Rodrigo Queiro wrote:
> You're looking for mapM_
> mapM_ :: (Monad m) => (a -> m b) -> [a] -> m ()
> (see also sequence_ :: (Monad m) => [m a] -> m () )
>
> I don't think that it is possible to have a 1-tuples, just 2 and up. ()
> is a unit rather than a 0-tuple, apparently:
>
http://www.haskell.org/onlinereport/basic.html#sect6.1.4
>
> On 10/03/2008, *Paulo J. Matos* <
pocm@soton.ac.uk
> <mailto:
pocm@soton.ac.uk>> wrote:
>
> 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,
>
> --