Property tests

Learn property-based testing with kotest-property.

Property testing with kotest-property

kotest-property tests that a property holds for randomly generated inputs. You write a condition with forAll; the library tries hundreds of random values to find a counterexample.

Example: forAll<Int> { n -> (n * 2) % 2 == 0 }. This checks that doubling any integer yields an even number. If a counterexample is found, the test fails and the example is shown. This pattern catches edge cases classic unit tests miss.

forAll and Arb

forAll takes a property function. The type parameter controls random input generation; Arb defines custom generators.

Example: forAll<Int> { n -> n + 0 == n }. Here a random Int is generated. There are built-in generators like Arb.string() and Arb.positiveInt(). When a property fails, the library shows the smallest failing case; this makes understanding the failure easier.

Exercise Test the reverse property

In main call forAll<String> { s -> s.reversed().reversed() == s } and print println("ok"). Use the io.kotest.property.forAll import.

Show example solution

import io.kotest.property.forAll

fun main() {
    forAll<String> { s ->
        s.reversed().reversed() == s
    }
    println("ok")
}

Test yourself

What does kotest-property provide?
- Testing with a single fixed input
- Verifying a property with random inputs
- Only UI testing
- Only compile-time checks
Show answer

Verifying a property with random inputs — kotest-property verifies properties with random inputs.

Complete the forAll block

____<Int> { n ->
    (n * 2) % 2 == 0
}

Show answer

forAll<Int> { n ->
    (n * 2) % 2 == 0
}

Match the property-test parts

forAll<T> { }

Property block
Arb.string()

String generator
Int

Type parameter
counterexample

Failing case

Next lesson: Summary - Advanced testing

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