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From: hws@icsib1.Berkeley.EDU (Heinz Schmidt)
Newsgroups: comp.object
Subject: Re: OO Languages with "constraints" (Was: Subclassing vs. subtyping)
Message-ID: <18450@pasteur.Berkeley.EDU>
Date: 17 Oct 89 01:50:52 GMT
References: <CLINE.89Oct14182152@sun.clarkson.edu> <33009@cornell.UUCP> <18129@pasteur.Berkeley.EDU>
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Reply-To: hws@icsib1.Berkeley.EDU (Heinz Schmidt)
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Organization: International Computer Science Institute, Berkeley
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In article <CLINE.89Oct14182152@sun.clarkson.edu>,
cline@sun.soe.clarkson.edu (Marshall Cline) writes:
...
> >              HEIGHT=WIDTH
> >(some OO languages allow to express such constraints)....
> What languages?
...
>But one of the cries at OOPSLA'89 was the need to support this kind of
>semantic information in our OO Languages.  No one said "Language X does

In Eiffel, you write the constraint as a class invariant [B.  Meyer:
Object-oriented Software Construction, Prentice Hall, 88].  You would
have to implement it, though, but the system can do some checking.

In OBJ2, an executable OO specification language, you can express
equational constraints.  [Principles of OBJ2, Conf. Rec.  12-th ACM
Symp. POPL 85, pp.52--66; there is also a more recent techrep., don't
have the reference at hand, it goes something like Goguen et al:
Introduction to OBJ2, SRI, Menlo Park, CA, 88?]

You might also want to have a closer look at OO logic programming
languages where classes are sets of facts (recent posting in this
group about ObjVProlog-D, for instance.) As I have limited experience
with PROLOG only, I can figure that one adds clausal constraints
easily.

-- Heinz Schmidt  (hws@icsi.berkeley.edu)