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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 Constraints (WRT an Ada IR)
Date: Sun, 11 Dec 2016 21:51:43 +0100
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Xref: news.eternal-september.org comp.lang.ada:32729
Date: 2016-12-11T21:51:43+01:00
List-Id: <comp.lang.ada>

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.

>> 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).