Add (Kleisli) Arrows

[turion]

We could define Kleisli as:

newtype Kleisli m f a b = Kleisli { runKleisli :: a -> m f b }

It would be an instance of a generalised Arrow instance which would yet have to be defined.

[turion]

The corresponding Arrow type class would look something like:

class CategoryEffect (cat :: k -> * -> * -> *) where
  type Unit cat :: k
  type Plus cat (f :: k) (g :: k) :: k
  type Inv cat (f :: k) (g :: k) :: Constraint
  type Inv cat f g = ()

  id :: cat (Unit cat) a a
  (>>>) :: Inv cat f g => cat f a b -> cat g b c -> cat (Plus cat f g) a c

class CategoryEffect a => ArrowEffect (a :: k -> * -> * -> *) where
  arr :: (b -> c) -> a (Unit a) b c
  first :: a f b c -> a f (b, d) (c, d)

[dorchard]

Cool idea! Let's do it! Do you want to a PR or should I move this code into a new module?

[turion]

I can make a PR.