From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Assignment access type with discriminants
Date: Thu, 23 Mar 2023 20:53:02 +0100 [thread overview]
Message-ID: <tviams$19ejf$1@dont-email.me> (raw)
In-Reply-To: <k83llkF8ubmU2@mid.individual.net>
On 2023-03-23 19:55, Niklas Holsti wrote:
> Perhaps it would be better to make the assignment P := PF2.all'Access
> illegal, because it in effect converts a constrained access value (PF2)
> to an unconstrained access subtype (P), and so in some sense violates
> the prohibition of constrained subtypes of general access types.
Yes this is substitutability violation. Such cases never go without a
punishment. In this case it is an implementation overhead.
Consider:
procedure Set (Destination : in out Foo; Source : Foo) is
begin
Destination := Source;
end Set;
The compiler cannot implement Set in a natural way, because Destination
might be arbitrarily constrained by the caller. E.g. when the actual for
Destination is P.all. So, the constraint must be passed together with
the actual. A quite burden.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
next prev parent reply other threads:[~2023-03-23 19:53 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-22 9:19 Assignment access type with discriminants Dmitry A. Kazakov
2023-03-22 9:31 ` Björn Lundin
2023-03-22 14:10 ` G.B.
2023-03-23 11:51 ` Dmitry A. Kazakov
2023-03-23 16:53 ` AdaMagica
2023-03-23 18:09 ` Niklas Holsti
2023-03-23 17:04 ` J-P. Rosen
2023-03-23 18:55 ` Niklas Holsti
2023-03-23 19:53 ` Dmitry A. Kazakov [this message]
2023-03-24 9:41 ` J-P. Rosen
2023-03-25 8:51 ` Randy Brukardt
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