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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Need Help On Ada95 Problem
Date: Fri, 10 Feb 2012 15:03:26 -0500
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Date: 2012-02-10T15:03:26-05:00
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"Alex" <alex@foo.invalid> writes:

> I'm curious about your code sample:
>
> X : String(0 .. -1000);
>
> I tried it and it works, but I don't understand how it's possible.
> String is defined in ARM 3.6.3 as
>
> type String is array(Positive range <>) of Character;
>
> and your range is definitely outside of Positive. If I try 
>
> X : String(0 .. 500); 
>
> I get the expected compiler error.

Constraint_Error at run-time, I assume.
And maybe a compile-time warning (which you wouldn't
get if 500 were known only at run time).

>...What's going on that allows the
> range to be from 0 to a negative number?

There is a check that the bounds belong to the index subtype,
but that only applies if it's not a null range.  For String,
you certainly want to allow 1..0 for an empty string.

Allowing 0..0 or 0..-1000 for String is a language
design flaw, which has been discussed here many times.
For that matter, allowing 5..10 is a flaw.
This causes needless inefficiency and obscure bugs.

- Bob