Package

zio.test

laws

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package laws

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.

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Type Members

  1. trait GenF[-R, F[_]] extends AnyRef

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    A GenF knows how to construct a generator of F[A] values given a generator of A values for any A.

    A GenF knows how to construct a generator of F[A] values given a generator of A values for any A. For example, a GenF of List values knows how to generate lists with elements given a generator of elements of that type. You can think of GenF as a "recipe" for building generators for parameterized types.

  2. trait GenF2[-R, F[_, _]] extends AnyRef

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    A GenF knows how to construct a generator of F[A,B] values given a generator of A and generator of B values.

    A GenF knows how to construct a generator of F[A,B] values given a generator of A and generator of B values. For example, a GenF2 of Function1 values knows how to generate functions A => B with elements given a generator of elements of that type B.

  3. type Lawful[-Caps[_]] = ZLawful[Caps, Any]

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  4. type Lawful2[-CapsBoth[_, _], -CapsLeft[_], -CapsRight[_]] = ZLawful2[CapsBoth, CapsLeft, CapsRight, Any]

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  5. type Laws[-Caps[_]] = ZLaws[Caps, Any]

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  6. type Laws2[-CapsBoth[_, _], -CapsLeft[_], -CapsRight[_]] = ZLaws2[CapsBoth, CapsLeft, CapsRight, Any]

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  7. trait ZLawful[-Caps[_], -R] extends AnyRef

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    ZLawful[Caps, R] describes a capability that is expected to satisfy a set of laws.

    ZLawful[Caps, R] describes a capability that is expected to satisfy a set of laws. Lawful instances can be combined using + to describe a set of capabilities and all of the laws that those capabilities are expected to satisfy.

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

object Equal extends Lawful[Equal] {
  val laws = ???
}

  8. trait ZLawful2[-CapsBoth[_, _], -CapsLeft[_], -CapsRight[_], -R] extends AnyRef

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  9. abstract class ZLaws[-Caps[_], -R] extends AnyRef

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    ZLaws[Caps, R] represents a set of laws that values with capabilities Caps are expected to satisfy.

    ZLaws[Caps, R] represents a set of laws that values with capabilities Caps are expected to satisfy. Laws can be run by providing a generator of values of a type A with the required capabilities to return a test result. Laws can be combined using + to produce a set of laws that require both sets of laws to be satisfied.

  10. abstract class ZLaws2[-CapsBoth[_, _], -CapsLeft[_], -CapsRight[_], -R] extends AnyRef

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Value Members

  1. object GenF

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  2. object GenF2 extends FunctionVariants

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  3. object LawfulF

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  4. object LawfulF2

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  5. object Laws

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  6. object Laws2

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  7. object LawsF

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  8. object LawsF2

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  9. object ZLawfulF

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    ZLawfulF[CapsF, Caps, R] describes a set of laws that a parameterized type F[A] with capabilities CapsF is expected to satisfy with respect to all types A that have capabilities Caps.

    ZLawfulF[CapsF, Caps, R] describes a set of laws that a parameterized type F[A] with capabilities CapsF is expected to satisfy with respect to all types A that have capabilities Caps. Lawful instances can be combined using + to describe a set of capabilities and all of the laws that those capabilities are expected to satisfy.

  10. object ZLawfulF2

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  11. object ZLaws

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  12. object ZLaws2

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  13. object ZLawsF

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    ZLaws[CapsF, Caps, R] describes a set of laws that a parameterized type F[A] with capabilities CapsF is expected to satisfy with respect to all types A that have capabilities Caps.

    ZLaws[CapsF, Caps, R] describes a set of laws that a parameterized type F[A] with capabilities CapsF is expected to satisfy with respect to all types A that have capabilities Caps. Laws can be run by providing a GenF that is capable of generating F[A] values given a generator of A values and a generator of values of some type A. Laws can be combined using + to produce a set of laws that require both sets of laws to be satisfied.

  14. object ZLawsF2

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  15. def checkAllLaws[CapsF[_[_]], Caps[_], R, R1 <: R, F[_], A](lawful: Invariant[CapsF, Caps, R])(genF: GenF[R1, F], gen: Gen[R1, A])(implicit arg0: CapsF[F], arg1: Caps[A], trace: Trace): ZIO[R1, Nothing, TestResult]

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    Checks that all values generated by a the specified generator satisfy the expected behavior of the lawful instance.

  16. def checkAllLaws[CapsF[_[-_]], Caps[_], R, R1 <: R, F[-_], A](lawful: Contravariant[CapsF, Caps, R])(genF: GenF[R1, F], gen: Gen[R1, A])(implicit arg0: CapsF[F], arg1: Caps[A], trace: Trace): ZIO[R1, Nothing, TestResult]

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    Checks that all values generated by a the specified generator satisfy the expected behavior of the lawful instance.

  17. def checkAllLaws[CapsF[_[+_]], Caps[_], R, R1 <: R, F[+_], A](lawful: Covariant[CapsF, Caps, R])(genF: GenF[R1, F], gen: Gen[R1, A])(implicit arg0: CapsF[F], arg1: Caps[A], trace: Trace): ZIO[R1, Nothing, TestResult]

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  18. def checkAllLaws[CapsBoth[_, _], CapsLeft[_], CapsRight[_], R, R1 <: R, A, B](lawful: ZLawful2[CapsBoth, CapsLeft, CapsRight, R])(a: Gen[R1, A], b: Gen[R1, B])(implicit arg0: CapsLeft[A], arg1: CapsRight[B], CapsBoth: CapsBoth[A, B], trace: Trace): ZIO[R1, Nothing, TestResult]

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    Checks that all values generated by a the specified generator satisfy the expected behavior of the lawful instance.

  19. def checkAllLaws[Caps[_], R, R1 <: R, A](lawful: ZLawful[Caps, R])(gen: Gen[R1, A])(implicit arg0: Caps[A], trace: Trace): ZIO[R1, Nothing, TestResult]

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    Checks that all values generated by a the specified generator satisfy the expected behavior of the lawful instance.

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