Skip to main content

Custom Generators

To write your own generator for a type T, you just create an instance of Arb<T> or Exhaustive<T>.


When writing a custom arbitrary we can use the arbitrary builder which accepts a lambda that must return the type we are generating for. The parameter to this lambda is a RandomSource parameter which contains the seed and the Random instance. We should typically use the provided RandomSource if we need access to a kotlin.Random instance, as this instance will have been seeded by the framework to allow for repeatable tests.

For example, here is a custom arb that generates a random int between 3 and 6 using the arbitrary builder.

val sillyArb = arbitrary { rs: RandomSource ->

In addition to the RandomSource parameter, the arbitrary builder lambda also provides the ArbitraryBuilderSyntax context which we can leverage to compose other arbitraries when building ours.

For example, here is an Arbitrary that supports a custom class called Person, delegating to a String arbitrary and an Int arbitrary.

data class Person(val name: String, val age: Int)

val personArb = arbitrary {
val name = Arb.string(10..12).bind()
val age =, 150).bind()
Person(name, age)

The resulting arbitrary produced using this syntax is equivalent to using map, flatMap and bind.


When writing a custom exhaustive we can use the exhaustive() extension function on a List. Nothing more to it than that really!

val singleDigitPrimes = listOf(2,3,5,7).exhaustive()
class PropertyExample: StringSpec({
"testing single digit primes" {
checkAll(singleDigitPrimes) { prime ->
isPrime(prime) shouldBe true
isPrime(prime * prime) shouldBe false