-- tree data structure
data Tree a = Leaf | Node (Tree a) a (Tree a)

-- base case, add to empty tree
insert Leaf item = Node (Leaf) item (Leaf)
-- add to an inner node
insert (Node left this right) item =
  if item < this then Node (insert left item) this right
  else Node left this (insert right item)

-- traverse the list (return a string)
traverse Leaf = ""
traverse (Node left this right) =
  (traverse left) ++ (show this) ++ " " ++ (traverse right)

-- a main function
main = do
  let tree = foldl insert (Leaf) [5, 6, 1, 99, 23, 42, 2, 18, 78]
  putStr (traverse tree)