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 ->

We can also use this random if we are composing other arbs 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 { rs ->
val name = Arb.string(10..12).next(rs)
val age =, 150).next(rs)
Person(name, age)

Although in the real world this type of arb would use bind, it demonstrates the principle of composing.


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