There is a need to find the height of a binary tree. Meaning beginning from root (0),  the maximum hopes to "highest" leaf.
Having the definition: 
 data Tree a = Node a (Tree a) (Tree a) | Empty
showing a Tree could be a node with two (node or empty ) or empty, the following recursive algorithm takes six lines of code in Haskell:
 height tree = tall tree 0
   where tall node h = case node of
                       Empty -> h
                       Node _ left right -> if l > r then l else r
                                            where l = tall left (h + 1)
                                                  r = tall right (h +1)
After, I found that:
 treeHeight :: Tree a -> Int
 treeHeight Empty = 0
 treeHeight (Node _ t1 t2) = 1 + max (treeHeight t1) (treeHeight t2)