Нумеруем узлы дерева с помощью монады State

traversalTree.hs

traversalTree :: Tree () -> State Integer (Tree Integer)
traversalTree (Leaf _) = do
  n <- get
  put (n + 1)
  return (Leaf n)
traversalTree (Fork left cur right) = do
  left' <- traversalTree left
  n <- get
  put (n + 1)
  right' <- traversalTree right
  return (Fork left' n right')

numberTree :: Tree () -> Tree Integer
numberTree tree = evalState (traversalTree tree) 1