/
Async.scala
635 lines (501 loc) · 21.4 KB
/
Async.scala
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/*
* Copyright 2020-2023 Typelevel
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package cats.effect.kernel
import cats.{~>, Monoid, Semigroup}
import cats.arrow.FunctionK
import cats.data.{EitherT, Ior, IorT, Kleisli, OptionT, WriterT}
import cats.implicits._
import scala.annotation.{nowarn, tailrec}
import scala.concurrent.{ExecutionContext, Future}
import java.util.concurrent.Executor
import java.util.concurrent.atomic.AtomicReference
/**
* A typeclass that encodes the notion of suspending asynchronous side effects in the `F[_]`
* context
*
* An asynchronous task is one whose results are computed somewhere else (eg by a
* [[scala.concurrent.Future]] running on some other threadpool). We await the results of that
* execution by giving it a callback to be invoked with the result.
*
* That computation may fail hence the callback is of type `Either[Throwable, A] => ()`. This
* awaiting is semantic only - no threads are blocked, the current fiber is simply descheduled
* until the callback completes.
*
* This leads us directly to the simplest asynchronous FFI
* {{{
* def async_[A](k: (Either[Throwable, A] => Unit) => Unit): F[A]
* }}}
*
* {{{async(k)}}} is semantically blocked until the callback is invoked.
*
* `async_` is somewhat contrained however. We can't perform any `F[_]` effects in the process
* of registering the callback and we also can't register a finalizer to eg cancel the
* asynchronous task in the event that the fiber running `async_` is canceled.
*
* This leads us directly to the more general asynchronous FFI
* {{{
* def async[A](k: (Either[Throwable, A] => Unit) => F[Option[F[Unit]]]): F[A]
* }}}
*
* As evidenced by the type signature, `k` may perform `F[_]` effects and it returns an
* `Option[F[Unit]]` which is an optional finalizer to be run in the event that the fiber
* running {{{async(k)}}} is canceled.
*/
trait Async[F[_]] extends AsyncPlatform[F] with Sync[F] with Temporal[F] {
/**
* The asynchronous FFI.
*
* `k` takes a callback of type `Either[Throwable, A] => Unit` to signal the result of the
* asynchronous computation. The execution of `async(k)` is semantically blocked until the
* callback is invoked.
*
* `k` returns an `Option[F[Unit]]` which is an optional finalizer to be run in the event that
* the fiber running {{{async(k)}}} is canceled.
*/
def async[A](k: (Either[Throwable, A] => Unit) => F[Option[F[Unit]]]): F[A] = {
val body = new Cont[F, A, A] {
def apply[G[_]](implicit G: MonadCancel[G, Throwable]) = { (resume, get, lift) =>
G.uncancelable { poll =>
lift(k(resume)) flatMap {
case Some(fin) => G.onCancel(poll(get), lift(fin))
case None => poll(get)
}
}
}
}
cont(body)
}
/**
* A convenience version of [[Async.async]] for when we don't need to perform `F[_]` effects
* or perform finalization in the event of cancelation.
*/
def async_[A](k: (Either[Throwable, A] => Unit) => Unit): F[A] =
async[A](cb => as(delay(k(cb)), None))
/**
* An effect that never terminates.
*
* Polymorphic so it can be used in situations where an arbitrary effect is expected eg
* [[Fiber.joinWithNever]]
*/
def never[A]: F[A] = async(_ => pure(none[F[Unit]]))
/**
* Shift execution of the effect `fa` to the execution context `ec`. Execution is shifted back
* to the previous execution context when `fa` completes.
*
* evalOn(executionContext, ec) <-> pure(ec)
*/
def evalOn[A](fa: F[A], ec: ExecutionContext): F[A]
/**
* [[Async.evalOn]] as a natural transformation.
*/
def evalOnK(ec: ExecutionContext): F ~> F =
new (F ~> F) {
def apply[A](fa: F[A]): F[A] = evalOn(fa, ec)
}
/**
* Start a new fiber on a different execution context.
*
* See [[GenSpawn.start]] for more details.
*/
def startOn[A](fa: F[A], ec: ExecutionContext): F[Fiber[F, Throwable, A]] =
evalOn(start(fa), ec)
/**
* Start a new background fiber on a different execution context.
*
* See [[GenSpawn.background]] for more details.
*/
def backgroundOn[A](
fa: F[A],
ec: ExecutionContext): Resource[F, F[Outcome[F, Throwable, A]]] =
Resource.make(startOn(fa, ec))(_.cancel)(this).map(_.join)
/**
* Obtain a reference to the current execution context.
*/
def executionContext: F[ExecutionContext]
/**
* Obtain a reference to the current execution context as a `java.util.concurrent.Executor`.
*/
def executor: F[Executor] = map(executionContext) {
case exec: Executor => exec
case ec => ec.execute(_)
}
/**
* Lifts a [[scala.concurrent.Future]] into an `F` effect.
*/
def fromFuture[A](fut: F[Future[A]]): F[A] =
flatMap(fut) { f =>
flatMap(executionContext) { implicit ec =>
async_[A](cb => f.onComplete(t => cb(t.toEither)))
}
}
/**
* Translates this `F[A]` into a `G` value which, when evaluated, runs the original `F` to its
* completion, the `limit` number of stages, or until the first stage that cannot be expressed
* with [[Sync]] (typically an asynchronous boundary).
*
* Note that `syncStep` is merely a hint to the runtime system; implementations have the
* liberty to interpret this method to their liking as long as it obeys the respective laws.
* For example, a lawful implementation of this function is `G.pure(Left(fa))`, in which case
* the original `F[A]` value is returned unchanged.
*
* @param limit
* The maximum number of stages to evaluate prior to forcibly yielding to `F`
*/
@nowarn("msg=never used")
def syncStep[G[_], A](fa: F[A], limit: Int)(implicit G: Sync[G]): G[Either[F[A], A]] =
G.pure(Left(fa))
/*
* NOTE: This is a very low level api, end users should use `async` instead.
* See cats.effect.kernel.Cont for more detail.
*
* If you are an implementor, and you have `async`, `Async.defaultCont`
* provides an implementation of `cont` in terms of `async`.
* Note that if you use `defaultCont` you _have_ to override `async`.
*/
def cont[K, R](body: Cont[F, K, R]): F[R]
}
object Async {
def apply[F[_]](implicit F: Async[F]): F.type = F
def defaultCont[F[_], K, R](body: Cont[F, K, R])(implicit F: Async[F]): F[R] = {
sealed trait State
case class Initial() extends State
case class Value(v: Either[Throwable, K]) extends State
case class Waiting(cb: Either[Throwable, K] => Unit) extends State
F.delay(new AtomicReference[State](Initial())).flatMap { state =>
def get: F[K] =
F.defer {
state.get match {
case Value(v) => F.fromEither(v)
case Initial() =>
F.async { cb =>
val waiting = Waiting(cb)
@tailrec
def loop(): Unit =
state.get match {
case s @ Initial() =>
state.compareAndSet(s, waiting)
loop()
case Waiting(_) => ()
case Value(v) => cb(v)
}
def onCancel = F.delay(state.compareAndSet(waiting, Initial())).void
F.delay(loop()).as(onCancel.some)
}
case Waiting(_) =>
/*
* - `cont` forbids concurrency, so no other `get` can be in Waiting.
* - if a previous get has succeeded or failed and we are being sequenced
* afterwards, it means `resume` has set the state to `Value`.
* - if a previous `get` has been interrupted and we are running as part of
* its finalisers, the state would have been either restored to `Initial`
* by the finaliser of that `get`, or set to `Value` by `resume`
*/
sys.error("Impossible")
}
}
def resume(v: Either[Throwable, K]): Unit = {
@tailrec
def loop(): Unit =
state.get match {
case Value(_) => () /* idempotent, double calls are forbidden */
case s @ Initial() =>
if (!state.compareAndSet(s, Value(v))) loop()
else ()
case s @ Waiting(cb) =>
if (state.compareAndSet(s, Value(v))) cb(v)
else loop()
}
loop()
}
body[F].apply(resume, get, FunctionK.id)
}
}
implicit def asyncForOptionT[F[_]](implicit F0: Async[F]): Async[OptionT[F, *]] =
new OptionTAsync[F] {
override implicit protected def F: Async[F] = F0
}
implicit def asyncForEitherT[F[_], E](implicit F0: Async[F]): Async[EitherT[F, E, *]] =
new EitherTAsync[F, E] {
override implicit protected def F: Async[F] = F0
}
implicit def asyncForIorT[F[_], L](
implicit F0: Async[F],
L0: Semigroup[L]): Async[IorT[F, L, *]] =
new IorTAsync[F, L] {
override implicit protected def F: Async[F] = F0
override implicit protected def L: Semigroup[L] = L0
}
implicit def asyncForWriterT[F[_], L](
implicit F0: Async[F],
L0: Monoid[L]): Async[WriterT[F, L, *]] =
new WriterTAsync[F, L] {
override implicit protected def F: Async[F] = F0
override implicit protected def L: Monoid[L] = L0
}
implicit def asyncForKleisli[F[_], R](implicit F0: Async[F]): Async[Kleisli[F, R, *]] =
new KleisliAsync[F, R] {
override implicit protected def F: Async[F] = F0
}
private[effect] trait OptionTAsync[F[_]]
extends Async[OptionT[F, *]]
with Sync.OptionTSync[F]
with Temporal.OptionTTemporal[F, Throwable] {
implicit protected def F: Async[F]
override protected final def delegate = super.delegate
override protected final def C = F
override def unique: OptionT[F, Unique.Token] =
delay(new Unique.Token())
override def syncStep[G[_], A](fa: OptionT[F, A], limit: Int)(
implicit G: Sync[G]): G[Either[OptionT[F, A], A]] =
G.map(F.syncStep[G, Option[A]](fa.value, limit)) {
case Left(foption) => Left(OptionT(foption))
case Right(None) => Left(OptionT.none)
case Right(Some(a)) => Right(a)
}
def cont[K, R](body: Cont[OptionT[F, *], K, R]): OptionT[F, R] =
OptionT(
F.cont(
new Cont[F, K, Option[R]] {
override def apply[G[_]](implicit G: MonadCancel[G, Throwable])
: (Either[Throwable, K] => Unit, G[K], F ~> G) => G[Option[R]] =
(cb, ga, nat) => {
val natT: OptionT[F, *] ~> OptionT[G, *] =
new ~>[OptionT[F, *], OptionT[G, *]] {
override def apply[A](fa: OptionT[F, A]): OptionT[G, A] =
OptionT(nat(fa.value))
}
body[OptionT[G, *]].apply(cb, OptionT.liftF(ga), natT).value
}
}
)
)
def evalOn[A](fa: OptionT[F, A], ec: ExecutionContext): OptionT[F, A] =
OptionT(F.evalOn(fa.value, ec))
def executionContext: OptionT[F, ExecutionContext] = OptionT.liftF(F.executionContext)
override def never[A]: OptionT[F, A] = OptionT.liftF(F.never)
override def ap[A, B](
ff: OptionT[F, A => B]
)(fa: OptionT[F, A]): OptionT[F, B] = delegate.ap(ff)(fa)
override def pure[A](x: A): OptionT[F, A] = delegate.pure(x)
override def flatMap[A, B](fa: OptionT[F, A])(f: A => OptionT[F, B]): OptionT[F, B] =
delegate.flatMap(fa)(f)
override def tailRecM[A, B](a: A)(f: A => OptionT[F, Either[A, B]]): OptionT[F, B] =
delegate.tailRecM(a)(f)
override def raiseError[A](e: Throwable): OptionT[F, A] =
delegate.raiseError(e)
override def handleErrorWith[A](fa: OptionT[F, A])(
f: Throwable => OptionT[F, A]): OptionT[F, A] =
delegate.handleErrorWith(fa)(f)
}
private[effect] trait EitherTAsync[F[_], E]
extends Async[EitherT[F, E, *]]
with Sync.EitherTSync[F, E]
with Temporal.EitherTTemporal[F, E, Throwable] {
implicit protected def F: Async[F]
override protected final def delegate = super.delegate
override protected final def C = F
override def unique: EitherT[F, E, Unique.Token] =
delay(new Unique.Token())
override def syncStep[G[_], A](fa: EitherT[F, E, A], limit: Int)(
implicit G: Sync[G]): G[Either[EitherT[F, E, A], A]] =
G.map(F.syncStep[G, Either[E, A]](fa.value, limit)) {
case Left(feither) => Left(EitherT(feither))
case Right(Left(e)) => Left(EitherT.leftT(e))
case Right(Right(a)) => Right(a)
}
def cont[K, R](body: Cont[EitherT[F, E, *], K, R]): EitherT[F, E, R] =
EitherT(
F.cont(
new Cont[F, K, Either[E, R]] {
override def apply[G[_]](implicit G: MonadCancel[G, Throwable])
: (Either[Throwable, K] => Unit, G[K], F ~> G) => G[Either[E, R]] =
(cb, ga, nat) => {
val natT: EitherT[F, E, *] ~> EitherT[G, E, *] =
new ~>[EitherT[F, E, *], EitherT[G, E, *]] {
override def apply[A](fa: EitherT[F, E, A]): EitherT[G, E, A] =
EitherT(nat(fa.value))
}
body[EitherT[G, E, *]].apply(cb, EitherT.liftF(ga), natT).value
}
}
)
)
def evalOn[A](fa: EitherT[F, E, A], ec: ExecutionContext): EitherT[F, E, A] =
EitherT(F.evalOn(fa.value, ec))
def executionContext: EitherT[F, E, ExecutionContext] = EitherT.liftF(F.executionContext)
override def never[A]: EitherT[F, E, A] = EitherT.liftF(F.never)
override def ap[A, B](
ff: EitherT[F, E, A => B]
)(fa: EitherT[F, E, A]): EitherT[F, E, B] = delegate.ap(ff)(fa)
override def pure[A](x: A): EitherT[F, E, A] = delegate.pure(x)
override def flatMap[A, B](fa: EitherT[F, E, A])(
f: A => EitherT[F, E, B]): EitherT[F, E, B] =
delegate.flatMap(fa)(f)
override def tailRecM[A, B](a: A)(f: A => EitherT[F, E, Either[A, B]]): EitherT[F, E, B] =
delegate.tailRecM(a)(f)
override def raiseError[A](e: Throwable): EitherT[F, E, A] =
delegate.raiseError(e)
override def handleErrorWith[A](fa: EitherT[F, E, A])(
f: Throwable => EitherT[F, E, A]): EitherT[F, E, A] =
delegate.handleErrorWith(fa)(f)
}
private[effect] trait IorTAsync[F[_], L]
extends Async[IorT[F, L, *]]
with Sync.IorTSync[F, L]
with Temporal.IorTTemporal[F, L, Throwable] {
implicit protected def F: Async[F]
override protected final def delegate = super.delegate
override protected final def C = F
override def unique: IorT[F, L, Unique.Token] =
delay(new Unique.Token())
override def syncStep[G[_], A](fa: IorT[F, L, A], limit: Int)(
implicit G: Sync[G]): G[Either[IorT[F, L, A], A]] =
G.map(F.syncStep[G, Ior[L, A]](fa.value, limit)) {
case Left(fior) => Left(IorT(fior))
case Right(Ior.Right(a)) => Right(a)
case Right(ior) => Left(IorT.fromIor(ior))
}
def cont[K, R](body: Cont[IorT[F, L, *], K, R]): IorT[F, L, R] =
IorT(
F.cont(
new Cont[F, K, Ior[L, R]] {
override def apply[G[_]](implicit G: MonadCancel[G, Throwable])
: (Either[Throwable, K] => Unit, G[K], F ~> G) => G[Ior[L, R]] =
(cb, ga, nat) => {
val natT: IorT[F, L, *] ~> IorT[G, L, *] =
new ~>[IorT[F, L, *], IorT[G, L, *]] {
override def apply[A](fa: IorT[F, L, A]): IorT[G, L, A] =
IorT(nat(fa.value))
}
body[IorT[G, L, *]].apply(cb, IorT.liftF(ga), natT).value
}
}
)
)
def evalOn[A](fa: IorT[F, L, A], ec: ExecutionContext): IorT[F, L, A] =
IorT(F.evalOn(fa.value, ec))
def executionContext: IorT[F, L, ExecutionContext] = IorT.liftF(F.executionContext)
override def never[A]: IorT[F, L, A] = IorT.liftF(F.never)
override def ap[A, B](
ff: IorT[F, L, A => B]
)(fa: IorT[F, L, A]): IorT[F, L, B] = delegate.ap(ff)(fa)
override def pure[A](x: A): IorT[F, L, A] = delegate.pure(x)
override def flatMap[A, B](fa: IorT[F, L, A])(f: A => IorT[F, L, B]): IorT[F, L, B] =
delegate.flatMap(fa)(f)
override def tailRecM[A, B](a: A)(f: A => IorT[F, L, Either[A, B]]): IorT[F, L, B] =
delegate.tailRecM(a)(f)
override def raiseError[A](e: Throwable): IorT[F, L, A] =
delegate.raiseError(e)
override def handleErrorWith[A](fa: IorT[F, L, A])(
f: Throwable => IorT[F, L, A]): IorT[F, L, A] =
delegate.handleErrorWith(fa)(f)
}
private[effect] trait WriterTAsync[F[_], L]
extends Async[WriterT[F, L, *]]
with Sync.WriterTSync[F, L]
with Temporal.WriterTTemporal[F, L, Throwable] {
implicit protected def F: Async[F]
override protected final def delegate = super.delegate
override protected final def C = F
override def unique: WriterT[F, L, Unique.Token] =
delay(new Unique.Token())
override def syncStep[G[_], A](fa: WriterT[F, L, A], limit: Int)(
implicit G: Sync[G]): G[Either[WriterT[F, L, A], A]] =
G.pure(Left(fa))
def cont[K, R](body: Cont[WriterT[F, L, *], K, R]): WriterT[F, L, R] =
WriterT(
F.cont(
new Cont[F, K, (L, R)] {
override def apply[G[_]](implicit G: MonadCancel[G, Throwable])
: (Either[Throwable, K] => Unit, G[K], F ~> G) => G[(L, R)] =
(cb, ga, nat) => {
val natT: WriterT[F, L, *] ~> WriterT[G, L, *] =
new ~>[WriterT[F, L, *], WriterT[G, L, *]] {
override def apply[A](fa: WriterT[F, L, A]): WriterT[G, L, A] =
WriterT(nat(fa.run))
}
body[WriterT[G, L, *]].apply(cb, WriterT.liftF(ga), natT).run
}
}
)
)
def evalOn[A](fa: WriterT[F, L, A], ec: ExecutionContext): WriterT[F, L, A] =
WriterT(F.evalOn(fa.run, ec))
def executionContext: WriterT[F, L, ExecutionContext] = WriterT.liftF(F.executionContext)
override def never[A]: WriterT[F, L, A] = WriterT.liftF(F.never)
override def ap[A, B](
ff: WriterT[F, L, A => B]
)(fa: WriterT[F, L, A]): WriterT[F, L, B] = delegate.ap(ff)(fa)
override def pure[A](x: A): WriterT[F, L, A] = delegate.pure(x)
override def flatMap[A, B](fa: WriterT[F, L, A])(
f: A => WriterT[F, L, B]): WriterT[F, L, B] =
delegate.flatMap(fa)(f)
override def tailRecM[A, B](a: A)(f: A => WriterT[F, L, Either[A, B]]): WriterT[F, L, B] =
delegate.tailRecM(a)(f)
override def raiseError[A](e: Throwable): WriterT[F, L, A] =
delegate.raiseError(e)
override def handleErrorWith[A](fa: WriterT[F, L, A])(
f: Throwable => WriterT[F, L, A]): WriterT[F, L, A] =
delegate.handleErrorWith(fa)(f)
}
private[effect] trait KleisliAsync[F[_], R]
extends Async[Kleisli[F, R, *]]
with Sync.KleisliSync[F, R]
with Temporal.KleisliTemporal[F, R, Throwable] {
implicit protected def F: Async[F]
override protected final def delegate = super.delegate
override protected final def C = F
override def unique: Kleisli[F, R, Unique.Token] =
delay(new Unique.Token())
override def syncStep[G[_], A](fa: Kleisli[F, R, A], limit: Int)(
implicit G: Sync[G]): G[Either[Kleisli[F, R, A], A]] =
G.pure(Left(fa))
def cont[K, R2](body: Cont[Kleisli[F, R, *], K, R2]): Kleisli[F, R, R2] =
Kleisli(r =>
F.cont(
new Cont[F, K, R2] {
override def apply[G[_]](implicit G: MonadCancel[G, Throwable])
: (Either[Throwable, K] => Unit, G[K], F ~> G) => G[R2] =
(cb, ga, nat) => {
val natT: Kleisli[F, R, *] ~> Kleisli[G, R, *] =
new ~>[Kleisli[F, R, *], Kleisli[G, R, *]] {
override def apply[A](fa: Kleisli[F, R, A]): Kleisli[G, R, A] =
Kleisli(r => nat(fa.run(r)))
}
body[Kleisli[G, R, *]].apply(cb, Kleisli.liftF(ga), natT).run(r)
}
}
))
def evalOn[A](fa: Kleisli[F, R, A], ec: ExecutionContext): Kleisli[F, R, A] =
Kleisli(r => F.evalOn(fa.run(r), ec))
def executionContext: Kleisli[F, R, ExecutionContext] = Kleisli.liftF(F.executionContext)
override def never[A]: Kleisli[F, R, A] = Kleisli.liftF(F.never)
override def ap[A, B](
ff: Kleisli[F, R, A => B]
)(fa: Kleisli[F, R, A]): Kleisli[F, R, B] = delegate.ap(ff)(fa)
override def pure[A](x: A): Kleisli[F, R, A] = delegate.pure(x)
override def flatMap[A, B](fa: Kleisli[F, R, A])(
f: A => Kleisli[F, R, B]): Kleisli[F, R, B] =
delegate.flatMap(fa)(f)
override def tailRecM[A, B](a: A)(f: A => Kleisli[F, R, Either[A, B]]): Kleisli[F, R, B] =
delegate.tailRecM(a)(f)
override def raiseError[A](e: Throwable): Kleisli[F, R, A] =
delegate.raiseError(e)
override def handleErrorWith[A](fa: Kleisli[F, R, A])(
f: Throwable => Kleisli[F, R, A]): Kleisli[F, R, A] =
delegate.handleErrorWith(fa)(f)
}
}