Fix Composition (2/2)

Goal, find: ◎ :: (b -> F c) -> (a -> F b) -> (a -> F c)
f :: a -> F b, g :: b -> F c, f x :: F b:

• Use fmap :: (t -> u) -> (F t -> F u)!
• (fmap g) :: F b -> F (F c) ; (t=b, u=F c)
• (fmap g) (f x) :: F (F c) it almost WORKS!
• We lack an important component, join :: F (F c) -> F c
• (g ◎ f) x = join ((fmap g) (f x)) ☺
  ◎ is the Kleisli composition; in Haskell: <=< (in Control.Monad).