AssertionError.scala

object Main {

  trait Language { self: Literal =>
    import self._

    def add(l: F[Int], r: F[Int]): F[Int]
  }

  trait Literal {
    type F[T]

    def num(i: Int): F[Int]
  }

  def expression(adder: Language) = {
    import adder._

    add(num(1), num(2))
  }

  val idLanguage = new Language {
    type F[T] = T

    def add(l: Int, r: Int): Int = l + r
    def num(i: Int): Int = i
  }

  val printLanguage = new Language {
    type F[T] = String

    def add(l: String, r: String) = s"$l + $r"
    def num(i: Int) = s"$i"
  }


  def main(args: Array[String]): Unit = {
    println(expression(printLanguage))
    println(expression(idLanguage))
  }
}