Haskell mergesort

mergesort

mergesort :: (Ord a) => [a] -> [a]
mergesort [x] = [x]
mergesort xs = merge (mergesort (left xs)) (mergesort (right xs))
   where left  xs = take ((length xs) `div` 2) xs
         right xs = drop ((length xs) `div` 2) xs

         merge :: (Ord a) => [a] -> [a] -> [a]
         merge []  ys = ys
         merge xs  [] = xs
         merge (x:xs) (y:ys)
           | x <= y = x:(merge xs (y:ys))
           | x > y  = y:(merge ys (x:xs))