qsort li
  | null li = []
  | otherwise = qsort lt ++ [p] ++ qsort gt
  where
    (p:xs) = li
    lt = [x | x <- xs, x <= p]
    gt = [x | x <- xs, x > p]

qsort0 [] = []
qsort0 (p:xs) = qsort0 lt ++ [p] ++ qsort0 gt
  where
    lt = [x | x <- xs, x <= p]
    gt = [x | x <- xs, x > p]

qsort1 li =
  if null li then []
  else let 
    (p:xs) = li 
    lt = [x | x <- xs, x <= p]
    gt = [x | x <- xs, x > p]
  in qsort1 lt ++ [p] ++ qsort1 gt  

qsort2 li =
  if null li then []
  else qsort2 lt ++ [p] ++ qsort2 gt
  where
    (p:xs) = li 
    lt = [x | x <- xs, x <= p]
    gt = [x | x <- xs, x > p]
    
qsort4 li =
  case li of
    []      -> []
    (p:xs)  -> qsort4 lt ++ [p] ++ qsort4 gt
      where
        lt = [x | x <- xs, x <= p]
        gt = [x | x <- xs, x > p]    
    
main = putStrLn $ show $ qsort [5,1,4,2,3]