Covariance and contravariance

When an F type accepts a type parameter of A, we can add a + or - sign in front of the parameter declaration to indicate the variance of F on A:

• F[+A] makes F covariant on A. This means that if B <:< A (where B extends A), then F[B] <:< F[A].
• F[-A] makes F contravariant on A. If B <:< A, then F[A]  <:< F[B].
• F[A] makes F invariant on A. If B <:< A, then there is no relationship between F[A] and F[B].

InvariantDecoder

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...