mergesort.hs

-- mergesort
merge:: (Ord a) => [a] -> [a] -> [a]
merge xs [] = xs
merge [] ys = ys
merge f@(x:xs) s@(y:ys)
    | x > y = x: merge xs s
    | x < y = y: merge f ys
    | x == y = x: y: merge xs ys


mergeSort:: (Ord a) => [a] -> [a]
mergeSort [x] = [x]
mergeSort lst =
    merge (mergeSort first) (mergeSort last)
    where
        (first, last) = splitAt (quot (length lst) 2) lst


-- | The main entry point.
main :: IO ()
main = do
    putStrLn $ show $ mergeSort [3, 2, 1, 4, 4]

👍
take a look at another impls http://rosettacode.org/wiki/Sorting_algorithms/Merge_sort#Haskell

thx )