3 acc f (a, b) x = if f x then (x:a, b)
  4                         else (a, x:b)
  5
  6 splitWith f = foldl (acc f) ([], [])
  7
  8 quickSort [] = []
  9 quickSort (x:xs) = quickSort left ++ [x] ++ quickSort right
 10                 where (left, right) = splitWith (<=x) xs
 11