Andrei Alexandrescu Wrote:
> Hello,
>
>
> Today, overriding functions have covariant return types:
>
> class A {
> A clone();
> }
>
> class B : A {
> B clone(); // fine, overrides A.clone
> }
>
> That is entirely principled and cool. Now the entire story is that
> overriding function may have not only covariant return types, but also
> contravariant argument types:
>
> class A {
> A fun(B);
> }
>
> class B : A {
> B fun(A); // fine (in theory), overrides A.fun
> }
>
> Today D does not support contravariant arguments, but Walter told me
> once he'd be quite willing to implement them. It is definitely the right
> thing to do, but Walter would want to see a compelling example before
> getting to work.
>
> Is there interest in contravariant argument types? If so, do you know of
> a killer example?
>
>
> Thanks,
>
> Andrei
Yes please. Gimme, gimme, gimme. Andrei and Walter, you guys are legends.
Currently I'm developing a specialized collection library based on templated
quantified types. Such library is useful in pattern matching applications.
Now despite any (negative) comments I may have made on prior occasion regarding
Scala being too academic re covariance & contravariance, it is difficult
producing this type of library without language support for covariant and
contravariant typing.
Designing such support into D would bring even more credibility to the language
.. especially if you guys can come up with a neat syntax to effect this
feature. I'm impressed with the way Walter observed that the ! operator was
not a binary operator in C/C++ and then went about re-purposing it for template
syntax. Accordingly I wouldn't be surprised if you guys pulled another cool
rabbit-out-of-the-hat trick.
So yes there is extreme interest in my neck of the woods but as for a killer
example, I need to think this out in more time than this reply makes for at the
moment.
If you do this, I suspect that it will be the end of my D1 adventures and force
me into D2.
Cheers
Justin Johansson
