159 lines
8.3 KiB
HTML
159 lines
8.3 KiB
HTML
|
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
||
|
|
<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
||
|
|
<HEAD>
|
||
|
|
<TITLE>28.1.7.1. Determining the Effective Method</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Determining the Effective Method">
|
||
|
|
<meta name="keywords" value="clm">
|
||
|
|
<meta name="resource-type" value="document">
|
||
|
|
<meta name="distribution" value="global">
|
||
|
|
<P>
|
||
|
|
<b>Common Lisp the Language, 2nd Edition</b>
|
||
|
|
<BR> <HR><A NAME=tex2html5209 HREF="node285.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5207 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5201 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5211 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5212 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html5210 HREF="node285.html"> Standard Method Combination</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html5208 HREF="node283.html"> Method Selection and </A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html5202 HREF="node283.html"> Method Selection and </A>
|
||
|
|
<HR> <P>
|
||
|
|
<H3><A NAME=SECTION003217100000000000000>28.1.7.1. Determining the Effective Method</A></H3>
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
<A NAME=DeterminingtheEffectiveMethodSECTION>The</A>
|
||
|
|
effective method for a set of
|
||
|
|
arguments is determined by the following three-step procedure:
|
||
|
|
<P>
|
||
|
|
<OL><LI> Select the applicable methods.
|
||
|
|
<P>
|
||
|
|
<LI> Sort the applicable methods by precedence order, putting
|
||
|
|
the most specific method first.
|
||
|
|
<P>
|
||
|
|
<LI> Apply method combination to the sorted list of
|
||
|
|
applicable methods, producing the effective method.
|
||
|
|
<P>
|
||
|
|
</OL>
|
||
|
|
<P>
|
||
|
|
<b>Selecting the Applicable Methods.</b>
|
||
|
|
This step is described in section <A HREF="node279.html#IntroductiontoMethodsSECTION">28.1.6.2</A>.
|
||
|
|
<P>
|
||
|
|
<b>Sorting the Applicable Methods by Precedence Order.</b>
|
||
|
|
To compare the precedence of two methods, their parameter specializers
|
||
|
|
are examined in order. The default examination order is from left to
|
||
|
|
right, but an alternative order may be specified by the
|
||
|
|
<tt>:argument-precedence-order</tt> option to <tt>defgeneric</tt> or to any of
|
||
|
|
the other forms that specify generic function options.
|
||
|
|
<P>
|
||
|
|
The corresponding parameter specializers from each method are
|
||
|
|
compared. When a pair of parameter specializers are equal, the next
|
||
|
|
pair are compared for equality. If all corresponding parameter
|
||
|
|
specializers are equal, the two methods must have different
|
||
|
|
qualifiers; in this case, either method can be selected to precede the
|
||
|
|
other.
|
||
|
|
<P>
|
||
|
|
If some corresponding parameter specializers are not equal, the first
|
||
|
|
pair of parameter specializers that are not equal determines the
|
||
|
|
precedence. If both parameter specializers are classes, the more
|
||
|
|
specific of the two methods is the method whose parameter specializer
|
||
|
|
appears earlier in the class precedence list of the corresponding
|
||
|
|
argument. Because of the way in which the set of applicable methods
|
||
|
|
is chosen, the parameter specializers are guaranteed to be present in
|
||
|
|
the class precedence list of the class of the argument.
|
||
|
|
<P>
|
||
|
|
If just one parameter specializer is <tt>(<tt>eql</tt> <i>object</i>)</tt>, the method with that parameter specializer precedes the
|
||
|
|
other method. If both parameter specializers are <tt>eql</tt>
|
||
|
|
forms, the
|
||
|
|
specializers must be the same (otherwise the two methods would
|
||
|
|
not both have been applicable to this argument).
|
||
|
|
<P>
|
||
|
|
The resulting list of applicable methods has the most specific
|
||
|
|
method first and the least specific method last.
|
||
|
|
<P>
|
||
|
|
<b>Applying Method Combination to the Sorted List of Applicable Methods.</b>
|
||
|
|
In the simple case-if standard method combination is used and all
|
||
|
|
applicable methods are primary methods-the effective method is the
|
||
|
|
most specific method. That method can call the next most specific
|
||
|
|
method by using the function <tt>call-next-method</tt>. The method that
|
||
|
|
<tt>call-next-method</tt> will call is referred to as the <i>next
|
||
|
|
method</i>. The predicate <tt>next-method-p</tt> tests whether a next
|
||
|
|
method exists. If <tt>call-next-method</tt> is called and there is no
|
||
|
|
next most specific method, the generic function <tt>no-next-method</tt>
|
||
|
|
is invoked.
|
||
|
|
<P>
|
||
|
|
In general, the effective method is some combination of the applicable
|
||
|
|
methods. It is defined by a Lisp form that contains calls to some or all
|
||
|
|
of the applicable methods, returns the value or values that will be
|
||
|
|
returned as the value or values of the generic function, and optionally
|
||
|
|
makes some of the methods accessible by means of <tt>call-next-method</tt>.
|
||
|
|
This Lisp form is the body of the effective method; it is augmented with
|
||
|
|
an appropriate lambda-list to make it a function.
|
||
|
|
<P>
|
||
|
|
The role of each method in the effective method is determined by its
|
||
|
|
method qualifiers and the specificity of the method. A qualifier
|
||
|
|
serves to mark a method, and the meaning of a qualifier is
|
||
|
|
determined by the way that these marks are used by this step
|
||
|
|
of the procedure. If an applicable method has an unrecognized
|
||
|
|
qualifier, this step signals an error and does not include that method
|
||
|
|
in the effective method.
|
||
|
|
<P>
|
||
|
|
When standard method combination is used together with qualified methods,
|
||
|
|
the effective method is produced as described in
|
||
|
|
section <A HREF="node285.html#StandardMethodCombinationSECTION">28.1.7.2</A>.
|
||
|
|
<P>
|
||
|
|
Another type of method combination can be specified by using the
|
||
|
|
<tt>:method-combination</tt> option of <tt>defgeneric</tt> or of any of the other
|
||
|
|
forms that specify generic function options. In this way this step of
|
||
|
|
the procedure can be customized.
|
||
|
|
<P>
|
||
|
|
New types of method combination can be defined by using the
|
||
|
|
<tt>define-method-combination</tt> macro.
|
||
|
|
<P>
|
||
|
|
The meta-object level also offers a mechanism for defining new types
|
||
|
|
of method combination. The generic function
|
||
|
|
<tt>compute-effective-method</tt> receives as arguments the generic function,
|
||
|
|
the method combination object, and the sorted list of applicable
|
||
|
|
methods. It returns the Lisp form that defines the effective method.
|
||
|
|
A method for <tt>compute-effective-method</tt> can be defined directly by
|
||
|
|
using <tt>defmethod</tt> or indirectly by using
|
||
|
|
<tt>define-method-combination</tt>.
|
||
|
|
A <i>method combination object</i> is an
|
||
|
|
object that encapsulates the method combination type and options
|
||
|
|
specified by the <tt>:method-combination</tt> option to forms that
|
||
|
|
specify generic function options.
|
||
|
|
<P>
|
||
|
|
<hr>
|
||
|
|
<b>Implementation note:</b> In the simplest implementation, the generic function would compute
|
||
|
|
the effective method each time it was called. In practice, this will
|
||
|
|
be too inefficient for some implementations. Instead, these
|
||
|
|
implementations might employ a variety of optimizations of the
|
||
|
|
three-step procedure. Some illustrative examples of such optimizations
|
||
|
|
are the following:
|
||
|
|
<P>
|
||
|
|
<UL><LI> Use a hash table keyed by the class of the arguments to
|
||
|
|
store the effective method.
|
||
|
|
<P>
|
||
|
|
<LI> Compile the effective method and save the resulting
|
||
|
|
compiled function in a table.
|
||
|
|
<P>
|
||
|
|
<LI> Recognize the Lisp form as an instance of a pattern of
|
||
|
|
control structure and substitute a closure that implements
|
||
|
|
that structure.
|
||
|
|
<P>
|
||
|
|
<LI> Examine the parameter specializers of all methods for the
|
||
|
|
generic function and enumerate all possible effective methods.
|
||
|
|
Combine the effective methods, together with code to select from
|
||
|
|
among them, into a single function and compile that function. Call
|
||
|
|
that function whenever the generic function is called.
|
||
|
|
</UL>
|
||
|
|
<hr>
|
||
|
|
<img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<BR> <HR><A NAME=tex2html5209 HREF="node285.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5207 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5201 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5211 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5212 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html5210 HREF="node285.html"> Standard Method Combination</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html5208 HREF="node283.html"> Method Selection and </A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html5202 HREF="node283.html"> Method Selection and </A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|