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munit-cats-effect Continuous Integration Maven Central

Integration library for MUnit and cats-effect.


For versions 2.0.0-M1 and above:

libraryDependencies += "org.typelevel" %%% "munit-cats-effect" % version % "test"

Please note that only Cats Effect 3 is supported for versions 2.0.0-M1 and above.

For versions 1.0.7 and below:

Cats Effect 2 integration is provided via:

libraryDependencies += "org.typelevel" %%% "munit-cats-effect-2" % version % "test"

Cats Effect 3 integration is provided via:

libraryDependencies += "org.typelevel" %%% "munit-cats-effect-3" % version % "test"

Builds are available for Scala 2.12, 2.13, and 3 for both the JVM and Scala.js.

Getting Started

The munit.CatsEffectSuite trait provides the ability to write tests that return IO and SyncIO values without needing to call any unsafe methods (e.g. unsafeRunSync()).

import cats.effect.{IO, SyncIO}
import munit.CatsEffectSuite

class ExampleSuite extends CatsEffectSuite {

  test("tests can return IO[Unit] with assertions expressed via a map") {
    IO(42).map(it => assertEquals(it, 42))

  test("alternatively, assertions can be written via assertIO") {
    assertIO(IO(42), 42)

  test("or via assertEquals syntax") {

  test("or via plain assert syntax on IO[Boolean]") {

  test("SyncIO works too") {

  import cats.effect.std.Dispatcher

  val dispatcher = ResourceFixture(Dispatcher.parallel[IO])

  dispatcher.test("resources can be lifted to munit fixtures") { dsp =>

There are more assertion functions like interceptIO and interceptMessageIO as well as syntax versions intercept and interceptMessage. See the CatsEffectAssertions trait for full details.

Every assertion in CatsEffectAssertions for IO-value or SyncIO-value is an IO computation under the hood. If you are planning to use multiple assertions per one test suite, therefore, they should be composed. Otherwise will calculate only the last assertion.

import cats.syntax.all._
import cats.effect.{IO, SyncIO}
import munit.CatsEffectSuite

class MultipleAssertionsExampleSuite extends CatsEffectSuite {
  test("multiple IO-assertions should be composed") {
    assertIO(IO(42), 42) *>

  test("multiple IO-assertions should be composed via for-comprehension") {
    for {
      _ <- assertIO(IO(42), 42)
      _ <- assertIO_(IO.unit)
    } yield ()       

  test("multiple SyncIO-assertions should be composed") {
    assertSyncIO(SyncIO(42), 42) *>
  test("multiple SyncIO-assertions should be composed via for-comprehension") {
    for {
      _ <- assertSyncIO(SyncIO(42), 42)
      _ <- assertSyncIO_(SyncIO.unit)
    } yield ()       

Suite-local fixtures

MUnit supports reusable suite-local fixtures that are instantiated only once for the entire test suite. This is useful when an expensive resource (like an HTTP client) is needed for each test case but it is undesirable to allocate a new one each time.

import cats.effect.{IO, Resource}

class SuiteLocalExampleSuite extends CatsEffectSuite {

  val myFixture = ResourceSuiteLocalFixture(
    Resource.make(IO.unit)(_ => IO.unit)
  val tempFileFixture = ResourceSuitLocalFixture(

  override def munitFixtures = List(myFixture, tempFileFixture)

  test("first test") {

  test("second test") {
  test("third test") {
    IO(tempFileFixture()).flatMap { file =>


Notice that this integration is not pure; myFixture is mutated internally when the framework initializes the fixture, so the same reference that is used from test cases must be specified in munitFixtures. Otherwise an exception FixtureNotInstantiatedException will be thrown.