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From: "Nasser M. Abbasi" <nma@12000.org>
Newsgroups: comp.lang.ada
Subject: Re: about the new Ada 2012 pre/post conditions
Date: Wed, 27 Jun 2012 08:41:55 -0500
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Date: 2012-06-27T08:41:55-05:00
List-Id: <comp.lang.ada>

On 6/27/2012 7:19 AM, Dmitry A. Kazakov wrote:

>
>> A:
>>     if Pre (...) then
>>        A (X) := 1;
>>     else
>>        raise Assertion_Failure;
>>     end if;
>>
>> B:
>>     A (X) := 1;
>>
>> A and B can be shown to both be implementations of the behavior
>> that some contract stipulates.

>> A and B have *essentially* the same effect once the program
>> is correct.
>

> I don't know what "essentially same effect" is, but whatever formal
> definition of essential you took you would have to prove that two programs
> are equivalent according to the definition. That will require proving that
> the exception is not propagated or else handled to an "essentially" same
> result. Good luck with that.
>

Hello;

I did not check, but I assumed all along that the evaluation
of pre() can have no side effects?

i.e. in the process of executing

        with pre ==> boolean valued expression

Then X could not be _modified_ during this check?

Because if X is modified, as a side effect of the check, from
say 3 to 5 then A(X):=1 will act differently  if pre() was NOT
present.

Hence both code examples shown above will not be equivalent.

Therefore, I assume this can not happen, and it is guaranteed by
the language and compiler that pre() and post() do not have
side-effects?

Else this becomes like Heisenberg uncertainty principle.

thanks,
--Nasser