Expectation

An Expectation[R] is an immutable tree structure that represents expectations on environment R.

Defining Expectations

ZIO Test has a variety of expectations, such as value, unit, failure, and never. In this section we are going to learn each of these expectations and their variant, by mocking the UserService service. So let's assume we have the following service:

import zio._
import zio.mock._
import zio.test._

case class User(id: String, name: String)

trait UserService {
  def insert(user: User): IO[String, Unit]
  def remove(id: String): IO[String, Unit]
  def totalUsers: IO[String, Int]
  def recentUsers(n: Int): IO[String, List[User]]
  def removeAll: IO[String, Unit]
}

object UserService {
  def totalUsers: ZIO[UserService, String, Int] =
    ZIO.serviceWithZIO(_.totalUsers)

  def insert(user: User): ZIO[UserService, String, Unit] =
    ZIO.serviceWithZIO(_.insert(user))

  def recentUsers(n: Int): ZIO[UserService, String, List[User]] =
    ZIO.serviceWithZIO(_.recentUsers(n))

  def remove(id: String): ZIO[UserService, String, Unit] =
    ZIO.serviceWithZIO(_.remove(id))
   
  def removeAll: ZIO[UserService, String, Unit] = 
    ZIO.serviceWithZIO(_.removeAll) 
}

We can write the mock version of this class as below:

object MockUserService extends Mock[UserService] {

  object Insert      extends Effect[User, String, Unit]
  object Remove      extends Effect[String, String, Unit]
  object RecentUsers extends Effect[Int, String, List[User]]
  object TotalUsers  extends Effect[Unit, String, Int]
  object RemoveAll   extends Effect[Unit, String, Unit]

  val compose: URLayer[mock.Proxy, UserService] =
    ZLayer {
      for {
        proxy <- ZIO.service[mock.Proxy]
      } yield new UserService {
        override def insert(user: User):  IO[String, Unit]       = proxy(Insert, user)
        override def remove(id: String):  IO[String, Unit]       = proxy(Remove, id)
        override def recentUsers(n: Int): IO[String, List[User]] = proxy(RecentUsers, n)
        override def totalUsers:          IO[String, Int]        = proxy(TotalUsers)
        override def removeAll:           IO[String, Unit]       = proxy(RemoveAll)
      }

    }
}

To create expectations we use the previously defined capability tags.

1. For methods that take input, the first argument will be an assertion on input, and the second the predefined result.

import zio.mock._
import zio.test._

val exp01 = MockUserService.RecentUsers( // capability to build an expectation for
  Assertion.equalTo(5), // assertion of the expected input argument
  Expectation.value(List(User("1", "Jane Doe"), User("2", "John Doe"))) // result, that will be returned
)

2. For methods that take no input, we only define the expected output:

val exp02 = MockUserService.TotalUsers(Expectation.value(42))

3. For methods that may return Unit, we may skip the predefined result (it will default to successful value) or use unit helper:

val exp03 = MockUserService.Remove(
  Assertion.equalTo("1"),
  Expectation.unit
)

4. For methods that may return Unit and take no input we can skip both:

val exp04 = MockUserService.RemoveAll()

Providing Mocked Environment

Each expectation can be taught of a mocked environment. They can be converted to a ZLayer implicitly. Therefore, we can compose them together and provide them to the environment of the SUT (System Under Test).

import zio.test._

import zio._
import zio.mock._
import zio.test.{test, _}

test("expecting simple value on call to nextInt") {
  val sut     = Random.nextInt
  val mockEnv = MockRandom.NextInt(Expectation.value(5))
  for {
    total <- sut.provideLayer(mockEnv)
  } yield assertTrue(total == 5)
} 

Mocking Unused Collaborators

Often the dependency on a collaborator is only in some branches of the code. To test the correct behaviour of branches without dependencies, we still have to provide it to the environment, but we would like to assert it was never called. With the Mock.empty method we can obtain a ZLayer with an empty service (no calls expected):

import zio.mock._
import zio.test._

object MaybeConsoleSpec extends MockSpecDefault {
  def spec = suite("processEvent")(
    test("expect no call") {
      def maybeConsole(invokeConsole: Boolean) =
        ZIO.when(invokeConsole)(Console.printLine("foo"))

      val sut1     = maybeConsole(false).unit
      val mockEnv1 = MockConsole.empty

      val sut2     = maybeConsole(true).unit
      val mockEnv2 = MockConsole.PrintLine(
        Assertion.equalTo("foo"),
        Expectation.unit
      )

      for {
        _ <- sut1.provideLayer(mockEnv1)
        _ <- sut2.provideLayer(mockEnv2)
      } yield assertTrue(true)
    }
  )
}

Mocking Multiple Collaborators

In some cases we have more than one collaborating service being called. We can create mocks for rich environments and as you enrich the environment by using capability tags from another service, the underlying mocked layer will be updated.

import zio._
import zio.mock._
import zio.test.{test, _}

test("mocking multiple collaborators") {
  val sut =
    for {
      _ <- Console.printLine("What is your name?")
      name <- Console.readLine.orDie
      num <- Random.nextInt
      _ <- Console.printLine(s"$name, your lucky number today is $num!")
    } yield ()

  val mockEnv: ULayer[Console with Random] = MockConsole.PrintLine(
    Assertion.equalTo("What is your name?"),
    Expectation.unit
  ) ++ MockConsole.ReadLine(Expectation.value("Mike")) ++
    MockRandom.NextInt(Expectation.value(42)) ++
    MockConsole.PrintLine(
      Assertion.equalTo("Mike, your lucky number today is 42!"),
      Expectation.unit
    )
    
  for {
    _ <- sut.provideLayer(mockEnv)
  } yield assertTrue(true)
} 

Expectations

In the most robust example, the result can be either a successful value or a failure. In this section we are going to introduce all these cases, by using the proper expectation from zio.mock.Expectation companion object:

value

Expecting a simple value:

import zio._
import zio.mock._
import zio.test.{test, _}

test("expecting simple value") {
  val sut     = UserService.totalUsers
  val mockEnv = MockUserService.TotalUsers(Expectation.value(14))
  for {
    total <- sut.provideLayer(mockEnv)
  } yield assertTrue(total == 14)
} 

valueF

Expecting a value based on input arguments:

import zio._
import zio.mock._
import zio.test.{test, _}

test("an expectation based on input arguments") {
  val sut     = UserService.recentUsers(3)
  val mockEnv = MockUserService.RecentUsers(
    Assertion.isPositive,
    Expectation.valueF(n =>
      (1 to n).map(id => User(id.toString, "name")).toList
    )
  )

  for {
    users <- sut.provideLayer(mockEnv)
  } yield assertTrue(users.map(_.id) == List("1", "2", "3"))
}

valueZIO

Expecting a value based on the input arguments and also the result of an effectful operation:

import zio._
import zio.mock._
import zio.test.{test, _}

test("effectful expectation") {
  val sut     = UserService.recentUsers(3)
  val mockEnv = MockUserService.RecentUsers(
    Assertion.isPositive,
    Expectation.valueZIO(n =>
      ZIO.foreach(1 to n) { n =>
        Random
          .nextUUID
          .map(id => User(id.toString, s"name-$n"))
      }.map(_.toList)
    )
  )

  for {
    users <- sut.provideLayer(mockEnv)
  } yield assertTrue(List("name-1", "name-2", "name-3") == users.map(_.name))
}

unit

Expecting simple unit value:

import zio._
import zio.mock._
import zio.test.{test, _}

test("expecting unit") {
  val sut     = UserService.remove("1")
  val mockEnv = MockUserService.Remove(
    Assertion.isNonEmptyString,
    Expectation.unit
  )
  
  for {
    res <- sut.provideLayer(mockEnv).exit
  } yield assertTrue(
    res match {
      case Exit.Success(()) => true
      case _ => false
    }
  )
}

failure

Expecting a failure:

import zio._
import zio.mock._
import zio.test.{test, _}

test("failure expectation") {
  val sut = UserService.totalUsers
  val mockEnv = MockUserService.TotalUsers(Expectation.failure("connection failed"))
  
  for {
    total <- sut.provideLayer(mockEnv).exit
  } yield assertTrue(
    total match {
      case Exit.Success(_) =>
        false
      case Exit.Failure(cause) =>
        cause.contains(Cause.fail("connection failed"))
    }
  )
}

There are also failureF and failureZIO variants like what we described for value expectation.

never

This expectation simulates a never-ending loop:

import zio._
import zio.mock._
import zio.test.{test, _}

test("never ending expectation") {
  val sut     = UserService.totalUsers
  val mockEnv = MockUserService.TotalUsers(
    Expectation.never
  )

  for {
    r <- Live.live(
      sut.provideLayer(mockEnv).timeout(500.millis)
    )
  } yield assertTrue(r.isEmpty)
}

Composing Expectations

We can combine our expectation to build complex scenarios using combinators defined in zio.mock.Expectation.

and

The and (alias &&) operator composes two expectations, producing a new expectation to satisfy both in any order:

import zio._
import zio.mock._
import zio.test.{test, _}

test("satisfy both expectations with a logical `and` operator") {
  val sut     = UserService.recentUsers(5) *> UserService.totalUsers
  val mockEnv = MockUserService.TotalUsers(Expectation.value(1)).and(
    MockUserService.RecentUsers(
      Assertion.isPositive,
      Expectation.value(List(User("1", "user"))))
  )

  for {
    total <- sut.provideLayer(mockEnv)
  } yield assertTrue(total == 1)
}

or

The or (alias ||) operator composes two expectations, producing a new expectation to satisfy only one of them:

import zio._
import zio.mock._
import zio.test.{test, _}

test("satisfy one of expectations with a logical `or` operator") {
  val sut     = UserService.totalUsers
  val mockEnv = MockUserService.TotalUsers(Expectation.value(1)).or(
    MockUserService.RecentUsers(
      Assertion.isPositive,
      Expectation.value(List(User("1", "user"))))
  )

  for {
    total <- sut.provideLayer(mockEnv)
  } yield assertTrue(total == 1)
}

andThen

The andThen (alias ++) operator composes two expectations, producing a new expectation to satisfy both sequentially:

import zio._
import zio.mock._
import zio.test.{test, _}

test("satisfy sequence of two expectations with `andThen` operator") {
  val sut     = UserService.recentUsers(5) *> UserService.totalUsers
  val mockEnv = MockUserService.RecentUsers(
    Assertion.isPositive,
    Expectation.value(List(User("1", "user")))
  ) andThen MockUserService.TotalUsers(Expectation.value(1))

  for {
    total <- sut.provideLayer(mockEnv)
  } yield assertTrue(total == 1)
}

In the example above, changing the SUT to UserService.totalUsers *> UserService.remove(1) will fail the test.

Exact Repetition

1. exactly — Produces a new expectation to satisfy itself exactly the given number of times:

import zio._
import zio.mock._
import zio.test.{test, _}

test("satisfying exact repetition of a method call") {
  val sut     = ZIO.foreach(List("1", "2", "3", "4"))(id => UserService.remove(id))
  val mockEnv = MockUserService.Remove(
    Assertion.isNonEmptyString
  ).exactly(4)

  for {
    _ <- sut.provideLayer(mockEnv)
  } yield assertTrue(true)
}

2. twice and thrice — Aliases for exactly(2) and exactly(3).

Range of Repetitions

1. Expectation#repeats(range: Range) — Repeats this expectation within given bounds, producing a new expectation to satisfy itself sequentially given number of times:

import zio._
import zio.mock._
import zio.test.{test, _}

test("expect repeated calls") {
  val sut     = Random.nextInt *> Random.nextInt
  val mockEnv = MockRandom.NextInt(Expectation.value(42)).repeats(2 to 4)

  for {
    _ <- sut.provideLayer(mockEnv)
  } yield assertTrue(true)
}

In the example above, if we repeat nextInt less than 2 or over 4 times, the test will fail.

Another note on repetitions is that, if we compose expectations with andThen/++, once another repetition starts executing, it must be completed in order to satisfy the composite expectation. For example (A ++ B).repeats(1, 2) will be satisfied by either A->B (one repetition) or A->B->A->B (two repetitions), but will fail on A->B->A (incomplete second repetition):

import zio._
import zio.mock._
import zio.mock.Expectation._
import zio.test.{test, _}

test("if another repetition starts executing, it must be completed") {
  val sut     = Random.nextInt *> Random.nextBoolean *> Random.nextInt
  val mockEnv = (MockRandom.NextInt(value(42)) ++ MockRandom.NextBoolean(value(true)))
    .repeats(1 to 2)
  for {
    _ <- sut.provideLayer(mockEnv)
  } yield assertTrue(true)
} @@ TestAspect.failing

2. The atLeast(min: Int), atMost(max: Int), and optional expectations are other variants for repeats expectation.

• The atLeast operator is a lower-bounded variant of repeated, produces a new expectation to satisfy itself sequentially at least given number of times.
• The atMost operator is an upper-bounded variant of repeated, produces a new expectation to satisfy itself sequentially at most given number of times.
• The optional operator is an alias for atMost(1), produces a new expectation to satisfy itself at most once.

MockClock is a built-in mock version of the [Clock] service.

Here is an example of mocking Clock.nanoTime capability:

import zio._
import zio.mock._
import zio.mock.Expectation._
import zio.test.{test, _}

test("calling mocked nanoTime should return expected time") {
  val app = Clock.nanoTime
  val env = MockClock.NanoTime(value(1000L))
  val out = app.provideLayer(env)
  out.map(r => assertTrue(r == 1000L))
}

MockConsole is a built-in mock version of the [Console] service.

Here is an example of mocking Console.readLine capability:

import zio._
import zio.mock._
import zio.test.{test, _}

test("calling mocked readline should return expected value") {
  for {
    line <- Console.readLine.provideLayer(
      MockConsole.ReadLine(Expectation.value("foo"))
    )
  } yield assertTrue(line == "foo")
}

MockRandom is a built-in mock version of the [Random] service. It mocks all the random service capabilities.

Here's how we can mock the MockRandom.nextIntBounded capability:

import zio._
import zio.mock._
import zio.test.{test, _}

test("expect call with input satisfying assertion and transforming it into output") {
  for {
    out <- Random.nextIntBounded(1).provideLayer(
      MockRandom.NextIntBounded(
        Assertion.equalTo(1),
        Expectation.valueF(_ + 41)
      )
    )
  } yield assertTrue(out == 42)
}

MockSystem is a built-in mock version of the [System] service. It mocks all the system service capabilities.

Here's how we can mock the MockSystem.property capability:

import zio._
import zio.mock._
import zio.test.{test, _}

test("calling mocked property should return expected property") {
  for {
    property <- System.property("java.vm.name").provideLayer(
      MockSystem.Property(
        Assertion.equalTo("java.vm.name"),
        Expectation.value(Some("OpenJDK 64-Bit Server VM"))
      )
    )
  } yield assertTrue(property.get.contains("OpenJDK"))
}