Continuation in Kotlin

Simon Vergauwen · November 28, 2021

Coroutines is one of the most liked features in Kotlin. Most people are familiar with the KotlinX Coroutines implementation, and many are also familiar with suspendCoroutine from the Kotlin Standard Library and then have also come across Continuation.

Kotlin has support for CPS, or Continuation Passing Style, which means that the compiler has a style in which it can automatically pass Continuation between functions that are marked with suspend.

This means that in every suspend fun we have access to an implicit Continuation parameter through suspendCoroutine { effect: Continuation -> ... }, or any of it’s lower level variants.

This Continuation only has a single type parameter Continuation<A>, and the result of running this suspend/Continuation based program is either A, or Throwable. This is represented by the Result<A> type from the Kotlin Standard Library.

In functional programming we often prefer to interrupt the program with a typed value R instead of Throwable. So we’d like a Continuation that instead of only being able to result in A | Throwable can result in A | R | Throwable, you can never escape that a program can also result in Throwable and suspend explicitly takes Throwable into account to operate correctly.

To achieve this we define a new type interface Effect<R, A>, which represents a function of suspend () -> A that can fail with R (and Throwable). It’s defined by suspend fun <B> fold(f: suspend (R) -> B, g: suspend (A) -> B): B.

What is interesting about the Effect<R, A> type is it’s a suspend function fold, so it doesn’t rely on any wrappers such as Either, Ior or Validated. Only when fold is called it will create a Continuation and run the computation, this means Effect can encode side-effects as pure values. This makes Effect<R, A> a very efficient generic runtime.

To construct a Effect<R, A> we simply call the effect<R, A> { } constructor, which exposes a rich syntax through the lambda receiver suspend EffectScope<R>.() -> A.

Working with Effect<R, A>

Let’s write a small program to read a file from disk, and instead of having the program work exception based we want to turn it into a polymorphic type-safe program.

We start by defining a small function that accepts a String, and does some simply validation to check that the path is not empty. If the path is empty, we want to program to result in EmptyPath. So we’re immediately going to see how we can raise an error of any arbitrary type R by using the function shift. The name shift comes shifting (or changing, especially unexpectedly), away from the computation and finishing the Continuation with R.

object EmptyPath

fun readFile(path: String): Effect<EmptyPath, Unit> = effect {
  if (path.isEmpty()) shift(EmptyPath) else Unit

Here we see how we can define a Effect<R, A> which has EmptyPath for the shifted type R, and Unit for the success type A.

Patterns like validating a Boolean is very common, and the Effect DSL offers utility functions like kotlin.require and kotlin.requireNotNull. They’re named ensure and ensureNotNull to avoid conflicts with the kotlin namespace. So let’s rewrite the function from above to use the DSL instead.

fun readFile2(path: String?): Effect<EmptyPath, Unit> = effect {
  ensureNotNull(path) { EmptyPath }
  ensure(path.isNotEmpty()) { EmptyPath }

You can get the full code here.

Now that we have the path, we can read from the File and return it as a domain model Content. We also want to take a look at what exceptions reading from a file might occur FileNotFoundException & SecurityError, so lets make some domain errors for those too. Grouping them as a sealed interface is useful since that way we can resolve all errors in a type safe manner.

value class Content(val body: List<String>)

sealed interface FileError
@JvmInline value class SecurityError(val msg: String?) : FileError
@JvmInline value class FileNotFound(val path: String) : FileError
object EmptyPath : FileError {
  override fun toString() = "EmptyPath"

We can finish our function, but we need to refactor our return value from Unit to Content and our error type from EmptyPath to FileError.

fun readFile(path: String?): Effect<FileError, Content> = effect {
  ensureNotNull(path) { EmptyPath }
  ensure(path.isNotEmpty()) { EmptyPath }
  try {
    val lines = File(path).readLines()
  } catch (e: FileNotFoundException) {
  } catch (e: SecurityException) {

The Effect returned by readFile function represents a suspend fun that will return:

  • the Content of a given path
  • a FileError
  • An unexpected fatal Throwable (OutOfMemoryException)

The returned Effect<FileError, Content> can executed into a value.

suspend fun test() {
  readFile("").toEither() shouldBe Either.Left(EmptyPath)
  readFile("").toValidated() shouldBe  Validated.Invalid(FileNotFound(""))
  readFile("").toIor() shouldBe Ior.Left(FileNotFound(""))
  readFile("README.MD").toOption { None } shouldBe None

    .fold({ _: FileError -> null }, { it })

You can get the full code here.

Handling errors

Handling errors of type R is the same as handling errors for any other data type in Arrow. Effect<R, A> offers handleError, handleErrorWith, redeem, redeemWith and attempt.

As you can see in the examples below it is possible to resolve errors of R or Throwable in Effect<R, A>.

val failed: Effect<String, Int> =
  effect { shift("failed") }

val resolved: Effect<Nothing, Int> =
  failed.handleError { it.length }

val newError: Effect<List<Char>, Int> =
  failed.handleErrorWith { str ->
    effect { shift(str.reversed().toList()) }

val redeemed: Effect<Nothing, Int> =
  failed.redeem({ str -> str.length }, ::identity)

val captured: Effect<String, Result<Int>> = effect<String, Int> {

suspend fun test() {
  failed.toEither() shouldBe Either.Left("failed")
  resolved.toEither() shouldBe Either.Right(6)
  newError.toEither() shouldBe Either.Left(listOf('d', 'e', 'l', 'i', 'a', 'f'))
  redeemed.toEither() shouldBe Either.Right(6)
  captured.toEither() shouldBe Either.Right(Result.success(1))

You can get the full code here.

Effect<E, A> and Either<E, A>

We’ve briefly mentioned the difference between Either<E, A> and Effect<E, A> but let’s compare them in some more detail.

Either<String, Int> represents a value of either a String or an Int, while Effect<String, Int> represents a function that will result in an Int, interrupt with a String, or throw a Throwable.

So Effect<String, Int> can represent a function that results in either a String, an Int or throw a Throwable. That is exactly what the either DSL of Arrow Core does, and let’s see what the implementation might look like with Effect.

Since Effect<E, A> can interrupt with a String we can implement bind for Either<String, Int>. Here we implement bind how it’s implemented in the library, it’s defined in EffectScope<E> which is the DSL available inside effect { }.

interface EffectScope<E> {
  suspend fun <A> shift(e: e): A
  suspend fun <A> Either<E, A>.bind(): A =
    when(this) {
      is Either.Right -> value
      is Either.Left -> shift(value)

When we encounter Either.Right we can simply return the value, and when we encounter Either.Left we short-circuit (interrupt) the function. This allows us to use the effect { } DSL similarly to how we use either { } from Arrow Core.

suspend fun test() {
  effect<String, Int> {
    val x: Int = 1.right().bind()
    val y: Int = shift("Failed")
  }.toEither() shouldBe Either.Left("failed")

You can get the full code here.

At the end of our DSL we call toEither to finally turn the Effect result into Either. So we can re-define the either DSL from Arrow with following function:

suspend fun <E, A> either(block: suspend EffectScope<E>.() -> A): Either<E, A> = effect(block).toEither()

So we can consider Effect a higher abstraction to write programs that can result in Either, Validated, Option, Ior, Result, etc

Structured Concurrency

Effect<R, A> can automatically operate with KotlinX’s Structured Concurrency by leveraging kotlin.cancellation.CancellationException introduced in Kotlin 1.4. It’s used by shift to raise error values of type R inside the Continuation since it effectively cancels/short-circuits it. For this reason shift adheres to the same rules as Structured Concurrency

For more details see the Effect<R, A> documentation


The Effect type will soon become available in Arrow! There is also RestrictedCont which exposes the same functionality without requiring suspend.

Thank you for reading the first informal (test) blog on my Github Pages, I hope you enjoyed it! Feel free to reach out on KotlinLang Slack for feedback or questions.

Stay tuned for more informal blogs!

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