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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 Constraints (WRT an Ada IR)
Date: Mon, 12 Dec 2016 09:27:16 +0100
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Xref: news.eternal-september.org comp.lang.ada:32738
Date: 2016-12-12T09:27:16+01:00
List-Id: <comp.lang.ada>

On 11/12/2016 21:51, G.B. wrote:
> On 11/12/2016 16:40, Dmitry A. Kazakov wrote:
>> On 2016-12-11 14:31, G.B. wrote:
>
>>> Seeing
>>>
>>>     function X (A, B: T) return T
>>>       with Pre => A > B;
>>>
>>> I must first establish A > B before calling X,
>>> because "Pre" tells me so.
>>
>> I still do not understand you question about caller "doing things"
>> with parameters. Caller passes parameters that's all.
>
>   If Algorithm_Establishing_A_Gt_B then
>      Y (X (A, B));
>   else
>      --  proverbial "This shouldn't happen!"
>      raise Program_Error;
>   end if;
>
>
> I.e., the programmer writing Y (X (A, B)) has ensured that
> the assumption expressed as X'Pre is actually true before
> calling X.

So what? Programmers program programs, and how is that related to 
contracts etc?

>>> How much should a function declaration tell a programmer about
>>> the relation between parameters A and B in the following?
>>
>> Remainder is not a relation. Relation is a Boolean-valued function.
>
> Given parameters A, B : T and
>
>    Pre => A > B
>
> the ">" is the relation, ((A, B) ∈ ">")  ≣  (A > B).

Right, ">" is a relation, Remainder is not. And the point was?

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