guile/doc/tutorial/guile-tut.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename guile-tut.info
@settitle Guile Tutorial
@set guile-tut

@include version.texi

@dircategory The Algorithmic Language Scheme
@direntry
* Guile Tutorial: (guile-tut).  The Guile tutorial.
@end direntry

@setchapternewpage off
@c Choices for setchapternewpage are {on,off,odd}.
@paragraphindent 2
@c %**end of header

@iftex
@finalout
@c DL: lose the egregious vertical whitespace, esp. around examples
@c but paras in @defun-like things don't have parindent
@parskip 4pt plus 1pt
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@titlepage
@title Guile Tutorial
@subtitle For use with Guile @value{VERSION}
@subtitle Last updated @value{UPDATED}

@author Mark Galassi
@author Cygnus Solutions and Los Alamos National Laboratory
@author @email{rosalia@@nis.lanl.gov}

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1997, 1998, 2004, 2006 Free Software Foundation

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the author.
@end titlepage


@ifnottex
@node Top
@top Guile Tutorial
@end ifnottex

@ifinfo
This file gives a tutorial introduction to Guile.

Copyright (C) 1997, 2004, 2006 Free Software Foundation

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the author.
@end ifinfo


@menu
* Jump Start::
* Introduction::
* Using Guile to program in Scheme::
* Guile in a Library::
* Regular Expression Support::
* UNIX System Programming::
* Where to find more Guile/Scheme resources::
* Concept Index::
* Procedure and Macro Index::
* Variable Index::
* Type Index::
@end menu


@node Jump Start
@chapter Jump Start

@noindent
Before giving an overview of Guile, I present some simple commands and
programs that you can type to get going immediately.

Start by invoking the Guile interpreter.  Usually you do this by just
typing @code{guile}.  Then type (or paste) the following expressions at
the prompt; the interpreter's response is preceded (in this manual) by
@result{}.

@example
<shell-prompt> guile
@end example
@lisp
(+ 20 35)
@result{} 55
(define (recursive-factorial n)
  (if (zero? n)
      1
      (* n (recursive-factorial (- n 1)))))
(recursive-factorial 5)
@result{} 120
(quit)
@end lisp

In this example we did some simple arithmetic @code{(+ 20 35)} and got
the answer @code{55}.  Then we coded the classic (and rather wasteful)
factorial algorithm and computed the factorial of @code{55}.  Finally we
quit with @code{(quit)}.

@cindex bignumbers
We can find out about some of Scheme's nice features by asking for the
factorial of some big number, say @code{500}.  On some systems the
correct answer will be returned (I do not indicate calling and leaving
the guile session anymore).

@lisp
(recursive-factorial 500)
@result{} 1220136825991110068701238785423046926253574342803192842192413588
   3858453731538819976054964475022032818630136164771482035841633787
   2207817720048078520515932928547790757193933060377296085908627042
   9174547882424912726344305670173270769461062802310452644218878789
   4657547771498634943677810376442740338273653974713864778784954384
   8959553753799042324106127132698432774571554630997720278101456108
   1188373709531016356324432987029563896628911658974769572087926928
   8712817800702651745077684107196243903943225364226052349458501299
   1857150124870696156814162535905669342381300885624924689156412677
   5654481886506593847951775360894005745238940335798476363944905313
   0623237490664450488246650759467358620746379251842004593696929810
   2226397195259719094521782333175693458150855233282076282002340262
   6907898342451712006207714640979456116127629145951237229913340169
   5523638509428855920187274337951730145863575708283557801587354327
   6888868012039988238470215146760544540766353598417443048012893831
   3896881639487469658817504506926365338175055478128640000000000000
   0000000000000000000000000000000000000000000000000000000000000000
   00000000000000000000000000000000000000000000000
@end lisp

The result is an example of Scheme's @emph{bignumbers}.  However, there
are operating environments that provide (by default) too little stack
space.  They will instead produce an error message like this:

@lisp
(recursive-factorial 500)
@print{}
ERROR: Stack overflow
ABORT: (stack-overflow)
@end lisp

Rather than enlarging the system's stack, we can implement the algorithm
such that it does not consume increasing stack space.  This is called a
@emph{tail recursive} implementation.  The following definition is tail
recursive and so should work on all systems.

@lisp
(define (tail-recursive-factorial n)
  (define (loop k l)
    (if (zero? k) l
	(loop (- k 1) (* k l))))
  (loop n 1))

(tail-recursive-factorial 500)
@result{} 1220136825991110068701238785423046926253574342803192842192413588
        ;; ... skipped
@end lisp

This is the most basic use of Guile: a simple Scheme interpreter.  In
the rest of this tutorial I will show you how Guile has many facets: it
is also an @emph{extensible} interpreter (to which many features can be
easilly added) and an @emph{embeddable} interpreter (which can be
invoked from your C programs).


@node Introduction
@chapter Introduction

@noindent
@dfn{Guile} (which can stand for @emph{GNU Ubiquitous Intelligent
Language Extension}) is the GNU extension language.  It started out as
an embeddable Scheme interpreter, and has rapidly evolved into a
kitchen-sink package including a standalone Scheme interpreter, an
embeddable Scheme interpreter, several graphics options, other languages
that can be used along with Scheme (for now just @emph{ctax} and
@emph{Tcl}), and hooks for much more.


@menu
* What are scripting and extension languages::
* History of Guile and its motivations::
* How to characterize Guile::
@end menu

@node What are scripting and extension languages
@section What are scripting and extension languages
@cindex scripting languages
@cindex extension languages

A @dfn{scripting language} is a programming language which serves as
glue between other system programs.  In the UNIX world, the traditional
scripting language is the @emph{Bourne shell}, which allows many UNIX
commands to be executed in sequence, or in a pipeline.  Traditional UNIX
commands are cleverly written to work well when put together in a
script.

Other examples of UNIX scripting languages are AWK, Perl, Scsh (the
Scheme Shell: a Scheme interpreter enhanced to do good scripting),
Python, Tcl, Java @dots{}
@cindex scripting languages - examples

UNIX programmers noticed, more than 25 years ago, that scripting
languages can do serious work, so the Bourne shell was written to have
variables, operators and control structures, just like a full-featured
programming language.
@cindex Bourne shell

What scripting languages have, that traditional programming languages do
not, is the ability to easily run an external program (or a pipeline of
external programs) and use the returned values and output from that
program in useful ways.

An @dfn{extension language} is a programming language interpreter
offered by an application program, so that users can write macros or
even full-fledged programs to extend the original application.
Extension languages have a C interface (it is usually C, but it could be
any other compiled language), and can be given access to the C data
structures.  Likewise, there are C routines to access the extension
language data structures.

Extension languages abound in the software world, even though the name
@emph{extension language} is seldom used.  Examples are:
@cindex extension languages - examples

@itemize @bullet
@item
Emacs Lisp, the language used to program and customize GNU Emacs.
@cindex Emacs Lisp

@item
Tcl, John Ousterhout's general-purpose scripting and extension language.
@cindex Tcl

@item
The Lotus 1-2-3 macro language (any spreadsheet macro language,
really).  I mention this one first because it is a classic, even though
it is seldom used any more.
@cindex Lotus 1-2-3

@item
Other spreadsheet and database macro languages.

@item
The Dominion empire-style game's @emph{exec} files.
@cindex Dominion

@item
Any syntax for a ".*rc" file you might have used.  Almost all programs
end up parsing some kind of startup or configuration file.  The syntax
for those can get pretty involved, thus justifying calling them
"extension languages".  The @emph{fvwm} window manager, for example,
parses a rather elaborate @file{.fvwmrc} file.

@item
Brent Benson's libscheme.a, an embeddable Scheme interpreter.
@cindex Benson, Brent
@cindex libscheme

@item
Guile, the GNU extension language, which is the subject of this
tutorial.

@end itemize

One lesson we can learn from looking at classical large software
applications is that "writers of large programs" always end up throwing
in some kind of parser for configuration or scripting.

Of the examples listed above, Emacs Lisp, Tcl, Libscheme and Guile have
an important property: they are not added as an afterthought for a
specific application.  They are general-purpose languages which a user
can learn (even in college courses) and then use to customize the
application program.

This is a recent and (in my opinion) very exciting direction in
large-program software engineering: program designers can link in the
Guile or Tcl library from the very beginning, and tell their users "You
want to customize this program?  Just use Scheme (or Tcl, or whatever
language), which you already know!"
@cindex large programs


@node History of Guile and its motivations
@section History of Guile and its motivations

A few separate threads of events led to the development of Guile.

In the fall of 1994, Richard Stallman, director of the GNU project,
posted an article with the subject "Why you should not use Tcl", in
which he argued that Tcl is inadequate as an extension language.  This
generated a flurry of flames (available in the hypermail archive
(@url{http://www.vanderburg.org/Tcl/war/}) @strong{The Tcl War}).
@cindex Stallman, Richard
@cindex GNU project
@cindex Tcl

The result was that Stallman then proposed his design for the GNU
Extension Language, first called GEL and then renamed Guile.  The
discussion triggered by that article is also available in a hypermail
archive, @url{http://www.vanderburg.org/Tcl/war2/}.

One interesting feature of this GNU Extension Language plan was that
users should have a @emph{choice} of languages to use in extending their
program.  The basic language would be a slightly modified Scheme, and
translators would be written to convert other languages (like Tcl,
Python, Perl, C-like languages @dots{}) into Scheme.

Tom Lord started working on this project immediately, taking Aubrey
Jaffer's small and portable implementation of Scheme, SCM, and making it
into an embeddable interpreter: callable from C and allowing new Scheme
procedures to be written in C.
@cindex Lord, Tom
@cindex Jaffer, Aubrey

In the spring of 1995, the guile-ii snapshot was released.  This made it
possible to start writing code in C and Scheme using the guile
facilities.

The guile-iii snapshot was released the summer of 1995, and it had fixed
enough problems so that the access to Scheme data structures from C was
almost complete.

After this, Cygnus Support added many features to Guile and finished
implementing others, so that Guile acquired thread support, a regular
expression matcher, a Tk interface, an interface to the SGI OpenGL
graphics system, an @emph{applet} formalism, and some other packages.
This was all in the Cygnus Guile r0.3 and r0.4 releases.
@cindex Cygnus Support

Meanwhile, Tom Lord left the project after having produced a divergent
version of Guile: 1.0b2.  The Free Software Foundation hired Jim Blandy
to coordinate Guile development.  The FSF released its first version of
Guile in January 1997.  In the future, many of the Cygnus packages will
be re-integrated into Guile.
@cindex Blandy, Jim
@cindex Free Software Foundation



@node How to characterize Guile
@section How to characterize Guile

I have already mentioned that Guile has become a kitchen sink package;
here you can see how Guile freely takes new commands and constructs from
the portable Scheme library @emph{slib}, the @emph{Tk} widget set, a
posix library (useful for UNIX systems programming), the regular
expression library @emph{rx}, and many more @dots{}
@cindex slib
@cindex Tk
@cindex POSIX
@c @cindex OpenGL
@cindex rx

So Guile has many more primitive procedures available to it than those
specified in @ref{Standard Procedures, Revised(5) Report on the
Algorithmic Language Scheme, , r5rs, Revised(5) Report on the
Algorithmic Language Scheme}.  On top of that, Guile will interpret
almost all standard Scheme programs.  The only incompatible difference
between the basic Guile language and R5RS Scheme is that Guile is case
sensitive, whereas R5RS is case insensitive.  We hope that few people
have written Scheme programs that depend on case insensitivity.
@cindex case sensitivity
@cindex Revised(5) Report on the Algorithmic Language Scheme
@cindex report on Scheme
@cindex Scheme language - report
@cindex Scheme language - definition

Here is a possible view of the @emph{sum of the parts} in Guile:
@cindex extensions to standard Scheme
@cindex extensions to R5RS
@cindex Scheme extensions
@example
guile   =       standard Scheme (R5RS)
        PLUS    extensions to R5RS offered by SCM
        PLUS    some extra primitives offered by Guile (catch/throw)
        PLUS    portable Scheme library (SLIB)
        PLUS    embeddable Scheme interpreter library (libguile)
        PLUS    Tk toolkit
        PLUS    threads
        PLUS    Posix library
@c         PLUS    OpenGL library (mesa)
@c         PLUS    OpenGL toolkit (glut)
        PLUS    Regular expression library (rx)
@c         PLUS    Applet formalism
        PLUS    Tcl library
@end example


@node Using Guile to program in Scheme
@chapter Using Guile to program in Scheme
@cindex Scheme programming tutorial
@cindex tutorial on Scheme programming

In this section I give a tutorial introduction to programming in Scheme,
with a slant toward the interesting things that can be done in Guile.

@c Applets are so @emph{chic} that they get their own section, but this
This section will try to touch on many of the interesting and cool
aspects of Guile, showing you how new types of problems can be solved
with Guile.  Note that using Guile as a library with @code{libguile.a}
is described in its own chapter (@pxref{Guile in a Library}).  Also note
that some small examples are given in @ref{Jump Start}.

To get started you need to know how to program in @dfn{Scheme} (a
dialect of LISP).  Fortunately Scheme is a small, clean language and is
not hard to learn.  It is also used in many undergraduate courses to
introduce computer programming.
@cindex lisp dialects

I will not try to teach you Scheme here (although you might end up
learning by example), since there are many good books on the subject,
listed in @ref{Where to find more Guile/Scheme resources}. @footnote{To
get started, look at the books @cite{Simply Scheme} and @cite{The Little
Schemer} from that list.}


@subsection Hello World
@cindex hello world

Our first program is the typical Scheme "hello world" program.  Put the
following code in a file called @code{hello.scm} (this can be find in
@file{examples/scheme/hello.scm}).

@smalllisp
#!/usr/local/bin/guile -s
!#

(display "hello world")
(newline)
@end smalllisp

Then run guile on it.  One way to do so is to start up guile and load
this file:

@smallexample
<shell-prompt> @kbd{guile}
guile> @kbd{(load "hello")}
@end smallexample

Another way is to make the file executable and execute it directly.
Notice how Guile recognizes a @code{-s} option which tells it to run a
script and then exit.  Guile also has a new type of block comment
enclosed by @code{#!} and @code{!#}, so that you can make executable
Scheme scripts with the standard UNIX @code{#!} mechanism.

In the given example, the first line is used to invoke the Guile
interpreter (make sure you correct the path if you installed Guile in
something other than /usr/local/bin).  Once Guile is invoked on this
file, it will understand that the first line is a comment.  The comment
is then terminated with @code{!#} on the second line so as to not
interfere with the execution mechanism.


@subsection A bunch of operations in Scheme

Here is some code you can type at the @code{guile>} prompt to see some
of the Scheme data types at work (mostly lists and vectors).  I have
inserted brief comments @emph{before} each line of code explaining what
happens.

@smalllisp
;; @r{make a list and bind it to the symbol @code{ls}}
guile> @kbd{(define ls (list 1 2 3 4 5 6 7))}
       @result{}
;; @r{display the list}
guile> @kbd{ls}
       @result{} (1 2 3 4 5 6 7)
;; @r{ask if @code{ls} is a vector; @code{#f} means it is not}
guile> @kbd{(vector? ls)}
       @result{} #f
;; @r{ask if @code{ls} is a list; @code{#t} means it is}
guile> @kbd{(list? ls)}
       @result{} #t
;; @r{ask for the length of @code{ls}}
guile> @kbd{(length ls)}
       @result{} 7
;; @r{pick out the first element of the list}
guile> @kbd{(car ls)}
       @result{} 1
;; @r{pick the rest of the list without the first element}
guile> @kbd{(cdr ls)}
       @result{} (2 3 4 5 6 7)
;; @r{this should pick out the 3rd element of the list}
guile> @kbd{(car (cdr (cdr ls)))}
       @result{} 3
;; @r{a shorthand for doing the same thing}
guile> @kbd{(caddr ls)}
       @result{} 3
;; @r{append the given list onto @code{ls}, print the result}
;; @r{@strong{NOTE:} the original list @code{ls} is @emph{not} modified}
guile> @kbd{(append ls (list 8 9 10))}
       @result{} (1 2 3 4 5 6 7 8 9 10)
guile> @kbd{(reverse ls)}
       @result{} (7 6 5 4 3 2 1)
;; @r{ask if 12 is in the list --- it obviously is not}
guile> @kbd{(memq 12 ls)}
       @result{} #f
;; @r{ask if 4 is in the list --- returns the list from 4 on.}
;; @r{Notice that the result will behave as true in conditionals}
guile> @kbd{(memq 4 ls)}
       @result{} (4 5 6 7)
;; @r{an @code{if} statement using the aforementioned result}
guile> @kbd{(if (memq 4 ls)
           (display "hey, it's true!\n")
           (display "dude, it's false\n"))}
       @print{hey, it's true!}
       @result{}
guile> @kbd{(if (memq 12 ls)
           (display "hey, it's true!\n")
           (display "dude, it's false\n"))}
       @print{dude, it's false}
       @result{}
guile> @kbd{(memq 4 (reverse ls))}
       @result{} (4 3 2 1)
;; @r{make a smaller list @code{ls2} to work with}
guile> @kbd{(define ls2 (list 2 3 4))}
;; @r{make a list in which the function @code{sin} has been}
;; @r{applied to all elements of @code{ls2}}
guile> @kbd{(map sin ls2)}
       @result{} (0.909297426825682 0.141120008059867 -0.756802495307928)
;; @r{make a list in which the squaring function has been}
;; @r{applied to all elements of @code{ls}}
guile> @kbd{(map (lambda (n) (* n n)) ls)}
       @result{} (1 4 9 16 25 36 49)
@end smalllisp

@smalllisp
;; @r{make a vector and bind it to the symbol @code{v}}
guile> @kbd{(define v '#(1 2 3 4 5 6 7))}
guile> @kbd{v}
       @result{} #(1 2 3 4 5 6 7)
guile> @kbd{(vector? v)}
       @result{} #t
guile> @kbd{(list? v)}
       @result{} #f
guile> @kbd{(vector-length v)}
       @result{} 7
;; @r{vector-ref allows you to pick out elements by index}
guile> @kbd{(vector-ref v 2)}
       @result{} 3
;; @r{play around with the vector: make it into a list, reverse}
;; @r{the list, go back to a vector and take the second element}
guile> @kbd{(vector-ref (list->vector (reverse (vector->list v))) 2)}
       @result{} 5
;; @r{this demonstrates that the entries in a vector do not have}
;; @r{to be of uniform type}
guile> @kbd{(vector-set! v 4 "hi there")}
       @result{} "hi there"
guile> @kbd{v}
       @result{} #(1 2 3 4 "hi there" 6 7)
@end smalllisp


@subsection Using recursion to process lists
@cindex recursion
@cindex list processing

Here are some typical examples of using recursion to process a list.

@smalllisp
;; @r{this is a rather trivial way of reversing a list}
(define (my-reverse l)
  (if (null? l)
      l
      (append (my-reverse (cdr l)) (list (car l)))))
(my-reverse '(27 32 33 40))
@result{} (40 33 32 27)
@end smalllisp


@subsection Processing matrices

Suppose you have a matrix represented as a list of lists:

@smalllisp
(define m
  (list
   (list 7 2 1 3 2 8 5 3 6)
   (list 4 1 1 1 3 8 9 8 1)
   (list 5 5 4 8 1 8 2 2 4)))
@end smalllisp

Then you could apply a certain function to each element of the matrix in
the following manner:
@smalllisp
;; @r{apply the function func to the matrix m element-by-element;}
;; @r{return a matrix with the result.}
(define (process-matrix m func)
  (map (lambda (l)
         (map func l))
       m))
@end smalllisp
Notice that I have used the Scheme @code{map} procedure because I am
interested in the matrix that results from the application of
@code{func}, rather than in the side effects associated with applying
@code{func}.

This could be invoked with @code{(process-matrix m sin)} or
@code{(process-matrix m (lambda (x) (* x x)))}; for example:

@smalllisp
(process-matrix m (lambda (x) (* x x)))
@result{} ((49 4 1 9 4 64 25 9 36) (16 1 1 1 9 64 81 64 1) (25 25 16 64 1 64 4 4 16))
@end smalllisp

To print a representation of the matrix, we could define a generalized
routine:
@smalllisp
;; @r{proc is a procedure to represent the single element,}
;; @r{row-proc is a procedure that is invoked after each row.}
;; @r{Example: proc could be (lambda (x) (begin (display x) (display " ")))}
;; @r{and row-proc could be (lambda (l) (display "\n"))}
(define (represent-matrix m proc row-proc)
  (for-each (lambda (l)
              (begin
                (for-each proc l)
                (row-proc l)))
            m))
@end smalllisp
@findex represent-matrix

And then invoke it with
@smalllisp
(represent-matrix m
                  (lambda (x) (begin (display x) (display " ")))
                  (lambda (l) (begin (display "\n"))))
@print{7 2 1 3 2 8 5 3 6}
@print{4 1 1 1 3 8 9 8 1}
@print{5 5 4 8 1 8 2 2 4}
@end smalllisp

@cindex objects

Now we write a helper routine that uses Scheme @dfn{closures} to make
objects with state that then receive messages to draw little squares.
@cindex closures
@cindex syntactic closures

But let us take it one step at a time.  I will start by showing you a
simple example of object in Scheme.  The object I make here represents a
cell, which could be a cell in a matrix.  The cell responds to commands
to draw itself, to return the next cell, and so forth.  @emph{Guile does
not currently have a Tk interface, so I will leave the hooks for
graphical rendering.  In a future release of Guile I will add graphical
rendering messages to the cell object.}

@smallexample
;; @r{cell-object.scm: routines for creating and manipulating cell objects}

;; @r{(the-x, the-y) is the initial position of the cell.}
;; @r{the-color is a string representing a color; must be something Tk can grok.}
;; @r{square-size is the size of the square that gets drawn.}
;; @r{(sizex, sizey) is the size of the matrix.}
(define (MAKE-CELL the-x the-y the-color square-size sizex sizey)
  (define (get-x) the-x)
  (define (get-y) the-y)

  (define (set-x! new-x)
    (set! the-x new-x)
    the-x)
  (define (set-y! new-y)
    (set! the-y new-y)
    the-y)
  (define (get-color) the-color)
  (define (set-color! new-color)
    (set! the-color new-color)
    the-color)
  (define (next!)
    (set! the-x (+ the-x 1))
    (if (>= the-x sizex)
	(begin
	  (set! the-x 0)
	  (set! the-y (+ the-y 1))))
	(if (>= the-y sizey)
	    (begin
	      (display "CELL next!: value of y is too big; not changing it\n")
	      (set! the-y (- the-y 1))))
	(cons the-x the-y))
  (define (draw)
    (let* ((x0 (* the-x square-size))
	   (y0 (* the-y square-size))
	   (x1 (+ x0 square-size))
	   (y1 (+ y0 square-size)))
      (display "I should draw a ")
      (display the-color)
      (display " rectangle with corners at ")
      (display x0) (display y0) (display x1) (display y1)
      ))

  ;; self is the dispatch procedure
  (define (self message)
    (case message
      ((x)            get-x)
      ((y)            get-y)
      ((set-x!)       set-x!)
      ((set-y!)       set-y!)
      ((color)        get-color)
      ((set-color!)   set-color!)
      ((next!)        next!)
      ((draw)         draw)
      (else (error "CELL: Unknown message -> " message))))
  ;; and now return the dispatch procedure
  self
  )
@end smallexample
@cindex cell-object
@findex MAKE-CELL

What does this procedure do?  It returns another procedure
(@code{self}) which receives a message (x, y, set-x!, set-y!, @dots{})
and takes an action to return or modify its state.  The state consists
of the values of variables @code{the-x}, @code{the-y}, @code{the-color}
and so forth.

Here are some examples of how to use MAKE-CELL and the cell object it
creates:
@smallexample
(define c (MAKE-CELL 0 0 "red" 10 7 9))

;; @r{retrieve the x and y coordinates}
((c 'x))
@result{} 0
((c 'y))
@result{} 0
;; @r{change the x coordinate}
((c 'set-x!) 5)
@result{} 5
((c 'x))
@result{} 5
;; @r{change the color}
((c 'color))
@result{} "red"
((c 'set-color!) "green")
@result{} "green"
((c 'color))
@result{} "green"
;; @r{now use the next! message to move to the next cell}
((c 'next!))
@result{} (6 . 0)
((c 'x))
@result{} 6
((c 'y))
@result{} 0
;; @r{now make things wrap around}
((c 'next!))
@result{} (0 . 1)
((c 'next!))
@result{} (1 . 1)
((c 'next!))
@result{} (2 . 1)
((c 'x))
@result{} 2
((c 'y))
@result{} 1
@end smallexample

You will notice that expressions like @code{(c 'next)} return procedures
that do the job, so we have to use extra parentheses to make the job
happen.  This syntax is rather awkward; one way around it is to define a
@code{send} procedure:

@smallexample
;; @r{send makes object syntax a bit easier; instead of saying}
;; @r{    ((my-cell 'set-x!) 4)}
;; @r{you can say}
;; @r{    (send my-cell 'set-x! 4)}
(define (send obj . args)
  (let ((first-eval (apply obj (list (car args)))))
    (if (null? (cdr args))
	(first-eval)
	(apply first-eval (cdr args)))))
@end smallexample
@findex send

You can see that @code{send} passes the message to the object, making
sure that things are evaluated the proper number of times.  You can now
type:

@smallexample
(define c2 (MAKE-CELL 0 0 "red" 10 7 9))
(send c2 'x)
@result{} 0
(send c2 'set-x! 5)
@result{} 5
(send c2 'color)
@result{} "red"
(send c2 'set-color! "green")
@result{} "green"
(send c2 'next!)
@result{} (1 . 0)
(send c2 'x)
@result{} 1
(send c2 'y)
@result{} 0
@end smallexample

@cindex object-based programming
@cindex object-oriented programming

This is the simplest way of implementing objects in Scheme, but it does
not really allow for full @emph{object-oriented programming} (for
example, there is no inheritance).  But it is useful for
@emph{object-based programming}.

Guile comes with a couple more complete object-oriented extensions to
Scheme: these are part of slib (@pxref{Object, , , slib, SLIB: the
portable Scheme library} and @pxref{Yasos, , , slib, SLIB: the portable
Scheme library}).

@node Guile in a Library
@chapter Guile in a Library

@iftex
@nobreak
@end iftex
In the previous chapters Guile was used to write programs entirely in
Scheme, and no C code was seen; but I have been claiming @emph{ad
nauseam} that Guile is an @emph{extension} language.  Here we see how
that is done, and how that can be useful.
@cindex libguile
@cindex extending C programs


@menu
* Two world views::
* What is libguile::
* How to get started with libguile::
* More interesting programming with libguile::
* Further examples::
@end menu

@node Two world views
@section Two world views
@cindex master world

In this manual, I usually jump into examples and explain them as you
type in the code; here I will digress and ramble for a few paragraphs to
set some concepts straight, and then let you type (or paste) in fun
examples.

In 1995, I implemented a large program, @dfn{Gnudl}, using Guile quite
extensively.  In the design phase of Gnudl, I found I had to make a
choice: should the fundamental data structures be C or Scheme data
structures?
@cindex gnudl
@cindex GNU Data Language
@cindex Galassi, Mark

Guile allows C to see its data structures (scalar types, lists, vectors,
strings @dots{}).  C also allows Guile to see its data structures.  As a
large program designer, you have to decide which of those capabilities
to use.  You have two main choices:

@enumerate 1
@item
You can write your software mostly in Scheme.  In this case, your C
software will mostly parse the Scheme code with Guile calls, and provide
some new primitive procedures to be used by Scheme.  This is what Gnudl
does.

@item
You can write your software mostly in C, occasionally allowing Scheme
code to be parsed by Guile, either to allow the user to modify data
structures, or to parse a configuration file, @dots{}
@end enumerate

Mixing the two approaches seems unwise: the overall layout would be
confusing.  But who knows?  There might be problems that are best solved
by a hybrid approach.  Please let me know if you think of such a
problem.

If you use the former approach, we will say that the @dfn{master world}
is Scheme, and the C routines serve Scheme and access Scheme data
structures.  In the latter case, the master world is C, and Scheme
routines serve the C code and access C data structures.

In both approaches the @code{libguile.a} library is the same, but a
predominantly different set of routines will be used.  When we go
through examples of libguile use, we will point out which is the master
world in order to clarify these two approaches.


@node What is libguile
@section What is libguile
@cindex libguile
@cindex gh interface
@cindex scm interface

@dfn{Libguile} is the library which allows C programs to start a Scheme
interpreter and execute Scheme code.  There are also facilities in
libguile to make C data structures available to Scheme, and vice versa.

The interface provided by the libguile C library is somewhat specific to
the implementation of the Scheme interpreter.  This low-level libguile
interface is usually referred to as the @code{scm_} interface, since its
public calls (API) all have the @code{scm_} prefix.

There is also a higher-level libguile interface, which is usually
referred to as the @code{gh_} interface (libGuile High).  Its public
calls all have the @code{gh_} prefix.  The @code{gh_} library interface
is designed to hide the implementation details, thus making it easier to
assimilate and portable to other underlying Scheme implementations.

People extending Guile by adding bindings to C libraries (like OpenGL or
Rx) are encouraged to use the @code{gh_} interface, so their work will
be portable to other Scheme systems.  The @code{gh_} interface should be
more stable, because it is simpler.

The @code{scm_} interface is necessary if you want to poke into the
innards of Scheme data structures, or do anything else that is not
offered by the @code{gh_} interface.  It is not covered in this
tutorial, but is covered extensively in @ref{Data representation,, Data
Representation in Guile, guile, Guile Reference Manual}.

This chapter gives a gentle introduction to the @code{gh_} interface,
presenting some @emph{hello world}-style programs which I wrote while
teaching myself to use libguile.
@cindex hello world

The @cite{Guile Programmer's Manual} gives more examples of programs
written using libguile, illustrating diverse applications.  You can also
consult my @emph{Gnudl} documentation at
@url{http://nis-www.lanl.gov/~rosalia/mydocs/} to see a large scale
project that uses C and Scheme code together.


@node How to get started with libguile
@section How to get started with libguile
@cindex learn0

Here is an elementary first program, @code{learn0}, to get going with
libguile.  The program (which uses Scheme as a master world) is in a
single source file, @code{learn0.c}:

@smallexample
/* @r{test the new libgh.a (Guile High-level library) with a trivial
   program} */

#include <stdio.h>

#include <guile/gh.h>

void main_prog(int argc, char *argv[]);

main(int argc, char *argv[])
@{
  gh_enter(argc, argv, main_prog);
@}

void main_prog(int argc, char *argv[])
@{
  int done;
  char input_str[200];

  gh_eval_str("(display \"hello Guile\")");
  gh_eval_str("(newline)");

  /* @r{for fun, evaluate some simple Scheme expressions here} */
  gh_eval_str("(define (square x) (* x x))");
  gh_eval_str("(define (fact n) (if (= n 1) 1 (* n (fact (- n 1)))))");
  gh_eval_str("(square 9)");

  /* @r{now sit in a Scheme eval loop: I input the expressions, have
     Guile evaluate them, and then get another expression.} */
  done = 0;
  fputs("learn0> ", stdout);
  while (fgets(input_str, 199, stdin) != NULL) @{
    gh_eval_str(input_str);
    fputs("\nlearn0> ", stdout);
  @}

  exit(0);
@}
@end smallexample

If you name this program @code{learn0.c}, it can now be compiled with:
@smallexample
gcc -g -c learn0.c -o learn0.o
gcc -o learn0 learn0.o -lguile -lm
@end smallexample

@c @emph{NOTE: If you are in the Guile development tree, you can simply do
@c ``cd doc/examples/c; make; ./learn0''.}

The program is simple: it creates a Scheme interpreter, passes a couple
of strings to it that define new Scheme functions @code{square} and
@code{factorial}, and then a couple of strings that invoke those
functions.

It then goes into a read-eval-print-loop (REPL), so you could type
one-line Scheme expressions to it and have them evaluated.  For example:
@smallexample
<shell-prompt> ./learn0
hello Guile
learn0> (display (sin 1.3))
963.558185417193e-3
learn0> (display (fact 10))
3628800
learn0> (quit)
<shell-prompt>
@end smallexample

You should notice the key steps involved in this @code{learn0} program:

@cartouche
@enumerate
@item
@code{#include <guile/gh.h>}
@item
You need to invoke the initialization routine @code{gh_enter()}.  This
starts up a Scheme interpreter, handling many implementation-specific
details.
@item
Your main() function should be almost empty: the real main program goes
in a separate function main_prog() which is passed to gh_enter().  This
rather arcane convention is due to the way Guile's garbage collector
works: the whole program has to run in the dynamic context of
@code{gh_enter()}.
@item
You pass strings to the Scheme interpreter with the @code{gh_eval_str()}
routine.
@item
You link your program with @code{-lguile}.
@end enumerate
@end cartouche


@node More interesting programming with libguile
@section More interesting programming with libguile
@cindex learn1
@cindex callback
@cindex builtin functions

The @code{learn0} program shows how you can invoke Scheme commands from
a C program.  This is not such a great achievement: the same could have
been done by opening a pipe to SCM or any other Scheme interpreter.

A true extension language must allow @dfn{callbacks}.  Callbacks allow
you to write C routines that can be invoked as Scheme procedures, thus
adding new primitive procedures to Scheme.  This also means that a
Scheme procedure can modify a C data structure.

Guile allows you to define new Scheme procedures in C, and provides a
mechanism to go back and forth between C and Scheme data types.

Here is a second program, @code{learn1}, which demonstrates these
features.  It is split into three source files: @code{learn1.c},
@code{c_builtins.h} and @code{c_builtins.c}.  I am including the code
here.
@c , but you might just want to look at the online source code and the
@c Makefile.am that come with Guile in the
@c @file{doc/examples/c} directory.

Notice that @code{learn1} uses a Scheme master world, and the C routines
in @code{c_builtins.c} are simply adding new primitives to Scheme.

@menu
* learn1.c::
* c_builtins.h::
* c_builtins.c::
* What learn1 is doing::
* Compiling and running learn1::
@end menu

@node learn1.c
@subsection learn1.c

Here is @file{learn1.c}:
@smallexample
#include <stdio.h>

#include <guile/gh.h>

#include "c_builtins.h"

void main_prog(int argc, char *argv[]);

main(int argc, char *argv[])
@{
  gh_enter(argc, argv, main_prog);
@}

void main_prog(int argc, char *argv[])
@{
  char input_str[200];		/* @r{ugly hack: assume strlen(line) < 200} */
  int done;

  /* @r{for fun, evaluate some simple Scheme expressions here} */
  gh_eval_str("(define (square x) (* x x))");
  gh_eval_str("(define (fact n) (if (= n 1) 1 (* n (fact (- n 1)))))");
  gh_eval_str("(square 9)");
  gh_eval_str("(fact 100)");

  /* @r{now try to define some new builtins, coded in C, so that they are
     available in Scheme.} */
  gh_new_procedure1_0("c-factorial", c_factorial);
  gh_new_procedure1_0("c-sin", c_sin);
  gh_new_procedure1_0("v-t", vector_test);

  /* @r{now sit in a Scheme eval loop: I input the expressions, have
     Guile evaluate them, and then get another expression.}  */
  done = 0;
  fputs("learn1> ", stdout);
  while (!done) @{
    if (gets(input_str) == NULL) @{
      done = 1;
    @} else @{
      gh_eval_str(input_str);
      fputs("learn1> ", stdout);
    @}
  @}

  exit(0);
@}
@end smallexample

@node c_builtins.h
@subsection c_builtins.h

Here is @file{c_builtins.h}:
@smallexample
/* @r{builtin function prototypes} */

#include <guile/gh.h>

SCM c_factorial(SCM n);
SCM c_sin(SCM n);
SCM vector_test(SCM s_length);
@end smallexample

@node c_builtins.c
@subsection c_builtins.c

Here is @file{c_builtins.c}:
@smallexample
#include <stdio.h>
#include <math.h>

#include <guile/gh.h>

#include "c_builtins.h"

/* @r{this is a factorial routine in C, made to be callable by Scheme} */
SCM c_factorial(SCM s_n)
@{
  int i;
  unsigned long result = 1, n;

  n = gh_scm2ulong(s_n);

  gh_defer_ints();
  for (i = 1; i <= n; ++i) @{
    result = result*i;
  @}
  gh_allow_ints();
  return gh_ulong2scm(result);
@}

/* @r{a sin routine in C, callable from Scheme.  it is named c_sin() to
   distinguish it from the default Scheme sin function} */
SCM c_sin(SCM s_x)
@{
  double x = gh_scm2double(s_x);

  return gh_double2scm(sin(x));
@}

/* @r{play around with vectors in Guile: this routine creates a vector of
   the given length, initializes it all to zero except element 2 which
   is set to 1.9.}  */
SCM vector_test(SCM s_length)
@{
  SCM xvec;

  c_length = gh_scm2ulong(s_length);
  printf("requested length for vector: %ld\n", gh_scm2ulong(s_length));

  /* create a vector */
  xvec = gh_make_vector(s_length, gh_double2scm(0.0));
  /* set the second element in it */
  gh_vector_set_x(xvec, gh_int2scm(2), gh_double2scm(1.9));

  return xvec;
@}
@end smallexample

@node What learn1 is doing
@subsection What learn1 is doing
@cindex registering callbacks
@cindex registering C functions
@cindex primitive procedures

If you compare learn1 to learn0, you will find that learn1 uses a new
Guile construct: the function @code{gh_new_procedure()}, and its
siblings:

@smallexample
  /* @r{now try to define some new builtins, coded in C, so that they are
     available in Scheme.} */
  gh_new_procedure1_0("c-factorial", c_factorial);
  gh_new_procedure1_0("c-sin", c_sin);
  gh_new_procedure1_0("v-t", vector_test);
@end smallexample

It is clear that @code{gh_new_procedure()} adds a new builtin
routine written in C which can be invoked from Scheme.  We can now
revise our checklist for programming with libguile, so it includes
adding callbacks.
@cindex libguile - step by step

@cartouche
@enumerate
@item
@code{#include <guile/gh.h>}
@item
You need to invoke the initialization routine @code{gh_enter()}.  This
starts up a Scheme interpreter, handling many details.
@item
Your main() function should be almost empty: the real main program goes
in a separate function main_prog() which is passed to gh_enter().  This
rather arcane convention is due to the way Guile's garbage collector
works: the whole program has to run in the dynamic context of
@code{gh_enter()}.
@item
You pass strings to the Scheme interpreter with the @code{gh_eval_str()}
routine.
@item
@strong{[new]} You can now define new builtin Scheme functions;
i.e. define new builtin Scheme functions, with the
@code{gh_new_procedure()} routine.
@item
You pass strings to the Scheme interpreter with the
@code{gh_eval_str()} routine.
@item
You link your program with @code{-lguile}.
@end enumerate
@end cartouche

I breezed by the issue of how to write your C routines that are
registered to be called from Scheme.  This is non-trivial, and is
discussed at length in the @cite{Guile Programmer's Manual}.


@node Compiling and running learn1
@subsection Compiling and running learn1

@smallexample
gcc -g -c learn1.c -o learn1.o
gcc -g -c c_builtins.c -o c_builtins.o
gcc -o learn1 learn1.o c_builtins.o -lguile -lm
@end smallexample

If you run @code{learn1}, it will prompt you for a one-line Scheme
expression, just as @code{learn0} did.  The difference is that you can
use the new C builtin procedures (@code{c-factorial}, @code{c-sin},
@code{v-t}).

@smallexample
<shell-prompt> ./learn1
welcome to Guile
hello Guile
learn1> (display (c-factorial 6))
720
learn1> (display (c-factorial 20))
2192834560
learn1> (display (c-factorial 100))
0
learn1> (display (c-sin 1.5))
0.997494986604054
learn1> (display (v-t 10))
requested length for vector: 10
#(0.0 0.0 1.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0)
learn1> (display (v-t 15))
requested length for vector: 15
#(0.0 0.0 1.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0)
learn1> (quit)
<shell-prompt>
@end smallexample

As you see, taking @code{(c-factorial 100)} does not use bignumbers and
returns a bogus answer.

@node Further examples
@section Further examples

Further ``idealized'' examples are included in the @code{doc/examples/c}
distribution.  They include programs to:

@c [FIXME: still have to write some of these; then I will revise the list.]

@itemize @bullet
@item
Parse a startup file (C is the master world).
@item
Set up initial conditions for an n-body simulation (C is the master
world).
@item
Implement a Scheme interpreter with all of Guile's goodies, @emph{plus}
the readline library @emph{and} a fast Fourier transform routine
provided in C (Scheme is the master world).
@end itemize

@node Regular Expression Support
@chapter Regular Expression Support

@node UNIX System Programming
@chapter UNIX System Programming

@node Where to find more Guile/Scheme resources
@chapter Where to find more Guile/Scheme resources


@node Concept Index
@unnumbered Concept Index

@printindex cp

@node Procedure and Macro Index
@unnumbered Procedure and Macro Index

This is an alphabetical list of all the procedures and macros in Dominion.

@printindex fn

@node Variable Index
@unnumbered Variable Index

This is an alphabetical list of the major global variables in Dominion.

@printindex vr

@node Type Index
@unnumbered Type Index

This is an alphabetical list of the major data structures in Dominion.

@printindex tp

@contents

@bye