Merge sort in haskell

mergesort.hs

mergesort [] = error "Empty list"
mergesort [x] = [x]
mergesort xs =
  merge (mergesort left) (mergesort right)
  where
  (left,right) = splitAt (length xs `div` 2) xs
  merge [] ys = ys
  merge xs [] = xs
  merge xs@(x:xs') ys@(y:ys') 
    | x<=y = x: merge xs' ys
    | otherwise = y: merge xs ys'