103 lines
4.1 KiB
HTML
103 lines
4.1 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>Serial Number (Guile Reference Manual)</title>
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<meta name="description" content="Serial Number (Guile Reference Manual)">
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<meta name="keywords" content="Serial Number (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="Shared-Variable.html" rel="next" title="Shared Variable">
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<link href="Closure.html" rel="prev" title="Closure">
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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="Serial-Number">
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<div class="nav-panel">
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<p>
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Next: <a href="Shared-Variable.html" accesskey="n" rel="next">Example 2: A Shared Persistent Variable</a>, Previous: <a href="Closure.html" accesskey="p" rel="prev">Closure</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-1_003a-A-Serial-Number-Generator"><span>3.4.6 Example 1: A Serial Number Generator<a class="copiable-link" href="#Example-1_003a-A-Serial-Number-Generator"> ¶</a></span></h4>
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<p>This example uses closure to create a procedure with a variable binding
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that is private to the procedure, like a local variable, but whose value
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persists between procedure calls.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (make-serial-number-generator)
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(let ((current-serial-number 0))
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(lambda ()
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(set! current-serial-number (+ current-serial-number 1))
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current-serial-number)))
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(define entry-sn-generator (make-serial-number-generator))
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(entry-sn-generator)
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⇒
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1
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(entry-sn-generator)
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⇒
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2
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</pre></div>
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<p>When <code class="code">make-serial-number-generator</code> is called, it creates a local
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environment with a binding for <code class="code">current-serial-number</code> whose
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initial value is 0, then, within this environment, creates a procedure.
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The local environment is stored within the created procedure object and
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so persists for the lifetime of the created procedure.
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</p>
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<p>Every time the created procedure is invoked, it increments the value of
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the <code class="code">current-serial-number</code> binding in the captured environment and
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then returns the current value.
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</p>
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<p>Note that <code class="code">make-serial-number-generator</code> can be called again to
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create a second serial number generator that is independent of the
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first. Every new invocation of <code class="code">make-serial-number-generator</code>
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creates a new local <code class="code">let</code> environment and returns a new procedure
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object with an association to this environment.
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</p>
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</div>
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</body>
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</html>
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