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copied!<p>If you want to combine two <code>IO</code>-generated streams, then Gabriel's comment is the solution.</p> <p>Otherwise, you can't wait for both streams, which one produces a value first. Conduits are single-threaded and deterministic - it processes only one pipe at a time. But you could create a function that interleaves two streams, letting them decide when to switch:</p> <pre><code>{-# OPTIONS_GHC -fwarn-incomplete-patterns #-} import Control.Monad (liftM) import Data.Conduit.Internal ( Pipe (..), Source, Sink, injectLeftovers, ConduitM (..), mapOutput, mapOutputMaybe ) -- | Alternate two given sources, running one until it yields `Nothing`, -- then switching to the other one. merge :: Monad m => Source m (Maybe a) -> Source m (Maybe b) -> Source m (Either a b) merge (ConduitM l) (ConduitM r) = ConduitM $ goL l r where goL :: Monad m => Pipe () () (Maybe a) () m () -> Pipe () () (Maybe b) () m () -> Pipe () () (Either a b) () m () goL (Leftover l ()) r = goL l r goL (NeedInput _ c) r = goL (c ()) r goL (PipeM mx) r = PipeM $ liftM (`goL` r) mx goL (Done _) r = mapOutputMaybe (liftM Right) r goL (HaveOutput c f (Just o)) r = HaveOutput (goL c r) f (Left o) goL (HaveOutput c f Nothing) r = goR c r -- This is just a mirror copy of goL. We should combine them together to -- avoid code repetition. goR :: Monad m => Pipe () () (Maybe a) () m () -> Pipe () () (Maybe b) () m () -> Pipe () () (Either a b) () m () goR l (Leftover r ()) = goR l r goR l (NeedInput _ c) = goR l (c ()) goR l (PipeM mx) = PipeM $ liftM (goR l) mx goR l (Done _) = mapOutputMaybe (liftM Left) l goR l (HaveOutput c f (Just o)) = HaveOutput (goR l c) f (Right o) goR l (HaveOutput c f Nothing) = goL l c </code></pre> <p>It processes one source until it returns <code>Nothing</code>, then switches to another, etc. If one source finishes, the other one is processed to the end.</p> <p>As an example, we can combine and interleave two lists:</p> <pre><code>import Control.Monad.Trans import Data.Conduit (($$), awaitForever) import Data.Conduit.List (sourceList) main = (merge (sourceList $ concatMap (\x -> [Just x, Just x, Nothing]) [ 1..10]) (sourceList $ concatMap (\x -> [Just x, Nothing]) [101..110]) ) $$ awaitForever (\x -> lift $ print x) </code></pre> <hr> <p>If you need multiple sources, <code>merge</code> could be adapted to something like</p> <pre><code>mergeList :: Monad m => [Source m (Maybe a)] -> Source m a </code></pre> <p>which would cycle through the given list of sources until all of them are finished.</p>

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