Short Mergesort in Haskell

mergesort.hs

msort :: Ord a => [a] -> [a]
msort [] = []
msort [a] = [a]
msort as =  let half = div (length as) 2; merge [] ys = ys; merge xs [] = xs
                merge (x:xs) (y:ys) = if x <= y then x:merge xs (y:ys) else y:merge (x:xs) ys 
            in merge (msort $ take half as) (msort $ drop half as)