Merge sort in Haskell

Main.hs

module Main where

import System.Environment

main :: IO ()
main = do
  args <- getArgs
  mapM_ putStrLn $ sort args

sort :: (Ord a) => [a] -> [a]
sort [x] = [x]
sort xs  = let (x1, x2) = bisect xs
               s1 = sort x1
               s2 = sort x2
           in merge s1 s2

merge :: (Ord a) => [a] -> [a] -> [a]
merge xs ys = mergeIter [] (reverse xs) (reverse ys)

mergeIter :: (Ord a) => [a] -> [a] -> [a] -> [a]
mergeIter acc [] []     = acc
mergeIter acc (x:xs) [] = mergeIter (x:acc) xs []
mergeIter acc [] (y:ys) = mergeIter (y:acc) [] ys
mergeIter acc (x:xs) (y:ys)
    | x > y     = mergeIter (x:acc) xs (y:ys)
    | otherwise = mergeIter (y:acc) (x:xs) ys

bisect :: [a] -> ([a], [a])
bisect xs = balance ([], xs)

balance :: ([a], [a]) -> ([a], [a])
balance (xs, ys) = if length xs >= length ys
                   then (xs, ys)
                   else let x = head ys
                            xs' = x:xs
                            ys' = tail ys
                        in balance (xs', ys')