singleton-example.scala

sealed trait A {
  type Typeclass[T] <: { def apply(): Unit }
}
  
case class B() extends A {
  trait Typeclass[T] { def apply(): Unit }
  
  object Typeclass {
    implicit val intClass: Typeclass[Int] = new Typeclass[Int] {
      def apply() { println("B.Int") }
    }
    
    implicit val longClass: Typeclass[Long] = new Typeclass[Long] {
      def apply() { println("B.Long") }
    }
  }
}
  
case class C() extends A {
  trait Typeclass[T] { def apply(): Unit }
  
  object Typeclass {
    implicit val intClass: Typeclass[Int] = new Typeclass[Int] {
      def apply() { println("C.Int") }
    }
  }
}
  
// The Singleton part is needed to keep D(b) from compiling.
case class D[X <: A with Singleton](a: X) {
  def hey[T](implicit imp: X#Typeclass[T]) { imp() }
}
  
val b = B()
val d = D[b.type](b)
  
d.hey[Int]