144 lines
6.3 KiB
HTML
144 lines
6.3 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Definition (Guile Reference Manual)</title>
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<meta name="description" content="Definition (Guile Reference Manual)">
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<meta name="keywords" content="Definition (Guile Reference Manual)">
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<meta name="Generator" content=".texi2any-real">
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<meta name="viewport" content="width=device-width,initial-scale=1">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="About-Data.html" rel="up" title="About Data">
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<link href="Values-and-Variables.html" rel="prev" title="Values and Variables">
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<style type="text/css">
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a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Definition">
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<div class="nav-panel">
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<p>
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Previous: <a href="Values-and-Variables.html" accesskey="p" rel="prev">Values and Variables</a>, Up: <a href="About-Data.html" accesskey="u" rel="up">Data Types, Values and Variables</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsection" id="Defining-and-Setting-Variables"><span>3.1.3 Defining and Setting Variables<a class="copiable-link" href="#Defining-and-Setting-Variables"> ¶</a></span></h4>
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<p>To define a new variable, you use Scheme’s <code class="code">define</code> syntax like
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this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define <var class="var">variable-name</var> <var class="var">value</var>)
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</pre></div>
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<p>This makes a new variable called <var class="var">variable-name</var> and stores
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<var class="var">value</var> in it as the variable’s initial value. For example:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">;; Make a variable `x' with initial numeric value 1.
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(define x 1)
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;; Make a variable `organization' with an initial string value.
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(define organization "Free Software Foundation")
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</pre></div>
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<p>(In Scheme, a semicolon marks the beginning of a comment that continues
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until the end of the line. So the lines beginning <code class="code">;;</code> are
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comments.)
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</p>
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<p>Changing the value of an already existing variable is very similar,
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except that <code class="code">define</code> is replaced by the Scheme syntax <code class="code">set!</code>,
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like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(set! <var class="var">variable-name</var> <var class="var">new-value</var>)
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</pre></div>
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<p>Remember that variables do not have fixed types, so <var class="var">new-value</var> may
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have a completely different type from whatever was previously stored in
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the location named by <var class="var">variable-name</var>. Both of the following
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examples are therefore correct.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">;; Change the value of `x' to 5.
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(set! x 5)
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;; Change the value of `organization' to the FSF's street number.
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(set! organization 545)
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</pre></div>
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<p>In these examples, <var class="var">value</var> and <var class="var">new-value</var> are literal numeric
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or string values. In general, however, <var class="var">value</var> and <var class="var">new-value</var>
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can be any Scheme expression. Even though we have not yet covered the
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forms that Scheme expressions can take (see <a class="pxref" href="About-Expressions.html">Expressions and Evaluation</a>), you
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can probably guess what the following <code class="code">set!</code> example does…
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(set! x (+ x 1))
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</pre></div>
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<p>(Note: this is not a complete description of <code class="code">define</code> and
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<code class="code">set!</code>, because we need to introduce some other aspects of Scheme
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before the missing pieces can be filled in. If, however, you are
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already familiar with the structure of Scheme, you may like to read
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about those missing pieces immediately by jumping ahead to the following
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references.
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</p>
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<ul class="itemize mark-bullet">
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<li><a class="ref" href="Lambda-Alternatives.html">Lambda Alternatives</a>, to read about an alternative form of the
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<code class="code">define</code> syntax that can be used when defining new procedures.
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</li><li><a class="ref" href="Procedures-with-Setters.html">Procedures with Setters</a>, to read about an alternative form of the
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<code class="code">set!</code> syntax that helps with changing a single value in the depths
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of a compound data structure.)
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</li><li>See <a class="xref" href="Internal-Definitions.html">Internal definitions</a>, to read about using <code class="code">define</code> other
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than at top level in a Scheme program, including a discussion of when it
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works to use <code class="code">define</code> rather than <code class="code">set!</code> to change the value
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of an existing variable.
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</li></ul>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Previous: <a href="Values-and-Variables.html">Values and Variables</a>, Up: <a href="About-Data.html">Data Types, Values and Variables</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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