main :: IO ()
main = do
  let unsorted = [10,9..1]
  putStrLn $ show $ mergesort unsorted

mergesort :: Ord a => [a] -> [a]
mergesort [] = []
mergesort [x] = [x]
mergesort l =
  let distribute l1 l2 []     = (l1, l2)
      distribute l1 l2 (x:xs) = distribute l2 (x:l1) xs
      (l1, l2) = distribute [] [] l
  in mergesort l1 `merge` mergesort l2

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