124 lines
5.3 KiB
HTML
124 lines
5.3 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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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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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>Callback Closure (Guile Reference Manual)</title>
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<meta name="description" content="Callback Closure (Guile Reference Manual)">
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<meta name="keywords" content="Callback Closure (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-Closure.html" rel="up" title="About Closure">
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<link href="OO-Closure.html" rel="next" title="OO Closure">
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<link href="Shared-Variable.html" rel="prev" title="Shared Variable">
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<style type="text/css">
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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="Callback-Closure">
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<div class="nav-panel">
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<p>
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Next: <a href="OO-Closure.html" accesskey="n" rel="next">Example 4: Object Orientation</a>, Previous: <a href="Shared-Variable.html" accesskey="p" rel="prev">Example 2: A Shared Persistent Variable</a>, Up: <a href="About-Closure.html" accesskey="u" rel="up">The Concept of Closure</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="Example-3_003a-The-Callback-Closure-Problem"><span>3.4.8 Example 3: The Callback Closure Problem<a class="copiable-link" href="#Example-3_003a-The-Callback-Closure-Problem"> ¶</a></span></h4>
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<p>A frequently used programming model for library code is to allow an
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application to register a callback function for the library to call when
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some particular event occurs. It is often useful for the application to
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make several such registrations using the same callback function, for
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example if several similar library events can be handled using the same
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application code, but the need then arises to distinguish the callback
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function calls that are associated with one callback registration from
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those that are associated with different callback registrations.
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</p>
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<p>In languages without the ability to create functions dynamically, this
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problem is usually solved by passing a <code class="code">user_data</code> parameter on the
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registration call, and including the value of this parameter as one of
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the parameters on the callback function. Here is an example of
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declarations using this solution in C:
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</p>
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<div class="example">
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<pre class="example-preformatted">typedef void (event_handler_t) (int event_type,
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void *user_data);
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void register_callback (int event_type,
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event_handler_t *handler,
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void *user_data);
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</pre></div>
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<p>In Scheme, closure can be used to achieve the same functionality without
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requiring the library code to store a <code class="code">user-data</code> for each callback
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registration.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">;; In the library:
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(define (register-callback event-type handler-proc)
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...)
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;; In the application:
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(define (make-handler event-type user-data)
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(lambda ()
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...
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<code referencing event-type and user-data>
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...))
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(register-callback event-type
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(make-handler event-type ...))
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</pre></div>
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<p>As far as the library is concerned, <code class="code">handler-proc</code> is a procedure
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with no arguments, and all the library has to do is call it when the
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appropriate event occurs. From the application’s point of view, though,
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the handler procedure has used closure to capture an environment that
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includes all the context that the handler code needs —
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<code class="code">event-type</code> and <code class="code">user-data</code> — to handle the event
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correctly.
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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="OO-Closure.html">Example 4: Object Orientation</a>, Previous: <a href="Shared-Variable.html">Example 2: A Shared Persistent Variable</a>, Up: <a href="About-Closure.html">The Concept of Closure</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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