124 lines
5.8 KiB
HTML
124 lines
5.8 KiB
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<!DOCTYPE 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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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>Procedures as Values (Guile Reference Manual)</title>
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<meta name="description" content="Procedures as Values (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="About-Procedures.html" rel="up" title="About Procedures">
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<link href="Simple-Invocation.html" rel="next" title="Simple Invocation">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Procedures-as-Values">
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<div class="nav-panel">
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<p>
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Next: <a href="Simple-Invocation.html" accesskey="n" rel="next">Simple Procedure Invocation</a>, Up: <a href="About-Procedures.html" accesskey="u" rel="up">The Representation and Use of Procedures</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="Procedures-as-Values-1"><span>3.2.1 Procedures as Values<a class="copiable-link" href="#Procedures-as-Values-1"> ¶</a></span></h4>
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<p>One of the great simplifications of Scheme is that a procedure is just
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another type of value, and that procedure values can be passed around
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and stored in variables in exactly the same way as, for example, strings
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and lists. When we talk about a built-in standard Scheme procedure such
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as <code class="code">open-input-file</code>, what we actually mean is that there is a
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pre-defined top level variable called <code class="code">open-input-file</code>, whose
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value is a procedure that implements what R5RS says that
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<code class="code">open-input-file</code> should do.
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</p>
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<p>Note that this is quite different from many dialects of Lisp —
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including Emacs Lisp — in which a program can use the same name with
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two quite separate meanings: one meaning identifies a Lisp function,
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while the other meaning identifies a Lisp variable, whose value need
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have nothing to do with the function that is associated with the first
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meaning. In these dialects, functions and variables are said to live in
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different <em class="dfn">namespaces</em>.
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</p>
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<p>In Scheme, on the other hand, all names belong to a single unified
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namespace, and the variables that these names identify can hold any kind
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of Scheme value, including procedure values.
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</p>
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<p>One consequence of the “procedures as values” idea is that, if you
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don’t happen to like the standard name for a Scheme procedure, you can
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change it.
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</p>
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<p>For example, <code class="code">call-with-current-continuation</code> is a very important
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standard Scheme procedure, but it also has a very long name! So, many
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programmers use the following definition to assign the same procedure
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value to the more convenient name <code class="code">call/cc</code>.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define call/cc call-with-current-continuation)
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</pre></div>
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<p>Let’s understand exactly how this works. The definition creates a new
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variable <code class="code">call/cc</code>, and then sets its value to the value of the
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variable <code class="code">call-with-current-continuation</code>; the latter value is a
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procedure that implements the behavior that R5RS specifies under the
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name “call-with-current-continuation”. So <code class="code">call/cc</code> ends up
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holding this value as well.
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</p>
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<p>Now that <code class="code">call/cc</code> holds the required procedure value, you could
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choose to use <code class="code">call-with-current-continuation</code> for a completely
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different purpose, or just change its value so that you will get an
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error if you accidentally use <code class="code">call-with-current-continuation</code> as a
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procedure in your program rather than <code class="code">call/cc</code>. For example:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(set! call-with-current-continuation "Not a procedure any more!")
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</pre></div>
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<p>Or you could just leave <code class="code">call-with-current-continuation</code> as it was.
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It’s perfectly fine for more than one variable to hold the same
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procedure value.
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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="Simple-Invocation.html">Simple Procedure Invocation</a>, Up: <a href="About-Procedures.html">The Representation and Use of Procedures</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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