primeFactors :: Int -> [Int]

primeFactors x = factor x primes
    where primes = filterPrime [2..]
          filterPrime (p:xs) = p:filterPrime [x | x <- xs, x `mod` p /= 0]
          factor x l@(p:xs)
            | p > x = []
            | x `mod` p == 0 = p:factor (x `div` p) l
            | otherwise = factor x xs