Covariant.scala

object Main extends App {

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

  trait Shape {}

  case class Circle(radius: Double) extends Shape

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

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

  def func(c: Covariant[Shape]): Unit = c.print()

  func(covariantCircle)
  func(covariantShape)
  
}