Can I make a pipe that consumes all of the producer output and passes it on as a list?

Question

I'm trying to use the Pipes library to model a workflow. In that workflow, I would like to accumulate all of that output from a producer, and then pass that on. In this case, I know that my producer produces a finite amount of output.

So if I have:

prod :: Producer a m ()
accum :: Pipe a [a] m r
groupConsumer :: Consumer [a] m r

how do I model accum so that I can do:

runEffect $ prod >-> accum >-> groupConsumer

Thanks!

Answer 1

You can use Pipes.Prelude.toListM to collect a Producer into a list:

Pipes.Prelude.toListM :: (Monad m) => Producer a m () -> m [a]

Pipes.Prelude.toListM prod :: (Monad m) => m [a]

Then you just feed that list to your groupConsumer:

runEffect $ (lift (Pipes.Prelude.toListM prod) >>= yield) >-> groupConsumer