Actions, calculations, and data

We will learn many skills on our journey into the world of functional programming. But first we must equip ourselves with a fundamental skill. That skill is the ability to distinguish three categories of code: actions, calculations, and data. Once we learn that distinction, a vast landscape of powerful ideas is available to us. We will learn to refactor actions to become calculations, making our code easier to test and read. We will improve the design of actions to be more reusable. We will make data immutable so that we can rely on it for keeping records. And we will learn a way to organize and understand our code as layers of meaning. But first, we must learn to see actions, calculations, and data.