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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Array slices and types
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Date: Thu, 21 Aug 2008 17:46:08 +0200
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Date: 2008-08-21T17:46:11+02:00
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On Thu, 21 Aug 2008 07:44:29 -0700 (PDT), Adam Beneschan wrote:

> Sorry, Dmitry, but if you'll pardon the expression, you're a little
> off-base here.  S'Base and S are subtypes of the same *type*.

No need to excuse, you are right, I don't see any semantic difference
between subtypes of "same" type and derived tagged types. Consequently, I
don't think it is worth to further maintain such difference in the
language.

> They
> may be different subtypes (have different constraints), but they're
> the same type.  You're asking for an attribute that would give you a
> different type.

Yep, for that matter, tag is merely a constraint put on T'Class. Ada 83
model is capable to incorporate OO! We always knew that! (:-))

> There may be merit in having such an attribute (I
> haven't looked into it closely), but calling it 'Base would be a bad
> idea.  S'Parent might be better.

Integer'Parent were Universal_Integer, I guess (:-))

> Niklas is asking for S'Base to be the unconstrained array subtype of
> the (possibly constrained) array subtype S.  It would still fit into
> the definition of 'Base (3.5(15)).  I'm not sure why this wasn't
> defined for array subtypes, or for any other type that could have
> discriminants; offhand I don't see how this would cause any problems.

I think that the reason was that S'Base was thought as a very low-level
thing, something like the least constrained type of the *same* internal
representation. For unconstrained arrays the representation is likely not
the same, they might have a dope. For other types nobody seemingly cared.
(:-))

IMO, if S'Base need to be extended, then that would require a bit firmer
foundation than the above.

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