Well, after some thought, I decided to try re-writing the boolean primes map program only using a list instead of an array. I came up with this program: primes :: Int -> [Int] primes how_much = (iterate 2 initial_map) where initial_map :: [Bool] initial_map = (map (\x -> True) [ 0 .. how_much]) iterate :: Int -> [Bool] -> [Int] iterate p (a:as) | p > mybound = process_map p (a:as) | a = p:(iterate (p+1) (mymark (p+1) step (2*p) as)) | (not a) = (iterate (p+1) as) where step :: Int step = if p == 2 then p else 2*p mymark :: Int -> Int -> Int -> [Bool] -> [Bool] mymark cur_pos step next_pos [] = [] mymark cur_pos step next_pos (a:as) = if (cur_pos == next_pos) then False:(mymark (cur_pos+1) step (cur_pos+step) as) else a:(mymark (cur_pos+1) step next_pos as) mybound :: Int mybound = ceiling(sqrt(fromIntegral(how_much))) process_map :: Int -> [Bool] -> [Int] process_map cur_pos [] = [] process_map cur_pos (a:as) | a = cur_pos:(process_map (cur_pos+1) as) | (not a) = (process_map (cur_pos+1) as) I don't know too much about Haskell yet, so it is possible this program can be further optimized using some Haskell built-ins. Now, this program can scale to 100,000 and beyond, as opposed to the array version which only got until 30,000 or 40,000. It's a pity Haskell doesn't handle arrays very well, but I guess every language has its faults. Regards, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif@vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ Home E-mail: shlomif@techie.com The prefix "God Said" has the extraordinary logical property of converting any statement that follows it into a true one.