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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Silly and stupid post-condition or not ?
Date: Fri, 3 Feb 2012 14:18:40 +0100
Organization: cbb software GmbH
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Date: 2012-02-03T14:18:40+01:00
List-Id: <comp.lang.ada>

On Fri, 03 Feb 2012 12:40:36 +0100, Yannick Duch�ne (Hibou57) wrote:

> Le Fri, 03 Feb 2012 12:09:10 +0100, Dmitry A. Kazakov  
> <mailbox@dmitry-kazakov.de> a �crit:
> 
>> On Fri, 03 Feb 2012 10:48:34 +0100, Yannick Duch�ne (Hibou57) wrote:
>>
>>> Le Fri, 03 Feb 2012 10:12:25 +0100, Dmitry A. Kazakov
>>> <mailbox@dmitry-kazakov.de> a �crit:
>>>> In the OP example,
>>>> Format_Error of unspecified semantics is to be used heavily.
>>>
>>> Format_Error as a specified semantic: Error. Which turns to disallow any
>>> access to any parsed Instance.
>>
>> If that is *the semantics* then the prostcondition is
>>
>>    Result.Status = Format_Error => Result.Instance raise Constraint_Error
> 
> Format_Error is a possible value of a discriminant. The postcondition  
> applies to the Image function. I see no way to drift from the latter to  
> the former. I understood you asked me what is the meaning of the value  
> Format_Error, not what is the meaning of the postcondition.

Supposed to specify the semantics of the operation in terms of the
parameter and result values. Not in Ada 2012, of course...

>> which is of Ada's variant record type. Are you designing compiler tests?
> Excuse me, I don't understand.

You are confusing the semantics of the language types with the problem
domain. In the problem domain Format_Error should tell something about the
image object, rather than to disallow anything. Your postcondition is not
to deliver that message.

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