Add some Contravariant instances #590
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vikraman
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The ContravariantTests don't work because of the Eq requirement. |
Current coverage is
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| @@ -21,6 +21,9 @@ final case class Kleisli[F[_], A, B](run: A => F[B]) { self => | |||
| def map[C](f: B => C)(implicit F: Functor[F]): Kleisli[F, A, C] = | |||
| Kleisli(a => F.map(run(a))(f)) | |||
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| def contramap[C](f: C => B)(implicit F: Contravariant[F]): Kleisli[F, A, C] = | |||
| Kleisli(a => F.contramap(run(a))(f)) | |||
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I think the contramap that I would expect for Kleisli would be the local method that already exists on it. I think in practice the F in a Kleisli is often going to be covariant (not contravariant) in nature (Future, Task, Xor, etc), so I don't know how often this would be helpful. If you have some good examples of how you might use this, I'd definitely be interested in them.
Otherwise, should we get rid of this one and use local for the Contravariant instance?
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Yes, that does seem more useful. Is there a way to keep both the instances?
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For simplicity, I'd be inclined to just keep the one until someone runs into a compelling use-case for the other.
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Thanks a bunch for working on this, @vikraman! |
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For the missing implicit def kleisliEq[F[_], A:Arbitrary, B](implicit FB: Eq[F[B]]): Eq[Kleisli[F, A, B]] =
function1Eq[A, F[B]].on(_.run) |
This is an attempt to fix #579