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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: Silly and stupid post-condition or not ?
References: <op.v8w6ptk5ule2fv@douda-yannick>
 	<82wr86fzos.fsf@stephe-leake.org>
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Date: Thu, 02 Feb 2012 04:40:20 -0500
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Date: 2012-02-02T04:40:20-05:00
List-Id: <comp.lang.ada>

Adam Beneschan <adam@irvine.com> writes:

> On Feb 1, 12:36 am, Stephen Leake <stephen_le...@stephe-leake.org>
> wrote:
>> "Yannick Duchêne (Hibou57)" <yannick_duch...@yahoo.fr> writes:
>>
>>
>>
>>
>>
>> >    type Parsed_Type (Status : Parsed_Status_Type) is record
>> >       case Status is
>> >          when Parsed =>
>> >             Instance : Instance_Type;
>> >          when Format_Error =>
>> >             null;
>> >       end case;
>> >    end record; -- Parsed_Type
>>
>> >    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 ?
>>
>> > Keep in mind there may be many other cases where "Parsed'Result.Status
>> > = Format_Error" could be True; the one in the Post is not the only
>> > possible  one.
>>
>> That means the post condition is misleading, and therefore bad.
>
> I don't see this.  It would be misleading to someone who doesn't
> understand the fallacy of the converse, but I'd hope that computer
> science students would be taught not to make this mistake by the end
> of the first or second year.

It is true that this post condition leaves open the possibility of other
causes of failure. But they should be documented as well; why is only
this one singled out?

I agree with Dmitry; post conditions should be _complete_. If the Ada
post condition language is not strong enough to be complete, don't use
it; use natural language comments.

-- 
-- Stephe