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POpipes 3.0 : non linear topologies
 

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    <p>I am having a look at the pipes 3.0 package for stream processing. <a href="http://hackage.haskell.org/packages/archive/pipes/3.0.0/doc/html/Control-Proxy-Tutorial.html" rel="noreferrer">The tutorial</a> is very well done and very clear, except that I cannot wrap my head around the "zip and merge" section.</p> <p>My goal is to combine pipes a bit like ArrowChoice allows to do:</p> <ul> <li>I have a unique producer of Either a a</li> <li>I would like to apply a first pipe to Left values and another one to Right values</li> <li>I would then like to merge the results, and continue piping</li> </ul> <pre><code> +----------+ +------+ - filterLeft -> pipe1 -> +------------+ | producer | - (Either a a) -> | fork | | mergeD (?) | +----------+ +------+ - filterRight -> pipe2 -> +------------+ </code></pre> <p>I define <code>fork</code> like in the tutorial:</p> <pre><code>fork () = runIdentityP . hoist (runIdentityP . hoist runIdentityP) $ forever $ do a &lt;- request () lift $ respond a lift $ lift $ respond a oddOrEven x = if odd x then Left x else Right x producer = fromListS [1..0] &gt;-&gt; mapD oddOrEven isLeft (Left _) = True isLeft (Right _) = False isRight = not . isLeft filterLeft = filterD isLeft filterRight = filterD isRight pipe1 = mapD (\x -&gt; ("seen on left", x)) pipe2 = mapD (\x -&gt; ("seen on right", x)) p1 = producer &gt;-&gt; fork </code></pre> <p>The problem is that I cannot make the types right. The tutorial seems only to show how to run the inner (lifted) pipe chain as a self contained session, but I would like to be able to reinject its values to the pipe, not just apply an effect on them. I of course tried to follow the types, but they get a bit hairy very quickly.</p> <p>Does Anyone can help me on this ? Thanks in advance.</p> <p>(PS: an example of this kind of topology would be a nice addition to the tutorial, or even better a section on how to emulate the <code>Control.Arrow</code> stuff using pipes)</p>
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    1. COHm, not enough time right now to answer fully, but here's some food for thought: http://hpaste.org/80426
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    2. COAre you looking for bidirectionally along both branches or is this strictly a one way flow left to right?
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    3. CO@Dan Burton, thanks for the pastie. I am not really at ease with Kleisli stuff, and that was in fact the reason I am looking at pipes ! everything looked much simpler with it !
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