hablapps / puretest   0.3.2


Purely functional testing in Scala

Scala versions: 2.12

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Puretest allows you to write purely functional tests for purely functional programs. Purely functional tests have two major advantages:

  • They can be fully reused both in unit and integration testing scenarios
  • They abstract away from Scalatest, Specs2, uTest and any other testing framework

Getting started

To add Puretest to your project and start using it, you just need to add our resolver and the cats or scalaz dependency, according to your preferences.

resolvers += "Habla repo - releases" at "http://repo.hablapps.com/releases"

libraryDependencies += "org.hablapps" %% "puretest-cats" % "0.3.2"
libraryDependencies += "org.hablapps" %% "puretest-scalaz" % "0.3.2"

What you're gonna get

Tic-tac-toe game

This is an example of how we can test a tagless final implementation of tic-tac-toe using puretest. The complete tic-tac-toe example can be found here.

import puretest._

trait TicTacToeSpec[P[_]] extends FunSpec[P] {

  /* Evidences */

  val ticTacToe: TicTacToe[P]
  import ticTacToe._

  implicit val RE: RaiseError[P, PuretestError[TicTacToe.Error]]

  /* Tests */

  Describe("Reset Spec") {

    It("First turn is X") {
      reset >>
      currentTurnIs(X) shouldBe true


  Describe("Place Spec") {

    Holds("Turn must change") {
      (reset >>
        place(X, (1, 1)) >>
        currentTurnIs(O)) &&
      (place(O, (1, 2)) >>

    It("should place stone in the specified location") {
      for {
        _ <- reset
        None <- in((1, 1))
        _ <- place(X, (1, 1))
        Some(X) <- in((1, 1))
      } yield ()

    It("should not be possible to place more than one stone at the same place") {
      reset >>
      place(X, (1, 1)) >>
      place(O, (1, 1)) shouldFailWith OccupiedPosition((1, 1))

First of all, note that these tests are fully general and work for any interpretation P of the TicTacToe[P[_]] system. They'll work if we instantiate it with the State monad, in order to do unit testing; and they'll work if we want to test a web service instance of TicTacToe based on Futures.

Second, the tests build upon an abstract BDD style (Describe, Holds and It) and an abstract set of purely functional matchers (shouldBe, shouldFailWith, etc.). This allow us to abstract away from ScalaTest, Specs2, etc.

In the following sections, we will describe these abstract components and will show how to run the tests with ScalaTest.

Purely functional matchers

Matchers from conventional testing frameworks typically execute a given program and then checks whether the execution satisfied a given condition. If the condition is not met, then the matcher fails and an exception is thrown. Purely functional matchers work similarly, but in a more declarative fashion, and just for purely functional programs. Currently, support is provided only for purely functional programs written in a tagless-final style.

For instance, the following test checks that the given program returns successfully the value 1:

// The corresponding scalaz or cats dependencies are assumed to be in scope
// in the following examples
import puretest._

def testOne[P[_]: HandleError[?[_], Throwable]
                : RaiseError[?[_], PuretestError[Throwable]]
                : Monad](program: P[Int]): P[Int] =
  program shouldBe 1

This test will pass if program actually executed without errors and the integer value returned is exactly 1. If the program returned a different value, or it simply failed, then the test will fail as well and a testing error will be raised. This desired functionality is possible thanks to the following capabilities declared in the signature:

  • HandleError[P, Throwable]. It allows us to check wether the program failed or not. In this particular case, we assume that the application program fails with an error of type Throwable, but it could be any type you want.
  • RaiseError[P, PuretestError[Throwable]]. It allows us to raise a testing error in case that the test doesn't pass. The argument of PuretestError refers to the type of error of the program being tested.

HandleError and RaiseError are type classes which simply provide the corresponding operations of MonadError. The required evidences of these type classes can be obtained automatically from MonadError[P, PuretestError[Throwable]].

For instance, if programs are to be interpreted as Either values, we may obtain the following outcomes:

scala> type P[A] = Either[PuretestError[Throwable], A]
defined type alias P

scala> testOne(1.point[P])
res0: P[Int] = Right(1)

scala> testOne(0.point[P])
res1: P[Int] = Left(Value 1 expected but found value 0 (<console>:18))

scala> testOne((new Throwable()).raiseError[P, Int])
res2: P[Int] = Left(Value 1 expected but found error java.lang.Throwable (<console>:18))

Besides shouldBe, there are a number of other functional matchers. This is the complete list:

Matcher Test pass iff
shouldSucceed[E] Program executes without errors of type E
shouldBe[E](value: A) Program returns successfully the specified value
shouldMatch[E](pattern: A => Boolean) Program returns successfully a value that matches the specified pattern
shouldFail[E] Program fails with an error of type E
shouldFailWith[E](error: E) Program fails exactly with error
shouldMatchFailure[E](pattern: E => Boolean) Program fails and the error matches the specified pattern

For-comprehension syntax

The shouldMatch pattern and some implicit declarations allows us to use for-comprehension syntax as follows:

def testWithForC[P[_]: HandleError[?[_], Throwable]
                     : RaiseError[?[_], PuretestError[Throwable]]
                     : Monad](program: P[Int]): P[Unit] =
  for {
    1 <- program
  } yield ()

where the for-comprehension expression is equivalent to the following one:

program shouldMatch{ case 1 => true; case _ => false } as(())

Specification-style tests

We can group and specify tests in a BDD style using the trait FunSpec[P[_]]. Basically, this trait gives us the possibility to assign textual descriptions to tests as follows:

import puretest._

trait Test[P[_]] extends FunSpec[P]{

  implicit val ME: MonadError[P, Throwable] // HandleError will be derived automatically from this
  implicit val RE: RaiseError[P, PuretestError[Throwable]]

  Describe("Working program"){
    It("should succeed"){
      1.point[P] shouldSucceed

    It("should succeed with the specific value"){
      1.point[P] shouldBe 2

    Holds("should succeed with boolean values"){

  Describe("Failing program"){
    It("should fail"){
      (new Throwable()).raiseError[P, Int] shouldFail

    It("should fail with the specific error thrown"){
      (new Throwable("error")).raiseError[P, Int] shouldMatchFailure[Throwable]{
        _.getMessage == "error2"

The Holds(...){ p } style is used with boolean programs. It is essentially equivalent to It(...){ p shouldBe true }.

ScalaTest binding

Once we have our tests defined in an abstract and purely functional way, the next step is running them with a specific testing framework, and for a specific interpretation P[_]. The recommended practice is fixing first the testing framework, and then instantiate the resulting class for any interpretation we wish. ScalaTest is the only framework supported so far.

For instance, the test suite above could be instantiated for ScalaTest as follows:

object Test {
  class ScalaTest[P[_]](implicit
    val ME: MonadError[P, Throwable],
    val RE: RaiseError[P, PuretestError[Throwable]]
    val Tester: Tester[P, PuretestError[Throwable]],
  ) extends scalatestImpl.FunSpec[P, Throwable] with Test[P]

The scalatestImpl.FunSpec trait extends the ScalaTest FunSpec API, in such a way that an instance of Test.ScalaTest will be a regular ScalaTest test. The abstract Describe and It instructions of puretest are implemented in terms of the ScalaTest describe and it operations of ScalaTest FunSpec, and simply check that the corresponding testing expressions run without errors.

Note that besides the evidences of the test suite (MonadError and RaiseError), we also need a Tester instance for P. In essence, an evidence of Tester[P[_], E] is just a natural transformation P ~> Either[E, ?]. There are instances of this type class for some of the most common program types, these being:

  • Tester[Either[E, ?], E]
  • Tester[Future, Throwable]
  • Tester[Validated[E, ?], E]

So, creating a ScalaTest instance for type Either[PuretestError[Throwable], ?] is really easy:

scala> object ScalatestTest extends Test.ScalaTest[Either[PuretestError[Throwable], ?]]
defined object ScalatestTest

scala> ScalatestTest.execute()
Working program
- should succeed
- should succeed with the specific value *** FAILED ***
  Value 2 expected but found value 1 (<pastie>:36) (FunSpec.scala:18)
Failing program
- should fail
- should fail with the specific error thrown *** FAILED ***
  Expected pattern doesn't match found error java.lang.Throwable: error (<pastie>:46) (FunSpec.scala:18)

Testing stateful interpretations

Some interpretations will require an initial state to run, and to that end we have another type class named StateTester[P[_], S, E]. This class is very similar to Tester; in fact it just offers a function that given an initial state S returns a regular Tester[P, E].

Same as with Tester, there are some basic instances already defined in puretest:

  • (Tester[F, E], Monad[F]) => StateTester[StateT[F, S, ?], S, E]
  • (Tester[F, E]) => StateTester[ReaderT[F, S, ?], S, E]

Here we show an example of a stateful specification and its instantiation for ScalaTest:

import puretest._

trait StateTest[P[_]] extends FunSpec[P] {
  implicit val MS: MonadState[P, Int]
  implicit val HE: HandleError[P, Throwable]
  implicit val RE: RaiseError[P, PuretestError[Throwable]]

  Describe("MonadState program"){
    It("should satisfy Put-Get law"){
      (MS.put(1) >> MS.get) shouldBe 1

    It("should satisfy Put-Put law"){
      (MS.put(1) >> MS.put(2) >> MS.get) shouldBe 2

object StateTest {
  class ScalaTest[P[_]](
    val Tester: Tester[P, PuretestError[Throwable]]
    val MS: MonadState[P, Int],
    val HE: HandleError[P, Throwable],
    val RE: RaiseError[P, PuretestError[Throwable]],
  ) extends scalatestImpl.FunSpec[P, Throwable] with StateTest[P]

And this is how we can run this test for an stateful interpretation (using StateTester to obtain the corresponding Tester instance):

scala> type P[A] = StateT[Either[PuretestError[Throwable], ?], Int, A]
defined type alias P

scala> object StateTestScalaTest extends StateTest.ScalaTest(StateTester[P, Int, PuretestError[Throwable]].apply(0))
defined object StateTestScalaTest

scala> StateTestScalaTest.execute()
MonadState program
- should satisfy Put-Get law
- should satisfy Put-Put law

Examples & talks

So far we have one example where you can see puretest in action. Don't waste any time and take a look at our awesome TicTacToe.

Some events where we have talked about puretest:

  • Typelevel unconference - Lambda World 2017 (slides)

Current status

Puretest is very much in progress right now, so contributions are welcome. Some of the areas that need urgent attention are the following ones:

  • Support for testing purely functional programs using GADTs (scalaz/cats Free, freestyle's @free, Eff, etc.)
  • Integrations with Specs2, uTest, and other testing frameworks


Puretest is licensed under the Apache License, Version 2.0.