tree.hs

data Tree' a = E' | Tree' (Tree' a) a (Tree' a)

foo' = Tree' E' 0 E'
foo = Tree' (Tree' E' 1 E' ) 0 (Tree' E' 1 (Tree' E' 0 E'))
foo'' = Tree' E' 0 (Tree' E' 1 E')

height t = case t of 
  E' -> 0
  Tree' s1 x s2 -> if h1 > h2 then h1 + 1 else h2 + 1
    where
      h1 = height s1
      h2 = height s2