Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!usc!trwind!gumby!wiley!alice.coyote.trw.com!scott
From: scott@alice.coyote.trw.com (Scott Simpson)
Newsgroups: comp.lang.lisp
Subject: Re: CLOS questions
Summary: Create a new class
Keywords: classes, discriminated records, public domain CLOS
Message-ID: <2726886C.2C1B@wilbur.coyote.trw.com>
Date: 25 Oct 90 06:38:36 GMT
References: <28175@bellcore.bellcore.com>
Sender: news@wilbur.coyote.trw.com (News Software)
Organization: TRW Inc., Redondo Beach, CA
Lines: 72

In article <28175@bellcore.bellcore.com> dsm@ctt.bellcore.com (David S. Miller) writes:
>I'm thinking of using CLOS to implement a project, and I have a few
>(1) Are there any public domain (ftp'able) versions of CLOS out there?
>(for a Sun 3)?

PCL (Portable Common Loops) from arisia.xerox.com. I believe it is
very close to standard CLOS.

>(2) Is it possible for an object to be an instance of two disjoint classes?

Not that I know of.

>(e.g.) Supposing I have classes representing groups of people such as
>employees, students, and athletes.  What if I want to represent an individual
>student-employee, or student-athlete, without creating a new class?

Why don't you want to create a new class? Assuming that employees,
students and athletes are all disjoint, you could have a hierarchy
like

        		 		person
					   |
            ------------------------------------------------------
       	   /				   |		          \
	employee			student		        athlete
           \    			 | \                       / |
	    \   			 |  -----------------------  |
	     \  			 |      	|            |
	      \ 			 |          student-athlete  |
	       ---------------- 	 |      	             |
			        \        |      	             |
				 \ 	 | 		             /
				  \	 | 	          -----------
				student-employee-athlete /

>As far as I can tell, it doesn't seem possible for an object to be an
>instance of more than one class.  Yet, it seems inappropriate to have
>to create a new class every time you want a new instance to inherit
>attributes from more than one existing class.

You can inherit attributes from more than one class by using multiple
inheritance. You still have to create a new class to do multiple
inheritance though. Why is creating a new class inappropriate?

>(3) Is there any mechanism in CLOS for automatically classifying an
>object into a class based on attribute values?
>
>(e.g.) In the previous example, suppose the 'student' class has an
>"athletic-prowess' slot.  If the value of that slot was set to 'high
>for a particular instance, then I'd want that instance to be
>classified into the 'athlete' class as well.

Why couldn't you write a routine to take a parameter that is the
athletic-prowess? You then compute on this parameter and create either
a student or athlete object and return it. Athlete would inherit from
student and student would contain the athletic-prowess slot.

>The best way that I can think of to handle this is to define a method
>such as:
>(defmethod (setf athletic-prowess) ((new-value (eql 'high)) ...)
>	...classify this instance as an athlete...  )

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. If you didn't create a brand new object,
then anybody pointing to the old object would suddenly be pointing to
a new object of a different type! Ada has this problem with
dynamically allocated discriminated record types. Consequently, in
Ada, you cannot change the discriminant after the record is
dynamically allocated.
--
Scott Simpson			TRW			scott@coyote.trw.com