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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: abstract types and subprograms
Date: Tue, 20 May 2014 14:45:22 +0300
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Date: 2014-05-20T14:45:22+03:00
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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