PointAdder and scala type class

Point.scala

object Point {

  trait PointAdder[P1, P2] {
    def add(p1: P1, p2: P2): Point3D
  }

  case class Point2D(x: Int, y: Int)
  case class Point3D(x: Int, y: Int, z: Int)

  implicit object Point2DAdder extends PointAdder[Point2D, Point2D] {
    def add(p1: Point2D, p2: Point2D): Point3D = Point3D(p1.x + p2.x, p1.y + p2.y, 0)
  }

  implicit object Point3DAdder extends PointAdder[Point3D, Point3D] {
    def add(p1: Point3D, p2: Point3D): Point3D = Point3D(p1.x + p2.x, p1.y + p2.y, p1.z + p2.z)
  }

  implicit object Point2DPoint3DAdder extends PointAdder[Point2D, Point3D] {
    def add(p1: Point2D, p2: Point3D): Point3D = Point3D(p1.x + p2.x, p1.y + p2.y, p2.z)
  }

  implicit object Point3DPoint2DAdder extends PointAdder[Point3D, Point2D] {
    def add(p1: Point3D, p2: Point2D): Point3D = Point3D(p1.x + p2.x, p1.y + p2.y, p1.z)
  }

  def main(args: Array[String]) {
    val point1 = Point2D(1, 2)
    val point2 = Point3D(1, 2, 3)

    println(implicitly[PointAdder[Point2D, Point3D]].add(point1, point2))
  }

}

Sinon dans ta première version tu ajoutes simplement une méthode add :

def add[P1, P2](p1: P1, p2: P2)(implicit adder: PointAdder[P1, P2]): Point3D = adder.add(p1, p2)

c'est en gros le pattern CanBuildFrom utilisé dans les collections