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ScalaCheck

MUnit supports writing property-based tests using ScalaCheck.

Getting started

ScalaCheck support is provided as a separate module. You can add it to your build via:

libraryDependencies += "org.scalameta" %% "munit-scalacheck" % "0.7.1" % Test

You can then extend ScalaCheckSuite and write ScalaCheck property tests:

import munit.ScalaCheckSuite
import org.scalacheck.Prop._

class IntegerSuite extends ScalaCheckSuite {

  property("addition is commutative") {
    forAll { (n1: Int, n2: Int) =>
      n1 + n2 == n2 + n1
    }
  }

  property("0 is the identity of addition") {
    forAll { (n: Int) =>
      n + 0 == n
    }
  }

}

👉 property is almost identical to FunSuite's test, but it additionally ensures that the test value is a ScalaCheck Prop.

It supports all of test's features, such as tagging and marking the property as expected to fail:

property("my property".tag(WindowsOnly)) {
  forAll { (n: Int) => n * 0 == 0 }
}

property("issue-123".fail) {
  forAll { (n: Int) => buggyFunction(n) }
}

Using assertions

ScalaCheck supports writing property checks as boolean expressions (as in the examples above).

In MUnit you can also use assertions to verify properties:

import munit.ScalaCheckSuite
import org.scalacheck.Prop._

class IntegerSuite extends ScalaCheckSuite {

  property("integer identities") {
    forAll { (n: Int) =>
      assertEquals(n + 0, n)
      assertEquals(n * 1, n)
    }
  }

}

The property above is equivalent to

property("integer identities") {
  forAll { (n: Int) =>
    (n + 0 == n) && (n * 1 == n)
  }
}

but it provides better error reporting by including the source location of the failed assertion, which is not possible when using boolean expressions.

Configuring ScalaCheck

You can override the scalaCheckTestParameters property of ScalaCheckSuite to customize how ScalaCheck checks the properties:

import munit.ScalaCheckSuite
import org.scalacheck.Prop._

class IntegerSuite extends ScalaCheckSuite {

  override def scalaCheckTestParameters =
    super.scalaCheckTestParameters
      .withMinSuccessfulTests(200)
      .withMaxDiscardRatio(10)

  property("addition is commutative") {
    forAll { (n1: Int, n2: Int) =>
      n1 + n2 == n2 + n1
    }
  }

}

Migrating from ScalaTest

ScalaTest provides two styles for writing property-based tests, which are both front-ends to ScalaCheck.

If you are using the "ScalaCheck style" API, i.e. the Checkers trait, switching to MUnit requires minor mechanical changes:

-import org.scalatest.prop.Checkers
-import org.scalatest.FunSuite
+import munit.ScalaCheckSuite
 import org.scalacheck.Prop._

-class IntegerSuite extends FunSuite with Checkers {
+class IntegerSuite extends ScalaCheckSuite {

-  test("addition is commutative") {
+  property("addition is commutative") {
-    check { (n1: Int, n2: Int) =>
+    forAll { (n1: Int, n2: Int) =>
       n1 + n2 == n2 + n1
     }
   }

}

If you're using the "ScalaTest style" API, i.e. the GeneratorDrivenPropertyChecks trait, switching to MUnit requires again a few changes, typically including the property body.

For example, if you are using ScalaTest matchers you will need to convert them to MUnit assertions:

-import org.scalatest.Matchers
-import org.scalatest.prop.GeneratorDrivenPropertyChecks
-import org.scalatest.FunSuite
+import munit.ScalaCheckSuite
+import org.scalacheck.Prop._

-class IntegerSuite extends FunSuite with GeneratorDrivenPropertyChecks with Matchers {
+class IntegerSuite extends ScalaCheckSuite {

-  test("addition and multiplication are commutative") {
+  property("addition and multiplication are commutative") {
     forAll { (n1: Int, n2: Int) =>
     forAll { (n1: Int, n2: Int) =>
-      (n1 + n2) should be (n2 + n1)
-      (n1 * n2) should be (n2 * n1)
+      assertEquals(n1 + n2, n2 + n1)
+      assertEquals(n1 * n2, n2 * n1)
     }
   }

}