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From: axel@coma.uucp (Axel Mahler)
Newsgroups: comp.object
Subject: Inheriting from generic classes
Message-ID: <1994@opal.tubopal.UUCP>
Date: 4 Oct 90 17:20:39 GMT
Sender: news@tubopal.UUCP
Reply-To: axel@coma.uucp (Axel Mahler)
Organization: Technical University of Berlin, Germany
Lines: 49
Nntp-Posting-Host: coma.cs.tu-berlin.de
Originator: axel@coma.cs.tu-berlin.de

I am wondering about the semantics of inheritance from generic classes.
More precisely, it's the semantics of constrained genericity. Consider
the following situation:

  class C [P : T1]

This notation is not intended to resemble any existing programming language.
I think some new version of Eiffel will (has already ?) introduce this
notation for constrained genericity. The meaning is: declare a class C
with a formal generic parameter P that is constrained to class T1, i.e.
any actual parameter that will substitute P must be "kind-of" T1.

Now let:

  class D [Q : T2] inherit C

and finally:

  classdef E D[T3]

The latter construct shall denote a generic instantiation of generic class
D with T3 as actual parameter (yielding a "real" class E).

Do you think, this scenario makes any sense ? How shall both constraints
be interpreted ?  I came up with the opinion that the described setup 
does make sense if one adds both constraints, i.e.:

  T3 is-subclass-of T1 AND T3 is-subclass-of T2

I had an argument with a couple of collegues the other day, who insisted
that T2 should be a subclass of T1. One collegue proposed that the 
generic paramter lists of C and D should be semantically concatenated, so
that in the case of the generic instantiation two actual paramter classes
shall be provided: 

  classdef E D[T1a, T2a]

and T1a, and T2a satisfying the individual constraints of C and D 
respectively.

Unfortunately, there are only few discussions of genericity and inheritance.
What do you think ??

cheers, Axel Mahler
-- 
===============================================================================
Axel Mahler, Tech.Univ. Berlin

 E-Mail: axel@coma.cs.tu-berlin.de (axel@db0tui62.bitnet)