Contravariant.scala

object Main extends App {

  /* 
    Contravariant:  F[B] is a subtype of F[A] if A is a subtype of B
  */

  trait Shape {}

  case class Circle(radius: Double) extends Shape

  /* 
    if we change this to [A] or [+A] we cannot pass 
    covariantShape into func 
  */
  trait Contravariant[-A] {
    def print(): Unit
  }

  val covariantCircle: Contravariant[Circle] = () => println(s"Circle")
  val covariantShape: Contravariant[Shape] = () => println(s"Shape")

  def func(c: Contravariant[Circle]): Unit = c.print()

  func(covariantCircle)
  func(covariantShape)

}