[Haskell-cafe] IO () and IO [()]

[Rodrigo Queiro]

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 at 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,
>
> --
> 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
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