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Fiber.scala
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Fiber.scala
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/*
* Copyright (c) 2017-2018 The Typelevel Cats-effect Project Developers
*
* 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
import cats.syntax.apply._
import cats.syntax.applicative._
import cats.syntax.flatMap._
import cats.{Applicative, Apply, Monoid, Semigroup, ~>}
/**
* `Fiber` represents the (pure) result of an [[Async]] data type (e.g. [[IO]])
* being started concurrently and that can be either joined or canceled.
*
* You can think of fibers as being lightweight threads, a fiber being a
* concurrency primitive for doing cooperative multi-tasking.
*
* For example a `Fiber` value is the result of evaluating [[IO.start]]:
*
* {{{
* val io = IO.shift *> IO(println("Hello!"))
*
* val fiber: IO[Fiber[IO, Unit]] = io.start
* }}}
*
* Usage example:
*
* {{{
* for {
* fiber <- IO.shift *> launchMissiles.start
* _ <- runToBunker.handleErrorWith { error =>
* // Retreat failed, cancel launch (maybe we should
* // have retreated to our bunker before the launch?)
* fiber.cancel *> IO.raiseError(error)
* }
* aftermath <- fiber.join
* } yield {
* aftermath
* }
* }}}
*/
trait Fiber[F[_], A] {
/**
* Triggers the cancellation of the fiber.
*
* Returns a new task that will trigger the cancellation upon
* evaluation. Depending on the implementation, this task might
* await for all registered finalizers to finish, but this behavior
* is implementation dependent.
*
* Note that if the background process that's evaluating the result
* of the underlying fiber is already complete, then there's nothing
* to cancel.
*/
def cancel: CancelToken[F]
/**
* Returns a new task that will await for the completion of the
* underlying fiber, (asynchronously) blocking the current run-loop
* until that result is available.
*/
def join: F[A]
}
object Fiber extends FiberInstances {
/**
* Given a `join` and `cancel` tuple, builds a [[Fiber]] value.
*/
def apply[F[_], A](join: F[A], cancel: CancelToken[F]): Fiber[F, A] =
Tuple[F, A](join, cancel)
private final case class Tuple[F[_], A](join: F[A], cancel: CancelToken[F])
extends Fiber[F, A]
implicit class FiberOps[F[_], A](val self: Fiber[F, A]) extends AnyVal{
/**
* Modify the context `F` using transformation `f`.
*/
def mapK[G[_]](f: F ~> G): Fiber[G, A] = new Fiber[G, A] {
def cancel: CancelToken[G] = f(self.cancel)
def join: G[A] = f(self.join)
}
}
}
private[effect] abstract class FiberInstances extends FiberLowPriorityInstances {
implicit def fiberApplicative[F[_]](implicit F: Concurrent[F]): Applicative[Fiber[F, ?]] = new Applicative[Fiber[F, ?]] {
final override def pure[A](x: A): Fiber[F, A] =
Fiber(F.pure(x), F.unit)
final override def ap[A, B](ff: Fiber[F, A => B])(fa: Fiber[F, A]): Fiber[F, B] =
map2(ff, fa)(_(_))
final override def map2[A, B, Z](fa: Fiber[F, A], fb: Fiber[F, B])(f: (A, B) => Z): Fiber[F, Z] = {
val fa2 = F.guaranteeCase(fa.join) { case ExitCase.Error(_) => fb.cancel; case _ => F.unit }
val fb2 = F.guaranteeCase(fb.join) { case ExitCase.Error(_) => fa.cancel; case _ => F.unit }
Fiber(
F.racePair(fa2, fb2).flatMap {
case Left((a, fiberB)) => (a.pure[F], fiberB.join).mapN(f)
case Right((fiberA, b)) => (fiberA.join, b.pure[F]).mapN(f)
},
F.map2(fa.cancel, fb.cancel)((_, _) => ()))
}
final override def product[A, B](fa: Fiber[F, A], fb: Fiber[F, B]): Fiber[F, (A, B)] =
map2(fa, fb)((_, _))
final override def map[A, B](fa: Fiber[F, A])(f: A => B): Fiber[F, B] =
Fiber(F.map(fa.join)(f), fa.cancel)
final override val unit: Fiber[F, Unit] =
Fiber(F.unit, F.unit)
}
implicit def fiberMonoid[F[_]: Concurrent, M[_], A: Monoid]: Monoid[Fiber[F, A]] =
Applicative.monoid[Fiber[F, ?], A]
}
private[effect] abstract class FiberLowPriorityInstances {
implicit def fiberSemigroup[F[_]: Concurrent, A: Semigroup]: Semigroup[Fiber[F, A]] =
Apply.semigroup[Fiber[F, ?], A]
}