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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Run-time accessibility checks
Newsgroups: comp.lang.ada
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Date: Sun, 7 Dec 2008 09:44:13 +0100
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On Sat, 6 Dec 2008 09:10:49 -0800 (PST), Ludovic Brenta wrote:

> Dmitry A. Kazakov wrote:

>> It is absolutely unacceptable to me that a correct
>> program might fail at run-time because of accessibility checks.
> 
> I differ here; to me, a program that fails an accessibility check at
> run time is incorrect.

Ah, I used to think this way too. But then I realized that this form of
constraint is not a semantic one. Comparing to the constraint range 1..10,
accessibility constraint is an implementation detail. It has no semantic
meaning. When you assign a value outside the range that is semantically
incorrect. When you assign a pointer that potentially might become
dangling, that has no semantic meaning. The pointer is not yet dangling.
The program correctness depends on whether it will become dangling.

>> The only
>> case I could buy it, if exceptions where contracted, so that I would get an
>> compile-time error at some other place. Like "Constraint_Error may be
>> propagated, but the contract states otherwise."
> 
> In my understanding, there is an implicit contract stating that all
> subprograms may raise at least Program_Error, Storage_Error or
> Constraint_Error.

That must be changed.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de