122 lines
4.9 KiB
HTML
122 lines
4.9 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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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Coding With Keywords (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="Keywords.html" rel="up" title="Keywords">
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<link href="Keyword-Read-Syntax.html" rel="next" title="Keyword Read Syntax">
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<link href="Why-Use-Keywords_003f.html" rel="prev" title="Why Use Keywords?">
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<div class="subsubsection-level-extent" id="Coding-With-Keywords">
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<p>
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Next: <a href="Keyword-Read-Syntax.html" accesskey="n" rel="next">Keyword Read Syntax</a>, Previous: <a href="Why-Use-Keywords_003f.html" accesskey="p" rel="prev">Why Use Keywords?</a>, Up: <a href="Keywords.html" accesskey="u" rel="up">Keywords</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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<h4 class="subsubsection" id="Coding-With-Keywords-1"><span>6.6.7.2 Coding With Keywords<a class="copiable-link" href="#Coding-With-Keywords-1"> ¶</a></span></h4>
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<p>If a procedure wants to support keywords, it should take a rest argument
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and then use whatever means is convenient to extract keywords and their
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corresponding arguments from the contents of that rest argument.
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</p>
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<p>The following example illustrates the principle: the code for
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<code class="code">make-window</code> uses a helper procedure called
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<code class="code">get-keyword-value</code> to extract individual keyword arguments from
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the rest argument.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (get-keyword-value args keyword default)
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(let ((kv (memq keyword args)))
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(if (and kv (>= (length kv) 2))
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(cadr kv)
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default)))
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(define (make-window . args)
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(let ((depth (get-keyword-value args #:depth screen-depth))
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(bg (get-keyword-value args #:bg "white"))
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(width (get-keyword-value args #:width 800))
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(height (get-keyword-value args #:height 100))
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...)
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...))
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</pre></div>
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<p>But you don’t need to write <code class="code">get-keyword-value</code>. The <code class="code">(ice-9
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optargs)</code> module provides a set of powerful macros that you can use to
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implement keyword-supporting procedures like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(use-modules (ice-9 optargs))
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(define (make-window . args)
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(let-keywords args #f ((depth screen-depth)
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(bg "white")
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(width 800)
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(height 100))
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...))
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</pre></div>
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<p>Or, even more economically, like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(use-modules (ice-9 optargs))
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(define* (make-window #:key (depth screen-depth)
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(bg "white")
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(width 800)
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(height 100))
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...)
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</pre></div>
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<p>For further details on <code class="code">let-keywords</code>, <code class="code">define*</code> and other
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facilities provided by the <code class="code">(ice-9 optargs)</code> module, see
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<a class="ref" href="Optional-Arguments.html">Optional Arguments</a>.
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</p>
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<p>To handle keyword arguments from procedures implemented in C,
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use <code class="code">scm_c_bind_keyword_arguments</code> (see <a class="pxref" href="Keyword-Procedures.html">Keyword Procedures</a>).
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</p>
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</div>
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