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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: array from static predicate on enumerated type
Date: Wed, 17 Mar 2021 08:08:33 +0100
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On 2021-03-17 05:05, Matt Borchers wrote:

> It feels like an unfinished addition or incomplete feature to actively prevent 'First, 'Last, etc. from returning something reasonable about the sub-type in question.  Return SOMETHING, even if it pertains to the parent type, and let the programmer decide how it can be useful to them.  Using my LETTERS and CURVED_LETTERS example, I still don't see why the following would be problematic:
> 
> A <= LETTERS'First
> K <= LETTERS'Last
> 11 <= LETTERS'Length
> 1 <= LETTERS'Pos(B)
> 10 <= LETTERS'Pos(J)
> 
> B <= CURVED_LETTERS'First
> J <= CURVED_LETTERS'Last
> 5 <= CURVED_LETTERS'Length
> 0 <= CURVED_LETTERS'Pos(B)
> 4 <= CURVED_LETTERS'Pos(J)
> 
> I didn't address 'Succ and 'Pred, but these also seem reasonable.

Consider this:

    function Count (A : LETTERS) return Natural is
       Count : Natural := 0;
    begin
       for I in A'First..A'Last loop
          Count := Count + 1;
       end loop;
    end Count;

How this supposed to be compiled? Note that CURVED_LETTERS is a subtype 
of LETTERS. So you can pass CURVED_LETTERS to Count.

Count cannot use LETTERS'First, 'Succ etc in its body, they might be 
overridden they are like in CURVED_LETTERS.

In short it must either dispatch on the actual argument or else the 
argument must be converted to LETTERS in a way that LETTERS operations 
would yield the correct result.

Neither is possible.

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