When an F type accepts a type parameter of A, we can add a+or-sign in front of the parameter declaration to indicate thevarianceofF on A:
We are now going to explore this variance concept with an example. Let's start with a simple class hierarchy, as shown in the following code:
trait Animal case class Cat(name: String) extends Animal case class Dog(name: String) extends Animal
With this hierarchy, I can declare a variable of an Animal type and assign it to an instance of aCatorDog type. The following code will compile:
val animal1: Animal = Cat("Max")
val animal2: Animal = Dog("Dolly")
implicitly[Dog <:< Animal]More generally, the assignment val a: A = b: B compiles...