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154 lines
8.9 KiB
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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>Object Properties (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Object-Properties">
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<p>
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Next: <a href="Sorting.html" accesskey="n" rel="next">Sorting</a>, Previous: <a href="Equality.html" accesskey="p" rel="prev">Equality</a>, Up: <a href="Utility-Functions.html" accesskey="u" rel="up">General Utility Functions</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="Object-Properties-1"><span>6.9.2 Object Properties<a class="copiable-link" href="#Object-Properties-1"> ¶</a></span></h4>
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<p>It’s often useful to associate a piece of additional information with a
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Scheme object even though that object does not have a dedicated slot
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available in which the additional information could be stored. Object
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properties allow you to do just that.
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</p>
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<p>Guile’s representation of an object property is a procedure-with-setter
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(see <a class="pxref" href="Procedures-with-Setters.html">Procedures with Setters</a>) that can be used with the generalized
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form of <code class="code">set!</code> to set and retrieve that property for any Scheme object. So, setting a
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property looks like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(set! (my-property obj1) value-for-obj1)
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(set! (my-property obj2) value-for-obj2)
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</pre></div>
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<p>And retrieving values of the same property looks like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(my-property obj1)
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⇒
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value-for-obj1
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(my-property obj2)
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⇒
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value-for-obj2
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</pre></div>
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<p>To create an object property in the first place, use the
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<code class="code">make-object-property</code> procedure:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define my-property (make-object-property))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dobject_002dproperty"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-object-property</strong><a class="copiable-link" href="#index-make_002dobject_002dproperty"> ¶</a></span></dt>
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<dd><p>Create and return an object property. An object property is a
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procedure-with-setter that can be called in two ways. <code class="code">(set!
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(<var class="var">property</var> <var class="var">obj</var>) <var class="var">val</var>)</code> sets <var class="var">obj</var>’s <var class="var">property</var>
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to <var class="var">val</var>. <code class="code">(<var class="var">property</var> <var class="var">obj</var>)</code> returns the current
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setting of <var class="var">obj</var>’s <var class="var">property</var>.
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</p></dd></dl>
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<p>A single object property created by <code class="code">make-object-property</code> can
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associate distinct property values with all Scheme values that are
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distinguishable by <code class="code">eq?</code> (ruling out numeric values).
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</p>
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<p>Internally, object properties are implemented using a weak key hash
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table. This means that, as long as a Scheme value with property values
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is protected from garbage collection, its property values are also
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protected. When the Scheme value is collected, its entry in the
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property table is removed and so the (ex-) property values are no longer
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protected by the table.
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</p>
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<p>Guile also implements a more traditional Lispy interface to properties,
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in which each object has an list of key-value pairs associated with it.
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Properties in that list are keyed by symbols. This is a legacy
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interface; you should use weak hash tables or object properties instead.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-object_002dproperties"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">object-properties</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-object_002dproperties"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fobject_005fproperties"><span class="category-def">C Function: </span><span><strong class="def-name">scm_object_properties</strong> <var class="def-var-arguments">(obj)</var><a class="copiable-link" href="#index-scm_005fobject_005fproperties"> ¶</a></span></dt>
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<dd><p>Return <var class="var">obj</var>’s property list.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-set_002dobject_002dproperties_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">set-object-properties!</strong> <var class="def-var-arguments">obj alist</var><a class="copiable-link" href="#index-set_002dobject_002dproperties_0021"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fset_005fobject_005fproperties_005fx"><span class="category-def">C Function: </span><span><strong class="def-name">scm_set_object_properties_x</strong> <var class="def-var-arguments">(obj, alist)</var><a class="copiable-link" href="#index-scm_005fset_005fobject_005fproperties_005fx"> ¶</a></span></dt>
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<dd><p>Set <var class="var">obj</var>’s property list to <var class="var">alist</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-object_002dproperty"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">object-property</strong> <var class="def-var-arguments">obj key</var><a class="copiable-link" href="#index-object_002dproperty"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fobject_005fproperty"><span class="category-def">C Function: </span><span><strong class="def-name">scm_object_property</strong> <var class="def-var-arguments">(obj, key)</var><a class="copiable-link" href="#index-scm_005fobject_005fproperty"> ¶</a></span></dt>
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<dd><p>Return the property of <var class="var">obj</var> with name <var class="var">key</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-set_002dobject_002dproperty_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">set-object-property!</strong> <var class="def-var-arguments">obj key value</var><a class="copiable-link" href="#index-set_002dobject_002dproperty_0021"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fset_005fobject_005fproperty_005fx"><span class="category-def">C Function: </span><span><strong class="def-name">scm_set_object_property_x</strong> <var class="def-var-arguments">(obj, key, value)</var><a class="copiable-link" href="#index-scm_005fset_005fobject_005fproperty_005fx"> ¶</a></span></dt>
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<dd><p>In <var class="var">obj</var>’s property list, set the property named <var class="var">key</var>
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to <var class="var">value</var>.
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</p></dd></dl>
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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="Sorting.html">Sorting</a>, Previous: <a href="Equality.html">Equality</a>, Up: <a href="Utility-Functions.html">General Utility Functions</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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