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144 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Customizing Class Redefinition (Guile Reference Manual)</title>
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<link href="Redefining-a-Class.html" rel="up" title="Redefining a Class">
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<div class="subsection-level-extent" id="Customizing-Class-Redefinition">
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<p>
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Previous: <a href="Default-Class-Redefinition-Behavior.html" accesskey="p" rel="prev">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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<hr>
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<h4 class="subsection" id="Customizing-Class-Redefinition-1"><span>8.12.3 Customizing Class Redefinition<a class="copiable-link" href="#Customizing-Class-Redefinition-1"> ¶</a></span></h4>
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<p>When <code class="code">define-class</code> notices that a class is being redefined, it
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constructs the new class metaobject as usual, then invokes the
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<code class="code">class-redefinition</code> generic function with the old and new classes
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as arguments. Therefore, if the old or new classes have metaclasses
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other than the default <code class="code"><redefinable-class></code>, class redefinition
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behavior can be customized by defining a <code class="code">class-redefinition</code>
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method that is specialized for the relevant metaclasses.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-class_002dredefinition"><span class="category-def">generic: </span><span><strong class="def-name">class-redefinition</strong><a class="copiable-link" href="#index-class_002dredefinition"> ¶</a></span></dt>
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<dd><p>Handle the class redefinition from <var class="var">old</var> to <var class="var">new</var>, and return
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the new class metaobject that should be bound to the variable specified
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by <code class="code">define-class</code>’s first argument.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-class_002dredefinition-1"><span class="category-def">method: </span><span><strong class="def-name">class-redefinition</strong> <var class="def-var-arguments">(old <top>) (new <class>)</var><a class="copiable-link" href="#index-class_002dredefinition-1"> ¶</a></span></dt>
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<dd><p>Not all classes are redefinable, and not all previous bindings are
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classes. See <a class="xref" href="Redefinable-Classes.html">Redefinable Classes</a>. This default method just returns
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<var class="var">new</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-class_002dredefinition-2"><span class="category-def">method: </span><span><strong class="def-name">class-redefinition</strong> <var class="def-var-arguments">(old <redefinable-class>) (new <redefinable-class>)</var><a class="copiable-link" href="#index-class_002dredefinition-2"> ¶</a></span></dt>
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<dd><p>This method implements GOOPS’ default class redefinition behavior, as
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described in <a class="ref" href="Default-Class-Redefinition-Behavior.html">Default Class Redefinition Behavior</a>. Returns the
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metaobject for the new class definition.
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</p></dd></dl>
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<p>The <code class="code">class-redefinition</code> method for classes with metaclass
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<code class="code"><redefinable-class></code> calls the following generic functions, which
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could of course be individually customized.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-remove_002dclass_002daccessors_0021"><span class="category-def">generic: </span><span><strong class="def-name">remove-class-accessors!</strong> <var class="def-var-arguments">old</var><a class="copiable-link" href="#index-remove_002dclass_002daccessors_0021"> ¶</a></span></dt>
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<dd><p>The default <code class="code">remove-class-accessors!</code> method removes the accessor
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methods of the old class from all classes which they specialize.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-update_002ddirect_002dmethod_0021"><span class="category-def">generic: </span><span><strong class="def-name">update-direct-method!</strong> <var class="def-var-arguments">method old new</var><a class="copiable-link" href="#index-update_002ddirect_002dmethod_0021"> ¶</a></span></dt>
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<dd><p>The default <code class="code">update-direct-method!</code> method substitutes the new
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class for the old in all methods specialized to the old class.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-update_002ddirect_002dsubclass_0021"><span class="category-def">generic: </span><span><strong class="def-name">update-direct-subclass!</strong> <var class="def-var-arguments">subclass old new</var><a class="copiable-link" href="#index-update_002ddirect_002dsubclass_0021"> ¶</a></span></dt>
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<dd><p>The default <code class="code">update-direct-subclass!</code> method invokes
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<code class="code">class-redefinition</code> recursively to handle the redefinition of
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subclasses.
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</p></dd></dl>
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<p>An alternative class redefinition strategy could be to leave all
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existing instances as instances of the old class, but accepting that the
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old class is now “nameless”, since its name has been taken over by the
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new definition. In this strategy, any existing subclasses could also
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be left as they are, on the understanding that they inherit from a nameless
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superclass.
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</p>
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<p>This strategy is easily implemented in GOOPS, by defining a new metaclass,
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that will be used as the metaclass for all classes to which the strategy
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should apply, and then defining a <code class="code">class-redefinition</code> method that
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is specialized for this metaclass:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-class <can-be-nameless> (<redefinable-class>))
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(define-method (class-redefinition (old <can-be-nameless>)
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(new <class>))
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new)
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</pre></div>
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<p>When customization can be as easy as this, aren’t you glad that GOOPS
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implements the far more difficult strategy as its default!
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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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Previous: <a href="Default-Class-Redefinition-Behavior.html">Default Class Redefinition Behavior</a>, Up: <a href="Redefining-a-Class.html">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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