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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 13:00:17 +0100
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On 2021-01-12 23:56, Randy Brukardt wrote:
> "Shark8" <onewingedshark@gmail.com> wrote in message
> news:50f68100-8909-4fdb-ad26-14bcbc010775n@googlegroups.com...

>> You mention the case where the difference in the indices is 1 as being
>> separate; why?
> 
> Because the "natural" implementation of the length of an array works when
> the indices are one apart; you don't need extra code to deal with
> String(1..0) - subtracting the bounds and adding 1 (the usual length
> formula) works fine.

Nothing prevents implementation from using one of the bounds and the 
length in the array's dope vector. It is a question of optimization.

Then, maybe I am wrong, but I do not see anything that must prevent 
bounds on a null-array from sliding. They could slide to some canonical 
range making the "natural" implementation safe.

>> Also, would the sometimes talked about idea of a "null range" have helped
>> the situation out?
>> (I don't think so, since the one is the required implementation, and the
>> other is a syntax- and partially semantic-issue.)
> 
> Right, the problem is that the syntax allows too much, and any rule to avoid
> the problem necessarily will take code to make some sort of check.

Syntax can never be the problem, only semantics can.

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