haskell 4 recursive

 :: (Ord a) => [a] -> a
 [] = error "max of empty list"
 [x] = x
 (x:xs)
  | x > maxTail = x
  | otherwise = maxTail
  where maxTail = max xs

deplicate :: (Num i, Ord i) => i -> a -> [a]
deplicate n x
  | n <= 0 = []
  | otherwise = x:replicate (n - 1) x
zip' :: [a] -> [b] -> [(a,b)]  
zip' _ [] = []  
zip' [] _ = []  
zip' (x:xs) (y:ys) = (x,y):zip' xs ys

quicksort :: (Ord a) => [a] -> [a]  
quicksort [] = []  
quicksort (x:xs) =  
  let smallerSorted = quicksort [a | a <- xs, a <= x] 
      biggerSorted = quicksort [a | a <- xs, a > x]  
  in smallerSorted ++ [x] ++ biggerSorted