TreeInstances.hs

instance Functor Tree where
  fmap f (Leaf a) = Leaf $ f a
  fmap f (Node a b) = Node (f a) (map (fmap f) b)
  
instance Comonad Tree where
  coreturn (Leaf a) = a
  coreturn (Node a _) = a
  cojoin l@(Leaf a)   = Leaf l
  cojoin n@(Node a b) = Node n (map cojoin b)
  x =>> f = fmap f $ cojoin x