Add an ApplyK type class?

[LukaJCB]

I'd love to see a very simple ApplyK typeclass that'd let us use something like this:

def map2K[F[_], G[_], H[_]](af: A[F], ag: A[G])(f: Tuple2K[F, G, ?] ~> H): A[H]

And it should also be able to be derived fairly easily, right?

[kailuowang]

Is this also isomorphic to CartesianK (or SemigroupalK) + FunctorK ?

[LukaJCB]

Yes exactly :)

[kailuowang]

so we have at least 3 options to provide derivation here.
Option 1, in ApplyK companion, we provide a method that derives an instance from FunctorK and SemigroupalK instances. Then users have to add both @autoFunctorK and @autoSemigroupalK to their algebra.
Option 2, we provide a new @autoApplyK annotation and reuse the code from the @autoFunctorK and @autoSemigroupalK.

WDYT?

[LukaJCB]

I think I prefer Option 2, seems more consistent and also more convenient for the end user. I'd gladly contribute the typeclass + laws + docs, but I'm not sure if I can help with the macros. If you want I could create a PR that adds those things :)

[kailuowang]

@LukaJCB, it would be awesome if you could add the typeclass + laws + docs.
Actually, if you want you could even add the derivation method in the companion mentioned in Option 1 (should be just a couple of lines of code I think) so that you can start using it while waiting on the @autoApplyK macro.

[LukaJCB]

Will do, thanks! :)

[LukaJCB]

Btw for a use case, you can look here :) typelevel/typelevel.github.com#220

[kailuowang]

Noice, another awesome one from you.