123 lines
4.5 KiB
HTML
123 lines
4.5 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>Hook Example (Guile Reference Manual)</title>
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<meta name="description" content="Hook Example (Guile Reference Manual)">
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<meta name="keywords" content="Hook Example (Guile Reference Manual)">
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<meta name="viewport" content="width=device-width,initial-scale=1">
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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="Hooks.html" rel="up" title="Hooks">
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<link href="Hook-Reference.html" rel="next" title="Hook Reference">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Hook-Example">
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<div class="nav-panel">
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<p>
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Next: <a href="Hook-Reference.html" accesskey="n" rel="next">Hook Reference</a>, Up: <a href="Hooks.html" accesskey="u" rel="up">Hooks</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="subsubsection" id="Hook-Usage-by-Example"><span>6.9.6.1 Hook Usage by Example<a class="copiable-link" href="#Hook-Usage-by-Example"> ¶</a></span></h4>
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<p>Hook usage is shown by some examples in this section. First, we will
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define a hook of arity 2 — that is, the procedures stored in the hook
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will have to accept two arguments.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define hook (make-hook 2))
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hook
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⇒ #<hook 2 40286c90>
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</pre></div>
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<p>Now we are ready to add some procedures to the newly created hook with
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<code class="code">add-hook!</code>. In the following example, two procedures are added,
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which print different messages and do different things with their
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arguments.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(add-hook! hook (lambda (x y)
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(display "Foo: ")
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(display (+ x y))
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(newline)))
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(add-hook! hook (lambda (x y)
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(display "Bar: ")
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(display (* x y))
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(newline)))
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</pre></div>
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<p>Once the procedures have been added, we can invoke the hook using
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<code class="code">run-hook</code>.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(run-hook hook 3 4)
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-| Bar: 12
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-| Foo: 7
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</pre></div>
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<p>Note that the procedures are called in the reverse of the order with
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which they were added. This is because the default behavior of
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<code class="code">add-hook!</code> is to add its procedure to the <em class="emph">front</em> of the
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hook’s procedure list. You can force <code class="code">add-hook!</code> to add its
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procedure to the <em class="emph">end</em> of the list instead by providing a third
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<code class="code">#t</code> argument on the second call to <code class="code">add-hook!</code>.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(add-hook! hook (lambda (x y)
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(display "Foo: ")
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(display (+ x y))
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(newline)))
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(add-hook! hook (lambda (x y)
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(display "Bar: ")
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(display (* x y))
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(newline))
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#t) ; <span class="r"><- Change here!</span>
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(run-hook hook 3 4)
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-| Foo: 7
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-| Bar: 12
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</pre></div>
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</div>
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</body>
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</html>
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