No Functor implementation for Fn* Family?

[ekinimo]

Hey!

I was browsing your code and noticed that there isnt an implementation of Functor for functions.

Another thing that i noticed is that you took the bi/tri/quad ... Functor generalization. I wonder if it would make sense to user specify whether fmap be covariant or contravariant, maybe that way profunctors/lenses etc. becomes represantable

[JonathanBrouwer]

Hi @ekinimo, thanks for showing interest in the crate!

I think making an implementation for functions is difficult, because there are three different function traits Fn, FnMut, and FnOnce. Because of coherence checking we could only implement the trait for one of these function traits.
We could implement the functor for function pointers fn(...) -> ..., but this wouldn't work because it would need the capture the impl Fn(A) -> B which it cannot do because the Target type cannot refer to.
If you can come up with a smart way around this, please let me know :)

I'm open to having contravariant functors, but don't have the time/energy to implement it nicely. If you or someone else feels like implementing it I'm happy to review and get it into this crate

[ekinimo]

Thank you for the reply @JonathanBrouwer

I'm not as well versed as i want to be in generic magic yet and have no knowledge of proc macros yet so i'm afraid it will take a while to get comfy with the codebase to make any meaningful contribution.

Though i wonder if one could bypass the coherence checking by something like this