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POScala multiple implicit conversions?
 

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  1. POScala multiple implicit conversions?
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    <p>I don't understand the apparent contradictory behavior I'm seeing in the following code (Scala 2.9):</p> <pre><code>class Pimp1(val x : Double) { def pluss(that : Pimp1) = x + that.x def &lt;(that : Pimp1) = x &lt; that.x } object Pimp1 { implicit def d2pimp(d : Double) = new Pimp1(d) implicit def d2pimp(od : Option[Double]) = new Pimp1(od.get) } object Scratch2 extends App { import Pimp1._ 5.0.pluss(Some(5.0)) 5.0 &lt; Some(5.0) } </code></pre> <p>The line '5.0.pluss(Some(5.0))' compiles, but the line after it does not compile with the following error message:</p> <blockquote> <p>overloaded method value &lt; with alternatives: (x: Double)Boolean (x: Float)Boolean (x: Long)Boolean (x: Int)Boolean (x: Char)Boolean (x: Short)Boolean (x: Byte)Boolean cannot be applied to (Some[Double])</p> </blockquote> <p>If I add explicit &lt; operator to the Pimp class that takes an Option[Double]:</p> <pre><code>def &lt;(that : Option[Double]) = x &lt; that.get </code></pre> <p>Everything compiles fine.</p> <p>Now, the way I understand Scala implicit conversion rules, this makes perfect sense:</p> <ol> <li>The compiler understands that there's no '&lt;' operator on Double that accepts Option[Double]</li> <li>It considers the implicit conversion to Pimp1.</li> <li>If Pimp1 has a suitable operator, it works, otherwise, it generates an error.</li> <li>Importantly, this demonstrates that the compiler does <strong>not</strong> consider applying a second (available) implicit conversion, from Option[Double] to Pimp.</li> </ol> <p>This is how I expected things to work.</p> <p>However, this seems to be contradicted by the first example, where:</p> <ol> <li>The compiler sees that there's no pluss method on Double.</li> <li>The compiler tries the implicit conversion to Pimp, which does have such a method.</li> <li>However, in order to make the operator work, the compiler has to apply a second implicit conversion, on the argument, to convert it to Pimp.</li> </ol> <p>According to the logic above, this should not compile, but it does. Do the implicit conversion rules treat non-existing methods and non-matching methods differently? </p>
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    1. COIt's interesting, because if you reverse it, it works: Some(5.0) < 5.0
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