guile/doc/ref/intro.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996-1997,2000-2004,2006,2008-2011,2013,2019
@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{http://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, 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::  
* Organisation of this Manual::
* 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.

In 2007, the Scheme community agreed upon and published R6RS, a
significant installment in the RnRS series.  R6RS expands the core
Scheme language, and standardises many non-core functions that
implementations---including Guile---have previously done in different
ways.  Over time, Guile has been updated to incorporate almost all of
the features of R6RS, and to adjust some existing features to conform to
the R6RS specification.  @xref{R6RS Support}, for full details.

In parallel to official standardization efforts, the SRFI process
(@url{http://srfi.schemers.org/}) standardises interfaces for many
practical needs, such as multithreaded programming and multidimensional
arrays.  Guile supports many SRFIs, as documented in detail in @ref{SRFI
Support}.

The process that led to the R6RS standard brought a split in the Scheme
community to the surface.  The implementors that wrote R6RS considered
that it was impossible to write useful, portable programs in R5RS, and
that only an ambitious standard could solve this problem.  However, part
of the Scheme world saw the R6RS effort as too broad, and as having
included some components that would never be adopted by more
minimalistic Scheme implementations.  This second group succeeded in
taking control of the official Scheme standardization track and in 2013
released a more limited R7RS, essentially consisting of R5RS, plus a
module system.  Guile supports R7RS also.  @xref{R7RS Support}.

With R6RS and R7RS, the unified Scheme standardization process appears
to have more or less run its course.  There will continue to be more
code written in terms of both systems, and modules defined using the
SRFI process, and Guile will support both.  However for future
directions, Guile takes inspiration from other related language
communities: Racket, Clojure, Concurrent ML, and so on.

In summary, Guile supports writing and running code written to the R5RS,
R6RS, and R7RS Scheme standards, and also supports a number of SRFI
modules.  However for most users, until a need for cross-implementation
portability has been identified, we recommend using the parts of Guile
that are useful in solving the problem at hand, regardless of whether
they proceed from a standard or whether they are Guile-specific.


@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.

The idea of extensibility is closely related to the GNU project's
primary goal, that of promoting software freedom.  Software freedom
means that people receiving a software package can modify or enhance
it to their own desires, including in ways that may not have occurred
at all to the software's original developers.  For programs written in
a compiled language like C, this freedom covers modifying and
rebuilding the C code; but if the program also provides an extension
language, that is usually a much friendlier and lower-barrier-of-entry
way for the user to start making their own changes.

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

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

@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/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 Organisation of this Manual
@section Organisation of this Manual

The rest of this manual is organised into the following chapters.

@table @strong
@item Chapter 2: Hello Guile!
A whirlwind tour shows how Guile can be used interactively and as
a script interpreter, how to link Guile into your own applications,
and how to write modules of interpreted and compiled code for use with
Guile.  Everything introduced here is documented again and in full by
the later parts of the manual.

@item Chapter 3: Hello Scheme!
For readers new to Scheme, this chapter provides an introduction to the basic
ideas of the Scheme language.  This material would apply to any Scheme
implementation and so does not make reference to anything Guile-specific.

@item Chapter 4: Programming in Scheme
Provides an overview of programming in Scheme with Guile.  It covers how to
invoke the @code{guile} program from the command-line and how to write scripts
in Scheme.  It also introduces the extensions that Guile offers beyond standard
Scheme.

@item Chapter 5: Programming in C
Provides an overview of how to use Guile in a C program.  It
discusses the fundamental concepts that you need to understand to
access the features of Guile, such as dynamic types and the garbage
collector.  It explains in a tutorial like manner how to define new
data types and functions for the use by Scheme programs.

@item Chapter 6: Guile API Reference
This part of the manual documents the Guile @acronym{API} in
functionality-based groups with the Scheme and C interfaces presented
side by side.

@item Chapter 7: Guile Modules
Describes some important modules, distributed as part of the Guile
distribution, that extend the functionality provided by the Guile
Scheme core.

@item Chapter 8: GOOPS
Describes GOOPS, an object oriented extension to Guile that provides
classes, multiple inheritance and generic functions.

@end table

@node Typographical Conventions
@section Typographical Conventions

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

The symbol @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{}}).


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