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    <p>One of the disadvantages of making <code>RList</code> an inner trait of a producer is that it becomes less pleasant to write methods or functions with <code>RList</code> arguments outside of the producer—you end up with a lot of this:</p> <pre><code>def foo[P &lt;: RListProducer, T](rl: P#RList[T]) = ??? </code></pre> <p>An alternative is to give <code>RList</code> a type member that will be unique for each "source" <code>RList</code>, but that each source will pass on to its "children". Something like this:</p> <pre><code>trait RList[T] { outer =&gt; type S protected val wrapped: List[T] def map[V](f: T =&gt; V) = new RList[V] { type S = outer.S protected val wrapped = outer.wrapped.map(f) } def zip[V](r: RList[V] { type S = outer.S }) = new RList[(T, V)] { type S = outer.S protected val wrapped = outer.wrapped.zip(r.wrapped) } } object RList { def toRList[T](ts: List[T]) = new RList[T] { type S = this.type protected val wrapped = ts } } </code></pre> <p>Now assume we have the following:</p> <pre><code>val a = RList.toRList(1 :: 2 :: 3 :: Nil) val b = a.map(_.toString) val c = RList.toRList("1" :: "2" :: "3" :: Nil) </code></pre> <p>Now <code>a zip b</code> (or <code>a zip b zip a zip a</code>, etc.) will compile, but if you throw in a <code>c</code> you'll get a compiler error.</p> <hr> <p><strong>Note:</strong> I'd originally written <code>zip</code> as follows:</p> <pre><code>def zip[V](r: RList[V])(implicit ev: r.S =:= S) = new RList[(T, V)] { ... } </code></pre> <p>This gives a slightly nicer compiler error message, but if you're working pre-2.10 it requires turning on dependent method types with <code>-Ydependent-method-types</code>.</p>
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    1. COI'm not sure if there's any guarantee that dependent method types aren't going to disappear in a future release of scala, but if you're willing to work with that understanding, this is indeed the better way to do it.
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    2. CO@AlecZorab: See my edit—you don't really need dependent method types here. In any case they're enabled by default in 2.10, so I don't think they're going anywhere.
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    3. COyes, I just discovered that for myself when I tried it. Much better solution than mine.
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