ConstraintsOnAssociativeType2.scala

// This file must be compiled under Scala 2.8.0 with -Xexperimental
case class Hoge() 
case class Foo()
case class Dummy()

trait C[A] {
  type Conv
  def method(a: A): Conv
}

trait Show[A] { def format(a: A): String }

object InstanceDeclarations {
  implicit object HogeInstance extends C[Hoge] {
    type Conv = String
    def method(a: Hoge): String = "Hoge"
  }
  implicit object FooInstance extends C[Foo] {
    type Conv = Dummy
    def method(a: Foo): Dummy = Dummy()
  }
  implicit object ShowString extends Show[String] {
    def format(a: String): String  = a
  }
}

import InstanceDeclarations._

object ConstraintsOnAssociativeType2 {
  class Workaround[S](s: S) {
    def apply(implicit cs: C[S]) = new {
      def apply(implicit c: Show[cs.Conv]) = c.format(cs.method(s))
    }
  }
  def main(args: Array[String]) {
    println(new Workaround(Hoge()).apply.apply) // OK
    //println(new Workaround(Foo()).apply.apply) // Error
  }
}