On Wed, Apr 4, 2012 at 4:48 AM, Paul Liu <nine...@gmail.com> wrote: > Is there any follow up on this? > > I was wondering what is the best way to sequence a number of sources > together. Anybody gave a further thought on this? >
I believe sequence sources together can already be done by `Monoid` instance. > > Regards, > Paul Liu > > On Tue, Dec 27, 2011 at 3:45 PM, Aristid Breitkreuz > <arist...@googlemail.com> wrote: > > Hi all, > > > > As you may have noticed, Michael Snoyman has been working on an > > alternative approach to I/O, called conduits. You can find it here: > > > > https://github.com/snoyberg/conduit > > > > When looking at the Source type (explained here: > > http://www.yesodweb.com/blog/2011/12/conduits), I noticed that they > > seem to behave "like lists", and naturally wondered if I could write a > > Monad instance for them. But first, let's have a brief look at the > > definition of Source: > > > > data SourceResult a = Open a | Closed > > > > data PreparedSource m a = PreparedSource > > { sourcePull :: ResourceT m (SourceResult a) > > , sourceClose :: ResourceT m () > > } > > > > newtype Source m a = Source { prepareSource :: ResourceT m > > (PreparedSource m a) } > > > > ResourceT deals with resource acquisition and releasing (making sure > > that all resources are released), and provides a an abstraction over > > IORef/STRef. For our purposes here, ResourceT is probably close enough > > to IO. > > > > > > So now the question again is, can we write a Monad instance for this? > > I have been able to write join (concatenate) and return (a source with > > a single non-repeated) element. > > > > https://gist.github.com/1525471 > > > > I _think_ it behaves properly like a Monad, but I'm not quite sure, > > and neither was Michael. Greg Weber then suggested bringing the > > question to this forum. What made the question difficult for me is > > that this would be a stateful Monad transformer, so I'm not quite sure > > how to test the Monad laws properly. > > > > > > There's a second part to this question: If Source turns out not to be > > a Monad, is it possibly a ZipList-like Applicative? And either way, > > which is more useful: The list-like or the ziplist-like instances (of > > Applicative/Monad)? > > > > > > > > Thank you, > > > > Aristid > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > -- > Regards, > Paul Liu > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- http://yi-programmer.com/
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
