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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Extending discriminant types
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Date: Thu, 20 Nov 2008 15:03:42 +0100
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Date: 2008-11-20T15:03:43+01:00
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On Thu, 20 Nov 2008 12:45:27 +0100, Georg Bauhaus wrote:

> Just out of curiosity, given
> 
>    type T is tagged ...;
>    type D is new T with ...;
>    type E is new D with ...;
> 
> what would E'Base be?

In Ada 83 S'Base was meant as something like "a least constrained
supertype", assuming specialization as a types algebra operation. Under
generalization is should be E itself. When inheritance both specializes,
generalizes and deviates a bit, there might be no S'Base. So the concept
was wrong.

Alternatively S'Base could mean the closest parent of S, different from S.
Then there is no problem, except that it would not work with MI. Wrong
again.

[ The right concept is class-wide (contravariant) operations, when
specialization need to be dealt with. E.g.

   function "+" (L, R : Integer) return Integer'Class;
      -- Contravariant in the result. This is not Ada!
]

> Assuming that D is privately tagged, would E'Base
> have different effects depending on whether E's tag
> is visible or not?

Visibility of S'Base is of no concern, because S'Base is itself visible.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de