From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Subtypes and type invariants in Ada2012
Date: Mon, 21 Aug 2017 18:52:16 +0200
Date: 2017-08-21T18:52:16+02:00 [thread overview]
Message-ID: <onf342$dsg$1@gioia.aioe.org> (raw)
In-Reply-To: onf1q0$bbm$1@gioia.aioe.org
On 2017-08-21 18:29, Victor Porton wrote:
> Dmitry A. Kazakov wrote:
>
>> On 2017-08-21 17:38, Victor Porton wrote:
>>
>>> So, my question: Is it possible to restrict a subtype with something like
>>> a type invariant?
>>
>> Type invariant is not a restriction of public interface/properties, it
>> is an implementation detail. So it should never produce a [sub]type.
>>
>>> I want a subtype restricted by an arbitrary condition. Is it possible?
>>
>> Arbitrary condition breaks type operations. E.g. condition Is_Prime (X)
>> breaks more or less everything (assuming covariance). All restrictions
>> do. A few don't break much, e.g. range constraints.
>
> I don't get. What do you mean saying "breaks"?
Prime + Prime not in Prime
Properties of the set of prime numbers are not ones of the set of
integer numbers.
> For me the condition Is_Prime added to a subtype of Natural seems quite
> natural.
The new set loses properties of Natural.
Natural + 1 in Natural
is false for Prime.
> It is quite natural to have a variable which can hold only prime
> numbers. This may appear in some math software.
Subtyping is not about individual values it is about sets of values as a
whole. Properties of prime numbers require a new type with other
operations. E.g.
Prime + Prime return Natural (contra-variant)
Then
X : Prime := 13;
Y : Prime := X + 1; -- Type error (not a subtype!)
Z : Natural := X + 1; -- OK
> I would propose to add public subtype invariants (not only private tagged
> type invariants as now) to Ada2020. Anything particular against this?
It would not make sense.
--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
next prev parent reply other threads:[~2017-08-21 16:52 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-21 15:38 Subtypes and type invariants in Ada2012 Victor Porton
2017-08-21 16:10 ` J-P. Rosen
2017-08-21 16:18 ` Dmitry A. Kazakov
2017-08-21 16:29 ` Victor Porton
2017-08-21 16:52 ` Dmitry A. Kazakov [this message]
2017-08-21 17:41 ` Victor Porton
2017-08-21 19:55 ` Dmitry A. Kazakov
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