Expressions, not statements

A statement is a block of code that doesn't return anything. An expression, on the other hand, returns a new value. Since statements produce no results, the only way for them to be useful is to mutate state. And functional programming tries to avoid mutating the state as much as possible. Theoretically, the more we rely on expressions, the more our functions will be pure, with all the benefits of functional purity.

We've used the if expression many times already, so one of its benefits should be clear: it's less verbose and, for that reason, less error-prone than the if statement.

Pattern matching

The concept of pattern matching is like switch/case on steroids for someone who comes from Java. We've already seen how when expression can be used, inChapter 1, Getting Started with Kotlin, so let's briefly discuss why this concept is important for the functional paradigm.

As you may know, switch in Java accepts only some primitive types, strings, or enums.

Consider the...