IO

A data type for encoding side effects as pure values, capable of expressing both synchronous and asynchronous computations.

Introduction

A value of type IO[A] is a computation which, when evaluated, can perform effects before returning a value of type A.

IO values are pure, immutable values and thus preserves 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
2. can end in either success or failure and in case of failure flatMap chains get short-circuited (IO implementing the algebra of MonadError)
3. can be canceled, but note this capability relies on the user to provide cancellation 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:

import cats.effect.IO

val ioa = IO { println("hey!") }

val program: IO[Unit] =
  for {
     _ <- ioa
     _ <- ioa
  } yield ()

program.unsafeRunSync()
//=> hey!
//=> hey!
()

The above example prints "hey!" twice, as the effect re-runs each time it is sequenced in the monadic chain.

On Referential Transparency and Lazy Evaluation

IO can suspend side effects and is thus a lazily evaluated data type, being many times compared with Future from the standard library and to understand the landscape in terms of the evaluation model (in Scala), consider this classification:

	Eager	Lazy
Synchronous	A	() => A
		Eval[A]
Asynchronous	(A => Unit) => Unit	() => (A => Unit) => Unit
	Future[A]	IO[A]

In comparison with Scala's Future, the IO data type preserves referential transparency even when dealing with side effects and is lazily evaluated. In an eager language like Scala, this is the difference between a result and the function producing it.

Similar with Future, with IO you can reason about the results of asynchronous processes, but due to its purity and laziness IO can be thought of as a specification (to be evaluated at the "end of the world"), yielding more control over the evaluation model and being more predictable, for example when dealing with sequencing vs parallelism, when composing multiple IOs or when dealing with failure.

Note laziness goes hand in hand with referential transparency. Consider this example:

for {
  _ <- addToGauge(32)
  _ <- addToGauge(32)
} yield ()

If we have referential transparency, we can rewrite that example as:

val task = addToGauge(32)

for {
  _ <- task
  _ <- task
} yield ()

This doesn't work with Future, but works with IO and this ability is essential for functional programming.

Stack Safety

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(a)
  }

IO implements all the typeclasses shown in the hierarchy. Therefore all those operations are available for IO, in addition to some others.

Describing Effects

IO is a potent abstraction that can efficiently describe multiple kinds of effects:

Pure Values — IO.pure & IO.unit

You can lift pure values into IO, yielding IO values that are "already evaluated", the following function being defined on IO's companion:

def pure[A](a: A): IO[A] = ???

Note that the given parameter is passed by value, not by name.

For example we can lift a number (pure value) into IO and compose it with another IO that wraps a side a effect in a safe manner, as nothing is going to be executed:

IO.pure(25).flatMap(n => IO(println(s"Number is: $n")))

It should be obvious that IO.pure cannot suspend side effects, because IO.pure is eagerly evaluated, with the given parameter being passed by value, so don't do this:

IO.pure(println("THIS IS WRONG!"))

In this case the println will trigger a side effect that is not suspended in IO and given this code that probably is not our intention.

IO.unit is simply an alias for IO.pure(()), being a reusable reference that you can use when an IO[Unit] value is required, but you don't need to trigger any other side effects:

val unit: IO[Unit] = IO.pure(())

Given IO[Unit] is so prevalent in Scala code, the Unit type itself being meant to signal completion of side effectful routines, this proves useful as a shortcut and as an optimization, since the same reference is returned.

Synchronous Effects — IO.apply

It's probably the most used builder and the equivalent of Sync[IO].delay, describing IO operations that can be evaluated immediately, on the current thread and call-stack:

def apply[A](body: => A): IO[A] = ???

Note the given parameter is passed ''by name'', its execution being "suspended" in the IO context.

An example would be reading / writing from / to the console, which on top of the JVM uses blocking I/O, so their execution is immediate:

def putStrLn(value: String) = IO(println(value))
val readLn = IO(scala.io.StdIn.readLine())

And then we can use that to model interactions with the console in a purely functional way:

for {
  _ <- putStrLn("What's your name?")
  n <- readLn
  _ <- putStrLn(s"Hello, $n!")
} yield ()

Asynchronous Effects — IO.async & IO.cancelable

IO can describe asynchronous processes via the IO.async and IO.cancelable builders.

IO.async is the operation that complies with the laws of Async#async (see Async) and can describe simple asynchronous processes that cannot be canceled, its signature being:

def async[A](k: (Either[Throwable, A] => Unit) => Unit): IO[A] = ???

The provided registration function injects a callback that you can use to signal either successful results (with Right(a)), or failures (with Left(error)). Users can trigger whatever asynchronous side effects are required, then use the injected callback to signal completion.

For example, you don't need to convert Scala's Future, because you already have a conversion operation defined in IO.fromFuture, however the code for converting a Future would be straightforward:

import scala.concurrent.{Future, ExecutionContext}
import scala.util.{Success, Failure}

def convert[A](fa: => Future[A])(implicit ec: ExecutionContext): IO[A] =
  IO.async { cb =>
    // This triggers evaluation of the by-name param and of onComplete, 
    // so it's OK to have side effects in this callback
    fa.onComplete {
      case Success(a) => cb(Right(a))
      case Failure(e) => cb(Left(e))
    }
  }

Cancelable Processes

For building cancelable IO tasks you need to use the IO.cancelable builder, this being compliant with Concurrent#cancelable (see Concurrent) and has this signature:

def cancelable[A](k: (Either[Throwable, A] => Unit) => IO[Unit]): IO[A] = ???

So it is similar with IO.async, but in that registration function the user is expected to provide an IO[Unit] that captures the required cancellation logic.

Important: cancellation is the ability to interrupt an IO task before completion, possibly releasing any acquired resources, useful in race conditions to prevent leaks.

As example suppose we want to describe a sleep operation that depends on Java's ScheduledExecutorService, delaying a tick for a certain time duration:

import java.util.concurrent.ScheduledExecutorService
import scala.concurrent.duration._

def delayedTick(d: FiniteDuration)
  (implicit sc: ScheduledExecutorService): IO[Unit] = {
 
  IO.cancelable { cb =>
    val r = new Runnable { def run() = cb(Right(())) }
    val f = sc.schedule(r, d.length, d.unit)
    
    // Returning the cancellation token needed to cancel 
    // the scheduling and release resources early
    IO(f.cancel(false)).void
  }
}

Note this delayed tick is already described by IO.sleep (via Timer), so you don't need to do it.

More on dealing with ''cancellation'' below.

IO.never

IO.never represents a non-terminating IO defined in terms of async, useful as shortcut and as a reusable reference:

val never: IO[Nothing] = IO.async(_ => ())

This is useful in order to use non-termination in certain cases, like race conditions. For example, given IO.race, we have these equivalences:

IO.race(lh, IO.never) <-> lh.map(Left(_))

IO.race(IO.never, rh) <-> rh.map(Right(_))

Deferred Execution — IO.defer (previously suspend)

The IO.defer builder has this equivalence:

IO.defer(f) <-> IO(f).flatten

So it is useful for suspending effects, but that defers the completion of the returned IO to some other reference. It's also useful for modeling stack safe, tail recursive loops:

import cats.effect.IO

def fib(n: Int, a: Long, b: Long): IO[Long] =
  IO.defer {
    if (n > 0)
      fib(n - 1, b, a + b)
    else
      IO.pure(a)
  }

Normally a function like this would eventually yield a stack overflow error on top of the JVM. By using IO.defer and doing all of those cycles using IO's run-loop, its evaluation is lazy and it's going to use constant memory. This would work with flatMap as well, of course, suspend being just nicer in this example.

We could describe this function using Scala's @tailrec mechanism, however by using IO we can also preserve fairness by inserting asynchronous boundaries:

import cats.effect._

def fib(n: Int, a: Long, b: Long)(implicit cs: ContextShift[IO]): IO[Long] =
  IO.defer {
    if (n == 0) IO.pure(a) else {
      val next = fib(n - 1, b, a + b)
      // Every 100 cycles, introduce a logical thread fork
      if (n % 100 == 0)
        cs.shift *> next
      else
        next
    }
  }

And now we have something more interesting than a @tailrec loop. As can be seen, IO allows very precise control over the evaluation.

Concurrency and Cancellation

IO can describe interruptible asynchronous processes. As an implementation detail:

1. not all IO tasks are cancelable. Cancellation status is only checked after asynchronous boundaries. It can be achieved in the following way:

• Building it with IO.cancelable, IO.async, IO.asyncF or IO.bracket
• Using IO.cancelBoundary or IO.shift

Note that the second point is the consequence of the first one and anything that involves those operations is also possible to cancel. It includes, but is not limited to waiting on Mvar.take, Mvar.put and Deferred.get.

We should also note that flatMap chains are only cancelable only if the chain happens after an asynchronous boundary. After an asynchronous boundary, cancellation checks are performed on every N flatMap. The value of N is hardcoded to 512.

Here is an example,

import cats.effect.{ContextShift, IO}
import scala.concurrent.ExecutionContext

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

def retryUntilRight[A, B](io: IO[Either[A, B]]): IO[B] = {
  io.flatMap {
    case Right(b) => IO.pure(b)
    case Left(_) => retryUntilRight(io)
  }
}

// non-terminating IO that is NOT cancelable
val notCancelable: IO[Int] = retryUntilRight(IO(Left(0)))

// non-terminating IO that is cancelable because there is an
// async boundary created by IO.shift before `flatMap` chain
val cancelable: IO[Int] = IO.shift *> retryUntilRight(IO(Left(0)))

2. IO tasks that are cancelable, usually become non-terminating on cancel

Also this might be a point of confusion for folks coming from Java and that expect the features of Thread.interrupt or of the old and deprecated Thread.stop:

IO cancellation does NOT work like that, as thread interruption in Java is inherently unsafe, unreliable and not portable!

Next subsections describe cancellation-related operations in more depth.

Building cancelable IO tasks

Cancelable IO tasks can be described via the IO.cancelable builder. The delayedTick example making use of the Java's ScheduledExecutorService was already given above, but to recap:

import java.util.concurrent.ScheduledExecutorService

import cats.effect.IO

import scala.concurrent.duration.FiniteDuration

def sleep(d: FiniteDuration)
  (implicit sc: ScheduledExecutorService): IO[Unit] = {

  IO.cancelable { cb => 
    val r = new Runnable { def run() = cb(Right(())) }
    val f = sc.schedule(r, d.length, d.unit)
    // Returning a function that can cancel our scheduling
    IO(f.cancel(false)).void
  }
}

Important: if you don't specify cancellation logic for a task, then the task is NOT cancelable. So for example, using Java's blocking I/O still:

import java.io._

import cats.effect.IO

import scala.concurrent.ExecutionContext
import scala.util.control.NonFatal

def unsafeFileToString(file: File) = {
  // Freaking Java :-)
  val in = new BufferedReader(
    new InputStreamReader(new FileInputStream(file), "utf-8"))
  
  try {
    // Uninterruptible loop
    val sb = new StringBuilder()
    var hasNext = true
    while (hasNext) {
      hasNext = false
      val line = in.readLine()
      if (line != null) {
        hasNext = true
        sb.append(line)
      }
    }
    sb.toString
  } finally {
    in.close()
  }
}

def readFile(file: File)(implicit ec: ExecutionContext) =
  IO.async[String] { cb =>
    ec.execute(() => {
      try {
        // Signal completion
        cb(Right(unsafeFileToString(file)))
      } catch {
        case NonFatal(e) =>
          cb(Left(e))
      }
    })
  }

This is obviously not cancelable and there's no magic that the IO implementation does to make that loop cancelable. No, we are not going to use Java's Thread.interrupt, because that would be unsafe and unreliable and besides, whatever the IO does has to be portable between platforms.

But there's a lot of flexibility in what can be done, including here. We could simply introduce a variable that changes to false, to be observed in that while loop:

import java.io.File
import java.util.concurrent.atomic.AtomicBoolean

import cats.effect.IO

import scala.concurrent.ExecutionContext
import scala.io.Source
import scala.util.control.NonFatal

def unsafeFileToString(file: File, isActive: AtomicBoolean) = {
  val sc = new StringBuilder
  val linesIterator = Source.fromFile(file).getLines()
  var hasNext = true
  while (hasNext && isActive.get) {
    sc.append(linesIterator.next())
    hasNext = linesIterator.hasNext
  }
  sc.toString
}

def readFile(file: File)(implicit ec: ExecutionContext) =
  IO.cancelable[String] { cb =>
    val isActive = new AtomicBoolean(true)
    
    ec.execute(() => {
      try {
        // Signal completion
        cb(Right(unsafeFileToString(file, isActive)))
      } catch {
        case NonFatal(e) =>
          cb(Left(e))
      }
    })    
    // On cancel, signal it
    IO(isActive.set(false)).void
  }

Gotcha: Cancellation is a Concurrent Action!

This is not always obvious, not from the above examples, but you might be tempted to do something like this:

import java.io._

import cats.effect.IO

import scala.concurrent.ExecutionContext
import scala.util.control.NonFatal

def readLine(in: BufferedReader)(implicit ec: ExecutionContext) =
  IO.cancelable[String] { cb =>
    ec.execute(() => cb(
      try Right(in.readLine()) 
      catch { case NonFatal(e) => Left(e) }))
      
    // Cancellation logic is not thread-safe!
    IO(in.close()).void
  }

An operation like this might be useful in streaming abstractions that stream I/O chunks via IO (via libraries like FS2, Monix, or others).

But the described operation is incorrect, because in.close() is concurrent with in.readLine, which can lead to thrown exceptions and in many cases it can lead to data corruption. This is a big no-no. We want to interrupt whatever it is that the IO is doing, but not at the cost of data corruption.

Therefore the user needs to handle thread safety concerns. So here's one way of doing it:

import java.io._
import java.util.concurrent.atomic.AtomicBoolean

import cats.effect.IO

import scala.util.control.NonFatal
import scala.concurrent.ExecutionContext

def readLine(in: BufferedReader)(implicit ec: ExecutionContext) =
  IO.cancelable[String] { cb =>
    val isActive = new AtomicBoolean(true)
    ec.execute { () => 
      if (isActive.getAndSet(false)) {
        try cb(Right(in.readLine()))
        catch { case NonFatal(e) => cb(Left(e)) }
      }
      // Note there's no else; if cancellation was executed
      // then we don't call the callback; task becoming 
      // non-terminating ;-)
    }
    // Cancellation logic
    IO {
      // Thread-safe gate
      if (isActive.getAndSet(false))
        in.close()
    }.void
  }

In this example it is the cancellation logic itself that calls in.close(), but the call is safe due to the thread-safe guard that we're creating by usage of an atomic getAndSet.

This is using an AtomicBoolean for thread-safety, but don't shy away from using intrinsic locks / mutexes via synchronize blocks or whatever else concurrency primitives the JVM provides, whatever is needed in these side effectful functions. And don't worry, this is usually needed only in IO.cancelable, IO.async or IO.apply, as these builders represents the FFI for interacting with the impure world, aka the dark side, otherwise once you're in IO's context, you can compose concurrent tasks using higher level tools.

Shared memory concurrency is unfortunately both the blessing and the curse of working with kernel threads. Not a big problem on N:1 platforms like JavaScript, but there you don't get in-process CPU parallelism either. Such is life, a big trail of tradeoffs.

Concurrent start + cancel

You can use IO as a green-threads system, with the "fork" operation being available via IO#start, the operation that's compliant with Concurrent#start. This is a method with the following signature:

def start: IO[Fiber[IO, A]]

Returned is a Fiber. You can think of fibers as being lightweight threads, a fiber being the pure and light equivalent of a thread that can be either joined (via join) or interrupted (via cancel).

Example:

import cats.effect.{ContextShift, IO}

import scala.concurrent.ExecutionContext

// Needed for IO.start to do a logical thread fork
implicit val cs: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

val launchMissiles: IO[Unit] = IO.raiseError(new Exception("boom!"))
val runToBunker = IO(println("To the bunker!!!"))

for {
  fiber <- 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

start is defined on IO with overloads accepting an implicit ContextShift or an explicit ExecutionContext, but it's also available via the Concurrent type class. To get an instance of Concurrent[IO], you need a ContextShift[IO] in implicit scope.

runCancelable & unsafeRunCancelable

The above is the pure cancel, accessible via Fiber. However the second way to access cancellation token and thus interrupt tasks is via runCancelable (the pure version) and unsafeRunCancelable (the unsafe version).

Example relying on the side-effecting unsafeRunCancelable and note this kind of code is impure and should be used with care:

import cats.effect.IO

import scala.concurrent.ExecutionContext
import scala.concurrent.duration._

// Needed for `sleep`
implicit val timer = IO.timer(ExecutionContext.global)

// Delayed println
val io: IO[Unit] = IO.sleep(10.seconds) *> IO(println("Hello!"))

val cancel: IO[Unit] = 
  io.unsafeRunCancelable(r => println(s"Done: $r"))

// ... if a race condition happens, we can cancel it,
// thus canceling the scheduling of `IO.sleep`
cancel.unsafeRunSync()

The runCancelable alternative is the operation that's compliant with the laws of ConcurrentEffect. Same idea, only the actual execution is suspended in SyncIO:

import cats.effect.SyncIO
import cats.syntax.flatMap._

val pureResult: SyncIO[IO[Unit]] = io.runCancelable { r => 
  IO(println(s"Done: $r"))
}

// On evaluation, this will first execute the source, then it 
// will cancel it, because it makes perfect sense :-)
pureResult.toIO.flatten

uncancelable marker

Given a cancelable IO, we can turn it into an IO that cannot be canceled:

import cats.effect.IO

import scala.concurrent.ExecutionContext
import scala.concurrent.duration._

// Needed for `sleep`
implicit val timer = IO.timer(ExecutionContext.global)

// Our reference from above
val io: IO[Unit] = IO.sleep(10.seconds) *> IO(println("Hello!"))

// This IO can't be canceled, even if we try
io.uncancelable

Sometimes you need to ensure that an IO's execution is atomic, or in other words, either all of it executes, or none of it. And this is what this operation does — cancelable IOs are by definition not atomic and in certain cases we need to make them atomic.

This law is compliant with the laws of Concurrent#uncancelable (see Concurrent).

IO.cancelBoundary

Returns a cancelable boundary — an IO task that checks for the cancellation status of the run-loop and does not allow for the bind continuation to keep executing in case cancellation happened.

This operation is very similar to IO.shift, as it can be dropped in flatMap chains in order to make such long loops cancelable:

import cats.effect.IO

def fib(n: Int, a: Long, b: Long): IO[Long] =
  IO.defer {
    if (n <= 0) IO.pure(a) else {
      val next = fib(n - 1, b, a + b)

      // Every 100-th cycle check cancellation status
      if (n % 100 == 0)
        IO.cancelBoundary *> next
      else
        next
    }
  }

As mentioned at the very beginning of this section, fairness needs to be managed explicitly, the protocol being easy to follow and predictable in a WYSIWYG fashion.

Comparison to IO.shift

IO.cancelBoundary is essentially lighter version of IO.shift without ability to shift into different thread pool. It is lighter in the sense that it will avoid doing logical fork.

Race Conditions — race & racePair

A race condition is a piece of logic that creates a race between two or more tasks, with the winner being signaled immediately, with the losers being usually canceled.

IO provides two operations for races in its companion:

// simple version
def race[A, B](lh: IO[A], rh: IO[B])
  (implicit cs: ContextShift[IO]): IO[Either[A, B]]
  
// advanced version
def racePair[A, B](lh: IO[A], rh: IO[B])
  (implicit cs: ContextShift[IO]): IO[Either[(A, Fiber[IO, B]), (Fiber[IO, A], B)]]

The simple version, IO.race, will cancel the loser immediately, whereas the second version gives you a Fiber, letting you decide what to do next.

So race can be derived with racePair like so:

import cats.effect.{ContextShift, IO}

def race[A, B](lh: IO[A], rh: IO[B])
  (implicit cs: ContextShift[IO]): IO[Either[A, B]] = {
  
  IO.racePair(lh, rh).flatMap {
    case Left((a, fiber)) => 
      fiber.cancel.map(_ => Left(a))
    case Right((fiber, b)) => 
      fiber.cancel.map(_ => Right(b))
  }
}

Using race we could implement a "timeout" operation:

import cats.effect.{ContextShift, Timer, IO}

import scala.concurrent.CancellationException
import scala.concurrent.duration.FiniteDuration

def timeoutTo[A](fa: IO[A], after: FiniteDuration, fallback: IO[A])
  (implicit timer: Timer[IO], cs: ContextShift[IO]): IO[A] = {

  IO.race(fa, timer.sleep(after)).flatMap {
    case Left(a) => IO.pure(a)
    case Right(_) => fallback
  }
}

def timeout[A](fa: IO[A], after: FiniteDuration)
  (implicit timer: Timer[IO], cs: ContextShift[IO]): IO[A] = {

  val error = new CancellationException(after.toString)
  timeoutTo(fa, after, IO.raiseError(error))
}

See Parallelism section above for how to obtain a Timer[IO]

Comparison with Haskell's "async interruption"

Haskell treats interruption with what they call "asynchronous exceptions", providing the ability to interrupt a running task by throwing an exception from another thread (concurrently).

For cats.effect, for the "cancel" action, what happens is that whatever you specify in the IO.cancelable builder gets executed. And depending on the implementation of an IO.cancelable task, it can become non-terminating. If we'd need to describe our cancel operation with an impure signature, it would be:

() => Unit

By comparison Haskell (and possibly the upcoming Scalaz 8 IO), sends an error, a Throwable on interruption and canceled tasks get completed with that Throwable. Their impure cancel is:

Throwable => Unit

Throwable => Unit allows the task's logic to know the cancellation reason, however cancellation is about cutting the connection to the producer, closing all resources as soon as possible, because you're no longer interested in the result, due to some race condition that happened.

Throwable => Unit is also a little confusing, being too broad in scope. Users might be tricked into sending messages back to the producer via this channel, in order to steer it, to change its outcome - however cancellation is cancellation, we're doing it for the purpose of releasing resources and the implementation of race conditions will end up closing the connection, disallowing the canceled task to send anything downstream.

Therefore it's confusing for the user and the only practical use is to release resources differently, based on the received error. But that's not a use-case that's worth pursuing, given the increase in complexity.

Safe Resource Acquisition and Release

Status Quo

In mainstream imperative languages you usually have try / finally statements at disposal for acquisition and safe release of resources. Pattern goes like this:

import java.io._

def javaReadFirstLine(file: File): String = {
  val in = new BufferedReader(new FileReader(file))
  try {
    in.readLine()
  } finally {
    in.close()
  }
}

It does have problems like:

1. this statement is obviously meant for side-effectful computations and can't be used by FP abstractions
2. it's only meant for synchronous execution, so we can't use it when working with abstractions capable of asynchrony (e.g. IO, Task, Future)
3. finally executes regardless of the exception type, indiscriminately, so if you get an out of memory error it still tries to close the file handle, unnecessarily delaying a process crash
4. if the body of try throws an exception, then followed by the body of finally also throwing an exception, then the exception of finally gets rethrown, hiding the original problem

bracket

Via the bracket operation we can easily describe the above:

import java.io._

import cats.effect.IO

def readFirstLine(file: File): IO[String] =
  IO(new BufferedReader(new FileReader(file))).bracket { in =>
    // Usage (the try block)
    IO(in.readLine())
  } { in =>
    // Releasing the reader (the finally block)
    IO(in.close()).void
  }

Notes:

1. this is pure, so it can be used for FP
2. this works with asynchronous IO actions
3. the release action will happen regardless of the exit status of the use action, so it will execute for successful completion, for thrown errors or for canceled execution
4. if the use action throws an error and then the release action throws an error as well, the reported error will be that of use, whereas the error thrown by release will just get logged (via System.err)

Of special consideration is that bracket calls the release action on cancellation as well. Consider this sample:

import java.io._

import cats.effect.{ContextShift, IO}

import scala.concurrent.ExecutionContext

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

def readFile(file: File): IO[String] = {
  // Opens file with an asynchronous boundary before it, 
  // ensuring that processing doesn't block the "current thread"
  val acquire = IO.shift *> IO(new BufferedReader(new FileReader(file)))
    
  acquire.bracket { in =>
    // Usage (the try block)
    IO {
      // Ugly, low-level Java code warning!
      val content = new StringBuilder()
      var line: String = null
      do {
        line = in.readLine()
        if (line != null) content.append(line)
      } while (line != null)
      content.toString()
    }
  } { in =>
    // Releasing the reader (the finally block)
    // This is problematic if the resulting `IO` can get 
    // canceled, because it can lead to data corruption
    IO(in.close()).void
  }
}

That loop can be slow, we could be talking about a big file and as described in the "Concurrency and Cancellation" section, cancellation is a concurrent action with whatever goes on in use.

And in this case, on top of the JVM that is capable of multi-threading, calling io.close() concurrently with that loop can lead to data corruption. Depending on use-case synchronization might be needed to prevent it:

import java.io._

import cats.effect.{ContextShift, IO}

import scala.concurrent.ExecutionContext

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

def readFile(file: File): IO[String] = {
  // Opens file with an asynchronous boundary before it, 
  // ensuring that processing doesn't block the "current thread"
  val acquire = IO.shift *> IO(new BufferedReader(new FileReader(file)))
    
  // Suspended execution because we are going to mutate 
  // a shared variable
  IO.defer {
    // Shared state meant to signal cancellation
    var isCanceled = false
    
    acquire.bracket { in =>
      IO {
        val content = new StringBuilder()
        var line: String = null
        do {
        // Synchronized access to isCanceled and to the reader
          line = in.synchronized {
            if (!isCanceled)
              in.readLine()
            else
              null
          }
          if (line != null) content.append(line)
        } while (line != null)
        content.toString()
      }
    } { in =>
      IO {
        // Synchronized access to isCanceled and to the reader
        in.synchronized {
          isCanceled = true
          in.close()
        }
      }.void
    }
  }
}

bracketCase

The bracketCase operation is the generalized bracket, also receiving an ExitCase in release in order to distinguish between:

1. successful completion
2. completion in error
3. cancellation

Usage sample:

import java.io.BufferedReader
import cats.effect.IO
import cats.effect.ExitCase.{Completed, Error, Canceled}

def readLine(in: BufferedReader): IO[String] =
  IO.pure(in).bracketCase { in =>
    IO(in.readLine())
  } { 
    case (_, Completed | Error(_)) =>
      // Do nothing
      IO.unit
    case (in, Canceled) =>
      IO(in.close())
  }

In this example we are only closing the passed resource in case cancellation occurred. As to why we're doing this — consider that the BufferedReader reference was given to us and usually the producer of such a resource should also be in charge of releasing it. If this function would release the given BufferedReader on a successful result, then this would be a flawed implementation.

Remember the age old C++ idiom of "resource acquisition is initialization (RAII)", which says that the lifetime of a resource should be tied to the lifetime of its parent.

But in case we detect cancellation, we might want to close that resource, because in the case of a cancellation event, we might not have a "run-loop" active after this IO returns its result, so there might not be anybody available to release it.

Conversions

There are two useful operations defined in the IO companion object to lift both a scala Future and an Either into IO.

fromFuture

Constructs an IO which evaluates the given Future and produces either a result or a failure. It is defined as follow:

import cats.effect.IO
import scala.concurrent.Future

def fromFuture[A](iof: IO[Future[A]]): IO[A] = ???

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.

Lazy evaluation, equivalent with by-name parameters:

import cats.effect.{ContextShift, IO}

import scala.concurrent.Future
import scala.concurrent.ExecutionContext
import ExecutionContext.Implicits.global

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

IO.fromFuture(IO {
  Future(println("I come from the Future!"))
})

Eager evaluation:

val f = Future.successful("I come from the Future!")

IO.fromFuture(IO.pure(f))

fromEither

Lifts an Either[Throwable, A] into the IO[A] context raising the throwable if it exists.

import cats.effect.IO

def fromEither[A](e: Either[Throwable, A]): IO[A] = e.fold(IO.raiseError, IO.pure)

Error Handling

Since there is an instance of MonadError[IO, Throwable] available in Cats Effect, all the error handling is done through it. This means you can use all the operations available for MonadError and thus for ApplicativeError on IO as long as the error type is a Throwable. Operations such as raiseError, attempt, handleErrorWith, recoverWith, etc. Just make sure you have the syntax import in scope such as cats.syntax.all._.

raiseError

Constructs an IO which sequences the specified exception.

import cats.effect.IO

val boom: IO[Unit] = IO.raiseError(new Exception("boom"))
boom.unsafeRunSync()

attempt

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. Example:

import cats.effect.IO

val boom: IO[Unit] = IO.raiseError(new Exception("boom"))
boom.attempt.unsafeRunSync()

Look at the MonadError typeclass for more.

Example: Retrying with Exponential Backoff

With IO you can easily model a loop that retries evaluation until success or some other condition is met.

For example here's a way to implement retries with exponential back-off:

import cats.effect.{IO, Timer}

import scala.concurrent.duration._

def retryWithBackoff[A](ioa: IO[A], initialDelay: FiniteDuration, maxRetries: Int)
  (implicit timer: Timer[IO]): IO[A] = {

  ioa.handleErrorWith { error =>
    if (maxRetries > 0)
      IO.sleep(initialDelay) *> retryWithBackoff(ioa, initialDelay * 2, maxRetries - 1)
    else
      IO.raiseError(error)
  }
}

Thread Shifting

IO provides a function shift to give you more control over the execution of your operations.

shift

Note there are 2 overloads of the IO.shift function:

• One that takes a ContextShift that manages the thread-pool used to trigger async boundaries.
• Another that takes a Scala ExecutionContext as the thread-pool.

Please use the former by default and use the latter only for fine-grained control over the thread pool in use.

By default, Cats Effect provides an instance of ContextShift[IO] that manages thread-pools, but only inside an implementation of IOApp. Custom instances of ContextShift[IO] can be created using an ExecutionContext:

import cats.effect.{ContextShift, IO}
import scala.concurrent.ExecutionContext.Implicits.global

implicit val contextShift: ContextShift[IO] = IO.contextShift(global)

We can introduce an asynchronous boundary in the flatMap chain before a certain task:

val task = IO(println("task"))

IO.shift(contextShift).flatMap(_ => task)

Note that the ContextShift value is taken implicitly from the context so you can just do this:

IO.shift.flatMap(_ => task)

Or using Cats syntax:

IO.shift *> task
// equivalent to
implicitly[ContextShift[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
// equivalent to
task <* implicitly[ContextShift[IO]].shift

Example of where this might be useful:

import java.util.concurrent.Executors

import cats.effect.IO
import scala.concurrent.ExecutionContext

val cachedThreadPool = Executors.newCachedThreadPool()
val BlockingFileIO   = ExecutionContext.fromExecutor(cachedThreadPool)
implicit val Main = ExecutionContext.global

val ioa: IO[Unit] =
  for {
    _     <- IO(println("Enter your name: "))
    _     <- IO.shift(BlockingFileIO)
    name  <- IO(scala.io.StdIn.readLine())
    _     <- IO.shift(Main)
    _     <- IO(println(s"Welcome $name!"))
    _     <- IO(cachedThreadPool.shutdown())
  } yield ()

We start by asking the user to enter its name and next we thread-shift to the BlockingFileIO execution context because we expect the following action to block on the thread for a long time and we don't want that to happen in the main thread of execution. After the expensive IO operation (readLine) gets back with a response we thread-shift back to the main execution context defined as an implicit value, and finally the program ends by showing a message in the console and shutting down a thread pool, all actions run in the main execution context.

Another somewhat less common application of shift is to reset the thread stack and yield control back to the underlying pool. For example:

import cats.effect.{ContextShift, IO}
import scala.concurrent.ExecutionContext

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

lazy val doStuff = IO(println("stuff"))

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. Although the thread-shifting is not completely necessary, it might help in some cases to alleviate the use of the main thread pool.

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.
• Preventing an overflow of the call stack in the case of improperly constructed async actions.

IO is trampolined for all synchronous and asynchronous joins. This means that you can safely call flatMap in a recursive function of arbitrary depth, without fear of blowing the stack. So you can do this for example:

import cats.effect.IO

def signal[A](a: A): IO[A] = IO.async(_(Right(a)))

def loop(n: Int): IO[Int] =
  signal(n).flatMap { x =>
    if (x > 0) loop(n - 1) else IO.pure(0)
  }

Parallelism

Since the introduction of the Parallel typeclasss in the Cats library and its IO instance, it became possible to execute two or more given IOs in parallel.

Note: all parallel operations require an implicit ContextShift[IO] in scope (see ContextShift). You have a ContextShift in scope if:

1. via usage of IOApp that gives you a ContextShift by default
2. the user provides a custom ContextShift, which can be created using IO.contextShift(executionContext)

parMapN

It has the potential to run an arbitrary number of IOs in parallel, and it allows you to apply a function to the result (as in map). It finishes processing when all the IOs are completed, either successfully or with a failure. For example:

import cats.effect.{ContextShift, IO}
import cats.syntax.all._

import scala.concurrent.ExecutionContext

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

val ioA = IO(println("Running ioA"))
val ioB = IO(println("Running ioB"))
val ioC = IO(println("Running ioC"))

// make sure that you have an implicit ContextShift[IO] in scope. 
val program = (ioA, ioB, ioC).parMapN { (_, _, _) => () }

program.unsafeRunSync()
//=> Running ioB
//=> Running ioC
//=> Running ioA
()

If any of the IOs completes with a failure then the result of the whole computation will be failed, while the unfinished tasks get cancelled. Example:

import cats.effect.{ContextShift, ExitCase, IO}
import cats.syntax.all._

import scala.concurrent.ExecutionContext
import scala.concurrent.duration._

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)
implicit val timer = IO.timer(ExecutionContext.global)

val a = IO.raiseError[Unit](new Exception("boom")) <* IO(println("Running ioA"))
val b = (IO.sleep(1.second) *> IO(println("Running ioB")))
  .guaranteeCase {
    case ExitCase.Canceled => IO(println("ioB was canceled!"))
    case _ => IO.unit
  }

val parFailure = (a, b).parMapN { (_, _) => () }

parFailure.attempt.unsafeRunSync()
//=> ioB was canceled!
//=> java.lang.Exception: boom
//=>  ... 43 elided
()

If one of the tasks fails immediately, then the other gets canceled and the computation completes immediately, so in this example the pairing via parMapN will not wait for 10 seconds before emitting the error:

import cats.effect.{ContextShift, Timer, IO}
import cats.syntax.all._

import scala.concurrent.ExecutionContext
import scala.concurrent.duration._

implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)
implicit val timer: Timer[IO] = IO.timer(ExecutionContext.global)

val ioA = IO.sleep(10.seconds) *> IO(println("Delayed!"))
val ioB = IO.raiseError[Unit](new Exception("dummy"))

(ioA, ioB).parMapN((_, _) => ())

parSequence

If you have a list of IO, and you want a single IO with the result list you can use parSequence which executes the IO tasks in parallel.

import cats.data.NonEmptyList
import cats.effect.{ContextShift, Timer, IO}
import cats.syntax.parallel._

import scala.concurrent.ExecutionContext 

// Needed for IO.start to do a logical thread fork
implicit val cs: ContextShift[IO] = IO.contextShift(ExecutionContext.global)
implicit val timer: Timer[IO] = IO.timer(ExecutionContext.global)

val anIO = IO(1)

val aLotOfIOs = 
  NonEmptyList.of(anIO, anIO)

val ioOfList = aLotOfIOs.parSequence

There is also cats.Traverse.sequence which does this synchronously.

parTraverse

If you have a list of data and a way of turning each item into an IO, but you want a single IO for the results you can use parTraverse to run the steps in parallel.

import cats.data.NonEmptyList
import cats.effect.{ContextShift, Timer, IO}
import cats.syntax.parallel._

import scala.concurrent.ExecutionContext 

// Needed for IO.start to do a logical thread fork
implicit val cs: ContextShift[IO] = IO.contextShift(ExecutionContext.global)
implicit val timer: Timer[IO] = IO.timer(ExecutionContext.global)

val results = NonEmptyList.of(1, 2, 3).parTraverse { i =>
  IO(i)
}

There is also cats.Traverse.traverse which will run each step synchronously.

"Unsafe" Operations

We have been using some "unsafe" operations pretty much everywhere in the previous examples but we never explained any of them, so here it goes. All of the operations prefixed with unsafe are impure functions and perform side effects (for example Haskell has unsafePerformIO). But don't be scared by the name! You should write your programs in a monadic way using functions such as map and flatMap to compose other functions and ideally you should just call one of these unsafe operations only once, at the very end of your program.

unsafeRunSync

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.

IO(println("Sync!")).unsafeRunSync()
// Sync!

unsafeRunAsync

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.

IO(println("Async!")).unsafeRunAsync(_ => ())
// Async!

unsafeRunCancelable

Evaluates the source IO, passing the result of the encapsulated effects to the given callback. Note that this has the potential to be interrupted.

IO(println("Potentially cancelable!")).unsafeRunCancelable(_ => ())
// Potentially cancelable!
// res59: cats.effect.package.CancelToken[IO] = Suspend(
//   thunk = cats.effect.internals.IOConnection$Impl$$Lambda$11656/0x00000001030e7040@24ab8e87,
//   trace = StackTrace(
//     stackTrace = List(
//       cats.effect.internals.IOTracing$.buildFrame(IOTracing.scala:48),
//       cats.effect.internals.IOTracing$.buildCachedFrame(IOTracing.scala:39),
//       cats.effect.internals.IOTracing$.cached(IOTracing.scala:34),
//       cats.effect.IO$.defer(IO.scala:1157),
//       cats.effect.internals.IOConnection$Impl.<init>(IOConnection.scala:103),
//       cats.effect.internals.IOConnection$.apply(IOConnection.scala:81),
//       cats.effect.IO.unsafeRunCancelable(IO.scala:304),
//       repl.MdocSession$App15.<init>(io.md:468),
//       repl.MdocSession$App13.<init>(io.md:446),
//       repl.MdocSession$App12.<init>(io.md:414),
//       repl.MdocSession$App11.<init>(io.md:372),
//       repl.MdocSession$App10.<init>(io.md:343),
//       repl.MdocSession$App9.<init>(io.md:289),
//       repl.MdocSession$App8.<init>(io.md:233),
//       repl.MdocSession$App7.<init>(io.md:206),
//       repl.MdocSession$App6.<init>(io.md:177),
//       repl.MdocSession$App5.<init>(io.md:155),
//       repl.MdocSession$App.<init>(io.md:137),
//       repl.MdocSession$.app(io.md:3),
//       mdoc.internal.document.DocumentBuilder$$doc$.$anonfun$build$2(DocumentBuilder.scala:89),
//       scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18),
//       scala.util.DynamicVariable.withValue(DynamicVariable.scala:59),
//       scala.Console$.withErr(Console.scala:193),
//       mdoc.internal.document.DocumentBuilder$$doc$.$anonfun$build$1(DocumentBuilder.scala:89),
//       scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18),
//       scala.util.DynamicVariable.withValue(DynamicVariable.scala:59),
//       scala.Console$.withOut(Console.scala:164),
//       mdoc.internal.document.DocumentBuilder$$doc$.build(DocumentBuilder.scala:88),
//       mdoc.internal.markdown.MarkdownBuilder$.$anonfun$buildDocument$2(MarkdownBuilder.scala:47),
//       mdoc.internal.markdown.MarkdownBuilder$$anon$1.run(MarkdownBuilder.scala:104)
//     )
//   )
// )

unsafeRunTimed

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 purposes; 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).

import scala.concurrent.duration._

IO(println("Timed!")).unsafeRunTimed(5.seconds)

unsafeToFuture

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.

IO("Gimme a Future!").unsafeToFuture()

Best Practices

This section presents some best practices for working with IO:

Keep Granularity

It's better to keep the granularity, so please don't do something like this:

IO {
  readingFile
  writingToDatabase
  sendBytesOverTcp
  launchMissiles
}

In FP we embrace reasoning about our programs and since IO is a Monad you can compose bigger programs from small ones in a for-comprehension. For example:

val program =
  for {
    data <- readFile
    _    <- writeToDatabase(data)
    _    <- sendBytesOverTcp(data)
    _    <- launchMissiles
  } yield ()

Each step of the comprehension is a small program, and the resulting program is a composition of all those small steps, which is compositional with other programs. IO values compose.

Use pure functions in map / flatMap

When using map or flatMap it is not recommended to pass a side effectful function, as mapping functions should also be pure. So this should be avoided:

IO.pure(123).map(n => println(s"NOT RECOMMENDED! $n"))

This too should be avoided, because the side effect is not suspended in the returned IO value:

IO.pure(123).flatMap { n =>
  println(s"NOT RECOMMENDED! $n")
  IO.unit
}

The correct approach would be this:

IO.pure(123).flatMap { n =>
  // Properly suspending the side effect
  IO(println(s"RECOMMENDED! $n"))
}

Note that as far as the actual behavior of IO is concerned, something like IO.pure(x).map(f) is equivalent with IO(f(x)) and IO.pure(x).flatMap(f) is equivalent with IO.defer(f(x)).

But you should not rely on this behavior, because it is NOT described by the laws required by the Sync type class and those laws are the only guarantees of behavior that you get. For example the above equivalence might be broken in the future in regards to error handling. So this behavior is currently there for safety reasons, but you should regard it as an implementation detail that could change in the future.

Stick with pure functions.