Kotest Reflective Arbs

When running tests on the JVM, Kotest supports automatically generating more complex Arbs.
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Reflective Arbs

When running tests on the JVM, Kotest supports automatically generating more complex Arb instances. The generated Arb fills class parameters using built-in primitive Arbs and additional reflective Arbs. If you only need generation and instantiation and do not need filtering, you can use the class type directly in a checkAll or forAll call. If you want to obtain an Arb for further manipulation or to filter invalid values, you can use Arb.bind with a type argument to get the Arb. If the required type depends on a type that is not supported by default, you can provide an Arb for that type when calling Arb.bind.

Example:

enum class Currency {
  USD, GBP, EUR
}

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:

  • Non-private classes or data classes, when the primary constructor is not private and the constructor parameters are also supported types.
  • Pair, when items 1 and 2 are in this category.
  • Primitives.
  • Enums.
  • Sealed classes, when the subtypes and their primary constructors are not private.
  • LocalDate, LocalDateTime, LocalTime, Period, and Instant from java.time.
  • BigDecimal and BigInteger.
  • Collections: Set, List, and Map.
  • Classes whose Arb is provided through supplied Arbs.

References