djspiewak / cats-effect

The IO monad for cats


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For when purity just isn't impure enough.

This project aims to provide a standard IO type for the cats ecosystem, as well as a set of typeclasses (and associated laws!) which characterize general effect types. This project was explicitly designed with the constraints of the JVM and of JavaScript in mind. Critically, this means two things:

  • Manages both synchronous and asynchronous (callback-driven) effects
  • Compatible with a single-threaded runtime

In this way, IO is more similar to common Task implementations than it is to the classic scalaz.effect.IO or even Haskell's IO, both of which are purely synchronous in nature. As Haskell's runtime uses green threading, a synchronous IO (and the requisite thread blocking) makes a lot of sense. With Scala though, we're either on a runtime with native threads (the JVM) or only a single thread (JavaScript), meaning that asynchronous effects are every bit as important as synchronous ones.

This project does not attempt to provide any tools for concurrency or parallelism. The only function which does any sort of thread or thread-pool manipulation of any sort is shift (on both IO and Effect), which takes a given computation and shifts it to a different thread pool. This function mostly exists because it turns out to be somewhat difficult to do useful things with IO if you don't have this function. At any rate, it is impossible to define safe and practical concurrent primitives solely in terms of IO. If you want concurrency, you should use a streaming framework like fs2 or Monix. Note that both of these frameworks are, at least conceptually, compatible with IO.


The most current stable release of cats-effect is 0.5. We are confident in the quality of this release, and do consider it "production-ready". However, we will not be guaranteeing source compatibility until the 1.0 release, which will depend on cats-core 1.0 (when it is released). See compatibility and versioning for more information on our compatibility and semantic versioning policies.

libraryDependencies += "org.typelevel" %% "cats-effect" % "0.5"

If your project uses Scala.js, replace the double-% with a triple. Note that cats-effect has an upstream dependency on cats-core version 1.0.0-RC1.

Cross-builds are available for Scala 2.12, 2.11 and 2.10, Scala.js major version 0.6.

The most current snapshot (or major release) can be found in the maven badge at the top of this readme. If you are a very brave sort, you are free to depend on snapshots; they are stable versions, as they are derived from the git hash rather than an unstable -SNAPSHOT suffix, but they do not come with any particular confidence or compatibility guarantees.

Please see this document for information on how to cryptographically verify the integrity of cats-effect releases. You should absolutely be doing this! It takes five minutes and eliminates the need to trust a third-party with your classpath.


The cats-effect-laws artifact provides Discipline-style laws for the Async, Sync and Effect typeclasses (LiftIO is lawless, but highly parametric). It is relatively easy to use these laws to test your own implementations of these typeclasses. For an example of this, see IOTests.scala.

libraryDependencies += "org.typelevel" %% "cats-effect-laws" % "0.5" % "test"

These laws are compatible with both Specs2 and ScalaTest.


Most of the API documentation can be found in the scaladoc. To summarize though, the typeclass hierarchy looks something like this:

cats-effect typeclasses

All of the typeclasses are of kind (* -> *) -> *, as you would expect. MonadError is of course provided by cats-core, while the other four classes are in cats-effect. For concision and reference, the abstract methods of each typeclass are given below:

  • Sync[F]
    • def suspend[A](thunk: => F[A]): F[A]
  • Async[F]
    • def async[A](k: (Either[Throwable, A] => Unit) => Unit): F[A]
  • LiftIO[F]
    • def liftIO[A](ioa: IO[A]): F[A]
  • Effect[F]
    • def runAsync[A](fa: F[A])(cb: Either[Throwable, A] => IO[Unit]): IO[Unit]

The runAsync function is of particular interest here, since it returns the concrete type IO[Unit]. Effectively, what this type is asserting is that all effect-ish things must be able to asynchronous interpret into IO, which is the canonical parametric type for managing side-effecting computation. IO in turn has several functions for (unsafely!) running its constituent actions as side effects, for interoperability with legacy effectful APIs and for ending the world. Of course, concrete Effect implementations are free to define their own unsafe runner functions, and we expect that most of them will do exactly this.

Really, this type signature is saying that, ultimately, IO is side effects and side effects are IO, especially when taken together with the liftIO function.

JavaScript and unsafeRunSync()

One of the trickiest functions to design from the perspective of IO is the unsafeRunSync() function, which has the very revealing (and terrifying) type signature IO[A] => A. This function is extremely convenient for testing, as well as simplifying interoperability with legacy side-effecting APIs. Unfortunately, it is also impossible to implement safely on JavaScript.

The reason for this is the presence of async actions. For any IO constructed with the async function, we have to somehow extract the A (or the Throwable) which is received by the callback and move that value (or exception) back to the call-site for unsafeRunSync(). This is exactly as impossible as it sounds. On the JVM, we can use a CountDownLatch to simply block the thread, hoping that the async callback will eventually get fired and we will receive a result. Unfortunately, on JavaScript, this would be impossible since there is only one thread! If that thread is blocked awaiting a callback, then there is no thread on which to fire the callback. So we would have a deadlock.

IO solves this problem by providing users with a lawful implementation of runAsync. On a generic Effect, runAsync provides a means for asynchronously interpreting the effect into IO, a concrete type which can be run directly. On IO, runAsync provides a means for asynchronously interpreting IO into another IO which is guaranteed to be synchronous. Thus, it is safe – even on JavaScript! – to call unsafeRunSync() on the IO which results from runAsync. Though, notably, runAsync returns an IO[Unit] rather than an IO. So really calling runAsync on IO followed by unsafeRunSync() is the same as calling unsafeRunAsync.

It would be quite weird if this were not the case.

Testing and Timeouts

One of the four unsafe functions provided by IO is unsafeRunTimed. This function takes a scala.concurrent.duration.Duration and uses that value to set a timeout when blocking for an asynchronous result. Critically, this timeout does not come into play if the IO is entirely synchronous! Nor is it fully used when the IO is partially synchronous, and partially asynchronous (constructed via flatMap). Thus, the name is slightly misleading: it is not a timeout on the overall IO, it is a timeout on the thread blocking associated with awaiting any async action contained within the IO.

This function is useful for testing, and no where else! It is not an appropriate function to use for implementing timeouts. In fact, if you find this function anywhere in your production code path, you have a bug. But it is very useful for testing.

If the timeout is hit, the function will return None.


IO is stack-safe… to a point. Any IO actions which are synchronous are entirely stack-safe. In other words it is safe to flatMap, attempt, and otherwise futz with synchronous IO inside of loop-ish constructs, and the interpretation of that IO will use constant stack. The implementation of tailRecM, from the cats Monad, reflects this fact: it just delegates to flatMap!

However, there's a catch: any IO constructed with async will not be stack-safe. This is because async is fundamentally capturing a callback, and as the JVM does not support tail call elimination, it will always result in a new stack frame. This problem is best illustrated with a unit test:

// this is expected behavior
test("fail to provide stack safety with repeated async suspensions") {
  val result = (0 until 10000).foldLeft(IO(0)) { (acc, i) =>
    acc.flatMap(n => IO.async[Int](_(Right(n + 1))))

  intercept[StackOverflowError] {
    result.unsafeRunAsync(_ => ())

What we're doing here is constructing 10000 IO instances using async, each nested within the flatMap of the previous one. This will indeed blow the stack on any JVM with default parameters. This is obviously not an issue for JavaScript, though this test does appear to use an enormous amount of memory on V8 (around 4 GB).

Anyway, this stack unsafety is by design. Realistically, the only way to avoid this problem would be to thread-shift (via ExecutionContext) the results of any callback threaded through async. This would effectively reset the stack, avoiding the uneliminated tail call problem. However, this is not always what you want. Some frameworks, such as Netty, can provide a performance benefit to keeping short continuations on the event dispatch pool, rather than thread-shifting back to the application main pool. Other frameworks, such as SWT, outright require that callback continuations remain on the calling thread, and will throw exceptions if you attempt otherwise.

As IO is attempting to not be opinionated about applications or threading in general, it simply chooses not to thread-shift async. If you need thread-shifting behavior, it is relatively easy to implement yourself, or you can simply use the built-in shift function. The shift function takes an implicit ExecutionContext and moves as much of the IO as possible (its synchronous prefix and its asynchronous continuation) onto that thread pool. If we were to modify the above test by inserting a call to shift anywhere in the body of the fold, the resulting IO would be stack-safe.


We use the standard pull request driven github workflow. Pull requests are always welcome, even if it's for something as minor as a whitespace tweak! If you're a maintainer, you are expected to do your work in pull requests, rather than pushing directly to master. Ideally, someone other than yourself will merge and push your PR to master. However, if you've received at least one explicit 👍 from another maintainer (or significant volume of 👍 from the general cats community), you may merge your own PR in the interest of moving forward with important efforts. Please don't abuse this policy.

Do not rebase commits that have been PR'd! That history doesn't belong to you anymore, and it is not yours to rewrite. This goes for maintainers and contributors alike. Rebasing locally is completely fine (and encouraged), since linear history is pretty and checkpoint commits are not. Just don't rebase something that's already out there unless you've explicitly marked it as a work in progress (e.g. [WIP]) in some clear and unambiguous way.

cats-effect is a Typelevel project. This means we embrace pure, typeful, functional programming, and provide a safe and friendly environment for teaching, learning, and contributing as described in the Typelevel Code of Conduct.