109 lines
5.2 KiB
HTML
109 lines
5.2 KiB
HTML
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<!DOCTYPE html>
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<html>
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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>Eval Special (Guile Reference Manual)</title>
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<meta name="description" content="Eval Special (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="Evaluating.html" rel="up" title="Evaluating">
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<link href="Eval-Procedure.html" rel="prev" title="Eval Procedure">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Eval-Special">
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<div class="nav-panel">
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<p>
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Previous: <a href="Eval-Procedure.html" accesskey="p" rel="prev">Evaluating a Procedure Invocation Expression</a>, Up: <a href="Evaluating.html" accesskey="u" rel="up">Evaluating Expressions and Executing Programs</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="subsubsection" id="Evaluating-Special-Syntactic-Expressions"><span>3.3.1.4 Evaluating Special Syntactic Expressions<a class="copiable-link" href="#Evaluating-Special-Syntactic-Expressions"> ¶</a></span></h4>
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<p>When a procedure invocation expression is evaluated, the procedure and
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<em class="emph">all</em> the argument expressions must be evaluated before the
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procedure can be invoked. Special syntactic expressions are special
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because they are able to manipulate their arguments in an unevaluated
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form, and can choose whether to evaluate any or all of the argument
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expressions.
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</p>
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<p>Why is this needed? Consider a program fragment that asks the user
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whether or not to delete a file, and then deletes the file if the user
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answers yes.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(if (string=? (read-answer "Should I delete this file?")
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"yes")
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(delete-file file))
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</pre></div>
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<p>If the outermost <code class="code">(if …)</code> expression here was a procedure
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invocation expression, the expression <code class="code">(delete-file file)</code>, whose
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side effect is to actually delete a file, would already have been
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evaluated before the <code class="code">if</code> procedure even got invoked! Clearly this
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is no use — the whole point of an <code class="code">if</code> expression is that the
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<em class="dfn">consequent</em> expression is only evaluated if the condition of the
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<code class="code">if</code> expression is “true”.
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</p>
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<p>Therefore <code class="code">if</code> must be special syntax, not a procedure. Other
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special syntaxes that we have already met are <code class="code">define</code>, <code class="code">set!</code>
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and <code class="code">lambda</code>. <code class="code">define</code> and <code class="code">set!</code> are syntax because
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they need to know the variable <em class="emph">name</em> that is given as the first
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argument in a <code class="code">define</code> or <code class="code">set!</code> expression, not that
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variable’s value. <code class="code">lambda</code> is syntax because it does not
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immediately evaluate the expressions that define the procedure body;
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instead it creates a procedure object that incorporates these
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expressions so that they can be evaluated in the future, when that
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procedure is invoked.
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</p>
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<p>The rules for evaluating each special syntactic expression are specified
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individually for each special syntax. For a summary of standard special
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syntax, see See <a class="xref" href="Syntax-Summary.html">Summary of Common Syntax</a>.
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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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Previous: <a href="Eval-Procedure.html">Evaluating a Procedure Invocation Expression</a>, Up: <a href="Evaluating.html">Evaluating Expressions and Executing Programs</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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