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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: In-Out Parameters for functions
Date: Tue, 03 Feb 2004 16:32:53 +0100
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Date: 2004-02-03T16:32:53+01:00
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On Tue, 03 Feb 2004 09:06:52 -0500, Hyman Rosen <hyrosen@mail.com>
wrote:

>Dmitry A. Kazakov wrote:
> > + is commutative
>
>Mathematics went beyond dogmatic statements like that many years ago.
>Notice you didn't say that * is commutative. Very disingenuous.

Well, on numbers with overflow + is as non-commutative as * for
matrices. Commutativity is not a property of the sign "+". And
compilers do not try to optimize user-defined types just because they
have "+" defined, I hope.

>> You want to fix the circumstances, I prefer to fix the code.
>
>Once the circumstances are fixed, the code is no more broken
>than if the operands were evaluated in sequential statements.

Nope, false => true, but this does not make false premise true.

>> The results are illegal, if different [*].
>
>Can you point to where the ARM says so? I believe you are wrong.

I didn't talk about ARM. However, it mentions mathematically correct
result. If a+b differs from b+a more than accuracy requirements allow,
it is no more mathematically correct.

>> When a programmer writes: x:=a+b, the postcondition of that is:
>> { x=a+b & x=b+a }
>
>If I replace + with *, you would find no one agreeing with you if the
>operands were matrices, for example.

For matrices * is not commutative. We are discussing commutative
operators.

--
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de