[Haskell-cafe] Conduits: Is Source a valid instance of Monad?

[yi huang]

On Wed, Apr 4, 2012 at 4:48 AM, Paul Liu <ninegua at 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
> <aristidb at 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
> >
> > _______________________________________________
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> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
>
> --
> Regards,
> Paul Liu
>
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>



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