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