Skip to main content
Version: 2.0.x

Introduction to ZIO Test Assertions

Assertions are used to make sure that the assumptions on computations are exactly what we expect them to be. They are executable checks for a property that must be true in our code. Also, they can be seen as a specification of a program and facilitate understanding of programs.

An Assertion[A] is a statement that can be used to assert the predicate of type A => Boolean. It is a piece of code that checks whether a value of type A satisfies some condition. If the condition is satisfied, the assertion passes; otherwise, it fails. We can think of the Assertion[A] as a function from A to Boolean:

case class Assertion[-A](arrow: TestArrow[A, Boolean]) {
def test(value: A): Boolean = ???
def run(value: => A): TestResult = ???

Assertion has a companion object with lots of predefined assertions that can be used to test values of different types. For example, the Assertion.equalTo takes a value of type A and returns an assertion that checks whether the value is equal to the given value:

import zio.test._
import zio.test.Assertion

def sut = 40 + 2
val assertion: Assertion[Int] = Assertion.equalTo[Int, Int](42)
assertion.test(sut) // true

Behind the scenes, the Assertion type uses a TestArrow type to represent the function from A to Boolean. For example, instead of using a predefined equalTo assertion, we can create our assertion directly from a TestArrow:

import zio.test._

def sut = 40 + 2
val assertion: Assertion[Int] = Assertion(TestArrow.fromFunction(_ == 42))
assertion.test(sut) // true

Please note that the TestArrow is the fundamental building block of assertions specially the complex ones. Usually, as the end user, we do not require interacting with TestArrow directly. But it is good to know that it is there and how it works. We will see more about TestArrow in the next sections.

Built-in Assertions

The companion object of Assertion provides a comprehensive set of predefined assertions that can be used to test values of different types. We have a separate page for introducing the built-in assertions in ZIO Test.

Logical Operations

As a proposition, assertions compose using logical conjunction and disjunction and can be negated:

import zio.test._

val greaterThanZero: Assertion[Int] = Assertion.isPositive
val lessThanFive : Assertion[Int] = Assertion.isLessThan(5)
val equalTo10 : Assertion[Int] = Assertion.equalTo(10)

val assertion: Assertion[Int] = greaterThanZero && lessThanFive || !equalTo10

After composing them, we can run it on any expression:

import zio._

val result: TestResult =

Composable Nested Assertions

Besides the logical operators, we can also combine assertions like the following to have assertions on more complex types like Option[Int]:

val assertion: Assertion[Option[Int]] = Assertion.isSome(Assertion.equalTo(5))

test("optional value is some(5)") {
assert(Some(1 + 4))(assertion)

This nested assertion will pass only if the given value is Some(5).

We can also combine assertions on more complex types like Either[Int, Option[Int]]:

import zio.test._
import zio.test.Assertion.{isRight, isSome, equalTo, hasField}

test("either value is right(Some(5))") {
assert(Right(Some(1 + 4)))(isRight(isSome(equalTo(5))))

Here we're checking deeply nested values inside an Either and Option. Because Assertions compose this is not a problem. All layers are being peeled off tested for the condition until the final value is reached.

Here the expression Right(Some(1 + 4)) is of type Either[Any, Option[Int]] and our assertion isRight(isSome(equalTo(5))) is of type Assertion[Either[Any, Option[Int]]]


Under the hood, the above assertion uses the >>> operator of TestArrow to make the composition of two assertions sequentially:

import zio.test._
import zio.test.Assertion

def isRight[A]: TestArrow[Either[Any, A], A] =

def isSome[A]: TestArrow[Option[A], A] =

def equalTo[A, B](expected: B): TestArrow[B, Boolean] =
TestArrow.fromFunction((actual: B) => actual == expected)

val assertion: Assertion[Either[Any, Option[Int]]] = {
val arrow: TestArrow[Either[Any, Option[Int]], Boolean] =
isRight >>> // Either[Any, Option[Int]] => Option[Int]
isSome[Int] >>> // Option[Int] => Int
equalTo(5) // Int => Boolean

By composing an arrow of TestArrow[Either[Any, Option[Int]], Option[Int]] and TestArrow[Option[Int], Boolean] and TestArrow[Int, Boolean] we can create an arrow of TestArrow[Either[Any, Option[Int]], Boolean]. Using this technique, we can compose more arrows to create more and more complex assertions.

We can see that TestArrow has the same analogy as ZLayer. We are dealing with generalization of functions and composition of functions in a pure and declarative fashion, which is called "arrow" in functional programming. In other words, with TestArrow, we have reified the concept of a function and its composition, which allows us to manipulate functions as first-class values.

One of the benefits of reification of assertions into "arrows" is that we can write macros to generate assertions from pure Scala code. This is how the smart assertions work in ZIO Test.

Testing using Assertions

We have two types of methods for writing test assertions:

  1. Classic Assertions— This one is the classic way of asserting ordinary values (assert) and ZIO effects (assertZIO) without using macros.
  2. Smart Assertions— This is a unified syntax for asserting both ordinary values and ZIO effects using the assertTrue macro.