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POScala view application puzzler
 

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    <p>Say we have the following two traits:</p> <pre><code>trait Foo[A] { def howMany(xs: List[A]) = xs.size } trait Bar </code></pre> <p>And an implicit conversion from the second to the first:</p> <pre><code>implicit def bar2foo[A](bar: Bar) = new Foo[A] {} </code></pre> <p>We create a <code>Bar</code> and a list of integers:</p> <pre><code>val bar = new Bar {} val stuff = List(1, 2, 3) </code></pre> <p>Now I'd expect the following to work:</p> <pre><code>bar howMany stuff </code></pre> <p>But it doesn't:</p> <pre><code>scala&gt; bar howMany stuff &lt;console&gt;:13: error: type mismatch; found : List[Int] required: List[A] bar howMany stuff ^ </code></pre> <p>So we go to <a href="http://www.scala-lang.org/docu/files/ScalaReference.pdf" rel="nofollow noreferrer">the spec</a>, which has this to say (emphasis in bold is mine):</p> <blockquote> <p>Views are applied in three situations.</p> <ol> <li><p>[Isn't relevant here.]</p></li> <li><p>In a selection <em>e.m</em> with <em>e</em> of type <em>T</em>, <strong>if the selector <em>m</em> does not denote a member of <em>T</em></strong>. In this case, a view <em>v</em> is searched which is applicable to <em>e</em> and whose result contains a member named <em>m</em>. The search proceeds as in the case of implicit parameters, where the implicit scope is the one of <em>T</em>. If such a view is found, the selection <em>e.m</em> is converted to <em>v(e).m</em>.</p></li> <li><p>In a selection <em>e.m(args)</em> with <em>e</em> of type <em>T</em>, <strong>if the selector <em>m</em> denotes some member(s) of <em>T</em>, but none of these members is applicable to the arguments <em>args</em></strong>. In this case a view <em>v</em> is searched which is applicable to <em>e</em> and whose result contains a method <em>m</em> which is applicable to <em>args</em>. The search proceeds as in the case of implicit parameters, where the implicit scope is the one of <em>T</em>. If such a view is found, the selection <em>e.m</em> is converted to <em>v(e).m(args)</em>.</p></li> </ol> </blockquote> <p>So we try the following, thinking it must be too absurd to work:</p> <pre><code>trait Foo[A] { def howMany(xs: List[A]) = xs.size } trait Bar { def howMany = throw new Exception("I don't do anything!") } implicit def bar2foo[A](bar: Bar) = new Foo[A] {} val bar = new Bar {} val stuff = List(1, 2, 3) </code></pre> <p>But it does (on both 2.9.2 and 2.10.0-RC2, at least):</p> <pre><code>scala&gt; bar howMany stuff res0: Int = 3 </code></pre> <p>This leads to some really strange behavior, as for example in <a href="https://gist.github.com/4131492" rel="nofollow noreferrer">this workaround</a> for <a href="https://stackoverflow.com/q/13507202/334519">this problem</a>.</p> <p>I have three (closely related) questions:</p> <ol> <li>Is there a straightforward way (i.e., one that doesn't involve adding fake methods with the appropriate name) to have the view applied correctly in the original case above?</li> <li>Can someone provide a reading of the spec that accounts for this behavior?</li> <li>Assuming this is the intended behavior, does it make any sense at all?</li> </ol> <p>I'd also appreciate any links to previous discussions of this issue—I haven't been having much luck with Google.</p>
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