FoldableBoolean.kt

fun <R> Boolean.fold(whenFalse: R, whenTrue: R) = when (this) {
    true -> whenTrue
    false -> whenFalse
}

inline fun <R> Boolean.fold(whenFalse: () -> R, whenTrue: () -> R) = when (this) {
    true -> whenTrue()
    false -> whenFalse()
}

FoldableBooleanTest.kt

import org.assertj.core.api.Assertions.*
import org.junit.jupiter.api.Test

class FoldableBooleanTest {

    @Test
    fun should_return_true_parameter_when_fold_over_true() {
        val trueValue = true

        val should = trueValue.fold("It's false", "It's true")

        assertThat(should).isEqualTo("It's true")
    }

    @Test
    fun should_return_false_parameter_when_fold_over_false() {
        val falseValue = false

        val should = falseValue.fold("It's false", "It's true")

        assertThat(should).isEqualTo("It's false")
    }

    @Test
    fun should_apply_true_function_when_fold_over_true() {
        val trueValue = true
        
        trueValue.fold({ throw Exception() }, { assertThat(true).isTrue() })
    }

    @Test
    fun should_apply_false_function_when_fold_over_true() {
        val falseValue = false

        falseValue.fold({ assertThat(true).isTrue() }, { throw  Exception() })
    }
}