factors' :: Integral t => t -> [t]
factors' n
  | n < 0 = factors' (-n)
  | n > 0 = if 1 == n
               then []
               else let fac = mfac n 2 in fac : factors' (n `div` fac)
  where mfac m x
          | rem m x == 0 = x
          | x * x > m    = m
          | otherwise    = mfac m (if odd x then x + 2 else x + 1)