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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Is this expected behavior or not
Date: Tue, 12 Mar 2013 19:21:47 +0100
Organization: cbb software GmbH
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Date: 2013-03-12T19:21:47+01:00
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On Tue, 12 Mar 2013 19:04:31 +0100, Georg Bauhaus wrote:

> On 12.03.13 18:42, Dmitry A. Kazakov wrote:
>> On Tue, 12 Mar 2013 13:19:06 -0400, Robert A Duff wrote:
>> 
>>> > "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>>> > 
>>>> >> The semantics of 'Pred and 'Succ shall be properly defined in terms of
>>>> >> convariance of the argument and the result. There are arguments for and
>>>> >> against contravariant results. It goes straight to the circle-ellipse
>>>> >> controversy.
>>> > 
>>> > The profile of 'Succ is given in RM-3.5(23):
>>> > 
>>> >     function S'Succ (Arg: S'Base) return S'Base;
>>> > 
>>> > It's neither contravariant nor covariant -- it's invariant.
>> Invariant = contravariant.
> 
> In Ada? For non-tagged types?

Tagged is a special case, nothing more.

When the compiler allows instances of Prime in an operation of Prime'Base
that means that Prime inherits the operation <=> Prime is a subtype of
Prime'Base [in an interface having the operation]. In our case the
operation is 'Succ. Now if the argument, or result of 'Succ stays
Prime'Base that is the definition of the operation being contravariant in
it.

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