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Kotlin 2.0 and beyond @antonarhipov

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July 20, 2011 - Kotlin announced at JVMLS February 15, 2016 - Kotlin 1.0 May 21, 2024 - Kotlin 2.0

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July 20, 2011 - Kotlin announced at JVMLS February 15, 2016 - Kotlin 1.0 May 21, 2024 - Kotlin 2.0

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July 20, 2011 - Kotlin announced at JVMLS February 15, 2016 - Kotlin 1.0 2020 - Anton joined Kotlin team

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July 20, 2011 - Kotlin announced at JVMLS February 15, 2016 - Kotlin 1.0 2020 ANTON KOTLIN FANBOYS Reality ANTON JAVA DEVELOPERS Expectation Me becoming Kotlin advocate:

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July 20, 2011 - Kotlin announced at JVMLS February 15, 2016 - Kotlin 1.0 May 21, 2024 - Kotlin 2.0

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Features added after Kotlin 1.0: - Multiplatform projects - Coroutines - Inline / Value classes - Trailing comma - fun interfaces - ...

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Features added after Kotlin 1.0: - Multiplatform projects - Coroutines - Inline / Value classes - Trailing comma - fun interfaces - Type aliases - Sealed classes & interfaces - Contracts - break/continue inside when - Exhaustive when statements - Builder inference - . . < operator - Data objects

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Features added after Kotlin 1.0: - Multiplatform projects - Coroutines - Inline / Value classes - Trailing comma - fun interfaces - Type aliases - Sealed classes & interfaces - Contracts - break/continue inside when - Exhaustive when statements - Builder inference - ..< operator - Data objects - provideDelegate - Bound callable references - Destructuring in lambdas - Array literals in annotations - Local lateinit variables - Opt-in annotations - De fi nitely non-nullable types - Instantiation of annotation classes - Support for JSpecify - suspend functions as supertypes - Secondary constructors for inline value classes

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class MyFun(var param: T): () - > Result { override fun invoke(): Result { / /... } } fun handle(handler: () -> Result) { //. .. } ‘C’ for consistency!

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class MyFun(var param: T): () - > Result { override fun invoke(): Result { / /... } } fun handle(handler: () -> Result) { //. .. } ‘C’ for consistency! Functional type

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class MyFun(var param: T): suspend () - > Result { override suspend fun invoke(): Result { / /... } } fun handle(handler: suspend () -> Result) { //. .. } ‘C’ for consistency! Forbidden before 1.6

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class MyFun(var param: T): suspend () - > Result { override suspend fun invoke(): Result { / /... } } fun handle(handler: suspend () -> Result) { //. .. } ‘C’ for consistency! Fixed in 1.6

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fun process(supplier: Supplier) = supplier.get() fun main() { process { "hello!" } } ‘C’ for consistency!

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fun process(supplier: Supplier) = supplier.get() fun main() { process { "hello!" } } SAM type SAM conversion ‘C’ for consistency!

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fun process(supplier: Supplier = Supplier { "hello!" }) = supplier.get() fun main() { val supplier: Supplier = Supplier { "hello!" } process(supplier) } ‘C’ for consistency!

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fun process(supplier: Supplier = Supplier { "hello!" }) = supplier.get() fun main() { val supplier: Supplier = Supplier { "hello!" } process(supplier) } Type name required ‘C’ for consistency!

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What’s FE?

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FE stands for compiler’s Frontend

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K2: The new Kotlin compiler - why?

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K2: The new Kotlin compiler - why? 1. A few language features have appeared unexpectedly in Kotlin Hard to maintain and evolve the compiler 2. Interaction with compiler and IDEs Many ad-hoc solutions, no strict contracts, and no stable API 3. Compilation time performance

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K2: The new Kotlin compiler - why? 1. A few language features have appeared unexpectedly in Kotlin Hard to maintain and evolve the compiler 2. Interaction with compiler and IDEs Many ad-hoc solutions, no strict contracts, and no stable API 3. Compilation time performance

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K2: The new Kotlin compiler - why? 1. A few language features have appeared unexpectedly in Kotlin Hard to maintain and evolve the compiler 2. Interaction with compiler and IDEs Many ad-hoc solutions, no strict contracts, and no stable API 3. Compilation time performance

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https: // youtu.be/db19VFLZqJM https: // youtu.be/iTdJJq_LyoY

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Kotlin 2.0 More than 80 features in the di ff erent subsystems Around 25 and small improvements within the language Main focus is on correctness and performance

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Frontend Intermediate Representation (FIR)

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if (condition) { println("Hello") } for (n in list) { println(n) } val (a, b) = "a" to "b" Frontend Intermediate Representation (FIR)

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if (condition) { println("Hello") } when { condition -> println("Hello") } for (n in list) { println(n) } val = list.interator() while(.hasNext()){ val s = .next() println(s) } val (a, b) = "a" to "b" val = "a" to "b" val a = pair.component1() val b = pair.component2() Frontend Intermediate Representation (FIR)

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fun mutate(ml: MutableList) { ml[0] = ml[0] + 1 } Combination of Long and Integer Literal Types Frontend Intermediate Representation (FIR)

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fun mutate(ml: MutableList) { ml[0] = ml[0] + 1 } Combination of Long and Integer Literal Types Long Integer Literal Type Frontend Intermediate Representation (FIR)

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fun mutate(ml: MutableList) { ml[0] += 1 } Combination of Long and Integer Literal Types Error: 1L is required // Error in Kotlin 1.x Frontend Intermediate Representation (FIR)

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fun mutate(ml: MutableList) { ml[0] += 1 } Combination of Long and Integer Literal Types // OK in 2.0 Desugared into: ml.set(0, ml.get(0).plus(1)) Frontend Intermediate Representation (FIR)

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Combination of nullable operator-calls class Box(val ml: MutableList) fun mutate(box: Box?) { box ?. ml[0] += 1 // Error in 1.x box ?. ml[0] += 1L // Error in 1.x } Frontend Intermediate Representation (FIR)

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Combination of nullable operator-calls class Box(val ml: MutableList) fun mutate(box: Box?) { box ?. ml[0] += 1 // OK in 2.0 } box ?. run { ml.set(0, ml.get(0).plus(1))} Desugared into: Frontend Intermediate Representation (FIR)

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New control fl ow engine read: more smart-casts! - KT-7186 Smart cast for captured variables inside changing closures of inline functions - KT-4113 Smart casts for properties to not-null functional types at invoke calls - KT-25747 DFA variables: propagate smart cast results from local variables - KT-1982 Smart cast to a common supertype of subject types after || (OR operator) - ...

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Smart-casts

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class Cat { fun purr() { println("Purr purr") } } fun petAnimal(animal: Any) { if (animal is Cat) { animal.purr() } } Smart-casts

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class Cat { fun purr() { println("Purr purr") } } fun petAnimal(animal: Any) { if (animal is Cat) { animal.purr() } } Smart-casts

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class Cat { fun purr() { println("Purr purr") } } fun petAnimal(animal: Any) { val isCat = animal is Cat if (isCat) { animal.purr() // Error in Kotlin 1.x } } Smart-casts from variables

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class Cat { fun purr() { println("Purr purr") } } fun petAnimal(animal: Any) { val isCat = animal is Cat if (isCat) { animal.purr() // Error in Kotlin 1.x } } Smart-casts from variables Kotlin 1.x: variables don't carry any data fl ow information

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class Cat { fun purr() { println("Purr purr") } } fun petAnimal(animal: Any) { val isCat = animal is Cat if (isCat) { animal.purr() // OK in Kotlin 2.0 } } Smart-casts from variables Kotlin 2.0: synthetic data fl ow variables propagate information about smart-casts

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Smart-casts from variables class Card(val holder: String?) fun findHolder(card: Any): String { val cardWithHolder = card is Card && !card.holder.isNullOrEmpty() return when { cardWithHolder -> { card.holder } else -> "none" } }

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Smart-casts from variables class Card(val holder: String?) fun findHolder(card: Any): String { val cardWithHolder = card is Card && !card.holder.isNullOrEmpty() return when { cardWithHolder -> { card.holder } else -> "none" } } Any -> Card String? -> String Smart-cast to String

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Smart-casts inside closures of inline lambdas

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fun indexOfMax(a: IntArray): Int? { var maxI: Int? = null a.forEachIndexed { i, value -> if (maxI == null || a[maxI !! ] <= value) { maxI = i } } return maxI } Smart-casts inside closures of inline lambdas

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fun indexOfMax(a: IntArray): Int? { var maxI: Int? = null a.forEachIndexed { i, value -> if (maxI == null || a[maxI !! ] <= value) { maxI = i } } return maxI } Smart-casts inside closures of inline lambdas Kotlin 1.x: Smart cast to 'Int' is impossible. 'maxI' is a local variable that is captured by a changing closure

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fun indexOfMax(a: IntArray): Int? { var maxI: Int? = null a.forEachIndexed { i, value -> if (maxI == null || a[maxI] <= value) { maxI = i } } return maxI } Smart-casts inside closures of inline lambdas Kotlin 2.0: treats inline functions as having an implicit callInPlace contract

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Smart-casts after || interface Status { fun signal() } interface Ok : Status interface Postponed : Status interface Declined : Status

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interface Status { fun signal() } interface Ok : Status interface Postponed : Status interface Declined : Status Smart-casts after ||

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Smart-casts after || fun alert(signalStatus: Any) { if (signalStatus is Postponed || signalStatus is Declined) { signalStatus.signal() // Error 1.x: the inferred type is Any } }

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Smart-casts after || fun alert(signalStatus: Any) { if (signalStatus is Postponed || signalStatus is Declined) { signalStatus.signal() // OK in 2.0 } }

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Smart-casts after || fun alert(signalStatus: Any) { if (signalStatus is Postponed || signalStatus is Declined) { signalStatus.signal() // OK in 2.0 } } Kotlin 2.0: merge to a common supertype (e.g. Status)

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What's next for Kotlin?

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What's next for Kotlin? Guards: pattern matching without binding Context-sensitive resolution Data classes improvements E ff ect system capabilities Data recognition and decunstruction

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What's next for Kotlin? Guards: pattern matching without binding Context-sensitive resolution Data classes improvements E ff ect system capabilities Data recognition and decunstruction Language features for libraries Union types for errors "dataarg" classes Context parameters

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } val searchPanel = selectedSearchPanel() when { searchPanel is SearchPanel.NewsPanel && !searchPanel.isBlocked -> item { NewResult(lastData) } searchPanel == SearchPanel.SpeakersPanel - > item { } searchPanel == SearchPanel.TalksPanel -> item { } } } }

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } val searchPanel = selectedSearchPanel() when { searchPanel is SearchPanel.NewsPanel && !searchPanel.isBlocked -> item { NewResult(lastData) } searchPanel == SearchPanel.SpeakersPanel - > item { } searchPanel == SearchPanel.TalksPanel -> item { } } } }

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } val searchPanel = selectedSearchPanel() when { searchPanel is SearchPanel.NewsPanel && !searchPanel.isBlocked -> item { NewResult(lastData) } searchPanel == SearchPanel.SpeakersPanel - > item { } searchPanel == SearchPanel.TalksPanel -> item { } } } }

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } when(val searchPanel = selectedSearchPanel()) { is SearchPanel.NewsPanel & & !searchPanel.isBlocked - > item { NewResult(lastData) } is SearchPanel.SpeakersPanel -> item { } is SearchPanel.TalksPanel -> item { } } } }

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } when(val searchPanel = selectedSearchPanel()) { is SearchPanel.NewsPanel & & !searchPanel.isBlocked - > item { NewResult(lastData) } is SearchPanel.SpeakersPanel -> item { } is SearchPanel.TalksPanel -> item { } } } } Error: expecting ' -> ' &&

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } when(val searchPanel = selectedSearchPanel()) { is SearchPanel.NewsPanel if !searchPanel.isBlocked - > item { NewResult(lastData) } is SearchPanel.SpeakersPanel -> item { } is SearchPanel.TalksPanel -> item { } } } } Guarded conditions: KEEP-371 Coming as Beta in 2.1 if

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } when(val searchPanel = selectedSearchPanel()) { is SearchPanel.NewsPanel if !searchPanel.isBlocked - > item { NewResult(lastData) } is SearchPanel.SpeakersPanel -> item { } is SearchPanel.TalksPanel -> item { } } } } Can we do better?

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } when(val searchPanel = selectedSearchPanel()) { is SearchPanel.NewsPanel if !searchPanel.isBlocked - > item { NewResult(lastData) } is SearchPanel.SpeakersPanel -> item { } is SearchPanel.TalksPanel -> item { } } } } Can we do better?

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@Composable fun DisplayLastSearchResultByPanelType( searchData: Sequence, id: String ) { LazyColumn { val lastData = searchData.last { it.id == id } when(val searchPanel = selectedSearchPanel()) { is NewsPanel if !searchPanel.isBlocked - > item { NewResult(lastData) } is SpeakersPanel -> item { } is TalksPanel -> item { } } } } Coming as experimental in 2.2 Context-sensitive resolution KEEP-379 No extra quali fi er

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Context-sensitive resolution enum class Status { Ok, Fail } fun process(status: Status = Status.Ok) { ... }

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Context-sensitive resolution enum class Status { Ok, Fail } fun process(status: Status = Status.Ok) { ... }

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Context-sensitive resolution enum class Status { Ok, Fail } fun process(status: Status = Ok) { ... }

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Descructuring

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val user = User("Anton", "Arhipov") val (firstName, surname) = user println(surname) // Arhipov println(firstName) // Anton Descructuring data class User(val name: String, val lastName: String)

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val user = User("Anton", "Arhipov") val (surname, firstName) = user println(surname) // Anton println(firstName) // Arhipov Descructuring data class User(val name: String, val lastName: String)

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val user = User("Anton", "Arhipov") val (surname, firstName) = user Name-based descructuring Error in 2.x: -“surname” doesn’t match the property “name” -“ fi rstName” doesn’t match the property “lastName” data class User(val name: String, val lastName: String)

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How do we evolve libraries?

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LazyColumn { val lastData = searchData.last { it.id == id } val searchPanel = selectedSearchPanel() when { searchPanel is SearchPanel.NewsPanel && !searchPanel.isBlocked -> item { NewResult(lastData) } searchPanel == SearchPanel.SpeakersPanel - > item { } searchPanel == SearchPanel.TalksPanel -> item { } } }

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@Composable fun LazyColumn( modifier: Modifier = Modifier, state: LazyListState = rememberLazyListState(), contentPadding: PaddingValues = PaddingValues(0.dp), reverseLayout: Boolean = false, verticalArrangement: Arrangement.Vertical = if (!reverseLayout) Arrangement.Top else Arrangement.Bottom, horizontalAlignment: Alignment.Horizontal = Alignment.Start, flingBehavior: FlingBehavior = ScrollableDefaults.flingBehavior(), userScrollEnabled: Boolean = true, content: LazyListScope.() -> Unit ) { /* ... */ }

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API evolution of optional parameters Adding new parameter with a default value can be backwards incompatible, leading to new overlads of the function

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@Composable fun LazyColumn( modifier: Modifier = Modifier, state: LazyListState = rememberLazyListState(), contentPadding: PaddingValues = PaddingValues(0.dp), reverseLayout: Boolean = false, verticalArrangement: Arrangement.Vertical = if (!reverseLayout) Arrangement.Top else Arrangement.Bottom, horizontalAlignment: Alignment.Horizontal = Alignment.Start, flingBehavior: FlingBehavior = ScrollableDefaults.flingBehavior(), userScrollEnabled: Boolean = true, content: LazyListScope.() -> Unit ) { /* ... */ }

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dataarg class ColumnSettings( val contentPadding: PaddingValues = PaddingValues(0.dp), val reverseLayout: Boolean = false, val verticalArrangement: Arrangement.Vertical = if (!reverseLayout) Arrangement.Top else Arrangement.Bottom, val horizontalAlignment: Alignment.Horizontal = Alignment.Start, val userScrollEnabled: Boolean = true ) Extensible data arguments

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dataarg class ColumnSettings( val contentPadding: PaddingValues = PaddingValues(0.dp), val reverseLayout: Boolean = false, val verticalArrangement: Arrangement.Vertical = if (!reverseLayout) Arrangement.Top else Arrangement.Bottom, val horizontalAlignment: Alignment.Horizontal = Alignment.Start, val userScrollEnabled: Boolean = true ) @Composable fun LazyColumn( modifier: Modifier = Modifier, state: LazyListState = rememberLazyListState(), dataarg args: ColumnSettings, flingBehavior: FlingBehavior = .. . , content: @Composable RowScope.() -> Unit ) { /* .. . * / } LazyColumn(reverseLayout = true) { //... }

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LazyColumn { val lastData = searchData.last { it.id == id } val searchPanel = selectedSearchPanel() when { searchPanel is SearchPanel.NewsPanel && !searchPanel.isBlocked -> item { NewResult(lastData) } searchPanel == SearchPanel.SpeakersPanel - > item { } searchPanel == SearchPanel.TalksPanel -> item { } } }

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/** * Returns the last element matching the given [predicate]. */ public inline fun Sequence.last(predicate: (T) -> Boolean): T { var result: T? = null for (element in this) if (predicate(element)) result = element return result ? : throw NoSuchElementException("Not found") } Find last matching element in the sequence

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What if the predicate is '{ it == null }' /** * Returns the last element matching the given [predicate]. */ public inline fun Sequence.last(predicate: (T) -> Boolean): T { var last: T? = null var found = false for (element in this) { if (predicate(element)) { last = element found = true } } if (!found) throw NoSuchElementException("Not found") @Suppress("UNCHECKED_CAST") return last as T }

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private object NotFound fun Sequence.last(predicate: (T) -> Boolean): T { var result: Any? = NotFound for (element in this) if (predicate(element)) result = element if (result == = NotFound) throw NoSuchElementException("Not found") return result as T } Can we do better?

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private object NotFound fun Sequence.last(predicate: (T) -> Boolean): T { var result: Any? = NotFound for (element in this) if (predicate(element)) result = element if (result == = NotFound) throw NoSuchElementException("Not found") return result as T } Use of 'Any?' type Unchecked cast Can we do better?

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Union types for errors private error object NotFound fun Sequence.last(predicate: (T) -> Boolean): T { var result: T | NotFound = NotFound for (element in this) if (predicate(element)) result = element if (result is NotFound) throw NoSuchElementException("Not found") return result } Union types for errors Automatic smart-cast In research

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https://vimeo.com/747697423 val client = buildClient { firstName = "Anton" lastName = "Arhipov" twitter { handle = "@antonarhipov" } company { name = "JetBrains" city = "Tallinn" } } println("Created client is: $client")

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class Client(var name: String? = null, var birthday: LocalDate? = null) fun buildClient(init: Client.() - > Unit): Client { var client = Client() client.init() return client } Use case: type-safe builders (a.k.a DSLs)

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class Client(var name: String? = null, var birthday: LocalDate? = null) fun buildClient(init: Client.() - > Unit): Client { var client = Client() client.init() return client } Use case: type-safe builders DSL library

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class Client(var name: String? = null, var birthday: LocalDate? = null) fun buildClient(init: Client.() - > Unit): Client { var client = Client() client.init() return client } Use case: type-safe builders DSL library buildClient { name = "Bob" birthday = LocalDate.of(2000, 3, 10) } User code

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class Client(var name: String? = null, var birthday: LocalDate? = null) fun buildClient(init: Client.() - > Unit): Client { var client = Client() client.init() return client } Use case: type-safe builders DSL library buildClient { name = "Bob" birthday = LocalDate.of(2000, 3, 10) } User code Can we do better?

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class Client(var name: String? = null, var birthday: LocalDate? = null) fun buildClient(init: Client.() - > Unit): Client { var client = Client() client.init() return client } Use case: type-safe builders DSL library buildClient { name = "Bob" birthday = 10 March 2000 } User code infix fun Int.March(year: Int) = LocalDate.of(year, Month.MARCH, this) val dob = 10 March 2000

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class Client(var name: String? = null, var birthday: LocalDate? = null) fun buildClient(init: Client.() - > Unit): Client { var client = Client() client.init() return client } Use case: type-safe builders DSL library buildClient { name = "Bob" birthday = 10 March 2000 } User code infix fun Int.March(year: Int) = LocalDate.of(year, Month.MARCH, this) val dob = 10 March 2000 How can we restrict the scope?

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object ClientBuilderContext context(_: ClientBuilderContext) infix fun Int.March(year: Int) = LocalDate.of(year, Month.MARCH, this) DSL library buildClient { name = "Bob" birthday = 10 March 2000 } User code val dob = 10 March 2000 Context parameters (KEEP-367)

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DSL library buildClient { name = "Bob" birthday = 10 March 2000 } User code val dob = 10 March 2000 fun buildClient(init: context(ClientBuilderContext) Client.() - > Unit): Client = with(ClientBuilderContext()) { //. .. } object ClientBuilderContext context(_: ClientBuilderContext) infix fun Int.March(year: Int) = LocalDate.of(year, Month.MARCH, this) Context parameters (KEEP-367)

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DSL library buildClient { name = "Bob" birthday = 10 March 2000 } User code val dob = 10 March 2000 fun buildClient(init: context(ClientBuilderContext) Client.() - > Unit): Client = with(ClientBuilderContext()) { //. .. } object ClientBuilderContext context(_: ClientBuilderContext) infix fun Int.March(year: Int) = LocalDate.of(year, Month.MARCH, this) Context parameters (KEEP-367)

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DSL library buildClient { name = "Bob" birthday = 10 March 2000 } User code val dob = 10 March 2000 fun buildClient(init: context(ClientBuilderContext) Client.() - > Unit): Client = with(ClientBuilderContext()) { //. .. } object ClientBuilderContext context(_: ClientBuilderContext) infix fun Int.March(year: Int) = LocalDate.of(year, Month.MARCH, this) The required context is missing Required context available in this block Context parameters (KEEP-367)

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buildClient { name = "Bob" birthday = 10 March 2000 } Loos perfect! Or isn't?...

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buildClient { name = "Bob" // 'name' property stays uninitialized birthday = 10 March 2000 } Or... What if the user forgets to assign a property?

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E ff ect system capabilities buildClient { name = "Bob" birthday = 10 March 2000 }

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E ff ect system capabilities buildClient { name = "Bob" birthday = 10 March 2000 }

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E ff ect system capabilities buildClient { name = "Bob" birthday = 10 March 2000 } In research

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E ff ect system capabilities buildClient { name = "Bob" birthday = 10 March 2000 } fun buildClient(init: context(ClientBuilderContext) Client.() -> Unit): Client { contract { called(init@name, ONCE) called(init@birthday, ONCE) } with(ClientBuilderContext()) { // ... } } In research

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E ff ect system capabilities buildClient { name = "Bob" birthday = 10 March 2000 } fun buildClient(init: context(ClientBuilderContext) Client.() -> Unit): Client { contract { called(init@name, ONCE) called(init@birthday, ONCE) } with(ClientBuilderContext()) { // ... } } Contract: "Ensure that the 'name' property is assigned once in the 'init' block" In research

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Summary Kotlin 2.0: new compiler & more smart-casts More features are coming for working with data Stronger abstractions and improvements in the type system

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https://speakerdeck.com/antonarhipov @antonarhipov https://github.com/antonarhipov