Cats: add unzip to Functor

[julien-truffaut]

what do you think about adding

trait Functor[F[_]]{
  def unzip[A, B](fab: F[(A, B)]): (F[A], F[B]) = (fab.map(_._1), fab.map(_._2))
}

potentially we could have optimised version per type

reference https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-List-NonEmpty.html#v:unzip

[gagandeepkalra]

Is it okay if I pick this up?

[stremlenye]

Wouldn't the implementation @julien-truffaut proposed invoke computation twice for the effect F[_]?

[kubukoz]

Also concerned about this. This would probably be fine in Foldable, but for effects it'll require evaluating twice or break referential transparency.

[LukaJCB]

We could call it unzipDuplicate or something to make this clear from the naming. WDYT? :)

[kubukoz]

I'd probably like it more with a comment than that name tbh 😆 let's see what others think

[kailuowang]

The effect duplication seems pretty obvious in type (2 F[A]s out) to me. I agree that if we want to add a note a comment will suffice.

[joroKr21]

I don't think there is a way to unzip an effect without running it twice unless we do some memoization, but that's cats-effect territory.

[kubukoz]

@joroKr21 even in cats-effect memoization without an additional effect breaks RT.

[joroKr21]

Yes you would have to return F[(F[A], F[B])] which defeats the purpose.

[gagandeepkalra]

PR merged, thanks everyone :)