Monads are Trees with Grafting

Tree.hs

-- http://blog.sigfpe.com/2010/01/monads-are-trees-with-grafting.html
import Control.Applicative
import Control.Monad

data Tree a = Fork (Tree a) (Tree a) | Leaf a | Nil deriving Show

tree1 = Fork (Fork (Leaf 2) Nil) (Fork (Leaf 2) (Leaf 3))

instance Functor Tree where
  fmap = liftM

instance Applicative Tree where
  pure = return
  (<*>) = ap

instance Monad Tree where
  return a = Leaf a
  Nil >>= f = Nil
  Leaf a >>= f = f a
  Fork u v >>= f = Fork (u >>= f) (v >>= f)