Scala Programming Projects

CovariantDecoder

Let's follow the compiler's advice and create a new CovariantDecoder[+A], along with a CovariantCatDecoder instance that extends it, as shown in the following code:

trait CovariantDecoder[+A] {  def decode(s: String): Option[A]}object CovariantCatDecoder extends CovariantDecoder[Cat] { (...)}

We do not show the implementation of decode in CovariantCatDecoder; it is the same as in InvariantCatDecoder. With this covariant parameter, the following relationship is verified:

implicitly[CovariantDecoder[Cat] <:< CovariantDecoder[Animal]]

This time, we can assign the CovariantCatDecoder to an instance of CovariantDecoder[Animal], as shown in the following code:

val covariantAnimalDecoder: CovariantDecoder[Animal] = CovariantCatDecoder ...