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POScala Advanced Type Usage
 

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    <h2>The Background</h2> <p>I'm working on an event library in Scala. <a href="https://github.com/goldmar/CEScala/" rel="nofollow noreferrer">In my library</a> you can define events like this:</p> <pre><code>val e1 = new ImperativeEvent[Int] </code></pre> <p>You can trigger them like this:</p> <pre><code>e1(42) </code></pre> <p>You can create reactions like this:</p> <pre><code>val r1 = (i: Int) =&gt; println(i) </code></pre> <p>And attach them to the event like this:</p> <pre><code>e1 += r1 </code></pre> <p>There is also some other stuff (like event transformations, compositions etc). I use the <a href="http://esper.codehaus.org/" rel="nofollow noreferrer">Esper CEP engine</a> as the backend of my library. Esper uses an SQL-like language called EPL for most operations.</p> <h2>The Problem</h2> <p>I'm trying to implement some more advanced concepts like event joins. So now you can define events with multiple properties like this (using tuple types):</p> <pre><code>val e2 = new ImperativeEvent[(Int, String)] </code></pre> <p>And then join them like this:</p> <pre><code>val e3 = e1 join e2 windowLength (30) on "E1.P1 = E2.P1" </code></pre> <p>which performs a join of e1 and e2 on the last 30 occurrances of both on the condition that their respective first properties are equal.</p> <p>This is alright but I'd like to get rid of the strings in my implementation to make the event expressions type checkable. I'd like to change the join expression to something like this:</p> <pre><code>val e3 = e1 join e2 windowLength (30) on e1._1 === e2._1 </code></pre> <p>similar to the way it is done in eg. <a href="http://squeryl.org/" rel="nofollow noreferrer">Squeryl</a>. The problem with this is, I can't access the types of the elements of the tuple type...</p> <h2>The Question</h2> <p>How can I access the tuple types statically? Right now I've only managed to access them at run-time through reflection which does not help me. I'm pretty sure that what I want to achieve is not possible with tuples but I'm wondering if using HLists from the shapeless library or something similar might help in achieving my goal.</p>
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    1. USMark Goldenstein
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    1. COIs seems you have `e1.join(e2).windowLength(30).on(e1._1 === e2._1)`. What is `===` ?
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    2. CO**+=** seems an odd choice of operator for what you use it for here. It implies that **ImperativeEvent** is purely or principally a collection of reactions. If you have overridden **+** for adding any of the various things that you might add to an event, I guess that would be a bit more understandable but it still makes me queasy.
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      1. USitsbruce
    3. CO@pedrofurla E1 refers to the left event of the join, E2 to the right one. The `join` method replaces those with the appropriate internal event IDs. The actual join is made by Esper (http://esper.codehaus.org/), my library is mostly a wrapper around it. Esper is based on an SQL-like language called EPL, that's why it uses Strings. I'd like to add type checking to those Strings (similar to eg. Squeryl, that's where my `===` example comes from) but I'm not sure if this is possible with my current event representation (in which parameters are expressed as (tuple) types of the event).
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