opdracht.hs

is_sorted :: Ord a => Tree a -> Bool
is_sorted Leaf = True
is_sorted (Node e left right) 
-- de richting van <= en > is andersom, want prefixnotatie
  = is_sorted' ((>=) e) left && is_sorted' ((<) e) right
  where 
  is_sorted' :: Ord a => (a->Bool) -> Tree a -> Bool
  is_sorted' _ Leaf = True
  is_sorted' test (Node e left right) 
    | test e == False = False
    | otherwise = is_sorted left && is_sorted right 
                && is_sorted' test left && is_sorted' test right