101 lines
5.5 KiB
HTML
101 lines
5.5 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>Delayed Evaluation (Guile Reference Manual)</title>
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<meta name="description" content="Delayed Evaluation (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="Read_002fLoad_002fEval_002fCompile.html" rel="up" title="Read/Load/Eval/Compile">
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<link href="Local-Evaluation.html" rel="next" title="Local Evaluation">
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<link href="Character-Encoding-of-Source-Files.html" rel="prev" title="Character Encoding of Source Files">
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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="Delayed-Evaluation">
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<div class="nav-panel">
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<p>
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Next: <a href="Local-Evaluation.html" accesskey="n" rel="next">Local Evaluation</a>, Previous: <a href="Character-Encoding-of-Source-Files.html" accesskey="p" rel="prev">Character Encoding of Source Files</a>, Up: <a href="Read_002fLoad_002fEval_002fCompile.html" accesskey="u" rel="up">Reading and Evaluating Scheme Code</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="Delayed-Evaluation-1"><span>6.16.10 Delayed Evaluation<a class="copiable-link" href="#Delayed-Evaluation-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-delayed-evaluation"></a>
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<a class="index-entry-id" id="index-promises"></a>
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<p>Promises are a convenient way to defer a calculation until its result
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is actually needed, and to run such a calculation only once. Also
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see <a class="pxref" href="SRFI_002d45.html">SRFI-45 - Primitives for Expressing Iterative Lazy Algorithms</a>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-delay"><span class="category-def">syntax: </span><span><strong class="def-name">delay</strong> <var class="def-var-arguments">expr</var><a class="copiable-link" href="#index-delay"> ¶</a></span></dt>
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<dd><a class="index-entry-id" id="index-delay-3"></a>
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<p>Return a promise object which holds the given <var class="var">expr</var> expression,
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ready to be evaluated by a later <code class="code">force</code>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-promise_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">promise?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-promise_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fpromise_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_promise_p</strong> <var class="def-var-arguments">(obj)</var><a class="copiable-link" href="#index-scm_005fpromise_005fp"> ¶</a></span></dt>
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<dd><p>Return true if <var class="var">obj</var> is a promise.
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</p></dd></dl>
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<a class="index-entry-id" id="index-force-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-force"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">force</strong> <var class="def-var-arguments">p</var><a class="copiable-link" href="#index-force"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fforce"><span class="category-def">C Function: </span><span><strong class="def-name">scm_force</strong> <var class="def-var-arguments">(p)</var><a class="copiable-link" href="#index-scm_005fforce"> ¶</a></span></dt>
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<dd><p>Return the value obtained from evaluating the <var class="var">expr</var> in the given
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promise <var class="var">p</var>. If <var class="var">p</var> has previously been forced then its
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<var class="var">expr</var> is not evaluated again, instead the value obtained at that
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time is simply returned.
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</p>
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<p>During a <code class="code">force</code>, an <var class="var">expr</var> can call <code class="code">force</code> again on
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its own promise, resulting in a recursive evaluation of that
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<var class="var">expr</var>. The first evaluation to return gives the value for the
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promise. Higher evaluations run to completion in the normal way, but
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their results are ignored, <code class="code">force</code> always returns the first
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value.
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</p></dd></dl>
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</div>
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</body>
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</html>
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