169 lines
8.9 KiB
HTML
169 lines
8.9 KiB
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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>Customizing Class Definition (Guile Reference Manual)</title>
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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="The-Metaobject-Protocol.html" rel="up" title="The Metaobject Protocol">
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<link href="Method-Definition.html" rel="next" title="Method Definition">
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<link href="Class-Definition-Protocol.html" rel="prev" title="Class Definition Protocol">
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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="Customizing-Class-Definition">
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<div class="nav-panel">
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<p>
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Next: <a href="Method-Definition.html" accesskey="n" rel="next">Method Definition</a>, Previous: <a href="Class-Definition-Protocol.html" accesskey="p" rel="prev">Class Definition Protocol</a>, Up: <a href="The-Metaobject-Protocol.html" accesskey="u" rel="up">The Metaobject Protocol</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="Customizing-Class-Definition-1"><span>8.11.6 Customizing Class Definition<a class="copiable-link" href="#Customizing-Class-Definition-1"> ¶</a></span></h4>
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<p>If the metaclass of the new class is something more specialized than the
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default <code class="code"><class></code>, then the type of <var class="var">class</var> in the calls above
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is more specialized than <code class="code"><class></code>, and hence it becomes possible
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to define generic function methods, specialized for the new class’s
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metaclass, that can modify or override the default behavior of
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<code class="code">initialize</code>, <code class="code">compute-cpl</code> or <code class="code">compute-get-n-set</code>.
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</p>
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<p><code class="code">compute-cpl</code> computes the class precedence list (“CPL”) for the
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new class (see <a class="pxref" href="Class-Precedence-List.html">Class Precedence List</a>), and returns it as a list of
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class objects. The CPL is important because it defines a superclass
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ordering that is used, when a generic function is invoked upon an
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instance of the class, to decide which of the available generic function
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methods is the most specific. Hence <code class="code">compute-cpl</code> could be
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customized in order to modify the CPL ordering algorithm for all classes
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with a special metaclass.
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</p>
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<p>The default CPL algorithm is encapsulated by the <code class="code">compute-std-cpl</code>
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procedure, which is called by the default <code class="code">compute-cpl</code> method.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-compute_002dstd_002dcpl"><span class="category-def">procedure: </span><span><strong class="def-name">compute-std-cpl</strong> <var class="def-var-arguments">class</var><a class="copiable-link" href="#index-compute_002dstd_002dcpl"> ¶</a></span></dt>
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<dd><p>Compute and return the class precedence list for <var class="var">class</var> according
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to the algorithm described in <a class="ref" href="Class-Precedence-List.html">Class Precedence List</a>.
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</p></dd></dl>
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<p><code class="code">compute-slots</code> computes and returns a list of all slot definitions
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for the new class. By default, this list includes the direct slot
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definitions from the <code class="code">define-class</code> form, plus the slot definitions
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that are inherited from the new class’s superclasses. The default
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<code class="code">compute-slots</code> method uses the CPL computed by <code class="code">compute-cpl</code>
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to calculate this union of slot definitions, with the rule that slots
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inherited from superclasses are shadowed by direct slots with the same
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name. One possible reason for customizing <code class="code">compute-slots</code> would be
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to implement an alternative resolution strategy for slot name conflicts.
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</p>
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<p><code class="code">compute-get-n-set</code> computes the low-level closures that will be
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used to get and set the value of a particular slot, and returns them in
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a list with two elements.
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</p>
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<p>The closures returned depend on how storage for that slot is allocated.
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The standard <code class="code">compute-get-n-set</code> method, specialized for classes of
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type <code class="code"><class></code>, handles the standard GOOPS values for the
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<code class="code">#:allocation</code> slot option (see <a class="pxref" href="Slot-Options.html">allocation</a>). By
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defining a new <code class="code">compute-get-n-set</code> method for a more specialized
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metaclass, it is possible to support new types of slot allocation.
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</p>
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<p>Suppose you wanted to create a large number of instances of some class
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with a slot that should be shared between some but not all instances of
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that class - say every 10 instances should share the same slot storage.
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The following example shows how to implement and use a new type of slot
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allocation to do this.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-class <batched-allocation-metaclass> (<class>))
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(let ((batch-allocation-count 0)
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(batch-get-n-set #f))
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(define-method (compute-get-n-set
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(class <batched-allocation-metaclass>) s)
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(case (slot-definition-allocation s)
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((#:batched)
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;; If we've already used the same slot storage for 10 instances,
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;; reset variables.
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(if (= batch-allocation-count 10)
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(begin
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(set! batch-allocation-count 0)
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(set! batch-get-n-set #f)))
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;; If we don't have a current pair of get and set closures,
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;; create one. make-closure-variable returns a pair of closures
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;; around a single Scheme variable - see goops.scm for details.
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(or batch-get-n-set
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(set! batch-get-n-set (make-closure-variable)))
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;; Increment the batch allocation count.
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(set! batch-allocation-count (+ batch-allocation-count 1))
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batch-get-n-set)
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;; Call next-method to handle standard allocation types.
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(else (next-method)))))
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(define-class <class-using-batched-slot> ()
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...
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(c #:allocation #:batched)
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...
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#:metaclass <batched-allocation-metaclass>)
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</pre></div>
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<p>The usage of <code class="code">compute-getter-method</code> and <code class="code">compute-setter-method</code>
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is described in <a class="ref" href="Class-Definition-Protocol.html">Class Definition Protocol</a>.
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</p>
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<p><code class="code">compute-cpl</code> and <code class="code">compute-get-n-set</code> are called by the
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standard <code class="code">initialize</code> method for classes whose metaclass is
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<code class="code"><class></code>. But <code class="code">initialize</code> itself can also be modified, by
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defining an <code class="code">initialize</code> method specialized to the new class’s
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metaclass. Such a method could complete override the standard
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behavior, by not calling <code class="code">(next-method)</code> at all, but more
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typically it would perform additional class initialization steps before
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and/or after calling <code class="code">(next-method)</code> for the standard behavior.
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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="Method-Definition.html">Method Definition</a>, Previous: <a href="Class-Definition-Protocol.html">Class Definition Protocol</a>, Up: <a href="The-Metaobject-Protocol.html">The Metaobject Protocol</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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