typelevel / scalacheck-effect

Effectful property testing built on ScalaCheck

Version Matrix

ScalaCheck Effect

Continuous Integration Gitter Chat Latest version

ScalaCheck Effect is a library that extends the functionality of ScalaCheck to support "effectful" properties. An effectful property is one that evaluates each sample in some type constructor F[_]. For example:

import org.scalacheck.effect.PropF
import org.scalacheck.Test
import cats.effect.{ExitCode, IO, IOApp}

object Example extends IOApp {
  def run(args: List[String]): IO[ExitCode] = {
    val p: PropF[IO] = 
      PropF.forAllF { (x: Int) =>
        IO(x).start.flatMap(_.join).map(res => assert(res == x))

    val result: IO[Test.Result] = p.check()

    result.flatMap(r => IO(println(r))).as(ExitCode.Success)

Running this program results in the output: Result(Passed,100,0,Map(),0).

This library provides the org.scalacheck.effect.PropF type, which is the effectul analog to org.scalacheck.Prop. In this example, we use PropF.forAllF to write a property of the shape Int => IO[Unit]. This example uses cats.effect.IO as the type constructor, but any effect F[_] with an instance of MonadError[F, Throwable] can be used, including scala.concurrent.Future.

The key idea here is using the PropF.{forAllF, forAllNoShrinkF} methods to create PropF[F] instances. The check() method on PropF converts a PropF[F] to a F[Test.Result].

sbt dependency

libraryDependencies += "org.typelevel" %% "scalacheck-effect" % scalacheckEffectVersion

MUnit Integration

This project also provides support for checking PropF values from within MUnit based test suites.

import munit.{CatsEffectSuite, ScalaCheckEffectSuite}
import org.scalacheck.effect.PropF

// Example uses https://github.com/typelevel/munit-cats-effect

class ExampleSuite extends ScalaCheckEffectSuite with CatsEffectSuite {
  test("first PropF test") {
    PropF.forAllF { (x: Int) =>
      IO(x).start.flatMap(_.join).map(res => assert(res == x))

Design Goals

  • Support effectful properties without blocking.
  • Compatibility with Gen/Cogen/Arbitrary.
  • Parity with Prop features, including shrinking.
  • Follow same style as ScalaCheck and use ScalaCheck reporting.
  • Integrate well with popular test frameworks.
  • Non-goal: provide direct support for checking effectful properties directly from SBT or from standalone app.

Frequently Asked Questions

Why not just call Await.result / unsafeRunSync inside a property definition?

Calling Await.result, unsafeRunSync() or a similar blocking operation is not possible on Scala.js.


This library builds heavily on the ideas in ScalaCheck. It grew out of the FS2 AsyncPropertySuite, which only implemented a handful of features. The Weaver Test framework also has similar support for effectful properties. Finally, the Scala Hedgehog library has a prototype of similar functionality.