Scala example of declaration side Type Covariance

CovariantExample.scala

// take out the + and see the last few lines fail as doit then only takes Animal, and not its subtypes
// we output A, so + is fine, we are covariant
sealed trait ThisThatValue[+A]

object ThisThatValue {
  final case class This[+A](value: A) extends ThisThatValue[A]
  final case class That[+A](value: A) extends ThisThatValue[A]
}

sealed trait Animal {
  val name: String
}

object Animal {
  final case class Dog(name: String) extends Animal
  final case class Cat(name: String) extends Animal

  // if I added a new Animal class I would be warned
  // in below match about being not exhaustive

  //final case class Mouse(name:String) extends Animal
}

import ThisThatValue._
import Animal._

val dog1 = This(Dog("max"))
val cat1 = That(Cat("Kitty"))
val dog2 = That(Dog("Spot"))

def doit(cv: ThisThatValue[Animal]): Unit =
  cv match {
    case That(a: Dog) => println(s"that dog $a")
    case That(a: Cat) => println(s"that cat $a")
    case This(a) => println(s"this $a")
  }

doit(dog1)
doit(cat1)
doit(dog2)

output.txt

this Dog(max)
that cat Cat(Kitty)
that dog Dog(Spot)