From: Victor Porton <porton@narod.ru>
Subject: Re: abstract types and subprograms
Date: Tue, 20 May 2014 14:45:22 +0300
Date: 2014-05-20T14:45:22+03:00 [thread overview]
Message-ID: <llff8f$cql$1@speranza.aioe.org> (raw)
In-Reply-To: a24c0cf4-3e60-48b9-983d-768e0640622e@googlegroups.com
mockturtle wrote:
> On Tuesday, May 20, 2014 12:22:30 PM UTC+2, Victor Porton wrote:
>
>>
>>
>> rdf-base.ads:20:04: function that returns abstract type must be abstract
>> gnatmake: "/home/porton/Projects/librdf-ada/rdf-base.adb" compilation
>> error
>>
>> By the philosophy of programming From_Handle should be non-abstract but
>> return an abstract object, and From_Handle should be automatically
>> overridden for descendants non-abstract objects.
>>
>
> In my opinion, there is a contradiction: an abstract type is, by
> definition, a type "without objects," in the sense that you cannot have a
> value whose type is an abstract type. Therefore, you cannot have a
> non-abstract function that returns an abstract type, since by calling it
> you would create a value of an abstract type.
>
> Functions that return abstract types make sense. For example, they can be
> used to "force" who derives from you abstract type to provide a
> constructor for the derived type. I use them, for example, to implement
> "plugins" in order to have a standard constructor for the derived type.
>
> I hope this can help you a bit.
But does it make sense (philosophically, not accordingly the current
specification) to use a function which returns an abstract type for a record
extension aggregate?
In my humble opinion, this use may be useful in a future Ada standard.
--
Victor Porton - http://portonvictor.org
next prev parent reply other threads:[~2014-05-20 11:45 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-20 10:22 abstract types and subprograms Victor Porton
2014-05-20 10:51 ` mockturtle
2014-05-20 11:45 ` Victor Porton [this message]
2014-05-20 16:27 ` Robert A Duff
2014-05-20 19:52 ` Dmitry A. Kazakov
2014-05-24 18:49 ` Robert A Duff
2014-05-24 19:41 ` Dmitry A. Kazakov
2014-05-20 11:55 ` Dmitry A. Kazakov
2014-05-20 12:28 ` Victor Porton
2014-05-20 14:11 ` Dmitry A. Kazakov
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