Add a Functor instance to Kleisli

15 Apr 2019

      Hello everyone!

I would like to propose to add a `Functor` instance to the `Kleisli` data
type from the `Control.Arrow` module. The instance can look like this:

    instance Functor m => Functor (Kleisli m a) where
        fmap :: (b -> c) -> Kleisli m a b -> Kleisli m a c
        fmap f (Kleisli h) = Kleisli (fmap f . h)
        {-# INLINE fmap #-}

        (<$) :: c -> Kleisli m a b -> Kleisli m a c
        c <$ Kleisli h = Kleisli (\a -> c <$ h a)
        {-# INLINE (<$) #-}

Having this instance would be really helpful in improving the `profunctors`
package by adding QuantifiedConstraints to it. See more details in the
discussion below:

    https://github.com/ekmett/profunctors/pull/70#discussion_r267648958

Thanks,
Dmitrii Kovanikov

Add a Functor instance to Kleisli

Dmitriy Kovanikov