What is functional programming (FP)? Why is it talked about so much?

A programming paradigm is a style of programming. FP is a programming paradigm characterized by the absence of side effects.

In FP, functions are the primary means of structuring code. The FP paradigm advocates using pure functions and stresses on immutable data structures. So we don't mutate variables, but pass a state to function parameters. Functional languages give us lazy evaluation and use recursion instead of explicit loops. Functions are first-class citizens like numbers or strings. We pass functions as argument values, just like a numeric or string argument. This ability to pass functions as arguments allows us to compose behavior, that is, cobble together something entirely new from existing functions.

In this chapter, we will take a whirlwind tour of functional programming. We will look at bits of code and images to understand the concepts. This will also lay a nice foundation for the rest of the book. We will use the functional paradigm and see how it changes the way we think about data structures and algorithms.

This chapter starts with a look at the concept of abstraction. We will see why abstractions are important in programming. FP is a declarative style of programming, similar to Structured Query Language (SQL). Because it is declarative, we use it to tell what we want the computer to do, rather how it should do it. We will also see how this style helps us stay away from writing common, repetitive boilerplate code.

Passing functions as arguments to other, higher order functions is the central idea in FP; we look at this next. We will also see how to stay away from null checks. Controlled state change allows us to better reason our code. Being immutable is the key for creating code that would be easier to reason about.

Next, we will see how recursion helps us realize looping without mutating any variables. We will wrap up the chapter with a look at lazy evaluation, copy-on-write, and functional composition.