Re: abstract types and subprograms

[mockturtle <framefritti@gmail.com>]

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.

Riccardo