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POWriting applications with Scala actors in practice
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  1. POWriting applications with Scala actors in practice
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    copied!<p>I've now written a few applications using scala actors and I'm interested in how people have approached or dealt with some of the problems I've encountered.</p> <p><strong>A plethora of Message classes or !?</strong></p> <p>I have an actor which reacts to a user operation and must cause something to happen. Let's say it <code>react</code>s to a message <code>UserRequestsX(id)</code>. A continuing problem I have is that, because I want to modularize my programs, a single actor on its own is unable to complete the action without involving other actors. For example, suppose I need to use the <code>id</code> parameter to retrieve a bunch of values and then these need to be deleted via some other actor. If I were writing a normal Java program, I might do something like:</p> <pre><code>public void reportTrades(Date date) { Set&lt;Trade&gt; trades = persistence.lookup(date); reportService.report(trades); } </code></pre> <p>Which is simple enough. However, using actors this becomes a bit of a pain because I want to avoid using <strong><code>!?</code></strong>. One actor reacts to the <code>ReportTrades(date)</code> message but it must ask a <code>PersistenceActor</code> for the trades and then a <code>ReportActor</code> to report them. The only way I've found of doing this is to do:</p> <pre><code>react { case ReportTrades(date) =&gt; persistenceActor ! GetTradesAndReport(date) } </code></pre> <p>So that in my <code>PersistenceActor</code> I have a react block:</p> <pre><code>react { case GetTradesAndReport(date) =&gt; val ts = trades.get(date) //from persietent store reportActor ! ReportTrades(ts) } </code></pre> <p>But now I have 2 problems:</p> <ol> <li>I have to create extra message classes to represent <em>the same request</em> (i.e. "report trades"). In fact I have three in this scenario but I may have many more - it becomes a problem keeping track of these</li> <li>What should I call the first and third message <code>ReportTrades</code>? It's confusing to call them both <code>ReportTrades</code> (or if I do, I must put them in separate packages). Essentially there is no such thing as <code>overloading</code> a class by <code>val</code> type.</li> </ol> <p>Is there something I'm missing? Can I avoid this? Should I just give up and use <strong><code>!?</code></strong> Do people use some organizational structure to clarify what is going on?</p>
 

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