Book Image

Functional Kotlin

Book Image

Functional Kotlin

Overview of this book

Functional programming makes your application faster, improves performance, and increases your productivity. Kotlin supports many of the popular and advanced functional features of functional languages. This book will cover the A-Z of functional programming in Kotlin. This book bridges the language gap for Kotlin developers by showing you how to create and consume functional constructs in Kotlin. We also bridge the domain gap by showing how functional constructs can be applied in business scenarios. We’ll take you through lambdas, pattern matching, immutability, and help you develop a deep understanding of the concepts and practices of functional programming. If you want learn to address problems using Recursion, Koltin has support for it as well. You’ll also learn how to use the funKtionale library to perform currying and lazy programming and more. Finally, you’ll learn functional design patterns and techniques that will make you a better programmer.By the end of the book, you will be more confident in your functional programming skills and will be able to apply them while programming in Kotlin.
Table of Contents (22 chapters)
Title Page
Copyright and Credits
Packt Upsell


A monad is a functor type that defines a flatMap (or bind, in other languages) function, that receives a lambda that returns the same type. Let me explain it with an example. Luckily for us, List<T> defines a flatMap function:

fun main(args: Array<String>) {
    val result = listOf(1, 2, 3)
            .flatMap { i ->
                listOf(i * 2, i + 3)

    println(result) //2, 4, 4, 5, 6, 6

In a map function, we just transform the List value's content, but in flatMap, we can return a new List type with less or more items, making it a lot more potent than map.

So, a generic monad will look like this (just remember that we don't have higher-kinded types):

interface Monad<C<_>>: Functor<C> { //Invalid Kotlin code
    fun <A, B> flatMap(ca:C<A>, fm:(A) -> C<B>): C<B>

Now, we can write a flatMap function for our Option type:

fun <T, R> Option<T>.flatMap(fm: (T) -> Option...