guile/doc/ref/tour.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, 2011,
@c   2012, 2021 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@raisesections

@node Hello Guile!
@section Hello Guile!

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.  It also explains how best to report
any problems that you find.

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::
* Reporting Bugs::              
@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{scheme@@(guile-user)>} prompts:

@example
$ guile
scheme@@(guile-user)> (+ 1 2 3)                ; add some numbers
$1 = 6
scheme@@(guile-user)> (define (factorial n)    ; define a function
                       (if (zero? n) 1 (* n (factorial (- n 1)))))
scheme@@(guile-user)> (factorial 20)
$2 = 2432902008176640000
scheme@@(guile-user)> (getpwnam "root")        ; look in /etc/passwd
$3 = #("root" "x" 0 0 "root" "/root" "/bin/bash")
scheme@@(guile-user)> @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.  @xref{Guile Scripting}, for more details.

@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-@value{EFFECTIVE-VERSION}`
@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
scheme@@(guile-user)> (+ 1 2 3)
$1 = 6
scheme@@(guile-user)> (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{extension}, but it
really just is an ordinary object library.

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

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

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

void
init_bessel ()
@{
  scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
@}
@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 `pkg-config --cflags guile-@value{EFFECTIVE-VERSION}` \
  -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
scheme@@(guile-user)> (load-extension "./libguile-bessel" "init_bessel")
scheme@@(guile-user)> (j0 2)
$1 = 0.223890779141236
@end smallexample

For more on how to install your extension, @pxref{Installing Site
Packages}.


@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.

For more details on the module system beyond this introductory material,
@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
scheme@@(guile-user)> (use-modules (ice-9 popen))
scheme@@(guile-user)> (use-modules (ice-9 rdelim))
scheme@@(guile-user)> (define p (open-input-pipe "ls -l"))
scheme@@(guile-user)> (read-line p)
$1 = "total 30"
scheme@@(guile-user)> (read-line p)
$2 = "drwxr-sr-x    2 mgrabmue mgrabmue     1024 Mar 29 19:57 CVS"
@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/site/foo/bar.scm

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

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

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

For more on how to install your module, @pxref{Installing Site
Packages}.


@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/site/math/bessel.scm

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

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

$ file /usr/local/lib/guile/@value{EFFECTIVE-VERSION}/extensions/libguile-bessel.so
@dots{} ELF 32-bit LSB shared object @dots{}
$ guile
scheme@@(guile-user)> (use-modules (math bessel))
scheme@@(guile-user)> (j0 2)
$1 = 0.223890779141236
@end smallexample

@xref{Foreign Extensions}, for more information.

@lowersections

@node Reporting Bugs
@section Reporting Bugs

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

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

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

@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

Before reporting the bug, 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.

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. You can get this information from invoking
@samp{guile --version} at your shell, or calling @code{(version)} from
within Guile.

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

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

@item
If you installed Guile from a binary package, the version of that
package. On systems that use RPM, use @code{rpm -qa | grep guile}. On systems
that use DPKG, @code{dpkg -l | grep guile}. 

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

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

@item
A complete description of how to reproduce the bug.

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

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

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

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

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



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