qsort [] = []
qsort (p:xs) = qsort lt ++ [p] ++ qsort gt
  where (lt, gt) = partition' (\x -> x <= p) xs
    -- lt = [x | x <- xs, x <= p]
    -- gt = [x | x <- xs, x > p]

partition' _ [] = ([], [])  
partition' f (x:xs) 
  | f x = ((x:accepted, rejected))
  | otherwise = ((accepted, x:rejected))
  where (accepted, rejected) = partition' f xs
  
main = do
  print $ qsort [3,4,1,2]
  print $ partition' odd [1,2,3,4]