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JVMLS 2016. Coroutines in Kotlin
The document presents an overview of coroutines in Kotlin, detailing their implementation and benefits for asynchronous programming without using explicit callbacks or future combinators. It discusses coroutine builders, exception handling, and the compilation of coroutines to state machines for efficient execution. The document also highlights examples comparing Kotlin's coroutine features with other programming languages like C# and Scala.
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JVMLS 2016. Coroutines in Kotlin
- 1. Corou%nes in Kotlin Andrey.Breslav@JetBrains.com
- 2. This talk could have been named… • async/await/yield • fibers • [stackless] con%nua%ons Suspendable Computa%ons
- 3. Outline • Mo%va%on/Examples • Solu%ons in other languages • Kotlin’s Solu%on – Client code – Library code • Compiling Corou%nes • Excep%on Handling • Appendix. Serializable Corou%nes?
- 4. “Legal” • All I’m saying is no more final than Valhalla J
- 5. val image = loadImage(url) myUI.setImage(image) Mo%va%on Time-‐consuming opera%on UI Thread
- 6. Latency! • Blocking bad. Very bad.
- 7. asyncUI { val image = await(loadImage(url)) myUI.setImage(image) } Suspendable Computa%on Suspending call Con%nua%on Time-‐consuming opera%on await(…) loadImage(url) UI Thread Worker Thread setImage(…)
- 8.
- 9. Combining Futures • CompletableFuture .supplyAsync { loadImage(url) } .thenAccept(myUI::setImage) • so veeery func%onal J
- 10. asyncUI { val image = loadImageAsync(url) myUI.setImage(image) } Flexibility Asynchronous opera%on Con%nua%on interface Continuation<P> { fun resume(data: P) fun resumeWithException(e: Throwable) } <Image> Library Func%on (corou%ne builder)
- 11. asyncUI { val image = await(loadImage(url)) myUI.setImage(image) } Run%me Support Con%nua%on interface Continuation<P> { fun resume(data: P) fun resumeWithException(e: Throwable) } <Image>
- 12. Summary: Goals • Asynchronous programing (and more) – without explicit callbacks – without explicit Future combinators • Maximum flexibility for library designers – with minimal run%me support – and no macros J
- 13. Flavors of Corou%nes Stackless Stackful Language restric;ons Use in special contexts L Use anywhere J Implemented in C#, Scala, Kotlin, … Quasar, Javaflow, … Code transforma;on Local (compiler magic) J All over the place L Run;me support Lidle J Substan%al L
- 14. The C# Way async Task<String> work() { Thread.sleep(200); return “done”; } async Task moreWork() { Console.WriteLine(“Work started”); var str = await work(); Console.WriteLine($“Work completed: {str}”); }
- 15. Example: async/await (I) fun work(): CompletableFuture<String> = async {! Thread.sleep(200)! "done"! }! ! fun moreWork() = async {! println("Work started")! val str = await(work())! println("Work completed: $str")! } type is op%onal
- 16. Example: async/await (I) fun work() = async {! Thread.sleep(200)! "done"! }! ! fun moreWork() = async {! println("Work started")! val str = await(work())! println("Work completed: $str")! }
- 17. await() fun moreWork()= async {! println("Work started")! val str = await(work())! println("Work completed: $str")! }! ! suspend fun <V> await(f: CompletableFuture<V>, c: Continuation<V>) {! f.whenComplete { value, throwable ->! if (throwable == null)! c.resume(value)! else! c.resumeWithException(throwable)! }! }! !
- 18. async() fun moreWork()= async {! println("Work started")! val str = await(work())! println("Work completed: $str")! }! ! fun <T> async(! coroutine c: FutureController<T>.() -> Continuation<Unit>! ): CompletableFuture<T> {! val controller = FutureController<T>()! c(controller).resume(Unit)! return controller.future! }! implicit receiver λ has no params
- 19. Controller @AllowSuspendExtensions! class FutureController<T>{! internal val future = CompletableFuture<T>()! ! suspend fun <V> await(f: CompletableFuture<V>, c: Continuation<V>) {! f.whenComplete { value, throwable ->! if (throwable == null)! c.resume(value)! else! c.resumeWithException(throwable)! }! }! ! operator fun handleResult(value: T, c: Continuation<Nothing>) {! future.complete(value)! }! ! operator fun handleException(t: Throwable, c: Continuation<Nothing>) {! future.completeExceptionally(t)! }! }! fun work() = async {! Thread.sleep(200)! "done"! }!
- 20. Extensibility suspend fun<V> FutureController<*>.await(! lf: ListenableFuture<V>, c: Continuation<V>! ) {! Futures.addCallback(lf, object : FutureCallback<V> { ! override fun onSuccess(value: V) {! c.resume(value)! }! override fun onFailure(e: Throwable) {! c.resumeWithException(throwable)! }! })! }! ! // Example! async {! val res1 = await(getCompletableFuture()) ! val res2 = await(getListeableFuture())! }!
- 21. Summary: Corou%ne Libraries • fun async(coroutine c: …) – func%on with a coroutine parameter • suspend fun await(…, c: Continuation<…>) – func%on marked suspend ! – con%nua%on is implicitly passed in at the call site • class Controller – declares suspend func%ons • may allow suspend extensions – declares return/excep%on handlers
- 22. How Suspension Works fun moreWork() = async {! println("Work started")! val str = await(work())! println("Work completed: $str")! }! ! ! ! ! ! ! ! ! ! .! controller.await(! work(), ! current_continuation! )! return!
- 23. Yield (The C# Way) IEnumerable<int> Fibonacci() { var cur = 1; var next = 1; yield return 1; while (true) { yield return next; var tmp = cur + next; cur = next; next = tmp; } } Infinite (lazy) sequence of Fibonacci numbers
- 24. Example: Lazy Fibonacci val fibonacci: Sequence<Int> = generate {! var cur = 1! var next = 1! ! yield(1)! ! while (true) {! yield(next)! ! val tmp = cur + next! cur = next! next = tmp! }! }! ! assertEquals("1, 1, 2, 3, 5", fibonacci.take(5).joinToString())
- 25. fun <T> generate(! coroutinec: GeneratorController<T>.() -> Continuation<Unit>! ): Sequence<T> =! object : Sequence<T> {! override fun iterator(): Iterator<T> {! val iterator = GeneratorController<T>()! iterator.setNextStep(c(iterator))! return iterator! }! }! ! class GeneratorController<T> : AbstractIterator<T>() {! ...! suspend fun yield(value: T, c: Continuation<Unit>) {! setNext(value)! setNextStep(c)! }! ...! }
- 26. Compiling to State Machines generate {! var cur = 1! var next = 1! ! yield(1)! ! while (true) {! yield(next)! ! val tmp = cur + next! cur = next! next = tmp! }! } L0 L1 L2 var cur = 1! var next = 1 true val tmp = cur + next! cur = next! next = tmp!
- 27. val fibonacci =generate {! var cur = 1! var next = 1! ! yield(1)! ! while (true) {! yield(next)! ! val tmp = cur + next! cur = next! next = tmp! }! } Compiling Corou%nes (I) class fibonacci$1 implements Function1, ! Continuation {! volatile GeneratorController controller volatile int label = -‐2 volatile int cur volatile int next public Continuation<Unit> invoke( GeneratorController controller) public void resume(Object param) public void resumeWithException(Throwable e) private void doResume(Object param, Throwable e) } for shared local variables
- 28. Compiling Corou%nes (II) • Fields: – GeneratorController controller – int label • void doResume(Object param, Throwable e) – tableswitch (label) case 0: L0 case 1: L1 case 2: L2 – L0: ... label = 1 controller.yield(1, /* continuation = */ this) return – L1: ... label = 2 controller.yield(next, /* continuation = */ this) return – L2: ... L0 L1 L2 var cur = 1! var next = 1 true val tmp = cur + next! cur = next! next = tmp!
- 29. Compiling Corou%nes (III) – L0: var cur = 1 var next = 1 this.cur = cur this.next = next this.label = 1 this.controller.yield(1, this) return L0 L1 L2 var cur = 1! var next = 1
- 30. Compiling Corou%nes (IV) • void doResume(Object param, Throwable e) – L1: cur = this.cur next = this.next if (e != null) throw e // while (true) { this.cur = cur this.next = next this.label = 2 this.controller.yield(next, this) return L0 L1 L2 true
- 31. Summary: Compiling Corou%nes • Note: generate()/yield() can be expressed – flexibility: þ • Corou%ne body is compiled to a state machine • Only one instance is allocated at run%me
- 32. asyncUI { val image = await(loadImage(url)) myUI.setImage(image) } IOExcep%on CannotAwaitExcep%on WindowDisposedExcep%on Excep%on Handling
- 33. Throwing and Catching • Who can throw – Synchronous code (inside a corou%ne) – Asynchronous opera%ons (called from corou%ne) – Library code (that manages the corouitne) • Who can catch – The corou%ne itself (user code) – Library code
- 34. Controller.handleExcep%on(e) void doResume(Object param, Throwable e) tableswitch (label) case 0: L0 case 1: L1 case 2: L2 try { L0: ... label = 1 controller.await(..., /* continuation = */ this) return L1: ... label = 2 controller.await(..., /* continuation = */ this) return L2: ... } catch (Throwable e) { controller.handleException(e) }
- 35. Rou%ng Asynchronous Excep%ons • void doResume(Object param, Throwable e) ... – L1: // fields -‐> locals if (e != null) throw e ... // locals -‐> fields this.label = 2 this.controller.yield(next, this) return suspend fun await(f, c) {! f.whenComplete { value, e ->! if (throwable == null)! c.resume(value)! else! c.resumeWithException(e)
- 36. Example: Excep%on Handling asyncUI { val image = try { await( loadImage(url) ) } catch(e: Exception) { log(e) throw e } myUI.setImage(image) } Operation order: • loadImage(url) • -‐> tmp_future • -‐> actual_work() • await(tmp_future) • <suspend> • actual_work() completes • <resume> • myUI.setImage(image) actual_work(url) worker thread
- 37. Summary: Excep%on Handling • Uniform treatment of all excep%ons – both sync and async • Default handler: controller.handleException(e) • Not covered in this talk – Suspending in finally blocks – Calling finally blocks through Continuation<T>
- 38. Appendix. Serializable Corou%nes? serializableAsync(getDB()) { val newUser = showRegistrationForm() sendConfirmationEmail(newUser) if (awaitEmailAddressConfirmation(newUser)) { // registration confirmed in time confirmRegistration(getDB(), newUser) showWelcomeScreen(newUser) } else { // registration not confirmed cancelRegistration(getDB(), newUser) } } Suspending Calls Data to be serialized: • label (state of the SM) • newUser
Editor's Notes
- #2 A rarity: Language design talk As Brian and Dan mention every time in the last few years, we started as “here’s my language, look how cool it is”, and now JVMLS means more a summit of people who speak the JVM language
- #7 A car blocking others Blocking one’s driveway
- #10 It helps, but async/await is so much more straightforward