Path: utzoo!attcan!uunet!decwrl!ucbvax!agate!linus!linus!john
From: john@linus.mitre.org (John D. Burger)
Newsgroups: comp.lang.lisp
Subject: Re: CLOS questions
Message-ID: <124464@linus.mitre.org>
Date: 25 Oct 90 20:40:46 GMT
References: <28175@bellcore.bellcore.com> <2726886C.2C1B@wilbur.coyote.trw.com>
Reply-To: john@mitre.org
Organization: The MITRE Corporation, Bedford MA
Lines: 42

The reason a new class has to be created when you want to inherit from
two or more classes is that CLOS requires you to define the ORDER of
the inheritance.  Thus if #<Fred> is an instance of both
#<Standard-Class ATHLETE> and #<Standard-Class STUDENT>, it's
necessary to know the order in which #<Fred> should inherit from these
classes.  This must be done in CLOS by defining a NEW class, say
#<Standard-Class STUDENT-ATHLETE>, whose DIRECT-SUPERCLASSES slot
dictates the order of inheritance.

scott@alice.coyote.trw.com (Scott Simpson) writes:

>The problem is that you want to reclassify an object after creation
>time. The only way I know of doing this is to create a brand new
>object in a different class.

Actually, one can use CHANGE-CLASS to ``coerce'' an instance from its
current class to a new one.  Thus

(SETF FRED (MAKE-INSTANCE 'STUDENT :NAME "Fred"))
  #<Instance of STUDENT Fred 1ab3467>
(CLASS-OF FRED)
  #<Standard-Class STUDENT>
(CHANGE-CLASS FRED 'ATHLETE)
  #<Instance of ATHLETE Fred 1ab3467>
(CLASS-OF FRED)
  #<Standard-Class ATHLETE>

Note that Fred is the exact same object.

It's fairly easy to write CLOS code to cons up new mixtures of
existing classes when necessary.  For instance, posit a function
FIND-MIXTURE, which takes any number of classes as arguments, and
finds or creates a new class with exactly those supers.  Thus
(FIND-MIXTURE 'STUDENT 'ATHLETE) would create a new class, perhaps
called STUDENT-ATHLETE, assuming no such class already existed.  Then
one could call CHANGE-CLASS on Fred and this new class, whenever it
became apparent that Fred was not just a student.
-- 
John Burger                                               john@mitre.org

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