112 lines
4.3 KiB
HTML
112 lines
4.3 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>Shared Variable (Guile Reference Manual)</title>
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<meta name="description" content="Shared Variable (Guile Reference Manual)">
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<meta name="keywords" content="Shared Variable (Guile Reference Manual)">
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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-Closure.html" rel="up" title="About Closure">
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<link href="Callback-Closure.html" rel="next" title="Callback Closure">
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<link href="Serial-Number.html" rel="prev" title="Serial Number">
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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="Shared-Variable">
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<div class="nav-panel">
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<p>
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Next: <a href="Callback-Closure.html" accesskey="n" rel="next">Example 3: The Callback Closure Problem</a>, Previous: <a href="Serial-Number.html" accesskey="p" rel="prev">Example 1: A Serial Number Generator</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="Example-2_003a-A-Shared-Persistent-Variable"><span>3.4.7 Example 2: A Shared Persistent Variable<a class="copiable-link" href="#Example-2_003a-A-Shared-Persistent-Variable"> ¶</a></span></h4>
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<p>This example uses closure to create two procedures, <code class="code">get-balance</code>
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and <code class="code">deposit</code>, that both refer to the same captured local
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environment so that they can both access the <code class="code">balance</code> variable
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binding inside that environment. The value of this variable binding
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persists between calls to either procedure.
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</p>
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<p>Note that the captured <code class="code">balance</code> variable binding is private to
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these two procedures: it is not directly accessible to any other code.
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It can only be accessed indirectly via <code class="code">get-balance</code> or
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<code class="code">deposit</code>, as illustrated by the <code class="code">withdraw</code> procedure.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define get-balance #f)
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(define deposit #f)
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(let ((balance 0))
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(set! get-balance
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(lambda ()
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balance))
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(set! deposit
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(lambda (amount)
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(set! balance (+ balance amount))
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balance)))
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(define (withdraw amount)
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(deposit (- amount)))
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(get-balance)
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⇒
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0
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(deposit 50)
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⇒
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50
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(withdraw 75)
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⇒
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-25
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</pre></div>
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<p>An important detail here is that the <code class="code">get-balance</code> and
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<code class="code">deposit</code> variables must be set up by <code class="code">define</code>ing them at top
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level and then <code class="code">set!</code>ing their values inside the <code class="code">let</code> body.
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Using <code class="code">define</code> within the <code class="code">let</code> body would not work: this
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would create variable bindings within the local <code class="code">let</code> environment
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that would not be accessible at top level.
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</p>
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</div>
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</body>
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</html>
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