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Kotlin Academy · Lesson

Testing and Evolving DSLs Without Breaking Users

Design DSL APIs for stability and test them with readable assertion blocks.

Testing and Evolving DSLs Without Breaking Users is a free Kotlin Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why DSL Testing Is Different

A DSL is a public API. Changes to it can break every call site in user code. Testing a DSL means verifying both the output it produces and the structure it enforces — including that invalid constructs remain compile errors.

Testing DSL Output

The simplest test: build an object using the DSL and assert on the rendered result or the internal state of the builder.

@Test
fun `div contains a paragraph`() {
    val result = html {
        body {
            div { p("Hi") }
        }
    }
    assertTrue(result.render().contains("<p>"))
}

Testing Builder State

Instead of testing the rendered string, test the builder object graph directly. This is more robust to formatting changes:

@Test
fun `server config has correct port`() {
    val cfg = server {
        host = "example.com"
        port = 9090
    }
    assertEquals(9090, cfg.port)
    assertEquals("example.com", cfg.host)
}

Testing Nested Structures

Walk the object tree to verify nesting relationships:

@Test
fun `body contains one div`() {
    val page = html { body { div { } } }
    assertEquals(1, page.children
        .filterIsInstance<Body>().first()
        .children.filterIsInstance<Div>().size
    )
}

Compile-Error Testing

You cannot unit-test compile errors directly, but you can add comments like // This should NOT compile with the failing code commented out. Some projects use the Kotlin Compile Testing library to assert that certain code does NOT compile.

Evolving a DSL Safely: Additive Changes

Adding new optional parameters with defaults, or new builder functions, is backward-compatible. Existing call sites compile unchanged.

// Before
fun server(block: ServerConfig.() -> Unit): ServerConfig
// After — additive: new optional feature
fun server(enableMetrics: Boolean = false, block: ServerConfig.() -> Unit): ServerConfig

Breaking Change: Removing or Renaming

Removing or renaming a DSL function breaks call sites. If you must rename, provide a deprecated alias and remove it in a future major version:

@Deprecated("Use database{} instead", ReplaceWith("database(block)"))
fun db(block: DbConfig.() -> Unit) = database(block)

Versioning Your DSL

For library DSLs, follow semantic versioning. Breaking DSL changes (removed functions, changed receiver types) warrant a major version bump. Document them in a changelog.

Using @RequiresOptIn for Experimental DSL Features

Mark unstable DSL extensions with @RequiresOptIn. Users opt in explicitly, preventing accidental reliance on features that may change:

@RequiresOptIn(message = "This DSL feature is experimental and may change")
annotation class ExperimentalDsl

@ExperimentalDsl
fun ServerConfig.enableDebug() { /*...*/ }

Property Delegation in DSLs

DSLs can use property delegation to enforce required fields and provide clear error messages when a required value is missing:

class Required<T> {
    private var value: T? = null
    operator fun getValue(t: Any?, p: KProperty<*>): T = value ?: error("${p.name} is required")
    operator fun setValue(t: Any?, p: KProperty<*>, v: T) { value = v }
}

Contract Testing Across Versions

Keep a set of "golden" DSL usage snippets as tests. If a refactor breaks them, the test suite catches it before users do. These also serve as living documentation.

Quick Check

What is the safest kind of DSL change for backward compatibility?

Recap: Testing and Evolving DSLs

Key takeaways:

  • Test DSL output and builder object state in unit tests
  • Additive changes (new optional functions/parameters) are safe
  • Use @Deprecated(ReplaceWith=...) to rename without breaking users
  • Use @RequiresOptIn for experimental DSL features
  • Keep golden-usage tests to catch regressions across versions

Frequently asked questions

Is the “Testing and Evolving DSLs Without Breaking Users” lesson free?

Yes — the full text of “Testing and Evolving DSLs Without Breaking Users” is free to read here on the web, and the Kotlin Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Kotlin Academy course, upgrade to CoddyKit PRO.

What will I learn in “Testing and Evolving DSLs Without Breaking Users”?

Design DSL APIs for stability and test them with readable assertion blocks. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Kotlin Academy?

No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Testing and Evolving DSLs Without Breaking Users” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Kotlin Academy lesson?

Yes. Every Kotlin Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Lambda with Receiver: The DSL Foundation
  2. @DslMarker: Preventing Receiver Leakage
  3. Building a Type-Safe HTML/Config DSL
  4. Testing and Evolving DSLs Without Breaking Users
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