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<TITLE>Brief Guide to CLOS: Generic functions and methods</TITLE>
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<H2><A NAME="s6">6. Generic functions and methods</A></H2>
<P>Generic function in <CODE>CLOS</CODE> are the closest thing to "messages".
Instead of writing</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(SEND instance operation-name arg*)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>you write</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(operation-name instance arg*)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>The operations / messages are generic functions -- functions whose
behavior can be defined for instances of particular classes by
defining methods.</P>
<P><CODE>(DEFGENERIC function-name lambda-list)</CODE> can be used to define a generic
function. You don't have to call <CODE>DEFGENERIC</CODE>, however, because
<CODE>DEFMETHOD</CODE>
automatically defines the generic function if it has not been defined
already. On the other hand, it's often a good idea to use <CODE>DEFGENERIC</CODE>
as a declaration that an operation exists and has certain parameters.</P>
<P>Anyway, all of the interesting things happen in methods. A method is
defined by:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(DEFMETHOD generic-function-name specialized-lambda-list
form*)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>This may look fairly cryptic, but compare it to <CODE>DEFUN</CODE> described in
a similar way:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(DEFUN function-name lambda-list form*)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>A "lambda list" is just a list of formal parameters, plus things like
<CODE>&OPTIONAL</CODE> or <CODE>&REST</CODE>. It's because of such
complications that we say
"lambda-list" instead of "(parameter*)"
when describing the syntax.</P>
<P>[I won't say anything
about <CODE> &OPTIONAL </CODE>,
<CODE>&REST</CODE>, or <CODE>&KEY</CODE> in methods.
The rules are in CLtL2 or the HyperSpec, if you want to know them.]</P>
<P>So a normal function has a lambda list like <CODE>(var1 var2 ...)</CODE>.
A method has one in which each parameter can be "specialized"
to a particular class. So it looks like:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
((var1 class1) (var2 class2) ...)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>The specializer is optional. Omitting it means that the method
can apply to instances of any class, including classes that were
not defined by <CODE>DEFCLASS</CODE>. For example:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(defmethod change-subject ((teach teacher) new-subject)
(setf (teacher-subject teach) new-subject))
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>Here the new-subject could be any object. If you want to restrict
it, you might do something like:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(defmethod change-subject ((teach teacher) (new-subject string))
(setf (teacher-subject teach) new-subject))
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>Or you could define classes of subjects.</P>
<P>Methods in "classical" object-oriented programming specialize
only one parameter. In <CODE>CLOS</CODE>, you can specialize more than one.
If you do, the method is sometimes called a multi-method.</P>
<P>A method defined for a class <CODE>C</CODE> overrides any method defined
for a superclass of <CODE>C</CODE>. The method for <CODE>C</CODE> is
"more specific"
than the method for the superclass, because <CODE>C</CODE> is more specific
that the classes it inherits from (eg, dog is more specific
than animal).</P>
<P>For multi-methods, the determination of which method is more
specific involves more than one parameter. The parameters
are considered from left to right.</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(defmethod test ((x number) (y number))
'(num num))
(defmethod test ((i integer) (y number))
'(int num))
(defmethod test ((x number) (j integer))
'(num int))
(test 1 1) => (int num), not (num int)
(test 1 1/2) => (int num)
(test 1/2 1) => (num int)
(test 1/2 1/2) => (num num)
</PRE>
</CODE></BLOCKQUOTE>
</P>
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