On Sat, Dec 24, 2011 at 2:00 PM, Erik de Castro Lopo wrote:

Michael Craig 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 think part of the problem here is that Enumeratee is defined as:

   type Enumeratee ao ai m b = Step ai m b -> Iteratee ao m (Step ai m b)

If you expand out your type signature you get:

   (=$=) :: Monad m          => (Step a1 m b -> Iteratee a0 m (Step a1 m b))          -> (Step a2 m b -> Iteratee a1 m (Step a2 m b))          -> (Step a2 m b -> Iteratee a0 m (Step a2 m b))

which to me looks rather painful to implement.

...

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.

Thoughts on these issues?

I think these issues are actually common to all implementations of the Iteratee concept. Basically they do not compose as nicely and as cleanly as they would be expected to. I recently ran into this difficulty in composition in my project which was solved by nesting an iteratee inside an enumerator.

  https://github.com/erikd/http-proxy/commit/73775555c1cc695b21b7c13b823abc6c3...

There is work being done to address these issues. See Michael Snoyman's work on Conduits:

  https://github.com/snoyberg/conduit

Cheers. Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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By the way, conduit defines *precisely* that operator, for "fusing" (iterIO's term) two conduits together. Michael

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 wrote:

On 24 December 2011 05:47, Michael Craig 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-I...

cheers,

Conrad.