Minitest: Zero Crap Scala Testing Library

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Minitest is my minimal testing library that I’ve been using for developing Monix.

Raison d’être #

I dislike most testing frameworks, because of bloat and of heavy DSLs trying to mimic the English language. When I created Minitest, I wasn’t satisfied with any of the available alternatives.

Then I found that SBT can do all the heavy lifting (e.g. running the tests, reporting, etc), exposing a nice sbt/test-interface that you can integrate with. All you need to do is to build your own API on top. And so I did.

NOTE: My opinions in this article disagree with the design choices of popular testing libraries. I know that libraries like ScalaTest or Specs2 are the way they are because people want them that way. And those are awesome projects, having awesome authors. They are just not for me.

Portability #

The natural tendency of testing frameworks is to grow beyond all imagination in order to accommodate the various testing styles that people want, these testing frameworks also end up hard to port to new platforms - which is especially relevant in Scala due to new major versions and new targets released all the time (e.g. Scala.js, Scala Native, Dotty).

In 2014 I started to work on Monix and during that time Scala.js was also born.

This awesome Scala compiler that targets JavaScript was really fresh back then and I wanted to target it, however none of the testing libraries (e.g. ScalaTest) were supporting it yet, except for µTest. µTest seemed fine, but it had problems when displaying error messages, plus its DSL was and still is weird.

For some reason I don’t like magic in my tests — and by magic I mean expressions or statements that I don’t immediately understand. That, and I wanted an easy transition in Monix from ScalaTest’s FunSuite, which is what I was using.

Say No to DSLs #

I do not want to write x shouldBe greaterThan(y) or other such nonsense. I do not have the memory for that, I always forget the API and the IDE doesn’t help much due to the implicit conversions going on.

Unit tests might be business driven, but so is software in general, nothing makes tests a special snowflake to warrant the abuse of the programming language to make it look like English.

Yes, there are advantages to such a DSL. For example if you express the above with a simple assert, then you won’t get a meaningful error message back, depending on whether the implementation does or doesn’t do macros for assert, but if it does, then that’s a whole other can of worms:

assert(x > y)

Fact: most assertions that you need to do in testing are equality tests and for that rare moment in which you need inequality tests, you can simply add a custom error message:

assert(x > y, s"$x > $y")

Yes, it’s repetitive, but I don’t care, because this is such a rare event that I don’t want to optimize it with a special DSL or even with macros.

Another thing that I absolutely hate are tests marked with English words like “it”, forcing you to phrase the test’s description in a certain way. I frequently end up with “sentences” that makes no sense:

it("left identity") {
  // ...

This tendency actually stems from Java OOP, with its “kingdom of nouns”, the idea being that the things you’re testing are all nouns that interact with the world, aka objects. Well, I’m not testing just objects, so it is a bad trend.

And the idea that business folks might be writing tests, hell no, that almost never happens and if they are inclined to do that (like once in a million), then they can just learn programming. It’s not like an English-like DSL is any closer to natural language.

Minimal Implementation, Less is More #

Because the implementation is minimal, there’s nothing that I can’t fix in it, there’s nothing that I can’t implement should I need anything.

It’s also easy to port to new targets. I intend to port it to Scala Native as soon as it is available for Scala 2.12 (at the moment of writing, it isn’t).

In fact, if you want to build your own testing framework, Minitest can serve as a sample ;-)

Hypothesis #

All you need is the ability to express:

  1. synchronous tests, returning Unit (or an equivalent, as I ended up doing in order to avoid Scala’s annoying implicit conversion)
  2. asynchronous tests, returning Future
  3. the ability to setup an environment before every test, then tear it down after each test

All the asserts that you need:

  1. assert(boolean, string?): general purpose assertion for any condition
  2. assertEquals(received, expected): for equality testing with a nice error message
  3. intercept: for testing that exceptions are thrown
  4. fail(reason?): fails the current test
  5. ignore(reason?): ignores the current test
  6. cancel(reason?): cancels the current test

What you don’t need:

  1. nesting in tests
  2. an English-like DSL
  3. a purely functional base API

Tutorial #

Test suites MUST BE objects, not classes. To create a simple test suite, it could inherit from SimpleTestSuite. Here’s a simple test:

import minitest.SimpleTestSuite

object MySimpleSuite extends SimpleTestSuite {
  test("should be") {
    assertEquals(2, 1 + 1)

  test("should not be") {
    assert(1 + 1 != 3)

  test("should throw") {
    class DummyException extends RuntimeException("DUMMY")
    def test(): String = throw new DummyException

    intercept[DummyException] {

  test("test result of") {
    assertResult("hello world") {
      "hello" + " " + "world"

  test("should be ignored") {
    if (Platform.isJS) ignore("Blocking not supported on top of JS")
    val r = Await.result(Future(1), Duration.Inf)
    assertEquals(r, 1)

In case you want to setup an environment for each test and need setup and tearDown semantics, you could inherit from TestSuite. Then on each test definition, you’ll receive a fresh value:

import monix.execution.schedulers.TestScheduler
import minitest.TestSuite

object MyTestSuite extends TestSuite[TestScheduler] {
  def setup() = TestScheduler()

  def tearDown(env: TestScheduler): Unit =
    assert(env.state.tasks.isEmpty, "Scheduler should not have tasks left")

  test("simulated async") { implicit ec =>
    val f = Future(1).map(_ + 1)

    assertEquals(f.value, Some(Success(2)))

Minitest supports asynchronous results in tests, just use testAsync and return a Future[Unit]:


object MySimpleSuite extends SimpleTestSuite {
  testAsync("asynchronous execution") {
    val future = Future(100).map(_+1)

    for (result <- future) yield {
      assertEquals(result, 101)

Minitest has integration with ScalaCheck. So for property-based testing:

import minitest.laws.Checkers

object MyLawsTest extends SimpleTestSuite with Checkers {
  test("addition of integers is commutative") {
    check2((x: Int, y: Int) => x + y == y + x)

  test("addition of integers is transitive") {
    check3((x: Int, y: Int, z: Int) => (x + y) + z == x + (y + z))

That’s everything!

Common Complaints #

I do not like Future #

That’s too bad, because the Future is needed by the runtime and regardless what alternative you use (e.g. cats.effect.IO, monix.eval.Task), you’ll have to convert it into a Future anyway.

Besides, a good testing framework cannot have dependencies, because it would end in conflict with the project’s dependencies. It’s unwise to depend on Cats or Scalaz.

And you can always build your own testTask, testEffect or testIO utilities on top of testAsync.

I want a purely functional API #

Specs2 has a nice functional API. You might like that, however I don’t like it for all the reasons stated above.

And if pure FP is what you want, nothing stops you from implementing your own utilities and I recommend piggybacking on ScalaCheck, e.g:

import cats.effect.IO
import org.scalacheck.{Prop, Test}

trait PureTestSuite extends minitest.api.AbstractTestSuite {
  private[this] val ts = new SimpleTestSuite {}
  lazy val properties =

  def test(name: String)(f: => Prop): Unit =
    ts.test(name) {
      val result = Test.check(config, f)
      if (!result.passed) fail(Pretty.pretty(result))

  def testIO(name: String)(f: => IO[Prop]): Unit =
    ts.testAsync(name) { { result =>
        val result = Test.check(config, f)
        if (!result.passed) fail(Pretty.pretty(result))

There’s your purely functional API in just a couple of lines of code.

I don’t want that in Monix though - the integration that we have with ScalaCheck is minimal and enough.

It does not support Maven, CBT or others #

Sorry about that, but this library is meant to be minimal and stable, and I don’t have the time to expand support beyond SBT right now.

Pull requests open and only accepted if they don’t complicate the codebase much.

Final Words #

Forget DSLs.

All you need for testing are Minitest and ScalaCheck.

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Tags: Testing | Scala