Multiple subtyping with alias this and nested classes

[Andrei Alexandrescu]

Yigal Chripun wrote:
> On 06/10/2009 02:15, Andrei Alexandrescu wrote:
>> Yigal Chripun wrote:
>>>> I think 'alias this' is a powerful feature.
>>>>
>>>> Given the constraints like "easy for the compiler writer to implement"
>>>> and "we have many other things to do", I doubt traits or something
>>>> like that will be or needs to be in the language.
>>>
>>> Alias this is powerful black magic. being powerful doesn't make it any
>>> less dark.
>>
>> So how does this work? Alias this was meant very clearly for allowing
>> subtyping. Conversely, using it for subtyping is precisely how it was
>> meant to be used. How exactly did you decide it was a hack or black
>> magic or whatever?
>>
>> Andrei
> 
> D already has syntax for sub-typing reference types and alias this 
> introduces a completely different syntax doing the same for value types 
> which is bad human interface IMO.

Inheritance is a bit more than just subtyping, for example it entails 
the prefix property (otherwise it wouldn't be very practical). That's 
why D only allows single inheritance of state; multiple inheritance of 
state causes more problems than it solves. That's also why alias this is 
a very flexible complement to inheritance - it offers subtyping without 
requiring the prefix property and leaves layout to the discretion of the 
user.

> it also mixes two concepts - sub-typing and opImplicitCast (as i recall, 
> this is done by alias this to a function)

opImplicitCast hasn't been implemented and probably alias this will 
render it unnecessary. I don't see a dangerous conflation.

> we discussed this in the past already and IIRC you agreed with me about 
> implementing compile time inheritance for structs which I feel is much 
> cleaner and more consistent with the existing syntax of interfaces.

Maybe there's a misunderstanding. Inheritance of interfaces for structs 
(which is different from what alias this helps with) was something that 
Walter and I considered interesting to pursue for simplifying concept 
checking. However, recent developments clarified that that's not enough 
(I even deleted the related enhancement requests from bugzilla). 
Currently I'm considering a reflection-based concept checking, but it's 
too crude an idea to discuss it at the moment.


Andrei