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From: Simon Wright <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Contracted exceptions for Ada
Date: Sun, 09 Dec 2007 18:39:31 +0000
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Date: 2007-12-09T18:39:31+00:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Sun, 09 Dec 2007 16:11:13 +0100, Martin Krischik wrote:
>
>> Dmitry A. Kazakov wrote:
>> 
>>> (Maybe a wild guess, but this could save Ariane V. A program assumed
>>> Constraint_Error exception free was recompiled for the hardware where that
>>> was not the case.)
>> 
>> My understanding is that the system was brought down by an unexpected
>> hardware exception - i.E. one which the CPU's floating point unit
>> generated.
>
> Yes, and that would be detected by the compiler:
>
>    function Integer (X : Float) return Integer;
>
> which was contracted as exception-free on one platform will not be on
> another. So the compiler would reject either its implementation that raises
> Constraint_Error or else its use contracted as Constraint_Error-free. So
> the idea.

I don't see how the compiler/platform in use has anything to do with
this.

    In Ariane IV the maximum horizontal velocity is X.

    Therefore we can convert the hardware input to this-type without
    worrying about overflow.

    Therefore any exception will be caused by hardware error.

seems a proper analysis for Ariane IV; reusing precisely the same
software/platform in an environment where the maximum horizontal
velocity was much larger than X is an error that no amount of
*software* engineering is going to fix.