153 lines
8.2 KiB
HTML
153 lines
8.2 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>Top Level (Guile Reference Manual)</title>
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<meta name="description" content="Top Level (Guile Reference Manual)">
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<meta name="keywords" content="Top Level (Guile Reference Manual)">
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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="Binding-Constructs.html" rel="up" title="Binding Constructs">
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<link href="Local-Bindings.html" rel="next" title="Local Bindings">
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<style type="text/css">
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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="Top-Level">
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<div class="nav-panel">
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<p>
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Next: <a href="Local-Bindings.html" accesskey="n" rel="next">Local Variable Bindings</a>, Up: <a href="Binding-Constructs.html" accesskey="u" rel="up">Definitions and Variable Bindings</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="Top-Level-Variable-Definitions"><span>6.10.1 Top Level Variable Definitions<a class="copiable-link" href="#Top-Level-Variable-Definitions"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-variable-definition"></a>
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<p>At the top level of a program (i.e., not nested within any other
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expression), a definition of the form
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define a <var class="var">value</var>)
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</pre></div>
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<p>defines a variable called <code class="code">a</code> and sets it to the value <var class="var">value</var>.
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</p>
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<p>If the variable already exists in the current module, because it has
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already been created by a previous <code class="code">define</code> expression with the
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same name, its value is simply changed to the new <var class="var">value</var>. In this
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case, then, the above form is completely equivalent to
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(set! a <var class="var">value</var>)
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</pre></div>
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<p>This equivalence means that <code class="code">define</code> can be used interchangeably
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with <code class="code">set!</code> to change the value of variables at the top level of
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the REPL or a Scheme source file. It is useful during interactive
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development when reloading a Scheme file that you have modified, because
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it allows the <code class="code">define</code> expressions in that file to work as expected
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both the first time that the file is loaded and on subsequent occasions.
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</p>
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<p>Note, though, that <code class="code">define</code> and <code class="code">set!</code> are not always
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equivalent. For example, a <code class="code">set!</code> is not allowed if the named
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variable does not already exist, and the two expressions can behave
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differently in the case where there are imported variables visible from
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another module.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-define"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">define</strong> <var class="def-var-arguments">name value</var><a class="copiable-link" href="#index-define"> ¶</a></span></dt>
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<dd><p>Create a top level variable named <var class="var">name</var> with value <var class="var">value</var>.
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If the named variable already exists, just change its value. The return
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value of a <code class="code">define</code> expression is unspecified.
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</p></dd></dl>
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<p>The C API equivalents of <code class="code">define</code> are <code class="code">scm_define</code> and
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<code class="code">scm_c_define</code>, which differ from each other in whether the
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variable name is specified as a <code class="code">SCM</code> symbol or as a
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null-terminated C string.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-scm_005fdefine"><span class="category-def">C Function: </span><span><strong class="def-name">scm_define</strong> <var class="def-var-arguments">(sym, value)</var><a class="copiable-link" href="#index-scm_005fdefine"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fc_005fdefine"><span class="category-def">C Function: </span><span><strong class="def-name">scm_c_define</strong> <var class="def-var-arguments">(const char *name, value)</var><a class="copiable-link" href="#index-scm_005fc_005fdefine"> ¶</a></span></dt>
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<dd><p>C equivalents of <code class="code">define</code>, with variable name specified either by
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<var class="var">sym</var>, a symbol, or by <var class="var">name</var>, a null-terminated C string. Both
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variants return the new or preexisting variable object.
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</p></dd></dl>
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<p><code class="code">define</code> (when it occurs at top level), <code class="code">scm_define</code> and
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<code class="code">scm_c_define</code> all create or set the value of a variable in the top
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level environment of the current module. If there was not already a
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variable with the specified name belonging to the current module, but a
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similarly named variable from another module was visible through having
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been imported, the newly created variable in the current module will
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shadow the imported variable, such that the imported variable is no
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longer visible.
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</p>
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<p>Attention: Scheme definitions inside local binding constructs
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(see <a class="pxref" href="Local-Bindings.html">Local Variable Bindings</a>) act differently (see <a class="pxref" href="Internal-Definitions.html">Internal definitions</a>).
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</p>
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<p>Many people end up in a development style of adding and changing
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definitions at runtime, building out their program without restarting
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it. (You can do this using <code class="code">reload-module</code>, the <code class="code">reload</code> REPL
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command, the <code class="code">load</code> procedure, or even just pasting code into a
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REPL.) If you are one of these people, you will find that sometimes
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there are some variables that you <em class="emph">don’t</em> want to redefine all the
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time. For these, use <code class="code">define-once</code>.
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</p>
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<a class="index-entry-id" id="index-defvar"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-define_002donce"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">define-once</strong> <var class="def-var-arguments">name value</var><a class="copiable-link" href="#index-define_002donce"> ¶</a></span></dt>
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<dd><p>Create a top level variable named <var class="var">name</var> with value <var class="var">value</var>, but
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only if <var class="var">name</var> is not already bound in the current module.
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</p></dd></dl>
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<p>Old Lispers probably know <code class="code">define-once</code> under its Lisp name,
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<code class="code">defvar</code>.
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</p>
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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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Next: <a href="Local-Bindings.html">Local Variable Bindings</a>, Up: <a href="Binding-Constructs.html">Definitions and Variable Bindings</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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