Mergesort in Haskell

mergeSort.hs

mergeSort :: Ord a => [a] -> [a]
mergeSort []     = []
mergeSort (x:[]) = (x:[])
mergeSort xs = merge (mergeSort (fst (halfList xs))) (mergeSort (snd (halfList xs)))
    where   halfList xs = (take (div len_xs 2) xs, drop (div len_xs 2) xs)
                where len_xs = length xs
            merge [] [] = []
            merge [] ys = ys
            merge xs [] = xs
            merge (x:xs) (y:ys)
                | x < y     = x : (merge xs (y:ys))
                | otherwise = y : (merge (x:xs) ys)