172 lines
7.6 KiB
HTML
172 lines
7.6 KiB
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<!DOCTYPE html>
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<html>
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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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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>SRFI-26 (Guile Reference Manual)</title>
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<meta name="description" content="SRFI-26 (Guile Reference Manual)">
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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_002d27.html" rel="next" title="SRFI-27">
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<link href="SRFI_002d23.html" rel="prev" title="SRFI-23">
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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="SRFI_002d26">
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<div class="nav-panel">
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<p>
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Next: <a href="SRFI_002d27.html" accesskey="n" rel="next">SRFI-27 - Sources of Random Bits</a>, Previous: <a href="SRFI_002d23.html" accesskey="p" rel="prev">SRFI-23 - Error Reporting</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_002d26-_002d-specializing-parameters"><span>7.5.18 SRFI-26 - specializing parameters<a class="copiable-link" href="#SRFI_002d26-_002d-specializing-parameters"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-SRFI_002d26"></a>
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<a class="index-entry-id" id="index-parameter-specialize"></a>
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<a class="index-entry-id" id="index-argument-specialize"></a>
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<a class="index-entry-id" id="index-specialize-parameter"></a>
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<p>This SRFI provides a syntax for conveniently specializing selected
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parameters of a function. It can be used with,
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (srfi srfi-26))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-cut"><span class="category-def">library syntax: </span><span><strong class="def-name">cut</strong> <var class="def-var-arguments">slot1 slot2 …</var><a class="copiable-link" href="#index-cut"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-cute"><span class="category-def">library syntax: </span><span><strong class="def-name">cute</strong> <var class="def-var-arguments">slot1 slot2 …</var><a class="copiable-link" href="#index-cute"> ¶</a></span></dt>
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<dd><p>Return a new procedure which will make a call (<var class="var">slot1</var> <var class="var">slot2</var>
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…) but with selected parameters specialized to given expressions.
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</p>
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<p>An example will illustrate the idea. The following is a
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specialization of <code class="code">write</code>, sending output to
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<code class="code">my-output-port</code>,
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</p>
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<div class="example">
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<pre class="example-preformatted">(cut write <> my-output-port)
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⇒
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(lambda (obj) (write obj my-output-port))
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</pre></div>
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<p>The special symbol <code class="code"><></code> indicates a slot to be filled by an
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argument to the new procedure. <code class="code">my-output-port</code> on the other
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hand is an expression to be evaluated and passed, ie. it specializes
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the behavior of <code class="code">write</code>.
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</p>
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<dl class="table">
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<dt><code class="code"><></code></dt>
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<dd><p>A slot to be filled by an argument from the created procedure.
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Arguments are assigned to <code class="code"><></code> slots in the order they appear in
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the <code class="code">cut</code> form, there’s no way to re-arrange arguments.
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</p>
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<p>The first argument to <code class="code">cut</code> is usually a procedure (or expression
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giving a procedure), but <code class="code"><></code> is allowed there too. For example,
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</p>
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<div class="example">
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<pre class="example-preformatted">(cut <> 1 2 3)
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⇒
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(lambda (proc) (proc 1 2 3))
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</pre></div>
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</dd>
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<dt><code class="code"><...></code></dt>
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<dd><p>A slot to be filled by all remaining arguments from the new procedure.
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This can only occur at the end of a <code class="code">cut</code> form.
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</p>
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<p>For example, a procedure taking a variable number of arguments like
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<code class="code">max</code> but in addition enforcing a lower bound,
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</p>
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<div class="example">
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<pre class="example-preformatted">(define my-lower-bound 123)
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(cut max my-lower-bound <...>)
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⇒
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(lambda arglist (apply max my-lower-bound arglist))
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</pre></div>
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</dd>
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</dl>
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<p>For <code class="code">cut</code> the specializing expressions are evaluated each time
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the new procedure is called. For <code class="code">cute</code> they’re evaluated just
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once, when the new procedure is created. The name <code class="code">cute</code> stands
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for “<code class="code">cut</code> with evaluated arguments”. In all cases the
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evaluations take place in an unspecified order.
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</p>
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<p>The following illustrates the difference between <code class="code">cut</code> and
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<code class="code">cute</code>,
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</p>
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<div class="example">
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<pre class="example-preformatted">(cut format <> "the time is ~s" (current-time))
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⇒
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(lambda (port) (format port "the time is ~s" (current-time)))
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(cute format <> "the time is ~s" (current-time))
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⇒
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(let ((val (current-time)))
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(lambda (port) (format port "the time is ~s" val))
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</pre></div>
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<p>(There’s no provision for a mixture of <code class="code">cut</code> and <code class="code">cute</code>
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where some expressions would be evaluated every time but others
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evaluated only once.)
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</p>
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<p><code class="code">cut</code> is really just a shorthand for the sort of <code class="code">lambda</code>
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forms shown in the above examples. But notice <code class="code">cut</code> avoids the
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need to name unspecialized parameters, and is more compact. Use in
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functional programming style or just with <code class="code">map</code>, <code class="code">for-each</code>
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or similar is typical.
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</p>
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<div class="example">
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<pre class="example-preformatted">(map (cut * 2 <>) '(1 2 3 4))
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(for-each (cut write <> my-port) my-list)
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</pre></div>
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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_002d27.html">SRFI-27 - Sources of Random Bits</a>, Previous: <a href="SRFI_002d23.html">SRFI-23 - Error Reporting</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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</html>
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