Add a Functor instance to Kleisli

[Tony Morris]

+1

Including others such as Applicative, Alternative, etc.

On 15/4/19 4:34 pm, Fumiaki Kinoshita wrote:
> Hello,
>
> This has been proposed as a part of my
> proposal: https://mail.haskell.org/pipermail/libraries/2019-April/029478.html
>
> 2019年4月15日(月) 15:30 Dmitriy Kovanikov <kovanikov at gmail.com
> <mailto:kovanikov at gmail.com>>:
>
>     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
>
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