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

[Lennart Augustsson]

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 <westondan at imageworks.com>
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
>
> 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 at soton.ac.uk
> > <mailto: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://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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> >
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