guile/doc/ref/intro.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2010
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Introduction
@chapter Introduction

Guile is an implementation of the Scheme programming language.  Scheme
(@url{schemers.org}) is an elegant and conceptually simple dialect of
Lisp, originated by Guy Steele and Gerald Sussman, and since evolved
by the series of reports known as RnRS (the
@tex
Revised$^n$
@end tex
@ifnottex
Revised^n
@end ifnottex
Reports on Scheme).

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

@itemize
@item
it is easy to combine with other code written in C
@item
it has a historical and continuing connection with the GNU Project
@item
it emphasizes interactive and incremental programming
@item
it actually supports several languages, not just Scheme.
@end itemize

@noindent
The next few sections explain what we mean by these points.  The
sections after that cover how you can obtain and install Guile, a tour
of the ways that you can use it, how to report any problems that you
encounter, and the typographical conventions that we use in this
manual.

@menu
* Guile and Scheme::              
* Combining with C::              
* Guile and the GNU Project::
* Interactive Programming::
* Supporting Multiple Languages::
* Obtaining and Installing Guile::  
* Whirlwind Tour::              
* Reporting Bugs::              
* Typographical Conventions::          
@end menu

@node Guile and Scheme
@section Guile and Scheme

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

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

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

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

@node Combining with C
@section Combining with C Code

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

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

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

@node Guile and the GNU Project
@section Guile and the GNU Project

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

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

@node Interactive Programming
@section Interactive Programming

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

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

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

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

@node Supporting Multiple Languages
@section Supporting Multiple Languages

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

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

@node Obtaining and Installing Guile
@section Obtaining and Installing Guile

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

@url{ftp://ftp.gnu.org/pub/gnu/guile/guile-@value{VERSION}.tar.gz}

To unbundle Guile use the instruction

@example
zcat guile-@value{VERSION}.tar.gz | tar xvf -
@end example

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

@example
cd guile-@value{VERSION}
./configure
make
make install
@end example

This will install the Guile executable @file{guile}, the Guile library
@file{libguile} and various associated header files and support libraries. It
will also install the Guile reference manual.

@c [[include instructions for getting R5RS]]

Since this manual frequently refers to the Scheme ``standard'', also
known as R5RS, or the
@tex
``Revised$^5$ Report on the Algorithmic Language Scheme'',
@end tex
@ifnottex
``Revised^5 Report on the Algorithmic Language Scheme'',
@end ifnottex
we have included the report in the Guile distribution; see
@ref{Top, , Introduction, r5rs, Revised(5) Report on the Algorithmic
Language Scheme}.
This will also be installed in your info directory.

@node Whirlwind Tour
@section A Whirlwind Tour

This chapter presents a quick tour of all the ways that Guile can be
used.  There are additional examples in the @file{examples/}
directory in the Guile source distribution.

The following examples assume that Guile has been installed in
@code{/usr/local/}.

@menu
* Running Guile Interactively::
* Running Guile Scripts::
* Linking Guile into Programs::
* Writing Guile Extensions::
* Using the Guile Module System::
@end menu


@node Running Guile Interactively
@subsection 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 @code{$} and
@code{guile>} prompts:

@example
$ guile
guile> (+ 1 2 3)                ; add some numbers
6
guile> (define (factorial n)    ; define a function
         (if (zero? n) 1 (* n (factorial (- n 1)))))
guile> (factorial 20)
2432902008176640000
guile> (getpwnam "jimb")        ; find my entry in /etc/passwd
#("jimb" ".0krIpK2VqNbU" 4008 10 "Jim Blandy" "/u/jimb"
  "/usr/local/bin/bash")
guile> @kbd{C-d}
$
@end example


@node Running Guile Scripts
@subsection Running Guile Scripts

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

Here is a trivial Guile script, for more details @xref{Guile Scripting}.

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


@node Linking Guile into Programs
@subsection 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 @file{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
@code{my-hostname}.

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

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

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

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

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

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

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

@example
$ ./simple-guile
guile> (+ 1 2 3)
6
guile> (my-hostname)
"burns"
@end example

@node Writing Guile Extensions
@subsection 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 @dfn{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 @code{j0} function available to Scheme code.

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

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

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

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

@smallexample
gcc -shared -o libguile-bessel.so -fPIC bessel.c
@end smallexample

For creating shared libraries portably, we recommend the use of GNU
Libtool (@pxref{Top, , Introduction, libtool, GNU Libtool}).

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

@smallexample
$ guile
guile> (load-extension "./libguile-bessel" "init_bessel")
guile> (j0 2)
0.223890779141236
@end smallexample


@node Using the Guile Module System
@subsection Using the Guile Module System

Guile has support for dividing a program into @dfn{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
@xref{Modules}.

@menu
* Using Modules::
* Writing new Modules::
* Putting Extensions into Modules::
@end menu


@node Using Modules
@subsubsection 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
@code{(ice-9 popen)} module which provides the means for communicating
with other processes over pipes together with the @code{(ice-9
rdelim)} module that provides the function @code{read-line}.

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

@node Writing new Modules
@subsubsection Writing new Modules

You can create new modules using the syntactic form
@code{define-module}.  All definitions following this form until the
next @code{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.

@smallexample
$ cat /usr/local/share/guile/foo/bar.scm

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

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

$ guile
guile> (use-modules (foo bar))
guile> (frob 12)
24
@end smallexample

@node Putting Extensions into Modules
@subsubsection 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 @code{load-extension} directly in the body of the module.

@smallexample
$ cat /usr/local/share/guile/math/bessel.scm

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

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

$ file /usr/local/lib/libguile-bessel.so
@dots{} ELF 32-bit LSB shared object @dots{}
$ guile
guile> (use-modules (math bessel))
guile> (j0 2)
0.223890779141236
@end smallexample

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.

@node Reporting Bugs
@section Reporting Bugs

Any problems with the installation should be reported to
@email{bug-guile@@gnu.org}.

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.

@itemize @bullet
@item
Whenever documentation and actual behavior differ, you have certainly
found a bug, either in the documentation or in the program.

@item
When Guile crashes, it is a bug.

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

@item
When calculations produce wrong results, it is a bug.

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

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

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

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.

@itemize @bullet
@item
The version number of Guile.  Without this, we won't know whether there
is any point in looking for the bug in the current version of Guile.

You can get the version number by invoking the command

@example
$ guile --version
Guile 1.9.0
Copyright (c) 1995, 1996, 1997, 2000, 2001, 2002, 2003, 2004,
2005, 2006, 2007, 2008, 2009 Free Software Foundation
Guile may be distributed under the terms of the GNU Lesser General
Public Licence.  For details, see the files `COPYING.LESSER' and
`COPYING', which are included in the Guile distribution.  There is
no warranty, to the extent permitted by law.
@end example

@item
The type of machine you are using, and the operating system name and
version number.  On GNU systems, you can get it with @file{uname}.

@example
$ uname -a
Linux tortoise 2.2.17 #1 Thu Dec 21 17:29:05 CET 2000 i586 unknown
@end example

@item
The operands given to the @file{configure} command when Guile was
installed.  It's often useful to augment this with the output of the
command @code{guile-config info}.

@item
A complete list of any modifications you have made to the Guile source.
(We may not have time to investigate the bug unless it happens in an
unmodified Guile.  But if you've made modifications and you don't tell
us, you are sending us on a wild goose chase.)

Be precise about these changes.  A description in English is not
enough---send a context diff for them.

Adding files of your own, or porting to another machine, is a
modification of the source.

@item
Details of any other deviations from the standard procedure for
installing Guile.

@item
The complete text of any source files needed to reproduce the bug.

If you can tell us a way to cause the problem without loading any source
files, please do so.  This makes it much easier to debug.  If you do
need files, make sure you arrange for us to see their exact contents.

@item
The precise Guile invocation command line we need to type to reproduce
the bug.

@item
A description of what behavior you observe that you believe is
incorrect.  For example, "The Guile process gets a fatal signal," or,
"The resulting output is as follows, which I think is wrong."

Of course, if the bug is that Guile gets a fatal signal, then one can't
miss it.  But if the bug is incorrect results, the maintainer might fail
to notice what is wrong.  Why leave it to chance?

If the manifestation of the bug is a Guile error message, it is
important to report the precise text of the error message, and a
backtrace showing how the Scheme program arrived at the error.

This can be done using the procedure @code{backtrace} in the REPL.

@item
Check whether any programs you have loaded into Guile, including your
@file{.guile} file, set any variables that may affect the functioning of
Guile.  Also, see whether the problem happens in a freshly started Guile
without loading your @file{.guile} file (start Guile with the @code{-q}
switch to prevent loading the init file).  If the problem does
@emph{not} occur then, you must report the precise contents of any
programs that you must load into Guile in order to cause the problem to
occur.

@item
If the problem does depend on an init file or other Scheme programs that
are not part of the standard Guile distribution, then you should make
sure it is not a bug in those programs by complaining to their
maintainers first.  After they verify that they are using Guile in a way
that is supposed to work, they should report the bug.

@item
If you wish to mention something in the Guile source, show the line of
code with a few lines of context.  Don't just give a line number.

The line numbers in the development sources might not match those in your
sources.  It would take extra work for the maintainers to determine what
code is in your version at a given line number, and we could not be
certain.

@item
Additional information from a C debugger such as GDB might enable
someone to find a problem on a machine which he does not have available.
If you don't know how to use GDB, please read the GDB manual---it is not
very long, and using GDB is easy.  You can find the GDB distribution,
including the GDB manual in online form, in most of the same places you
can find the Guile distribution.  To run Guile under GDB, you should
switch to the @file{libguile} subdirectory in which Guile was compiled, then
do @code{gdb guile} or @code{gdb .libs/guile} (if using GNU Libtool).

However, you need to think when you collect the additional information
if you want it to show what causes the bug.

For example, many people send just a backtrace, but that is not very
useful by itself.  A simple backtrace with arguments often conveys
little about what is happening inside Guile, because most of the
arguments listed in the backtrace are pointers to Scheme objects.  The
numeric values of these pointers have no significance whatever; all that
matters is the contents of the objects they point to (and most of the
contents are themselves pointers).
@end itemize


@node Typographical Conventions
@section Typographical Conventions

We use some conventions in this manual.

@itemize @bullet

@item
For some procedures, notably type predicates, we use ``iff'' to mean
``if and only if''.  The construct is usually something like: `Return
@var{val} iff @var{condition}', where @var{val} is usually
``@nicode{#t}'' or ``non-@nicode{#f}''.  This typically means that
@var{val} is returned if @var{condition} holds, and that @samp{#f} is
returned otherwise.  To clarify: @var{val} will @strong{only} be
returned when @var{condition} is true.
@cindex iff

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

The symbol @samp{@result{}} is used to tell which value is returned by
an evaluation:

@lisp
(+ 1 2)
@result{} 3
@end lisp

Some procedures produce some output besides returning a value.  This
is denoted by the symbol @samp{@print{}}.

@lisp
(begin (display 1) (newline) 'hooray)
@print{} 1
@result{} hooray
@end lisp

As you can see, this code prints @samp{1} (denoted by
@samp{@print{}}), and returns @code{hooray} (denoted by
@samp{@result{}}).  Do not confuse the two.

@c Add other conventions here.

@end itemize


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