Re: Primitive subprogram for a subtype?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 21/08/2017 14:32, Victor Porton wrote:
> Dmitry A. Kazakov wrote:
> 
>> On 21/08/2017 00:45, Victor Porton wrote:
>>> Is it possible to define a primitive subprogram for a subtype rather than
>>> the base type?
>>
>> No, a subtype (Ada subtype) would have same tag.
> 
> How is it related to having the same tag?

Because a primitive subprogram must have a unique slot in the 
dispatching table. If tags are same dispatch cannot lead to different 
bodies.

> I do NOT speak about defining a
> primitive subprogram for both the type and its subtype but for the subtype
> only.

A subtype is a type that inherits subprograms = considered substitutable 
for its supertype. If it does not, then it is not a subtype in any 
sense, including Ada's, Liskov's, whatever flavor of.

P.S. Considering non-primitive subprograms and non-tagged types. Ada 
subtype is an "equivalent" type. It is both sub- and supertype of its 
base type and all other subtypes of whatever subtypes and their subtypes 
(a closure set). Thus anything you define on it is exported to the base 
type and to all subtypes from this type equivalence class.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de