145 lines
6.6 KiB
HTML
145 lines
6.6 KiB
HTML
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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>SRFI-2 (Guile Reference Manual)</title>
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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="SRFI-Support.html" rel="up" title="SRFI Support">
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<link href="SRFI_002d4.html" rel="next" title="SRFI-4">
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<div class="subsection-level-extent" id="SRFI_002d2">
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<div class="nav-panel">
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<p>
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Next: <a href="SRFI_002d4.html" accesskey="n" rel="next">SRFI-4 - Homogeneous numeric vector datatypes</a>, Previous: <a href="SRFI_002d1.html" accesskey="p" rel="prev">SRFI-1 - List library</a>, Up: <a href="SRFI-Support.html" accesskey="u" rel="up">SRFI Support Modules</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="SRFI_002d2-_002d-and_002dlet_002a"><span>7.5.4 SRFI-2 - and-let*<a class="copiable-link" href="#SRFI_002d2-_002d-and_002dlet_002a"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-SRFI_002d2"></a>
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<p>The following syntax can be obtained with
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(use-modules (srfi srfi-2))
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</pre></div>
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<p>or alternatively
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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 and-let-star))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-and_002dlet_002a"><span class="category-def">library syntax: </span><span><strong class="def-name">and-let*</strong> <var class="def-var-arguments">(clause …) body …</var><a class="copiable-link" href="#index-and_002dlet_002a"> ¶</a></span></dt>
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<dd><p>A combination of <code class="code">and</code> and <code class="code">let*</code>.
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</p>
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<p>Each <var class="var">clause</var> is evaluated in turn, and if <code class="code">#f</code> is obtained
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then evaluation stops and <code class="code">#f</code> is returned. If all are
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non-<code class="code">#f</code> then <var class="var">body</var> is evaluated and the last form gives the
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return value, or if <var class="var">body</var> is empty then the result is <code class="code">#t</code>.
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Each <var class="var">clause</var> should be one of the following,
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</p>
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<dl class="table">
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<dt><code class="code">(symbol expr)</code></dt>
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<dd><p>Evaluate <var class="var">expr</var>, check for <code class="code">#f</code>, and bind it to <var class="var">symbol</var>.
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Like <code class="code">let*</code>, that binding is available to subsequent clauses.
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</p></dd>
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<dt><code class="code">(expr)</code></dt>
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<dd><p>Evaluate <var class="var">expr</var> and check for <code class="code">#f</code>.
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</p></dd>
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<dt><code class="code">symbol</code></dt>
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<dd><p>Get the value bound to <var class="var">symbol</var> and check for <code class="code">#f</code>.
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</p></dd>
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</dl>
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<p>Notice that <code class="code">(expr)</code> has an “extra” pair of parentheses, for
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instance <code class="code">((eq? x y))</code>. One way to remember this is to imagine
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the <code class="code">symbol</code> in <code class="code">(symbol expr)</code> is omitted.
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</p>
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<p><code class="code">and-let*</code> is good for calculations where a <code class="code">#f</code> value means
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termination, but where a non-<code class="code">#f</code> value is going to be needed in
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subsequent expressions.
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</p>
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<p>The following illustrates this, it returns text between brackets
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‘<samp class="samp">[...]</samp>’ in a string, or <code class="code">#f</code> if there are no such brackets
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(ie. either <code class="code">string-index</code> gives <code class="code">#f</code>).
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (extract-brackets str)
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(and-let* ((start (string-index str #\[))
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(end (string-index str #\] start)))
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(substring str (1+ start) end)))
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</pre></div>
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<p>The following shows plain variables and expressions tested too.
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<code class="code">diagnostic-levels</code> is taken to be an alist associating a
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diagnostic type with a level. <code class="code">str</code> is printed only if the type
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is known and its level is high enough.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (show-diagnostic type str)
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(and-let* (want-diagnostics
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(level (assq-ref diagnostic-levels type))
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((>= level current-diagnostic-level)))
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(display str)))
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</pre></div>
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<p>The advantage of <code class="code">and-let*</code> is that an extended sequence of
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expressions and tests doesn’t require lots of nesting as would arise
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from separate <code class="code">and</code> and <code class="code">let*</code>, or from <code class="code">cond</code> with
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<code class="code">=></code>.
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</p>
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</dd></dl>
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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="SRFI_002d4.html">SRFI-4 - Homogeneous numeric vector datatypes</a>, Previous: <a href="SRFI_002d1.html">SRFI-1 - List library</a>, Up: <a href="SRFI-Support.html">SRFI Support Modules</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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