Thanks! I failed to notice this instance declaration in the document. But I'm still curious as to whether a Monad instance for Source makes any sense, since in 4.0 all of Source/Conduit/Sink would share the same implementation.
Regards, Paul Liu On Tue, Apr 3, 2012 at 6:25 PM, yi huang <yi.codepla...@gmail.com> wrote: > 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/ -- Regards, Paul Liu _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
