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POReactive-banana: up-to-date values from fromPoll
 

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    <p>I'm writing a music player in Haskell with reactive-banana. One problem I have is fetching up-to-date values with fromPoll. I want to enable the user to optionally select a part of the track <strong>while playing</strong>. My code looks something like this:</p> <pre><code>makePlayNetworkDescr :: Player a =&gt; AddHandler Command -&gt; a -&gt; NetworkDescription t () makePlayNetworkDescr addCmdEvent player = do bPosition &lt;- fromPoll (getPosition player) eCmds &lt;- fromAddHandler addCmdEvent let eSetStart = filterE (isJust) $ bPosition &lt;@ filterE (==SetStart) eCmds eSetEnd = filterE (isJust) $ bPosition &lt;@ filterE (==SetEnd) eCmds eClearRange = filterE (==ClearRange) eCmds bStart = accumB Nothing ((const &lt;$&gt; eSetStart) `union` (const Nothing &lt;$ eClearRange)) bEnd = accumB Nothing ((const &lt;$&gt; eSetEnd) `union` (const Nothing &lt;$ eClearRange)) </code></pre> <p>Above, <em>getPosition</em> is a partial function, returning <em>Nothing</em> before the playback actually starts. The problem is that once the <em>addCmdEvent</em> fires for the first time, <em>bPosition</em> will still hold a <em>Nothing</em> value. <em>eSetStart/End</em> calculate their values based on this. Only then does <em>bPosition</em> get updated, and this is the value which will be used next time that <em>addCmdEvent</em> fires. And so on, the value will always be "off by one", so to speak. </p> <p>There is a related <a href="https://stackoverflow.com/questions/8618069/reactive-banana-firing-event-that-contain-the-most-up-to-date-value-of-a-behavi">SO question</a>, but in that case there exists a "trigger" event which can be used to calculate the new value of the behavior. Is anything like that possible with fromPoll?</p>
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    1. COWhen you're finished, you should put the result on Hackage. Looking forward to playing my music from Haskell!
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    2. COI do plan to put it on Hackage once (hah) I'm finished. However it's a "scratch my own itch" kind of music player to aid with transcribing music, so I'm not sure how interesting would it be to other people.
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