Haskell MergeSort Implementation

mergesort.hs

import qualified Data.List as L

testList = [5,3,1,1,2,-1,0,33,10]
main = print $ mergeSort testList `isSameAs` L.sort testList


isSameAs = (==)

mergeSort :: (Ord a) => [a] -> [a]
mergeSort [] = []
mergeSort [x] = [x]
mergeSort xs = merge left right
        where
                left  = mergeSort $ take (length xs `div` 2) xs
                right = mergeSort $ drop (length xs `div` 2) xs

                merge :: (Ord a) => [a] -> [a] -> [a]
                merge [] [] = []
                merge [] xs | not $ null xs = xs
                merge xs [] | not $ null xs = xs
                merge (l:lxs) (r:rxs) =
                        if (l < r)
                        then (l : merge lxs (r:rxs))
                        else (r : merge (l:lxs) rxs)