Poly.scala

object Poly {
  trait Case[A] {
    type Result
    def at(x: A): Result
  }

  trait PolyDef {
    def apply[A : Case](x: A): Case[A]#Result
  }

  object Length extends PolyDef {
    override def apply[A : Case](x: A): Case[A]#Result = implicitly[Case[A]].at(x)
  }

  implicit object IntCase extends Case[Int] {
    override type Result = Int
    override def at(x: Int) = 4
  }

  implicit object StringCase extends Case[String] {
    override type Result = Int
    override def at(x: String) = x.length
  }

  def main(args: Array[String]): Unit = {
    println(Length(1))
    println(Length("hello"))
  }
}