Thanks for the replies, all. It's good to see that the other iteratee packages out there are addressing this issue.

I still don't get why it's an issue in the first place. It seems to me like a pretty simple thing to implement:

(=$=) :: (Monad m)
      => Enumeratee a0 a1 m (Step a2 m b) -> Enumeratee a1 a2 m b
      -> Enumeratee a0 a2 m b
(=$=) e01 e12 step = Iteratee $ do
    step' <- runIteratee $ e12 step
    runIteratee . joinI $ e01 step'

This puts a type restriction on the LHS enumeratee, but enumeratees are generally polymorphic in the last type param anyway. (And joinE has a similar restriction when composing an enumerator with an enumeratee.)

Is there a good reason why enumerator doesn't export this or something analogous?

Mike Craig


On Sun, Dec 25, 2011 at 10:20 PM, Conrad Parker <conrad@metadecks.org> wrote:
On 24 December 2011 05:47, Michael Craig <mkscrg@gmail.com> wrote:
> I've been looking for a way to compose enumeratees in the enumerator
> package, but I've come up with nothing so far. I want this function
>
> (=$=) :: Monad m => Enumeratee a0 a1 m b -> Enumeratee a1 a2 m b ->
> Enumeratee a0 a2 m b
>
> I'm building a modular library on top of enumerator that facilitates reading
> time series data from a DB, applying any number of transformations to it,
> and then writing it back / doing something else with it. I'd like to be able
> to write simple transformations (enumeratees) and compose them without
> binding them to either a db reader (enumerator) or db writer (iteratee).
>
> I've been looking at the iterIO package as a possible alternative, because
> it seems to allow easy composition of Inums (enumeratees). I'm a little
> skittish of it because it seems unpopular next to enumerator.

Hi Michael,

You could also look at the iteratee package. This is the signature of
the (><>) operator:

(><>) :: (Nullable s1, Monad m) => (forall x. Enumeratee s1 s2 m x) ->
Enumeratee s2 s3 m a -> Enumeratee s1 s3 m a

it's quite useful for composing enumeratees, likewise its friend (<><)
swims the other way.

http://hackage.haskell.org/packages/archive/iteratee/0.8.7.5/doc/html/Data-Iteratee-Iteratee.html

cheers,

Conrad.