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POExceptions in Yesod
 

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    <p>I had made a daemon that used a very primitive form of <code>ipc</code> (telnet and send a String that had certain words in a certain order). I snapped out of it and am now using <code>JSON</code> to pass messages to a <code>Yesod</code> server. However, there were some things I really liked about my design, and I'm not sure what my choices are now.</p> <p>Here's what I was doing:</p> <pre><code>buildManager :: Phase -&gt; IO () buildManager phase = do let buildSeq = findSeq phase jid = JobID $ pack "8" config = MkConfig $ Just jid flip C.catch exceptionHandler $ runReaderT (sequence_ $ buildSeq &lt;*&gt; stages) config -- ^^ I would really like to keep the above line of code, or something like it. return () </code></pre> <p>each function in buildSeq looked like this</p> <pre><code>foo :: Stage -&gt; ReaderT Config IO () data Config = MkConfig (Either JobID Product) BaseDir JobMap </code></pre> <p><code>JobMap</code> is a <code>TMVar Map</code> that tracks information about current jobs.</p> <p>so now, what I have are Handlers, that all look like this</p> <pre><code>foo :: Handler RepJson </code></pre> <p><code>foo</code> represents a command for my daemon, each handler may have to process a different JSON object.</p> <p>What I would like to do is send one <code>JSON</code> object that represents success, and another JSON object that espresses information about some exception.</p> <p>I would like <code>foo</code>s helper function to be able to return an <code>Either</code>, but I'm not sure how I get that, plus the ability to terminate evaluation of my list of actions, <code>buildSeq</code>.</p> <p>Here's the only choice I see</p> <p>1) make sure <code>exceptionHandler</code> is in Handler. Put <code>JobMap</code> in the <code>App</code> record. Using <code>getYesod</code> alter the appropriate value in <code>JobMap</code> indicating details about the exception, which can then be accessed by <code>foo</code></p> <p>Is there a better way?</p> <p>What are my other choices?</p> <p>Edit: For clarity, I will explain the role of<code>Handler RepJson</code>. The server needs some way to accept commands such as <code>build</code> <code>stop</code> <code>report</code>. The client needs some way of knowing the results of these commands. I have chosen JSON as the medium with which the server and client communicate with each other. I'm using the Handler type just to manage the JSON in/out and nothing more.</p>
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