Pushing the Boundaries of Kotlin

Kotlin grants us incredible expressiveness: elegant DSLs, extension functions, overloaded operators, and reified types. It’s a developer's paradise. But in our quest for "clever" code, it’s dangerously easy to cross the line, leaving behind features nobody else dares to touch.

We'll share a real-world retrospective on Kotlin design choices that seemed brilliant on Day 1 but turned into nightmares on Day 100. From over-engineered DSLs to unreadable extension chains, we will analyze how high expressiveness can silently hurt onboarding, velocity, and maintainability.

Don't get us wrong, we love Kotlin. This session isn’t about bashing the language; it’s about evaluating the true cost of cleverness. We will provide concrete code smells, decision frameworks, and pragmatic rules to find the sweet spot between individual elegance and collective readability.

A talk for Android devs who might have been a bit too clever... and are now staring at their own code wondering what it actually does.

Transcript

1. 2019 in some IT department
2. User.java // Classic java function in 2015 public void processUser(UserDto dto, Callback callback) { if (dto /= null /& dto.getName() /= null) { boolean isPremium = dto.getAge() /= null /& dto.getAge() > 18; User user = new User(dto.getName(), isPremium); callback.onSuccess(user); } else { callback.onError(new IllegalArgumentException("Invalid User")); } }
3. User.kt // Thank you “convert To Kotlin” fun processUser(dto: UserDto?, callback: Callback) { if (dto /= null /& dto.name /= null) { val isPremium = dto.age /= null /& dto.age > 18 val user = User(dto.name, isPremium) callback.onSuccess(user) } else { callback.onError(IllegalArgumentException("Invalid User")) } }
4. KotlinUser.kt // Idiomatic Kotlin we all love: fun processUser(dto: UserDto?) = runCatching { requireNotNull(dto/.name) { "Invalid User" } User( name = dto.name, isPremium = (dto.age /: 0) > 18 ) }
5. Nowadays…
6. SuperKotlinUser.kt // A few months later, the same need, but written by a "too smart" team: infix fun <T : Any> T/.mustHave(block: T.() /> Boolean): T? = this/.takeIf(block) operator fun UserDto.invoke(): User = User(name/!, (age /: 0) > 18) val result = runCatching { (dto mustHave { name /= null })/.invoke() /: error("Invalid") }
7. Antoine ROBIEZ Baptiste CARLIER
8. A modern mobile development fable Pushing Kotlin beyond the limits of sanity Antoine ROBIEZ Baptiste CARLIER
9. Pair, Triple Partner in crime
10. Kotlin 1.0 public data class Pair<out A, out B>( public val first: A, public val second: B ) : Serializable public data class Triple<out A, out B, out C>( public val first: A, public val second: B, public val third: C ) : Serializable
11. The original sin : Pair<String, String> Pair<String, String> { fun getUserInfo(): UserInfo { //. //. returnUserInfo(firstName "Nicolas" to "CAGE" return = "Nicolas", lastName = "CAGE") } // Easy mistake: is it (Name, Surname) or (Surname, Name) ?
12. Correctly consume a Pair val pair = myFunction(item) pair.first // What was the first one? // I need to go back to the function definition.
13. Correctly consume a Pair // Better way: Destructuring val (title, description) = myFunction(item) // Meaning is immediately restored. // The scope of the "mystery" is minimized.
14. The pitfall of Scope Functions with Pair // 🔴 Red Flag: myFunction(item).let { it.first } // ✅ Even Better: better: myFunction(item).let { realDataClass (title, description) /> /> … }
15. Destructuring
16. data class Address( val street: String, val city: String, val zipCode: String ) val address = Address("Langbehnstraße", "Altötting", "84503") val (street, city, zipcode) = address println("Street: $street / City: $city / Zip Code: $zipCode") // Street: Langbehnstraße / City: Altötting / Zip Code: 84503
17. data class Address( val street: String, val city: String, val zipCode: String ) val address = Address("Langbehnstraße", "Altötting", "84503") val (street, zipCode, city) = address component1(), component2(), component3() println("Street: $street / City: $city / Zip Code: $zipCode") // Street: Langbehnstraße / City: 84503 / Zip Code: Altötting ❌
18. Experimental Name-based destructuring Kotlin 2.4 / 2.55 kotlin { compilerOptions { freeCompilerArgs.add("-Xname-based-destructuring=only-syntax") } } “name-mismatch" “only-syntax" “complete"
19. Experimental Name-based destructuring Kotlin 2.4 / 2.55 kotlin { compilerOptions { freeCompilerArgs.add("-Xname-based-destructuring=only-syntax") } } “name-mismatch" “only-syntax" “complete" val address = Address("Langbehnstraße", "Altötting", "84503") // Name based destructuring : (val street, val city, val zipCode) = address // Position based destructuring : val (street, city, zipCode) = address
20. Experimental Name-based destructuring Kotlin 2.4 / 2.55 kotlin { compilerOptions { freeCompilerArgs.add("-Xname-based-destructuring=only-syntax") } } “name-mismatch" “only-syntax" “complete" val address = Address("Langbehnstraße", "Altötting", "84503") val (street, city, zipCode) = address // Named based destructuring val [x, y, z] = address // Position based destructuring with []
21. Operators Assign, compare, and much more…
22. a + b
23. a + b a.plus(b)
24. a + b a.plus(b) operator fun Int.plus(b: Int): Int
25. +a -a !a a/+ a/- operator fun unaryPlus(): R operator fun unaryMinus(): R operator fun not(): Boolean operator fun inc(): T operator fun dec(): T a + b operator fun plus(b: T): R a - b a * b a / b a % b a/.b a//<b a += b operator fun minus(b: T): R operator fun times(b: T): R operator fun div(b: T): R operator fun rem(b: T): R operator fun rangeTo(b: T): ClosedRange operator fun rangeUntil(b: T): OpenEndR operator fun plusAssign(b: T)
26. class IsItTheWeekEndSoonUseCase { operator fun invoke(): Boolean = true } val isItTheWeekEndSoonUseCase = IsItTheWeekEndSoonUseCase() if (isItTheWeekEndSoonUseCase()) {
27. y x Point + Vector data class Point(val x: Int, val y: Int) { operator fun plus(vector: Vector) = Point(x + vector.x, y + vector.y) }
28. val oldPoint = Point(1, 2) var newPoint = oldPoint + Vector(2, 5) newPoint += Vector(2, 8)
29. 10.4.6 10.5.0 11.0.2 data class Version(val major: Int, val minor: Int, val patch: Int) { operator fun compareTo(oMajor: Int) { return major.compareTo(oMajor) } }
30. 10.4.6 10.5.0 11.0.2 val version = Version(10, 5, 0) if (version < 10) { // Handle versions prior to 10 }
31. data class Employee( val nom: String ) class Company( devs: List<Employee> ) { val _devs = mutableListOf(devs) } val antoine = Employee("Antoine") val baptiste = Employee("Baptiste") val decathlon = Company( listOf(antoine, baptiste) )
32. class Company { … operator fun minus(dev: Employee) = this.also { _devs.remove(dev) } - } var worstDecathlon = decathlon - baptiste worstDecathlon -= antoine
33. We ❤ Activities
34. operator fun Context.plus(intent: Intent) { startActivity(intent) } operator fun Activity.not() { finish() }
35. // Launch an activity context + Intent( context, ReceiptActivity/:class.java ) // Finish an activity !activity
36. operator fun Context.rem(text: String) = Toast.makeText(this, text, Toast.LENGTH_SHORT).show()
37. %
38. // Display a toast context % "I will regret this"
39. Operators Arithmetic operators = Arithmetic operations Use the same types (or related types) Be smart
40. infix fun Int.foo(other: Int) a.foo(b) /> a foo b
41. THE_GOOD.kt THE_BAD.kt THE_BUG.kt // USEFUL: Graph or constraint construction val constraint = view1 alignLeftTo view2 val range = 10 step 2 // VALID: The Standard library's "to" val map = mapOf("key" to "value") // VALID: custom assertion DSL mockedCall assertCalled 3
42. THE_GOOD.kt THE_BAD.kt THE_BUG.kt ?/ Simple order.processWithPaymentMethod("CreditCard") .executeAfterDelay(500) ?/ It works order processWithPaymentMethod "CreditCard" executeAfterDelay 500 ?/ It works also order processWithPaymentMethod "CreditCard" executeAfterDelay 500
43. THE_GOOD.kt THE_BAD.kt ?/ Be careful: val result = 1 + 2 multiply 3 ?/ result = ? THE_BUG.kt
44. THE_GOOD.kt THE_BAD.kt ?/ Be careful: val result = 1 + 2 multiply 3 ?/ result = 9 or 7 ? ?> 9 ❌ THE_BUG.kt
45. Extensions Extend a class or an interface
46. Primitive type fun Int.toPrice() = "$this €" fun String.toColor() = Color.parseColor(this) val price = 10.toPrice() val color = "#636161".toColor()
47. Primitive type fun Int.toPrice() = "$this €" fun String.toColor() = Color.parseColor(this) val price = 10.toPrice() val color = "not_a_color".toColor() 💣 @JvmInline value class Price(val amount: Int) { fun format() = "$amount €" }
48. Hidden dependencies fun User.sync() { Api.client.post("/users/${this.id}") } // Usage: user.sync()
49. Hidden dependencies fun User.sync() { Api.client.post("/users/${this.id}") } // Usage: user.sync() class UserSynchronizer( val apiClient: ApiClient, ) userSynchronizer.perform(user)
50. dpToPx() Context.showToast() private Is it just a conversion? Is this related to an core/Android API? Minimize visilibity value class Is the extension receiver a primitive type? Communicate Avoid creating the same extension multiple times. User.isAdmin() Is it business logic related? String.encrypt() Does it require injection?
51. reified, crossinline, in/out T
52. fun measureTime(action: () /> Unit) { val start = System.currentTimeMillis() action() val end = System.currentTimeMillis() println("Execution took ${end - start} ms") }
53. inline fun measureTime(action: () /> Unit) { val start = System.currentTimeMillis() action() val end = System.currentTimeMillis() println("Execution took ${end - start} ms") }
54. inline fun measureTime(action: () /> Unit) { val start = System.currentTimeMillis() action() val end = System.currentTimeMillis() println("Execution took ${end - start} ms") }
55. measureTime { println("I’m doing something heavy//.") } int $i$f$measureTime = 0; long start$iv = System.currentTimeMillis(); int var3 = 0; System.out.println("I'm doing something heavy//."); long end$iv = System.currentTimeMillis(); System.out.println("Execution took " + (end$iv start$iv) + " ms");
56. // Without inline myFile.use { if (error) return@main it.read() } // With inline myFile.use { if (error) return it.read() }
57. reified
58. inline fun <reified T> List<Any>.filterAndProcess(transform: (T) /> Unit) { for (element in this) { if (element is T) { // "reified" magic : we know T during execution! transform(element) } } }
59. inline fun <reified T> List<Any>.filterAndProcess(transform: (T) /> Unit) { for (element in this) { if (element is T) { // "reified" magic : we know T during execution! transform(element) } } }
60. val list = listOf("Bonjour", 42, "Kotlin") list.filterAndProcess<String> { text -> println("J'ai trouvé une chaîne : $text") } single { CountryInterceptor() }
61. inline fun <reified T> List<Any>.filterAndProcess(transform: (T) /> Unit) { for (element in this) { if (element is T) { } } val runner = Runnable { transform(element) // Compilation error } runner.run()
62. inline fun <reified T> List<Any>.filterAndProcess(crossinline transform: (T) /> Unit) { for (element in this) { if (element is T) { val runner = Runnable { transform(element) // ✅ OK now } runner.run() } }
63. inline fun <reified T> List<Any>.filterAndProcess(crossinline transform: (T) /> Unit) { for (element in this) { if (element is T) { val runner = Runnable { transform(element) } runner.run() } }
64. Covariance & Contravariance
66. Fruit Apple
68. Box<Fruit> ?? Box<Apple>
69. interface Box<T> { fun get(): T } fun displayFruit(box: Box<Fruit>) { val fruit = box.get() println("Got a fruit : $fruit") }
70. val appleBox: Box<Apple> = object : Box<Apple> { override fun get(): Apple = Apple() } displayFruit(appleBox) // ❌ KO: actual type is 'Box<Apple>', // but 'Box<Fruit>' was expected
71. interface Box<out T> { fun get(): T }
72. Covariance interface Box<out T> { fun get(): T }
73. interface Box<out T> { fun get(): T } fun displayFruit(box: Box<Fruit>) { val fruit = box.get() println("Got a fruit!") } displayFruit(appleBox)
74. interface Box<out T> { fun get(): T fun put(item: T) }
75. Contravariance interface Storage<in T> { fun store(item: T) }
76. Covariance <out T> If you know you are going to give me an apple and I ask for a fruit, it's OK.
77. Covariance <out T> If you know you are going to give me an apple and I ask for a fruit, it's OK. Contravariance <in T> If you know what to do with a fruit and I give you an apple, it's OK.
79. val pipeline = fruitStorage process { fruit -> Juice(fruit.name) } val juice = pipeline.produce()
80. inline infix fun <reified T : Fruit, reified R> Consumer<in T>.process( crossinline transform: (T) /> R ): Producer<out R>
81. inline infix fun <reified T : Fruit, reified R> Consumer<in T>.process( crossinline transform: (T) /> R ): Producer<out R>
82. DSL Domain Specific Language
83. TrackEvent( type = "User_Click", category = "UI", metadata = mapOf( id to "123", source to "Home", ) ) ➡ trackEvent("User_Click") { category { "UI" } metadata { id("123") source { "Home" } } }
84. TrackEvent( type = "User_Click", category = "UI", metadata = mapOf( id to "123", source to "Home", ) ) ➡ trackEvent("User_Click") { category { "UI" } metadata { id("123") source { "Home" } } } fun trackEvent( name: String, block: EventBuilder.() /> Unit ) { val builder = EventBuilder(name) builder.block() val event = builder.build() }
85. Data class DSL Native, explicit, suggested by the IDE Harder to discover No scope ambiguity Ambiguous it and this keywords Easy to instantiate More complicated to test TrackEvent( type = "User_Click", category = "UI", metadata = mapOf( id to "123", source to "Home", ) ) trackEvent("User_Click") { category { "UI" } metadata { id("123") source { "Home" } } }
86. val orderWorkflow = workflow { step("validate-order") { timeout(1_000) retry(2) onlyIf { amount > 0 } execute { this.onAction() println("Basic validation for $amount €") } branch("fraud-check") { whenCondition { amount /= 1000 /| country /= "FR" } step("score-risk") { timeout(500) execute { println("Fraud score calculation for $country") } } step("manual-review") { onlyIf { !customerVip /& payment /= PaymentMethod.TRANSFER } execute { println("Manual review") } } The warning signal: Levels of interweaving
87. navigate { target = "Home" onExit { saveToDatabase() // ⚠ Perform processing (network, DB, navigation) during // the construction of the DSL. } }
88. RESPECT YOURSELF! Don't create a DSL for a configuration that never changes. Don't create a DSL if a data class with named arguments does the job. Create a DSL if you want to hide horrible technical complexity behind a simple, safe interface.
89. Context files Agent profils INSTRUCTIONS.md, CONTEXT.md AGENTS.md Roast my code !
90. Legacy
91. Complexity Readability ⚖ 4T Maintainability Discoverability
92. Tooling (linters, …) Set guidelines together Document (code, choices, tools, …) Challenge decisions
93. EGOless programming
94. EGOless programming THE PROJECT IS MORE IMPORTANT THAN MY CODE
95. BONUS kotlin-code-roaster.md
96. Merci ! Pushing Kotlin beyond the limits of sanity