compress :: (Eq a) => [a] -> [a]
compress = foldr elim []
  where elim e [] = [e]
        elim e ns
          | e == head ns = ns 
          | otherwise = e:ns

compress' xs = foldr f (const []) xs Nothing
  where
    f x r a@(Just q) 
      | x == q = r a
    f x r _ = x : r (Just x)