[Haskell-cafe] Composing Enumeratees in enumerator

[Michael Snoyman]

On Sat, Dec 24, 2011 at 2:00 PM, Erik de Castro Lopo
<mle+hs at mega-nerd.com> 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/73775555c1cc695b21b7c13b823abc6c3358a978
>
> 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