Embed async within a Monad implementing MonadIO

Question

I have some pipes-concurrency code that looks like this:

-- this won't compile but will give you the gist of what's happening
pipeline :: MonadIO m => Consumer a m ()
main = do
    (output, input) <- spawn Unbounded
    async $ do runEffect $ fromInput input >-> pipeline
               performGC
    -- skipped the `output` pipeline code.

Question 1: This obviously won't compile since runEffect will return MonadIO m => m () and async requires IO a. Is there a way to do this? Or am I stuck forcing my pipeline to contain effects within the IO monad?

Question 2: does it even make sense to embed an async within a Monad implementing MonadIO? Not sure if I'm articulating myself well here.

Thank you!

Answer 1

This obviously won't compile since runEffect will return MonadIO m => m () and async requires IO a

That's not quite right. IO is an instance of MonadIO so your output of runEffect could be passed to async or e.g. a function taking MaybeT IO () (also a MonadIO instance).

I think what you're looking for is liftIO :: IO a -> m a, which will let you "lift" the specific IO (Async a) returned by async to a polymorphic MonadIO type as called for in your type signature.

...
liftIO $ async $ do runEffect $ fromInput input >-> pipeline
           performGC

Haven't tried to compile this; you may need to lift other IO parts of your functions as well.

Question 2: does it even make sense to embed an async within a Monad implementing MonadIO? Not sure if I'm articulating myself well here.

Sure, if you want to do concurrency in some monad stack why not?