merge sort v2

fastMergeSort.hs

fastMergeSort :: (Ord a) => [a] -> [a]
fastMergeSort xs = fastMergeSort' (length xs) xs
  where
    fastMergeSort' 0 []  = []
    fastMergeSort' 1 [x] = [x]
    fastMergeSort' s xs  = merge (fastMergeSort' i l) (fastMergeSort' j r)
      where
        i = s `div` 2
        j = s - i
        l = take i xs
        r = drop i xs
        merge x [] = x
        merge [] y = y
        merge xs@(x:xs') ys@(y:ys')
          | x < y     = x:merge xs' ys
          | otherwise = y:merge xs  ys'