the use of GNU Libtool (see Introduction).

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

2.3.5 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.

(Although the module system implementation is in flux, feel free to use it anyway. Guile will provide reasonable backwards compatibility.)

Details on the module system beyond this introductory material can be found in See Modules.

2.3.5.1 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.

2.3.5.2 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.

2.3.5.3 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.

There is also a way to manipulate the module system from C but only Scheme files can be autoloaded. Thus, we recommend that you define your modules in Scheme.

2.4 Discouraged and Deprecated

From time to time functions and other features of Guile become obsolete. Guile has some mechanisms in place that can help you cope with this.

Guile has two levels of obsoleteness: things can be deprecated, meaning that their use is considered harmful and should be avoided, even in old code; or they can be merely discouraged, meaning that they are fine in and of themselves, but that there are better alternatives that should be used in new code.

When you use a feature that is deprecated, you will likely get a warning message at run-time. Also, deprecated features are not ready for production use: they might be very slow. When something is merely discouraged, it performs normally and you wont get any messages at run-time.

The primary source for information about just what things are discouraged or 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 and/or discouraged features from the public API of Guile, and how to control the warning messages for deprecated features.

The idea behind those mechanisms is that normally all deprecated and discouraged features are available, but that you can omit them on purpose to check whether your code still relies on them.

2.5 Reporting Bugs

Whenever you have found a bug in Guile you are encouraged to 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.

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.

3 Programming in Scheme

Guile's core language is Scheme, and an awful lot can be achieved simply by using Guile to write and run Scheme programs. 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 readers who are not yet familiar with the Scheme language, this part includes a chapter that presents the basic concepts of the language, and gives references to freely available Scheme tutorial material on the web.

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

3.1 Basic Ideas in 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 reference chapters that follow.

The style of this introductory material aims about halfway between the terse precision of R5RS and the discursive randomness of a Scheme tutorial.

3.1.1 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.

3.1.1.1 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.

3.1.1.2 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.

3.1.1.3 Defining and Setting Variables

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

(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:

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.

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 (see About Expressions), you can probably guess what the following set! example does...

(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.

3.1.2 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.

3.1.2.1 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.

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:

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.

3.1.2.2 Simple Procedure Invocation

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

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

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

(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:

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.)

3.1.2.3 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.

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:

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:

This is a valid procedure invocation expression, and its result is the string "Name=FSF:Address=Cambridge".

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.

3.1.2.4 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.

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

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:

(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.)

3.1.3 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:

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.

3.1.3.1 Evaluating Expressions and Executing Programs

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

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.

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

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.

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 (see Creating a Procedure) or implicitly using the shorthand form of define (see Lambda Alternatives).

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.

the value of the expression key is the string value

"Paul
Evans"

. If key is then modified by

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

Evaluating a Procedure Invocation Expression

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

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

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.

In the outermost expression, procedure is string-length and arg1 is (string-append "/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.

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 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 See Syntax Summary.

3.1.3.2 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,

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 (see SRFI-1 Selectors) returning the last element of a list,

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.

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.

3.1.3.3 Using the Guile REPL

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

The REPL is a useful mechanism for exploring the evaluation behaviour described in the previous subsection. If you type string-append, for example, the REPL replies

#<primitive-procedure
string-append>

, illustrating the relationship between the variable string-append and the procedure value stored in that variable.

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

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

3.1.3.4 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.

define (see Top Level) is used to create a new variable and set its initial value.

let, let* and letrec (see 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 See About Closure.

begin (see 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 (see if cond case) provide conditional evaluation of argument expressions depending on whether one or more conditions evaluate to “true” or “false”.

case (see if cond case) provides conditional evaluation of argument expressions depending on whether a variable has one of a specified group of values.

and (see and or) executes a sequence of expressions in order until either there are no expressions left, or one of them evaluates to “false”.

or (see and or) executes a sequence of expressions in order until either there are no expressions left, or one of them evaluates to “true”.

3.1.4 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.

3.1.4.1 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

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.

3.1.4.2 Local Variables and Environments

We have seen how to create top level variables using the define syntax (see 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 (see 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.

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.

3.1.4.3 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:

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.

3.1.4.4 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.

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.

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.

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.

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.

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:

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.

3.1.4.5 Closure

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.

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.

3.1.4.6 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.

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.

3.1.4.7 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.

An important detail here is that the get-balance and deposit variables must be set up by defineing 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.

3.1.4.8 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:

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.

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.

3.1.4.9 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.

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

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

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.

3.2 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. The current latest revision of RnRS is version 5 (see R5RS), and Guile 1.4 is fully compliant with the Scheme specification in this revision.

But Guile, like most Scheme implementations, also goes beyond R5RS in many ways, because R5RS does not give specifications (or even recommendations) regarding many issues that are important in practical programming. Some of the areas where Guile extends R5RS are:

3.3 Guile Scripting

3.3.1 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:

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

3.3.2 Invoking Guile

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 Scripting Examples.

3.3.3 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:

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.

When Guile sees the meta switch \, it parses command-line argument from the script file according to the following rules:

3.3.4 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.

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

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:

(Note the use of the meta switch \ so that the script invocation can include more than one Guile option: See 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:

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, See getopt-long.

3.3.5 Scripting Examples

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:

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:

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.

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:

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:

3.4 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.

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.

3.4.1 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.

It's a good idea to put these two lines (without the “guile>” prompts) in your .guile file. Guile reads this file when it starts up interactively, so anything in this file has the same effect as if you type it in by hand at the “guile>” prompt.

3.4.2 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, ..., and you can then use these variables in subsequent expressions.

To enable value history, type (use-modules (ice-9 history)) at the Guile prompt, or add this to your .guile file. (It is not enabled by default, to avoid the possibility of conflicting with some other use you may have for the variables $1, $2, ..., and also because it prevents the stored evaluation results from being garbage collected, which some people may not want.)

3.4.3 Error Handling

When code being evaluated from the REPL hits an error, Guile remembers the execution context where the error occurred and can give you three levels of information about what the error was and exactly where it occurred.

By default, Guile displays only the first level, which is the most immediate information about where and why the error occurred, for example:

However, as the message above says, you can obtain more information about the context of the error by typing (backtrace) or (debug).

(backtrace) displays the Scheme call stack at the point where the error occurred:

In a more complex scenario than this one, this can be extremely useful for understanding where and why the error occurred. You can make Guile show the backtrace automatically by adding

(debug-enable
'backtrace)

to your .guile.

(debug) takes you into Guile's interactive debugger, which provides commands that allow you to

3.4.4 Using the Interactive Debugger

Guile's interactive debugger is a command line application that accepts commands from you for examining the stack and, if stopped at a trap, for continuing program execution in various ways. Unlike in the normal Guile REPL, commands are typed mostly without parentheses.

When you first enter the debugger, it introduces itself with a message like this:

“debug>” is the debugger's prompt, and a reminder that you are not in the normal Guile REPL. In case you find yourself in the debugger by mistake, the quit command will return you to the REPL.

— Debugger Command: quit

Exit the debugger.

3.4.4.1 Display Backtrace

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

— Debugger Command: backtrace [count]
— Debugger Command: bt [count]
— Debugger Command: where [count]

Print backtrace of all stack frames, or of the innermost count frames. With a negative argument, print the outermost -count frames. If the number of frames isn't explicitly given, the debug option depth determines the maximum number of frames printed.

The format of the displayed backtrace is the same as for the display-backtrace procedure (see Examining the Stack).

3.4.4.2 Frame Selection

A call stack consists of a sequence of stack frames, with each frame describing one level of the nested evaluations and applications that the program was executing when it hit a breakpoint or an error. Frames are numbered such that frame 0 is the outermost — i.e. the operation on the call stack that began least recently — and frame N-1 the innermost (where N is the total number of frames on the stack).

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:

— Debugger Command: up [n]

Move n frames up the stack. For positive n, this advances toward the outermost frame, to lower frame numbers, to frames that have existed longer. n defaults to one.

— Debugger Command: down [n]

Move n frames down the stack. For positive n, this advances toward the innermost frame, to higher frame numbers, to frames that were created more recently. n defaults to one.

— Debugger Command: frame [n]

Select and print a stack frame. With no argument, print the selected stack frame. (See also “info frame”.) An argument specifies the frame to select; it must be a stack-frame number.

3.4.4.3 Frame Information

The following commands return detailed information about the currently selected frame.

— Debugger Command: info frame

Display a verbose description of the selected frame. The information that this command provides is equivalent to what can be deduced from the one line summary for the frame that appears in a backtrace, but is presented and explained more clearly.

— Debugger Command: info args

Display the argument variables of the current stack frame. Arguments can also be seen in the backtrace, but are presented more clearly by this command.

— Debugger Command: position

Display the name of the source file that the current expression comes from, and the line and column number of the expression's opening parenthesis within that file. This information is only available when the positions read option is enabled (see Reader options).

3.4.4.4 Frame Evaluation

The evaluate command is most useful for querying the value of a variable, either global or local, in the environment of the selected stack frame, but it can be used more generally to evaluate any expression.

— Debugger Command: evaluate expression

Evaluate an expression in the environment of the selected stack frame. The expression must appear on the same line as the command, however it may be continued over multiple lines.

3.4.4.5 Single Stepping and Continuing Execution

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.

— Debugger Command: step [n]

Tell the debugged program to do n more steps from its current position. One step means executing until the next frame entry or exit of any kind. n defaults to 1.

— Debugger Command: next [n]

Tell the debugged program to do n more steps from its current position, but only counting frame entries and exits where the corresponding source code comes from the same file as the current stack frame. (See Step Traps for the details of how this works.) If the current stack frame has no source code, the effect of this command is the same as of step. n defaults to 1.

— Debugger 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 command line debugger.

— Debugger Command: continue

Tell the program being debugged to continue running. (In fact this is the same as the quit command, because it exits the debugger command loop and so allows whatever code it was that invoked the debugger to continue.)

3.5 Using Guile in Emacs

There are several options for working on Guile Scheme code in Emacs. The simplest are to use Emacs's standard scheme-mode for editing code, and to run the interpreter when you need it by typing “guile” at the prompt of a *shell* buffer, but there are Emacs libraries available which add various bells and whistles to this. The following diagram shows these libraries and how they relate to each other, with the arrows indicating “builds on” or “extends”. For example, the Quack library builds on cmuscheme, which in turn builds on the standard scheme mode.

scheme, written by Bill Rozas and Dave Love, is Emacs's standard mode for Scheme code files. It provides Scheme-sensitive syntax highlighting, parenthesis matching, indentation and so on.

cmuscheme, written by Olin Shivers, provides a comint-based Scheme interaction buffer, so that you can run an interpreter more directly than with the *shell* buffer approach by typing M-x run-scheme. It also extends scheme-mode so that there are key presses for sending selected bits of code from a Scheme buffer to this interpreter. This means that when you are writing some code and want to check what an expression evaluates to, you can easily select that code and send it to the interpreter for evaluation, then switch to the interpreter to see what the result is. cmuscheme is included in the standard Emacs distribution.

Quack, written by Neil Van Dyke, adds a number of incremental improvements to the scheme/cmuscheme combination: convenient menu entries for looking up Scheme-related references (such as the SRFIs); enhanced indentation rules that are customized for particular Scheme interpreters, including Guile; an enhanced version of the run-scheme command that knows the names of the common Scheme interpreters and remembers which one you used last time; and so on. Quack is available from http://www.neilvandyke.org/quack.

GDS, written by Neil Jerram, also builds on the scheme/cmuscheme combination, but with a change to the way that Scheme code fragments are sent to the interpreter for evaluation. cmuscheme and Quack send code fragments to the interpreter's standard input, on the assumption that the interpreter is expecting to read Scheme expressions there, and then monitor the interpreter's standard output to infer what the result of the evaluation is. GDS doesn't use standard input and output like this. Instead, it sets up a socket connection between the Scheme interpreter and Emacs, and sends and receives messages using a simple protocol through this socket. The messages include requests to evaluate Scheme code, and responses conveying the results of an evaluation, thus providing similar function to cmuscheme or Quack. They also include requests for stack exploration and debugging, which go beyond what cmuscheme or Quack can do. The price of this extra power, however, is that GDS is Guile-specific. GDS requires the Scheme interpreter to run some GDS-specific library code; currently this code is written as a Guile module and uses features that are specific to Guile. GDS is now included in the Guile distribution; for previous Guile releases (1.8.4 and earlier) it can be obtained as part of the guile-debugging package from http://www.ossau.uklinux.net/guile.

Finally, xscheme is similar to cmuscheme — in that it starts up a Scheme interaction process and sends commands to that process's standard input — and to GDS — in that it has support beyond cmuscheme or Quack for exploring the Scheme stack when an error has occurred — but is implemented specifically for MIT/GNU Scheme. Hence it isn't really relevant to Guile work in Emacs, except as a reference for useful features that could be implemented in one of the other libraries mentioned here.

In summary, the best current choice for working on Guile code in Emacs is either Quack or GDS, depending on which of these libraries' features you find most important. For more information on Quack, please see the website referenced above. GDS is documented further in the rest of this section.

3.5.1 GDS Introduction

GDS aims to allow you to work on Guile Scheme code in the same kind of way that Emacs allows you to work on Emacs Lisp code: providing easy access to help, evaluating arbitrary fragments of code, a nice debugging interface, and so on. The thinking behind the GDS library is that you will usually be doing one of two things.

GDS can provide these facilities for any number of Guile Scheme programs (which we often refer to as “clients”) at once, and these programs can be started either independently of GDS, including outside Emacs, or specifically by GDS.

Communication between each Guile client program and GDS uses a TCP socket, which means that it is orthogonal to any other interfaces that the client program has. In particular GDS does not interfere with a program's standard input and output.

3.5.2 GDS Architecture

In order to understand the following documentation fully it will help to have a picture in mind of how GDS works, so we briefly describe that here. GDS consists of three components.

The data exchanged between client and server components, and between server and interface, is a sequence of sexps (parenthesised expressions) that are designed so as to be directly readable by both Scheme and Emacs Lisp. The use of a TCP connection means that the server and Emacs interface can theoretically be on a different computer from the client programs, but in practice there are currently two problems with this. Firstly the GDS API doesn't provide any way of specifying a non-local server to connect to, and secondly there is no security or authentication mechanism in the GDS protocol. These are issues that should be addressed in the future.

3.5.3 Getting Started with GDS

somewhere in your .emacs file. This will cause Emacs to load the GDS Emacs Lisp code when starting up, and to start the inferior GDS server process so that it is ready and waiting for any Guile programs that want to use GDS.

(If GDS's Scheme code is not installed in one of the locations in Guile's load path, you may find that the server process fails to start. When this happens you will see an error message from Emacs:

and the gds-debug buffer will contain a Scheme backtrace ending with the message:

The solution for this is to customize the Emacs variable gds-scheme-directory so that it specifies where the GDS Scheme code is installed. Then either restart Emacs or type M-x gds-run-debug-server to try starting the GDS server process again.)

For evaluations, help and completion from Scheme code buffers that you are working on, this is all you need. The first time you do any of these things, GDS will automatically start a new Guile client program as an Emacs subprocess. This Guile program does nothing but wait for and act on instructions from GDS, and we refer to it as a utility Guile client. Over time this utility client will accumulate the code that you ask it to evaluate, and you can also tell it to load complete files or modules by sending it load or use-modules expressions.

When you want to use GDS to work on an independent Guile application, you need to add something to that application's Scheme code to cause it to connect to and interact with GDS at the right times. The following subsections describe the ways of doing this.

3.5.3.1 Invoking GDS when an Exception Occurs

One option is to use GDS to catch and display any exceptions that are thrown by the application's code. If you already have a lazy-catch or with-throw-handler around the area of code that you want to monitor, you just need to add the following to the handler code:

where key and args are the first and rest arguments that Guile passes to the handler. (In other words, they assume the handler signature (lambda (key . args) ...).) With Guile 1.8 or later, you can also do this with a catch, by adding this same code to the catch's pre-unwind handler.

If you don't already have any of these, insert a whole with-throw-handler expression (or lazy-catch if your Guile is pre-1.8) around the code of interest like this:

Either way, you will need to use the (ice-9 gds-client) and (ice-9 debugging traps) modules.

Two special cases of this are the lazy-catch that the Guile REPL code uses to catch exceptions in user code, and the lazy-catch inside the stack-catch utility procedure that is provided by the (ice-9 stack-catch) module. Both of these use a handler called lazy-handler-dispatch (defined in boot-9.scm), which you can hook into such that it calls GDS to display the stack when an exception occurs. To do this, use the on-lazy-handler-dispatch procedure as follows.

After this the program will use GDS to display the stack whenever it hits an exception that is protected by a lazy-catch using lazy-handler-dispatch.

3.5.3.2 Accepting GDS Instructions at Any Time

In addition to setting an exception handler as described above, a Guile program can in principle set itself up to accept new instructions from GDS at any time, not just when it has stopped at an exception. This would allow the GDS user to evaluate code in the context of the running program, without having to wait for the program to stop first.

gds-accept-input causes the calling program to loop processing instructions from GDS, until GDS sends the continue instruction. This blocks the thread that calls it, however, so it will normally be more practical for the program to set up a dedicated GDS thread and call gds-accept-input from that thread.

For select-driven applications, an alternative approach would be for the GDS client code to provide an API which allowed the application to

3.5.3.3 Utility Guile Implementation

The “utility” Guile client mentioned above is a simple combination of the mechanisms that we have just described. In fact the code for the utility Guile client is essentially just this:

The named-module-use! line ensures that the client can process help and apropos expressions, to implement lookups in Guile's online help. The #f parameter to gds-accept-input means that the continue instruction will not cause the instruction loop to exit, which makes sense here because the utility client has nothing to do except to process GDS instructions.

The utility client does not use on-lazy-handler-dispatch at its top level, because it has its own mechanism for catching and reporting exceptions in the code that it is asked to evaluate. This mechanism summarizes the exception and gives the user a button they can click to see the full stack, so the end result is very similar to what on-lazy-handler-dispatch provides. Deep inside gds-accept-input, in the part that handles evaluating expressions from Emacs, the GDS client code uses throw->trap-context and gds-debug-trap to implement this.

3.5.4 Working with GDS in Scheme Buffers

The following subsections describe the facilities and key sequences that GDS provides for working on code in scheme-mode buffers.

3.5.4.1 Access to Guile Help and Completion

The following keystrokes provide fast and convenient access to Guile's built in help, and to completion with respect to the set of defined and accessible symbols.

3.5.4.2 Evaluating Scheme Code

The following keystrokes and commands provide various ways of sending code to a Guile client process for evaluation.

If you type C-u before one of these commands, GDS will immediately pop up a Scheme stack buffer, showing the requested evaluation, so that you can single step through it. (This is achieved by setting a <source-trap> trap at the start of the requested evaluation; see Source Traps for more on how those work.) The Scheme stack display, and the options for continuing through the code, are described in the next two sections.

3.5.5 Displaying the Scheme Stack

When you specify gds-debug-trap as the behaviour for a trap and the Guile program concerned hits that trap, GDS displays the stack and the relevant Scheme source code in Emacs, allowing you to explore the state of the program and then decide what to do next. The same applies if the program calls

(on-lazy-handler-dispatch
gds-debug-trap)

and then throws an exception that passes through lazy-handler-dispatch, except that in this case you can only explore; it isn't possible to continue normal execution after an exception.

The following commands are available in the stack buffer for exploring the state of the program.

Selecting a frame means that GDS will display the source code corresponding to that frame in the adjacent window, and that subsequent frame-sensitive commands, such as gds-evaluate (see below) and gds-step-over (see Continuing Execution), will refer to that frame.

S (gds-proc-source) is useful when the procedure being called was created by an anonymous (lambda ...) expression. Such procedures appear in the stack trace as

<procedure #f
(...)>

, which doesn't give you much clue as to what will happen next. S will show you the procedure's code, which is usually enough for you to identify it.

3.5.6 Continuing Execution

If it makes sense to continue execution from the stack which is being displayed, GDS provides the following further commands in the stack buffer.

3.5.7 Associating Buffers with Clients

The first time that you use one of GDS's evaluation, help or completion commands from a given Scheme mode buffer, GDS will ask which Guile client program you want to use for the operation, or if you want to start up a new “utility” client. After that GDS considers the buffer to be “associated” with the selected client, and so sends all further requests to that client, but you can override this by explicitly associating the buffer with a different client, or by removing the default association.

When a buffer is associated with a client program, the buffer's modeline shows whether the client is currently able to accept instruction from GDS. This is done by adding one of the following suffixes to the “Scheme” major mode indicator:

3.5.8 An Example GDS Session

Create a file, testgds.scm say, for experimenting with GDS and Scheme code, and type this into it:

Now select all of this code and type C-c C-r to send the selected region to Guile for evaluation. GDS will ask you which Guile process to use; unless you know that you already have another Guile application running and connected to GDS, choose the “Start a new Guile” option, which starts one of the “utility” processes described in GDS Getting Started.

this tells you that the evaluation was successful but that the return values were unspecified. Its effect was to load a module of example functions and set a trap on one of these functions, fact1, that calculates the factorial of its argument.

If you now call fact1, you can see the trap and GDS's stack display in action. To do this add

to your testgds.scm buffer and type C-x C-e (which evaluates the expression that the cursor is just after the end of). The result should be that a GDS stack window like the following appears:

This stack tells you that Guile is about to call the fact1 procedure, with argument 4, and you can step through this call in detail by pressing i once and then <SPC> (see Continuing Execution).

(i is needed as the first keystroke rather than <SPC>, because the aim here is to step through code in the

(ice-9
debugging example-fns)

module, whose source file is .../ice-9/debugging/example-fns.scm, but the initial (fact1 4) call comes from the Guile session, whose “source file” Guile presents as standard input. If the user starts by pressing <SPC> instead of i, the effect is that the program runs until it hits the first recursive call

(fact1 (- n
1))

, where it stops because of the trap on fact1 firing again. At this point, the source file is .../ice-9/debugging/example-fns.scm, because the recursive (fact1 (- n 1)) call comes from code in that file, so further pressing of <SPC> successfully single-steps through this file.)

3.6 Further Reading

4 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 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.

There is also a pedagogical yet detailed explanation of how the data representation of Guile is implemented, See 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), See API Reference.

4.1 Linking Programs With Guile

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

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

On most systems, you should not need to tell the compiler and linker explicitly where they can find libguile.h and libguile. When Guile has been installed in a peculiar way, or when you are on a peculiar system, things might not be so easy and you might need to pass additional -I or -L options to the compiler. Guile provides the utility program guile-config to help you find the right values for these options. You would typically run guile-config during the configuration phase of your program and use the obtained information in the Makefile.

4.1.1 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.

4.1.2 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.

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 guile-config to learn about the necessary compiler and linker flags.

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. Guile also provides the GUILE_FLAGS macro for autoconf that performs all necessary checks. Here is a configure.in file for simple-guile that uses this macro. Autoconf can use this file as a template to generate a configure script. In order for Autoconf to find the GUILE_FLAGS macro, you will need to run aclocal first (see Invoking aclocal).

Here is a Makefile.in template, from which the configure script produces a Makefile customized for the host system:

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

4.2 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.

4.2.1 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.

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

For creating shared libraries portably, we recommend the use of GNU Libtool (see Introduction).

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.

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.

4.3 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 (see 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.

4.3.1 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.

Often, you need to convert between SCM values and approriate 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:

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 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 Data Representation, but you will encounter it occasionally in low-level user code as well.

4.3.2 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 (see 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 funtions 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 behabvior: 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 Remembering During Operations.

4.3.3 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 (see 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.)

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 determing whether a non-negative integer n is even or odd.

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' (see Dynamic Wind).

Instead of coping with non-local control flow, you can also prevent it by erecting a continuation barrier, See Continuation Barriers. The function scm_c_with_continuation_barrier, for example, is guaranteed to return exactly once.

4.3.4 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 See System asyncs.

Since signal handling in Guile relies on safe points, you need to make sure that your functions do offer enough of them. Normally, calling libguile functions in the normal course of action is all that is needed. But when a thread might spent a long time in a code section that calls no libguile function, it is good to include explicit safe points. This can allow the user to interrupt your code with <C-c>, for example.

You can do this with the macro SCM_TICK. This macro is syntactically a statement. That is, you could use it like this:

Frequent execution of a safe point is even more important in multi threaded programs, See Multi-Threading.

4.3.5 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.

While in guile mode, a thread promises to reach a safe point reasonably frequently (see Asynchronous Signals). In addition to running signal handlers, these points are also potential rendezvous points of all guile mode threads where Guile can orchestrate global things like garbage collection. Consequently, when a thread in guile mode blocks and does no longer frequent safe points, it might cause all other guile mode threads to block as well. To prevent this from happening, a guile mode thread should either only block in libguile functions (who know how to do it right), or should temporarily leave guile mode with scm_without_guile.

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 non-sensical 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:

The first thing to note is that storing into a SCM location concurrently from multiple threads is guaranteed to be robust: you don't know which value wins but it will in any case be a valid SCM value.

But there is no guarantee that the list referenced by list is not modified in another thread while the loop iterates over it. Thus, while copying its elements into the vector, the list might get longer or shorter. For this reason, the loop must check both that it doesn't overrun the vector (SCM_SIMPLE_VECTOR_SET does no range-checking) and that it doesn't overrung the list (SCM_CAR and SCM_CDR likewise do no type checking).

It is safe to use SCM_CAR and SCM_CDR on the local variable list once it is known that the variable contains a pair. The contents of the pair might change spontaneously, but it will always stay a valid pair (and a local variable will of course not spontaneously point to a different Scheme object).

Likewise, a simple vector such as the one returned by scm_make_vector is guaranteed to always stay the same length so that it is safe to only use SCM_SIMPLE_VECTOR_LENGTH once and store the result. (In the example, vector is safe anyway since it is a fresh object that no other thread can possibly know about until it is returned from my_list_to_vector.)

Of course the behavior of my_list_to_vector is suboptimal when list does indeed get asynchronously lengthened or shortened in another thread. But it is robust: it will always return a valid vector. That vector might be shorter than expected, or its last elements might be unspecified, but it is a valid vector and if a program wants to rule out these cases, it must avoid modifying the list asynchronously.

This version uses the type-checking and thread-robust functions scm_car and scm_cdr instead of the faster, but less robust macros SCM_CAR and SCM_CDR. When the list is shortened (that is, when list holds a non-pair), scm_car will throw an error. This might be preferable to just returning a half-initialized vector.

The API for accessing vectors and arrays of various kinds from C takes a slightly different approach to thread-robustness. In order to get at the raw memory that stores the elements of an array, you need to reserve that array as long as you need the raw memory. During the time an array is reserved, its elements can still spontaneously change their values, but the memory itself and other things like the size of the array are guaranteed to stay fixed. Any operation that would change these parameters of an array that is currently reserved will signal an error. In order to avoid these errors, a program should of course put suitable synchronization mechanisms in place. As you can see, Guile itself is again only concerned about robustness, not about correctness: without proper synchronization, your program will likely not be correct, but the worst consequence is an error message.

Real thread-safeness often requires that a critical section of code is executed in a certain restricted manner. A common requirement is that the code section is not entered a second time when it is already being executed. Locking a mutex while in that section ensures that no other thread will start executing it, blocking asyncs ensures that no asynchronous code enters the section again from the current thread, and the error checking of Guile mutexes guarantees that an error is signalled when the current thread accidentally reenters the critical section via recursive function calls.

Guile provides two mechanisms to support critical sections as outlined above. You can either use the macros SCM_CRITICAL_SECTION_START and SCM_CRITICAL_SECTION_END for very simple sections; or use a dynwind context together with a call to scm_dynwind_critical_section.

The macros only work reliably for critical sections that are guaranteed to not cause a non-local exit. They also do not detect an accidental reentry by the current thread. Thus, you should probably only use them to delimit critical sections that do not contain calls to libguile functions or to other external functions that might do complicated things.

The function scm_dynwind_critical_section, on the other hand, will correctly deal with non-local exits because it requires a dynwind context. Also, by using a separate mutex for each critical section, it can detect accidental reentries.

4.4 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.)

4.4.1 Describing a New Type

To define a new type, the programmer must write four functions to manage instances of the type:

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:

4.4.2 Creating 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. See Double Smobs, for their discussion.

Guile provides functions for managing memory which are often helpful when implementing smobs. See 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:

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 competely 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:

The conversions of s_width and s_height to ints might fail and signal an error, thus causing a non-local exit. This is not a problem since no resources have been allocated yet that would have to be freed.

The allocation of image in step 1 might fail, but this is likewise no problem.

Step 2 can not exit non-locally. At the end of it, the image struct is in a valid state for the mark_image and free_image functions (see below).

Step 3 can not exit non-locally either. This is guaranteed by Guile. After it, smob contains a valid smob that is properly initialized and protected, and in turn can properly protect the Scheme values in its image struct.

But before the smob is completely created, SCM_NEWSMOB might cause the garbage collector to run. During this garbage collection, the SCM values in the image struct would be invisible to Guile. It only gets to know about them via the mark_image function, but that function can not yet do its job since the smob has not been created yet. Thus, it is important to not store SCM values in the image struct until after the smob has been created.

Step 4, finally, might fail and cause a non-local exit. In that case, the complete creation of the smob has not been successful, but it does nevertheless exist in a valid state. It will eventually be freed by the garbage collector, and all the resources that have been allocated for it will be correctly freed by free_image.

4.4.3 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.

4.4.4 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 (see 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.

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:

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.

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.

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.

4.4.5 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:

4.4.6 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.

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 (see 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.)

4.4.7 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. (see Data Representation for the details.) One word of the two-word cells is used for SCM_SMOB_DATA (or SCM_SMOB_OBJECT), the other contains the 16-bit type tag and the 16 extra bits.

In addition to the fundamental two-word cells, Guile also has four-word cells, which are appropriately called double cells. You can use them for double smobs and get two more immediate words of type scm_t_bits.

A double smob is created with SCM_NEWSMOB2 or SCM_NEWSMOB3 instead of SCM_NEWSMOB. Its immediate words can be retrieved as scm_t_bits with SCM_SMOB_DATA_2 and SCM_SMOB_DATA_3 in addition to SCM_SMOB_DATA. Unsurprisingly, the words can be set to scm_t_bits values with SCM_SET_SMOB_DATA_2 and SCM_SET_SMOB_DATA_3.

Of course there are also SCM_SMOB_OBJECT_2, SCM_SMOB_OBJECT_3, SCM_SET_SMOB_OBJECT_2, and SCM_SET_SMOB_OBJECT_3.

4.4.8 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.)

Here is a sample build and interaction with the code from the example-smob directory, on the author's machine:

4.5 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 initialiation actions, not just for collecting calls to scm_c_define_gsubr. For a full list of what can be done, See Snarfing Macros.

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:

Here is how you might define the Scheme function clear-image, implemented by the C function clear_image:

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.

For historical reasons, the SCM_DEFINE macro also defines a static array of characters named s_clear_image, initialized to the string "clear-image". You shouldn't use this array, but you might need to be aware that it exists.

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:

This scans image-type.c for SCM_DEFINE declarations, and writes to image-type.x the output:

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:

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.

4.6 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.

4.6.1 How One Might Extend Dia Using Guile

Dia is a free software program for drawing schematic diagrams like flow charts and floor plans (http://www.gnome.org/projects/dia/). This section conducts the thought experiment of adding Guile to Dia. In so doing, it aims to illustrate several of the steps and considerations involved in adding Guile to applications in general.

4.6.1.1 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?

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:

4.6.1.2 Four Steps Required to Add Guile

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.

4.6.1.3 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.

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.

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):

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.

4.6.1.4 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:

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_SMOB_DATA and scm_from_bool are from the standard Guile API. SCM_VALIDATE_SHAPE is a macro that you should define as part of your SMOB definition: it checks that the passed parameter is of the expected type. 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 (see 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:

For where to put this call, see the subsection after next on the structure of Guile-enabled code (see Dia Structure).

4.6.1.5 Providing a Hook for the Evaluation of Scheme Code

To make the Guile integration useful, you have to design some kind of hook into your application that application users can use to cause their Scheme code to be evaluated.

Technically, this is straightforward; you just have to decide on a mechanism that is appropriate for your application. Think of Emacs, for example: when you type <ESC> :, you get a prompt where you can type in any Elisp code, which Emacs will then evaluate. Or, again like Emacs, you could provide a mechanism (such as an init file) to allow Scheme code to be associated with a particular key sequence, and evaluate the code when that key sequence is entered.

In either case, once you have the Scheme code that you want to evaluate, as a null terminated string, you can tell Guile to evaluate it by calling the scm_c_eval_string function.

4.6.1.6 Top-level Structure of Guile-enabled Dia

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:

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.

4.6.1.7 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:

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:

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:

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 ...

4.6.2 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:

In the case of Scheme, particular features that make programming easier — and more fun! — are its powerful mechanisms for abstracting parts of programs (closures — see About Closure) and for iteration (see 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.

4.6.3 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”² 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.)

4.6.4 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:

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.

4.6.4.1 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:

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.

4.6.4.2 Functional and Performance Constraints

4.6.4.3 Your Preferred Programming Style

4.6.4.4 What Controls Program Execution?

4.6.5 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.³ For example...

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.

5 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.

5.1 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:

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.

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 bot for the return value and for the arguments.

5.2 The SCM Type

Guile represents all Scheme values with the single C type SCM. For an introduction to this topic, See 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.

5.3 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, See 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 (see 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. See Invoking Guile, for more details.
Since this function does not return, you must do all application-specific initialization before calling this function.

5.4 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, See 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.

5.5 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 Compound Data Types.

5.5.1 Booleans

Boolean values are returned by predicate procedures, such as the general equality predicates eq?, eqv? and equal? (see Equality) and numerical and string comparison operators like string=? (see String Comparison) and <= (see Comparison).

In test condition contexts like if and cond (see if cond case), where a group of subexpressions will be evaluated only if a condition expression evaluates to “true”, “true” means any value at all except #f.

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.

5.5.2 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 Numbers.

5.5.2.1 Scheme's Numerical “Tower”

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.

— 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.

5.5.2.2 Integers

Integers are whole numbers, that is numbers with no fractional part, such as 2, 83, and −3789.

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.

The infinities `+inf.0' and `-inf.0' are considered to be inexact integers. They are explained in detail in the next section, together with reals and rationals.

C has a host of different integer types, and Guile offers a host of functions to convert between them and the SCM representation. For example, a C int can be handled with scm_to_int and scm_from_int. Guile also defines a few C integer types of its own, to help with differences between systems.

C integer types that are not covered can be handled with the generic scm_to_signed_integer and scm_from_signed_integer for signed types, or with scm_to_unsigned_integer and scm_from_unsigned_integer for unsigned types.

Scheme integers can be exact and inexact. For example, a number written as 3.0 with an explicit decimal-point is inexact, but it is also an integer. The functions integer? and scm_is_integer report true for such a number, but the functions scm_is_signed_integer and scm_is_unsigned_integer only allow exact integers and thus report false. Likewise, the conversion functions like scm_to_signed_integer only accept exact integers.

The motivation for this behavior is that the inexactness of a number should not be lost silently. If you want to allow inexact integers, you can explicitly insert a call to inexact->exact or to its C equivalent scm_inexact_to_exact. (Only inexact integers will be converted by this call into exact integers; inexact non-integers will become exact fractions.)

— Scheme Procedure: integer? x
— C Function: scm_integer_p (x)

Return #t if x is an exact or inexact integer number, else #f.

          (integer? 487)
          => #t
          
          (integer? 3.0)
          => #t
          
          (integer? -3.4)
          => #f
          
          (integer? +inf.0)
          => #t

— C Function: int scm_is_integer (SCM x)

This is equivalent to scm_is_true (scm_integer_p (x)).

— C Type: scm_t_int8
— C Type: scm_t_uint8
— C Type: scm_t_int16
— C Type: scm_t_uint16
— C Type: scm_t_int32
— C Type: scm_t_uint32
— C Type: scm_t_int64
— C Type: scm_t_uint64
— C Type: scm_t_intmax
— C Type: scm_t_uintmax

The C types are equivalent to the corresponding ISO C types but are defined on all platforms, with the exception of scm_t_int64 and scm_t_uint64, which are only defined when a 64-bit type is available. For example, scm_t_int8 is equivalent to int8_t.
You can regard these definitions as a stop-gap measure until all platforms provide these types. If you know that all the platforms that you are interested in already provide these types, it is better to use them directly instead of the types provided by Guile.

— C Function: int scm_is_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
— C Function: int scm_is_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)

Return 1 when x represents an exact integer that is between min and max, inclusive.
These functions can be used to check whether a SCM value will fit into a given range, such as the range of a given C integer type. If you just want to convert a SCM value to a given C integer type, use one of the conversion functions directly.

— C Function: scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
— C Function: scm_t_uintmax scm_to_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)

When x represents an exact integer that is between min and max inclusive, return that integer. Else signal an error, either a `wrong-type' error when x is not an exact integer, or an `out-of-range' error when it doesn't fit the given range.

— C Function: SCM scm_from_signed_integer (scm_t_intmax x)
— C Function: SCM scm_from_unsigned_integer (scm_t_uintmax x)

Return the SCM value that represents the integer x. This function will always succeed and will always return an exact number.

— C Function: char scm_to_char (SCM x)
— C Function: signed char scm_to_schar (SCM x)
— C Function: unsigned char scm_to_uchar (SCM x)
— C Function: short scm_to_short (SCM x)
— C Function: unsigned short scm_to_ushort (SCM x)
— C Function: int scm_to_int (SCM x)
— C Function: unsigned int scm_to_uint (SCM x)
— C Function: long scm_to_long (SCM x)
— C Function: unsigned long scm_to_ulong (SCM x)
— C Function: long long scm_to_long_long (SCM x)
— C Function: unsigned long long scm_to_ulong_long (SCM x)
— C Function: size_t scm_to_size_t (SCM x)
— C Function: ssize_t scm_to_ssize_t (SCM x)
— C Function: scm_t_int8 scm_to_int8 (SCM x)
— C Function: scm_t_uint8 scm_to_uint8 (SCM x)
— C Function: scm_t_int16 scm_to_int16 (SCM x)
— C Function: scm_t_uint16 scm_to_uint16 (SCM x)
— C Function: scm_t_int32 scm_to_int32 (SCM x)
— C Function: scm_t_uint32 scm_to_uint32 (SCM x)
— C Function: scm_t_int64 scm_to_int64 (SCM x)
— C Function: scm_t_uint64 scm_to_uint64 (SCM x)
— C Function: scm_t_intmax scm_to_intmax (SCM x)
— C Function: scm_t_uintmax scm_to_uintmax (SCM x)

When x represents an exact integer that fits into the indicated C type, return that integer. Else signal an error, either a `wrong-type' error when x is not an exact integer, or an `out-of-range' error when it doesn't fit the given range.
The functions scm_to_long_long, scm_to_ulong_long, scm_to_int64, and scm_to_uint64 are only available when the corresponding types are.

— C Function: SCM scm_from_char (char x)
— C Function: SCM scm_from_schar (signed char x)
— C Function: SCM scm_from_uchar (unsigned char x)
— C Function: SCM scm_from_short (short x)
— C Function: SCM scm_from_ushort (unsigned short x)
— C Function: SCM scm_from_int (int x)
— C Function: SCM scm_from_uint (unsigned int x)
— C Function: SCM scm_from_long (long x)
— C Function: SCM scm_from_ulong (unsigned long x)
— C Function: SCM scm_from_long_long (long long x)
— C Function: SCM scm_from_ulong_long (unsigned long long x)
— C Function: SCM scm_from_size_t (size_t x)
— C Function: SCM scm_from_ssize_t (ssize_t x)
— C Function: SCM scm_from_int8 (scm_t_int8 x)
— C Function: SCM scm_from_uint8 (scm_t_uint8 x)
— C Function: SCM scm_from_int16 (scm_t_int16 x)
— C Function: SCM scm_from_uint16 (scm_t_uint16 x)
— C Function: SCM scm_from_int32 (scm_t_int32 x)
— C Function: SCM scm_from_uint32 (scm_t_uint32 x)
— C Function: SCM scm_from_int64 (scm_t_int64 x)
— C Function: SCM scm_from_uint64 (scm_t_uint64 x)
— C Function: SCM scm_from_intmax (scm_t_intmax x)
— C Function: SCM scm_from_uintmax (scm_t_uintmax x)

Return the SCM value that represents the integer x. These functions will always succeed and will always return an exact number.

— C Function: void scm_to_mpz (SCM val, mpz_t rop)

Assign val to the multiple precision integer rop. val must be an exact integer, otherwise an error will be signalled. rop must have been initialized with mpz_init before this function is called. When rop is no longer needed the occupied space must be freed with mpz_clear. See Initializing Integers, for details.

— C Function: SCM scm_from_mpz (mpz_t val)

Return the SCM value that represents val.

5.5.2.3 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 can not represent 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:

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:

The limited precision of Guile's encoding means that any “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.

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.

The infinities are written `+inf.0' and `-inf.0', respectivly. This syntax is also recognized by read as an extension to the usual Scheme syntax.

Dividing zero by zero yields something that is not a number at all: `+nan.0'. This is the special `not a number' value.

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.

The infinities are inexact integers and are considered to be both even and odd. While `+nan.0' is not = to itself, it is eqv? to itself.

— 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.
Since Guile can not represent irrational numbers, every number satisfying real? also satisfies rational? in Guile.

— 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 x is either `+inf.0' or `-inf.0', #f otherwise.

— Scheme Procedure: nan? x
— C Function: scm_nan_p (x)

Return #t if x is `+nan.0', #f otherwise.

— Scheme Procedure: nan
— C Function: scm_nan ()

Return NaN.

— Scheme Procedure: inf
— C Function: scm_inf ()

Return Inf.

— 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 representats val. The returned value is inexact according to the predicate inexact?, but it will be exactly equal to val.

5.5.2.4 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.

Guile represents a complex number with a non-zero imaginary part as a pair of inexact rationals, so the real and imaginary parts of a complex number have the same properties of inexactness and limited precision as single inexact rational numbers. Guile can not represent exact complex numbers with non-zero imaginary parts.

— Scheme Procedure: complex? z
— C Function: scm_complex_p (z)

Return #t if x is a complex number, #f otherwise. Note that the sets of real, rational and integer values form subsets of the set of complex numbers, i. e. the predicate will also be fulfilled if x is a real, rational or integer number.

— C Function: int scm_is_complex (SCM val)

Equivalent to scm_is_true (scm_complex_p (val)).

5.5.2.5 Exact and Inexact Numbers

R5RS requires that 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 real 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.

— 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

— Scheme Procedure: inexact? z
— C Function: scm_inexact_p (z)

Return #t if the number z is inexact, #f else.

— 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.

5.5.2.6 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:

If the base code is omitted, the integer is assumed to be decimal. The following examples show how these base codes are used.

The codes for indicating exactness (which can, incidentally, be applied to all numerical values) are:

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.

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'.

5.5.2.7 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
     

— 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

— 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
     

5.5.2.8 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.

5.5.2.9 Converting Numbers To and From Strings

— 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).

5.5.2.10 Complex Number Operations

— Scheme Procedure: make-rectangular real imaginary
— C Function: scm_make_rectangular (real, imaginary)

Return a complex number constructed of the given real and imaginary parts.

— Scheme Procedure: make-polar x y
— C Function: scm_make_polar (x, y)

Return the complex number x * e^(i * y).

— 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 doubles 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.

5.5.2.11 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 od (- 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.

5.5.2.12 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 the 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.

5.5.2.13 Primitive Numeric Functions

Many of Guile's numeric procedures which accept any kind of numbers as arguments, including complex numbers, are implemented as Scheme procedures that use the following real number-based primitives. These primitives signal an error if they are called with complex arguments.

— Scheme Procedure: $abs x

Return the absolute value of x.

— Scheme Procedure: $sqrt x

Return the square root of x.

— Scheme Procedure: $expt x y
— C Function: scm_sys_expt (x, y)

Return x raised to the power of y. This procedure does not accept complex arguments.

— Scheme Procedure: $sin x

Return the sine of x.

— Scheme Procedure: $cos x

Return the cosine of x.

— Scheme Procedure: $tan x

Return the tangent of x.

— Scheme Procedure: $asin x

Return the arcsine of x.

— Scheme Procedure: $acos x

Return the arccosine of x.

— Scheme Procedure: $atan x

Return the arctangent of x in the range −PI/2 to PI/2.

— Scheme Procedure: $atan2 x y
— C Function: scm_sys_atan2 (x, y)

Return the arc tangent of the two arguments x and y. This is similar to calculating the arc tangent of x / y, except that the signs of both arguments are used to determine the quadrant of the result. This procedure does not accept complex arguments.

— Scheme Procedure: $exp x

Return e to the power of x, where e is the base of natural logarithms (2.71828...).

— Scheme Procedure: $log x

Return the natural logarithm of x.

— Scheme Procedure: $sinh x

Return the hyperbolic sine of x.

— Scheme Procedure: $cosh x

Return the hyperbolic cosine of x.

— Scheme Procedure: $tanh x

Return the hyperbolic tangent of x.

— Scheme Procedure: $asinh x

Return the hyperbolic arcsine of x.

— Scheme Procedure: $acosh x

Return the hyperbolic arccosine of x.

— Scheme Procedure: $atanh x

Return the hyperbolic arctangent of x.

C functions for the above are provided by the standard mathematics library. Naturally these expect and return double arguments (see Mathematics).

	Scheme Procedure	C Function
	`$abs`	`fabs`
	`$sqrt`	`sqrt`
	`$sin`	`sin`
	`$cos`	`cos`
	`$tan`	`tan`
	`$asin`	`asin`
	`$acos`	`acos`
	`$atan`	`atan`
	`$atan2`	`atan2`
	`$exp`	`exp`
	`$expt`	`pow`
	`$log`	`log`
	`$sinh`	`sinh`
	`$cosh`	`cosh`
	`$tanh`	`tanh`
	`$asinh`	`asinh`
	`$acosh`	`acosh`
	`$atanh`	`atanh`

asinh, acosh and atanh are C99 standard but might not be available on older systems. Guile provides the following equivalents (on all systems).

— C Function: double scm_asinh (double x)
— C Function: double scm_acosh (double x)
— C Function: double scm_atanh (double x)

Return the hyperbolic arcsine, arccosine or arctangent of x respectively.

5.5.2.14 Bitwise Operations

For the following bitwise functions, negative numbers are treated as infinite precision twos-complements. For instance -6 is bits ...111010, with infinitely many ones on the left. It can be seen that adding 6 (binary 110) to such a bit pattern gives all zeros.

— Scheme Procedure: logand n1 n2 ...
— C Function: scm_logand (n1, n2)

Return the bitwise and of the integer arguments.

          (logand) => -1
          (logand 7) => 7
          (logand #b111 #b011 #b001) => 1

— Scheme Procedure: logior n1 n2 ...
— C Function: scm_logior (n1, n2)

Return the bitwise or of the integer arguments.

          (logior) => 0
          (logior 7) => 7
          (logior #b000 #b001 #b011) => 3

— Scheme Procedure: logxor n1 n2 ...
— C Function: scm_loxor (n1, n2)

Return the bitwise xor of the integer arguments. A bit is set in the result if it is set in an odd number of arguments.
          (logxor) => 0
          (logxor 7) => 7
          (logxor #b000 #b001 #b011) => 2
          (logxor #b000 #b001 #b011 #b011) => 1
     

— Scheme Procedure: lognot n
— C Function: scm_lognot (n)

Return the integer which is the ones-complement of the integer argument, ie. each 0 bit is changed to 1 and each 1 bit to 0.
          (number->string (lognot #b10000000) 2)
             => "-10000001"
          (number->string (lognot #b0) 2)
             => "-1"
     

— Scheme Procedure: logtest j k
— C Function: scm_logtest (j, k)

Test whether j and k have any 1 bits in common. This is equivalent to (not (zero? (logand j k))), but without actually calculating the logand, just testing for non-zero.
          (logtest #b0100 #b1011) => #f
          (logtest #b0100 #b0111) => #t
     

— Scheme Procedure: logbit? index j
— C Function: scm_logbit_p (index, j)

Test whether bit number index in j is set. index starts from 0 for the least significant bit.

          (logbit? 0 #b1101) => #t
          (logbit? 1 #b1101) => #f
          (logbit? 2 #b1101) => #t
          (logbit? 3 #b1101) => #t
          (logbit? 4 #b1101) => #f

— Scheme Procedure: ash n cnt
— C Function: scm_ash (n, cnt)

Return n shifted left by cnt bits, or shifted right if cnt is negative. This is an “arithmetic” shift.
This is effectively a multiplication by 2^cnt, and when cnt is negative it's a division, rounded towards negative infinity. (Note that this is not the same rounding as quotient does.)
With n viewed as an infinite precision twos complement, ash means a left shift introducing zero bits, or a right shift dropping bits.
          (number->string (ash #b1 3) 2)     => "1000"
          (number->string (ash #b1010 -1) 2) => "101"
          
          ;; -23 is bits ...11101001, -6 is bits ...111010
          (ash -23 -2) => -6
     

— Scheme Procedure: logcount n
— C Function: scm_logcount (n)

Return the number of bits in integer n. If n is positive, the 1-bits in its binary representation are counted. If negative, the 0-bits in its two's-complement binary representation are counted. If zero, 0 is returned.
          (logcount #b10101010)
             => 4
          (logcount 0)
             => 0
          (logcount -2)
             => 1
     

— Scheme Procedure: integer-length n
— C Function: scm_integer_length (n)

Return the number of bits necessary to represent n.
For positive n this is how many bits to the most significant one bit. For negative n it's how many bits to the most significant zero bit in twos complement form.
          (integer-length #b10101010) => 8
          (integer-length #b1111)     => 4
          (integer-length 0)          => 0
          (integer-length -1)         => 0
          (integer-length -256)       => 8
          (integer-length -257)       => 9
     

— Scheme Procedure: integer-expt n k
— C Function: scm_integer_expt (n, k)

Return n raised to the power k. k must be an exact integer, n can be any number.
Negative k is supported, and results in 1/n^abs(k) in the usual way. n^0 is 1, as usual, and that includes 0^0 is 1.
          (integer-expt 2 5)   => 32
          (integer-expt -3 3)  => -27
          (integer-expt 5 -3)  => 1/125
          (integer-expt 0 0)   => 1
     

— Scheme Procedure: bit-extract n start end
— C Function: scm_bit_extract (n, start, end)

Return the integer composed of the start (inclusive) through end (exclusive) bits of n. The startth bit becomes the 0-th bit in the result.
          (number->string (bit-extract #b1101101010 0 4) 2)
             => "1010"
          (number->string (bit-extract #b1101101010 4 9) 2)
             => "10110"
     

5.5.2.15 Random Number Generation

Pseudo-random numbers are generated from a random state object, which can be created with seed->random-state. 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.

— 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:

For security applications, such as password generation, you should use more bits of seed. Otherwise an open source password generator could be attacked by guessing the seed... but that's a subject for another manual.

5.5.3 Characters

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.

Most of the “control characters” (those below codepoint 32) in the ASCII character set, as well as the space, may be referred to by longer names: for example, #\tab, #\esc, #\stx, and so on. The following table describes the ASCII names for each character.

0 = `#\nul`	1 = `#\soh`	2 = `#\stx`	3 = `#\etx`
4 = `#\eot`	5 = `#\enq`	6 = `#\ack`	7 = `#\bel`
8 = `#\bs`	9 = `#\ht`	10 = `#\nl`	11 = `#\vt`
12 = `#\np`	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 “delete” character (octal 177) may be referred to with the name #\del.

Alias	Original
`#\space`	`#\sp`
`#\newline`	`#\nl`
`#\tab`	`#\ht`
`#\backspace`	`#\bs`
`#\return`	`#\cr`
`#\page`	`#\np`
`#\null`	`#\nul`

— Scheme Procedure: char? x
— C Function: scm_char_p (x)

Return #t iff x is a character, else #f.

— Scheme Procedure: char=? x y

Return #t iff x is the same character as y, else #f.

— Scheme Procedure: char<? x y

Return #t iff x is less than y in the ASCII sequence, else #f.

— Scheme Procedure: char<=? x y

Return #t iff x is less than or equal to y in the ASCII sequence, else #f.

— Scheme Procedure: char>? x y

Return #t iff x is greater than y in the ASCII sequence, else #f.

— Scheme Procedure: char>=? x y

Return #t iff x is greater than or equal to y in the ASCII sequence, else #f.

— Scheme Procedure: char-ci=? x y

Return #t iff x is the same character as y ignoring case, else #f.

— Scheme Procedure: char-ci<? x y

Return #t iff x is less than y in the ASCII sequence ignoring case, else #f.

— Scheme Procedure: char-ci<=? x y

Return #t iff x is less than or equal to y in the ASCII sequence ignoring case, else #f.

— Scheme Procedure: char-ci>? x y

Return #t iff x is greater than y in the ASCII sequence ignoring case, else #f.

— Scheme Procedure: char-ci>=? x y

Return #t iff x is greater than or equal to y in the ASCII sequence ignoring case, else #f.

— Scheme Procedure: char-alphabetic? chr
— C Function: scm_char_alphabetic_p (chr)

Return #t iff chr is alphabetic, else #f.

— Scheme Procedure: char-numeric? chr
— C Function: scm_char_numeric_p (chr)

Return #t iff chr is numeric, else #f.

— Scheme Procedure: char-whitespace? chr
— C Function: scm_char_whitespace_p (chr)

Return #t iff chr is whitespace, else #f.

— Scheme Procedure: char-upper-case? chr
— C Function: scm_char_upper_case_p (chr)

Return #t iff chr is uppercase, else #f.

— Scheme Procedure: char-lower-case? chr
— C Function: scm_char_lower_case_p (chr)

Return #t iff chr is lowercase, else #f.

— Scheme Procedure: char-is-both? chr
— C Function: scm_char_is_both_p (chr)

Return #t iff chr is either uppercase or lowercase, else #f.

— Scheme Procedure: char->integer chr
— C Function: scm_char_to_integer (chr)

Return the number corresponding to ordinal position of chr in the ASCII sequence.

— Scheme Procedure: integer->char n
— C Function: scm_integer_to_char (n)

Return the character at position n in the ASCII sequence.

— 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.

5.5.4 Character Sets

The data type charset implements sets of characters (see 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.

The Guile implementation of character sets currently deals only with 8-bit characters. In the future, when Guile gets support for international character sets, this will change, but the functions provided here will always then be able to efficiently cope with very large character sets.

5.5.4.1 Character Set Predicates/Comparison

Use these procedures for testing whether an object is a character set, or whether several character sets are equal or subsets of each other. char-set-hash can be used for calculating a hash value, maybe for usage in fast lookup procedures.

— Scheme Procedure: char-set? obj
— C Function: scm_char_set_p (obj)

Return #t if obj is a character set, #f otherwise.

— Scheme Procedure: char-set= . char_sets
— C Function: scm_char_set_eq (char_sets)

Return #t if all given character sets are equal.

— Scheme Procedure: char-set<= . char_sets
— C Function: scm_char_set_leq (char_sets)

Return #t if every character set csi is a subset of character set csi+1.

— Scheme Procedure: char-set-hash cs [bound]
— C Function: scm_char_set_hash (cs, bound)

Compute a hash value for the character set cs. If bound is given and non-zero, it restricts the returned value to the range 0 ... bound - 1.

5.5.4.2 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.

5.5.4.3 Creating Character Sets

— Scheme Procedure: char-set-copy cs
— C Function: scm_char_set_copy (cs)

Return a newly allocated character set containing all characters in cs.

— Scheme Procedure: char-set . rest
— C Function: scm_char_set (rest)

Return a character set containing all given characters.

— Scheme Procedure: list->char-set list [base_cs]
— C Function: scm_list_to_char_set (list, base_cs)

Convert the character list list to a character set. If the character set base_cs is given, the character in this set are also included in the result.

— Scheme Procedure: list->char-set! list base_cs
— C Function: scm_list_to_char_set_x (list, base_cs)

Convert the character list list to a character set. The characters are added to base_cs and base_cs is returned.

— Scheme Procedure: string->char-set str [base_cs]
— C Function: scm_string_to_char_set (str, base_cs)

Convert the string str to a character set. If the character set base_cs is given, the characters in this set are also included in the result.

— Scheme Procedure: string->char-set! str base_cs
— C Function: scm_string_to_char_set_x (str, base_cs)

Convert the string str to a character set. The characters from the string are added to base_cs, and base_cs is returned.

— Scheme Procedure: char-set-filter pred cs [base_cs]
— C Function: scm_char_set_filter (pred, cs, base_cs)

Return a character set containing every character from cs so that it satisfies pred. If provided, the characters from base_cs are added to the result.

— Scheme Procedure: char-set-filter! pred cs base_cs
— C Function: scm_char_set_filter_x (pred, cs, base_cs)

Return a character set containing every character from cs so that it satisfies pred. The characters are added to base_cs and base_cs is returned.

— Scheme Procedure: ucs-range->char-set lower upper [error [base_cs]]
— C Function: scm_ucs_range_to_char_set (lower, upper, error, base_cs)

Return a character set containing all characters whose character codes lie in the half-open range [lower,upper).
If error is a true value, an error is signalled if the specified range contains characters which are not contained in the implemented character range. If error is #f, these characters are silently left out of the resultung 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 resultung 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.

5.5.4.4 Querying Character Sets

Access the elements and other information of a character set with these procedures.

— Scheme Procedure: char-set-size cs
— C Function: scm_char_set_size (cs)

Return the number of elements in character set cs.

— Scheme Procedure: char-set-count pred cs
— C Function: scm_char_set_count (pred, cs)

Return the number of the elements int the character set cs which satisfy the predicate pred.

— Scheme Procedure: char-set->list cs
— C Function: scm_char_set_to_list (cs)

Return a list containing the elements of the character set cs.

— Scheme Procedure: char-set->string cs
— C Function: scm_char_set_to_string (cs)

Return a string containing the elements of the character set cs. The order in which the characters are placed in the string is not defined.

— Scheme Procedure: char-set-contains? cs ch
— C Function: scm_char_set_contains_p (cs, ch)

Return #t iff the character ch is contained in the character set cs.

— Scheme Procedure: char-set-every pred cs
— C Function: scm_char_set_every (pred, cs)

Return a true value if every character in the character set cs satisfies the predicate pred.

— Scheme Procedure: char-set-any pred cs
— C Function: scm_char_set_any (pred, cs)

Return a true value if any character in the character set cs satisfies the predicate pred.

5.5.4.5 Character-Set Algebra

Character sets can be manipulated with the common set algebra operation, such as union, complement, intersection etc. All of these procedures provide side-effecting variants, which modify their character set argument(s).

— Scheme Procedure: char-set-adjoin cs . rest
— C Function: scm_char_set_adjoin (cs, rest)

Add all character arguments to the first argument, which must be a character set.

— Scheme Procedure: char-set-delete cs . rest
— C Function: scm_char_set_delete (cs, rest)

Delete all character arguments from the first argument, which must be a character set.

— Scheme Procedure: char-set-adjoin! cs . rest
— C Function: scm_char_set_adjoin_x (cs, rest)

Add all character arguments to the first argument, which must be a character set.

— Scheme Procedure: char-set-delete! cs . rest
— C Function: scm_char_set_delete_x (cs, rest)

Delete all character arguments from the first argument, which must be a character set.

— Scheme Procedure: char-set-complement cs
— C Function: scm_char_set_complement (cs)

Return the complement of the character set cs.

— Scheme Procedure: char-set-union . rest
— C Function: scm_char_set_union (rest)

Return the union of all argument character sets.

— Scheme Procedure: char-set-intersection . rest
— C Function: scm_char_set_intersection (rest)

Return the intersection of all argument character sets.

— Scheme Procedure: char-set-difference cs1 . rest
— C Function: scm_char_set_difference (cs1, rest)

Return the difference of all argument character sets.

— Scheme Procedure: char-set-xor . rest
— C Function: scm_char_set_xor (rest)

Return the exclusive-or of all argument character sets.

— Scheme Procedure: char-set-diff+intersection cs1 . rest
— C Function: scm_char_set_diff_plus_intersection (cs1, rest)

Return the difference and the intersection of all argument character sets.

— Scheme Procedure: char-set-complement! cs
— C Function: scm_char_set_complement_x (cs)

Return the complement of the character set cs.

— Scheme Procedure: char-set-union! cs1 . rest
— C Function: scm_char_set_union_x (cs1, rest)

Return the union of all argument character sets.

— Scheme Procedure: char-set-intersection! cs1 . rest
— C Function: scm_char_set_intersection_x (cs1, rest)

Return the intersection of all argument character sets.

— Scheme Procedure: char-set-difference! cs1 . rest
— C Function: scm_char_set_difference_x (cs1, rest)

Return the difference of all argument character sets.

— Scheme Procedure: char-set-xor! cs1 . rest
— C Function: scm_char_set_xor_x (cs1, rest)

Return the exclusive-or of all argument character sets.

— Scheme Procedure: char-set-diff+intersection! cs1 cs2 . rest
— C Function: scm_char_set_diff_plus_intersection_x (cs1, cs2, rest)

Return the difference and the intersection of all argument character sets.

5.5.4.6 Standard Character Sets

In order to make the use of the character set data type and procedures useful, several predefined character set variables exist.

Currently, the contents of these character sets are recomputed upon a successful setlocale call (see Locales) in order to reflect the characters available in the current locale's codeset. For instance, char-set:letter contains 52 characters under an ASCII locale (e.g., the default C locale) and 117 characters under an ISO-8859-1 (“Latin-1”) locale.

— 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

This is empty, because ASCII has no titlecase characters.

— Scheme Variable: char-set:letter
— C Variable: scm_char_set_letter

All letters, e.g. the union of char-set:lower-case and char-set:upper-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, that is #\space and #\tab.

— Scheme Variable: char-set:iso-control
— C Variable: scm_char_set_iso_control

The ISO control characters with the codes 0–31 and 127.

— Scheme Variable: char-set:punctuation
— C Variable: scm_char_set_punctuation

The characters !"#%&'()*,-./:;?@[\\]_{}

— Scheme Variable: char-set:symbol
— C Variable: scm_char_set_symbol

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:full
— C Variable: scm_char_set_full

This character set contains all possible characters.

5.5.5 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 accidently 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.

5.5.5.1 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 rest are Guile extensions, notice they follow C string syntax.

5.5.5.2 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 (see 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 (see 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.

5.5.5.3 String Constructors

The string constructor procedures create new string objects, possibly initializing them with some specified character data. See also See String Selection, for ways to create strings from existing strings.

— Scheme Procedure: string char...

Return a newly allocated string made from the given character arguments.
          (string #\x #\y #\z) => "xyz"
          (string)             => ""
     

— Scheme Procedure: list->string lst
— C Function: scm_string (lst)

Return a newly allocated string made from a list of characters.
          (list->string '(#\a #\b #\c)) => "abc"
     

— Scheme Procedure: reverse-list->string lst
— C Function: scm_reverse_list_to_string (lst)

Return a newly allocated string made from a list of characters, in reverse order.
          (reverse-list->string '(#\a #\B #\c)) => "cBa"
     

— Scheme Procedure: make-string k [chr]
— C Function: scm_make_string (k, chr)

Return a newly allocated string of length k. If chr is given, then all elements of the string are initialized to chr, otherwise the contents of the string are unspecified.

— C Function: SCM scm_c_make_string (size_t len, SCM chr)

Like scm_make_string, but expects the length as a size_t.

— Scheme Procedure: string-tabulate proc len
— C Function: scm_string_tabulate (proc, len)

proc is an integer->char procedure. Construct a string of size len by applying proc to each index to produce the corresponding string element. The order in which proc is applied to the indices is not specified.

— Scheme Procedure: string-join ls [delimiter [grammar]]
— C Function: scm_string_join (ls, delimiter, grammar)

Append the string in the string list ls, using the string delim as a delimiter between the elements of ls. grammar is a symbol which specifies how the delimiter is placed between the strings, and defaults to the symbol infix.

infix
Insert the separator between list elements. An empty string will produce an empty list.
string-infix
Like infix, but will raise an error if given the empty list.
suffix
Insert the separator after every list element.
prefix
Insert the separator before each list element.

5.5.5.4 List/String conversion

When processing strings, it is often convenient to first convert them into a list representation by using the procedure string->list, work with the resulting list, and then convert it back into a string. These procedures are useful for similar tasks.

— Scheme Procedure: string->list str [start [end]]
— C Function: scm_substring_to_list (str, start, end)
— C Function: scm_string_to_list (str)

Convert the string str into a list of characters.

— Scheme Procedure: string-split str chr
— C Function: scm_string_split (str, chr)

Split the string str into the a list of the substrings delimited by appearances of the character chr. Note that an empty substring between separator characters will result in an empty string in the result list.
          (string-split "root:x:0:0:root:/root:/bin/bash" #\:)
          =>
          ("root" "x" "0" "0" "root" "/root" "/bin/bash")
          
          (string-split "::" #\:)
          =>
          ("" "" "")
          
          (string-split "" #\:)
          =>
          ("")
     

5.5.5.5 String Selection

Portions of strings can be extracted by these procedures. string-ref delivers individual characters whereas substring can be used to extract substrings from longer strings.

— Scheme Procedure: string-length string
— C Function: scm_string_length (string)

Return the number of characters in string.

— C Function: size_t scm_c_string_length (SCM str)

Return the number of characters in str as a size_t.

— Scheme Procedure: string-ref str k
— C Function: scm_string_ref (str, k)

Return character k of str using zero-origin indexing. k must be a valid index of str.

— C Function: SCM scm_c_string_ref (SCM str, size_t k)

Return character k of str using zero-origin indexing. k must be a valid index of str.

— Scheme Procedure: string-copy str [start [end]]
— C Function: scm_substring_copy (str, start, end)
— C Function: scm_string_copy (str)

Return a copy of the given string str.
The returned string shares storage with str initially, but it is copied as soon as one of the two strings is modified.

— Scheme Procedure: substring str start [end]
— C Function: scm_substring (str, start, end)

Return a new string formed from the characters of str beginning with index start (inclusive) and ending with index end (exclusive). str must be a string, start and end must be exact integers satisfying:
0 <= start <= end <= (string-length str).
The returned string shares storage with str initially, but it is copied as soon as one of the two strings is modified.

— Scheme Procedure: substring/shared str start [end]
— C Function: scm_substring_shared (str, start, end)

Like substring, but the strings continue to share their storage even if they are modified. Thus, modifications to str show up in the new string, and vice versa.

— Scheme Procedure: substring/copy str start [end]
— C Function: scm_substring_copy (str, start, end)

Like substring, but the storage for the new string is copied immediately.

— Scheme Procedure: substring/read-only str start [end]
— C Function: scm_substring_read_only (str, start, end)

Like substring, but the resulting string can not be modified.

— C Function: SCM scm_c_substring (SCM str, size_t start, size_t end)
— C Function: SCM scm_c_substring_shared (SCM str, size_t start, size_t end)
— C Function: SCM scm_c_substring_copy (SCM str, size_t start, size_t end)
— C Function: SCM scm_c_substring_read_only (SCM str, size_t start, size_t end)

Like scm_substring, etc. but the bounds are given as a size_t.

— Scheme Procedure: string-take s n
— C Function: scm_string_take (s, n)

Return the n first characters of s.

— Scheme Procedure: string-drop s n
— C Function: scm_string_drop (s, n)

Return all but the first n characters of s.

— Scheme Procedure: string-take-right s n
— C Function: scm_string_take_right (s, n)

Return the n last characters of s.

— Scheme Procedure: string-drop-right s n
— C Function: scm_string_drop_right (s, n)

Return all but the last n characters of s.

— Scheme Procedure: string-pad s len [chr [start [end]]]
— Scheme Procedure: string-pad-right s len [chr [start [end]]]
— C Function: scm_string_pad (s, len, chr, start, end)
— C Function: scm_string_pad_right (s, len, chr, start, end)

Take characters start to end from the string s and either pad with char or truncate them to give len characters.
string-pad pads or truncates on the left, so for example
          (string-pad "x" 3)     => "  x"
          (string-pad "abcde" 3) => "cde"
     
string-pad-right pads or truncates on the right, so for example
          (string-pad-right "x" 3)     => "x  "
          (string-pad-right "abcde" 3) => "abc"
     

— Scheme Procedure: string-trim s [char_pred [start [end]]]
— Scheme Procedure: string-trim-right s [char_pred [start [end]]]
— Scheme Procedure: string-trim-both s [char_pred [start [end]]]
— C Function: scm_string_trim (s, char_pred, start, end)
— C Function: scm_string_trim_right (s, char_pred, start, end)
— C Function: scm_string_trim_both (s, char_pred, start, end)

Trim occurrances 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 (see 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"
     

5.5.5.6 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 "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.

5.5.5.7 String Comparison

The procedures in this section are similar to the character ordering predicates (see 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 no ending ?. The predicates ending in -ci ignore the character case when comparing strings.

— Scheme Procedure: string=? s1 s2

Lexicographic equality predicate; return #t if the two strings are the same length and contain the same characters in the same positions, otherwise return #f.
The procedure string-ci=? treats upper and lower case letters as though they were the same character, but string=? treats upper and lower case as distinct characters.

— Scheme Procedure: string<? s1 s2

Lexicographic ordering predicate; return #t if s1 is lexicographically less than s2.

— Scheme Procedure: string<=? s1 s2

Lexicographic ordering predicate; return #t if s1 is lexicographically less than or equal to s2.

— Scheme Procedure: string>? s1 s2

Lexicographic ordering predicate; return #t if s1 is lexicographically greater than s2.

— Scheme Procedure: string>=? s1 s2

Lexicographic ordering predicate; return #t if s1 is lexicographically greater than or equal to s2.

— Scheme Procedure: string-ci=? s1 s2

Case-insensitive string equality predicate; return #t if the two strings are the same length and their component characters match (ignoring case) at each position; otherwise return #f.

— Scheme Procedure: string-ci<? s1 s2

Case insensitive lexicographic ordering predicate; return #t if s1 is lexicographically less than s2 regardless of case.

— Scheme Procedure: string-ci<=? s1 s2

Case insensitive lexicographic ordering predicate; return #t if s1 is lexicographically less than or equal to s2 regardless of case.

— Scheme Procedure: string-ci>? s1 s2

Case insensitive lexicographic ordering predicate; return #t if s1 is lexicographically greater than s2 regardless of case.

— Scheme Procedure: string-ci>=? s1 s2

Case insensitive lexicographic ordering predicate; return #t if s1 is lexicographically greater than or equal to s2 regardless of case.

— Scheme Procedure: string-compare s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
— C Function: scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)

Apply proc_lt, proc_eq, proc_gt to the mismatch index, depending upon whether s1 is less than, equal to, or greater than s2. The mismatch index is the largest index i such that for every 0 <= j < i, s1[j] = s2[j] – that is, i is the first position that does not match.

— Scheme Procedure: string-compare-ci s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
— C Function: scm_string_compare_ci (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)

Apply proc_lt, proc_eq, proc_gt to the mismatch index, depending upon whether s1 is less than, equal to, or greater than s2. The mismatch index is the largest index i such that for every 0 <= j < i, s1[j] = s2[j] – that is, i is the first position that does not match. The character comparison is done case-insensitively.

— 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).

5.5.5.8 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 occurence of a character which

equals char_pred, if it is character,
satisifies the predicate char_pred, if it is a procedure,
is in the set char_pred, if it is a character set.

— 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 occurence of a character which

equals char_pred, if it is character,
satisifies the predicate char_pred, if it is a procedure,
is in the set if char_pred is a character set.

— 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 occurence of a character which

equals char_pred, if it is character,
satisifies the predicate char_pred, if it is a procedure,
is in the set if char_pred is a character set.

— 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 occurence of a character which

does not equal char_pred, if it is character,
does not satisify 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 occurence 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,
satisifies 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.

5.5.5.9 Alphabetic Case Mapping

These are procedures for mapping strings to their upper- or lower-case equivalents, respectively, or for capitalizing strings.

— 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.

5.5.5.10 Reversing and Appending Strings

— Scheme Procedure: string-reverse str [start [end]]
— C Function: scm_string_reverse (str, start, end)

Reverse the string str. The optional arguments start and end delimit the region of str to operate on.

— Scheme Procedure: string-reverse! str [start [end]]
— C Function: scm_string_reverse_x (str, start, end)

Reverse the string str in-place. The optional arguments start and end delimit the region of str to operate on. The return value is unspecified.

— Scheme Procedure: string-append . args
— C Function: scm_string_append (args)

Return a newly allocated string whose characters form the concatenation of the given strings, args.
          (let ((h "hello "))
            (string-append h "world"))
          => "hello world"
     

— Scheme Procedure: string-append/shared . ls
— C Function: scm_string_append_shared (ls)

Like string-append, but the result may share memory with the argument strings.

— Scheme Procedure: string-concatenate ls
— C Function: scm_string_concatenate (ls)

Append the elements of ls (which must be strings) together into a single string. Guaranteed to return a freshly allocated string.

— Scheme Procedure: string-concatenate-reverse ls [final_string [end]]
— C Function: scm_string_concatenate_reverse (ls, final_string, end)

Without optional arguments, this procedure is equivalent to
          (string-concatenate (reverse ls))
     
If the optional argument final_string is specified, it is consed onto the beginning to ls before performing the list-reverse and string-concatenate operations. If end is given, only the characters of final_string up to index end are used.
Guaranteed to return a freshly allocated string.

— Scheme Procedure: string-concatenate/shared ls
— C Function: scm_string_concatenate_shared (ls)

Like string-concatenate, but the result may share memory with the strings in the list ls.

— Scheme Procedure: string-concatenate-reverse/shared ls [final_string [end]]
— C Function: scm_string_concatenate_reverse_shared (ls, final_string, end)

Like string-concatenate-reverse, but the result may share memory with the the strings in the ls arguments.

5.5.5.11 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) ).

5.5.5.12 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 s char_pred [start [end]]
— C Function: scm_string_filter (s, char_pred, 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 s char_pred [start [end]]
— C Function: scm_string_delete (s, char_pred, 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.

5.5.5.13 Conversion to/from C

When creating a Scheme string from a C string or when converting a Scheme string to a C string, the concept of character encoding becomes important.

In C, a string is just a sequence of bytes, and the character encoding describes the relation between these bytes and the actual characters that make up the string. For Scheme strings, character encoding is not an issue (most of the time), since in Scheme you never get to see the bytes, only the characters.

Well, ideally, anyway. Right now, Guile simply equates Scheme characters and bytes, ignoring the possibility of multi-byte encodings completely. This will change in the future, where Guile will use Unicode codepoints as its characters and UTF-8 or some other encoding as its internal encoding. When you exclusively use the functions listed in this section, you are `future-proof'.

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, See 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 current locale character encoding.
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.

— 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 in the current locale encoding with the same contents as str. The C string must be freed with free eventually, maybe by using scm_dynwind_free, See 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.

— 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.

5.5.6 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 (see Syntax of Regular Expressions), 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))

5.5.6.1 Regexp Functions

By default, Guile supports POSIX extended regular expressions. That means that the characters `(', `)', `+' and `?' are special, and must be escaped if you wish to match the literal characters.

This regular expression interface was modeled after that implemented by SCSH, the Scheme Shell. It is intended to be upwardly compatible with SCSH regular expressions.

Zero bytes (#\nul) cannot be used in regex patterns or input strings, since the underlying C functions treat that as the end of string. If there's a zero byte an error is thrown.

Patterns and input strings are treated as being in the locale character set if setlocale has been called (see Locales), and in a multibyte locale this includes treating multi-byte sequences as a single character. (Guile strings are currently merely bytes, though this may change in the future, See Conversion to/from C.)

— Scheme Procedure: string-match pattern str [start]

Compile the string pattern into a regular expression and compare it with str. The optional numeric argument start specifies the position of str at which to begin matching.
string-match returns a match structure which describes what, if anything, was matched by the regular expression. See 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.

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 (see 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 (see 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.

— 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
     

— 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)"
     

5.5.6.2 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.

— 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:

— 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”).

— 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.

— 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.

5.5.6.3 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:

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:

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.

5.5.7 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:

Notice how this differs from a string with contents “multiply-by-2”, which is written with double quotation marks, like this:

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.

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 (see Symbol Variables).

5.5.7.1 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:

You can make the mapping more explicit and the code more readable by defining constants:

But the simplest and clearest approach is not to use numbers at all, but symbols whose names specify the colours that they refer to:

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:

Then a car's combined property set could be naturally represented and manipulated as a list of symbols:

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.

5.5.7.2 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:

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:

With a representation like this, it is easy to use the efficient assq-XXX family of procedures (see Association Lists) to extract or change individual pieces of information:

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 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.

5.5.7.3 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 Binding Constructs, for how associations between symbols and variables are created, and Modules, for how those associations are affected by Guile's module system.

5.5.7.4 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 (see 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 string->symbol and string-ci->symbol procedures:

— 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:

From C, there are lower level functions that construct a Scheme symbol from a C string in the current locale encoding.

When you want to do more from C, you should convert between symbols and strings using scm_symbol_to_string and scm_string_to_symbol and work with the strings.

— C Function: scm_from_locale_symbol (const char *name)
— C Function: scm_from_locale_symboln (const char *name, size_t len)

Construct and return a Scheme symbol whose name is specified by name. For scm_from_locale_symbol, name must be null terminated; for scm_from_locale_symboln the length of name is specified explicitly by len.

— 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.

— 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 (see Symbol Uninterned), though they can't be usefully written out and read back in.

5.5.7.5 Function Slots and Property Lists

In traditional Lisp dialects, symbols are often understood as having three kinds of value at once:

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. for a symbol's property list, and 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 Object Properties.

5.5.7.6 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 (see 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.

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.

Although Guile provides this extended read syntax for symbols, widespread usage of it is discouraged because it is not portable and not very readable.

5.5.7.7 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.

5.5.8 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 :.

5.5.8.1 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:

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:

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:

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

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.

5.5.8.2 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.

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:

For further details on let-keywords, define* and other facilities provided by the (ice-9 optargs) module, see Optional Arguments.

5.5.8.3 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 (see 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: (see 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 in User level options interfaces and Reader options. Note that the prefix and postfix syntax are mutually exclusive.

5.5.8.4 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 *str)
— C Function: SCM scm_from_locale_keywordn (const char *str, size_t len)

Equivalent to scm_symbol_to_keyword (scm_from_locale_symbol (str)) and scm_symbol_to_keyword (scm_from_locale_symboln (str, len)), respectively.

5.5.9 “Functionality-Centric” Data Types

Procedures and macros are documented in their own chapter: see Procedures and Macros.

Variable objects are documented as part of the description of Guile's module system: see Variables.

Asyncs, dynamic roots and fluids are described in the chapter on scheduling: see Scheduling.

5.6 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.

5.6.1 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 (see 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.

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 Expression Syntax. The correct way to try these examples is as follows.

A new pair is made by calling the procedure cons with two arguments. Then the argument values are stored into a newly allocated pair, and the pair is returned. The name cons stands for "construct". Use the procedure pair? to test whether a given Scheme object is a pair or not.

— Scheme Procedure: cons x y
— C Function: scm_cons (x, y)

Return a newly allocated pair whose car is x and whose cdr is y. The pair is guaranteed to be different (in the sense of eq?) from every previously existing object.

— Scheme Procedure: pair? x
— C Function: scm_pair_p (x)

Return #t if x is a pair; otherwise return #f.

— C Function: int scm_is_pair (SCM x)

Return 1 when x is a pair; otherwise return 0.

The two parts of a pair are traditionally called car and cdr. They can be retrieved with procedures of the same name (car and cdr), and can be modified with the procedures set-car! and set-cdr!. Since a very common operation in Scheme programs is to access the car of a car of a pair, or the car of the cdr of a pair, etc., the procedures called caar, cadr and so on are also predefined.

— Scheme Procedure: car pair
— Scheme Procedure: cdr pair
— C Function: scm_car (pair)
— C Function: scm_cdr (pair)

Return the car or the cdr of pair, respectively.

— C Macro: SCM SCM_CAR (SCM pair)
— C Macro: SCM SCM_CDR (SCM pair)

These two macros are the fastest way to access the car or cdr of a pair; they can be thought of as compiling into a single memory reference.
These macros do no checking at all. The argument pair must be a valid pair.

— Scheme Procedure: cddr pair
— Scheme Procedure: cdar pair
— Scheme Procedure: cadr pair
— Scheme Procedure: caar pair
— Scheme Procedure: cdddr pair
— Scheme Procedure: cddar pair
— Scheme Procedure: cdadr pair
— Scheme Procedure: cdaar pair
— Scheme Procedure: caddr pair
— Scheme Procedure: cadar pair
— Scheme Procedure: caadr pair
— Scheme Procedure: caaar pair
— Scheme Procedure: cddddr pair
— Scheme Procedure: cdddar pair
— Scheme Procedure: cddadr pair
— Scheme Procedure: cddaar pair
— Scheme Procedure: cdaddr pair
— Scheme Procedure: cdadar pair
— Scheme Procedure: cdaadr pair
— Scheme Procedure: cdaaar pair
— Scheme Procedure: cadddr pair
— Scheme Procedure: caddar pair
— Scheme Procedure: cadadr pair
— Scheme Procedure: cadaar pair
— Scheme Procedure: caaddr pair
— Scheme Procedure: caadar pair
— Scheme Procedure: caaadr pair
— Scheme Procedure: caaaar pair
— C Function: scm_cddr (pair)
— C Function: scm_cdar (pair)
— C Function: scm_cadr (pair)
— C Function: scm_caar (pair)
— C Function: scm_cdddr (pair)
— C Function: scm_cddar (pair)
— C Function: scm_cdadr (pair)
— C Function: scm_cdaar (pair)
— C Function: scm_caddr (pair)
— C Function: scm_cadar (pair)
— C Function: scm_caadr (pair)
— C Function: scm_caaar (pair)
— C Function: scm_cddddr (pair)
— C Function: scm_cdddar (pair)
— C Function: scm_cddadr (pair)
— C Function: scm_cddaar (pair)
— C Function: scm_cdaddr (pair)
— C Function: scm_cdadar (pair)
— C Function: scm_cdaadr (pair)
— C Function: scm_cdaaar (pair)
— C Function: scm_cadddr (pair)
— C Function: scm_caddar (pair)
— C Function: scm_cadadr (pair)
— C Function: scm_cadaar (pair)
— C Function: scm_caaddr (pair)
— C Function: scm_caadar (pair)
— C Function: scm_caaadr (pair)
— C Function: scm_caaaar (pair)

These procedures are compositions of car and cdr, where for example caddr could be defined by
          (define caddr (lambda (x) (car (cdr (cdr x)))))
     
cadr, caddr and cadddr pick out the second, third or fourth elements of a list, respectively. SRFI-1 provides the same under the names second, third and fourth (see 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.

5.6.2 Lists

A very important data type in Scheme—as well as in all other Lisp dialects—is the data type list.⁴

5.6.2.1 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.⁵.

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:

This example also shows that lists have to be quoted when written (see Expression Syntax), because they would otherwise be mistakingly taken as procedure applications (see Simple Invocation).

5.6.2.2 List Predicates

Often it is useful to test whether a given Scheme object is a list or not. List-processing procedures could use this information to test whether their input is valid, or they could do different things depending on the datatype of their arguments.

— Scheme Procedure: list? x
— C Function: scm_list_p (x)

Return #t iff x is a proper list, else #f.

The predicate null? is often used in list-processing code to tell whether a given list has run out of elements. That is, a loop somehow deals with the elements of a list until the list satisfies null?. Then, the algorithm terminates.

— Scheme Procedure: null? x
— C Function: scm_null_p (x)

Return #t iff x is the empty list, else #f.

— C Function: int scm_is_null (SCM x)

Return 1 when x is the empty list; otherwise return 0.

5.6.2.3 List Constructors

This section describes the procedures for constructing new lists. list simply returns a list where the elements are the arguments, cons* is similar, but the last argument is stored in the cdr of the last pair of the list.

— Scheme Procedure: list elem1 ... elemN
— C Function: scm_list_1 (elem1)
— C Function: scm_list_2 (elem1, elem2)
— C Function: scm_list_3 (elem1, elem2, elem3)
— C Function: scm_list_4 (elem1, elem2, elem3, elem4)
— C Function: scm_list_5 (elem1, elem2, elem3, elem4, elem5)
— C Function: scm_list_n (elem1, ..., elemN, SCM_UNDEFINED)

Return a new list containing elements elem1 to elemN.
scm_list_n takes a variable number of arguments, terminated by the special SCM_UNDEFINED. That final SCM_UNDEFINED is not included in the list. None of elem1 to elemN can themselves be SCM_UNDEFINED, or scm_list_n will terminate at that point.

— Scheme Procedure: cons* arg1 arg2 ...

Like list, but the last arg provides the tail of the constructed list, returning (cons arg1(consarg2 (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 (see Copying).

5.6.2.4 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.

5.6.2.5 Append and Reverse

append and append! are used to concatenate two or more lists in order to form a new list. reverse and reverse! return lists with the same elements as their arguments, but in reverse order. The procedure variants with an ! directly modify the pairs which form the list, whereas the other procedures create new pairs. This is why you should be careful when using the side-effecting variants.

— Scheme Procedure: append lst1 ... lstN
— Scheme Procedure: append! lst1 ... lstN
— C Function: scm_append (lstlst)
— C Function: scm_append_x (lstlst)

Return a list comprising all the elements of lists lst1 to lstN.
          (append '(x) '(y))          =>  (x y)
          (append '(a) '(b c d))      =>  (a b c d)
          (append '(a (b)) '((c)))    =>  (a (b) (c))
     
The last argument lstN may actually be any object; an improper list results if the last argument is not a proper list.
          (append '(a b) '(c . d))    =>  (a b c . d)
          (append '() 'a)             =>  a
     
append doesn't modify the given lists, but the return may share structure with the final lstN. append! modifies the given lists to form its return.
For scm_append and scm_append_x, lstlst is a list of the list operands lst1 ... lstN. That lstlst itself is not modified or used in the return.

— Scheme Procedure: reverse lst
— Scheme Procedure: reverse! lst [newtail]
— C Function: scm_reverse (lst)
— C Function: scm_reverse_x (lst, newtail)

Return a list comprising the elements of lst, in reverse order.
reverse constructs a new list, reverse! modifies lst in constructing its return.
For reverse!, the optional newtail is appended to 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.

5.6.2.6 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 (SRFI-1 Deleting), and also an lset-difference which can delete multiple items in one call (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.

5.6.2.7 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 (SRFI-1 Searching).

5.6.2.8 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 (SRFI-1 Fold and Map).

5.6.3 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 (see Arrays).

5.6.3.1 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.

5.6.3.2 Dynamic Vector Creation and Validation

Instead of creating a vector implicitly by using the read syntax just described, you can create a vector dynamically by calling one of the vector and list->vector primitives with the list of Scheme values that you want to place into a vector. The size of the vector thus created is determined implicitly by the number of arguments given.

— Scheme Procedure: vector . l
— Scheme Procedure: list->vector l
— C Function: scm_vector (l)

Return a newly allocated vector composed of the given arguments. Analogous to list.
          (vector 'a 'b 'c) => #(a b c)
     

— 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.

5.6.3.3 Accessing and Modifying Vector Contents

vector-length and vector-ref return information about a given vector, respectively its size and the elements that are contained in the vector.

— Scheme Procedure: vector-length vector
— C Function: scm_vector_length vector

Return the number of elements in vector as an exact integer.

— C Function: size_t scm_c_vector_length (SCM v)

Return the number of elements in vector as a size_t.

— Scheme Procedure: vector-ref vector k
— C Function: scm_vector_ref vector k

Return the contents of position k of vector. k must be a valid index of vector.

          (vector-ref '#(1 1 2 3 5 8 13 21) 5) => 8
          (vector-ref '#(1 1 2 3 5 8 13 21)
              (let ((i (round (* 2 (acos -1)))))
                (if (inexact? i)
                  (inexact->exact i)
                     i))) => 13

— C Function: SCM scm_c_vector_ref (SCM v, size_t k)

Return the contents of position k (a size_t) of vector.

A vector created by one of the dynamic vector constructor procedures (see Vector Creation) can be modified using the following procedures.

NOTE: According to R5RS, it is an error to use any of these procedures on a literally read vector, because such vectors should be considered as constants. Currently, however, Guile does not detect this error.

— Scheme Procedure: vector-set! vector k obj
— C Function: scm_vector_set_x vector k obj

Store obj in position k of vector. k must be a valid index of vector. The value returned by `vector-set!' is unspecified.
          (let ((vec (vector 0 '(2 2 2 2) "Anna")))
            (vector-set! vec 1 '("Sue" "Sue"))
            vec) =>  #(0 ("Sue" "Sue") "Anna")
     

— C Function: void scm_c_vector_set_x (SCM v, size_t k, SCM obj)

Store obj in position k (a size_t) of v.

— Scheme Procedure: vector-fill! v fill
— C Function: scm_vector_fill_x (v, fill)

Store fill in every position of vector. The value returned by vector-fill! is unspecified.

— Scheme Procedure: vector-copy vec
— C Function: scm_vector_copy (vec)

Return a copy of vec.

— Scheme Procedure: vector-move-left! vec1 start1 end1 vec2 start2
— C Function: scm_vector_move_left_x (vec1, start1, end1, vec2, start2)

Copy elements from vec1, positions start1 to end1, to vec2 starting at position start2. start1 and start2 are inclusive indices; end1 is exclusive.
vector-move-left! copies elements in leftmost order. Therefore, in the case where vec1 and vec2 refer to the same vector, vector-move-left! is usually appropriate when start1 is greater than start2.

— Scheme Procedure: vector-move-right! vec1 start1 end1 vec2 start2
— C Function: scm_vector_move_right_x (vec1, start1, end1, vec2, start2)

Copy elements from vec1, positions start1 to end1, to vec2 starting at position start2. start1 and start2 are inclusive indices; end1 is exclusive.
vector-move-right! copies elements in rightmost order. Therefore, in the case where vec1 and vec2 refer to the same vector, vector-move-right! is usually appropriate when start1 is less than start2.

5.6.3.4 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 (see 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 mustr 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 (see 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);
     

5.6.4 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.

Strings could be regarded as uniform vectors of characters, See Strings. Likewise, bit vectors could be regarded as uniform vectors of bits, See Bit Vectors. Both are sufficiently different from uniform numeric vectors that the procedures described here do not apply to these two data types. However, both strings and bit vectors are generalized vectors, See Generalized Vectors, and arrays, See Arrays.

Uniform numeric vectors are the special case of one dimensional uniform numeric arrays.

Uniform numeric vectors can be useful since they consume less memory than the non-uniform, general vectors. Also, since the types they can store correspond directly to C types, it is easier to work with them efficiently on a low level. Consider image processing as an example, where you want to apply a filter to some image. While you could store the pixels of an image in a general vector and write a general convolution function, things are much more efficient with uniform vectors: the convolution function knows that all pixels are unsigned 8-bit values (say), and can use a very tight inner loop.

That is, when it is written in C. Functions for efficiently working with uniform numeric vectors from C are listed at the end of this section.

Procedures similar to the vector procedures (see Vectors) are provided for handling these uniform vectors, but they are distinct datatypes and the two cannot be inter-mixed. If you want to work primarily with uniform numeric vectors, but want to offer support for general vectors as a convenience, you can use one of the scm_any_to_* functions. They will coerce lists and vectors to the given type of uniform vector. Alternatively, you can write two versions of your code: one that is fast and works only with uniform numeric vectors, and one that works with any kind of vector but is slower.

One set of the procedures listed below is a generic one: it works with all types of uniform numeric vectors. In addition to that, there is a set of procedures for each type that only works with that type. Unless you really need to the generality of the first set, it is best to use the more specific functions. They might not be that much faster, but their use can serve as a kind of declaration and makes it easier to optimize later on.

The generic set of procedures uses uniform in its names, the specific ones use the tag from the following table.

The external representation (ie. read syntax) for these vectors is similar to normal Scheme vectors, but with an additional tag from the table above indiciating the vector's type. For example,

Note that the read syntax for floating-point here conflicts with #f for false. In Standard Scheme one can write (1 #f3) for a three element list (1 #f 3), but for Guile (1 #f3) is invalid. (1 #f 3) is almost certainly what one should write anyway to make the intention clear, so this is rarely a problem.

— Scheme Procedure: uniform-vector? obj
— Scheme Procedure: u8vector? obj
— Scheme Procedure: s8vector? obj
— Scheme Procedure: u16vector? obj
— Scheme Procedure: s16vector? obj
— Scheme Procedure: u32vector? obj
— Scheme Procedure: s32vector? obj
— Scheme Procedure: u64vector? obj
— Scheme Procedure: s64vector? obj
— Scheme Procedure: f32vector? obj
— Scheme Procedure: f64vector? obj
— Scheme Procedure: c32vector? obj
— Scheme Procedure: c64vector? obj
— C Function: scm_uniform_vector_p (obj)
— C Function: scm_u8vector_p (obj)
— C Function: scm_s8vector_p (obj)
— C Function: scm_u16vector_p (obj)
— C Function: scm_s16vector_p (obj)
— C Function: scm_u32vector_p (obj)
— C Function: scm_s32vector_p (obj)
— C Function: scm_u64vector_p (obj)
— C Function: scm_s64vector_p (obj)
— C Function: scm_f32vector_p (obj)
— C Function: scm_f64vector_p (obj)
— C Function: scm_c32vector_p (obj)
— C Function: scm_c64vector_p (obj)

Return #t if obj is a homogeneous numeric vector of the indicated type.

— Scheme Procedure: make-u8vector n [value]
— Scheme Procedure: make-s8vector n [value]
— Scheme Procedure: make-u16vector n [value]
— Scheme Procedure: make-s16vector n [value]
— Scheme Procedure: make-u32vector n [value]
— Scheme Procedure: make-s32vector n [value]
— Scheme Procedure: make-u64vector n [value]
— Scheme Procedure: make-s64vector n [value]
— Scheme Procedure: make-f32vector n [value]
— Scheme Procedure: make-f64vector n [value]
— Scheme Procedure: make-c32vector n [value]
— Scheme Procedure: make-c64vector n [value]
— C Function: scm_make_u8vector n [value]
— C Function: scm_make_s8vector n [value]
— C Function: scm_make_u16vector n [value]
— C Function: scm_make_s16vector n [value]
— C Function: scm_make_u32vector n [value]
— C Function: scm_make_s32vector n [value]
— C Function: scm_make_u64vector n [value]
— C Function: scm_make_s64vector n [value]
— C Function: scm_make_f32vector n [value]
— C Function: scm_make_f64vector n [value]
— C Function: scm_make_c32vector n [value]
— C Function: scm_make_c64vector n [value]

Return a newly allocated homogeneous numeric vector holding n elements of the indicated type. If value is given, the vector is initialized with that value, otherwise the contents are unspecified.

— Scheme Procedure: u8vector value ...
— Scheme Procedure: s8vector value ...
— Scheme Procedure: u16vector value ...
— Scheme Procedure: s16vector value ...
— Scheme Procedure: u32vector value ...
— Scheme Procedure: s32vector value ...
— Scheme Procedure: u64vector value ...
— Scheme Procedure: s64vector value ...
— Scheme Procedure: f32vector value ...
— Scheme Procedure: f64vector value ...
— Scheme Procedure: c32vector value ...
— Scheme Procedure: c64vector value ...
— C Function: scm_u8vector (values)
— C Function: scm_s8vector (values)
— C Function: scm_u16vector (values)
— C Function: scm_s16vector (values)
— C Function: scm_u32vector (values)
— C Function: scm_s32vector (values)
— C Function: scm_u64vector (values)
— C Function: scm_s64vector (values)
— C Function: scm_f32vector (values)
— C Function: scm_f64vector (values)
— C Function: scm_c32vector (values)
— C Function: scm_c64vector (values)

Return a newly allocated homogeneous numeric vector of the indicated type, holding the given parameter values. The vector length is the number of parameters given.

— Scheme Procedure: uniform-vector-length vec
— Scheme Procedure: u8vector-length vec
— Scheme Procedure: s8vector-length vec
— Scheme Procedure: u16vector-length vec
— Scheme Procedure: s16vector-length vec
— Scheme Procedure: u32vector-length vec
— Scheme Procedure: s32vector-length vec
— Scheme Procedure: u64vector-length vec
— Scheme Procedure: s64vector-length vec
— Scheme Procedure: f32vector-length vec
— Scheme Procedure: f64vector-length vec
— Scheme Procedure: c32vector-length vec
— Scheme Procedure: c64vector-length vec
— C Function: scm_uniform_vector_length (vec)
— C Function: scm_u8vector_length (vec)
— C Function: scm_s8vector_length (vec)
— C Function: scm_u16vector_length (vec)
— C Function: scm_s16vector_length (vec)
— C Function: scm_u32vector_length (vec)
— C Function: scm_s32vector_length (vec)
— C Function: scm_u64vector_length (vec)
— C Function: scm_s64vector_length (vec)
— C Function: scm_f32vector_length (vec)
— C Function: scm_f64vector_length (vec)
— C Function: scm_c32vector_length (vec)
— C Function: scm_c64vector_length (vec)

Return the number of elements in vec.

— Scheme Procedure: uniform-vector-ref vec i
— Scheme Procedure: u8vector-ref vec i
— Scheme Procedure: s8vector-ref vec i
— Scheme Procedure: u16vector-ref vec i
— Scheme Procedure: s16vector-ref vec i
— Scheme Procedure: u32vector-ref vec i
— Scheme Procedure: s32vector-ref vec i
— Scheme Procedure: u64vector-ref vec i
— Scheme Procedure: s64vector-ref vec i
— Scheme Procedure: f32vector-ref vec i
— Scheme Procedure: f64vector-ref vec i
— Scheme Procedure: c32vector-ref vec i
— Scheme Procedure: c64vector-ref vec i
— C Function: scm_uniform_vector_ref (vec i)
— C Function: scm_u8vector_ref (vec i)
— C Function: scm_s8vector_ref (vec i)
— C Function: scm_u16vector_ref (vec i)
— C Function: scm_s16vector_ref (vec i)
— C Function: scm_u32vector_ref (vec i)
— C Function: scm_s32vector_ref (vec i)
— C Function: scm_u64vector_ref (vec i)
— C Function: scm_s64vector_ref (vec i)
— C Function: scm_f32vector_ref (vec i)
— C Function: scm_f64vector_ref (vec i)
— C Function: scm_c32vector_ref (vec i)
— C Function: scm_c64vector_ref (vec i)

Return the element at index i in vec. The first element in vec is index 0.

— Scheme Procedure: uniform-vector-set! vec i value
— Scheme Procedure: u8vector-set! vec i value
— Scheme Procedure: s8vector-set! vec i value
— Scheme Procedure: u16vector-set! vec i value
— Scheme Procedure: s16vector-set! vec i value
— Scheme Procedure: u32vector-set! vec i value
— Scheme Procedure: s32vector-set! vec i value
— Scheme Procedure: u64vector-set! vec i value
— Scheme Procedure: s64vector-set! vec i value
— Scheme Procedure: f32vector-set! vec i value
— Scheme Procedure: f64vector-set! vec i value
— Scheme Procedure: c32vector-set! vec i value
— Scheme Procedure: c64vector-set! vec i value
— C Function: scm_uniform_vector_set_x (vec i value)
— C Function: scm_u8vector_set_x (vec i value)
— C Function: scm_s8vector_set_x (vec i value)
— C Function: scm_u16vector_set_x (vec i value)
— C Function: scm_s16vector_set_x (vec i value)
— C Function: scm_u32vector_set_x (vec i value)
— C Function: scm_s32vector_set_x (vec i value)
— C Function: scm_u64vector_set_x (vec i value)
— C Function: scm_s64vector_set_x (vec i value)
— C Function: scm_f32vector_set_x (vec i value)
— C Function: scm_f64vector_set_x (vec i value)
— C Function: scm_c32vector_set_x (vec i value)
— C Function: scm_c64vector_set_x (vec i value)

Set the element at index i in vec to value. The first element in vec is index 0. The return value is unspecified.

— Scheme Procedure: uniform-vector->list vec
— Scheme Procedure: u8vector->list vec
— Scheme Procedure: s8vector->list vec
— Scheme Procedure: u16vector->list vec
— Scheme Procedure: s16vector->list vec
— Scheme Procedure: u32vector->list vec
— Scheme Procedure: s32vector->list vec
— Scheme Procedure: u64vector->list vec
— Scheme Procedure: s64vector->list vec
— Scheme Procedure: f32vector->list vec
— Scheme Procedure: f64vector->list vec
— Scheme Procedure: c32vector->list vec
— Scheme Procedure: c64vector->list vec
— C Function: scm_uniform_vector_to_list (vec)
— C Function: scm_u8vector_to_list (vec)
— C Function: scm_s8vector_to_list (vec)
— C Function: scm_u16vector_to_list (vec)
— C Function: scm_s16vector_to_list (vec)
— C Function: scm_u32vector_to_list (vec)
— C Function: scm_s32vector_to_list (vec)
— C Function: scm_u64vector_to_list (vec)
— C Function: scm_s64vector_to_list (vec)
— C Function: scm_f32vector_to_list (vec)
— C Function: scm_f64vector_to_list (vec)
— C Function: scm_c32vector_to_list (vec)
— C Function: scm_c64vector_to_list (vec)

Return a newly allocated list holding all elements of vec.

— Scheme Procedure: list->u8vector lst
— Scheme Procedure: list->s8vector lst
— Scheme Procedure: list->u16vector lst
— Scheme Procedure: list->s16vector lst
— Scheme Procedure: list->u32vector lst
— Scheme Procedure: list->s32vector lst
— Scheme Procedure: list->u64vector lst
— Scheme Procedure: list->s64vector lst
— Scheme Procedure: list->f32vector lst
— Scheme Procedure: list->f64vector lst
— Scheme Procedure: list->c32vector lst
— Scheme Procedure: list->c64vector lst
— C Function: scm_list_to_u8vector (lst)
— C Function: scm_list_to_s8vector (lst)
— C Function: scm_list_to_u16vector (lst)
— C Function: scm_list_to_s16vector (lst)
— C Function: scm_list_to_u32vector (lst)
— C Function: scm_list_to_s32vector (lst)
— C Function: scm_list_to_u64vector (lst)
— C Function: scm_list_to_s64vector (lst)
— C Function: scm_list_to_f32vector (lst)
— C Function: scm_list_to_f64vector (lst)
— C Function: scm_list_to_c32vector (lst)
— C Function: scm_list_to_c64vector (lst)

Return a newly allocated homogeneous numeric vector of the indicated type, initialized with the elements of the list lst.

— Scheme Procedure: any->u8vector obj
— Scheme Procedure: any->s8vector obj
— Scheme Procedure: any->u16vector obj
— Scheme Procedure: any->s16vector obj
— Scheme Procedure: any->u32vector obj
— Scheme Procedure: any->s32vector obj
— Scheme Procedure: any->u64vector obj
— Scheme Procedure: any->s64vector obj
— Scheme Procedure: any->f32vector obj
— Scheme Procedure: any->f64vector obj
— Scheme Procedure: any->c32vector obj
— Scheme Procedure: any->c64vector obj
— C Function: scm_any_to_u8vector (obj)
— C Function: scm_any_to_s8vector (obj)
— C Function: scm_any_to_u16vector (obj)
— C Function: scm_any_to_s16vector (obj)
— C Function: scm_any_to_u32vector (obj)
— C Function: scm_any_to_s32vector (obj)
— C Function: scm_any_to_u64vector (obj)
— C Function: scm_any_to_s64vector (obj)
— C Function: scm_any_to_f32vector (obj)
— C Function: scm_any_to_f64vector (obj)
— C Function: scm_any_to_c32vector (obj)
— C Function: scm_any_to_c64vector (obj)

Return a (maybe newly allocated) uniform numeric vector of the indicated type, initialized with the elements of obj, which must be a list, a vector, or a uniform vector. When obj is already a suitable uniform numeric vector, it is returned unchanged.

— C Function: int scm_is_uniform_vector (SCM uvec)

Return non-zero when uvec is a uniform numeric vector, zero otherwise.

— C Function: SCM scm_take_u8vector (const scm_t_uint8 *data, size_t len)
— C Function: SCM scm_take_s8vector (const scm_t_int8 *data, size_t len)
— C Function: SCM scm_take_u16vector (const scm_t_uint16 *data, size_t len)
— C Function: SCM scm_take_s168vector (const scm_t_int16 *data, size_t len)
— C Function: SCM scm_take_u32vector (const scm_t_uint32 *data, size_t len)
— C Function: SCM scm_take_s328vector (const scm_t_int32 *data, size_t len)
— C Function: SCM scm_take_u64vector (const scm_t_uint64 *data, size_t len)
— C Function: SCM scm_take_s64vector (const scm_t_int64 *data, size_t len)
— C Function: SCM scm_take_f32vector (const float *data, size_t len)
— C Function: SCM scm_take_f64vector (const double *data, size_t len)
— C Function: SCM scm_take_c32vector (const float *data, size_t len)
— C Function: SCM scm_take_c64vector (const double *data, size_t len)

Return a new uniform numeric vector of the indicated type and length that uses the memory pointed to by data to store its elements. This memory will eventually be freed with free. The argument len specifies the number of elements in data, not its size in bytes.
The c32 and c64 variants take a pointer to a C array of floats or doubles. The real parts of the complex numbers are at even indices in that array, the corresponding imaginary parts are at the following odd index.

— C Function: size_t scm_c_uniform_vector_length (SCM uvec)

Return the number of elements of uvec as a size_t.

— C Function: const void * scm_uniform_vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_uint8 * scm_u8vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_int8 * scm_s8vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_uint16 * scm_u16vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_int16 * scm_s16vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_uint32 * scm_u32vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_int32 * scm_s32vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_uint64 * scm_u64vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const scm_t_int64 * scm_s64vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const float * scm_f23vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const double * scm_f64vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const float * scm_c32vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: const double * scm_c64vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)

Like scm_vector_elements (see Vector Accessing from C), but returns a pointer to the elements of a uniform numeric vector of the indicated kind.

— C Function: void * scm_uniform_vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_uint8 * scm_u8vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_int8 * scm_s8vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_uint16 * scm_u16vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_int16 * scm_s16vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_uint32 * scm_u32vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_int32 * scm_s32vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_uint64 * scm_u64vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: scm_t_int64 * scm_s64vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: float * scm_f23vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: double * scm_f64vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: float * scm_c32vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
— C Function: double * scm_c64vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)

Like scm_vector_writable_elements (see Vector Accessing from C), but returns a pointer to the elements of a uniform numeric vector of the indicated kind.

— Scheme Procedure: uniform-vector-read! uvec [port_or_fd [start [end]]]
— C Function: scm_uniform_vector_read_x (uvec, port_or_fd, start, end)

Fill the elements of uvec by reading raw bytes from port-or-fdes, using host byte order.
The optional arguments start (inclusive) and end (exclusive) allow a specified region to be read, leaving the remainder of the vector unchanged.
When port-or-fdes is a port, all specified elements of uvec are attempted to be read, potentially blocking while waiting formore input or end-of-file. When port-or-fd is an integer, a single call to read(2) is made.
An error is signalled when the last element has only been partially filled before reaching end-of-file or in the single call to read(2).
uniform-vector-read! returns the number of elements read.
port-or-fdes may be omitted, in which case it defaults to the value returned by (current-input-port).

— Scheme Procedure: uniform-vector-write uvec [port_or_fd [start [end]]]
— C Function: scm_uniform_vector_write (uvec, port_or_fd, start, end)

Write the elements of uvec as raw bytes to port-or-fdes, in the host byte order.
The optional arguments start (inclusive) and end (exclusive) allow a specified region to be written.
When port-or-fdes is a port, all specified elements of uvec are attempted to be written, potentially blocking while waiting for more room. When port-or-fd is an integer, a single call to write(2) is made.
An error is signalled when the last element has only been partially written in the single call to write(2).
The number of objects actually written is returned. port-or-fdes may be omitted, in which case it defaults to the value returned by (current-output-port).

5.6.5 Bit Vectors

Bit vectors are zero-origin, one-dimensional arrays of booleans. They are displayed as a sequence of 0s and 1s prefixed by #*, e.g.,

Bit vectors are are also generalized vectors, See Generalized Vectors, and can thus be used with the array procedures, See Arrays. Bit vectors are the special case of one dimensional bit arrays.

— Scheme Procedure: bitvector? obj
— C Function: scm_bitvector_p (obj)

Return #t when obj is a bitvector, else return #f.

— C Function: int scm_is_bitvector (SCM obj)

Return 1 when obj is a bitvector, else return 0.

— Scheme Procedure: make-bitvector len [fill]
— C Function: scm_make_bitvector (len, fill)

Create a new bitvector of length len and optionally initialize all elements to fill.

— C Function: SCM scm_c_make_bitvector (size_t len, SCM fill)

Like scm_make_bitvector, but the length is given as a size_t.

— Scheme Procedure: bitvector . bits
— C Function: scm_bitvector (bits)

Create a new bitvector with the arguments as elements.

— Scheme Procedure: bitvector-length vec
— C Function: scm_bitvector_length (vec)

Return the length of the bitvector vec.

— C Function: size_t scm_c_bitvector_length (SCM vec)

Like scm_bitvector_length, but the length is returned as a size_t.

— Scheme Procedure: bitvector-ref vec idx
— C Function: scm_bitvector_ref (vec, idx)

Return the element at index idx of the bitvector vec.

— C Function: SCM scm_c_bitvector_ref (SCM obj, size_t idx)

Return the element at index idx of the bitvector vec.

— Scheme Procedure: bitvector-set! vec idx val
— C Function: scm_bitvector_set_x (vec, idx, val)

Set the element at index idx of the bitvector vec when val is true, else clear it.

— C Function: SCM scm_c_bitvector_set_x (SCM obj, size_t idx, SCM val)

Set the element at index idx of the bitvector vec when val is true, else clear it.

— Scheme Procedure: bitvector-fill! vec val
— C Function: scm_bitvector_fill_x (vec, val)

Set all elements of the bitvector vec when val is true, else clear them.

— Scheme Procedure: list->bitvector list
— C Function: scm_list_to_bitvector (list)

Return a new bitvector initialized with the elements of list.

— Scheme Procedure: bitvector->list vec
— C Function: scm_bitvector_to_list (vec)

Return a new list initialized with the elements of the bitvector vec.

— Scheme Procedure: bit-count bool bitvector
— C Function: scm_bit_count (bool, bitvector)

Return a count of how many entries in bitvector are equal to bool. For example,
          (bit-count #f #*000111000)  => 6
     

— Scheme Procedure: bit-position bool bitvector start
— C Function: scm_bit_position (bool, bitvector, start)

Return the index of the first occurrance 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 (see 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.

5.6.6 Generalized Vectors

Guile has a number of data types that are generally vector-like: strings, uniform numeric vectors, bitvectors, and of course ordinary vectors of arbitrary Scheme values. These types are disjoint: a Scheme value belongs to at most one of the four types listed above.

If you want to gloss over this distinction and want to treat all four types with common code, you can use the procedures in this section. They work with the generalized vector type, which is the union of the four vector-like types.

— Scheme Procedure: generalized-vector? obj
— C Function: scm_generalized_vector_p (obj)

Return #t if obj is a vector, string, bitvector, or uniform numeric vector.

— Scheme Procedure: generalized-vector-length v
— C Function: scm_generalized_vector_length (v)

Return the length of the generalized vector v.

— Scheme Procedure: generalized-vector-ref v idx
— C Function: scm_generalized_vector_ref (v, idx)

Return the element at index idx of the generalized vector v.

— Scheme Procedure: generalized-vector-set! v idx val
— C Function: scm_generalized_vector_set_x (v, idx, val)

Set the element at index idx of the generalized vector v to val.

— Scheme Procedure: generalized-vector->list v
— C Function: scm_generalized_vector_to_list (v)

Return a new list whose elements are the elements of the generalized vector v.

— C Function: int scm_is_generalized_vector (SCM obj)

Return 1 if obj is a vector, string, bitvector, or uniform numeric vector; else return 0.

— C Function: size_t scm_c_generalized_vector_length (SCM v)

Return the length of the generalized vector v.

— C Function: SCM scm_c_generalized_vector_ref (SCM v, size_t idx)

Return the element at index idx of the generalized vector v.

— C Function: void scm_c_generalized_vector_set_x (SCM v, size_t idx, SCM val)

Set the element at index idx of the generalized vector v to val.

— C Function: void scm_generalized_vector_get_handle (SCM v, scm_t_array_handle *handle)

Like scm_array_get_handle but an error is signalled when v is not of rank one. You can use scm_array_handle_ref and scm_array_handle_set to read and write the elements of v, or you can use functions like scm_array_handle_<foo>_elements to deal with specific types of vectors.

5.6.7 Arrays

Arrays are a collection of cells organized into an arbitrary number of dimensions. Each cell can be accessed in constant time by supplying an index for each dimension.

In the current implementation, an array uses a generalized vector for the actual storage of its elements. Any kind of generalized vector will do, so you can have arrays of uniform numeric values, arrays of characters, arrays of bits, and of course, arrays of arbitrary Scheme values. For example, arrays with an underlying c64vector might be nice for digital signal processing, while arrays made from a u8vector might be used to hold gray-scale images.

The number of dimensions of an array is called its rank. Thus, a matrix is an array of rank 2, while a vector has rank 1. When accessing an array element, you have to specify one exact integer for each dimension. These integers are called the indices of the element. An array specifies the allowed range of indices for each dimension via an inclusive lower and upper bound. These bounds can well be negative, but the upper bound must be greater than or equal to the lower bound minus one. When all lower bounds of an array are zero, it is called a zero-origin array.

Arrays can be of rank 0, which could be interpreted as a scalar. Thus, a zero-rank array can store exactly one object and the list of indices of this element is the empty list.

Arrays contain zero elements when one of their dimensions has a zero length. These empty arrays maintain information about their shape: a matrix with zero columns and 3 rows is different from a matrix with 3 columns and zero rows, which again is different from a vector of length zero.

Generalized vectors, such as strings, uniform numeric vectors, bit vectors and ordinary vectors, are the special case of one dimensional arrays.

5.6.7.1 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.

where <rank> is a positive integer in decimal giving the rank of the array. It is omitted when the rank is 1 and the array is non-shared and has zero-origin (see below). For shared arrays and for a non-zero origin, the rank is always printed even when it is 1 to dinstinguish them from ordinary vectors.

The <vectag> part is the tag for a uniform numeric vector, like u8, s16, etc, b for bitvectors, or a for strings. It is empty for ordinary vectors.

The <@lower> part is a `@' character followed by a signed integer in decimal giving the lower bound of a dimension. There is one <@lower> for each dimension. When all lower bounds are zero, all <@lower> parts are omitted.

The <:len> part is a `:' character followed by an unsigned integer in decimal giving the length of a dimension. Like for the lower bounds, there is one <:len> for each dimension, and the <:len> part always follows the <@lower> part for a dimension. Lengths are only then printed when they can't be deduced from the nested lists of elements of the array literal, which can happen when at least one length is zero.

As a special case, an array of rank 0 is printed as #0<vectag>(<scalar>), where <scalar> is the result of printing the single element of the array.

5.6.7.2 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:

The range of each dimension can also be given explicitly, e.g., another way to create the same array:

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 underlaying storage vector is created according to type, which must be a symbol whose name is the `vectag' of the array as explained above, or #t for ordinary, non-specialized arrays.
For example, using the symbol f64 for type will create an array that uses a f64vector for storing its elements, and a will use a string.
When fill is not the special unspecified value, the new array is filled with fill. Otherwise, the initial contents of the array is unspecified. The special unspecified value is stored in the variable *unspecified* so that for example (make-typed-array 'u32 *unspecified* 4) creates a uninitialized u32 vector of length 4.
Each bound may be a positive non-zero integer N, in which case the index for that dimension can range from 0 through N-1; or an explicit index range specifier in the form (LOWER UPPER), where both lower and upper are integers, possibly less than zero, and possibly the same number (however, lower cannot be greater than upper).

— Scheme Procedure: list->array dimspec list

Equivalent to (list->typed-array #t dimspec list).

— Scheme Procedure: list->typed-array type dimspec list
— C Function: scm_list_to_typed_array (type, dimspec, list)

Return an array of the type indicated by type with elements the same as those of list.
The argument dimspec determines the number of dimensions of the array and their lower bounds. When dimspec is an exact integer, it gives the number of dimensions directly and all lower bounds are zero. When it is a list of exact integers, then each element is the lower index bound of a dimension, and there will be as many dimensions as elements in the list.

— Scheme Procedure: array-type array

Return the type of array. This is the `vectag' used for printing array (or #t for ordinary arrays) and can be used with make-typed-array to create an array of the same kind as array.

— Scheme Procedure: array-ref array idx ...

Return the element at (idx ...) in array.

          (define a (make-array 999 '(1 2) '(3 4)))
          (array-ref a 2 4) => 999

— Scheme Procedure: array-in-bounds? array idx ...
— C Function: scm_array_in_bounds_p (array, idxlist)

Return #t if the given index would be acceptable to array-ref.

          (define a (make-array #f '(1 2) '(3 4)))
          (array-in-bounds? a 2 3) => #t
          (array-in-bounds? a 0 0) => #f

— Scheme Procedure: array-set! array obj idx ...
— C Function: scm_array_set_x (array, obj, idxlist)

Set the element at (idx ...) in array to obj. The return value is unspecified.
          (define a (make-array #f '(0 1) '(0 1)))
          (array-set! a #t 1 1)
          a => #2((#f #f) (#f #t))
     

— Scheme Procedure: enclose-array array dim1 ...
— C Function: scm_enclose_array (array, dimlist)

dim1, dim2 ... should be nonnegative integers less than the rank of array. enclose-array returns an array resembling an array of shared arrays. The dimensions of each shared array are the same as the dimth dimensions of the original array, the dimensions of the outer array are the same as those of the original array that did not match a dim.
An enclosed array is not a general Scheme array. Its elements may not be set using array-set!. Two references to the same element of an enclosed array will be equal? but will not in general be eq?. The value returned by array-prototype when given an enclosed array is unspecified.
For example,
          (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1)
          =>
          #<enclosed-array (#1(a d) #1(b e) #1(c f)) (#1(1 4) #1(2 5) #1(3 6))>
          
          (enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 0)
          =>
          #<enclosed-array #2((a 1) (d 4)) #2((b 2) (e 5)) #2((c 3) (f 6))>
     

— Scheme Procedure: array-shape array
— Scheme Procedure: array-dimensions array
— C Function: scm_array_dimensions (array)

Return a list of the bounds for each dimenson of array.
array-shape gives (lower upper) for each dimension. array-dimensions instead returns just upper+1 for dimensions with a 0 lower bound. Both are suitable as input to make-array.
For example,
          (define a (make-array 'foo '(-1 3) 5))
          (array-shape a)      => ((-1 3) (0 4))
          (array-dimensions a) => ((-1 3) 5)
     

— Scheme Procedure: array-rank obj
— C Function: scm_array_rank (obj)

Return the rank of array.

— C Function: size_t scm_c_array_rank (SCM array)

Return the rank of array as a size_t.

— Scheme Procedure: array->list array
— C Function: scm_array_to_list (array)

Return a list consisting of all the elements, in order, of array.

— Scheme Procedure: array-copy! src dst
— Scheme Procedure: array-copy-in-order! src dst
— C Function: scm_array_copy_x (src, dst)

Copy every element from vector or array src to the corresponding element of dst. dst must have the same rank as src, and be at least as large in each dimension. The return value is unspecified.

— Scheme Procedure: array-fill! array fill
— C Function: scm_array_fill_x (array, fill)

Store fill in every element of array. The value returned is unspecified.

— Scheme Procedure: array-equal? array1 array2 ...

Return #t if all arguments are arrays with the same shape, the same type, and have corresponding elements which are either equal? or array-equal?. This function differs from equal? (see Equality) in that a one dimensional shared array may be array-equal? but not equal? to a vector or uniform vector.

— Scheme Procedure: array-map! dst proc src1 ... srcN
— Scheme Procedure: array-map-in-order! dst proc src1 ... srcN
— C Function: scm_array_map_x (dst, proc, srclist)

Set each element of the dst array to values obtained from calls to proc. The value returned is unspecified.
Each call is (proc elem1 ... elemN), where each elem is from the corresponding src array, at the dst index. array-map-in-order! makes the calls in row-major order, array-map! makes them in an unspecified order.
The src arrays must have the same number of dimensions as dst, and must have a range for each dimension which covers the range in dst. This ensures all dst indices are valid in each src.

— Scheme Procedure: array-for-each proc src1 ... srcN
— C Function: scm_array_for_each (proc, src1, srclist)

Apply proc to each tuple of elements of src1 ... srcN, in row-major order. The value returned is unspecified.

— Scheme Procedure: array-index-map! dst proc
— C Function: scm_array_index_map_x (dst, proc)

Set each element of the dst array to values returned by calls to proc. The value returned is unspecified.
Each call is (proc i1 ... iN), where i1...iN is the destination index, one parameter for each dimension. The order in which the calls are made is unspecified.
For example, to create a 4x4 matrix representing a cyclic group,
              / 0 1 2 3 \
              | 1 2 3 0 |
              | 2 3 0 1 |
              \ 3 0 1 2 /
     
          (define a (make-array #f 4 4))
          (array-index-map! a (lambda (i j)
                                (modulo (+ i j) 4)))
     

— Scheme Procedure: uniform-array-read! ra [port_or_fd [start [end]]]
— C Function: scm_uniform_array_read_x (ra, port_or_fd, start, end)

Attempt to read all elements of ura, in lexicographic order, as binary objects from port-or-fdes. If an end of file is encountered, the objects up to that point are put into ura (starting at the beginning) and the remainder of the array is unchanged.
The optional arguments start and end allow a specified region of a vector (or linearized array) to be read, leaving the remainder of the vector unchanged.
uniform-array-read! returns the number of objects read. port-or-fdes may be omitted, in which case it defaults to the value returned by (current-input-port).

— Scheme Procedure: uniform-array-write v [port_or_fd [start [end]]]
— C Function: scm_uniform_array_write (v, port_or_fd, start, end)

Writes all elements of ura as binary objects to port-or-fdes.
The optional arguments start and end allow a specified region of a vector (or linearized array) to be written.
The number of objects actually written is returned. port-or-fdes may be omitted, in which case it defaults to the value returned by (current-output-port).

5.6.7.3 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 (see Array Procedures).
mapfunc translates coordinates from the new array to the oldarray. It's called as (mapfunc newidx1 ...) with one parameter for each dimension of the new array, and should return a list of indices for oldarray, one for each dimension of oldarray.
mapfunc must be affine linear, meaning that each oldarray index must be formed by adding integer multiples (possibly negative) of some or all of newidx1 etc, plus a possible integer offset. The multiples and offset must be the same in each call.
     
     
One good use for a shared array is to restrict the range of some dimensions, so as to apply say array-for-each or array-fill! to only part of an array. The plain list function can be used for mapfunc in this case, making no changes to the index values. For example,
          (make-shared-array #2((a b c) (d e f) (g h i)) list 3 2)
          => #2((a b) (d e) (g h))
     
The new array can have fewer dimensions than oldarray, for example to take a column from an array.
          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i) (list i 2))
                             '(0 2))
          => #1(c f i)
     
A diagonal can be taken by using the single new array index for both row and column in the old array. For example,
          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i) (list i i))
                             '(0 2))
          => #1(a e i)
     
Dimensions can be increased by for instance considering portions of a one dimensional array as rows in a two dimensional array. (array-contents below can do the opposite, flattening an array.)
          (make-shared-array #1(a b c d e f g h i j k l)
                             (lambda (i j) (list (+ (* i 3) j)))
                             4 3)
          => #2((a b c) (d e f) (g h i) (j k l))
     
By negating an index the order that elements appear can be reversed. The following just reverses the column order,
          (make-shared-array #2((a b c) (d e f) (g h i))
                             (lambda (i j) (list i (- 2 j)))
                             3 3)
          => #2((c b a) (f e d) (i h g))
     
A fixed offset on indexes allows for instance a change from a 0 based to a 1 based array,
          (define x #2((a b c) (d e f) (g h i)))
          (define y (make-shared-array x
                                       (lambda (i j) (list (1- i) (1- j)))
                                       '(1 3) '(1 3)))
          (array-ref x 0 0) => a
          (array-ref y 1 1) => a
     
A multiple on an index allows every Nth element of an array to be taken. The following is every third element,
          (make-shared-array #1(a b c d e f g h i j k l)
                             (lambda (i) (list (* i 3)))
                             4)
          => #1(a d g j)
     
The above examples can be combined to make weird and wonderful selections from an array, but it's important to note that because mapfunc must be affine linear, arbitrary permutations are not possible.
In the current implementation, mapfunc is not called for every access to the new array but only on some sample points to establish a base and stride for new array indices in oldarray data. A few sample points are enough because mapfunc is linear.

— Scheme Procedure: shared-array-increments array
— C Function: scm_shared_array_increments (array)

For each dimension, return the distance between elements in the root vector.

— Scheme Procedure: shared-array-offset array
— C Function: scm_shared_array_offset (array)

Return the root vector index of the first element in the array.

— Scheme Procedure: shared-array-root array
— C Function: scm_shared_array_root (array)

Return the root vector of a shared array.

— Scheme Procedure: array-contents array [strict]
— C Function: scm_array_contents (array, strict)

If array may be unrolled into a one dimensional shared array without changing their order (last subscript changing fastest), then array-contents returns that shared array, otherwise it returns #f. All arrays made by make-array and make-typed-array may be unrolled, some arrays made by make-shared-array may not be.
If the optional argument strict is provided, a shared array will be returned only if its elements are stored internally contiguous in memory.

— Scheme Procedure: transpose-array array dim1 ...
— C Function: scm_transpose_array (array, dimlist)

Return an array sharing contents with array, but with dimensions arranged in a different order. There must be one dim argument for each dimension of array. dim1, dim2, ... should be integers between 0 and the rank of the array to be returned. Each integer in that range must appear at least once in the argument list.
The values of dim1, dim2, ... correspond to dimensions in the array to be returned, and their positions in the argument list to dimensions of array. Several dims may have the same value, in which case the returned array will have smaller rank than array.
          (transpose-array '#2((a b) (c d)) 1 0) => #2((a c) (b d))
          (transpose-array '#2((a b) (c d)) 0 0) => #1(a d)
          (transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) =>
                          #2((a 4) (b 5) (c 6))
     

5.6.7.4 Accessing Arrays from C

Arrays, especially uniform numeric arrays, are useful to efficiently represent large amounts of rectangularily organized information, such as matrices, images, or generally blobs of binary data. It is desirable to access these blobs in a C like manner so that they can be handed to external C code such as linear algebra libraries or image processing routines.

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 (see Dynamic Wind).

In addition, array reserving and unreserving must be properly paired. For instance, when reserving two or more arrays in a certain order, you need to unreserve them in the opposite order.

Once you have reserved an array and have retrieved the pointer to its elements, you must figure out the layout of the elements in memory. Guile allows slices to be taken out of arrays without actually making a copy, such as making an alias for the diagonal of a matrix that can be treated as a vector. Arrays that result from such an operation are not stored contiguously in memory and when working with their elements directly, you need to take this into account.

The layout of array elements in memory can be defined via a mapping function that computes a scalar position from a vector of indices. The scalar position then is the offset of the element with the given indices from the start of the storage block of the array.

In Guile, this mapping function is restricted to be affine: all mapping functions of Guile arrays can be written as

p = b +
c[0]*i[0] + c[1]*i[1] + ... + c[n-1]*i[n-1]

where i[k] is the kth index and n is the rank of the array. For example, a matrix of size 3x3 would have b == 0,

c[0] ==
3

and c[1] == 1. When you transpose this matrix (with transpose-array, say), you will get an array whose mapping function has b == 0, c[0] == 1 and c[1] == 3.

The function scm_array_handle_dims gives you (indirect) access to the coefficients c[k].

Note that there are no functions for accessing the elements of a character array yet. Once the string implementation of Guile has been changed to use Unicode, we will provide them.

— C Type: scm_t_array_handle

This is a structure type that holds all information necessary to manage the reservation of arrays as explained above. Structures of this type must be allocated on the stack and must only be accessed by the functions listed below.

— C Function: void scm_array_get_handle (SCM array, scm_t_array_handle *handle)

Reserve array, which must be an array, and prepare handle to be used with the functions below. You must eventually call scm_array_handle_release on handle, and do this in a properly nested fashion, as explained above. The structure pointed to by handle does not need to be initialized before calling this function.

— C Function: void scm_array_handle_release (scm_t_array_handle *handle)

End the array reservation represented by handle. After a call to this function, handle might be used for another reservation.

— C Function: size_t scm_array_handle_rank (scm_t_array_handle *handle)

Return the rank of the array represented by handle.

— C Type: scm_t_array_dim

This structure type holds information about the layout of one dimension of an array. It includes the following fields:

ssize_t lbnd
ssize_t ubnd
The lower and upper bounds (both inclusive) of the permissible index range for the given dimension. Both values can be negative, but lbnd is always less than or equal to ubnd.
ssize_t inc
The distance from one element of this dimension to the next. Note, too, that this can be negative.

— C Function: const scm_t_array_dim * scm_array_handle_dims (scm_t_array_handle *handle)

Return a pointer to a C vector of information about the dimensions of the array represented by handle. This pointer is valid as long as the array remains reserved. As explained above, the scm_t_array_dim structures returned by this function can be used calculate the position of an element in the storage block of the array from its indices.
This position can then be used as an index into the C array pointer returned by the various scm_array_handle_<foo>_elements functions, or with scm_array_handle_ref and scm_array_handle_set.
Here is how one can compute the position pos of an element given its indices in the vector indices:
          ssize_t indices[RANK];
          scm_t_array_dim *dims;
          ssize_t pos;
          size_t i;
          
          pos = 0;
          for (i = 0; i < RANK; i++)
            {
              if (indices[i] < dims[i].lbnd || indices[i] > dims[i].ubnd)
                out_of_range ();
              pos += (indices[i] - dims[i].lbnd) * dims[i].inc;
            }
     

— C Function: ssize_t scm_array_handle_pos (scm_t_array_handle *handle, SCM indices)

Compute the position corresponding to indices, a list of indices. The position is computed as described above for scm_array_handle_dims. The number of the indices and their range is checked and an approrpiate error is signalled for invalid indices.

— C Function: SCM scm_array_handle_ref (scm_t_array_handle *handle, ssize_t pos)

Return the element at position pos in the storage block of the array represented by handle. Any kind of array is acceptable. No range checking is done on pos.

— C Function: void scm_array_handle_set (scm_t_array_handle *handle, ssize_t pos, SCM val)

Set the element at position pos in the storage block of the array represented by handle to val. Any kind of array is acceptable. No range checking is done on pos. An error is signalled when the array can not store val.

— C Function: const SCM * scm_array_handle_elements (scm_t_array_handle *handle)

Return a pointer to the elements of a ordinary array of general Scheme values (i.e., a non-uniform array) for reading. This pointer is valid as long as the array remains reserved.

— C Function: SCM * scm_array_handle_writable_elements (scm_t_array_handle *handle)

Like scm_array_handle_elements, but the pointer is good for reading and writing.

— C Function: const void * scm_array_handle_uniform_elements (scm_t_array_handle *handle)

Return a pointer to the elements of a uniform numeric array for reading. This pointer is valid as long as the array remains reserved. The size of each element is given by scm_array_handle_uniform_element_size.

— C Function: void * scm_array_handle_uniform_writable_elements (scm_t_array_handle *handle)

Like scm_array_handle_uniform_elements, but the pointer is good reading and writing.

— C Function: size_t scm_array_handle_uniform_element_size (scm_t_array_handle *handle)

Return the size of one element of the uniform numeric array represented by handle.

— C Function: const scm_t_uint8 * scm_array_handle_u8_elements (scm_t_array_handle *handle)
— C Function: const scm_t_int8 * scm_array_handle_s8_elements (scm_t_array_handle *handle)
— C Function: const scm_t_uint16 * scm_array_handle_u16_elements (scm_t_array_handle *handle)
— C Function: const scm_t_int16 * scm_array_handle_s16_elements (scm_t_array_handle *handle)
— C Function: const scm_t_uint32 * scm_array_handle_u32_elements (scm_t_array_handle *handle)
— C Function: const scm_t_int32 * scm_array_handle_s32_elements (scm_t_array_handle *handle)
— C Function: const scm_t_uint64 * scm_array_handle_u64_elements (scm_t_array_handle *handle)
— C Function: const scm_t_int64 * scm_array_handle_s64_elements (scm_t_array_handle *handle)
— C Function: const float * scm_array_handle_f32_elements (scm_t_array_handle *handle)
— C Function: const double * scm_array_handle_f64_elements (scm_t_array_handle *handle)
— C Function: const float * scm_array_handle_c32_elements (scm_t_array_handle *handle)
— C Function: const double * scm_array_handle_c64_elements (scm_t_array_handle *handle)

Return a pointer to the elements of a uniform numeric array of the indicated kind for reading. This pointer is valid as long as the array remains reserved.
The pointers for c32 and c64 uniform numeric arrays point to pairs of floating point numbers. The even index holds the real part, the odd index the imaginary part of the complex number.

— C Function: scm_t_uint8 * scm_array_handle_u8_writable_elements (scm_t_array_handle *handle)
— C Function: scm_t_int8 * scm_array_handle_s8_writable_elements (scm_t_array_handle *handle)
— C Function: scm_t_uint16 * scm_array_handle_u16_writable_elements (scm_t_array_handle *handle)
— C Function: scm_t_int16 * scm_array_handle_s16_writable_elements (scm_t_array_handle *handle)
— C Function: scm_t_uint32 * scm_array_handle_u32_writable_elements (scm_t_array_handle *handle)
— C Function: scm_t_int32 * scm_array_handle_s32_writable_elements (scm_t_array_handle *handle)
— C Function: scm_t_uint64 * scm_array_handle_u64_writable_elements (scm_t_array_handle *handle)
— C Function: scm_t_int64 * scm_array_handle_s64_writable_elements (scm_t_array_handle *handle)
— C Function: float * scm_array_handle_f32_writable_elements (scm_t_array_handle *handle)
— C Function: double * scm_array_handle_f64_writable_elements (scm_t_array_handle *handle)
— C Function: float * scm_array_handle_c32_writable_elements (scm_t_array_handle *handle)
— C Function: double * scm_array_handle_c64_writable_elements (scm_t_array_handle *handle)

Like scm_array_handle_<kind>_elements, but the pointer is good for reading and writing.

— C Function: const scm_t_uint32 * scm_array_handle_bit_elements (scm_t_array_handle *handle)

Return a pointer to the words that store the bits of the represented array, which must be a bit array.
Unlike other arrays, bit arrays have an additional offset that must be figured into index calculations. That offset is returned by scm_array_handle_bit_elements_offset.
To find a certain bit you first need to calculate its position as explained above for scm_array_handle_dims and then add the offset. This gives the absolute position of the bit, which is always a non-negative integer.
Each word of the bit array storage block contains exactly 32 bits, with the least significant bit in that word having the lowest absolute position number. The next word contains the next 32 bits.
Thus, the following code can be used to access a bit whose position according to scm_array_handle_dims is given in pos:
          SCM bit_array;
          scm_t_array_handle handle;
          scm_t_uint32 *bits;
          ssize_t pos;
          size_t abs_pos;
          size_t word_pos, mask;
          
          scm_array_get_handle (&bit_array, &handle);
          bits = scm_array_handle_bit_elements (&handle);
          
          pos = ...
          abs_pos = pos + scm_array_handle_bit_elements_offset (&handle);
          word_pos = abs_pos / 32;
          mask = 1L << (abs_pos % 32);
          
          if (bits[word_pos] & mask)
            /* bit is set. */
          
          scm_array_handle_release (&handle);
     

— C Function: scm_t_uint32 * scm_array_handle_bit_writable_elements (scm_t_array_handle *handle)

Like scm_array_handle_bit_elements but the pointer is good for reading and writing. You must take care not to modify bits outside of the allowed index range of the array, even for contiguous arrays.

5.6.8 Records

A record type is a first class object representing a user-defined data type. A record is an instance of a record type.

— Scheme Procedure: record? obj

Return #t if obj is a record of any type and #f otherwise.
Note that record? may be true of any Scheme value; there is no promise that records are disjoint with other Scheme types.

— Scheme Procedure: make-record-type type-name field-names [print]

Create and return a new record-type descriptor.
type-name is a string naming the type. Currently it's only used in the printed representation of records, and in diagnostics. field-names is a list of symbols naming the fields of a record of the type. Duplicates are not allowed among these symbols.
          (make-record-type "employee" '(name age salary))
     
The optional print argument is a function used by display, write, etc, for printing a record of the new type. It's called as (print record port) and should look at record and write to port.

— Scheme Procedure: record-constructor rtd [field-names]

Return a procedure for constructing new members of the type represented by rtd. The returned procedure accepts exactly as many arguments as there are symbols in the given list, field-names; these are used, in order, as the initial values of those fields in a new record, which is returned by the constructor procedure. The values of any fields not named in that list are unspecified. The field-names argument defaults to the list of field names in the call to make-record-type that created the type represented by rtd; if the field-names argument is provided, it is an error if it contains any duplicates or any symbols not in the default list.

— Scheme Procedure: record-predicate rtd

Return a procedure for testing membership in the type represented by rtd. The returned procedure accepts exactly one argument and returns a true value if the argument is a member of the indicated record type; it returns a false value otherwise.

— Scheme Procedure: record-accessor rtd field-name

Return a procedure for reading the value of a particular field of a member of the type represented by rtd. The returned procedure accepts exactly one argument which must be a record of the appropriate type; it returns the current value of the field named by the symbol field-name in that record. The symbol field-name must be a member of the list of field-names in the call to make-record-type that created the type represented by rtd.

— Scheme Procedure: record-modifier rtd field-name

Return a procedure for writing the value of a particular field of a member of the type represented by rtd. The returned procedure accepts exactly two arguments: first, a record of the appropriate type, and second, an arbitrary Scheme value; it modifies the field named by the symbol field-name in that record to contain the given value. The returned value of the modifier procedure is unspecified. The symbol field-name must be a member of the list of field-names in the call to make-record-type that created the type represented by rtd.

— Scheme Procedure: record-type-descriptor record

Return a record-type descriptor representing the type of the given record. That is, for example, if the returned descriptor were passed to record-predicate, the resulting predicate would return a true value when passed the given record. Note that it is not necessarily the case that the returned descriptor is the one that was passed to record-constructor in the call that created the constructor procedure that created the given record.

— Scheme Procedure: record-type-name rtd

Return the type-name associated with the type represented by rtd. The returned value is eqv? to the type-name argument given in the call to make-record-type that created the type represented by rtd.

— Scheme Procedure: record-type-fields rtd

Return a list of the symbols naming the fields in members of the type represented by rtd. The returned value is equal? to the field-names argument given in the call to make-record-type that created the type represented by rtd.

5.6.9 Structures

A structure is a first class data type which holds Scheme values or C words in fields numbered 0 upwards. A vtable represents a structure type, giving field types and permissions, and an optional print function for write etc.

Structures are lower level than records (see Records) but have some extra features. The vtable system allows sets of types be constructed, with class data. The uninterpreted words can inter-operate with C code, allowing arbitrary pointers or other values to be stored along side usual Scheme SCM values.

5.6.9.1 Vtables

A vtable is a structure type, specifying its layout, and other information. A vtable is actually itself a structure, but there's no need to worray about that initially (see Vtable Contents.)

— Scheme Procedure: make-vtable fields [print]

Create a new vtable.
fields is a string describing the fields in the structures to be created. Each field is represented by two characters, a type letter and a permissions letter, for example "pw". The types are as follows.

p – a Scheme value. “p” stands for “protected” meaning it's protected against garbage collection.
u – an arbitrary word of data (an scm_t_bits). At the Scheme level it's read and written as an unsigned integer. “u” stands for “uninterpreted” (it's not treated as a Scheme value), or “unprotected” (it's not marked during GC), or “unsigned long” (its size), or all of these things.
s – a self-reference. Such a field holds the SCM value of the structure itself (a circular reference). This can be useful in C code where you might have a pointer to the data array, and want to get the Scheme SCM handle for the structure. In Scheme code it has no use.

The second letter for each field is a permission code,

w – writable, the field can be read and written.
r – read-only, the field can be read but not written.
o – opaque, the field can be neither read nor written at the Scheme level. This can be used for fields which should only be used from C code.
W,R,O – a tail array, with permissions for the array fields as per w,r,o.

A tail array is further fields at the end of a structure. The last field in the layout string might be for instance `pW' to have a tail of writable Scheme-valued fields. The `pW' field itself holds the tail size, and the tail fields come after it.
Here are some examples.
          (make-vtable "pw")      ;; one writable field
          (make-vtable "prpw")    ;; one read-only and one writable
          (make-vtable "pwuwuw")  ;; one scheme and two uninterpreted
          
          (make-vtable "prpW")    ;; one fixed then a tail array
     
The optional print argument is a function called by display and write (etc) to give a printed representation of a structure created from this vtable. It's called (print struct port) and should look at struct and write to port. The default print merely gives a form like `#<struct ADDR:ADDR>' with a pair of machine addresses.
The following print function for example shows the two fields of its structure.
          (make-vtable "prpw"
                       (lambda (struct port)
                         (display "#<")
                         (display (struct-ref 0))
                         (display " and ")
                         (display (struct-ref 1))
                         (display ">")))
     

5.6.9.2 Structure Basics

This section describes the basic procedures for working with structures. make-struct creates a structure, and struct-ref and struct-set! access write fields.

— Scheme Procedure: make-struct vtable tail-size [init...]
— C Function: scm_make_struct (vtable, tail_size, init_list)

Create a new structure, with layout per the given vtable (see Vtables).
tail-size is the size of the tail array if vtable specifies a tail array. tail-size should be 0 when vtable doesn't specify a tail array.
The optional init... arguments are initial values for the fields of the structure (and the tail array). This is the only way to put values in read-only fields. If there are fewer init arguments than fields then the defaults are #f for a Scheme field (type p) or 0 for an uninterpreted field (type u).
Type s self-reference fields, permission o opaque fields, and the count field of a tail array are all ignored for the init arguments, ie. an argument is not consumed by such a field. An s is always set to the structure itself, an o is always set to #f or 0 (with the intention that C code will do something to it later), and the tail count is always the given tail-size.
For example,
          (define v (make-vtable "prpwpw"))
          (define s (make-struct v 0 123 "abc" 456))
          (struct-ref s 0) => 123
          (struct-ref s 1) => "abc"
     
          (define v (make-vtable "prpW"))
          (define s (make-struct v 6 "fixed field" 'x 'y))
          (struct-ref s 0) => "fixed field"
          (struct-ref s 1) => 2    ;; tail size
          (struct-ref s 2) => x    ;; tail array ...
          (struct-ref s 3) => y
          (struct-ref s 4) => #f
     

— Scheme Procedure: struct? obj
— C Function: scm_struct_p (obj)

Return #t if obj is a structure, or #f if not.

— Scheme Procedure: struct-ref struct n
— C Function: scm_struct_ref (struct, n)

Return the contents of field number n in struct. The first field is number 0.
An error is thrown if n is out of range, or if the field cannot be read because it's o opaque.

— Scheme Procedure: struct-set! struct n value
— C Function: scm_struct_set_x (struct, n, value)

Set field number n in struct to value. The first field is number 0.
An error is thrown if n is out of range, or if the field cannot be written because it's r read-only or o opaque.

— Scheme Procedure: struct-vtable struct
— C Function: scm_struct_vtable (struct)

Return the vtable used by struct.
This can be used to examine the layout of an unknown structure, see Vtable Contents.

5.6.9.3 Vtable Contents

A vtable is itself a structure, with particular fields that hold information about the structures to be created. These include the fields of those structures, and the print function for them. The variables below allow access to those fields.

— Scheme Procedure: struct-vtable? obj
— C Function: scm_struct_vtable_p (obj)

Return #t if obj is a vtable structure.
Note that because vtables are simply structures with a particular layout, struct-vtable? can potentially return true on an application structure which merely happens to look like a vtable.

— Scheme Variable: vtable-index-layout
— C Macro: scm_vtable_index_layout

The field number of the layout specification in a vtable. The layout specification is a symbol like pwpw formed from the fields string passed to make-vtable, or created by make-struct-layout (see Vtable Vtables).
          (define v (make-vtable "pwpw" 0))
          (struct-ref v vtable-index-layout) => pwpw
     
This field is read-only, since the layout of structures using a vtable cannot be changed.

— Scheme Variable: vtable-index-vtable
— C Macro: scm_vtable_index_vtable

A self-reference to the vtable, ie. a type s field. This is used by C code within Guile and has no use at the Scheme level.

— Scheme Variable: vtable-index-printer
— C Macro: scm_vtable_index_printer

The field number of the printer function. This field contains #f if the default print function should be used.
          (define (my-print-func struct port)
            ...)
          (define v (make-vtable "pwpw" my-print-func))
          (struct-ref v vtable-index-printer) => my-print-func
     
This field is writable, allowing the print function to be changed dynamically.

— Scheme Procedure: struct-vtable-name vtable
— Scheme Procedure: set-struct-vtable-name! vtable name
— C Function: scm_struct_vtable_name (vtable)
— C Function: scm_set_struct_vtable_name_x (vtable, name)

Get or set the name of vtable. name is a symbol and is used in the default print function when printing structures created from vtable.
          (define v (make-vtable "pw"))
          (set-struct-vtable-name! v 'my-name)
          
          (define s (make-struct v 0))
          (display s) -| #<my-name b7ab3ae0:b7ab3730>
     

— Scheme Procedure: struct-vtable-tag vtable
— C Function: scm_struct_vtable_tag (vtable)

Return the tag of the given vtable.

5.6.9.4 Vtable Vtables

As noted above, a vtable is a structure and that structure is itself described by a vtable. Such a “vtable of a vtable” can be created with make-vtable-vtable below. This can be used to build sets of related vtables, possibly with extra application fields.

This second level of vtable can be a little confusing. The ball example below is a typical use, adding a “class data” field to the vtables, from which instance structures are created. The current implementation of Guile's own records (see Records) does something similar, a record type descriptor is a vtable with room to hold the field names of the records to be created from it.

— Scheme Procedure: make-vtable-vtable user-fields tail-size [print]
— C Function: scm_make_vtable_vtable (user_fields, tail_size, print_and_init_list)

Create a “vtable-vtable” which can be used to create vtables. This vtable-vtable is also a vtable, and is self-describing, meaning its vtable is itself. The following is a simple usage.
          (define vt-vt (make-vtable-vtable "" 0))
          (define vt    (make-struct vt-vt 0
                                     (make-struct-layout "pwpw"))
          (define s     (make-struct vt 0 123 456))
          
          (struct-ref s 0) => 123
     
make-struct is used to create a vtable from the vtable-vtable. The first initializer is a layout object (field vtable-index-layout), usually obtained from make-struct-layout (below). An optional second initializer is a printer function (field vtable-index-printer), used as described under make-vtable (see Vtables).
     
     
user-fields is a layout string giving extra fields to have in the vtables. A vtable starts with some base fields as per Vtable Contents, and user-fields is appended. The user-fields start at field number vtable-offset-user (below), and exist in both the vtable-vtable and in the vtables created from it. Such fields provide space for “class data”. For example,
          (define vt-of-vt (make-vtable-vtable "pw" 0))
          (define vt       (make-struct vt-of-vt 0))
          (struct-set! vt vtable-offset-user "my class data")
     
tail-size is the size of the tail array in the vtable-vtable itself, if user-fields specifies a tail array. This should be 0 if nothing extra is required or the format has no tail array. The tail array field such as `pW' holds the tail array size, as usual, and is followed by the extra space.
          (define vt-vt (make-vtable-vtable "pW" 20))
          (define my-vt-tail-start (1+ vtable-offset-user))
          (struct-set! vt-vt (+ 3 my-vt-tail-start) "data in tail")
     
The optional print argument is used by display and write (etc) to print the vtable-vtable and any vtables created from it. It's called as (print vtable port) and should look at vtable and write to port. The default is the usual structure print function, which just gives machine addresses.

— Scheme Procedure: make-struct-layout fields
— C Function: scm_make_struct_layout (fields)

Return a structure layout symbol, from a fields string. fields is as described under make-vtable (see Vtables). An invalid fields string is an error.
          (make-struct-layout "prpW") => prpW
          (make-struct-layout "blah") => ERROR
     

— Scheme Variable: vtable-offset-user
— C Macro: scm_vtable_offset_user

The first field in a vtable which is available for application use. Such fields only exist when specified by user-fields in make-vtable-vtable above.

5.6.10 Dictionary Types

A dictionary object is a data structure used to index information in a user-defined way. In standard Scheme, the main aggregate data types are lists and vectors. Lists are not really indexed at all, and vectors are indexed only by number (e.g. (vector-ref foo 5)). Often you will find it useful to index your data on some other type; for example, in a library catalog you might want to look up a book by the name of its author. Dictionaries are used to help you organize information in such a way.

An association list (or alist for short) is a list of key-value pairs. Each pair represents a single quantity or object; the car of the pair is a key which is used to identify the object, and the cdr is the object's value.

A hash table also permits you to index objects with arbitrary keys, but in a way that makes looking up any one object extremely fast. A well-designed hash system makes hash table lookups almost as fast as conventional array or vector references.

Alists are popular among Lisp programmers because they use only the language's primitive operations (lists, car, cdr and the equality primitives). No changes to the language core are necessary. Therefore, with Scheme's built-in list manipulation facilities, it is very convenient to handle data stored in an association list. Also, alists are highly portable and can be easily implemented on even the most minimal Lisp systems.

However, alists are inefficient, especially for storing large quantities of data. Because we want Guile to be useful for large software systems as well as small ones, Guile provides a rich set of tools for using either association lists or hash tables.

5.6.11 Association Lists

An association list is a conventional data structure that is often used to implement simple key-value databases. It consists of a list of entries in which each entry is a pair. The key of each entry is the car of the pair and the value of each entry is the cdr.

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.

5.6.11.1 Alist Key Equality

All of Guile's dedicated association list procedures, apart from acons, come in three flavours, depending on the level of equality that is required to decide whether an existing key in the association list is the same as the key that the procedure call uses to identify the required entry.

acons is an exception because it is used to build association lists which do not require their entries' keys to be unique.

5.6.11.2 Adding or Setting Alist Entries

acons adds a new entry to an association list and returns the combined association list. The combined alist is formed by consing the new entry onto the head of the alist specified in the acons procedure call. So the specified alist is not modified, but its contents become shared with the tail of the combined alist that acons returns.

In the most common usage of acons, a variable holding the original association list is updated with the combined alist:

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.

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:

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:

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.

5.6.11.3 Retrieving Alist Entries

assq, assv and assoc find the entry in an alist for a given key, and return the (key . value) pair. assq-ref, assv-ref and assoc-ref do a similar lookup, but return just the value.

— Scheme Procedure: assq key alist
— Scheme Procedure: assv key alist
— Scheme Procedure: assoc key alist
— C Function: scm_assq (key, alist)
— C Function: scm_assv (key, alist)
— C Function: scm_assoc (key, alist)

Return the first entry in alist with the given key. The return is the pair (KEY . VALUE) from alist. If there's no matching entry the return is #f.
assq compares keys with eq?, assv uses eqv? and assoc uses equal?. See also SRFI-1 which has an extended assoc (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 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.

5.6.11.4 Removing Alist Entries

To remove the element from an association list whose key matches a specified key, use assq-remove!, assv-remove! or assoc-remove! (depending, as usual, on the level of equality required between the key that you specify and the keys in the association list).

As with assq-set! and friends, the specified alist may or may not be modified destructively, and the only safe way to update a variable containing the alist is to set! it to the value that assq-remove! and friends return.

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:

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.

5.6.11.5 Sloppy Alist Functions

sloppy-assq, sloppy-assv and sloppy-assoc behave like the corresponding non-sloppy- procedures, except that they return #f when the specified association list is not well-formed, where the non-sloppy- versions would signal an error.

Specifically, there are two conditions for which the non-sloppy- procedures signal an error, which the sloppy- procedures handle instead by returning #f. Firstly, if the specified alist as a whole is not a proper list:

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.

5.6.11.6 Alist Example

5.6.12 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 (see Lists) for working with them.

Guile provides two types of hashtables. One is an abstract data type that can only be manipulated with the functions in this section. The other type is concrete: it uses a normal vector with alists as elements. The advantage of the abstract hash tables is that they will be automatically resized when they become too full or too empty.

5.6.12.1 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.

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.

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.

There is no procedure for calculating the number of key/value-pairs in a hash table, but hash-fold can be used for doing exactly that.

5.6.12.2 Hash Table Reference

Like the association list functions, the hash table functions come in several varieties, according to the equality test used for the keys. Plain hash- functions use equal?, hashq- functions use eq?, hashv- functions use eqv?, and the hashx- functions use an application supplied test.

A single make-hash-table creates a hash table suitable for use with any set of functions, but it's imperative that just one set is then used consistently, or results will be unpredictable.

Hash tables are implemented as a vector indexed by a hash value formed from the key, with an association list of key/value pairs for each bucket in case distinct keys hash together. Direct access to the pairs in those lists is provided by the -handle- functions. The abstract kind of hash tables hide the vector in an opaque object that represents the hash table, while for the concrete kind the vector is the hashtable.

When the number of table entries in an abstract hash table goes above a threshold, the vector is made larger and the entries are rehashed, to prevent the bucket lists from becoming too long and slowing down accesses. When the number of entries goes below a threshold, the vector is shrunk to save space.

A abstract hash table is created with make-hash-table. To create a vector that is suitable as a hash table, use (make-vector size '()), for example.

For the hashx- “extended” routines, an application supplies a hash function producing an integer index like hashq etc below, and an assoc alist search function like assq etc (see Retrieving Alist Entries). Here's an example of such functions implementing case-insensitive hashing of string keys,

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 (see Symbol Keys), string-hash and string-hash-ci (see String Comparison), and char-set-hash (see Character Set Predicates/Comparison).

— Scheme Procedure: make-hash-table [size]

Create a new abstract hash table object, with an optional minimum vector size.
When size is given, the table vector will still grow and shrink automatically, as described above, but with size as a minimum. If an application knows roughly how many entries the table will hold then it can use size to avoid rehashing when initial entries are added.

— Scheme Procedure: hash-table? obj
— C Function: scm_hash_table_p (obj)

Return #t if obj is a abstract hash table object.

— Scheme Procedure: hash-clear! table
— C Function: scm_hash_clear_x (table)

Remove all items from table (without triggering a resize).

— Scheme Procedure: hash-ref table key [dflt]
— Scheme Procedure: hashq-ref table key [dflt]
— Scheme Procedure: hashv-ref table key [dflt]
— Scheme Procedure: hashx-ref hash assoc table key [dflt]
— C Function: scm_hash_ref (table, key, dflt)
— C Function: scm_hashq_ref (table, key, dflt)
— C Function: scm_hashv_ref (table, key, dflt)
— C Function: scm_hashx_ref (hash, assoc, table, key, dflt)

Lookup key in the given hash table, and return the associated value. If key is not found, return dflt, or #f if dflt is not given.

— Scheme Procedure: hash-set! table key val
— Scheme Procedure: hashq-set! table key val
— Scheme Procedure: hashv-set! table key val
— Scheme Procedure: hashx-set! hash assoc table key val
— C Function: scm_hash_set_x (table, key, val)
— C Function: scm_hashq_set_x (table, key, val)
— C Function: scm_hashv_set_x (table, key, val)
— C Function: scm_hashx_set_x (hash, assoc, table, key, val)

Associate val with key in the given hash table. If key is already present then it's associated value is changed. If it's not present then a new entry is created.

— Scheme Procedure: hash-remove! table key
— Scheme Procedure: hashq-remove! table key
— Scheme Procedure: hashv-remove! table key
— Scheme Procedure: hashx-remove! hash assoc table key
— C Function: scm_hash_remove_x (table, key)
— C Function: scm_hashq_remove_x (table, key)
— C Function: scm_hashv_remove_x (table, key)
— C Function: scm_hashx_remove_x (hash, assoc, table, key)

Remove any association for key in the given hash table. If key is not in table then nothing is done.

— Scheme Procedure: hash key size
— Scheme Procedure: hashq key size
— Scheme Procedure: hashv key size
— C Function: scm_hash (key, size)
— C Function: scm_hashq (key, size)
— C Function: scm_hashv (key, size)

Return a hash value for key. This is a number in the range 0 to size-1, which is suitable for use in a hash table of the given size.
Note that hashq and hashv may use internal addresses of objects, so if an object is garbage collected and re-created it can have a different hash value, even when the two are notionally eq?. For instance with symbols,
          (hashq 'something 123)   => 19
          (gc)
          (hashq 'something 123)   => 62
     
In normal use this is not a problem, since an object entered into a hash table won't be garbage collected until removed. It's only if hashing calculations are somehow separated from normal references that its lifetime needs to be considered.

— Scheme Procedure: hash-get-handle table key
— Scheme Procedure: hashq-get-handle table key
— Scheme Procedure: hashv-get-handle table key
— Scheme Procedure: hashx-get-handle hash assoc table key
— C Function: scm_hash_get_handle (table, key)
— C Function: scm_hashq_get_handle (table, key)
— C Function: scm_hashv_get_handle (table, key)
— C Function: scm_hashx_get_handle (hash, assoc, table, key)

Return the (key . value) pair for key in the given hash table, or #f if key is not in table.

— Scheme Procedure: hash-create-handle! table key init
— Scheme Procedure: hashq-create-handle! table key init
— Scheme Procedure: hashv-create-handle! table key init
— Scheme Procedure: hashx-create-handle! hash assoc table key init
— C Function: scm_hash_create_handle_x (table, key, init)
— C Function: scm_hashq_create_handle_x (table, key, init)
— C Function: scm_hashv_create_handle_x (table, key, init)
— C Function: scm_hashx_create_handle_x (hash, assoc, table, key, init)

Return the (key . value) pair for key in the given hash table. If key is not in table then create an entry for it with init as the value, and return that pair.

— Scheme Procedure: hash-map->list proc table
— Scheme Procedure: hash-for-each proc table
— C Function: scm_hash_map_to_list (proc, table)
— C Function: scm_hash_for_each (proc, table)

Apply proc to the entries in the given hash table. Each call is (proc key value). hash-map->list returns a list of the results from these calls, hash-for-each discards the results and returns an unspecified value.
Calls are made over the table entries in an unspecified order, and for hash-map->list the order of the values in the returned list is unspecified. Results will be unpredictable if table is modified while iterating.
For example the following returns a new alist comprising all the entries from mytable, in no particular order.
          (hash-map->list cons mytable)
     

— Scheme Procedure: hash-for-each-handle proc table
— C Function: scm_hash_for_each_handle (proc, table)

Apply proc to the entries in the given hash table. Each call is (proc handle), where handle is a (key . value) pair. Return an unspecified value.
hash-for-each-handle differs from hash-for-each only in the argument list of proc.

— Scheme Procedure: hash-fold proc init table
— C Function: scm_hash_fold (proc, init, table)

Accumulate a result by applying proc to the elements of the given hash table. Each call is (proc key value prior-result), where key and value are from the table and prior-result is the return from the previous proc call. For the first call, prior-result is the given init value.
Calls are made over the table entries in an unspecified order. Results will be unpredictable if table is modified while hash-fold is running.
For example, the following returns a count of how many keys in mytable are strings.
          (hash-fold (lambda (key value prior)
                       (if (string? key) (1+ prior) prior))
                     0 mytable)
     

5.7 Smobs

This chapter contains reference information related to defining and working with smobs. See 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 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_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.
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_free (scm_t_bits tc, size_t (*free) (SCM obj))

This function sets the smob freeing procedure 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.

— C Function: void scm_set_smob_print (scm_t_bits tc, int (*print) (SCM obj, SCM port, scm_print_state* pstate))

This function sets the smob printing procedure for the smob type specified by the tag tc. tc is the tag returned by scm_make_smob_type.
The print procedure should output a textual representation of the smob instance obj to port, using information in pstate.
The textual representation should be of the form #<name ...>. This ensures that read will not interpret it as some other Scheme value.
It is often best to ignore pstate and just print to port with scm_display, scm_write, scm_simple_format, and scm_puts.

— C Function: void scm_set_smob_equalp (scm_t_bits tc, SCM (*equalp) (SCM obj1, SCM obj1))

This function sets the smob equality-testing predicate for the smob type specified by the tag tc. tc is the tag returned by scm_make_smob_type.
The equalp procedure should return SCM_BOOL_T when obj1 is equal? to obj2. Else it should return SCM_BOOL_F. Both obj1 and obj2 are instances of the smob type tc.

— C Function: void scm_assert_smob_type (scm_t_bits tag, SCM val)

When val is a smob of the type indicated by tag, do nothing. Else, signal an error.

— C Macro: int SCM_SMOB_PREDICATE (scm_t_bits tag, SCM exp)

Return true iff exp is a smob instance of the type indicated by tag. The expression exp can be evaluated more than once, so it shouldn't contain any side effects.

— C Macro: void SCM_NEWSMOB (SCM value, scm_t_bits tag, void *data)
— C Macro: void SCM_NEWSMOB2 (SCM value, scm_t_bits tag, void *data, void *data2)
— C Macro: void SCM_NEWSMOB3 (SCM value, scm_t_bits tag, void *data, void *data2, void *data3)

Make value contain a smob instance of the type with tag tag and smob data data, data2, and data3, as appropriate.
The tag is what has been returned by scm_make_smob_type. The initial values data, data2, and data3 are of type scm_t_bits; when you want to use them for SCM values, these values need to be converted to a scm_t_bits first by using SCM_UNPACK.
The flags of the smob instance start out as zero.

Since it is often the case (e.g., in smob constructors) that you will create a smob instance and return it, there is also a slightly specialized macro for this situation:

— C Macro: SCM_RETURN_NEWSMOB (scm_t_bits tag, void *data)
— C Macro: SCM_RETURN_NEWSMOB2 (scm_t_bits tag, void *data1, void *data2)
— C Macro: SCM_RETURN_NEWSMOB3 (scm_t_bits tag, void *data1, void *data2, void *data3)

This macro expands to a block of code that creates a smob instance of the type with tag tag and smob data data, data2, and data3, as with SCM_NEWSMOB, etc., and causes the surrounding function to return that SCM value. It should be the last piece of code in a block.

— C Macro: scm_t_bits SCM_SMOB_FLAGS (SCM obj)

Return the 16 extra bits of the smob obj. No meaning is predefined for these bits, you can use them freely.

— C Macro: scm_t_bits SCM_SET_SMOB_FLAGS (SCM obj, scm_t_bits flags)

Set the 16 extra bits of the smob obj to flags. No meaning is predefined for these bits, you can use them freely.

— C Macro: scm_t_bits SCM_SMOB_DATA (SCM obj)
— C Macro: scm_t_bits SCM_SMOB_DATA_2 (SCM obj)
— C Macro: scm_t_bits SCM_SMOB_DATA_3 (SCM obj)

Return the first (second, third) immediate word of the smob obj as a scm_t_bits value. When the word contains a SCM value, use SCM_SMOB_OBJECT (etc.) instead.

— C Macro: void SCM_SET_SMOB_DATA (SCM obj, scm_t_bits val)
— C Macro: void SCM_SET_SMOB_DATA_2 (SCM obj, scm_t_bits val)
— C Macro: void SCM_SET_SMOB_DATA_3 (SCM obj, scm_t_bits val)

Set the first (second, third) immediate word of the smob obj to val. When the word should be set to a SCM value, use SCM_SMOB_SET_OBJECT (etc.) instead.

— C Macro: SCM SCM_SMOB_OBJECT (SCM obj)
— C Macro: SCM SCM_SMOB_OBJECT_2 (SCM obj)
— C Macro: SCM SCM_SMOB_OBJECT_3 (SCM obj)

Return the first (second, third) immediate word of the smob obj as a SCM value. When the word contains a scm_t_bits value, use SCM_SMOB_DATA (etc.) instead.

— C Macro: void SCM_SET_SMOB_OBJECT (SCM obj, SCM val)
— C Macro: void SCM_SET_SMOB_OBJECT_2 (SCM obj, SCM val)
— C Macro: void SCM_SET_SMOB_OBJECT_3 (SCM obj, SCM val)

Set the first (second, third) immediate word of the smob obj to val. When the word should be set to a scm_t_bits value, use SCM_SMOB_SET_DATA (etc.) instead.

— C Macro: SCM * SCM_SMOB_OBJECT_LOC (SCM obj)
— C Macro: SCM * SCM_SMOB_OBJECT_2_LOC (SCM obj)
— C Macro: SCM * SCM_SMOB_OBJECT_3_LOC (SCM obj)

Return a pointer to the first (second, third) immediate word of the smob obj. Note that this is a pointer to SCM. If you need to work with scm_t_bits values, use SCM_PACK and SCM_UNPACK, as appropriate.

— Function: SCM scm_markcdr (SCM x)

Mark the references in the smob x, assuming that x's first data word contains an ordinary Scheme object, and x refers to no other objects. This function simply returns x's first data word.

5.8 Procedures and Macros

5.8.1 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 (see 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 evaluation 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.

The fact that the environment in effect when creating a procedure is enclosed in the procedure is shown with this example:

— 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 (see Procedures for On the Fly Evaluation).
body is a sequence of Scheme expressions which are evaluated in order when the procedure is invoked.

5.8.2 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. 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.

scm_c_make_gsubr and scm_c_define_gsubr automatically use scm_c_make_subr and also scm_makcclo if necessary. It is advisable to use the gsubr variants since they provide a slightly higher-level abstraction of the Guile implementation.

5.8.3 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 argument, but the remaining (optional) arguments can be only referenced as a list of values (see Lambda).

Guile comes with the module (ice-9 optargs), which makes using optional arguments much more convenient. In addition, this module provides syntax for handling keywords in argument lists (see Keywords).

Before using any of the procedures or macros defined in this section, you have to load the module (ice-9 optargs) with the statement:

5.8.3.1 let-optional Reference

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* (see Local Bindings).

— library syntax: let-optional rest-arg (binding ...) expr ...
— library syntax: let-optional* rest-arg (binding ...) expr ...

These two macros give you an optional argument interface that is very Schemey and introduces no fancy syntax. They are compatible with the scsh macros of the same name, but are slightly extended. Each of binding may be of one of the forms var or (var default-value). rest-arg should be the rest-argument of the procedures these are used from. The items in rest-arg are sequentially bound to the variable names are given. When rest-arg runs out, the remaining vars are bound either to the default values or #f if no default value was specified. rest-arg remains bound to whatever may have been left of rest-arg.
After binding the variables, the expressions expr ... are evaluated in order.

5.8.3.2 let-keywords Reference

let-keywords and let-keywords* extract values from keyword style argument lists, binding local variables to those values or to defaults.

— library syntax: let-keywords args allow-other-keys? (binding ...) body ...
— library syntax: let-keywords* args allow-other-keys? (binding ...) body ...

args is evaluated and should give a list of the form (#:keyword1 value1 #:keyword2 value2 ...). The bindings are variables and default expressions, with the variables to be set (by name) from the keyword values. The body forms are then evaluated and the last is the result. An example will make the syntax clearest,
          (define args '(#:xyzzy "hello" #:foo "world"))
          
          (let-keywords args #t
                ((foo  "default for foo")
                 (bar  (string-append "default" "for" "bar")))
            (display foo)
            (display ", ")
            (display bar))
          -| world, defaultforbar
     
The binding for foo comes from the #:foo keyword in args. But the binding for bar is the default in the let-keywords, since there's no #:bar in the args.
allow-other-keys? is evaluated and controls whether unknown keywords are allowed in the args list. When true other keys are ignored (such as #:xyzzy in the example), when #f an error is thrown for anything unknown.
let-keywords is like let (see Local Bindings) in that all bindings are made at once, the defaults expressions are evaluated (if needed) outside the scope of the let-keywords.
let-keywords* is like let*, each binding is made successively, and the default expressions see the bindings previously made. This is the style used by lambda* keywords (see lambda* Reference). For example,
          (define args '(#:foo 3))
          
          (let-keywords* args #f
                ((foo  99)
                 (bar  (+ foo 6)))
            (display bar))
          -| 9
     
The expression for each default is only evaluated if it's needed, ie. if the keyword doesn't appear in args. So one way to make a keyword mandatory is to throw an error of some sort as the default.
          (define args '(#:start 7 #:finish 13))
          
          (let-keywords* args #t
                ((start 0)
                 (stop  (error "missing #:stop argument")))
            ...)
          => ERROR: missing #:stop argument
     

5.8.3.3 lambda* Reference

When using optional and keyword argument lists, lambda for creating a procedure then let-optional or let-keywords is a bit lengthy. lambda* combines the features of those macros into a single convenient syntax.

— library syntax: lambda* ([var...][#:optional vardef...][#:key vardef... [#:allow-other-keys]][#:rest var | . var])body

     
     
Create a procedure which takes optional and/or keyword arguments specified with #:optional and #:key. For example,
          (lambda* (a b #:optional c d . e) '())
     
is a procedure with fixed arguments a and b, optional arguments c and d, and rest argument e. If the optional arguments are omitted in a call, the variables for them are bound to #f.
lambda* can also take keyword arguments. For example, a procedure defined like this:
          (lambda* (#:key xyzzy larch) '())
     
can be called with any of the argument lists (#:xyzzy 11), (#:larch 13), (#:larch 42 #:xyzzy 19), (). 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
          (lambda* (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,
          ((lambda* (#:key (heads 0) (tails 0))
             (display (list heads tails)))
           #: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, as per let-optional* and let-keywords*. 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)))
     

5.8.3.4 define* Reference

Just like define has a shorthand notation for defining procedures (see Lambda Alternatives), define* is provided as an abbreviation of the combination of define and lambda*.

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* formals body
— library syntax: define*-public formals body

define* and define*-public support optional arguments with a similar syntax to lambda*. They also support arbitrary-depth currying, just like Guile's define. Some examples:
          (define* (x y #:optional a (z 3) #:key w . u)
             (display (list y z u)))
     
defines a procedure x with a fixed argument y, an optional argument a, another optional argument z with default value 3, a keyword argument w, and a rest argument u.
          (define-public* ((foo #:optional bar) #:optional baz) '())
     
This illustrates currying. A procedure foo is defined, which, when called with an optional argument bar, returns a procedure that takes an optional argument baz.
Of course, define*[-public] also supports #:rest and #:allow-other-keys in the same way as lambda*.

— library syntax: defmacro* name formals body
— library syntax: defmacro*-public name formals body

These are just like defmacro and defmacro-public except that they take lambda*-style extended parameter lists, where #:optional, #:key, #:allow-other-keys and #:rest are allowed with the usual semantics. Here is an example of a macro with an optional argument:
          (defmacro* transmorgify (a #:optional b)
              (a 1))
     

5.8.4 Procedure Properties and Meta-information

Procedures always have attached the environment in which they were created and information about how to apply them to actual arguments. In addition to that, properties and meta-information can be stored with procedures. The procedures in this section can be used to test whether a given procedure satisfies a condition; and to access and set a procedure's property.

The first group of procedures are predicates to test whether a Scheme object is a procedure, or a special procedure, respectively. procedure? is the most general predicates, it returns #t for any kind of procedure. closure? does not return #t for primitive procedures, and thunk? only returns #t for procedures which do not accept any arguments.

— Scheme Procedure: procedure? obj
— C Function: scm_procedure_p (obj)

Return #t if obj is a procedure.

— Scheme Procedure: closure? obj
— C Function: scm_closure_p (obj)

Return #t if obj is a closure.

— Scheme Procedure: thunk? obj
— C Function: scm_thunk_p (obj)

Return #t if obj is a thunk.

Procedure properties are general properties to be attached to 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.

— Scheme Procedure: procedure-environment proc
— C Function: scm_procedure_environment (proc)

Return the environment of the procedure proc.

— Scheme Procedure: procedure-properties proc
— C Function: scm_procedure_properties (proc)

Return obj's property list.

— Scheme Procedure: procedure-property obj key
— C Function: scm_procedure_property (obj, key)

Return the property of obj with name key.

— Scheme Procedure: set-procedure-properties! proc alist
— C Function: scm_set_procedure_properties_x (proc, alist)

Set obj's property list to alist.

— Scheme Procedure: set-procedure-property! obj key value
— C Function: scm_set_procedure_property_x (obj, key, value)

In obj'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.

5.8.5 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.⁶

Also suppose that a corresponding setter procedure called foo-set! does exist.

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.

foo can from now an be used to either read from the data structure stored in f, or to write into the structure.

— 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 either a procedure with setter, or an operator struct.

— Scheme Procedure: setter proc

Return the setter of proc, which must be either a procedure with setter or an operator struct.

5.8.6 Lisp Style Macro Definitions

Macros are objects 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:

In Lisp-like languages, the traditional way to define macros 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:

The difference is analogous to the corresponding difference between Lisp's defun and Scheme's define.

false-if-exception, from the boot-9.scm file in the Guile distribution, is a good example of macro definition using defmacro:

The effect of this definition is that expressions beginning with the identifier false-if-exception are automatically transformed into a catch expression following the macro definition specification. For example:

5.8.7 The R5RS syntax-rules System

R5RS defines an alternative system for macro and syntax transformations using the keywords define-syntax, let-syntax, letrec-syntax and syntax-rules.

The main difference between the R5RS system and the traditional macros of the previous section is how the transformation is specified. In R5RS, rather than permitting a macro definition to return an arbitrary expression, the transformation is specified in a pattern language that

The last point is commonly referred to as being hygienic: the R5RS syntax-case system provides hygienic macros.

For example, the R5RS pattern language for the false-if-exception example of the previous section looks like this:

In Guile, the syntax-rules system is provided by the

(ice-9
syncase)

module. To make these facilities available in your code, include the expression (use-syntax (ice-9 syncase)) (see Using Guile Modules) before the first usage of define-syntax etc. If you are writing a Scheme module, you can alternatively include the form #:use-syntax (ice-9 syncase) in your define-module declaration (see Creating Guile Modules).

5.8.7.1 The syntax-rules Pattern Language

5.8.7.2 Top Level Syntax Definitions

expands at _compile_ or _read_ time (i.e. before any evaluation begins) into some expression that is given by the <transformer-spec>.

5.8.7.3 Local Syntax Definitions

5.8.8 Support for the syntax-case System

5.8.9 Internal Representation of Macros and Syntax

Internally, Guile uses three different flavors of macros. The three flavors are called acro (or syntax), macro and mmacro.

with foo being some flavor of macro, one of the following things will happen when the expression is evaluated.

The key difference between a macro and a mmacro is that the expression returned by a mmacro procedure is remembered (or memoized) so that the expansion does not need to be done again next time the containing code is evaluated.

The primitives procedure->syntax, procedure->macro and procedure->memoizing-macro are used to construct acros, macros and mmacros respectively. However, if you do not have a very special reason to use one of these primitives, you should avoid them: they are very specific to Guile's current implementation and therefore likely to change. Use defmacro, define-macro (see Macros) or define-syntax (see Syntax Rules) instead. (In low level terms, defmacro, define-macro and define-syntax are all implemented as mmacros.)

— Scheme Procedure: procedure->syntax code
— C Function: scm_makacro (code)

Return a macro which, when a symbol defined to this value appears as the first symbol in an expression, returns the result of applying code to the expression and the environment.

— Scheme Procedure: procedure->macro code
— C Function: scm_makmacro (code)

Return a macro which, when a symbol defined to this value appears as the first symbol in an expression, evaluates the result of applying code to the expression and the environment. For example:
          (define trace
            (procedure->macro
              (lambda (x env)
                `(set! ,(cadr x) (tracef ,(cadr x) ',(cadr x))))))
          
          (trace foo)
          ==
          (set! foo (tracef foo 'foo)).
     

— Scheme Procedure: procedure->memoizing-macro code
— C Function: scm_makmmacro (code)

Return a macro which, when a symbol defined to this value appears as the first symbol in an expression, evaluates the result of applying code to the expression and the environment. procedure->memoizing-macro is the same as procedure->macro, except that the expression returned by code replaces the original macro expression in the memoized form of the containing code.

In the following primitives, acro flavor macros are referred to as syntax transformers.

— Scheme Procedure: macro? obj
— C Function: scm_macro_p (obj)

Return #t if obj is a regular macro, a memoizing macro or a syntax transformer.

— Scheme Procedure: macro-type m
— C Function: scm_macro_type (m)

Return one of the symbols syntax, macro or macro!, depending on whether m is a syntax transformer, a regular macro, or a memoizing macro, respectively. If m is not a macro, #f is returned.

— Scheme Procedure: macro-name m
— C Function: scm_macro_name (m)

Return the name of the macro m.

— Scheme Procedure: macro-transformer m
— C Function: scm_macro_transformer (m)

Return the transformer of the macro m.

— Scheme Procedure: cons-source xorig x y
— C Function: scm_cons_source (xorig, x, y)

Create and return a new pair whose car and cdr are x and y. Any source properties associated with xorig are also associated with the new pair.

5.9 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.

5.9.1 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 (see 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 = (see 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. = (see Comparison) or char=? (see 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 == (see 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 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 (see 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?.

5.9.2 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 (see 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:

To create an object property in the first place, use the make-object-property procedure:

— 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.

5.9.2.1 Low Level Property Implementation.

— Scheme Procedure: primitive-make-property not-found-proc
— C Function: scm_primitive_make_property (not_found_proc)

Create a property token that can be used with primitive-property-ref and primitive-property-set!. See primitive-property-ref for the significance of not-found-proc.

— Scheme Procedure: primitive-property-ref prop obj
— C Function: scm_primitive_property_ref (prop, obj)

Return the property prop of obj.
When no value has yet been associated with prop and obj, the not-found-proc from prop is used. A call (not-found-proc prop obj) is made and the result set as the property value. If not-found-proc is #f then #f is the property value.

— Scheme Procedure: primitive-property-set! prop obj val
— C Function: scm_primitive_property_set_x (prop, obj, val)

Set the property prop of obj to val.

— Scheme Procedure: primitive-property-del! prop obj
— C Function: scm_primitive_property_del_x (prop, obj)

Remove any value associated with prop and obj.

5.9.2.2 An Older Approach to Properties

Traditionally, Lisp systems provide a different object property interface to that provided by make-object-property, in which the object property that is being set or retrieved is indicated by a symbol.

Guile includes this older kind of interface as well, but it may well be removed in a future release, as it is less powerful than make-object-property and so increases the size of the Guile library for no benefit. (And it is trivial to write a compatibility layer in Scheme.)

— 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.

5.9.3 Sorting

Sorting is very important in computer programs. Therefore, Guile comes with several sorting procedures built-in. As always, procedures with names ending in ! are side-effecting, that means that they may modify their parameters in order to produce their results.

The first group of procedures can be used to merge two lists (which must be already sorted on their own) and produce sorted lists containing all elements of the input lists.

— Scheme Procedure: merge alist blist less
— C Function: scm_merge (alist, blist, less)

Merge two already sorted lists into one. Given two lists alist and blist, such that (sorted? alist less?) and (sorted? blist less?), return a new list in which the elements of alist and blist have been stably interleaved so that (sorted? (merge alist blist less?) less?). Note: this does _not_ accept vectors.

— Scheme Procedure: merge! alist blist less
— C Function: scm_merge_x (alist, blist, less)

Takes two lists alist and blist such that (sorted? alist less?) and (sorted? blist less?) and returns a new list in which the elements of alist and blist have been stably interleaved so that (sorted? (merge alist blist less?) less?). This is the destructive variant of merge Note: this does _not_ accept vectors.

The following procedures can operate on sequences which are either vectors or list. According to the given arguments, they return sorted vectors or lists, respectively. The first of the following procedures determines whether a sequence is already sorted, the other sort a given sequence. The variants with names starting with stable- are special in that they maintain a special property of the input sequences: If two or more elements are the same according to the comparison predicate, they are left in the same order as they appeared in the input.

— Scheme Procedure: sorted? items less
— C Function: scm_sorted_p (items, less)

Return #t iff items is a list or a vector such that for all 1 <= i <= m, the predicate less returns true when applied to all elements i - 1 and i

— Scheme Procedure: sort items less
— C Function: scm_sort (items, less)

Sort the sequence items, which may be a list or a vector. less is used for comparing the sequence elements. This is not a stable sort.

— Scheme Procedure: sort! items less
— C Function: scm_sort_x (items, less)

Sort the sequence items, which may be a list or a vector. less is used for comparing the sequence elements. The sorting is destructive, that means that the input sequence is modified to produce the sorted result. This is not a stable sort.

— Scheme Procedure: stable-sort items less
— C Function: scm_stable_sort (items, less)

Sort the sequence items, which may be a list or a vector. less is used for comparing the sequence elements. This is a stable sort.

— Scheme Procedure: stable-sort! items less
— C Function: scm_stable_sort_x (items, less)

Sort the sequence items, which may be a list or a vector. less is used for comparing the sequence elements. The sorting is destructive, that means that the input sequence is modified to produce the sorted result. This is a stable sort.

The procedures in the last group only accept lists or vectors as input, as their names indicate.

— Scheme Procedure: sort-list items less
— C Function: scm_sort_list (items, less)

Sort the list items, using less for comparing the list elements. This is a stable sort.

— Scheme Procedure: sort-list! items less
— C Function: scm_sort_list_x (items, less)

Sort the list items, using less for comparing the list elements. The sorting is destructive, that means that the input list is modified to produce the sorted result. This is a stable sort.

— Scheme Procedure: restricted-vector-sort! vec less startpos endpos
— C Function: scm_restricted_vector_sort_x (vec, less, startpos, endpos)

Sort the vector vec, using less for comparing the vector elements. startpos (inclusively) and endpos (exclusively) delimit the range of the vector which gets sorted. The return value is not specified.

5.9.4 Copying Deep Structures

The procedures for copying lists (see 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 a 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.

5.9.5 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).

5.9.6 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.

5.9.6.1 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.

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.

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!.

5.9.6.2 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 (see C Hooks).

— Scheme Procedure: make-hook [n_args]
— C Function: scm_make_hook (n_args)

Create a hook for storing procedure of arity n_args. n_args defaults to zero. The returned value is a hook object to be used with the other hook procedures.

— Scheme Procedure: hook? x
— C Function: scm_hook_p (x)

Return #t if x is a hook, #f otherwise.

— Scheme Procedure: hook-empty? hook
— C Function: scm_hook_empty_p (hook)

Return #t if hook is an empty hook, #f otherwise.

— Scheme Procedure: add-hook! hook proc [append_p]
— C Function: scm_add_hook_x (hook, proc, append_p)

Add the procedure proc to the hook hook. The procedure is added to the end if append_p is true, otherwise it is added to the front. The return value of this procedure is not specified.

— Scheme Procedure: remove-hook! hook proc
— C Function: scm_remove_hook_x (hook, proc)

Remove the procedure proc from the hook hook. The return value of this procedure is not specified.

— Scheme Procedure: reset-hook! hook
— C Function: scm_reset_hook_x (hook)

Remove all procedures from the hook hook. The return value of this procedure is not specified.

— Scheme Procedure: hook->list hook
— C Function: scm_hook_to_list (hook)

Convert the procedure list of hook to a list.

— Scheme Procedure: run-hook hook . args
— C Function: scm_run_hook (hook, args)

Apply all procedures from the hook hook to the arguments args. The order of the procedure application is first to last. The return value of this procedure is not specified.

If, in C code, you are certain that you have a hook object and well formed argument list for that hook, you can also use scm_c_run_hook, which is identical to scm_run_hook but does no type checking.

— C Function: void scm_c_run_hook (SCM hook, SCM args)

The same as scm_run_hook but without any type checking to confirm that hook is actually a hook object and that args is a well-formed list matching the arity of the hook.

— C Macro: int SCM_HOOKP (x)

Return 1 if x is a Scheme-level hook, 0 otherwise.

5.9.6.3 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.

5.9.6.4 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:

— 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.

5.9.6.5 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 (see 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 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 (see 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.

5.9.6.6 Hooks into the Guile REPL

5.10 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.

5.10.1 Top Level Variable Definitions

On the top level of a program (i.e. when not inside the body of a procedure definition or a let, let* or letrec expression), a definition of the form

If the variable already exists, 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

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.

5.10.2 Local Variable Bindings

As opposed to definitions at the top level, which are visible in the whole program (or current module, when Guile modules are used), 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* 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 first local binding construct is let. The other constructs let* and letrec are specialized versions for usage where using plain let is a bit inconvenient.

— 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 storage for the variables is freed.

The init expressions are not allowed to refer to any of the variables.

— 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
     

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 (see Iteration)

5.10.3 Internal definitions

A define form which appears inside the body of a lambda, let, let*, letrec or equivalent expression is called an internal definition. An internal definition differs from a top level definition (see Top Level), because the definition is only visible inside the complete body of the enclosing form. Let us examine the following example.

Here the enclosing form is a let, so the defines 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 it's definition appears lexically after the definition of banana. This is because a sequence of internal definition acts as if it were a letrec expression.

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.

5.10.4 Querying variable bindings

Guile provides a procedure for checking whether a symbol is bound in the top level environment.

— Scheme Procedure: defined? sym [env]
— C Function: scm_defined_p (sym, env)

Return #t if sym is defined in the lexical environment env. When env is not specified, look in the top-level environment as defined by the current module.

5.11 Controlling the Flow of Program Execution

See Control Flow for a discussion of how the more general control flow of Scheme affects C code.