Eloquent Ruby

Chapter 13. Get the Behavior You Need with Singleton and Class Methods

Much of programming is about building models of the world. Social networking systems model the relationships between people. Accounting systems model the flow of money. Flight simulations model airplanes in flight (or slamming into the ground if I’m at the controls). You can, in fact, look at all of object oriented programming as a support system for this kind of modeling. We build classes to describe groups of similar things, and we have instances to represent the things themselves.

Usually, the class/instance approximation works fine: You define a class called American, and you define methods that indicate instances of this class like hamburgers and baseball. The problem ...