DSL Building on Kotlin Programming Language

A comprehensive guide to building Domain-Specific Languages (DSLs) in Kotlin, leveraging its powerful features for DSL development.

By İbrahim Korucuoğlu ( @siberoloji) | Sunday, February 23, 2025

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4 minute read

Introduction

Domain-Specific Languages (DSLs) are specialized programming languages designed for a particular domain or purpose. Unlike general-purpose languages (GPLs), which aim to be versatile and applicable across multiple domains, DSLs optimize for expressiveness and simplicity in specific areas like database querying, build automation, and UI design.

Kotlin, a modern and expressive programming language developed by JetBrains, provides robust support for DSL construction. Its features, such as extension functions, lambdas, infix notation, and type-safe builders, make it an excellent choice for DSL development. This blog post explores the concepts of DSLs, why Kotlin is well-suited for DSL building, and how you can create a custom DSL in Kotlin step by step.

What is a DSL?

DSLs can be classified into two main categories:

Internal DSLs - Embedded within an existing language (e.g., Kotlin DSL for Gradle, SQL-like queries in Kotlin).
External DSLs - Standalone languages with their own syntax and parsing logic (e.g., JSON, YAML, or SQL).

Kotlin excels in internal DSLs due to its rich syntax capabilities and powerful standard library. By leveraging Kotlin’s expressive constructs, developers can create concise and readable DSLs that enhance productivity.

Why Kotlin for DSLs?

Kotlin offers several language features that make it a great candidate for DSL development:

Type Safety: Kotlin ensures strong type safety, reducing runtime errors.
Lambdas with Receivers: These allow for a more natural, declarative style.
Extension Functions: Enable adding behavior to existing classes without modifying them.
Operator Overloading: Helps make DSLs more intuitive.
Infix Functions: Improve readability and mimic natural language syntax.
Scoped Functions (apply, let, run, etc.): Facilitate a fluent API design.

Building a DSL in Kotlin: Step-by-Step Guide

To demonstrate how to build a DSL in Kotlin, let’s create a simple HTML DSL that allows us to define HTML structures in a Kotlin-esque manner.

Step 1: Define a Basic HTML Structure

We need to create an element class that represents HTML elements.

class Element(val name: String) {
    private val children = mutableListOf<Element>()
    private val attributes = mutableMapOf<String, String>()

    fun setAttribute(name: String, value: String) {
        attributes[name] = value
    }

    fun addChild(element: Element) {
        children.add(element)
    }

    override fun toString(): String {
        val attrString = attributes.entries.joinToString(" ") { "${it.key}='${it.value}'" }
        val childrenString = children.joinToString("\n")
        return "<${name} ${attrString}>\n$childrenString\n</${name}>"
    }
}

Step 2: Utilize Lambdas with Receivers for a Fluent API

To make the DSL more natural, let’s use lambdas with receivers to allow nesting elements elegantly.

class HTML {
    private val root = Element("html")

    fun body(init: Element.() -> Unit) {
        val body = Element("body")
        body.init()
        root.addChild(body)
    }

    override fun toString(): String = root.toString()
}

fun html(init: HTML.() -> Unit): HTML {
    val html = HTML()
    html.init()
    return html
}

Step 3: Create a Readable and Intuitive DSL

Now, we can use our DSL to construct an HTML document:

val page = html {
    body {
        val header = Element("h1")
        header.setAttribute("style", "color: blue;")
        header.addChild(Element("Welcome to Kotlin DSL"))
        addChild(header)
    }
}

println(page)

This prints:

<html>
    <body>
        <h1 style='color: blue;'>
            Welcome to Kotlin DSL
        </h1>
    </body>
</html>

Advanced Techniques for DSL Building in Kotlin

1. Using Type-Safe Builders

Kotlin’s standard library provides @DslMarker, which prevents unintentional scope leaks in DSLs.

@DslMarker
annotation class HtmlDsl

@HtmlDsl
class Body {
    private val elements = mutableListOf<Element>()
    
    fun h1(text: String, init: Element.() -> Unit = {}) {
        val h1 = Element("h1")
        h1.addChild(Element(text))
        h1.init()
        elements.add(h1)
    }
}

2. Infix Functions for Better Readability

Kotlin supports infix notation, which can be leveraged to make DSLs more intuitive:

infix fun String.to(value: String) = Pair(this, value)

This allows attributes to be set more naturally:

header.setAttribute("style" to "color: red;")

3. Operator Overloading

Kotlin supports operator overloading, which can be used to build cleaner syntax:

operator fun Element.plus(child: Element) {
    this.addChild(child)
}

This allows us to write:

header + Element("p")

Real-World Examples of Kotlin DSLs

Several well-known DSLs are built using Kotlin:

Gradle Kotlin DSL - Used for build automation.
Ktor - A framework for building web applications.
Jetpack Compose - Declarative UI framework.

Conclusion

DSLs provide a powerful way to simplify and enhance domain-specific programming, and Kotlin is an excellent language for building them. By leveraging Kotlin’s expressive syntax and functional programming features, developers can create intuitive and readable DSLs that improve productivity and maintainability.

Whether you’re designing a DSL for HTML rendering, database querying, or UI development, Kotlin provides all the necessary tools to create a clean and elegant domain-specific language.

Ready to explore Kotlin DSLs? Try building your own and see the benefits firsthand!