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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Is this expected behavior or not
Date: Tue, 12 Mar 2013 13:19:06 -0400
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Date: 2013-03-12T13:19:06-04:00
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"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.
One way to think of it is that S'Succ is just a shorthand
for S'Base'Succ.  I admit that's somewhat confusing,
but it's just like operator "+" -- there's no "+" specific
to Positive, nor is there 'Succ specific to Positive -- both
operate on Integer'Base, and don't do any range checking on
their parameters or result.

> As Robert pointed out Ada tends to contravariance for numeric operations.
> But it is not a trivial choice for 'Pred, 'Succ, 'Range. Consider
>
>    for I in Prime'Range loop -- What is this supposed to mean?
>       ...

It doesn't mean anything -- it's illegal.

- Bob