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

Yes, I wish Haskell had a 1-tuple. The obvious syntax is already taken, but I could accept something different, like 'One a'. On Mon, Mar 10, 2008 at 11:17 PM, Dan Weston wrote:

I understand the lack of distinction between a unit type and a 0-tuple, since they are isomorphic. But it is strange that there is no 1-tuple, since _|_ and the 1-tuple (_|_) would be different things entirely, no?

Dan

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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* 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

Rodrigo Queiro wrote: this.

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Because mapM :: (Monad m) => (a -> m b) -> [a] -> m [b]

So I guess that it makes sense that you get IO [()] instead of IO

(),

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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://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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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