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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Silly and stupid =?UTF-8?B?cG9zdOKAkWNvbmRpdGlvbiBvciBub3TCoA==?=
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Date: Tue, 31 Jan 2012 11:22:31 +0100
Organization: cbb software GmbH
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Date: 2012-01-31T11:22:31+01:00
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On Tue, 31 Jan 2012 10:35:35 +0100, Georg Bauhaus wrote:

> On 31.01.12 09:54, Dmitry A. Kazakov wrote:
>> On Tue, 31 Jan 2012 00:11:43 +0100, Yannick Duchêne (Hibou57) wrote:
>>
>>>      function Parsed (S : String) return Parsed_Type
>>>         with Post =>
>>>           (if S'Length not in Image_Length_Type then
>>>               Parsed'Result.Status = Format_Error);
>>>            -- There may be other failure conditions.
>>>
>>>
>>> Is such a post‑condition a good or bad practice in your humble or
>>> authoritative opinion ?
>>
>> I would say it would be a bad practice. The reason is that IMO the
>> postcondition must be complete, it must tell all truth or nothing. Your
>> prostcondition is a partial truth, it does not describe all cases when
>> Status is mandated to be Format_Error.
> 
> One way to express a "hints only" postcondition is
> 
>    with Post => True or  -- yes, mathniks, just FTR:
>      (if ... then ...
> 
> The idea is that the partial truth, in the shape of a
> formal note, can later lead to an improved program
> structure.

Apply your idea to the case statement:

case Char is
   when 'A' => Put_Line ("Got A");
end case;

Fortunately Ada compiler would not let you further "improving" this program
structure, just rejecting the program. Such a nasty math-thinking on its
part!

Ada, at least Ada 83, considers program as a whole, trying minimize partial
truths where possible. Because where truth is partial, there is a place for
a lie. And in the case of software lies are always complete lies.

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