144 lines
6.4 KiB
HTML
144 lines
6.4 KiB
HTML
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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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<title>begin (Guile Reference Manual)</title>
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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="Control-Mechanisms.html" rel="up" title="Control Mechanisms">
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<link href="Conditionals.html" rel="next" title="Conditionals">
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<body lang="en">
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<div class="subsection-level-extent" id="begin">
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<div class="nav-panel">
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<p>
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Next: <a href="Conditionals.html" accesskey="n" rel="next">Simple Conditional Evaluation</a>, Up: <a href="Control-Mechanisms.html" accesskey="u" rel="up">Controlling the Flow of Program Execution</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="Sequencing-and-Splicing"><span>6.11.1 Sequencing and Splicing<a class="copiable-link" href="#Sequencing-and-Splicing"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-begin"></a>
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<a class="index-entry-id" id="index-sequencing"></a>
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<a class="index-entry-id" id="index-expression-sequencing"></a>
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<p>As an expression, the <code class="code">begin</code> syntax is used to evaluate a sequence
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of sub-expressions in order. Consider the conditional expression below:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(if (> x 0)
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(begin (display "greater") (newline)))
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</pre></div>
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<p>If the test is true, we want to display “greater” to the current
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output port, then display a newline. We use <code class="code">begin</code> to form a
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compound expression out of this sequence of sub-expressions.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-begin-1"><span class="category-def">syntax: </span><span><strong class="def-name">begin</strong> <var class="def-var-arguments">expr …</var><a class="copiable-link" href="#index-begin-1"> ¶</a></span></dt>
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<dd><p>The expression(s) are evaluated in left-to-right order and the values of
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the last expression are returned as the result of the
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<code class="code">begin</code>-expression. This expression type is used when the
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expressions before the last one are evaluated for their side effects.
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</p></dd></dl>
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<a class="index-entry-id" id="index-splicing"></a>
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<a class="index-entry-id" id="index-definition-splicing"></a>
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<p>The <code class="code">begin</code> syntax has another role in definition context
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(see <a class="pxref" href="Internal-Definitions.html">Internal definitions</a>). A <code class="code">begin</code> form in a definition
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context <em class="dfn">splices</em> its subforms into its place. For example,
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consider the following procedure:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (make-seal)
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(define-sealant seal open)
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(values seal open))
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</pre></div>
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<p>Let us assume the existence of a <code class="code">define-sealant</code> macro that
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expands out to some definitions wrapped in a <code class="code">begin</code>, like so:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (make-seal)
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(begin
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(define seal-tag
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(list 'seal))
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(define (seal x)
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(cons seal-tag x))
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(define (sealed? x)
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(and (pair? x) (eq? (car x) seal-tag)))
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(define (open x)
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(if (sealed? x)
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(cdr x)
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(error "Expected a sealed value:" x))))
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(values seal open))
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</pre></div>
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<p>Here, because the <code class="code">begin</code> is in definition context, its subforms
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are <em class="dfn">spliced</em> into the place of the <code class="code">begin</code>. This allows the
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definitions created by the macro to be visible to the following
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expression, the <code class="code">values</code> form.
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</p>
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<p>It is a fine point, but splicing and sequencing are different. It can
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make sense to splice zero forms, because it can make sense to have zero
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internal definitions before the expressions in a procedure or lexical
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binding form. However it does not make sense to have a sequence of zero
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expressions, because in that case it would not be clear what the value
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of the sequence would be, because in a sequence of zero expressions,
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there can be no last value. Sequencing zero expressions is an error.
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</p>
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<p>It would be more elegant in some ways to eliminate splicing from the
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Scheme language, and without macros (see <a class="pxref" href="Macros.html">Macros</a>), that would be a
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good idea. But it is useful to be able to write macros that expand out
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to multiple definitions, as in <code class="code">define-sealant</code> above, so Scheme
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abuses the <code class="code">begin</code> form for these two tasks.
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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="Conditionals.html">Simple Conditional Evaluation</a>, Up: <a href="Control-Mechanisms.html">Controlling the Flow of Program Execution</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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