Hello all, I find it funny that IO () is different from IO [()]. For example, if I define a function to output some lines with mapT, I would do: outputLines :: Int -> IO () outputLines i = mapM (putStrLn . show) (take i $ iterate ((+) 1) 1) However, this is in fact outputLines :: Int -> IO [()] I would like to know if in fact there's any difference in practice between (), [()], i.e. if in practice the difference matters. My first guess is that this is just a consequence of the Haskell type system and so that everything fits this really needs to be like this. Because mapM :: (Monad m) => (a -> m b) -> [a] -> m [b] So I guess that it makes sense that you get IO [()] instead of IO (), and adding an exception just to say that [()] == () isn't good. By the way, as a consequence can you possibly get IO (()) or IO ([()]) and are these all different from each other? Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK Sponsor ECS runners - Action against Hunger: http://www.justgiving.com/ecsrunslikethewind