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

[Paul Liu]

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?

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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-- 
Regards,
Paul Liu