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Effect.scala
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Effect.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
package effect
import simulacrum._
import cats.data.{EitherT, StateT, WriterT}
import scala.annotation.implicitNotFound
import scala.util.Either
/**
* A monad that can suspend side effects into the `F` context and
* that supports lazy and potentially asynchronous evaluation.
*
* This type class is describing data types that:
*
* 1. implement the [[Async]] algebra
* 1. implement a lawful [[Effect!.runAsync runAsync]] operation
* that triggers the evaluation (in the context of [[IO]])
*
* Note this is the safe and generic version of [[IO.unsafeRunAsync]]
* (aka Haskell's `unsafePerformIO`).
*/
@typeclass
@implicitNotFound("""Cannot find implicit value for Effect[${F}].
Building this implicit value might depend on having an implicit
s.c.ExecutionContext in scope, a Scheduler or some equivalent type.""")
trait Effect[F[_]] extends Async[F] {
/**
* Evaluates `F[_]`, with the effect of starting the run-loop
* being suspended in the `IO` context.
*
* Note that evaluating the returned `IO[Unit]` is guaranteed
* to execute immediately:
* {{{
* val io = F.runAsync(fa)(cb)
*
* // For triggering actual execution, guaranteed to be
* // immediate because it doesn't wait for the result
* io.unsafeRunSync
* }}}
*/
def runAsync[A](fa: F[A])(cb: Either[Throwable, A] => IO[Unit]): IO[Unit]
}
object Effect {
/**
* [[Effect]] instance built for `cats.data.WriterT` values initialized
* with any `F` data type that also implements `Effect`.
*/
implicit def catsEitherTEffect[F[_]: Effect]: Effect[EitherT[F, Throwable, ?]] =
new EitherTEffect[F] { def F = Effect[F] }
/**
* [[Effect]] instance built for `cats.data.StateT` values initialized
* with any `F` data type that also implements `Effect`.
*/
implicit def catsStateTEffect[F[_]: Effect, S: Monoid]: Effect[StateT[F, S, ?]] =
new StateTEffect[F, S] { def F = Effect[F]; def S = Monoid[S] }
/**
* [[Effect]] instance built for `cats.data.WriterT` values initialized
* with any `F` data type that also implements `Effect`.
*/
implicit def catsWriterTEffect[F[_]: Effect, L: Monoid]: Effect[WriterT[F, L, ?]] =
new WriterTEffect[F, L] { def F = Effect[F]; def L = Monoid[L] }
private[effect] trait EitherTEffect[F[_]] extends Effect[EitherT[F, Throwable, ?]]
with Async.EitherTAsync[F, Throwable] {
protected def F: Effect[F]
def runAsync[A](fa: EitherT[F, Throwable, A])(cb: Either[Throwable, A] => IO[Unit]): IO[Unit] =
F.runAsync(fa.value)(cb.compose(_.right.flatMap(x => x)))
}
private[effect] trait StateTEffect[F[_], S] extends Effect[StateT[F, S, ?]]
with Async.StateTAsync[F, S] {
protected def F: Effect[F]
protected def S: Monoid[S]
def runAsync[A](fa: StateT[F, S, A])(cb: Either[Throwable, A] => IO[Unit]): IO[Unit] =
F.runAsync(fa.runA(S.empty)(F))(cb)
}
private[effect] trait WriterTEffect[F[_], L] extends Effect[WriterT[F, L, ?]]
with Async.WriterTAsync[F, L] {
protected def F: Effect[F]
protected def L: Monoid[L]
def runAsync[A](fa: WriterT[F, L, A])(cb: Either[Throwable, A] => IO[Unit]): IO[Unit] =
F.runAsync(fa.run)(cb.compose(_.right.map(_._2)))
}
}