In this chapter, we’ll see how to write purely functional programs that manipulate state, using the simple domain of random number generation as the example. Although by itself it’s not the most compelling use case for the techniques in this chapter, the simplicity of random number generation makes it a good first example. We’ll see more compelling use cases in parts 3 and 4 of the book, especially part 4, where we’ll discuss dealing with state and effects in much greater detail. The goal here is to provide you with the basic pattern for making any stateful API purely functional. As you start writing your own functional APIs, you’ll likely run into many of the same questions we explore here.
If you need to generate random1 numbers in Scala, there’s a class in the standard library, called scala.util.Random (Scala API link: https://mng.bz/7W8g), with a pretty typical imperative API that relies on side effects. The following listing shows an example of its use.