[Haskell-cafe] Help with Data.Conduit.List.mapAccum

Michael Snoyman michael at snoyman.com
Wed Jun 15 06:34:29 UTC 2016

If I understand the question correctly, what you're saying is:

* My upstream (testC) is both returning a result _and_ yielding a stream of
values downstream
* My downstream (CL.consume) is collecting those values and returning a
result based on them
* How can I capture both the upstream and downstream results?

If so, this is the section of the docs with the relevant functions[1]. It
provides a few different fusion functions to combine an upstream and
downstream without throwing away the upstream result (which is the default
behavior of conduit, for various design and usability reasons). In your
case, this may look something like:

runConduit (fuseBoth testC CL.consume)



On Wed, Jun 15, 2016 at 8:53 AM, <bruno at ruomad.net> wrote:

> Hello,
> I'm trying to use Data.Conduit, but I get stuck with mapAccum as how to
> get and retrieve the result.
> If I use it, I get a simple conduit like :
> testC :: ConduitM () Int IO Int
> testC = Src =$= cdt where
> src = CL.sourceList [1..10]
> cdt = CL.mapAccum (\x s -> (x+s, x) 0
> I can get the output with
> testC $$ CL.consume
> but how do I retrieve the accumulated sum ?
> Bruno
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> Haskell-Cafe at haskell.org
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