The Joy of Kotlin

Introduction

The book «The Joy of Kotlin» (original title The Joy of Kotlin) by Pierre-Yves Saumont is not just another programming language guide. It is a deep dive into the world of functional programming implemented on the Kotlin platform. Published in 2019, the 480-page book offers the reader not only syntax but also the philosophy of writing clean, reliable, and expressive code.

The author is a renowned expert in functional programming, having previously written books on Java (notably «Functional Programming in Java»). In this work, he transfers his many years of experience to Kotlin — a modern language that is rapidly gaining popularity due to its conciseness and compatibility with Java.

What is this book about?

The book is dedicated to applying the principles of functional programming (FP) in the Kotlin language. Unlike many textbooks that limit themselves to the basics of syntax, Saumont consistently reveals how to write code free from side effects, using immutable data structures, higher-order functions, monads, and lazy evaluation.

Main topics covered in the book:

• Functions as first-class objects — creating and composing functions.
• Immutability — working with immutable collections and data.
• Error handling without exceptions — using Option, Either, and Try types.
• Data streams and laziness — efficiently working with infinite sequences.
• Monads — practical application of monads for managing computation context.
• Testing and debugging — how the functional approach simplifies writing tests.

Who is this book for?

The book is intended for experienced developers who are already familiar with the basics of Kotlin or Java and want to elevate their skill level. It will be useful for:

• Java programmers transitioning to Kotlin and wanting to master its functional capabilities.
• Developers who want to write more reliable and predictable code.
• Those interested in functional programming but unsure where to start in practice.

The book may seem challenging for programming beginners, as the author assumes knowledge of basic OOP concepts and the ability to read Java/Kotlin code.

What will the reader learn?

After reading the book, you will be able to:

• Write functions that have no side effects (pure functions).
• Use immutable data structures to prevent state errors.
• Apply functional programming patterns such as map, flatMap, fold.
• Build data processing chains without mutable variables.
• Handle errors declaratively, without try-catch blocks.
• Create lazy computations for performance optimization.
• Understand and use monads (Option, Either, State, IO).
• Design application architectures based on functional principles.

Strengths of the book

The first thing that stands out is the depth of exposition. The author does not simply list the language's capabilities but explains why the functional approach is better in certain situations. Each chapter contains numerous code examples that can be immediately tried out in an IDE.

The second important advantage is the practical focus. Saumont shows how to solve real-world problems (parsing, file handling, list processing) in a functional way. You don't just study theory; you see how it is applied in production code.

The third is the book's structure. The material is presented from simple to complex: starting with basic functions and ending with building your own functional library. This allows