It seems that the definition of split in System.Random is not really satisfactory. Imagine a tree-like computation of the form f gen = {- some expression using b, g1, g2 -} where b = fst (random gen) :: Bool (gen1, gen2) = split gen g1 = f gen1 g2 = f gen2 Let's look at the first 30 values of b produced along the right side of the tree: GHCi> length $ nub [ take 30 . map (fst . random) . iterate (snd . split) $ mkStdGen i :: [Bool] | i <- take 10000 . randoms $ mkStdGen 0 ] 10000 Great, we tried 10000 different initial gens and got 10000 different sequences. Now let's look at the left side: GHCi> length $ nub [ take 30 . map (fst . random) . iterate (fst . split) $ mkStdGen i :: [Bool] | i <- take 10000 . randoms $ mkStdGen 0 ] 8 This doesn't seem good. Michael's code (below) is effectively doing iterate (fst . split). Regards, Reid Barton On Sun, Oct 11, 2009 at 04:24:55PM -0700, michael rice wrote:
main = do gen <- getStdGen coinToss gen gen <- newStdGen main