[Haskell-cafe] Conduits: Is Source a valid instance of Monad?
Paul Liu
ninegua at gmail.com
Wed Apr 4 16:55:46 CEST 2012
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.codeplayer at gmail.com> wrote:
> 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
>> >
>> > _______________________________________________
>> > Haskell-Cafe mailing list
>> > Haskell-Cafe at haskell.org
>> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>>
>>
>> --
>> Regards,
>> Paul Liu
>>
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>> Haskell-Cafe at haskell.org
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>
>
>
>
> --
> http://yi-programmer.com/
--
Regards,
Paul Liu
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