Hello Haskell-Cafe! I've read about Control.Parallel and wanted to give it a try. Here's what I wrote: ----------- import Control.Parallel import Data.List splitList :: [Integer] -> [[Integer]] splitList = unfoldr f where f [] = Nothing f ~x = Just (splitAt 30000 x) map' :: (a->b) -> [a] -> [b] map' f (x:xs) =fx `par` (seq mfxs (fx:mfxs)) where fx = f x mfxs = map' f xs map' f [] = [] product' :: [Integer] -> Integer product' = foldr1 (\x y -> seq x (x*y)) fak :: Integer -> Integer fak n = product' $ map' product' (splitList [1..n]) main = putStrLn $ seq (fak 1000000) "done" ----------- This computes 1000000!. This version takes 8m29.189s to execute. Replace foldr1 with foldr and that goes down to 7m4.315s. Replace product' with the Prelude product and it takes only 6m17.685s. Why is that so? I'm using ghc 6.8.1 on Mac OS X. Regards, Adrian