Scala Cookbook

19.7. Selectively Adding New Behavior to a Closed Model

Problem

You have a closed model, and want to add new behavior to certain types within that model, while potentially excluding that behavior from being added to other types.

Solution

Implement your solution as a type class.

To demonstrate the problem and solution, when I first came to Scala, I thought it would be easy to write a single add method that would add any two numeric parameters, regardless of whether they were an Int, Double, Float, or other numeric value. Unfortunately I couldn’t get this to work—until I learned about type classes.

Because a Numeric type class already exists in the Scala library, it turns out that you can create an add method that accepts different numeric types like this:

def add[A](x: A, y: A)(implicit numeric: Numeric[A]): A = numeric.plus(x, y)

Once defined, this method can be used with different numeric types like this:

println(add(1, 1))
println(add(1.0, 1.5))
println(add(1, 1.5F))

The add method works because of some magic in the scala.math.Numeric trait. To see how this magic works, create your own type class.

Creating a type class

The process of creating a type class is a little complicated, but there is a formula:

• Usually you start with a need, such as having a closed model to which you want to add new behavior.

• To add the new behavior, you define a type class. The typical approach is to create a base trait, and then write specific implementations of that trait using implicit objects.

• Back in your main ...