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POPattern matching for abstracted case classes
 

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  1. POPattern matching for abstracted case classes
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    <p>I'm trying to abstract case classes in a module using dependent method types and a nightly build of the compiler (2.10.0.r26005-b20111114020239). I found some inspiration from <a href="https://stackoverflow.com/questions/7860163/what-are-some-compelling-use-cases-for-dependent-method-types">Miles Sabin' example</a>.</p> <p>I don't really understand what's wrong in the (self-contained) code below. The output depends on the order of the patterns in <code>foo</code>.</p> <pre><code>// afaik, the compiler doesn't not expose the unapply method // for a companion object trait Isomorphic[A, B] { def apply(x: A): B def unapply(x: B): Option[A] } // abstract module trait Module { // 3 types with some contraints type X type Y &lt;: X type Z &lt;: X // and their "companion" objects def X: Isomorphic[Int, X] def Y: Isomorphic[X, Y] def Z: Isomorphic[Y, Z] } // an implementation relying on case classes object ConcreteModule extends Module { sealed trait X { val i: Int = 42 } object X extends Isomorphic[Int, X] { def apply(_s: Int): X = new X { } def unapply(x: X): Option[Int] = Some(x.i) } case class Y(x: X) extends X // I guess the compiler could do that for me object Y extends Isomorphic[X, Y] case class Z(y: Y) extends X object Z extends Isomorphic[Y, Z] } object Main { def foo(t: Module)(x: t.X): Unit = { import t._ // the output depends on the order of the first 3 lines // I'm not sure what's happening here... x match { // unchecked since it is eliminated by erasure case Y(_y) =&gt; println("y "+_y) // unchecked since it is eliminated by erasure case Z(_z) =&gt; println("z "+_z) // this one is fine case X(_x) =&gt; println("x "+_x) case xyz =&gt; println("xyz "+xyz) } } def bar(t: Module): Unit = { import t._ val x: X = X(42) val y: Y = Y(x) val z: Z = Z(y) foo(t)(x) foo(t)(y) foo(t)(z) } def main(args: Array[String]) = { // call bar with the concrete module bar(ConcreteModule) } } </code></pre> <p>Any idea?</p>
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    1. COI've just tried with the latest trunk, and it doesn't compile for me: 35: error: illegal dependent method type. Using 2.10.0.r26037-b20111121020211. It's possible it wasn't meant to work?
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    2. COjust tested with 2.10.0.r26037-b20111121020211 and it compiles for me. I copy-pasted the whole block and then `Main.main(Array())`.
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