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From: KINDERMA@tubvm.cs.tu-berlin.de (Carsten Kindermann)
Newsgroups: comp.lang.eiffel
Subject: Class Grouping
Message-ID: <90338.181137KINDERMA@DB0TUI11.BITNET>
Date: 4 Dec 90 16:11:37 GMT
Organization: Technical University Berlin
Lines: 49


I am interested in constraining a formal generic parameter of a class
definition to more than a single class type.

As an illustration consider the (overly simplified) implementation of a
frame-like knowledge representation system:  FRAMEs have SLOTs, and the
value of a SLOT is a list of either FRAMEs, INTEGERs, or STRINGs.  The
problem now is to group the three classes FRAME, INTEGER, and STRING
together to get a single class SLOT-VALUE-LIST.  SLOT-VALUE-LIST would
then be a descendent of LIST, and might additionally be a sorted list.

My question thus is:  Is there any way to express in Eiffel that
SLOT-VALUE-LIST[T] does not hold list elements of any type T, but that T
must be exactly one of FRAME, INTEGER, STRING.

I could currently think of two solutions:

(1) Constraining a formal generic parameter by a list of allowable
Class_types.

    class SLOT-VALUE-LIST [T-> [FRAME,INTEGER,STRING] ]   ...

This seems not to be allowed in Eiffel 2.2, but actually is, I guess,
what I am looking for.

(2) Defining SLOT-VALUES, a common super-class of FRAME, INTEGER,
STRING.  Defining this class, however, would make it necessary to define
MYINTEGER (I assume the basic class INTEGER must not be defined as a
descendent of a user-defined class :-) ), but then the INTEGER slot-val-
ues would become objects rather than constants.

Thus, is there any way to do "class grouping" (I think that is how it is
called in the semantic data model area) in Eiffel?  If not, how would
you solve the problem?  Or maybe, could somebody just give the reasons,
*why* class grouping is not/cannot be included into Eiffel?

Thanks, Carsten Kindermann.


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Carsten Kindermann
TU Berlin, Projekt KIT-BACK            kinderma@tubvm.cs.tu-berlin.de
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