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Date: Tue, 27 Apr 2010 19:09:43 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Integer overflow is int overflow (Re: Web browser in Ada)
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Date: 2010-04-27T19:09:44+02:00
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On 27.04.10 18:13, Dmitry A. Kazakov wrote:
> On Tue, 27 Apr 2010 17:30:44 +0200, Georg Bauhaus wrote:
> 
>> My conclusions would be that:
>>
>> Standard.Integer is a constrained subtype. Its objects are
>> not like (what is commonly known as) integers, a set that can
>> be extended at will.
> 
> The set of integers is not extensible "at will." You probably meant that
> for any integer there exists at least one [actually an infinitely countable
> set] of greater or lesser integers. Nevertheless the set of integers (Z) is
> fixed.

Thanks for adding clarity. It is much needed.

> The value is determined by the expression and the
> language rules.

(OK, nitpicking, there is no value denoted by a named number
if determination does not actually happen. Words mean
nothing unless they are given meaning. ;-) ;-)

>> they are not objects, and they can hit compiler capacity limits.
> 
> Whether they are [run-time] objects is up to compiler. The compiler in Ada
> has certain freedom to "objectize" and "deobjectize" things.

Thanks, I didn't know this.


> You need overflow in order to define the behavior of the operations closed
> only in Z. E.g. "+".

True if it is not possible to define "+" in terms of saturation
or wrap-around, I think. Correct?

However, programmers do not define "+" in their C programs.
They use "+"!
Then overflow occurs, and not to prove the soundness of the
language definition, but as a mishap.