merge sort in haskell

merge-sort.hs

mergesort :: (Ord t) => [t] -> [t]
mergesort [] = []
mergesort [x] = [x]
mergesort xs = (mergesort ls) `merge` (mergesort rs)
    where (ls,rs) = split xs
          split (x:y:zs)  = let (xs,ys) = split zs in (x:xs,y:ys)
          split [x]       = ([x],[])
          split []        = ([],[])
          merge xs []     = xs
          merge [] ys     = ys
          merge xs@(x:xt) ys@(y:yt)
              | x <= y      = x : merge xt ys
              | otherwise   = y : merge xs yt