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From: anderson@sapir.cog.jhu.edu (Stephen R. Anderson)
Newsgroups: comp.lang.clos
Subject: Re: Compatible metaclasses?
Message-ID: <ANDERSON.90Nov13143110@sapir.cog.jhu.edu>
Date: 13 Nov 90 19:31:10 GMT
References: <EGDORF.90Nov13094839@zaphod.lanl.gov>
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Distribution: comp.lang.clos
Organization: Dept. of Cognitive Science, The Johns Hopkins University
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As it happens, I have copies of two other lisps that include CLOS
running on my Sun 4/110 at the moment, that I'm trying to choose
between. So I tried this little example out on each, and can provide a
couple of data points (even if no clarification). First, Harlequin's
LispWorks (2.06) behaves the same way as Lucid (though with a slightly
less informative error message: "Error: #<STANDARD-CLASS
STANDARD-OBJECT 92c3f4> is an invalid superclass of #<MY -METACLASS
TOP-DOG 79fc04>" where Lucid says "Error: Attempt to define a class
#<My-Special-Metaclass UNDER-DOG> with a super-class of #<My-Metaclass
TOP-DOG>; but their respective metaclasses #<Standard-Class
MY-SPECIAL-METACLASS> and #<Standard-Class MY-METACLASS> are
incompatible").  Franz' Allegro Common Lisp 4.0beta, in contrast,
accepts this situation and behaves as Skip wants it to, allowing me to
read new-slot of top-dog and under-dog and another-new-slot of
under-dog.  So which behavior ought I to prefer?

Steve Anderson

Dept. of Cognitive Science
The Johns Hopkins University