Chapter 5. Functional programming
This chapter covers
- Why functional programming matters
- Mixing functional programming with OOP
- Functions in various forms
- Monads and their practical examples
You’re already doing functional programming using Scala if you’ve been following the examples in the book. In some cases it’s explicitly mentioned or visible and in other cases it’s mixed with object-oriented constructs of the Scala language. This chapter focuses on functional programming concepts and how they can be implemented in Scala. The goal of the chapter is to make you comfortable with functional programming and help you to write code in functional programming style.
Functional programming is a programming paradigm that models computation as the evaluation of expressions. And expressions are built using functions that don’t have mutable state and side effects. Exploring the roots of functional programming is valuable.[1] Believe it or not, functional programming started around 1930 when Alonzo Church introduced lambda calculus.[2] A lambda calculus (λ calculus) is a formal mathematical system to investigate functions, function application, and function recursion. In lambda calculus functions are first-class values; functions can take other functions as a parameter and return functions as an output (higher-order functions). A function that adds two input parameters could be written like the following in lambda calculus: