Factorização em números primos

factores n = f n ft
  where
    f 1 _ = []
    f _ [] = []
    f n l@(x:xs) | n `mod` x == 0 = x : f (n `div` x) l
                 | otherwise      = f n xs
    ft = reverse $ takeWhile (<= n) allprimes
      where
        allprimes = 2 : (filter isPrime [3,5..])
        isPrime n = all (/= 0) . map (n  `mod`) $ [2 .. squareRoot n]
          where
            squareRoot = truncate . sqrt . fromIntegral