Scala Programming Projects

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 Animalcase class Cat(name: String) extends Animalcase 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 a Cat or Dog 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 if B <:< A.

You can check that type B extends type A with the expression implicitly[B <:< A]; if it compiles, then B is a subtype of A.

Then, we define an InvariantDecoder trait that has a single ...