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Version: 5.8

Reflective Arbs

When running tests on JVM, Kotest supports generating more complex Arbs automatically. The generated Arb relies on build-in default and further reflective Arbs to populate the class parameters. If you just need to create and instance and don't need filtering you can use the class type in the checkAll/forAll calls directly. When you want to obtain the Arb to manipulate it further or filter invalid values, you can use Arb.bind with the type argument to obtain the Arb. If the required type depends on types that are not supported by default, it is possible to provide Arbs for those types in the call to Arb.bind.


enum class Currency {

class CurrencyAmount(
val amount: Long,
val currency: Currency

context("Currencies converts to EUR") { // In some spec
checkAll(Arb.bind<CurrencyAmount>().filter { it.currency != EUR }) { currencyAmount ->
val converted = currencyAmount.convertTo(EUR)
converted.currency shouldBe EUR

context("Converting to a currency and back yields the same amount") { // In some spec
checkAll<CurrencyAmount, Currency>() { currencyAmount, currency ->
val converted = currencyAmount.convertTo(currency).convertTo(currencyAmount.currency)
converted.currency shouldBe currencyAmount.currency

Reflective binding is supported for:

  • Classes or dataclasses that are not private, which primary constructor is not private, and where constructor parameters are also supported types
  • Pair, where 1st and 2nd fall into this category
  • Primitives
  • Enums
  • Sealed classes, subtypes and their primary constructor must not be private
  • LocalDate, LocalDateTime, LocalTime, Period, Instant from java.time
  • BigDecimal, BigInteger
  • Collections (Set, List, Map)
  • Classes for which an Arb has been provided through providedArbs