zio-test

ZIO Test is a featherweight testing library for effectful programs.

The library imagines every spec as an ordinary immutable value, providing tremendous potential for composition. Thanks to tight integration with ZIO, specs can use resources (including those requiring disposal), have well- defined linear and parallel semantics, and can benefit from a host of ZIO combinators.

 import zio.test._
 import zio.Clock.nanoTime

 object MyTest extends ZIOSpecDefault {
   def spec = suite("clock")(
     test("time is non-zero") {
       for {
         time <- Live.live(nanoTime)
       } yield assertTrue(time >= 0L)
     }
   )
 }

The laws package provides functionality for describing laws as values. The fundamental abstraction is a set of ZLaws[Caps, R]. These laws model the laws that instances having a capability of type Caps are expected to satisfy. A capability Caps[_] is an abstraction describing some functionality that is common across different data types and obeys certain laws. For example, we can model the capability of two values of a type being compared for equality as follows:

trait Equal[-A] {
 def equal(a1: A, a2: A): Boolean
}

Definitions of equality are expected to obey certain laws:

1. Reflexivity - a1 === a1
2. Symmetry - a1 === a2 ==> a2 === a1
3. Transitivity - (a1 === a2) && (a2 === a3) ==> (a1 === a3)

These laws define what the capabilities mean and ensure that it is safe to abstract across different instances with the same capability.

Using ZIO Test, we can represent these laws as values. To do so, we define each law using one of the ZLaws constructors. For example:

val transitivityLaw = ZLaws.Laws3[Equal]("transitivityLaw") {
 def apply[A: Equal](a1: A, a2: A, a3: A): TestResult =
   ???
}

We can then combine laws using the + operator:

val reflexivityLaw: = ???
val symmetryLaw:    = ???

val equalLaws = reflexivityLaw + symmetryLaw + transitivityLaw

Laws have a run method that takes a generator of values of type A and checks that those values satisfy the laws. In addition, objects can extend ZLawful to provide an even more convenient syntax for users to check that instances satisfy certain laws.

object Equal extends Lawful[Equal]

object Hash extends Lawful[Hash]

object Ord extends Lawful[Ord]

checkAllLaws(Equal + Hash + Ord)(Gen.int)

Note that capabilities compose seamlessly because of contravariance. We can combine laws describing different capabilities to construct a set of laws requiring that instances having all of the capabilities satisfy each of the laws.