Building Domain Specific Languages in Kotlin
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Building Domain Specific Languages in Kotlin

Kotlin language features and software design patterns for creating DSLs and improving code

About the Book

If you are writing Kotlin code, inevitably you will encounter a Domain Specific Language (DSL). A DSL may be in your build tool, UI code, or somewhere else, but trust me it is there. Why do these DSLs exist? They allow you to create new simplified syntax for your APIs and libraries. With the ability to create this syntax you can go beyond object oriented design and create even more effective code that is easy to understand and has less errors. This book shows you the Kotlin features and design patterns you need to know to implement complex DSLs or make small improvements to anywhere in your code. Along the way you will gain a deeper understanding of some of Kotlin's language features such as lambdas and extension functions.

You will learn techniques for building DSLs

Building a DSL in Kotlin requires you to understand:

  1. DSL design: DSLs make assumptions to simplify syntax, such as assuming the code is part of a particular problem domain.
  2. Kotlin features: DSL implementations sometimes employ Kotlin features in ways that are unusual to achieve a desired syntax.
  3. Software design patterns: Software design patterns can help you create different syntax and also simplify DSLs.

The Kotlin features and design patterns this book describes are:

Kotlin features:

  • Lambda with receiver
  • Invoke operator
  • Operator overload
  • Extension functions
  • Custom property accessors
  • Infix notation

Design patterns:

  • Function chains
  • Nested builders
  • Context variables
  • Symbols

You will be able to write new kinds of code

When you understand the techniques in this book, you will be able to start code that once looked like this:

val dishes = mutableListOf<Dish>()

val ingredientsForBlt = listOf("bacon", "lettuce", "tomato")
val blt = Dish("blt", ingredientsForBlt)

val ingredientsForPizza = listOf("pepperoni", "mushrooms")
val pizza = Dish("pizza", ingredientsForPizza)

val menu = Menu("Sunrise Restaurant", dishes)

and turn it into this:

val builder = MenuBuilder("Sunrise Restaurant")
val menu = builder {
    dish("pizza") {
    dish("blt") {

or maybe even make the syntax look almost like plain sentences like this:

val builder = MenuBuilder("Sunrise Restaurant")
val menu = builder {
    "blt" with ingredients named "bacon" and "lettuce" and "tomato"
    "pizza" with ingredients named "cheese" and "pepperoni" and "mushroom"

You will see ample code examples

This book contains two parts to help you learn the necessary DSL skills. First, it has ample code examples to demonstrate each concept. Second, it has step by step exercises you can use to help you to practice what you have learned.

Are you excited?

Grab this book and start maximizing your knowledge of Kotlin and building some great new DSL syntax.

About the Author

Greg Milette
Greg Milette

Greg Milette is a Software Engineer who has worked on projects large and small, weird and simple. His interests include speech recognition, mobile sensors, IoT, building apps in Kotlin.

Table of Contents

  • Preface
    • Where is the code?
    • Exercises and Final Project
    • Go!
  • Design and Implement DSLs
    • Why learn about Kotlin DSLs?
    • Why are DSLs useful?
    • Features of Domain Specific Languages
    • Scenario: Creating Menus
    • How should you implement a DSL?
  • Apply design patterns to improve DSL syntax
    • Symbols
    • Function Chains
    • Context Variables
    • Nested builders
    • Summary
  • Create Structured Syntax using Lambdas
    • Lambda with Receiver
    • @DslMarker
    • Invoke
    • Summmary
  • Set properties using Kotlin features
    • Enums
    • Assignment
    • Operator Overload
    • Custom Accessors
    • Extension Functions
    • Summary
  • Create Sentences using Infix Notation
    • Infix and extension functions
    • Using infix in a DSL
    • Infix functions in the Menu DSL
    • Compound Clauses
    • Summary
    • Create a DSL today!
  • Project: Create a DSL for representing a car configuration

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