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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Overring function and its returned type
Date: Fri, 9 Nov 2012 14:11:38 +0100
Organization: cbb software GmbH
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Date: 2012-11-09T14:11:38+01:00
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On Fri, 09 Nov 2012 10:14:32 +0100, Yannick Duch�ne (Hibou57) wrote:

> Le Fri, 09 Nov 2012 09:24:18 +0100, Dmitry A. Kazakov  
> <mailbox@dmitry-kazakov.de> a �crit:
> 
>> On Fri, 09 Nov 2012 08:13:23 +0100, Yannick Duch�ne (Hibou57) wrote:
>>
>>>            overriding
>>>            function F
>>>              (E : T)
>>>               return R'Class -- Not overriding :-(
>>>               is abstract;
>>
>> What is this? "overriding" + "abstract" does not make sense.
> 
> Overriding sometime do nothing, just remind something, recall an inherited  
> definition explicitly.

No, overriding *always* does something. It overrides the part of the
polymorphic body it specifies.

------------
Alas, I overlooked that you used "R" for two different types. Yet another
argument for consequent use of "use" clauses! Down with "with" (:-))

I don't see a good reason why nominal type equivalence should be further
eroded. When you derive a new interface you get another type. It may have
operations of its own. So, it is not automatically compatible.

A more general question is about overriding out-operations to narrower
subtypes, and/or overriding in-operations to wider subtypes. That could
work.

Specifically regarding class-wide types they are not considered subtypes. I
believe there could be too many conflicts to make them subtypes. I mean a
model in which when S is derived from T, then S'Class is a subtype of
T'Class (constrained to S and its descendants).

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