contravariant argument types: wanna?

[Andrei Alexandrescu]

Justin Johansson wrote:
> 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

Thanks. To paraphrase Mark Twain, the reports of Walter and I being 
legend are greatly exaggerated.

Syntactically there is no need to change anything, just carry the change 
in the implementation.


Andrei