From: Matt Borchers <mattborchers@gmail.com>
Subject: Re: array from static predicate on enumerated type
Date: Wed, 17 Mar 2021 11:44:27 -0700 (PDT) [thread overview]
Message-ID: <ff7f8a0d-ff65-425a-8c01-6eb909d5cf60n@googlegroups.com> (raw)
In-Reply-To: <s2s9ti$qma$1@gioia.aioe.org>
Dimitry,
I understand what you are saying. But, we don't pass CURVED_LETTERS to Count, we pass a value of CURVED_LETTERS to Count. In my mind, the value should then assumed to be of the type as declared in the parameter to Count -- which is LETTERS in your example. Obviously any value of CURVED_LETTERS is actually a value of LETTERS. This is how Ada treats this case now.
To your question, I would say that the compiler treat the argument for A : LETTERS as a member of LETTERS such that A'First = 0 and A'Last = 11 even if a value of CURVED_LETTERS was passed in (as it does now).
Runtime dispatching wouldn't work obviously. It is also not legal to write two Count routines, one with A : LETTERS and the other with A : CURVED_LETTERS within the same scope.
If Count was written with a parameter A: CURVED_LETTERS then it can be assumed that A is a member of CURVED_LETTERS and A'First = 0 and A'Last = 4 and it would be up to the programmer to know that the sub-type Predicate has introduced additional constraints. However, this would be different from all prior behavior of sub-types where the result of 'Pos (and the like) on both the enumerated type and the sub-type produce the same result. In this sense, the introduction of the Predicate would alter the behavior and thus could, in some sense, be considered inconsistent. I now see that implementing this could cause run-time errors in existing code that used 'Pos and the rest on a sub-type to which a Predicate was added.
This discussion is helpful as I am now understanding that what I'm thinking is perhaps also equally inconsistent. My current thinking is that I preferred a value of a sub-type to continue to be comparable (with =) to a value of the parent type, but I also wanted it to behave like a full-fledged type when using its attributes. Nevertheless, it still feels like an unfinished feature as it is now.
Regards,
Matt
On Wednesday, March 17, 2021 at 3:08:38 AM UTC-4, Dmitry A. Kazakov wrote:
> 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
next prev parent reply other threads:[~2021-03-17 18:44 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-12 20:49 array from static predicate on enumerated type Matt Borchers
2021-03-12 21:22 ` Egil H H
2021-03-12 22:16 ` Jeffrey R. Carter
2021-03-12 22:41 ` Dmitry A. Kazakov
2021-03-13 2:06 ` Matt Borchers
2021-03-13 4:55 ` Randy Brukardt
2021-03-15 14:16 ` Matt Borchers
2021-03-15 17:53 ` Shark8
2021-03-16 6:58 ` Randy Brukardt
2021-03-13 8:04 ` Dmitry A. Kazakov
2021-03-15 14:11 ` Matt Borchers
2021-03-15 17:48 ` Shark8
2021-03-15 20:25 ` Dmitry A. Kazakov
2021-03-16 13:27 ` Shark8
2021-03-16 14:25 ` Dmitry A. Kazakov
2021-03-17 4:05 ` Matt Borchers
2021-03-17 7:08 ` Dmitry A. Kazakov
2021-03-17 18:44 ` Matt Borchers [this message]
2021-03-17 19:41 ` Dmitry A. Kazakov
2021-03-18 1:30 ` Matt Borchers
2021-03-18 8:20 ` Dmitry A. Kazakov
2021-03-19 0:10 ` Matt Borchers
2021-03-19 8:00 ` Dmitry A. Kazakov
2021-03-18 10:15 ` Niklas Holsti
2021-03-18 10:47 ` AdaMagica
2021-03-18 11:26 ` Dmitry A. Kazakov
2021-03-19 0:34 ` Matt Borchers
2021-03-19 0:49 ` Jeffrey R. Carter
2021-03-23 1:07 ` Matt Borchers
2021-03-23 3:43 ` Randy Brukardt
2021-03-22 19:09 ` Niklas Holsti
2021-03-17 15:08 ` Shark8
2021-03-17 19:08 ` Matt Borchers
2021-03-17 20:41 ` Shark8
2021-03-18 1:04 ` Matt Borchers
2021-03-18 14:25 ` Shark8
2021-03-18 23:36 ` Matt Borchers
2022-03-16 0:38 ` Thomas
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