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IO.scala
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IO.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 cats.arrow.FunctionK
import cats.effect.internals._
import cats.effect.internals.Callback.Extensions
import cats.effect.internals.TrampolineEC.immediate
import cats.effect.internals.IOPlatform.fusionMaxStackDepth
import scala.annotation.unchecked.uncheckedVariance
import scala.concurrent.{ExecutionContext, Future, Promise}
import scala.concurrent.duration._
import scala.util.{Failure, Left, Right, Success}
/**
* A pure abstraction representing the intention to perform a
* side effect, where the result of that side effect may be obtained
* synchronously (via return) or asynchronously (via callback).
*
* `IO` values are pure, immutable values and thus preserve
* referential transparency, being usable in functional programming.
* An `IO` is a data structure that represents just a description
* of a side effectful computation.
*
* `IO` can describe synchronous or asynchronous computations that:
*
* 1. on evaluation yield exactly one result
* 1. can end in either success or failure and in case of failure
* `flatMap` chains get short-circuited (`IO` implementing
* the algebra of `MonadError`)
* 1. can be canceled, but note this capability relies on the
* user to provide cancelation logic
*
* Effects described via this abstraction are not evaluated until
* the "end of the world", which is to say, when one of the "unsafe"
* methods are used. Effectful results are not memoized, meaning that
* memory overhead is minimal (and no leaks), and also that a single
* effect may be run multiple times in a referentially-transparent
* manner. For example:
*
* {{{
* val ioa = IO { println("hey!") }
*
* val program = for {
* _ <- ioa
* _ <- ioa
* } yield ()
*
* program.unsafeRunSync()
* }}}
*
* The above will print "hey!" twice, as the effect will be re-run
* each time it is sequenced in the monadic chain.
*
* `IO` is trampolined in its `flatMap` evaluation. This means that
* you can safely call `flatMap` in a recursive function of arbitrary
* depth, without fear of blowing the stack.
*
* {{{
* def fib(n: Int, a: Long = 0, b: Long = 1): IO[Long] =
* IO(a + b).flatMap { b2 =>
* if (n > 0)
* fib(n - 1, b, b2)
* else
* IO.pure(b2)
* }
* }}}
*/
sealed abstract class IO[+A] extends internals.IOBinaryCompat[A] {
import IO._
/**
* Functor map on `IO`. Given a mapping functions, it transforms the
* value produced by the source, while keeping the `IO` context.
*
* Any exceptions thrown within the function will be caught and
* sequenced into the `IO`, because due to the nature of
* asynchronous processes, without catching and handling exceptions,
* failures would be completely silent and `IO` references would
* never terminate on evaluation.
*/
final def map[B](f: A => B): IO[B] =
this match {
case Map(source, g, index) =>
// Allowed to do fixed number of map operations fused before
// resetting the counter in order to avoid stack overflows;
// See `IOPlatform` for details on this maximum.
if (index != fusionMaxStackDepth) Map(source, g.andThen(f), index + 1)
else Map(this, f, 0)
case _ =>
Map(this, f, 0)
}
/**
* Monadic bind on `IO`, used for sequentially composing two `IO`
* actions, where the value produced by the first `IO` is passed as
* input to a function producing the second `IO` action.
*
* Due to this operation's signature, `flatMap` forces a data
* dependency between two `IO` actions, thus ensuring sequencing
* (e.g. one action to be executed before another one).
*
* Any exceptions thrown within the function will be caught and
* sequenced into the `IO`, because due to the nature of
* asynchronous processes, without catching and handling exceptions,
* failures would be completely silent and `IO` references would
* never terminate on evaluation.
*/
final def flatMap[B](f: A => IO[B]): IO[B] =
Bind(this, f)
/**
* Materializes any sequenced exceptions into value space, where
* they may be handled.
*
* This is analogous to the `catch` clause in `try`/`catch`, being
* the inverse of `IO.raiseError`. Thus:
*
* {{{
* IO.raiseError(ex).attempt.unsafeRunAsync === Left(ex)
* }}}
*
* @see [[IO.raiseError]]
*/
def attempt: IO[Either[Throwable, A]] =
Bind(this, AttemptIO.asInstanceOf[A => IO[Either[Throwable, A]]])
/**
* Produces an `IO` reference that should execute the source on
* evaluation, without waiting for its result, being the safe
* analogue to [[unsafeRunAsync]].
*
* This operation is isomorphic to [[unsafeRunAsync]]. What it does
* is to let you describe asynchronous execution with a function
* that stores off the results of the original `IO` as a
* side effect, thus ''avoiding'' the usage of impure callbacks or
* eager evaluation.
*
* The returned `IO` is guaranteed to execute immediately,
* and does not wait on any async action to complete, thus this
* is safe to do, even on top of runtimes that cannot block threads
* (e.g. JavaScript):
*
* {{{
* // Sample
* val source = IO.shift *> IO(1)
* // Describes execution
* val start = source.runAsync
* // Safe, because it does not block for the source to finish
* start.unsafeRunSync
* }}}
*
* @return an `IO` value that upon evaluation will execute the source,
* but will not wait for its completion
*
* @see [[runCancelable]] for the version that gives you a cancelable
* token that can be used to send a cancel signal
*/
final def runAsync(cb: Either[Throwable, A] => IO[Unit]): IO[Unit] = IO {
unsafeRunAsync(cb.andThen(_.unsafeRunAsync(Callback.report)))
}
/**
* Produces an `IO` reference that should execute the source on evaluation,
* without waiting for its result and return a cancelable token, being the
* safe analogue to [[unsafeRunCancelable]].
*
* This operation is isomorphic to [[unsafeRunCancelable]]. Just like
* [[runAsync]], this operation avoids the usage of impure callbacks or
* eager evaluation.
*
* The returned `IO` boxes an `IO[Unit]` that can be used to cancel the
* running asynchronous computation (if the source can be cancelled).
*
* The returned `IO` is guaranteed to execute immediately,
* and does not wait on any async action to complete, thus this
* is safe to do, even on top of runtimes that cannot block threads
* (e.g. JavaScript):
*
* {{{
* val source: IO[Int] = ???
* // Describes interruptible execution
* val start: IO[IO[Unit]] = source.runCancelable
*
* // Safe, because it does not block for the source to finish
* val cancel: IO[Unit] = start.unsafeRunSync
*
* // Safe, because cancelation only sends a signal,
* // but doesn't back-pressure on anything
* cancel.unsafeRunSync
* }}}
*
* @return an `IO` value that upon evaluation will execute the source,
* but will not wait for its completion, yielding a cancelation
* token that can be used to cancel the async process
*
* @see [[runAsync]] for the simple, uninterruptible version
*/
final def runCancelable(cb: Either[Throwable, A] => IO[Unit]): IO[IO[Unit]] = IO {
val cancel = unsafeRunCancelable(cb.andThen(_.unsafeRunAsync(_ => ())))
IO.Delay(cancel)
}
/**
* Produces the result by running the encapsulated effects as impure
* side effects.
*
* If any component of the computation is asynchronous, the current
* thread will block awaiting the results of the async computation.
* On JavaScript, an exception will be thrown instead to avoid
* generating a deadlock. By default, this blocking will be
* unbounded. To limit the thread block to some fixed time, use
* `unsafeRunTimed` instead.
*
* Any exceptions raised within the effect will be re-thrown during
* evaluation.
*
* As the name says, this is an UNSAFE function as it is impure and
* performs side effects, not to mention blocking, throwing
* exceptions, and doing other things that are at odds with
* reasonable software. You should ideally only call this function
* *once*, at the very end of your program.
*/
final def unsafeRunSync(): A = unsafeRunTimed(Duration.Inf).get
/**
* Passes the result of the encapsulated effects to the given
* callback by running them as impure side effects.
*
* Any exceptions raised within the effect will be passed to the
* callback in the `Either`. The callback will be invoked at most
* *once*. Note that it is very possible to construct an IO which
* never returns while still never blocking a thread, and attempting
* to evaluate that IO with this method will result in a situation
* where the callback is *never* invoked.
*
* As the name says, this is an UNSAFE function as it is impure and
* performs side effects. You should ideally only call this
* function ''once'', at the very end of your program.
*/
final def unsafeRunAsync(cb: Either[Throwable, A] => Unit): Unit =
IORunLoop.start(this, cb)
/**
* Evaluates the source `IO`, passing the result of the encapsulated
* effects to the given callback.
*
* As the name says, this is an UNSAFE function as it is impure and
* performs side effects. You should ideally only call this
* function ''once'', at the very end of your program.
*
* @return an side-effectful function that, when executed, sends a
* cancelation reference to `IO`'s run-loop implementation,
* having the potential to interrupt it.
*/
final def unsafeRunCancelable(cb: Either[Throwable, A] => Unit): () => Unit = {
val conn = IOConnection()
IORunLoop.startCancelable(this, conn, cb)
conn.cancel
}
/**
* Similar to `unsafeRunSync`, except with a bounded blocking
* duration when awaiting asynchronous results.
*
* Please note that the `limit` parameter does not limit the time of
* the total computation, but rather acts as an upper bound on any
* *individual* asynchronous block. Thus, if you pass a limit of `5
* seconds` to an `IO` consisting solely of synchronous actions, the
* evaluation may take considerably longer than 5 seconds!
* Furthermore, if you pass a limit of `5 seconds` to an `IO`
* consisting of several asynchronous actions joined together,
* evaluation may take up to `n * 5 seconds`, where `n` is the
* number of joined async actions.
*
* As soon as an async blocking limit is hit, evaluation
* ''immediately'' aborts and `None` is returned.
*
* Please note that this function is intended for ''testing''; it
* should never appear in your mainline production code! It is
* absolutely not an appropriate function to use if you want to
* implement timeouts, or anything similar. If you need that sort
* of functionality, you should be using a streaming library (like
* fs2 or Monix).
*
* @see [[unsafeRunSync]]
*/
final def unsafeRunTimed(limit: Duration): Option[A] =
IORunLoop.step(this) match {
case Pure(a) => Some(a)
case RaiseError(e) => throw e
case self @ Async(_) =>
IOPlatform.unsafeResync(self, limit)
case _ =>
// $COVERAGE-OFF$
throw new AssertionError("unreachable")
// $COVERAGE-ON$
}
/**
* Evaluates the effect and produces the result in a `Future`.
*
* This is similar to `unsafeRunAsync` in that it evaluates the `IO`
* as a side effect in a non-blocking fashion, but uses a `Future`
* rather than an explicit callback. This function should really
* only be used if interoperating with legacy code which uses Scala
* futures.
*
* @see [[IO.fromFuture]]
*/
final def unsafeToFuture(): Future[A] = {
val p = Promise[A]
unsafeRunAsync(_.fold(p.failure, p.success))
p.future
}
/**
* Start execution of the source suspended in the `IO` context.
*
* This can be used for non-deterministic / concurrent execution.
* The following code is more or less equivalent with `parMap2`
* (minus the behavior on error handling and cancelation):
*
* {{{
* def par2[A, B](ioa: IO[A], iob: IO[B]): IO[(A, B)] =
* for {
* fa <- ioa.start
* fb <- iob.start
* a <- fa.join
* b <- fb.join
* } yield (a, b)
* }}}
*
* Note in such a case usage of `parMapN` (via `cats.Parallel`) is
* still recommended because of behavior on error and cancelation —
* consider in the example above what would happen if the first task
* finishes in error. In that case the second task doesn't get cancelled,
* which creates a potential memory leak.
*
* IMPORTANT — this operation does not start with an asynchronous boundary.
* But you can use [[IO.shift(implicit* IO.shift]] to force an async
* boundary just before start.
*/
final def start: IO[Fiber[IO, A @uncheckedVariance]] =
IOStart(this)
/**
* Returns a new `IO` that mirrors the source task for normal termination,
* but that triggers the given error on cancelation.
*
* Normally tasks that are cancelled become non-terminating.
*
* This `onCancelRaiseError` operator transforms a task that is
* non-terminating on cancelation into one that yields an error,
* thus equivalent with [[IO.raiseError]].
*/
final def onCancelRaiseError(e: Throwable): IO[A] =
IOCancel.raise(this, e)
/**
* Makes the source `IO` uninterruptible such that a [[Fiber.cancel]]
* signal has no effect.
*/
final def uncancelable: IO[A] =
IOCancel.uncancelable(this)
/**
* Converts the source `IO` into any `F` type that implements
* the [[LiftIO]] type class.
*/
final def to[F[_]](implicit F: LiftIO[F]): F[A @uncheckedVariance] =
F.liftIO(this)
/**
* Returns an `IO` action that treats the source task as the
* acquisition of a resource, which is then exploited by the `use`
* function and then `released`.
*
* The `bracket` operation is the equivalent of the
* `try {} catch {} finally {}` statements from mainstream languages.
*
* The `bracket` operation installs the necessary exception handler
* to release the resource in the event of an exception being raised
* during the computation, or in case of cancelation.
*
* If an exception is raised, then `bracket` will re-raise the
* exception ''after'' performing the `release`. If the resulting
* task gets cancelled, then `bracket` will still perform the
* `release`, but the yielded task will be non-terminating
* (equivalent with [[IO.never]]).
*
* Example:
*
* {{{
* import java.io._
*
* def readFile(file: File): IO[String] = {
* // Opening a file handle for reading text
* val acquire = IO(new BufferedReader(
* new InputStreamReader(new FileInputStream(file), "utf-8")
* ))
*
* acquire.bracket { in =>
* // Usage part
* IO {
* // Yes, ugly Java, non-FP loop;
* // side-effects are suspended though
* var line: String = null
* val buff = new StringBuilder()
* do {
* line = in.readLine()
* if (line != null) buff.append(line)
* } while (line != null)
* buff.toString()
* }
* } { in =>
* // The release part
* IO(in.close())
* }
* }
* }}}
*
* Note that in case of cancelation the underlying implementation
* cannot guarantee that the computation described by `use` doesn't
* end up executed concurrently with the computation from
* `release`. In the example above that ugly Java loop might end up
* reading from a `BufferedReader` that is already closed due to the
* task being cancelled, thus triggering an error in the background
* with nowhere to get signaled.
*
* In this particular example, given that we are just reading from a
* file, it doesn't matter. But in other cases it might matter, as
* concurrency on top of the JVM when dealing with I/O might lead to
* corrupted data.
*
* For those cases you might want to do synchronization (e.g. usage
* of locks and semaphores) and you might want to use [[bracketCase]],
* the version that allows you to differentiate between normal
* termination and cancelation.
*
* '''NOTE on error handling''': one big difference versus
* `try/finally` statements is that, in case both the `release`
* function and the `use` function throws, the error raised by `use`
* gets signaled.
*
* For example:
*
* {{{
* IO("resource").bracket { _ =>
* // use
* IO.raiseError(new RuntimeException("Foo"))
* } { _ =>
* // release
* IO.raiseError(new RuntimeException("Bar"))
* }
* }}}
*
* In this case the error signaled downstream is `"Foo"`, while the
* `"Bar"` error gets reported. This is consistent with the behavior
* of Haskell's `bracket` operation and NOT with `try {} finally {}`
* from Scala, Java or JavaScript.
*
* @see [[bracketCase]]
*
* @param use is a function that evaluates the resource yielded by
* the source, yielding a result that will get generated by
* the task returned by this `bracket` function
*
* @param release is a function that gets called after `use`
* terminates, either normally or in error, or if it gets
* cancelled, receiving as input the resource that needs to
* be released
*/
final def bracket[B](use: A => IO[B])(release: A => IO[Unit]): IO[B] =
bracketCase(use)((a, _) => release(a))
/**
* Returns a new `IO` task that treats the source task as the
* acquisition of a resource, which is then exploited by the `use`
* function and then `released`, with the possibility of
* distinguishing between normal termination and cancelation, such
* that an appropriate release of resources can be executed.
*
* The `bracketCase` operation is the equivalent of
* `try {} catch {} finally {}` statements from mainstream languages
* when used for the acquisition and release of resources.
*
* The `bracketCase` operation installs the necessary exception handler
* to release the resource in the event of an exception being raised
* during the computation, or in case of cancelation.
*
* In comparison with the simpler [[bracket]] version, this one
* allows the caller to differentiate between normal termination,
* termination in error and cancelation via an [[ExitCase]]
* parameter.
*
* @see [[bracket]]
*
* @param use is a function that evaluates the resource yielded by
* the source, yielding a result that will get generated by
* this function on evaluation
*
* @param release is a function that gets called after `use`
* terminates, either normally or in error, or if it gets
* canceled, receiving as input the resource that needs that
* needs release, along with the result of `use`
* (cancelation, error or successful result)
*/
def bracketCase[B](use: A => IO[B])(release: (A, ExitCase[Throwable]) => IO[Unit]): IO[B] =
IOBracket(this)(use)(release)
override def toString = this match {
case Pure(a) => s"IO($a)"
case RaiseError(e) => s"IO(throw $e)"
case _ => "IO$" + System.identityHashCode(this)
}
}
private[effect] abstract class IOParallelNewtype
extends internals.IOTimerRef with internals.IOCompanionBinaryCompat {
/** Newtype encoding for an `IO` datatype that has a `cats.Applicative`
* capable of doing parallel processing in `ap` and `map2`, needed
* for implementing `cats.Parallel`.
*
* Helpers are provided for converting back and forth in `Par.apply`
* for wrapping any `IO` value and `Par.unwrap` for unwrapping.
*
* The encoding is based on the "newtypes" project by
* Alexander Konovalov, chosen because it's devoid of boxing issues and
* a good choice until opaque types will land in Scala.
*/
type Par[+A] = Par.Type[A]
/** Newtype encoding, see the [[IO.Par]] type alias
* for more details.
*/
object Par extends IONewtype
}
private[effect] abstract class IOLowPriorityInstances extends IOParallelNewtype {
private[effect] class IOSemigroup[A: Semigroup] extends Semigroup[IO[A]] {
def combine(ioa1: IO[A], ioa2: IO[A]) =
ioa1.flatMap(a1 => ioa2.map(a2 => Semigroup[A].combine(a1, a2)))
}
implicit def ioSemigroup[A: Semigroup]: Semigroup[IO[A]] = new IOSemigroup[A]
}
private[effect] abstract class IOInstances extends IOLowPriorityInstances {
implicit val parApplicative: Applicative[IO.Par] = new Applicative[IO.Par] {
import IO.Par.unwrap
import IO.Par.{apply => par}
override def pure[A](x: A): IO.Par[A] =
par(IO.pure(x))
override def map2[A, B, Z](fa: IO.Par[A], fb: IO.Par[B])(f: (A, B) => Z): IO.Par[Z] =
par(IOParMap(unwrap(fa), unwrap(fb))(f))
override def ap[A, B](ff: IO.Par[A => B])(fa: IO.Par[A]): IO.Par[B] =
map2(ff, fa)(_(_))
override def product[A, B](fa: IO.Par[A], fb: IO.Par[B]): IO.Par[(A, B)] =
map2(fa, fb)((_, _))
override def map[A, B](fa: IO.Par[A])(f: A => B): IO.Par[B] =
par(unwrap(fa).map(f))
override def unit: IO.Par[Unit] =
par(IO.unit)
}
implicit val ioConcurrentEffect: ConcurrentEffect[IO] = new ConcurrentEffect[IO] {
override def pure[A](a: A): IO[A] =
IO.pure(a)
override def flatMap[A, B](ioa: IO[A])(f: A => IO[B]): IO[B] =
ioa.flatMap(f)
override def map[A, B](fa: IO[A])(f: A => B): IO[B] =
fa.map(f)
override def delay[A](thunk: => A): IO[A] =
IO(thunk)
override def unit: IO[Unit] =
IO.unit
override def attempt[A](ioa: IO[A]): IO[Either[Throwable, A]] =
ioa.attempt
override def handleErrorWith[A](ioa: IO[A])(f: Throwable => IO[A]): IO[A] =
IO.Bind(ioa, IOFrame.errorHandler(f))
override def raiseError[A](e: Throwable): IO[A] =
IO.raiseError(e)
override def suspend[A](thunk: => IO[A]): IO[A] =
IO.suspend(thunk)
override def start[A](fa: IO[A]): IO[Fiber[IO, A]] =
fa.start
override def uncancelable[A](fa: IO[A]): IO[A] =
fa.uncancelable
override def onCancelRaiseError[A](fa: IO[A], e: Throwable): IO[A] =
fa.onCancelRaiseError(e)
override def async[A](k: (Either[Throwable, A] => Unit) => Unit): IO[A] =
IO.async(k)
override def race[A, B](fa: IO[A], fb: IO[B]): IO[Either[A, B]] =
IO.race(fa, fb)
override def racePair[A, B](fa: IO[A], fb: IO[B]): IO[Either[(A, Fiber[IO, B]), (Fiber[IO, A], B)]] =
IO.racePair(fa, fb)
override def runAsync[A](ioa: IO[A])(cb: Either[Throwable, A] => IO[Unit]): IO[Unit] =
ioa.runAsync(cb)
override def cancelable[A](k: (Either[Throwable, A] => Unit) => IO[Unit]): IO[A] =
IO.cancelable(k)
override def runCancelable[A](fa: IO[A])(cb: Either[Throwable, A] => IO[Unit]): IO[IO[Unit]] =
fa.runCancelable(cb)
override def liftIO[A](ioa: IO[A]): IO[A] =
ioa
// this will use stack proportional to the maximum number of joined async suspensions
override def tailRecM[A, B](a: A)(f: A => IO[Either[A, B]]): IO[B] =
f(a) flatMap {
case Left(a) => tailRecM(a)(f)
case Right(b) => pure(b)
}
override def bracket[A, B](acquire: IO[A])
(use: A => IO[B])
(release: A => IO[Unit]): IO[B] =
acquire.bracket(use)(release)
override def bracketCase[A, B](acquire: IO[A])
(use: A => IO[B])
(release: (A, ExitCase[Throwable]) => IO[Unit]): IO[B] =
acquire.bracketCase(use)(release)
}
implicit val ioParallel: Parallel[IO, IO.Par] =
new Parallel[IO, IO.Par] {
override def applicative: Applicative[IO.Par] =
parApplicative
override def monad: Monad[IO] =
ioConcurrentEffect
override val sequential: ~>[IO.Par, IO] =
new FunctionK[IO.Par, IO] { def apply[A](fa: IO.Par[A]): IO[A] = IO.Par.unwrap(fa) }
override val parallel: ~>[IO, IO.Par] =
new FunctionK[IO, IO.Par] { def apply[A](fa: IO[A]): IO.Par[A] = IO.Par(fa) }
}
implicit def ioMonoid[A: Monoid]: Monoid[IO[A]] = new IOSemigroup[A] with Monoid[IO[A]] {
def empty = IO.pure(Monoid[A].empty)
}
implicit val ioSemigroupK: SemigroupK[IO] = new SemigroupK[IO] {
def combineK[A](a: IO[A], b: IO[A]): IO[A] =
ApplicativeError[IO, Throwable].handleErrorWith(a)(_ => b)
}
}
/**
* @define shiftDesc For example we can introduce an asynchronous
* boundary in the `flatMap` chain before a certain task:
* {{{
* IO.shift.flatMap(_ => task)
* }}}
*
* Or using Cats syntax:
* {{{
* import cats.syntax.all._
*
* IO.shift *> task
* }}}
*
* Or we can specify an asynchronous boundary ''after'' the
* evaluation of a certain task:
* {{{
* task.flatMap(a => IO.shift.map(_ => a))
* }}}
*
* Or using Cats syntax:
* {{{
* task <* IO.shift
* }}}
*
* Example of where this might be useful:
* {{{
* for {
* _ <- IO.shift(BlockingIO)
* bytes <- readFileUsingJavaIO(file)
* _ <- IO.shift(DefaultPool)
*
* secure = encrypt(bytes, KeyManager)
* _ <- sendResponse(Protocol.v1, secure)
*
* _ <- IO { println("it worked!") }
* } yield ()
* }}}
*
* In the above, `readFileUsingJavaIO` will be shifted to the
* pool represented by `BlockingIO`, so long as it is defined
* using `apply` or `suspend` (which, judging by the name, it
* probably is). Once its computation is complete, the rest
* of the `for`-comprehension is shifted ''again'', this time
* onto the `DefaultPool`. This pool is used to compute the
* encrypted version of the bytes, which are then passed to
* `sendResponse`. If we assume that `sendResponse` is
* defined using `async` (perhaps backed by an NIO socket
* channel), then we don't actually know on which pool the
* final `IO` action (the `println`) will be run. If we
* wanted to ensure that the `println` runs on `DefaultPool`,
* we would insert another `shift` following `sendResponse`.
*
* Another somewhat less common application of `shift` is to
* reset the thread stack and yield control back to the
* underlying pool. For example:
*
* {{{
* lazy val repeat: IO[Unit] = for {
* _ <- doStuff
* _ <- IO.shift
* _ <- repeat
* } yield ()
* }}}
*
* In this example, `repeat` is a very long running `IO`
* (infinite, in fact!) which will just hog the underlying
* thread resource for as long as it continues running. This
* can be a bit of a problem, and so we inject the `IO.shift`
* which yields control back to the underlying thread pool,
* giving it a chance to reschedule things and provide better
* fairness. This shifting also "bounces" the thread stack,
* popping all the way back to the thread pool and effectively
* trampolining the remainder of the computation. This sort
* of manual trampolining is unnecessary if `doStuff` is
* defined using `suspend` or `apply`, but if it was defined
* using `async` and does ''not'' involve any real
* concurrency, the call to `shift` will be necessary to avoid
* a `StackOverflowError`.
*
* Thus, this function has four important use cases:
*
* - shifting blocking actions off of the main compute pool,
* - defensively re-shifting asynchronous continuations back
* to the main compute pool
* - yielding control to some underlying pool for fairness
* reasons, and
* - preventing an overflow of the call stack in the case of
* improperly constructed `async` actions
*
* Note there are 2 overloads of this function:
*
* - one that takes an [[Timer]] ([[IO.shift(implicit* link]])
* - one that takes a Scala `ExecutionContext` ([[IO.shift(ec* link]])
*
* Use the former by default, use the later for fine grained
* control over the thread pool used.
*/
object IO extends IOInstances {
/**
* Suspends a synchronous side effect in `IO`.
*
* Any exceptions thrown by the effect will be caught and sequenced
* into the `IO`.
*/
def apply[A](body: => A): IO[A] = Delay(body _)
/**
* Suspends a synchronous side effect which produces an `IO` in `IO`.
*
* This is useful for trampolining (i.e. when the side effect is
* conceptually the allocation of a stack frame). Any exceptions
* thrown by the side effect will be caught and sequenced into the
* `IO`.
*/
def suspend[A](thunk: => IO[A]): IO[A] =
Suspend(thunk _)
/**
* Suspends a pure value in `IO`.
*
* This should ''only'' be used if the value in question has
* "already" been computed! In other words, something like
* `IO.pure(readLine)` is most definitely not the right thing to do!
* However, `IO.pure(42)` is correct and will be more efficient
* (when evaluated) than `IO(42)`, due to avoiding the allocation of
* extra thunks.
*/
def pure[A](a: A): IO[A] = Pure(a)
/** Alias for `IO.pure(())`. */
val unit: IO[Unit] = pure(())
/**
* A non-terminating `IO`, alias for `async(_ => ())`.
*/
val never: IO[Nothing] = async(_ => ())
/**
* Lifts an `Eval` into `IO`.
*
* This function will preserve the evaluation semantics of any
* actions that are lifted into the pure `IO`. Eager `Eval`
* instances will be converted into thunk-less `IO` (i.e. eager
* `IO`), while lazy eval and memoized will be executed as such.
*/
def eval[A](fa: Eval[A]): IO[A] = fa match {
case Now(a) => pure(a)
case notNow => apply(notNow.value)
}
/**
* Suspends an asynchronous side effect in `IO`.
*
* The given function will be invoked during evaluation of the `IO`
* to "schedule" the asynchronous callback, where the callback is
* the parameter passed to that function. Only the ''first''
* invocation of the callback will be effective! All subsequent
* invocations will be silently dropped.
*
* As a quick example, you can use this function to perform a
* parallel computation given an `ExecutorService`:
*
* {{{
* def fork[A](body: => A)(implicit E: ExecutorService): IO[A] = {
* IO async { cb =>
* E.execute(new Runnable {
* def run() =
* try cb(Right(body)) catch { case NonFatal(t) => cb(Left(t)) }
* })
* }
* }
* }}}
*
* The `fork` function will do exactly what it sounds like: take a
* thunk and an `ExecutorService` and run that thunk on the thread
* pool. Or rather, it will produce an `IO` which will do those
* things when run; it does *not* schedule the thunk until the
* resulting `IO` is run! Note that there is no thread blocking in
* this implementation; the resulting `IO` encapsulates the callback
* in a pure and monadic fashion without using threads.
*
* This function can be thought of as a safer, lexically-constrained
* version of `Promise`, where `IO` is like a safer, lazy version of
* `Future`.
*/
def async[A](k: (Either[Throwable, A] => Unit) => Unit): IO[A] = {
Async { (_, cb) =>
val cb2 = Callback.asyncIdempotent(null, cb)
try k(cb2) catch { case NonFatal(t) => cb2(Left(t)) }
}
}
/**
* Builds a cancelable `IO`.
*/
def cancelable[A](k: (Either[Throwable, A] => Unit) => IO[Unit]): IO[A] =
Async { (conn, cb) =>
val cb2 = Callback.asyncIdempotent(conn, cb)
val ref = ForwardCancelable()
conn.push(ref)
ref := (
try k(cb2) catch { case NonFatal(t) =>
cb2(Left(t))
IO.unit
})
}
/**
* Constructs an `IO` which sequences the specified exception.
*
* If this `IO` is run using `unsafeRunSync` or `unsafeRunTimed`,
* the exception will be thrown. This exception can be "caught" (or
* rather, materialized into value-space) using the `attempt`
* method.
*
* @see [[IO#attempt]]
*/
def raiseError[A](e: Throwable): IO[A] = RaiseError(e)
/**
* Constructs an `IO` which evaluates the given `Future` and
* produces the result (or failure).
*
* Because `Future` eagerly evaluates, as well as because it
* memoizes, this function takes its parameter as an `IO`,
* which could be lazily evaluated. If this laziness is
* appropriately threaded back to the definition site of the
* `Future`, it ensures that the computation is fully managed by
* `IO` and thus referentially transparent.
*
* Example:
*
* {{{
* // Lazy evaluation, equivalent with by-name params
* IO.fromFuture(IO(f))
*
* // Eager evaluation, for pure futures
* IO.fromFuture(IO.pure(f))
* }}}
*
* Roughly speaking, the following identities hold:
*
* {{{
* IO.fromFuture(IO(f)).unsafeToFuture() === f // true-ish (except for memoization)
* IO.fromFuture(IO(ioa.unsafeToFuture())) === ioa // true
* }}}
*
* @see [[IO#unsafeToFuture]]
*/
def fromFuture[A](iof: IO[Future[A]]): IO[A] =
iof.flatMap { f =>
IO.async { cb =>
f.onComplete(r => cb(r match {
case Success(a) => Right(a)
case Failure(e) => Left(e)
}))(immediate)
}
}
/**
* Lifts an Either[Throwable, A] into the IO[A] context raising the throwable
* if it exists.
*/
def fromEither[A](e: Either[Throwable, A]): IO[A] =
e match {
case Right(a) => pure(a)
case Left(err) => raiseError(err)
}
/**
* Asynchronous boundary described as an effectful `IO`, managed
* by the provided [[Timer]].
*
* This operation can be used in `flatMap` chains to "shift" the
* continuation of the run-loop to another thread or call stack.
*
* $shiftDesc
*
* @param timer is the [[Timer]] that's managing the thread-pool
* used to trigger this async boundary
*/
def shift(implicit timer: Timer[IO]): IO[Unit] =
timer.shift
/**
* Asynchronous boundary described as an effectful `IO`, managed
* by the provided Scala `ExecutionContext`.
*
* This operation can be used in `flatMap` chains to "shift" the
* continuation of the run-loop to another thread or call stack.
*
* $shiftDesc
*
* @param ec is the Scala `ExecutionContext` that's managing the
* thread-pool used to trigger this async boundary
*/
def shift(ec: ExecutionContext): IO[Unit] = {
IO.Async { (_, cb: Either[Throwable, Unit] => Unit) =>
ec.execute(new Runnable {
def run() = cb(Callback.rightUnit)
})
}
}
/**
* Creates an asynchronous task that on evaluation sleeps for the
* specified duration, emitting a notification on completion.
*
* This is the pure, non-blocking equivalent to:
*
* - `Thread.sleep` (JVM)
* - `ScheduledExecutorService.schedule` (JVM)
* - `setTimeout` (JavaScript)
*
* Similar with [[IO.shift(implicit* IO.shift]], you can combine it
* via `flatMap` to create delayed tasks:
*
* {{{
* val timeout = IO.sleep(10.seconds).flatMap { _ =>
* IO.raiseError(new TimeoutException)
* }
* }}}
*
* This operation creates an asynchronous boundary, even if the
* specified duration is zero, so you can count on this equivalence:
*
* {{{
* IO.sleep(Duration.Zero) <-> IO.shift
* }}}
*
* The created task is cancelable and so it can be used safely in race
* conditions without resource leakage.
*
* @param duration is the time span to wait before emitting the tick
*