Hi, This could gives you for any given integer the list of prime numbers. source is from a stack overflow snippet. helper function: isqrt :: Integral a => a -> a isqrt = floor . sqrt . fromIntegral main function: primes 1 = [] primes n = 2:[i | i <- [3,5..n], and [mod i k /= 0 | k <- primes (isqrt i)]] my main unclarity is how I should interpret i and k in de mod part. At every step of the recursion. example: primes 10. it should generate [2,3,5,7,9] if you ignore the second part. however, when I look at the second part then first I need to do this primes (isqrt 10), which give primes 3. which gives met [2,3]. first question) I haven't reached base case in this situations so can i or can i not meaningfully evaluate mod i k /= 0? Independent of that I now go deeper to reach base case. So I go to primes (isqrt 3) which gives me primes 1 which is []. Now I hit the base case and need to definitely evaluate mod i k /= 0. second question) but in this case what is i and what is k? If I have it completely wrong, then please be patience. I appreciate the input! best,