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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 Constraints (WRT an Ada IR)
Date: Wed, 14 Dec 2016 13:21:01 +0100
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Date: 2016-12-14T13:21:01+01:00
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On 13/12/2016 22:11, Dmitry A. Kazakov wrote:

>> Right, Pre aspects are *not* body!
>
> They are being executed at run time.

Actually, Pre aspects do not need to be executed at run-time.
Nor do they need to be part of "the program" if checking, so
not part of a body there. But they can be physically separate,
so they could even be prevented from having side effects.
Like the source of an interrupt that can be blocked.


> But Ada expression A>B is not a predicate

To any programmer concerned with the intent of ">", i.e. not
with the LRM exegesis of operation ">", nor with some "unruly"
user defined ">", etc. I bet that "A>B" is a sentence that has
two variables, universally quantified, the universe being T.
T is the type of both A and B and has the set of values for both
A and B.

So, as far as contract based programming by programmers goes,

    Pre => A > B

is to mean that

   (forall A)(forall B)[A in T -> (B in T -> A > B)]

SHALL be true so that the client gets what it deserves.
Including the effects of unruly ">" if that Pre is checked
somehow, somewhere, but that's for human lawyers and contract
law because a failure (a) needs to be attributed, and (b) can
actually be attributed: There is a contract stating Pre.

>> I only asked for a relation between A and B as inputs to Plus_Too.
>
> No, you asked for an Ada program that evaluates a relation in connection with the semantics of Plus_To.

And ... see above.