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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Lower bounds of Strings
Date: Wed, 13 Jan 2021 14:53:26 +0100
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On 2021-01-13 14:27, AdaMagica wrote:
> Dmitry A. Kazakov schrieb am Mittwoch, 13. Januar 2021 um 13:00:19 UTC+1:
>> Nothing prevents implementation from using one of the bounds and the
>> length in the array's dope vector. It is a question of optimization.
> 
> type UC is array (Integer range <>) of Something;
> procedure Proc (X: UC);
> 
> Here, no sliding is allowed in a call to Proc.
> And, as Randy said before, X: UC (+1234..-1234); must return the correct values for 'First and 'Last.

I am not sure that the invariant

    (I1..I2 => 0)'Last = I2

must be true while

    I1 + (I1..I2 => 0)'Length - 1 = I2

False.

Yes, it is kind of broken. One of them does not hold.

If we wanted to fix it we should respect mathematics where intervals of 
negative length are not empty intervals. So

    (0..-100 => 0)'Length = -99

or else

    Constraint_Error in the aggregate.

[And yes, that would break a lot of sloppy legacy code if we attempted 
to fix it]

> So an optimization would need three values, First, Last, Length. Why not?

Possibly. However, I see little problem with indices of null arrays. 
Semantically an index in the null array is meaningless and could be any 
[or none], because the whole idea of an index to point to an element. No 
elements, nothing to point to. Logically, Null_Array'Last should be 
Constraint_Error.

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