Binary tree

tree.hs

import           Control.Applicative

data Tree a = Leaf a
            | Node (Tree a) (Tree a)
            deriving Show

instance Functor Tree where
  fmap f (Leaf a) = Leaf (f a)
  fmap f (Node x y) = Node (fmap f x) (fmap f y)

instance Applicative Tree where
  pure = Leaf
  (Leaf f) <*> x = fmap f x
  (Node x y) <*> a = Node (x <*> a) (y <*> a)

instance Monad Tree where
  return = Leaf
  (Leaf a) >>= f = f a
  (Node x y) >>= f = Node (x >>= f) (y >>= f)