Merge sort in Haskell

MergeSort.hs

mergeSort :: Ord a => [a] -> [a]
mergeSort []  = []
mergeSort [x] = [x]
mergeSort xs  = combineSortedLists
                  (mergeSort firstHalf)
                  (mergeSort secondHalf)
                where (firstHalf, secondHalf) = splitInHalf xs

splitInHalf :: [a] -> ([a], [a])
splitInHalf xs = splitAt halfLength xs
                 where halfLength = (length xs) `div` 2

combineSortedLists :: Ord a => [a] -> [a] -> [a]
combineSortedLists = combineSortedLists_ []

-- Combine two lists with an accumulator in front, for tail-recursion.
combineSortedLists_ :: Ord a => [a] -> [a] -> [a] -> [a]
combineSortedLists_ acc []     ys     = (reverse acc)++ys
combineSortedLists_ acc xs     []     = (reverse acc)++xs
combineSortedLists_ acc (x:xs) (y:ys)
    | y < x     = combineSortedLists_ (y:acc) (x:xs) ys
    | otherwise = combineSortedLists_ (x:acc) xs (y:ys)