107 lines
5 KiB
HTML
107 lines
5 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>Local Variables (Guile Reference Manual)</title>
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<meta name="description" content="Local Variables (Guile Reference Manual)">
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<meta name="keywords" content="Local Variables (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="About-Closure.html" rel="up" title="About Closure">
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<link href="Chaining.html" rel="next" title="Chaining">
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<link href="About-Environments.html" rel="prev" title="About Environments">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Local-Variables">
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<div class="nav-panel">
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<p>
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Next: <a href="Chaining.html" accesskey="n" rel="next">Environment Chaining</a>, Previous: <a href="About-Environments.html" accesskey="p" rel="prev">Names, Locations, Values and Environments</a>, Up: <a href="About-Closure.html" accesskey="u" rel="up">The Concept of Closure</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="Local-Variables-and-Environments"><span>3.4.2 Local Variables and Environments<a class="copiable-link" href="#Local-Variables-and-Environments"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-local-variable"></a>
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<a class="index-entry-id" id="index-variable_002c-local"></a>
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<a class="index-entry-id" id="index-local-environment"></a>
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<a class="index-entry-id" id="index-environment_002c-local"></a>
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<p>We have seen how to create top level variables using the <code class="code">define</code>
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syntax (see <a class="pxref" href="Definition.html">Defining and Setting Variables</a>). It is often useful to create variables
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that are more limited in their scope, typically as part of a procedure
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body. In Scheme, this is done using the <code class="code">let</code> syntax, or one of
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its modified forms <code class="code">let*</code> and <code class="code">letrec</code>. These syntaxes are
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described in full later in the manual (see <a class="pxref" href="Local-Bindings.html">Local Variable Bindings</a>). Here
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our purpose is to illustrate their use just enough that we can see how
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local variables work.
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</p>
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<p>For example, the following code uses a local variable <code class="code">s</code> to
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simplify the computation of the area of a triangle given the lengths of
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its three sides.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define a 5.3)
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(define b 4.7)
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(define c 2.8)
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(define area
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(let ((s (/ (+ a b c) 2)))
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(sqrt (* s (- s a) (- s b) (- s c)))))
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</pre></div>
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<p>The effect of the <code class="code">let</code> expression is to create a new environment
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and, within this environment, an association between the name <code class="code">s</code>
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and a new location whose initial value is obtained by evaluating
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<code class="code">(/ (+ a b c) 2)</code>. The expressions in the body of the <code class="code">let</code>,
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namely <code class="code">(sqrt (* s (- s a) (- s b) (- s c)))</code>, are then evaluated
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in the context of the new environment, and the value of the last
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expression evaluated becomes the value of the whole <code class="code">let</code>
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expression, and therefore the value of the variable <code class="code">area</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="Chaining.html">Environment Chaining</a>, Previous: <a href="About-Environments.html">Names, Locations, Values and Environments</a>, Up: <a href="About-Closure.html">The Concept of Closure</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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