divides n d = n /= d && mod n d == 0

eratosthenes n =
  let
    filter_k_rec d acc =
      if d * d < n
      then filter_k_rec (d + 1) $ filter (\n -> not $ divides n d) acc
      else acc
  in filter_k_rec 2 [1..n]

main = do
  putStrLn "enter n: "
  n <- readLn :: IO Int
  print $ eratosthenes n