90 lines
3.9 KiB
HTML
90 lines
3.9 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>Redefinable Classes (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Redefinable-Classes">
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Next: <a href="Default-Class-Redefinition-Behavior.html" accesskey="n" rel="next">Default Class Redefinition Behavior</a>, Up: <a href="Redefining-a-Class.html" accesskey="u" rel="up">Redefining a Class</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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<h4 class="subsection" id="Redefinable-Classes-1"><span>8.12.1 Redefinable Classes<a class="copiable-link" href="#Redefinable-Classes-1"> ¶</a></span></h4>
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<p>The ability for a class to be redefined is a choice for a class author
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to make. By default, classes in GOOPS are <em class="emph">not</em> redefinable. A
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redefinable class is an instance of <code class="code"><redefinable-class></code>; that is
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to say, a class with <code class="code"><redefinable-class></code> as its metaclass.
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Accordingly, to define a redefinable class, add <code class="code">#:metaclass
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<redefinable-class></code> to its class definition:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-class <foo> ()
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#:metaclass <redefinable-class>)
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</pre></div>
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<p>Note that any subclass of <code class="code"><foo></code> is also redefinable, without the
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need to explicitly pass the <code class="code">#:metaclass</code> argument, so you only
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need to specify <code class="code">#:metaclass</code> for the roots of your application’s
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class hierarchy.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-class <bar> (<foo>))
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(class-of <bar>) ⇒ <redefinable-class>
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</pre></div>
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<p>Note that prior to Guile 3.0, all GOOPS classes were redefinable in
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theory. In practice, attempting to, for example, redefine
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<code class="code"><class></code> itself would almost certainly not do what you want.
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Still, redefinition is an interesting capability when building
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long-lived resilient systems, so GOOPS does offer this facility.
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</p>
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