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POI need a function that operates on child nodes
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  1. POI need a function that operates on child nodes
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    copied!<p>I'm working on the project I was working on here:</p> <p><a href="https://stackoverflow.com/questions/15441956/how-do-i-make-a-do-block-return-early">How do I make a do block return early?</a></p> <p>I went with the monad transformer method at my function looks something like this:</p> <pre><code>scrapePost :: String -&gt; IO () scrapePost url = liftM (fromMaybe ()) . runMaybeT $ do doc &lt;- lift $ fromUrl url -- get a bunch of stuff from the page -- send it to the db replies &lt;- lift . runX $ doc &gt;&gt;&gt; css ".post.reply" -- here is the problem mapM_ (parseReply url (fromJust page_id)) replies -- here is the problem </code></pre> <p><code>parseReply</code> is the function I need, but I can't seem to get it right. </p> <p>Here is my feeble attempt to start that function:</p> <pre><code>parseReply :: String -&gt; String -&gt; XNode -&gt; Maybe () parseReply url op_id reply = do reply_id &lt;- runX $ reply ! "id" return () </code></pre> <p>BTW, I am using HandsomeSoup</p> <p>I will operate just like the <code>scrapePost</code> function with a set css rules to scrape, drop replies that don't have all the values, and send them to the db. </p> <p>I want to use <code>mapM</code> because my hope is to replace all the <code>mapM</code> with <code>liftIO</code> and see the performance difference. </p> <p><strong>[UPDATE]</strong></p> <p>So it turns out I didn't need to do any type acrobatics, I just needed a way to turn the reply node into a root node which I found <a href="https://stackoverflow.com/a/9435726/334632">here</a>.</p> <p>Since <code>parseReply</code> is only used in the <code>MaybeT IO ()</code> context, its type doesn't need to change and <code>scrapePost</code> can stay the same.</p> <p><code>parseReply</code> becomes:</p> <pre><code>toRoot :: ArrowXml a =&gt; XmlTree -&gt; a n XmlTree toRoot node = root [] [constA node] parseReply :: String -&gt; String -&gt; XmlTree -&gt; MaybeT IO () parseReply url op_id reply = do let node = toRoot reply reply_id &lt;- lift . liftM (`atMay` 0) $ runX $ node &gt;&gt;&gt; css "div" ! "id" guard (isJust reply_id) return () </code></pre>
 

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