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   Re: EquivClass & Derived Type

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  • From: Stefan Haustein <haustein@kimo.cs.uni-dortmund.de>
  • To: Michael Anderson <michael@research.canon.com.au>
  • Date: Thu, 17 Feb 2000 12:43:18 +0100

Michael Anderson wrote:

> Thanks for all the replies to my previous questions on equivClass.  I
> now have a new problem based on my new understanding.  At the moment
> there exists equivClass and derived type, but I do not see why both are
> necessary as derived type could do the same job.

I do not see the need, too. You can find several messages
about this by looking at the thread "Schema concepts"

> By separating equivalence from inheritance, are we saying that even
> elements of the same type or subtypes of the same type are not
> necessarily substitutable by one another in every context.  For
> instance, even if elements "name" and "account number" are both of type
> string, they could not be used in place of one another in most context.
> Is this the main reason behind the separation of inheritance and
> equivalence?

"String" is a data type, so name and account number would
not be of the same element type, and the problem would
not exist. Also, you could allways hinder substitution 
by deriving a new element locally.

Best regards


Stefan Haustein
University of Dortmund
Computer Science VIII

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