pack' :: (Eq a) => [a] -> [[a]] -> [[a]]
pack' [] y = y
pack' (x:xs) [] = pack' xs [[x]]
pack' (x:xs) (y:ys) = if x == head y then pack' xs ((x:y):ys) else pack' xs ([[x], y]++ys)

pack :: (Eq a) => [a] -> [[a]]
pack x = reverse (pack' x [])