[Haskell-cafe] Composing Enumeratees in enumerator

[Michael Craig]

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 at metadecks.org>wrote:

> On 24 December 2011 05:47, Michael Craig <mkscrg at 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.
>
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