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Newsgroups: comp.lang.ada
Date: Thu, 31 May 2018 16:39:50 -0700 (PDT)
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Subject: Re: about inheritance of subtypes and entities (such as constants)
 related to a type in the same package
From: Mehdi Saada <00120260a@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:52830
Date: 2018-05-31T16:39:50-07:00
List-Id: <comp.lang.ada>

The idea was to define for a given (private) type, a series of subtypes wit=
h different type invariant, or with or without unknown discriminants (the b=
ox), to provide specific variant of the same ADT.
I know now that is meant to be done through tagged types, and that subtypes=
 are no good for that. But I would have liked a mechanism to generate, why =
deriving a new type from a parent, these tailored subtypes, with the same s=
ame, just like they would have been rewritten.
But it doesn't take much to see it wasn't a good idea, and that a lot probl=
em would occur... like: what happens when the derived type has constraints =
or invariants incompatible with subtypes associated with the parent ?

I should say "Copying" rather than inheriting. "Copying" constants would fa=
il for the same reason. I/one is better of with converting the constants de=
fined once, for the whole type hierarchy. Though the problem is only with u=
ntagged types, tagged private types have class-wide constants.