Scala Testing

Project Setup

Test libraries live in build.sbt under the Test configuration, so they are only on the classpath when running tests, never in the packaged artifact.

Example (build.sbt):

libraryDependencies += "org.scalatest" %% "scalatest" % "3.2.19" % Test
// or, for MUnit:
// libraryDependencies += "org.scalameta" %% "munit" % "1.0.0" % Test

ScalaTest Basics

The most common ScalaTest style is AnyFunSuite: each test is a call to test("description") { ... } containing one or more assertions.

Example:

import org.scalatest.funsuite.AnyFunSuite

class MathUtilsSuite extends AnyFunSuite {

  def add(a: Int, b: Int): Int = a + b

  test("add sums two positive numbers") {
    assert(add(2, 3) == 5)
  }

  test("add handles negative numbers") {
    assertResult(-1) {
      add(2, -3)
    }
  }

  test("division by zero throws") {
    assertThrows[ArithmeticException] {
      10 / 0
    }
  }
}

MUnit Basics

MUnit favors plain assertEquals calls and reads closer to JUnit. It starts faster and has fewer moving parts, which makes it a common default for new projects.

Example:

class MathUtilsSuite extends munit.FunSuite {

  def add(a: Int, b: Int): Int = a + b

  test("add sums two positive numbers") {
    assertEquals(add(2, 3), 5)
  }

  test("division by zero throws") {
    intercept[ArithmeticException] {
      10 / 0
    }
  }
}

Fixtures

A fixture sets up shared state before each test and tears it down after. In ScalaTest, override withFixture or mix in a fixture trait; in MUnit, override beforeEach/afterEach.

Example (MUnit):

class RepositorySuite extends munit.FunSuite {

  var data: List[Int] = _

  override def beforeEach(context: BeforeEach): Unit = {
    data = List(1, 2, 3)
  }

  test("data starts with three elements") {
    assertEquals(data.length, 3)
  }
}

Running Tests

Run the whole suite, a single test class, or watch and re-run on every file save.

sbt test                          # run every test
sbt "testOnly *MathUtilsSuite"    # run one suite
sbt ~test                         # re-run on file change