enzief

@kailuowang sure, I'll see what I can do.

Right. As there's no `contramapJson` for `Decoder` would it be correct to add ```scala trait Decoder[A] { def contramapJson(f: Json => Json): Decoder[A] // not sure about this signature }...

I didn't mean to enhance support for `scala.List`. The idea is defining implicits in the typeclasses' companion object would save some import for users and some boilerplate for maintainers. Instances...

`modify` is like Functor's `map` which has a "set" called `as`: ```scala def as[B](b: B): F[B] = map(_ => b) ``` That said, `replace` sounds very intuitive to me.

Sounds great! Let's see what @vovapolu thinks.