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POFusing conduits with multiple inputs
 

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    <p>I am trying to create a conduit that can consume multiple input streams. I need to be able to await on one or the other of the input streams in no particular order (e.g., not alternating) making zip useless. There is nothing parallel or non-deterministic going on here: I await on one stream or the other. I want to be able to write code similar to the following (where <code>awaitA</code> and <code>awaitB</code> await on the first or second input stream respectively):</p> <pre><code>do _ &lt;- awaitA x &lt;- awaitA y &lt;- awaitB yield (x,y) _ &lt;- awaitB _ &lt;- awaitB y' &lt;- awaitB yield (x,y') </code></pre> <p>The best solution I have is to make the inner monad another conduit, e.g.</p> <pre><code>foo :: Sink i1 (ConduitM i2 o m) () </code></pre> <p>Which then allows</p> <pre><code>awaitA = await awaitB = lift await </code></pre> <p>And this mostly works. Unfortunately, this seems to make it very difficult to fuse to the inner conduit before the outer conduit is fully connected. The first thing I tried was:</p> <pre><code>fuseInner :: Monad m =&gt; Conduit i2' m i2 -&gt; Sink i1 (ConduitM i2 o m) () -&gt; Sink i1 (ConduitM i2' o m) () fuseInner x = transPipe (x =$=) </code></pre> <p>But this doesn't work, at least when <code>x</code> is stateful since <code>(x =$=)</code> is run multiple times, effectively restarting <code>x</code> each time.</p> <p>Is there any way to write fuseInner, short of breaking into the internals of conduit (which looks like it would be pretty messy)? Is there some better way to handle multiple input streams? Am I just way to far beyond what conduit was designed for?</p> <p>Thanks!</p>
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    1. COI assume that you mean that you want to receive elements as they are generated from the two `IO` input streams. Use `stm-conduit` to do this.
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      1. USGabriel Gonzalez
    2. COI've read your updated question. Is [this](http://stackoverflow.com/questions/12496654/is-there-an-iteratee-like-concept-which-pulls-data-from-multiple-sources/12497593#12497593) closer to what you had in mind? If so, I can modify it to the equivalent `conduit` version and submit it as an answer.
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      1. USGabriel Gonzalez
    3. COI think that link describes exactly what I tried (`foo` is a `Sink` over a `ConduitM` monad). The problem is, I can't figure out how to fuse to the inner conduit with this strategy.
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