Re: T'Interface attribute

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

On 2017-08-08 00:59, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message

>> ...  In all contexts where both become visible must require explicit name
>> resolution, e.g. by renaming one of the types and/or primitive operations.
> 
> Ada doesn't have type renaming, so this is not possible in general.

Not exactly renaming, but a schema to qualify the parent:

    type T is new I as I1 and new I as I2 with null record;
    overriding I1 procedure Foo (X : T);
    overriding I2 procedure Foo (X : T);

>>> I've generally materialized that as "the root of an abstraction should
>>> always be declared abstract".
>>
>> Yes, but this is not enough, because it kills potential
>> multiple-inheritance cases.
> 
> As an implementer, knowing that multiple-inheritance is just barely possible
> to implement, I'm unwilling to consider that unless no other possibility
> exists. And some other way always exists. :-)

But Ada 2005 did it, so I am happily using it. (:-))

>>> In Claw, the only places that we got much benefit from OOP (beyond
>>> dispatching callbacks, the reason we used OOP in the first place) was in
>>> sharing implementations across related types. But that doesn't work for Ada
>>> interfaces, because you can't have any components in the type -- meaning
>>> that writing real implementations is impossible. One can use abstract types
>>> in this way (and we did so extensively).
>>
>> Reuse happens on the client's side, when you share interface-wide
>> implementations. This includes helper types which refer to [access]
>> I'Class. Surely you have lots of them to pass widgets around, in events,
>> in handlers etc.
> 
> Not really. The implementation has such things, but we only exposed
> parameters of Root_Window_Type'Class (with a couple of exceptions). The vast
> majority of routines only take a single object.

If you have a On_Button_Click event handler you want to pass there only 
implementations from Window_Clickable_Type'Class. Not just any 
Root_Window_Type'Class. And there is no way to place all they subclasses 
into single chain of inheritance. Some of Window_Clickable_Type are also 
Window_File_Selector_Type and Window_DnD_Type etc.

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