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

[Aristid Breitkreuz]

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