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POCan this be simplified?
 

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  1. POCan this be simplified?
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    <p>Referring to a <a href="https://stackoverflow.com/questions/5026055/mixing-in-generic-traits-in-parameterized-classes-without-duplicating-type-parame/5028335#5028335">previous answer of mine on stackoverflow</a></p> <p>The core of the complexity is illustrated in just one method:</p> <pre><code>implicit def traversableToFilterOps[CC[X] &lt;: Traversable[X], T] (xs: CC[T])(implicit witness: CC[T] &lt;:&lt; TraversableLike[T,CC[T]]) = new MoreFilterOperations[CC[T], T](xs) </code></pre> <p>With two questions:</p> <ol> <li><p>Is there any way to give the compiler a hint that <code>Map</code> meets the signature <code>CC[X] &lt;: Traversable[X]</code>? I would expect it to match as a <code>Traversable[Tuple2[_,_]]</code> but this doesn't happen. In the end, I had to write a second method taking <code>CC[KX,VX] &lt;: Map[KX,VX]</code>, but that feels redundant</p></li> <li><p><code>witness: CC[T] &lt;:&lt; TraversableLike[T,CC[T]]</code> also seems redundant given the first type parameter, my gut feeling is that this is enforced by the type of <code>Traversable</code> and must <em>always</em> hold true for any possible subclass or value of <code>X</code>, so there should be no reason to explicitly require it as a witness.</p></li> </ol> <p>If I test this using an existential type in the REPL, then the compiler seems to agree with me:</p> <pre><code>scala&gt; implicitly[Traversable[X] &lt;:&lt; TraversableLike[X,Traversable[X]] forSome { type X }] res8: &lt;:&lt;[Traversable[X],scala.collection.TraversableLike[X,Traversable[X]]] forSome { type X } = &lt;function1&gt; </code></pre> <p>Is there therefore any way to do away with the boilerplate?</p>
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    1. USKevin Wright
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    1. COThe answer lies somewhere in here: https://gist.github.com/445874
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