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Resource.scala
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Resource.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.kernel
import cats._
import cats.data.Kleisli
import cats.effect.kernel.Resource.Pure
import cats.effect.kernel.implicits._
import cats.effect.kernel.instances.spawn
import cats.syntax.all._
import scala.annotation.tailrec
import scala.annotation.unchecked.uncheckedVariance
import scala.concurrent.ExecutionContext
import scala.concurrent.duration.{Duration, FiniteDuration}
/**
* `Resource` is a data structure which encodes the idea of executing an action which has an
* associated finalizer that needs to be run when the action completes.
*
* Examples include scarce resources like files, which need to be closed after use, or
* concurrent abstractions like locks, which need to be released after having been acquired.
*
* There are several constructors to allocate a resource, the most common is
* [[Resource.make make]]:
*
* {{{
* def open(file: File): Resource[IO, BufferedReader] = {
* val openFile = IO(new BufferedReader(new FileReader(file)))
* Resource.make(acquire = openFile)(release = f => IO(f.close))
* }
* }}}
*
* and several methods to consume a resource, the most common is [[Resource!.use use]]:
*
* {{{
* def readFile(file: BufferedReader): IO[Content]
*
* open(file1).use(readFile)
* }}}
*
* Finalisation (in this case file closure) happens when the action passed to `use` terminates.
* Therefore, the code above is _not_ equivalent to:
*
* {{{
* open(file1).use(IO.pure).flatMap(readFile)
* }}}
*
* which will instead result in an error, since the file gets closed after `pure`, meaning that
* `.readFile` will then fail.
*
* Also note that a _new_ resource is allocated every time `use` is called, so the following
* code opens and closes the resource twice:
*
* {{{
* val file: Resource[IO, File]
* file.use(read) >> file.use(read)
* }}}
*
* If you want sharing, pass the result of allocating the resource around, and call `use` once.
* {{{
* file.use { file => read(file) >> read(file) }
* }}}
*
* The acquire and release actions passed to `make` are not interruptible, and release will run
* when the action passed to `use` succeeds, fails, or is interrupted. You can use
* [[Resource.makeCase makeCase]] to specify a different release logic depending on each of the
* three outcomes above.
*
* It is also possible to specify an interruptible acquire though [[Resource.makeFull makeFull]]
* but be warned that this is an advanced concurrency operation, which requires some care.
*
* Resource usage nests:
*
* {{{
* open(file1).use { in1 =>
* open(file2).use { in2 =>
* readFiles(in1, in2)
* }
* }
* }}}
*
* However, it is more idiomatic to compose multiple resources together before `use`, exploiting
* the fact that `Resource` forms a `Monad`, and therefore that resources can be nested through
* `flatMap`. Nested resources are released in reverse order of acquisition. Outer resources are
* released even if an inner use or release fails.
*
* {{{
* def mkResource(s: String) = {
* val acquire = IO(println(s"Acquiring $$s")) *> IO.pure(s)
* def release(s: String) = IO(println(s"Releasing $$s"))
* Resource.make(acquire)(release)
* }
*
* val r = for {
* outer <- mkResource("outer")
*
* inner <- mkResource("inner")
* } yield (outer, inner)
*
* r.use { case (a, b) =>
* IO(println(s"Using $$a and $$b"))
* }
* }}}
*
* On evaluation the above prints:
* {{{
* Acquiring outer
* Acquiring inner
* Using outer and inner
* Releasing inner
* Releasing outer
* }}}
*
* A `Resource` can also lift arbitrary actions that don't require finalisation through
* [[Resource.eval eval]]. Actions passed to `eval` preserve their interruptibility.
*
* Finally, `Resource` partakes in other abstractions such as `MonadError`, `Parallel`, and
* `Monoid`, so make sure to explore those instances as well as the other methods not covered
* here.
*
* `Resource` is encoded as a data structure, an ADT, described by the following node types:
*
* - [[Resource.Allocate Allocate]]
* - [[Resource.Bind Bind]]
* - [[Resource.Pure Pure]]
* - [[Resource.Eval Eval]]
*
* Normally users don't need to care about these node types, unless conversions from `Resource`
* into something else is needed (e.g. conversion from `Resource` into a streaming data type),
* in which case they can be interpreted through pattern matching.
*
* @tparam F
* the effect type in which the resource is allocated and released
* @tparam A
* the type of resource
*/
sealed abstract class Resource[F[_], +A] extends Serializable {
import Resource._
private[effect] def fold[B](
onOutput: A => F[B],
onRelease: F[Unit] => F[Unit]
)(implicit F: MonadCancel[F, Throwable]): F[B] = {
sealed trait Stack[AA]
case object Nil extends Stack[A]
final case class Frame[AA, BB](head: AA => Resource[F, BB], tail: Stack[BB])
extends Stack[AA]
// Indirection for calling `loop` needed because `loop` must be @tailrec
def continue[C](current: Resource[F, C], stack: Stack[C]): F[B] =
loop(current, stack)
// Interpreter that knows how to evaluate a Resource data structure;
// Maintains its own stack for dealing with Bind chains
@tailrec def loop[C](current: Resource[F, C], stack: Stack[C]): F[B] =
current match {
case Allocate(resource) =>
F.bracketFull(resource) {
case (a, _) =>
stack match {
case Nil => onOutput(a)
case Frame(head, tail) => continue(head(a), tail)
}
} {
case ((_, release), outcome) =>
onRelease(release(ExitCase.fromOutcome(outcome)))
}
case Bind(source, fs) =>
loop(source, Frame(fs, stack))
case Pure(v) =>
stack match {
case Nil => onOutput(v)
case Frame(head, tail) =>
loop(head(v), tail)
}
case Eval(fa) =>
fa.flatMap(a => continue(Resource.pure(a), stack))
}
loop(this, Nil)
}
/**
* Allocates a resource and supplies it to the given function. The resource is released as
* soon as the resulting `F[B]` is completed, whether normally or as a raised error.
*
* @param f
* the function to apply to the allocated resource
* @return
* the result of applying [F] to
*/
def use[B](f: A => F[B])(implicit F: MonadCancel[F, Throwable]): F[B] =
fold(f, identity)
/**
* For a resource that allocates an action (type `F[B]`), allocate that action, run it and
* release it.
*/
def useEval[B](implicit ev: A <:< F[B], F: MonadCancel[F, Throwable]): F[B] =
use(ev)
/**
* Allocates a resource with a non-terminating use action. Useful to run programs that are
* expressed entirely in `Resource`.
*
* The finalisers run when the resulting program fails or gets interrupted.
*/
def useForever(implicit F: Spawn[F]): F[Nothing] =
use[Nothing](_ => F.never)
/**
* Allocates a resource and closes it immediately.
*/
def use_(implicit F: MonadCancel[F, Throwable]): F[Unit] = use(_ => F.unit)
/**
* Allocates the resource and uses it to run the given Kleisli.
*/
def useKleisli[B >: A, C](usage: Kleisli[F, B, C])(
implicit F: MonadCancel[F, Throwable]): F[C] =
use(usage.run)
/**
* Creates a FunctionK that, when applied, will allocate the resource and use it to run the
* given Kleisli.
*/
def useKleisliK[B >: A](implicit F: MonadCancel[F, Throwable]): Kleisli[F, B, *] ~> F =
new (Kleisli[F, B, *] ~> F) {
def apply[C](fa: Kleisli[F, B, C]): F[C] = useKleisli(fa)
}
/**
* Allocates two resources concurrently, and combines their results in a tuple.
*
* The finalizers for the two resources are also run concurrently with each other, but within
* _each_ of the two resources, nested finalizers are run in the usual reverse order of
* acquisition.
*
* Note that `Resource` also comes with a `cats.Parallel` instance that offers more convenient
* access to the same functionality as `both`, for example via `parMapN`:
*
* {{{
* import scala.concurrent.duration._
* import cats.effect.{IO, Resource}
* import cats.effect.std.Random
* import cats.syntax.all._
*
* def mkResource(name: String) = {
* val acquire = for {
* n <- Random.scalaUtilRandom[IO].flatMap(_.nextIntBounded(1000))
* _ <- IO.sleep(n.millis)
* _ <- IO.println(s"Acquiring $$name")
* } yield name
*
* def release(name: String) =
* IO.println(s"Releasing $$name")
*
* Resource.make(acquire)(release)
* }
*
* val r = (mkResource("one"), mkResource("two"))
* .parMapN((s1, s2) => s"I have \$s1 and \$s2")
* .use(IO.println(_))
* }}}
*/
def both[B](
that: Resource[F, B]
)(implicit F: Concurrent[F]): Resource[F, (A, B)] = {
type Update = (F[Unit] => F[Unit]) => F[Unit]
def allocate[C](r: Resource[F, C], storeFinalizer: Update): F[C] =
r.fold(_.pure[F], release => storeFinalizer(F.guarantee(_, release)))
val bothFinalizers = F.ref(F.unit -> F.unit)
Resource.make(bothFinalizers)(_.get.flatMap(_.parTupled).void).evalMap { store =>
val thisStore: Update = f => store.update(_.bimap(f, identity))
val thatStore: Update = f => store.update(_.bimap(identity, f))
(allocate(this, thisStore), allocate(that, thatStore)).parTupled
}
}
/**
* Races the evaluation of two resource allocations and returns the result of the winner,
* except in the case of cancelation.
*/
def race[B](
that: Resource[F, B]
)(implicit F: Concurrent[F]): Resource[F, Either[A, B]] =
Concurrent[Resource[F, *]].race(this, that)
/**
* Implementation for the `flatMap` operation, as described via the `cats.Monad` type class.
*/
def flatMap[B](f: A => Resource[F, B]): Resource[F, B] =
Bind(this, f)
/**
* Given a mapping function, transforms the resource provided by this Resource.
*
* This is the standard `Functor.map`.
*/
def map[B](f: A => B): Resource[F, B] =
flatMap(a => Resource.pure[F, B](f(a)))
/**
* Given a natural transformation from `F` to `G`, transforms this Resource from effect `F` to
* effect `G`. The F and G constraint can also be satisfied by requiring a MonadCancelThrow[F]
* and MonadCancelThrow[G].
*/
def mapK[G[_]](
f: F ~> G
)(implicit F: MonadCancel[F, _], G: MonadCancel[G, _]): Resource[G, A] =
this match {
case Allocate(resource) =>
Resource.applyFull { (gpoll: Poll[G]) =>
gpoll {
f {
F.uncancelable { (fpoll: Poll[F]) => resource(fpoll) }
}
}.map {
case (a, release) =>
a -> ((r: ExitCase) => f(release(r)))
}
}
case Bind(source, f0) =>
// we insert a bind to get stack safety
suspend(G.unit >> source.mapK(f).pure[G]).flatMap(x => f0(x).mapK(f))
case Pure(a) =>
Resource.pure(a)
case Eval(fea) => Resource.eval(f(fea))
}
/**
* Runs `precede` before this resource is allocated.
*/
def preAllocate(precede: F[Unit]): Resource[F, A] =
Resource.eval(precede).flatMap(_ => this)
/**
* Runs `finalizer` when this resource is closed. Unlike the release action passed to
* `Resource.make`, this will run even if resource acquisition fails or is canceled.
*/
def onFinalize(finalizer: F[Unit])(implicit F: Applicative[F]): Resource[F, A] =
onFinalizeCase(_ => finalizer)
/**
* Like `onFinalize`, but the action performed depends on the exit case.
*/
def onFinalizeCase(f: ExitCase => F[Unit])(implicit F: Applicative[F]): Resource[F, A] =
Resource.makeCase(F.unit)((_, ec) => f(ec)).flatMap(_ => this)
/**
* Given a `Resource`, possibly built by composing multiple `Resource`s monadically, returns
* the acquired resource, as well as a cleanup function that takes an
* [[Resource.ExitCase exit case]] and runs all the finalizers for releasing it.
*
* If the outer `F` fails or is interrupted, `allocated` guarantees that the finalizers will
* be called. However, if the outer `F` succeeds, it's up to the user to ensure the returned
* `ExitCode => F[Unit]` is called once `A` needs to be released. If the returned `ExitCode =>
* F[Unit]` is not called, the finalizers will not be run.
*
* For this reason, this is an advanced and potentially unsafe api which can cause a resource
* leak if not used correctly, please prefer [[use]] as the standard way of running a
* `Resource` program.
*
* Use cases include interacting with side-effectful apis that expect separate acquire and
* release actions (like the `before` and `after` methods of many test frameworks), or complex
* library code that needs to modify or move the finalizer for an existing resource.
*/
def allocatedCase[B >: A](
implicit F: MonadCancel[F, Throwable]): F[(B, ExitCase => F[Unit])] = {
sealed trait Stack[AA]
case object Nil extends Stack[B]
final case class Frame[AA, BB](head: AA => Resource[F, BB], tail: Stack[BB])
extends Stack[AA]
// Indirection for calling `loop` needed because `loop` must be @tailrec
def continue[C](
current: Resource[F, C],
stack: Stack[C],
release: ExitCase => F[Unit]): F[(B, ExitCase => F[Unit])] =
loop(current, stack, release)
// Interpreter that knows how to evaluate a Resource data structure;
// Maintains its own stack for dealing with Bind chains
@tailrec def loop[C](
current: Resource[F, C],
stack: Stack[C],
release: ExitCase => F[Unit]): F[(B, ExitCase => F[Unit])] =
current match {
case Allocate(resource) =>
F uncancelable { poll =>
resource(poll) flatMap {
case (b, rel) =>
// Insert F.unit to emulate defer for stack-safety
val rel2 = (ec: ExitCase) => rel(ec).guarantee(F.unit >> release(ec))
stack match {
case Nil =>
/*
* We *don't* poll here because this case represents the "there are no flatMaps"
* scenario. If we poll in this scenario, then the following code will have a
* masking gap (where F = Resource):
*
* F.uncancelable(_(F.uncancelable(_ => foo)))
*
* In this case, the inner uncancelable has no trailing flatMap, so it will hit
* this case exactly. If we poll, we will create the masking gap and surface
* cancelation improperly.
*/
F.pure((b, rel2))
case Frame(head, tail) =>
poll(continue(head(b), tail, rel2))
.onCancel(rel(ExitCase.Canceled))
.onError { case e => rel(ExitCase.Errored(e)).handleError(_ => ()) }
}
}
}
case Bind(source, fs) =>
loop(source, Frame(fs, stack), release)
case Pure(v) =>
stack match {
case Nil =>
(v: B, release).pure[F]
case Frame(head, tail) =>
loop(head(v), tail, release)
}
case Eval(fa) =>
fa.flatMap(a => continue(Resource.pure(a), stack, release))
}
loop(this, Nil, _ => F.unit)
}
/**
* Given a `Resource`, possibly built by composing multiple `Resource`s monadically, returns
* the acquired resource, as well as an action that runs all the finalizers for releasing it.
*
* If the outer `F` fails or is interrupted, `allocated` guarantees that the finalizers will
* be called. However, if the outer `F` succeeds, it's up to the user to ensure the returned
* `F[Unit]` is called once `A` needs to be released. If the returned `F[Unit]` is not called,
* the finalizers will not be run.
*
* For this reason, this is an advanced and potentially unsafe api which can cause a resource
* leak if not used correctly, please prefer [[use]] as the standard way of running a
* `Resource` program.
*
* Use cases include interacting with side-effectful apis that expect separate acquire and
* release actions (like the `before` and `after` methods of many test frameworks), or complex
* library code that needs to modify or move the finalizer for an existing resource.
*/
def allocated[B >: A](implicit F: MonadCancel[F, Throwable]): F[(B, F[Unit])] =
F.uncancelable(poll =>
poll(allocatedCase).map { case (b, fin) => (b, fin(ExitCase.Succeeded)) })
/**
* Applies an effectful transformation to the allocated resource. Like a `flatMap` on `F[A]`
* while maintaining the resource context
*/
def evalMap[B](f: A => F[B]): Resource[F, B] =
this.flatMap(a => Resource.eval(f(a)))
/**
* Applies an effectful transformation to the allocated resource. Like a `flatTap` on `F[A]`
* while maintaining the resource context
*/
def evalTap[B](f: A => F[B]): Resource[F, A] =
this.flatMap(a => Resource.eval(f(a)).map(_ => a))
/**
* Acquires the resource, runs `gb` and closes the resource once `gb` terminates, fails or
* gets interrupted
*/
def surround[B](gb: F[B])(implicit F: MonadCancel[F, Throwable]): F[B] =
use(_ => gb)
/**
* Creates a FunctionK that can run `gb` within a resource, which is then closed once `gb`
* terminates, fails or gets interrupted
*/
def surroundK(implicit F: MonadCancel[F, Throwable]): F ~> F =
new (F ~> F) {
override def apply[B](gb: F[B]): F[B] = surround(gb)
}
def forceR[B](that: Resource[F, B])(implicit F: MonadCancel[F, Throwable]): Resource[F, B] =
Resource.applyFull { poll => poll(this.use_ !> that.allocatedCase) }
def !>[B](that: Resource[F, B])(implicit F: MonadCancel[F, Throwable]): Resource[F, B] =
forceR(that)
def onCancel(fin: Resource[F, Unit])(implicit F: MonadCancel[F, Throwable]): Resource[F, A] =
Resource.applyFull { poll => poll(this.allocatedCase).onCancel(fin.use_) }
def guaranteeCase(
fin: Outcome[Resource[F, *], Throwable, A @uncheckedVariance] => Resource[F, Unit])(
implicit F: MonadCancel[F, Throwable]): Resource[F, A] =
Resource.applyFull { poll =>
poll(this.allocatedCase).guaranteeCase {
case Outcome.Succeeded(ft) =>
fin(Outcome.Succeeded(Resource.eval(ft.map(_._1)))).use_.handleErrorWith { e =>
ft.flatMap(_._2(ExitCase.Errored(e))).handleError(_ => ()) >> F.raiseError(e)
}
case Outcome.Errored(e) =>
fin(Outcome.Errored(e)).use_.handleError(_ => ())
case Outcome.Canceled() =>
fin(Outcome.Canceled()).use_
}
}
/*
* 1. If the scope containing the *fiber* terminates:
* a) if the fiber is incomplete, run its inner finalizers when it completes
* b) if the fiber is succeeded, run its finalizers
* 2. If the fiber is canceled or errored, finalize
* 3. If the fiber succeeds and .cancel won the race, finalize eagerly and
* `join` results in `Canceled()`
* 4. If the fiber succeeds and .cancel lost the race or wasn't called,
* finalize naturally when the containing scope ends, `join` returns
* the value
*/
def start(
implicit
F: Concurrent[F]): Resource[F, Fiber[Resource[F, *], Throwable, A @uncheckedVariance]] = {
final case class State(
fin: F[Unit] = F.unit,
finalizeOnComplete: Boolean = false,
confirmedFinalizeOnComplete: Boolean = false)
Resource {
import Outcome._
F.ref[State](State()) flatMap { state =>
val finalized: F[A] = F uncancelable { poll =>
poll(this.allocated) guarantee {
// confirm that we completed and we were asked to clean up
// note that this will run even if the inner effect short-circuited
state update { s =>
if (s.finalizeOnComplete)
s.copy(confirmedFinalizeOnComplete = true)
else
s
}
} flatMap {
// if the inner F has a zero, we lose the finalizers, but there's no avoiding that
case (a, rel) =>
val action = state modify { s =>
if (s.confirmedFinalizeOnComplete)
(s, rel.handleError(_ => ()))
else
(s.copy(fin = rel), F.unit)
}
action.flatten.as(a)
}
}
F.start(finalized) map { outer =>
val fiber = new Fiber[Resource[F, *], Throwable, A] {
def cancel =
Resource eval {
F uncancelable { poll =>
// technically cancel is uncancelable, but separation of concerns and what not
poll(outer.cancel) *> state.update(_.copy(finalizeOnComplete = true))
}
}
def join =
Resource eval {
outer.join.flatMap[Outcome[Resource[F, *], Throwable, A]] {
case Canceled() =>
Outcome.canceled[Resource[F, *], Throwable, A].pure[F]
case Errored(e) =>
Outcome.errored[Resource[F, *], Throwable, A](e).pure[F]
case Succeeded(fp) =>
state.get map { s =>
if (s.confirmedFinalizeOnComplete)
Outcome.canceled[Resource[F, *], Throwable, A]
else
Outcome.succeeded(Resource.eval(fp))
}
}
}
}
val finalizeOuter =
state.modify(s => (s.copy(finalizeOnComplete = true), s.fin)).flatten
(fiber, finalizeOuter)
}
}
}
}
def evalOn(ec: ExecutionContext)(implicit F: Async[F]): Resource[F, A] =
Resource.applyFull { poll =>
poll(this.allocatedCase).evalOn(ec).map {
case (a, release) => (a, release.andThen(_.evalOn(ec)))
}
}
def attempt[E](implicit F: ApplicativeError[F, E]): Resource[F, Either[E, A]] =
this match {
case Allocate(resource) =>
Resource.applyFull { poll =>
resource(poll).attempt.map {
case Left(error) => (Left(error), (_: ExitCase) => F.unit)
case Right((a, release)) => (Right(a), release)
}
}
case Bind(source, f) =>
Resource.unit.flatMap(_ => source.attempt).flatMap {
case Left(error) => Resource.pure(error.asLeft)
case Right(s) => f(s).attempt
}
case p @ Pure(_) =>
Resource.pure(p.a.asRight)
case e @ Eval(_) =>
Resource.eval(e.fa.attempt)
}
def handleErrorWith[B >: A, E](f: E => Resource[F, B])(
implicit F: ApplicativeError[F, E]): Resource[F, B] =
attempt.flatMap {
case Right(a) => Resource.pure(a)
case Left(e) => f(e)
}
def combine[B >: A](that: Resource[F, B])(implicit A: Semigroup[B]): Resource[F, B] =
for {
x <- this
y <- that
} yield A.combine(x, y)
def combineK[B >: A](that: Resource[F, B])(
implicit F: MonadCancel[F, Throwable],
K: SemigroupK[F],
G: Ref.Make[F]): Resource[F, B] =
Resource.make(Ref[F].of(F.unit))(_.get.flatten).evalMap { finalizers =>
def allocate(r: Resource[F, B]): F[B] =
r.fold(_.pure[F], (release: F[Unit]) => finalizers.update(_.guarantee(release)))
K.combineK(allocate(this), allocate(that))
}
}
object Resource extends ResourceFOInstances0 with ResourceHOInstances0 with ResourcePlatform {
/**
* Creates a resource from an allocating effect.
*
* @see
* [[make]] for a version that separates the needed resource with its finalizer tuple in two
* parameters
*
* @tparam F
* the effect type in which the resource is acquired and released
* @tparam A
* the type of the resource
* @param resource
* an effect that returns a tuple of a resource and an effect to release it
*/
def apply[F[_], A](resource: F[(A, F[Unit])])(implicit F: Functor[F]): Resource[F, A] =
applyCase[F, A] {
resource.map {
case (a, release) =>
(a, (_: ExitCase) => release)
}
}
/**
* Creates a resource from an allocating effect, with a finalizer that is able to distinguish
* between [[ExitCase exit cases]].
*
* @see
* [[makeCase]] for a version that separates the needed resource with its finalizer tuple in
* two parameters
*
* @tparam F
* the effect type in which the resource is acquired and released
* @tparam A
* the type of the resource
* @param resource
* an effect that returns a tuple of a resource and an effectful function to release it
*/
def applyCase[F[_], A](resource: F[(A, ExitCase => F[Unit])]): Resource[F, A] =
applyFull(_ => resource)
/**
* Creates a resource from an allocating effect, with a finalizer that is able to distinguish
* between [[ExitCase exit cases]].
*
* The action takes a `Poll[F]` to allow for interruptible acquires, which is most often
* useful when acquiring lock-like structure: it should be possible to interrupt a fiber
* waiting on a lock, but if it does get acquired, release need to be guaranteed.
*
* Note that in this case the acquire action should know how to cleanup after itself in case
* it gets canceled, since Resource will only guarantee release when acquire succeeds and
* fails (and when the actions in `use` or `flatMap` fail, succeed, or get canceled)
*
* TODO make sure this api, which is more general than makeFull, doesn't allow for
* interruptible releases
*
* @see
* [[makeFull]] for a version that separates the needed resource with its finalizer tuple in
* two parameters
*
* @tparam F
* the effect type in which the resource is acquired and released
* @tparam A
* the type of the resource
* @param resource
* an effect that returns a tuple of a resource and an effectful function to release it,
* where acquisition can potentially be interrupted
*/
def applyFull[F[_], A](resource: Poll[F] => F[(A, ExitCase => F[Unit])]): Resource[F, A] =
Allocate(resource)
/**
* Given a `Resource` suspended in `F[_]`, lifts it in the `Resource` context.
*/
def suspend[F[_], A](fr: F[Resource[F, A]]): Resource[F, A] =
Resource.eval(fr).flatMap(x => x)
/**
* Creates a resource from an acquiring effect and a release function.
*
* @tparam F
* the effect type in which the resource is acquired and released
* @tparam A
* the type of the resource
* @param acquire
* an effect to acquire a resource
* @param release
* a function to effectfully release the resource returned by `acquire`
*/
def make[F[_], A](acquire: F[A])(release: A => F[Unit])(
implicit F: Functor[F]): Resource[F, A] =
apply[F, A](acquire.map(a => a -> release(a)))
/**
* Creates a resource from an acquiring effect and a release function that can discriminate
* between different [[ExitCase exit cases]].
*
* @tparam F
* the effect type in which the resource is acquired and released
* @tparam A
* the type of the resource
* @param acquire
* a function to effectfully acquire a resource
* @param release
* a function to effectfully release the resource returned by `acquire`
*/
def makeCase[F[_], A](
acquire: F[A]
)(release: (A, ExitCase) => F[Unit])(implicit F: Functor[F]): Resource[F, A] =
applyCase[F, A](acquire.map(a => (a, e => release(a, e))))
/**
* Creates a resource from an acquiring effect and a release function that can discriminate
* between different [[ExitCase exit cases]].
*
* The acquiring effect takes a `Poll[F]` to allow for interruptible acquires, which is most
* often useful when acquiring lock-like structures: it should be possible to interrupt a
* fiber waiting on a lock, but if it does get acquired, release need to be guaranteed.
*
* Note that in this case the acquire action should know how to cleanup after itself in case
* it gets canceled, since Resource will only guarantee release when acquire succeeds and
* fails (and when the actions in `use` or `flatMap` fail, succeed, or get canceled)
*
* @tparam F
* the effect type in which the resource is acquired and released
* @tparam A
* the type of the resource
* @param acquire
* an effect to acquire a resource, possibly interruptibly
* @param release
* a function to effectfully release the resource returned by `acquire`
*/
def makeFull[F[_], A](acquire: Poll[F] => F[A])(release: A => F[Unit])(
implicit F: Functor[F]): Resource[F, A] =
applyFull[F, A](poll => acquire(poll).map(a => (a, _ => release(a))))
/**
* Creates a resource from an acquiring effect and a release function that can discriminate
* between different [[ExitCase exit cases]].
*
* The acquiring effect takes a `Poll[F]` to allow for interruptible acquires, which is most
* often useful when acquiring lock-like structures: it should be possible to interrupt a
* fiber waiting on a lock, but if it does get acquired, release need to be guaranteed.
*
* Note that in this case the acquire action should know how to cleanup after itself in case
* it gets canceled, since Resource will only guarantee release when acquire succeeds and
* fails (and when the actions in `use` or `flatMap` fail, succeed, or get canceled)
*
* @tparam F
* the effect type in which the resource is acquired and released
* @tparam A
* the type of the resource
* @param acquire
* an effect to acquire a resource, possibly interruptibly
* @param release
* a function to effectfully release the resource returned by `acquire`
*/
def makeCaseFull[F[_], A](acquire: Poll[F] => F[A])(release: (A, ExitCase) => F[Unit])(
implicit F: Functor[F]): Resource[F, A] =
applyFull[F, A](poll => acquire(poll).map(a => (a, e => release(a, e))))
/**
* Lifts a pure value into a resource. The resource has a no-op release.
*
* @param a
* the value to lift into a resource
*/
def pure[F[_], A](a: A): Resource[F, A] =
Pure(a)
/**
* A resource with a no-op allocation and a no-op release.
*/
def unit[F[_]]: Resource[F, Unit] = pure(())
/**
* Lifts an applicative into a resource. The resource has a no-op release. Preserves
* interruptibility of `fa`.
*
* @param fa
* the value to lift into a resource
*/
def eval[F[_], A](fa: F[A]): Resource[F, A] =
Resource.Eval(fa)
/**
* Lifts a finalizer into a resource. The resource has a no-op allocation.
*/
def onFinalize[F[_]: Applicative](release: F[Unit]): Resource[F, Unit] =
unit.onFinalize(release)
/**
* Creates a resource that allocates immediately without any effects, but calls `release` when
* closing, providing the [[ExitCase the usage completed with]].
*/
def onFinalizeCase[F[_]: Applicative](release: ExitCase => F[Unit]): Resource[F, Unit] =
unit.onFinalizeCase(release)
/**
* Lifts an applicative into a resource as a `FunctionK`. The resource has a no-op release.
*/
def liftK[F[_]]: F ~> Resource[F, *] =
new (F ~> Resource[F, *]) {
def apply[A](fa: F[A]): Resource[F, A] = Resource.eval(fa)
}
/**
* Allocates two resources concurrently, and combines their results in a tuple.
*/
def both[F[_]: Concurrent, A, B](
rfa: Resource[F, A],
rfb: Resource[F, B]): Resource[F, (A, B)] =
rfa.both(rfb)
/**
* Races the evaluation of two resource allocations and returns the result of the winner,
* except in the case of cancelation.
*/
def race[F[_]: Concurrent, A, B](
rfa: Resource[F, A],
rfb: Resource[F, B]
): Resource[F, Either[A, B]] =
rfa.race(rfb)
/**
* Creates a [[Resource]] by wrapping a Java
* [[https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html AutoCloseable]].
*
* In most real world cases, implementors of AutoCloseable are blocking as well, so the close
* action runs in the blocking context.
*
* @example
* {{{
* import cats.effect._
* import scala.io.Source
*
* def reader(data: String): Resource[IO, Source] =
* Resource.fromAutoCloseable(IO.blocking {
* Source.fromString(data)
* })
* }}}
*
* @example
* {{{
* import cats.effect._
* import scala.io.Source
*
* def reader[F[_]](data: String)(implicit F: Sync[F]): Resource[F, Source] =
* Resource.fromAutoCloseable(F.blocking {
* Source.fromString(data)
* })
* }}}
*
* @param acquire
* The effect with the resource to acquire.
* @param F
* the effect type in which the resource was acquired and will be released
* @tparam F
* the type of the effect
* @tparam A
* the type of the autocloseable resource
* @return
* a Resource that will automatically close after use
*/
def fromAutoCloseable[F[_], A <: AutoCloseable](acquire: F[A])(
implicit F: Sync[F]): Resource[F, A] =
Resource.make(acquire)(autoCloseable => F.blocking(autoCloseable.close()))
def canceled[F[_]](implicit F: MonadCancel[F, _]): Resource[F, Unit] =
Resource.eval(F.canceled)
def uncancelable[F[_], A](body: Poll[Resource[F, *]] => Resource[F, A])(
implicit F: MonadCancel[F, Throwable]): Resource[F, A] =
Resource applyFull { poll =>
val inner = new Poll[Resource[F, *]] {
def apply[B](rfb: Resource[F, B]): Resource[F, B] =
Resource applyFull { innerPoll => innerPoll(poll(rfb.allocatedCase)) }
}
body(inner).allocatedCase
}
def unique[F[_]](implicit F: Unique[F]): Resource[F, Unique.Token] =
Resource.eval(F.unique)
def never[F[_], A](implicit F: GenSpawn[F, _]): Resource[F, A] =
Resource.eval(F.never[A])
def cede[F[_]](implicit F: GenSpawn[F, _]): Resource[F, Unit] =
Resource.eval(F.cede)
def deferred[F[_], A](
implicit F: GenConcurrent[F, _]): Resource[F, Deferred[Resource[F, *], A]] =
Resource.eval(F.deferred[A]).map(_.mapK(Resource.liftK[F]))
def ref[F[_], A](a: A)(implicit F: GenConcurrent[F, _]): Resource[F, Ref[Resource[F, *], A]] =
Resource.eval(F.ref(a)).map(_.mapK(Resource.liftK[F]))
def monotonic[F[_]](implicit F: Clock[F]): Resource[F, FiniteDuration] =
Resource.eval(F.monotonic)
def realTime[F[_]](implicit F: Clock[F]): Resource[F, FiniteDuration] =
Resource.eval(F.realTime)
def suspend[F[_], A](hint: Sync.Type)(thunk: => A)(implicit F: Sync[F]): Resource[F, A] =
Resource.eval(F.suspend(hint)(thunk))
def sleep[F[_]](time: Duration)(implicit F: GenTemporal[F, _]): Resource[F, Unit] =
Resource.eval(F.sleep(time))
@deprecated("Use overload with Duration", "3.4.0")
def sleep[F[_]](time: FiniteDuration, F: GenTemporal[F, _]): Resource[F, Unit] =
sleep(time: Duration)(F)
def cont[F[_], K, R](body: Cont[Resource[F, *], K, R])(implicit F: Async[F]): Resource[F, R] =
Resource.applyFull { poll =>
poll {
F.cont {
new Cont[F, K, (R, Resource.ExitCase => F[Unit])] {
def apply[G[_]](implicit G: MonadCancel[G, Throwable]) = { (cb, ga, nt) =>
type D[A] = Kleisli[G, Ref[G, ExitCase => F[Unit]], A]
val nt2 = new (Resource[F, *] ~> D) {
def apply[A](rfa: Resource[F, A]) =
Kleisli { r =>
nt(rfa.allocatedCase) flatMap {
case (a, fin) =>