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Coroutines for Kotlin (Revision 3.1)

  • Type: Informal description
  • Author: Andrey Breslav
  • Contributors: Vladimir Reshetnikov, Stanislav Erokhin, Ilya Ryzhenkov, Denis Zharkov, Roman Elizarov
  • Status: Implemented in Kotlin 1.1.0


This is a description of coroutines in Kotlin. This concept is also known as, or partly covers

  • generators/yield
  • async/await
  • composable сontinuations


  • No dependency on a particular implementation of Futures or other such rich library;
  • Cover equally the "async/await" use case and "generator blocks";
  • Make it possible to utilize Kotlin coroutines as wrappers for different existing asynchronous APIs (such as Java NIO, different implementations of Futures, etc).

Table of Contents

Use cases

A coroutine can be thought of as an instance of suspendable computation, i.e. the one that can suspend at some points and later resume execution possibly on another thread. Coroutines calling each other (and passing data back and forth) can form the machinery for cooperative multitasking.

Asynchronous computations

The first class of motivating use cases for coroutines are asynchronous computations (handled by async/await in C# and other languages). Let's take a look at how such computations are done with callbacks. As an inspiration, let's take asynchronous I/O (the APIs below are simplified):

// asynchronously read into `buf`, and when done run the lambda {
    // this lambda is executed when the reading completes
    bytesRead ->
    process(buf, bytesRead)

    // asynchronously write from `buf`, and when done run the lambda
    outChannel.write(buf) {
        // this lambda is executed when the writing completes

Note that we have a callback inside a callback here, and while it saves us from a lot of boilerplate (e.g. there's no need to pass the buf parameter explicitly to callbacks, they just see it as a part of their closure), the indentation levels are growing every time, and one can easily anticipate the problems that may come at nesting levels greater than one (google for "callback hell" to see how much people suffer from this in JavaScript).

This same computation can be expressed straightforwardly as a coroutine (provided that there's a library that adapts the I/O APIs to coroutine requirements):

launch(CommonPool) {
    // suspend while asynchronously reading
    val bytesRead = inChannel.aRead(buf) 
    // we only get to this line when reading completes
    process(buf, bytesRead)
    // suspend while asynchronously writing   
    // we only get to this line when writing completes  

The aRead() and aWrite() here are special suspending functions — they suspend execution (which does not mean blocking the thread it has been running on) and resume when the call has completed. If we squint our eyes just enough to imagine that all the code after aRead() has been wrapped in a lambda and passed to aRead() as a callback, and the same has been done for aWrite(), we can see that this code is the same as above, only more readable.

It is our explicit goal to support coroutines in a very generic way, so in this example, launch{}, .aRead(), and .aWrite() are just library functions geared for working with coroutines: launch is the coroutine builder — it builds and launches coroutine in some context (a CommonPool context is used in the example), while aRead/aWrite are special suspending functions which implicitly receive continuations (continuations are just generic callbacks).

The library code for launch{} is shown in coroutine builders section, and the library code for .aRead() is shown in wrapping callbacks section.

Note, that with explicitly passed callbacks having an asynchronous call in the middle of a loop can be tricky, but in a coroutine it is a perfectly normal thing to have:

launch(CommonPool) {
    while (true) {
        // suspend while asynchronously reading
        val bytesRead = inFile.aRead(buf)
        // continue when the reading is done
        if (bytesRead == -1) break
        process(buf, bytesRead)
        // suspend while asynchronously writing
        // continue when the writing is done

One can imagine that handling exceptions is also a bit more convenient in a coroutine.


There's another style of expressing asynchronous computations: through futures (and their close relatives — promises). We'll use an imaginary API here, to apply an overlay to an image:

val future = runAfterBoth(
    asyncLoadImage("...original..."), // creates a Future 
    asyncLoadImage("...overlay...")   // creates a Future
) {
    original, overlay ->
    applyOverlay(original, overlay)

With coroutines, this could be rewritten as

val future = future {
    val original = asyncLoadImage("...original...") // creates a Future
    val overlay = asyncLoadImage("...overlay...")   // creates a Future
    // suspend while awaiting the loading of the images
    // then run `applyOverlay(...)` when they are both loaded
    applyOverlay(original.await(), overlay.await())

The library code for future{} is shown in building futures section, and the library code for .await() is shown in suspending functions section.

Again, less indentation and more natural composition logic (and exception handling, not shown here), and no special keywords (like async and await in C#, JS and other languages) to support futures: future{} and .await() are just functions in a library.


Another typical use case for coroutines would be lazily computed sequences (handled by yield in C#, Python and many other languages). Such a sequence can be generated by seemingly sequential code, but at runtime only requested elements are computed:

// inferred type is Sequence<Int>
val fibonacci = buildSequence {
    yield(1) // first Fibonacci number
    var cur = 1
    var next = 1
    while (true) {
        yield(next) // next Fibonacci number
        val tmp = cur + next
        cur = next
        next = tmp

This code creates a lazy Sequence of Fibonacci numbers, that is potentially infinite (exactly like Haskel's infinite lists). We can request some of it, for example, through take():


This will print 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 You can try this code here

The strength of generators is in supporting arbitrary control flow, such as while (from the example above), if, try/catch/finally and everything else:

val seq = buildSequence {
    yield(firstItem) // suspension point

    for (item in input) {
        if (!item.isValid()) break // don't generate any more items
        val foo = item.toFoo()
        if (!foo.isGood()) continue
        yield(foo) // suspension point        

    try {
        yield(lastItem()) // suspension point
    finally {
        // some finalization code

The library code for buildSequence{} and yield() is shown in restricted suspension section.

Note that this approach also allows to express yieldAll(sequence) as a library function (as well as buildSequence{} and yield() are), which simplifies joining lazy sequences and allows for efficient implementation.

Asynchronous UI

A typical UI application has a single event dispatch thread where all UI operations happen. Modification of UI state from other threads is usually not allowed. All UI libraries provide some kind of primitive to move execution back to UI thread. Swing, for example, has SwingUtilities.invokeLater, JavaFX has Platform.runLater, Android has Activity.runOnUiThread, etc. Here is a snippet of code from a typical Swing application that does some asynchronous operation and then displays its result in the UI:

makeAsyncRequest {
    // this lambda is executed when the async request completes
    result, exception ->

    if (exception == null) {
        // display result in UI
        SwingUtilities.invokeLater {
    } else {
       // process exception

This is similar to callback hell that we've seen in asynchronous computations use case and it is elegantly solved by coroutines, too:

launch(Swing) {
    try {
        // suspend while asynchronously making request
        val result = makeRequest()
        // display result in UI, here Swing context ensures that we always stay in event dispatch thread
    } catch (exception: Throwable) {
        // process exception

The library code for Swing context is shown in the continuation interceptor section.

All exception handling is performed using natural language constructs.

More use cases

Coroutines can cover many more use cases, including these:

  • Channel-based concurrency (aka goroutines and channels);
  • Actor-based concurrency;
  • Background processes occasionally requiring user interaction, e.g., show a modal dialog;
  • Communication protocols: implement each actor as a sequence rather than a state machine;
  • Web application workflows: register a user, validate email, log them in (a suspended coroutine may be serialized and stored in a DB).

Coroutines overview

This section gives an overview of the language mechanisms that enable writing coroutines and the standard libraries that govern their semantics.

Experimental status of coroutines

Coroutines are experimental in Kotlin 1.1, because we expect the design to change. Kotlin compiler produces a warning on usage of coroutine-related features. There is an opt-in switch -Xcoroutines=enable that removes the warning.

All the APIs related to coroutines in kotlin-stdlib ship in a package named kotlin.coroutines.experimental. When the final design is ready, it will be published under kotlin.coroutines, while the experimental package will stay for a while, so that the old binaries will be compatible and continue to work.

Every library that uses coroutines in its public API should do the same,
so if you are writing a library that it here to stay and you care about the users of your future versions, you will also need to name your package something like And when the final design of coroutines comes, drop the experimental suffix from the main API, but keep the old package around for those of your users who might need it for binary compatibility.

More details can be found in this forum post


  • A coroutine — is an instance of suspendable computation. It is conceptually similar to a thread, in the sense that it takes a block of code to run and has a similar life-cycle — it is created and started, but it is not bound to any particular thread. It may suspend its execution in one thread and resume in another one. Moreover, like a future or promise, it may complete with some result or exception.

  • A suspending function — a function that is marked with suspend modifier. It may suspend execution of the code without blocking the current thread of execution by invoking other suspending functions. A suspending function cannot be invoked from a regular code, but only from other suspending functions and from suspending lambdas (see below). For example, .await() and yield(), as shown in use cases, are suspending functions that may be defined in a library. The standard library provides primitive suspending functions that are used to define all other suspending functions.

  • A suspending lambda — a block of code that can be run in a coroutine. It looks exactly like an ordinary lambda expression but its functional type is marked with suspend modifier. Just like a regular lambda expression is a short syntactic form for an anonymous local function, a suspending lambda is a short syntactic form for an anonymous suspending function. It may suspend execution of the code without blocking the current thread of execution by invoking suspending functions. For example, blocks of code in curly braces following launch, future, and buildSequence functions, as shown in use cases, are suspending lambdas.

Note: Suspending lambdas may invoke suspending functions in all places of their code where a non-local return statement from this lambda is allowed. That is, suspending function calls inside inline lambdas like apply{} block are allowed, but not in the noinline nor in crossinline inner lambda expressions. A suspension is treated as a special kind of non-local control transfer.

  • A suspending function type — is a function type for suspending functions and lambdas. It is just like a regular function type, but with suspend modifier. For example, suspend () -> Int is a type of suspending function without arguments that returns Int. A suspending function that is declared like suspend fun foo(): Int conforms to this function type.

  • A coroutine builder — a function that takes some suspending lambda as an argument, creates a coroutine, and, optionally, gives access to its result in some form. For example, launch{}, future{}, and buildSequence{} as shown in use cases, are coroutine builders defined in a library. The standard library provides primitive coroutine builders that are used to define all other coroutine builders.

Note: Some languages have hard-coded support for particular ways to create and start a coroutines that define how their execution and result are represented. For example, generate keyword may define a coroutine that returns a certain kind of iterable object, while async keyword may define a coroutine that returns a certain kind of promise or task. Kotlin does not have keywords or modifiers to define and start a coroutine. Coroutine builders are simply functions defined in a library. In case where a coroutine definition takes the form of a method body in another language, in Kotlin such method would typically be a regular method with an expression body, consisting of an invocation of some library-defined coroutine builder whose last argument is a suspending lambda:

fun asyncTask() = future { ... }
  • A suspension point — is a point during coroutine execution where the execution of the coroutine may be suspended. Syntactically, a suspension point is an invocation of suspending function, but the actual suspension happens when the suspending function invokes the standard library primitive to suspend the execution.

  • A continuation — is a state of the suspended coroutine at suspension point. It conceptually represents the rest of its execution after the suspension point. For example:

buildSequence {
    for (i in 1..10) yield(i * i)

Here, every time the coroutine is suspended at a call to suspending function yield(), the rest of its execution is represented as a continuation, so we have 10 continuations: first runs the loop with i = 2 and suspends, second runs the loop with i = 3 and suspends, etc, the last one prints "over" and completes the coroutine. The coroutine that is created, but is not started yet, is represented by its initial continuation of type Continuation<Unit> that consists of its whole execution.

As mentioned above, one of the driving requirements for coroutines is flexibility: we want to be able to support many existing asynchronous APIs and other use cases and minimize the parts hard-coded into the compiler. As a result, the compiler is only responsible for support of suspending functions, suspending lambdas, and the corresponding suspending function types. There are few primitives in the standard library and the rest is left to application libraries.

Continuation interface

Here is the definition of the standard library interface Continuation, which represents a generic callback:

interface Continuation<in T> {
   val context: CoroutineContext
   fun resume(value: T)
   fun resumeWithException(exception: Throwable)

The context is covered in details in coroutine context section and represents an arbitrary user-defined context that is associated with the coroutine. Functions resume and resumeWithException are completion callbacks that are used to provide either a successful result (via resume) or to report a failure (via resumeWithException) on coroutine completion.

Suspending functions

An implementation of a typical suspending function like .await() looks like this:

suspend fun <T> CompletableFuture<T>.await(): T =
    suspendCoroutine<T> { cont: Continuation<T> ->
        whenComplete { result, exception ->
            if (exception == null) // the future has been completed normally
            else // the future has completed with an exception

You can get this code here. Note: this simple implementation suspends coroutine forever if the future never completes. The actual implementation in kotlinx.coroutines is slightly more involved, because it supports cancellation.

The suspend modifier indicates that this is a function that can suspend execution of a coroutine. This particular function is defined as an extension function on CompletableFuture<T> type so that its usage reads naturally in the left-to-right order that corresponds to the actual order of execution:


A modifier suspend may be used on any function: top-level function, extension function, member function, or operator function.

Note, in the current release local functions, property getters/setters, and constructors cannot have suspend modifier. These restrictions will be lifted in the future.

Suspending functions may invoke any regular functions, but to actually suspend execution they must invoke some other suspending function. In particular, this await implementation invokes a suspending function suspendCoroutine that is defined in the standard library as a top-level suspending function in the following way:

suspend fun <T> suspendCoroutine(block: (Continuation<T>) -> Unit): T

When suspendCoroutine is called inside a coroutine (and it can only be called inside a coroutine, because it is a suspending function) it suspends the execution of coroutine, captures its state in a continuation instance and passes this continuation to the specified block as an argument. To resume execution of the coroutine, the block may call either continuation.resume() or continuation.resumeWithException() in this thread of in some other thread. Resuming the same continuation more than once is not allowed and produces IllegalStateException.

Note: That is the key difference between coroutines in Kotlin and first-class delimited continuations in functional languages like Scheme or continuation monad in Haskel. The choice to support only limited resume-once continuations is purely pragmatic as none of the intended uses cases need first-class continuations and we can more efficiently implement limited version of them. However, first-class continuations can be implemented as a separate library by cloning the state of the coroutine that is captured in continuation, so that its clone can be resumed again. This mechanism may be efficiently provided by the standard library in the future.

The value passed to continuation.resume() becomes the return value of suspendCoroutine(), which, in turn, becomes the return value of .await() when the coroutine resumes its execution.

Coroutine builders

Suspending functions cannot be invoked from regular functions, so the standard library provides functions to start coroutine execution from a regular non-suspending scope. Here is the implementation of a simple launch{} coroutine builder:

fun launch(context: CoroutineContext, block: suspend () -> Unit) =

private class StandaloneCoroutine(override val context: CoroutineContext): Continuation<Unit> {
    override fun resume(value: Unit) {}

    override fun resumeWithException(exception: Throwable) {
        val currentThread = Thread.currentThread()
        currentThread.uncaughtExceptionHandler.uncaughtException(currentThread, exception)

You can get this code here.

This implementation defines a simple class StandaloneCoroutine that represents this coroutine and implements Continuation interface to capture its completion. The completion of coroutine invokes its completion continuation. Its resume or resumeWithException functions are invoked when coroutine completes with the result or exception correspondingly. Because launch does "fire-and-forget" coroutine, it is defined for suspending functions with Unit return type and actually ignores this result in its resume function. If coroutine execution completes with exception, then the uncaught exception handler of the current thread is used to report it.

Note: this simple implementation returns Unit and provides no access to the state of the coroutine at all. The actual implementation in kotlinx.coroutines is more complex, because it returns an instance of Job interface that represents a coroutine and can be cancelled.

The context is covered in details in coroutine context section. It suffices to say here that it is a good style to include a context parameter in library-defined coroutine builders for better composition with other libraries that may define useful context elements.

The startCoroutine is defined in the standard library as an extension for suspending function type. Its signature is:

fun <T> (suspend  () -> T).startCoroutine(completion: Continuation<T>)

The startCoroutine creates coroutine and starts its execution immediately, in the current thread (but see remark below), until the first suspension point, then it returns. Suspension point is an invocation of some suspending function in the body of the coroutine and it is up to the code of the corresponding suspending function to define when and how the coroutine execution resumes.

Note: continuation interceptor (from the context) that is covered later, can dispatch the execution of the coroutine, including its initial continuation, into another thread.

Coroutine context

Coroutine context is a persistent set of user-defined objects that can be attached to the coroutine. It may include objects responsible for coroutine threading policy, logging, security and transaction aspects of the coroutine execution, coroutine identity and name, etc. Here is the simple mental model of coroutines and their contexts. Think of a coroutine as a light-weight thread. In this case, coroutine context is just like a collection of thread-local variables. The difference is that thread-local variables are mutable, while coroutine context is immutable, which is not a serious limitation for coroutines, because they are so light-weight that it is easy to launch a new coroutine when there is a need to change something in the context.

The standard library does not contain any concrete implementations of the context elements, but has interfaces and abstract classes so that all these aspects can be defined in libraries in a composable way, so that aspects from different libraries can coexist peacefully as elements of the same context.

Conceptually, coroutine context is an indexed set of elements, where each element has a unique key. It is a mix between a set and a map. Its elements have keys like in a map, but its keys are directly associated with elements, more like in a set. The standard library defines the minimal interface for CoroutineContext:

interface CoroutineContext {
    operator fun <E : Element> get(key: Key<E>): E?
    fun <R> fold(initial: R, operation: (R, Element) -> R): R
    operator fun plus(context: CoroutineContext): CoroutineContext
    fun minusKey(key: Key<*>): CoroutineContext

    interface Element : CoroutineContext {
        val key: Key<*>

    interface Key<E : Element>

The CoroutineContext itself has four core operations available on it:

  • Operator get provides type-safe access to an element for a given key. It can be used with [..] notation as explained in Kotlin operator overloading.
  • Function fold works likes Collection.fold extension in the standard library and provides means to iterate all elements in the context.
  • Operator plus works like extension in the standard library and returns a combination of two contexts with elements on the right-hand side of plus replacing elements with the same key on the left-hand side.
  • Function minusKey returns a context that does not contain a specified key.

An Element of the coroutine context is a context itself. It is a singleton context with this element only. This enables creation of composite contexts by taking library definitions of coroutine context elements and joining them with +. For example, if one library defines auth element with user authorization information, and some other library defines CommonPool object with some execution context information, then you can use a launch{} coroutine builder with the combined context using launch(auth + CommonPool) {...} invocation.

Note: kotlinx.coroutines provides several context elements, including CommonPool object that dispatches execution of coroutine onto a shared pool of background threads.

All library-defined context elements shall extend AbstractCoroutineContextElement class that is provided by the standard library. The following style is recommended for library defined context elements. The example below shows a hypothetical authorization context element that stores current user name:

class AuthUser(val name: String) : AbstractCoroutineContextElement(AuthUser) {
    companion object Key : CoroutineContext.Key<AuthUser>

The definition of context Key as a companion object of the corresponding element class enables fluent access to the corresponding element of the context. Here is a hypothetical implementation of suspending function that needs to check the name of the current user:

suspend fun secureAwait(): Unit = suspendCoroutine { cont ->
    val currentUser = cont.context[AuthUser]?.name
    // do something user-specific

Continuation interceptor

Let's recap asynchronous UI use case. Asynchronous UI applications must ensure that the coroutine body itself is always executed in UI thread, despite the fact that various suspending functions resume coroutine execution in arbitrary threads. This is accomplished using a continuation interceptor. First of all, we need to fully understand the lifecycle of a coroutine. Consider a snippet of code that uses launch{} coroutine builder:

launch(CommonPool) {
    initialCode() // execution of initial code
    f1.await() // suspension point #1
    block1() // execution #1
    f2.await() // suspension point #2
    block2() // execution #2

Coroutine starts with execution of its initialCode until the first suspension point. At the suspension point it suspends and, after some time, as defined by the corresponding suspending function, it resumes to execute block1, then it suspends again and resumes to execute block2, after which it completes.

Continuation interceptor has an option to intercept and wrap the continuation that corresponds to the execution of initialCode, block1, and block2 from their resumption to the subsequent suspension points. The initial code of the coroutine is treated as a resumption of its initial continuation. The standard library provides the following interface:

interface ContinuationInterceptor : CoroutineContext.Element {
    companion object Key : CoroutineContext.Key<ContinuationInterceptor>
    fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T>

The interceptContinuation function wraps the continuation of the coroutine. Whenever coroutine is suspended, coroutine framework uses the following line of code to wrap the actual continuation for the subsequent resumption:

val facade = continuation.context[ContinuationInterceptor]?.interceptContinuation(continuation) ?: continuation

Coroutine framework caches the resulting facade for each actual instance of continuation. See implementation details section for more details.

Let us take a look at a concrete example code for Swing interceptor that dispatches execution onto Swing UI event dispatch thread. We start with a definition of a SwingContinuation wrapper class that checks the current thread and makes sure that continuation resumes only in Swing event dispatch thread. If the execution already happens in UI thread, then Swing just invokes an appropriate cont.resume right away, otherwise it dispatches execution of the continuation onto Swing UI thread using SwingUtilities.invokeLater.

private class SwingContinuation<T>(val cont: Continuation<T>) : Continuation<T> by cont {
    override fun resume(value: T) {
        if (SwingUtilities.isEventDispatchThread()) cont.resume(value)
        else SwingUtilities.invokeLater { cont.resume(value) }

    override fun resumeWithException(exception: Throwable) {
        if (SwingUtilities.isEventDispatchThread()) cont.resumeWithException(exception)
        else SwingUtilities.invokeLater { cont.resumeWithException(exception) }

Then define Swing object that is going to serve as the corresponding context element and implement ContinuationInterceptor interface:

object Swing : AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
    override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> =

You can get this code here. Note: the actual implementation of Swing object in kotlinx.coroutines also supports coroutine debugging facilities that provide and display the identifier of the currently running coroutine in the name of the thread that is currently running this coroutine.

Now, one can use launch{} coroutine builder with Swing parameter to execute a coroutine that is running completely in Swing event dispatch thread:

launch(Swing) {
   // code in here can suspend, but will always resume in Swing EDT

Restricted suspension

A different kind of coroutine builder and suspension function is needed to implement buildSequence{} and yield() from generators use case. Here is the library code for buildSequence{} coroutine builder:

fun <T> buildSequence(block: suspend SequenceBuilder<T>.() -> Unit): Sequence<T> = Sequence {
    SequenceCoroutine<T>().apply {
        nextStep = block.createCoroutine(receiver = this, completion = this)

It uses a different primitive from the standard library called createCoroutine that creates coroutine, but does not start it. Instead, it returns its initial continuation as a reference to Continuation<Unit>. The other difference is that suspending lambda block for this builder is an extension lambda with SequenceBuilder<T> receiver. The SequenceBuilder interface provides the scope for the generator block and is defined in a library as:

interface SequenceBuilder<in T> {
    suspend fun yield(value: T)

To avoid creation of multiple objects, buildSequence{} implementation defines SequenceCoroutine<T> class that implements SequenceBuilder<T> and also implements Continuation<Unit>, so it can serve both as a receiver parameter for createCoroutine and as its completion continuation parameter. The simple implementation for SequenceCoroutine<T> is shown below:

private class SequenceCoroutine<T>: AbstractIterator<T>(), SequenceBuilder<T>, Continuation<Unit> {
    lateinit var nextStep: Continuation<Unit>

    // AbstractIterator implementation
    override fun computeNext() { nextStep.resume(Unit) }

    // Completion continuation implementation
    override val context: CoroutineContext get() = EmptyCoroutineContext
    override fun resume(value: Unit) { done() }
    override fun resumeWithException(exception: Throwable) { throw exception }

    // Generator implementation
    override suspend fun yield(value: T) {
        return suspendCoroutine { cont -> nextStep = cont }

You can get this code here

The implementation of yield uses suspendCoroutine suspending function to suspend the coroutine and to capture its continuation. Continuation is stored as nextStep to be resumed when the computeNext is invoked.

However, buildSequence{} and yield(), as shown above, are not ready for an arbitrary suspending function to capture the continuation in their scope. They work synchronously. They need absolute control on how continuation is captured, where it is stored, and when it is resumed. They form restricted suspension scope. The ability to restrict suspensions is provided by @RestrictsSuspension annotation that is placed on the scope class or interface, in the above example this scope interface is SequenceBuilder:

interface SequenceBuilder<in T> {
    suspend fun yield(value: T)

This annotation enforces certain restrictions on suspending functions that can be used in the scope of SequenceBuilder{} or similar synchronous coroutine builder. Any extension suspending lambda or function that has restricted suspension scope class or interface (marked with @RestrictsSuspension) as its receiver is called a restricted suspending function. Restricted suspending functions can only invoke member or extension suspending functions on the same instance of their restricted suspension scope. In particular, it means that no SequenceBuilder extension of lambda in its scope can invoke suspendContinuation or other general suspending function. To suspend the execution of a generate coroutine they must ultimately invoke SequenceBuilder.yield. The implementation of yield itself is a member function of Generator implementation and it does not have any restrictions (only extension suspending lambdas and functions are restricted).

It makes little sense to support arbitrary contexts for such a restricted coroutine builder as sequenceBuilder so it is hardcoded to always work with EmptyCoroutineContext.

More examples

This is a non-normative section that does not introduce any new language constructs or library functions, but shows how all the building blocks compose to cover a large variety of use-cases.

Wrapping callbacks

Many asynchronous APIs have callback-style interfaces. The suspendCoroutine suspending function from the standard library provides for an easy way to wrap any callback into a Kotlin suspending function.

There is a simple pattern. Assume that you have someLongComputation function with callback that receives Result of this computation.

fun someLongComputation(params: Params, callback: (Result) -> Unit)

You can convert it into a suspending function with the following straightforward code:

suspend fun someLongComputation(params: Params): Result = suspendCoroutine { cont ->
    someLongComputation(params) { cont.resume(it) }

Now the return type of this computation is explicit, but it is still asynchronous and does not block a thread.

For a more complex example let us take a look at aRead() function from asynchronous computations use case. It can be implemented as a suspending extension function for Java NIO AsynchronousFileChannel and its CompletionHandler callback interface with the following code:

suspend fun AsynchronousFileChannel.aRead(buf: ByteBuffer): Int =
    suspendCoroutine { cont ->
        read(buf, 0L, Unit, object : CompletionHandler<Int, Unit> {
            override fun completed(bytesRead: Int, attachment: Unit) {

            override fun failed(exception: Throwable, attachment: Unit) {

You can get this code here. Note: for large-scale production use coroutine-based IO libraries shall provide some means to cancel long-running IO operations.

If you are dealing with lots of functions that all share the same type of callback, then you can define a common wrapper function to easily convert all of them to suspending functions. For example, vert.x uses a particular convention that all its asynchronous functions receive Handler<AsyncResult<T>> as a callback. To simply the use of arbitrary vert.x functions from coroutines the following helper function can be defined:

inline suspend fun <T> vx(crossinline callback: (Handler<AsyncResult<T>>) -> Unit) = 
    suspendCoroutine<T> { cont ->
        callback(Handler { result: AsyncResult<T> ->
            if (result.succeeded()) {
            } else {

Using this helper function, an arbitrary asynchronous vert.x function, handler) can be invoked from a coroutine with vx {, it) }.

Building futures

The future{} builder from futures use-case can be defined for any future or promise primitive similarly to the launch{} builder as explained in coroutine builders section:

fun <T> future(context: CoroutineContext = CommonPool, block: suspend () -> T): CompletableFuture<T> =
        CompletableFutureCoroutine<T>(context).also { block.startCoroutine(completion = it) }

The first difference from launch{} is that it returns an implementation of CompletableFuture, and the other difference is that it is defined with a default CommonPool context, so that its default execution behaviour is similar to the CompletableFuture.supplyAsync method that runs its code in ForkJoinPool.commonPool. The basic implementation of CompletableFutureCoroutine is straightforward:

class CompletableFutureCoroutine<T>(override val context: CoroutineContext) : CompletableFuture<T>(), Continuation<T> {
    override fun resume(value: T) { complete(value) }
    override fun resumeWithException(exception: Throwable) { completeExceptionally(exception) }

You can get this code here. The actual implementation in kotlinx.coroutines is more advanced, because it propagates the cancellation of the resulting future to cancel the coroutine.

The completion of this coroutine invokes the corresponding complete methods of the future to record the result of this coroutine.

Non-blocking sleep

Coroutines should not use Thread.sleep, because it blocks a thread. However, it is quite straightforward to implement a suspending non-blocking delay function by using Java's ScheduledThreadPoolExecutor

private val executor = Executors.newSingleThreadScheduledExecutor {
    Thread(it, "scheduler").apply { isDaemon = true }

suspend fun delay(time: Long, unit: TimeUnit = TimeUnit.MILLISECONDS): Unit = suspendCoroutine { cont ->
    executor.schedule({ cont.resume(Unit) }, time, unit)

You can get this code here. Node: kotlinx.coroutines also provides delay function.

Note, that this kind of delay function resumes the coroutines that are using it in its single "scheduler" thread. The coroutines that are using interceptor like Swing will not stay to execute in this thread, as their interceptor dispatches them into an appropriate thread. Coroutines without interceptor will stay to execute in this scheduler thread. So this solution is convenient for demo purposes, but it is not the most efficient one. It is advisable to implement sleep natively in the corresponding interceptors.

For Swing interceptor that native implementation of non-blocking sleep shall use Swing Timer that is specifically designed for this purpose:

suspend fun Swing.delay(millis: Int): Unit = suspendCoroutine { cont ->
    Timer(millis) { cont.resume(Unit) }.apply {
        isRepeats = false

You can get this code here. Node: kotlinx.coroutines implementation of delay is aware of interceptor-specific sleep facilities and automatically uses the above approach where appropriate.

Cooperative single-thread multitasking

It is very convenient to write cooperative single-threaded applications, because you don't have to deal with concurrency and shared mutable state. JS, Python and many other languages do not have threads, but have cooperative multitasking primitives.

Coroutine interceptor provides a straightforward tool to ensure that all coroutines are confined to a single thread. The example code here defines newSingleThreadContext() function that creates a single-threaded execution services and adapts it to the coroutine interceptor requirements.

We will use it with future{} coroutine builder that was defined in building futures section in the following example that works in a single thread, despite the fact that it has two asynchronous tasks inside that are both active.

fun main(args: Array<String>) {
    log("Starting MyEventThread")
    val context = newSingleThreadContext("MyEventThread")
    val f = future(context) {
        log("Hello, world!")
        val f1 = future(context) {
            log("f1 is sleeping")
            delay(1000) // sleep 1s
            log("f1 returns 1")
        val f2 = future(context) {
            log("f2 is sleeping")
            delay(1000) // sleep 1s
            log("f2 returns 2")
        log("I'll wait for both f1 and f2. It should take just a second!")
        val sum = f1.await() + f2.await()
        log("And the sum is $sum")

You can get fully working example here. Node: kotlinx.coroutines has ready-to-use implementation of newSingleThreadContext.

If your whole application is based on a single-threaded execution, you can define your own helper coroutine builders with a hardcoded context for your single-threaded execution facilities.

Asynchronous sequences

The buildSequence{} coroutine builder that is shown in restricted suspension section is an example of a synchronous coroutine. Its producer code in the coroutine is invoked synchronously in the same thread as soon as its consumer invokes The buildSequence{} coroutine block is restricted and it cannot suspend its execution using 3rd-party suspending functions like asynchronous file IO as shown in wrapping callbacks section.

An asynchronous sequence builder is allowed to arbitrarily suspend and resume its execution. It means that its consumer shall be ready to handle the case, when the data is not produced yet. This is a natural use-case for suspending functions. Let us define SuspendingIterator interface that is similar to a regular Iterator interface, but its next() and hasNext() functions are suspending:

interface SuspendingIterator<out T> {
    suspend operator fun hasNext(): Boolean
    suspend operator fun next(): T

The definition of SuspendingSequence is similar to the standard Sequence but it returns SuspendingIterator:

interface SuspendingSequence<out T> {
    operator fun iterator(): SuspendingIterator<T>

We also define a scope interface for that is similar to a scope of a synchronous sequence builder, but it is not restricted in its suspensions:

interface SuspendingSequenceBuilder<in T> {
    suspend fun yield(value: T)

The builder function suspendingSequence{} is similar to a synchronous generate{}. Their differences lie in implementation details of SuspendingIteratorCoroutine and in the fact that it makes sense to accept an optional context in this case:

fun <T> suspendingSequence(
        context: CoroutineContext = EmptyCoroutineContext,
        block: suspend SuspendingSequenceBuilder<T>.() -> Unit
): SuspendingSequence<T> = object : SuspendingSequence<T> {
    override fun iterator(): SuspendingIterator<T> = suspendingIterator(context, block)


You can get full code here

Let us take newSingleThreadContext{} context from cooperative single-thread multitasking section and non-blocking delay function from non-blocking sleep section. This way we can write an implementation of a non-blocking sequence that yields integers from 1 to 10, sleeping 500 ms between them:

val seq = suspendingSequence(context) {
    for (i in 1..10) {

Now the consumer coroutine can consume this sequence at its own pace, while also suspending with other arbitrary suspending functions. Note, that Kotlin for loops work by convention, so there is no need for a special await for loop construct in the language. The regular for loop can be used to iterate over an asynchronous sequence that we've defined above. It is suspended whenever producer does not have a value:

for (value in seq) { // suspend while waiting for producer
    // do something with value here, may suspend here, too

You can find a worked out example with some logging that illustrates the execution here


Go-style type-safe channels can be implemented in Kotlin as a library. We can define an interface for send channel with suspending function send:

interface SendChannel<T> {
    suspend fun send(value: T)
    fun close()

and receiver channel with suspending function receive and an operator iterator in a similar style to asynchronous sequences:

interface ReceiveChannel<T> {
    suspend fun receive(): T
    suspend operator fun iterator(): ReceiveIterator<T>

The Channel<T> class implements both interfaces. The send suspends when the channel buffer is full, while receive suspends when the buffer is empty. It allows us to copy Go-style code into Kotlin almost verbatim. The fibonacci function that sends n fibonacci numbers in to a channel from the 4th concurrency example of a tour of Go would look like this in Kotlin:

suspend fun fibonacci(n: Int, c: SendChannel<Int>) {
    var x = 0
    var y = 1
    for (i in 0..n - 1) {
        val next = x + y
        x = y
        y = next

We can also define Go-style go {...} block to start the new coroutine in some kind of multi-threaded pool that dispatches an arbitrary number of light-weight coroutines onto a fixed number of actual heavy-weight threads. The example implementation here is trivially written on top of Java's common ForkJoinPool.

Using this go coroutine builder, the main function from the corresponding Go code would look like this, where mainBlocking is shortcut helper function for runBlocking with the same pool as go{} uses:

fun main(args: Array<String>) = mainBlocking {
    val c = Channel<Int>(2)
    go { fibonacci(10, c) }
    for (i in c) {

You can checkout working code here

You can freely play with the buffer size of the channel. For simplicity, only buffered channels are implemented in the example (with a minimal buffer size of 1), because unbuffered channels are conceptually similar to asynchronous sequences that were covered before.

Go-style select control block that suspends until one of the actions becomes available on one of the channels can be implemented as a Kotlin DSL, so that the 5th concurrency example of a tour of Go would look like this in Kotlin:

suspend fun fibonacci(c: SendChannel<Int>, quit: ReceiveChannel<Int>) {
    var x = 0
    var y = 1
    whileSelect {
        c.onSend(x) {
            val next = x + y
            x = y
            y = next
            true // continue while loop
        quit.onReceive {
            false // break while loop

You can checkout working code here

Example has an implementation of both select {...}, that returns the result of one of its cases like a Kotlin when expression, and a convenience whileSelect { ... } that is the same as while(select<Boolean> { ... }) with fewer braces.

The default selection case from the 6th concurrency example of a tour of Go just adds one more case into the select {...} DSL:

fun main(args: Array<String>) = mainBlocking {
    val tick = Time.tick(100)
    val boom = Time.after(500)
    whileSelect {
        tick.onReceive {
            true // continue loop
        boom.onReceive {
            false // break loop
        onDefault {
            println("    .")
            true // continue loop

You can checkout working code here

The Time.tick and Time.after are trivially implemented here with non-blocking delay function.

Other examples can be found here together with the links to the corresponding Go code in comments.

Note, that this sample implementation of channels is based on a single lock to manage its internal wait lists. It makes it easier to understand and reason about. However, it never runs user code under this lock and thus it is fully concurrent. This lock only somewhat limits its scalability to a very large number of concurrent threads. This implementation can be optimized to use lock-free disjoint-access-parallel data structures to scale to a large number of cores without changing its user-facing APIs. Channels are not built into the language, their implementation code be readily examined, improved, or replaced.

The other important observation is that this channel implementation is independent of the interceptor in the coroutine context. It can be used in UI applications under an event-thread interceptor as shown in the corresponding continuation interceptor section, or with any other one, or without an interceptor at all (in the later case, the execution thread is determined solely by the code of the other suspending functions used in a coroutine). The channel implementation just provides thread-safe non-blocking suspending functions.


Writing scalable asynchronous applications is a discipline that one follows, making sure that ones code never blocks, but suspends (using suspending functions), without actually blocking a thread. The Java concurrency primitives like ReentrantLock are thread-blocking and they should not be used in a truly non-blocking code. To control access to shared resources one can define Mutex class that suspends an execution of coroutine instead of blocking it. The header of the corresponding class would like this:

class Mutex {
    suspend fun lock()
    fun unlock()

You can get full implementation here

Using this implementation of non-blocking mutex the 9th concurrency example of a tour of Go can be translated into Kotlin using Kotlin's try-finally that serves the same purpose as Go's defer:

class SafeCounter {
    private val v = mutableMapOf<String, Int>()
    private val mux = Mutex()

    suspend fun inc(key: String) {
        try { v[key] = v.getOrDefault(key, 0) + 1 }
        finally { mux.unlock() }

    suspend fun get(key: String): Int? {
        return try { v[key] }
        finally { mux.unlock() }

You can checkout working code here

Advanced topics

This section covers some advanced topics dealing with resource management, concurrency, and programming style.

Resource management and GC

Coroutines don't use any off-heap storage and do not consume any native resources by themselves, unless the code that is running inside a coroutine does open a file or some other resource. While files opened in a coroutine must be closed somehow, the coroutine itself does not need to be closed. When coroutine is suspended its whole state is available by the reference to its continuation. If you lose the reference to suspended coroutine's continuation, then it will be ultimately collected by garbage collector.

Coroutines that open some closeable resources deserve a special attention. Consider the following coroutine that uses the buildSequence{} builder from restricted suspension section to produce a sequence of lines from a file:

fun sequenceOfLines(fileName: String) = buildSequence<String> {
    BufferedReader(FileReader(fileName)).use {
        while (true) {
            yield(it.readLine() ?: break)

This function returns a Sequence<String> and you can use this function to print all lines from a file in a natural way:


You can get full code here

It works as expected as long as you iterate the sequence returned by the sequenceOfLines function completely. However, if you print just a few first lines from this file like here:


then the coroutine resumes a few times to yield the first three lines and becomes abandoned. It is Ok for the coroutine itself to be abandoned but not for the open file. The use function will not have a chance to finish its execution and close the file. The file will be left open until collected by GC, because Java files have a finalizer that closes the file. It is not a big problem for a small slide-ware or a short-running utility, but it may be a disaster for a large backend system with multi-gigabyte heap, that can run out of open file handles faster than it runs out of memory to trigger GC.

This is a similar gotcha to Java's Files.lines method that produces a lazy stream of lines. It returns a closeable Java stream, but most stream operations do not automatically invoke the corresponding Stream.close method and it is up to the user to remember about the need to close the corresponding stream. One can define closeable sequence generators in Kotlin, but they will suffer from a similar problem that no automatic mechanism in the language can ensure that they are closed after use. It is explicitly out of the scope of Kotlin coroutines to introduce a language mechanism for an automated resource management.

However, usually this problem does not affect asynchronous use-cases of coroutines. An asynchronous coroutine is never abandoned, but ultimately runs until its completion, so if the code inside a coroutine properly closes its resources, then they will be ultimately closed.

Concurrency and threads

Each individual coroutine, just like a thread, is executed sequentially. It means that the following kind of code is perfectly safe inside a coroutine:

launch(CommonPool) { // starts a coroutine
    val m = mutableMapOf<String, String>()
    val v1 = someAsyncTask1().await() // suspends on await
    m["k1"] = v1 // modify map when resumed
    val v2 = someAsyncTask2().await() // suspends on await
    m["k2"] = v2 // modify map when resumed

You can use all the regular single-threaded mutable structures inside the scope of a particular coroutine. However, sharing mutable state between coroutines is potentially dangerous. If you use a coroutine builder that install a dispatcher to resume all coroutines JS-style in the single event-dispatch thread, like the Swing interceptor shown in continuation interceptor section, then you can safely work with all shared objects that are generally modified from this event-dispatch thread. However, if you work in multi-threaded environment or otherwise share mutable state between coroutines running in different threads, then you have to use thread-safe (concurrent) data structures.

Coroutines are like threads, albeit they are more lightweight. You can have millions of coroutines running on just a few threads. The running coroutine is always executed in some thread. However, a suspended coroutine does not consume a thread and it is not bound to a thread in any way. The suspending function that resumes this coroutine decides which thread the coroutine is resumed on by invoking Continuation.resume on this thread and coroutine's interceptor can override this decision and dispatch the coroutine's execution onto a different thread.

Asynchronous programming styles

There are different styles of asynchronous programming.

Callbacks were discussed in asynchronous computations section and are generally the least convenient style that coroutines are designed to replace. Any callback-style API can be wrapped into the corresponding suspending function as shown here.

Let us recap. For example, assume that you start with a hypothetical blocking sendEmail function with the following signature:

fun sendEmail(emailArgs: EmailArgs): EmailResult

It blocks execution thread for potentially long time while it operates.

To make it non-blocking you can use, for example, error-first node.js callback convention to represent its non-blocking version in callback-style with the following signature:

fun sendEmail(emailArgs: EmailArgs, callback: (Throwable?, EmailResult?) -> Unit)

However, coroutines enable other styles of asynchronous non-blocking programming. One of them is async/await style that is built into many popular languages. In Kotlin this style can be replicated by introducing future{} and .await() library functions that were shown as a part of futures use-case section.

This style is signified by the convention to return some kind of future object from the function instead of taking a callback as a parameter. In this async-style the signature of sendEmail is going to look like this:

fun sendEmailAsync(emailArgs: EmailArgs): Future<EmailResult>

As a matter of style, it is a good practice to add Async suffix to such method names, because their parameters are no different from a blocking version and it is quite easy to make a mistake of forgetting about asynchronous nature of their operation. The function sendEmailAsync starts a concurrent asynchronous operation and potentially brings with it all the pitfalls of concurrency. However, languages that promote this style of programming also typically have some kind of await primitive to bring the execution back into the sequence as needed.

Kotlin's native programming style is based on suspending functions. In this style, the signature of sendEmail looks naturally, without any mangling to its parameters or return type but with an additional suspend modifier:

suspend fun sendEmail(emailArgs: EmailArgs): EmailResult

The async and suspending styles can be easily converted into one another using the primitives that we've already seen. For example, sendEmailAsync can be implemented via suspending sendEmail using future coroutine builder:

fun sendEmailAsync(emailArgs: EmailArgs): Future<EmailResult> = future {

while suspending function sendEmail can be implemented via sendEmailAsync using .await() suspending function

suspend fun sendEmail(emailArgs: EmailArgs): EmailResult = 

So, in some sense, these two styles are equivalent and are both definitely superior to callback style in their convenience. However, let us look deeper at a difference between sendEmailAsync and suspending sendEmail.

Let us compare how they compose first. Suspending functions can be composed just like normal functions:

suspend fun largerBusinessProcess() {
    // a lot of code here, then somewhere inside
    // something else goes on after that

The corresponding async-style functions compose in this way:

fun largerBusinessProcessAsync() = future {
   // a lot of code here, then somewhere inside
   // something else goes on after that

Observe, that async-style function composition is more verbose and error prone. If you omit .await() invocation in async-style example, the code still compiles and works, but it now does email sending process asynchronously or even concurrently with the rest of a larger business process, thus potentially modifying some shared state and introducing some very hard to reproduce errors. On the contrary, suspending functions are sequential by default. With suspending functions, whenever you need any concurrency, you explicitly express it in the source code with some kind of future{} or a similar coroutine builder invocation.

Compare how these styles scale for a big project using many libraries. Suspending functions are a light-weight language concept in Kotlin. All suspending functions are fully usable in any unrestricted Kotlin coroutine. Async-style functions are framework-dependent. Every promises/futures framework must define its own async-like function that returns its own kind of promise/future class and its own await-like function, too.

Compare their performance. Suspending functions provide minimal overhead per invocation. You can checkout implementation details section. Async-style functions need to keep quite heavy promise/future abstraction in addition to all of that suspending machinery. Some future-like object instance must be always returned from async-style function invocation and it cannot be optimized away even if the function is very short and simple. Async-style is not well-suited for very fine-grained decomposition.

Compare their interoperability with JVM/JS code. Async-style functions are more interoperable with JVM/JS code that uses a matching type of future-like abstraction. In Java or JS they are just functions that return a corresponding future-like object. Suspending functions look strange from any language that does not support continuation-passing-style natively. However, you can see in the examples above how easy it is to convert any suspending function into an async-style function for any given promise/future framework. So, you can write suspending function in Kotlin just once, and then adapt it for interop with any style of promise/future with one line of code using an appropriate future{} coroutine builder function.

Implementation details

This section provides a glimpse into implementation details of coroutines. They are hidden behind the building blocks explained in coroutines overview section and their internal classes and code generation strategies are subject to change at any time as long as they don't break contracts of public APIs and ABIs.

Continuation passing style

Suspending functions are implemented via Continuation-Passing-Style (CPS). Every suspending function and suspending lambda has an additional Continuation parameter that is implicitly passed to it when it is invoked. Recall, that a declaration of await suspending function looks like this:

suspend fun <T> CompletableFuture<T>.await(): T

However, its actual implementation has the following signature after CPS transformation:

fun <T> CompletableFuture<T>.await(continuation: Continuation<T>): Any?

Its result type T has moved into a position of type argument in its additional continuation parameter. The implementation result type of Any? is designed to represent the action of the suspending function. When suspending function suspends coroutine, it returns a special marker value of COROUTINE_SUSPENDED. When a suspending function does not suspend coroutine but continues coroutine execution, it returns its result or throws an exception directly. This way, the Any? return type of the await implementation is actually a union of COROUTINE_SUSPENDED and T that cannot be expressed in Kotlin's type system.

The actual implementation of the suspending function is not allowed to invoke the continuation in its stack frame directly because that may lead to stack overflow on long-running coroutines. The suspendCoroutine function in the standard library hides this complexity from an application developer by tracking invocations of continuations and ensures conformance to the actual implementation contract of the suspending functions regardless of how and when the continuation is invoked.

State machines

It is crucial to implement coroutines efficiently, i.e. create as few classes and objects as possible. Many languages implement them through state machines and Kotlin does the same. In the case of Kotlin this approach results in the compiler creating only one class per suspending lambda that may have an arbitrary number of suspension points in its body.

Main idea: a suspending function is compiled to a state machine, where states correspond to suspension points. Example: let's take a suspending block with two suspension points:

val a = a()
val y = foo(a).await() // suspension point #1
val z = bar(a, y).await() // suspension point #2

There are three states for this block of code:

  • initial (before any suspension point)
  • after the first suspension point
  • after the second suspension point

Every state is an entry point to one of the continuations of this block (the initial continuation continues from the very first line).

The code is compiled to an anonymous class that has a method implementing the state machine, a field holding the current state of the state machine, and fields for local variables of the coroutine that are shared between states (there may also be fields for the closure of the coroutine, but in this case it is empty). Here's pseudo-Java code for the block above that uses continuation passing style for invocation of suspending functions await:

class <anonymous_for_state_machine> extends CoroutineImpl<...> implements Continuation<Object> {
    // The current state of the state machine
    int label = 0

    // local variables of the coroutine
    A a = null
    Y y = null

    void resume(Object data) {
        if (label == 0) goto L0
        if (label == 1) goto L1
        if (label == 2) goto L2
        else throw IllegalStateException()

        // data is expected to be `null` at this invocation
        a = a()
        label = 1
        data = foo(a).await(this) // 'this' is passed as a continuation 
        if (data == COROUTINE_SUSPENDED) return // return if await had suspended execution
        // external code has resumed this coroutine passing the result of .await() as data 
        y = (Y) data
        label = 2
        data = bar(a, y).await(this) // 'this' is passed as a continuation
        if (data == COROUTINE_SUSPENDED) return // return if await had suspended execution
        // external code has resumed this coroutine passing the result of .await() as data 
        Z z = (Z) data
        label = -1 // No more steps are allowed

Note that there is a goto operator and labels because the example depicts what happens in the byte code, not in the source code.

Now, when the coroutine is started, we call its resume()label is 0, and we jump to L0, then we do some work, set the label to the next state — 1, call .await() and return if the execution of the coroutine was suspended. When we want to continue the execution, we call resume() again, and now it proceeds right to L1, does some work, sets the state to 2, calls .await() and again returns in case of suspension. Next time it continues from L3 setting the state to -1 which means "over, no more work to do".

A suspension point inside a loop generates only one state, because loops also work through (conditional) goto:

var x = 0
while (x < 10) {
    x += nextNumber().await()

is generated as

class <anonymous_for_state_machine> extends CoroutineImpl<...> implements Continuation<Object> {
    // The current state of the state machine
    int label = 0

    // local variables of the coroutine
    int x

    void resume(Object data) {
        if (label == 0) goto L0
        if (label == 1) goto L1
        else throw IllegalStateException()

        x = 0
        if (x > 10) goto END
        label = 1
        data = nextNumber().await(this) // 'this' is passed as a continuation 
        if (data == COROUTINE_SUSPENDED) return // return if await had suspended execution
        // external code has resumed this coroutine passing the result of .await() as data 
        x += ((Integer) data).intValue()
        label = -1
        goto LOOP
        label = -1 // No more steps are allowed

Compiling suspending functions

The compiled code for suspending function depends on how and when it invokes other suspending functions. In the simplest case, a suspending function invokes other suspending functions only at tail positions making tail calls to them. This is a typical case for suspending functions that implement low-level synchronization primitives or wrap callbacks, as shown in suspending functions and wrapping callbacks sections. These functions invoke some other suspending function like suspendCoroutine at tail position. They are compiled just like regular non-suspending functions, with the only exception that the implicit continuation parameter they've got from CPS transformation is passed to the next suspending function in tail call.

Note: in the current implementation Unit-returning function must include an explicit return statement with the invocation of the other suspending function in order for it to be recognized as a tail call.

In a case when suspending invocations appear in non-tail positions, the compiler creates a state machine for the corresponding suspending function. An instance of the state machine object in created when suspending function is invoked and is discarded when it completes.

Note: in the future versions this compilation strategy may be optimized to create an instance of a state machine only at first suspension point.

This state machine object, in turn, serves as the completion continuation for the invocation of other suspending functions in non-tail positions. This state machine object instance is updated and reused when the function makes multiple invocations to other suspending functions. Compare this to other asynchronous programming styles, where each subsequent step of asynchronous processing is typically implemented with a separate, freshly allocated, closure object.

Coroutine intrinsics

The actual implementation of suspendCoroutine suspending function in the standard library is written in Kotlin itself and its source code is available as part of the standard library sources package. In order to provide for the safe and problem-free use of coroutines, it wraps the actual continuation of the state machine into an additional object on each suspension of coroutine. This is perfectly fine for truly asynchronous use cases like asynchronous computations and futures, since the runtime costs of the corresponding asynchronous primitives far outweigh the cost of an additional allocated object. However, for the generators use case this additional cost is prohibitive.

The kotlin.coroutines.experimental.intrinsics package in the standard library contains the function named suspendCoroutineOrReturn with the following signature:

suspend fun <T> suspendCoroutineOrReturn(block: (Continuation<T>) -> Any?): T

It provides direct access to continuation passing style of suspending functions and direct access to state machine implementation of coroutine. The user of suspendCoroutineOrReturn bears full responsibility of following CPS result convention, but gains slightly better performance as a result. This convention is usually easy to follow for buildSequence/yield-like coroutines, but attempts to write asynchronous await-like suspending functions on top of suspendCoroutineOrReturn are discouraged as they are extremely tricky to implement correctly without the help of suspendCoroutine and errors in these implementation attempts are typically heisenbugs that defy attempts to find and reproduce them via tests.

Optimized version of yield via suspendCoroutineOrReturn is shown below. Because yield always suspends to pass the control back to the consumer of the sequence, the corresponding block always returns COROUTINE_SUSPENDED.

// Generator implementation
override suspend fun yield(value: T) {
    return suspendCoroutineOrReturn { cont ->
        nextStep = cont

You can get full code here

Revision history

This section gives an overview of changes between various revisions of coroutines design.

Changes in revision 3.1

This revision is implemented in Kotlin 1.1.0 release.

  • kotlin.coroutines package is replaced with kotlin.coroutines.experimental.
  • Clarification on experimental status of coroutines added.

Changes in revision 3

This revision is implemented in Kotlin 1.1-Beta.

  • Suspending functions can invoke other suspending function at arbitrary points.
  • Coroutine dispatchers are generalized to coroutine contexts:
    • CoroutineContext interface is introduced.
    • ContinuationDispatcher interface is replaced with ContinuationInterceptor.
    • createCoroutine/startCoroutine parameter dispatcher is removed.
    • Continuation interface includes val context: CoroutineContext.
  • CoroutineIntrinsics object is replaced with kotlin.coroutines.intrinsics package.

Changes in revision 2

This revision is implemented in Kotlin 1.1-M04.

  • The coroutine keyword is replaced by suspending functional type.
  • Continuation for suspending functions is implicit both on call site and on declaration site.
  • suspendContinuation is provided to capture continuation is suspending functions when needed.
  • Continuation passing style transformation has provision to prevent stack growth on non-suspending invocations.
  • createCoroutine/startCoroutine coroutine builders are introduced.
  • The concept of coroutine controller is dropped:
    • Coroutine completion result is delivered via Continuation interface.
    • Coroutine scope is optionally available via coroutine receiver.
    • Suspending functions can be defined at top-level without receiver.
  • CoroutineIntrinsics object contains low-level primitives for cases where performance is more important than safety.