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MonadCancelLaws.scala
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MonadCancelLaws.scala
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/*
* Copyright 2020-2022 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
package laws
import cats.effect.kernel.MonadCancel
import cats.laws.MonadErrorLaws
import cats.syntax.all._
trait MonadCancelLaws[F[_], E] extends MonadErrorLaws[F, E] {
implicit val F: MonadCancel[F, E]
def guaranteeIsGuaranteeCase[A](fa: F[A], fin: F[Unit]) =
F.guarantee(fa, fin) <-> F.guaranteeCase(fa)(_ => fin)
// note that this implies the nested case as well
def uncancelablePollIsIdentity[A](fa: F[A]) =
F.uncancelable(_(fa)) <-> fa
def uncancelableIgnoredPollEliminatesNesting[A](fa: F[A]) =
F.uncancelable(_ => F.uncancelable(_ => fa)) <-> F.uncancelable(_ => fa)
// this law shows that inverted polls do not apply
def uncancelablePollInverseNestIsUncancelable[A](fa: F[A]) =
F.uncancelable(op => F.uncancelable(ip => op(ip(fa)))) <-> F.uncancelable(_ => fa)
// TODO F.uncancelable(p => F.canceled >> p(fa) >> fb) <-> F.uncancelable(p => p(F.canceled >> fa) >> fb)
def uncancelableEliminatesOnCancel[A](fa: F[A], fin: F[Unit]) =
F.uncancelable(_ => F.onCancel(fa, fin)) <-> F.uncancelable(_ => fa)
/*
* NB: This is effectively in violation of the monad laws, since
* we consider finalizers to associate over the boundary here, but
* we do NOT consider them to right-associate over map or flatMap.
* This simply stems from the fact that cancelation is fundamentally
* uncomposable, and it's better to pick a semantic for uncancelable
* which allows regional composition, since this avoids "gaps" in
* otherwise-safe code.
*
* The argument is that cancelation is a *hint* not a mandate. This
* holds for self-cancelation just as much as external-cancelation.
* Thus, laws about where the cancelation is visible are always going
* to be a bit off.
*/
def onCancelAssociatesOverUncancelableBoundary[A](fa: F[A], fin: F[Unit]) =
F.uncancelable(_ => F.onCancel(fa, fin)) <-> F.onCancel(F.uncancelable(_ => fa), fin)
def onCancelImpliesUncancelable[A](fa: F[A], fin1: F[Unit], fin2: F[Unit]) =
F.onCancel(F.onCancel(fa, F.uncancelable(_ => fin1)), fin2) <-> F.onCancel(
F.onCancel(fa, fin1),
fin2)
def forceRDiscardsPure[A, B](a: A, fa: F[B]) =
F.forceR(F.pure(a))(fa) <-> fa
def forceRDiscardsError[A](e: E, fa: F[A]) =
F.forceR(F.raiseError(e))(fa) <-> fa
def forceRCanceledShortCircuits[A](fa: F[A]) =
F.forceR(F.canceled)(fa) <-> F.productR(F.canceled)(fa)
def forceRAssociativity[A, B, C](fa: F[A], fb: F[B], fc: F[C]) =
F.forceR(fa)(F.forceR(fb)(fc)) <-> F.forceR(F.forceR(fa)(fb))(fc)
def uncancelableFinalizers[A](fin: F[Unit]) =
F.onCancel(F.canceled, F.uncancelable(_ => fin)) <-> F.onCancel(F.canceled, fin)
// only for root = Cancelable
def canceledSequencesOnCancelInOrder(fin1: F[Unit], fin2: F[Unit]) =
F.onCancel(F.onCancel(F.canceled, fin1), fin2) <->
F.forceR(F.uncancelable(_ => F.forceR(fin1)(fin2)))(F.canceled)
def uncancelableCanceledAssociatesRightOverFlatMapAttempt[A](fa: F[A]) =
(F.uncancelable(_ => F.canceled >> fa).attempt >> F.unit) <->
F.forceR(F.uncancelable(_ => fa))(F.canceled)
def canceledAssociatesLeftOverFlatMap[A](fa: F[A]) =
F.canceled >> fa.void <-> F.canceled
// only for root = Uncancelable
def uncancelableIdentity[A](fa: F[A]) =
F.uncancelable(_ => fa) <-> fa
def canceledUnitIdentity =
F.canceled <-> F.unit
}
object MonadCancelLaws {
def apply[F[_], E](implicit F0: MonadCancel[F, E]): MonadCancelLaws[F, E] =
new MonadCancelLaws[F, E] { val F = F0 }
}