104 lines
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104 lines
5.5 KiB
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<title>Method-Based Discriminating Functions - SBCL Internals</title>
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<link rel="up" href="Discriminating-Functions.html#Discriminating-Functions" title="Discriminating Functions">
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<link rel="prev" href="The-Initial-Discriminating-Function.html#The-Initial-Discriminating-Function" title="The Initial Discriminating Function">
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<link rel="next" href="Accessor-Discriminating-Functions.html#Accessor-Discriminating-Functions" title="Accessor Discriminating Functions">
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This manual is part of the SBCL software system. See the `README'
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file for more information.
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This manual is in the public domain and is provided with
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<p>
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<a name="Method-Based-Discriminating-Functions"></a>
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<a name="Method_002dBased-Discriminating-Functions"></a>
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Next: <a rel="next" accesskey="n" href="Accessor-Discriminating-Functions.html#Accessor-Discriminating-Functions">Accessor Discriminating Functions</a>,
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Previous: <a rel="previous" accesskey="p" href="The-Initial-Discriminating-Function.html#The-Initial-Discriminating-Function">The Initial Discriminating Function</a>,
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Up: <a rel="up" accesskey="u" href="Discriminating-Functions.html#Discriminating-Functions">Discriminating Functions</a>
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</div>
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<!-- node-name, next, previous, up -->
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<h3 class="section">3.2 Method-Based Discriminating Functions</h3>
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<p><a name="index-no_002dapplicable_002dmethod-3"></a>
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The method-based discriminating functions are used if all the methods of
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the generic function at the time of the first call are suitable:
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therefore, these discriminating function strategies do not transition
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into any of the other states unless the generic function is
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reinitialized. Of these discriminating functions, the simplest is the
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<code>SB-PCL::NO-METHODS</code>, which is appropriate when the generic
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function has no methods. In this case, the discriminating function
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simply performs an argument count check<a rel="footnote" href="Method_002dBased-Discriminating-Functions.html#fn-1" name="fnd-1"><sup>1</sup></a> and then calls
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<code>NO-APPLICABLE-METHOD</code> with the appropriate arguments.
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<p>If all of the specializers in all methods of the generic function are
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the root of the class hierarchy, <code>t</code>, then no discrimination need
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be performed: all of the methods are applicable on every
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call<a rel="footnote" href="Method_002dBased-Discriminating-Functions.html#fn-2" name="fnd-2"><sup>2</sup></a>. In this case, the <code>SB-PCL::DEFAULT-METHOD-ONLY</code>
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discriminating function can call the effective method directly, as it
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will be the same for every generic function call.<a rel="footnote" href="Method_002dBased-Discriminating-Functions.html#fn-3" name="fnd-3"><sup>3</sup></a>
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<p>If all methods of the generic function are known by the system to be
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side-effect-free and return constants, and the generic function has
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standard-method-combination and no eql-specialized methods, then the
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<code>SB-PCL::CONSTANT-VALUE</code> discriminating function can simply cache
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the return values for given argument types. Though this may initially
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appear to have limited applicability, type predicates are usually of
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this form, as in <a href="ex_003apred.html#ex_003apred">ex:pred</a><a rel="footnote" href="Method_002dBased-Discriminating-Functions.html#fn-4" name="fnd-4"><sup>4</sup></a>.
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<div class="float">
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<a name="ex_003apred"></a>
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<pre class="example"> (defgeneric foop (x))
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(defmethod foop ((foo foo)) t)
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(defmethod foop (object) nil)
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</pre>
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<p><strong class="float-caption">Example 3.1</strong></p></div>
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<p>More details of the cacheing mechanism are given in <a href="The-Cacheing-Mechanism.html#The-Cacheing-Mechanism">The Cacheing Mechanism</a> below.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="Method_002dBased-Discriminating-Functions.html#fnd-1">1</a>]</small> Actually, this bit
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isn't currently done. Oops.</p>
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<p class="footnote"><small>[<a name="fn-2" href="Method_002dBased-Discriminating-Functions.html#fnd-2">2</a>]</small> Hm, there might be another problem with argument count
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here.</p>
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<p class="footnote"><small>[<a name="fn-3" href="Method_002dBased-Discriminating-Functions.html#fnd-3">3</a>]</small> I wonder if
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we're invalidating this right if we define a method on
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compute-applicable-methods...</p>
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<p class="footnote"><small>[<a name="fn-4" href="Method_002dBased-Discriminating-Functions.html#fnd-4">4</a>]</small> There is vestigial code in SBCL
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for a currently unused specialization of <code>SB-PCL::CONSTANT-VALUE</code>
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for boolean values only.</p>
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<hr></div>
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