🚀 go-pugleaf

In article <pallas.669923769@red-dwarf> pallas@eng.sun.com (Joseph Pallas) writes:
>The problem is, no one has ever come up with a convincing reason why I
>should want my type system to handle the heterogeneous collection.
>This is because no one has come up with a convincing reason why I
>should want to write programs that contain heterogeneous collections.
>Needing collections of otherwise-unrelated objects generally signals a
>flaw in the design, not a failing of the type system.
>

Well, let me see how I can take a position on both sides of this
argument. I must admit that I missed the earlier segments, so if I am
reiterating something that has been already addressed, my apologies.

There are cases in object-oriented database systems where the results
of a query is a heterogeneous collection. Object algebras take as
input sets of objects and produce as output sets of objects (sometimes
new objects that were not in the database to begin with -- e.g.,
explicit join -- but that is not important here). It is quite possible
that the result set of objects is a heterogeneous collection. So,
there is a need for the ability to handle heterogeneous sets of
objects. 

On the other hand, I don't see why a type system and type checking
mechanism cannot be developed to handle these collections.  Some
object algebras impose what is called union compatibility on the
algebra operators [Shaw and Zdonik paper in the 6th Data Engineering
Conference, 1990]. Union compatibility requires that members of the
sets being operated on to be instances of types which are in a subtype
relationship with one another. However, this may be too restricted. In
last year's OOPSLA (1990), we presented a paper [pages 224-233] that
discusses a more general type system for a simplified object algebra.
The paper also discusses the issues of heterogeneous sets. This is by
no means the definitive word on the subject, but it demonstrates that
a type system, which is sometimes more restricted than one may like,
can be developed.

==Tamer



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M. Tamer Ozsu				Telephone:	(617) 466-2098	
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