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* <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> 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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