Effect

Effect represents a function of suspend () -> A, that short-circuit with a value of R (and Throwable), or completes with a value of A.

So Effect is defined by suspend fun <B> fold(f: suspend (R) -> B, g: suspend (A) -> B): B, to map both values of R and A to a value of B.

#writing-a-program-with-effect #handling-errors #structured-concurrency #arrow-fx-coroutines #parzip #partraverse #racen #bracketcase--resource #kotlinx #withcontext #async #launch #leaking-shift

Writing a program with Effect

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'll 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 an Effect<R, A> which has EmptyPath for the shift 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 EffectScope.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 }
}

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.

@JvmInline
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 the return type from Unit to Content and the 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()
    Content(lines)
  } catch (e: FileNotFoundException) {
    shift(FileNotFound(path))
  } catch (e: SecurityException) {
    shift(SecurityError(e.message))
  }
}

The readFile function defines a suspend fun that will return:

• the Content of a given path

• a FileError

• An unexpected fatal error (OutOfMemoryException)

Since these are the properties of our Effect function, we can turn it into a value.

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

   readFile("build.gradle.kts").fold({ _: FileError -> null }, { it })
     .shouldBeInstanceOf<Content>()
      .body.shouldNotBeEmpty()
}

The functions above are available out of the box, but it's easy to define your own extension functions in terms of fold. Implementing the toEither() operator is as simple as:

suspend fun <R, A> Effect<R, A>.toEither(): Either<R, A> =
  fold({ Either.Left(it) }) { Either.Right(it) }

suspend fun <A> Effect<None, A>.toOption(): Option<A> =
  fold(::identity) { Some(it) }

Adding your own syntax to EffectScope<R> is not advised, yet, but will be easy once "Multiple Receivers" become available.

context(EffectScope<R>)
suspend fun <R, A> Either<R, A>.bind(): A =
  when (this) {
    is Either.Left -> shift(value)
    is Either.Right -> value
  }

context(EffectScope<None>)
fun <A> Option<A>.bind(): A =
  fold({ shift(it) }, ::identity)

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> in a generic manner. There is no need to run Effect<R, A> into Either<R, A> before you can access R, you can simply call the same functions on Effect<R, A> as you would on Either<R, A> directly.

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> { 1 }.attempt()

suspend fun main() {
  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))
}

Note: Handling errors can also be done with try/catch but this is not recommended, it uses CancellationException which is used to cancel Coroutines and is advised not to capture in Kotlin. The CancellationException from Effect is ShiftCancellationException, this a public type, thus can be distinguished from any other CancellationException if necessary.

Structured Concurrency

Effect<R, A> relies on kotlin.cancellation.CancellationException to shift 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

Let's overview below how shift behaves with the different concurrency builders from Arrow Fx & KotlinX Coroutines. In the examples below we're going to be using a utility to show how sibling tasks get cancelled. The utility function show below called awaitExitCase will never finish suspending, and completes a Deferred with the ExitCase. ExitCase is a sealed class that can be a value of Failure(Throwable), Cancelled(CancellationException), or Completed. Since awaitExitCase suspends forever, it can only result in Cancelled(CancellationException).

suspend fun <A> awaitExitCase(exit: CompletableDeferred<ExitCase>): A =
  guaranteeCase(::awaitCancellation) { exitCase -> exit.complete(exitCase) }

Arrow Fx Coroutines

All operators in Arrow Fx Coroutines run in place, so they have no way of leaking shift. It's there always safe to compose effect with any Arrow Fx combinator. Let's see some small examples below.

parZip

 suspend fun main() {
   val error = "Error"
   val exit = CompletableDeferred<ExitCase>()
  effect<String, Int> {
    parZip({ awaitExitCase<Int>(exit) }, { shift<Int>(error) }) { a, b -> a + b }
  }.fold({ it shouldBe error }, { fail("Int can never be the result") })
  exit.await().shouldBeTypeOf<ExitCase>()
}

parTraverse

suspend fun main() {
  val error = "Error"
  val exits = (0..3).map { CompletableDeferred<ExitCase>() }
  effect<String, List<Unit>> {
    (0..4).parTraverse { index ->
      if (index == 4) shift(error)
      else awaitExitCase(exits[index])
    }
  }.fold({ msg -> msg shouldBe error }, { fail("Int can never be the result") })
  // It's possible not all parallel task got launched, and in those cases awaitCancellation never ran
  exits.forEach { exit -> exit.getOrNull()?.shouldBeTypeOf<ExitCase.Cancelled>() }
}

parTraverse will launch 5 tasks, for every element in 1..5. The last task to get scheduled will shift with "error", and it will cancel the other launched tasks before returning.

raceN

suspend fun main() {
  val error = "Error"
  val exit = CompletableDeferred<ExitCase>()
  effect<String, Int> {
    raceN({ awaitExitCase<Int>(exit) }) { shift<Int>(error) }
      .merge() // Flatten Either<Int, Int> result from race into Int
  }.fold({ msg -> msg shouldBe error }, { fail("Int can never be the result") })
  // It's possible not all parallel task got launched, and in those cases awaitCancellation never ran
  exit.getOrNull()?.shouldBeTypeOf<ExitCase.Cancelled>()
}

raceN races n suspend functions in parallel, and cancels all participating functions when a winner is found. We can consider the function that shifts the winner of the race, except with a shifted value instead of a successful one. So when a function in the race shifts, and thus short-circuiting the race, it will cancel all the participating functions.

bracketCase / Resource

suspend fun main() {
  val error = "Error"
  val exit = CompletableDeferred<ExitCase>()
  effect<String, Int> {
    bracketCase(
      acquire = { File("build.gradle.kts").bufferedReader() },
      use = { reader: BufferedReader -> shift(error) },
      release = { reader, exitCase ->
        reader.close()
        exit.complete(exitCase)
      }
    )
  }.fold({ it shouldBe error }, { fail("Int can never be the result") })
  exit.await().shouldBeTypeOf<ExitCase.Cancelled>()
}

suspend fun main() {
  val error = "Error"
  val exit = CompletableDeferred<ExitCase>()

  fun bufferedReader(path: String): Resource<BufferedReader> =
    Resource.fromAutoCloseable { File(path).bufferedReader() }
      .releaseCase { _, exitCase -> exit.complete(exitCase) }

  effect<String, Int> {
    val lineCount = bufferedReader("build.gradle.kts")
      .use { reader -> shift<Int>(error) }
    lineCount
  }.fold({ it shouldBe error }, { fail("Int can never be the result") })
  exit.await().shouldBeTypeOf<ExitCase.Cancelled>()
}

KotlinX

withContext

It's always safe to call shift from withContext since it runs in place, so it has no way of leaking shift. When shift is called from within withContext it will cancel all Jobs running inside the CoroutineScope of withContext.