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POFiltering / branching enumeratee
 

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    <p>I am using enumerator-0.4.10, and I need to distribute processing of different parts of the incoming stream to different iteratees (I am parsing a huge XML file, and different sub-trees have different processing logic). Only a single iteratee will be active at a time since the sub-trees don't intersect.</p> <p>I wrote a simple example that filters the stream and passes the result to one iteratee; please see below. However, with multiple nested iteratees it seems to me that I can no longer use an enumeratee. Do I need to write my own multi-enumeratee that holds multiple inner iteratees? Any better ideas?</p> <p>Here is my (beginner's) code for a single nested iteratee:</p> <pre><code>module Main ( main ) where import qualified Data.Enumerator as E ( Enumeratee, Step(..), Stream(..), checkDone, checkDoneEx, continue, enumList, joinI, run_, yield ) import Data.Enumerator ( ($$), (&gt;&gt;==) ) import qualified Data.Enumerator.List as EL ( consume ) -- cribbed from EL.concatMap concatMapAccum :: Monad m =&gt; (s -&gt; ao -&gt; (s, [ai])) -&gt; s -&gt; E.Enumeratee ao ai m b concatMapAccum f s0 = E.checkDone (E.continue . step s0) where step _ k E.EOF = E.yield (E.Continue k) E.EOF step s k (E.Chunks xs) = loop s k xs loop s k [] = E.continue (step s k) loop s k (x:xs) = case f s x of (s', ais) -&gt; k (E.Chunks $ ais) &gt;&gt;== E.checkDoneEx (E.Chunks xs) (\k' -&gt; loop s' k' xs) passFromTo :: Monad m =&gt; ((a -&gt; Bool), (a -&gt; Bool)) -&gt; Bool -&gt; E.Enumeratee a a m b passFromTo (from, to) pass0 = concatMapAccum updatePass pass0 where updatePass pass el = case (pass, from el, to el) of (True, _, to_el) -&gt; (not to_el, [el]) (False, True, _) -&gt; (True, [el]) (False, False, _) -&gt; (False, []) main :: IO() main = do E.run_ (E.enumList 3 [1..20] $$ E.joinI $ passFromTo ((\e -&gt; e == 3 || e == 13), (\e -&gt; e == 7 || e == 17)) False $$ EL.consume) &gt;&gt;= print $ ./dist/build/StatefulEnumeratee/StatefulEnumeratee [3,4,5,6,7,13,14,15,16,17] </code></pre>
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