when.kt

fun main() {
    
    val res = caseOf("string", {
        case({ contains("s") }) { 1 }
        case({ contains("t") }) { 2 }
    }, orElse = {
        3
    })
    
}

open class CaseOf<T, R>(private val receiver: T) {

    protected var result: R? = null

    fun case(condition: T.() -> Boolean, result: () -> R) {
        if (receiver.condition()) {
            this.result = result()
        }
        throw Interrupt
    }

    object Interrupt : Exception()

}

class ResultExtractor<T, R>(value: T) : CaseOf<T, R>(value) {
    
    fun extract() = result
    
}

fun <T, R> caseOf(value: T, body: CaseOf<T, R>.() -> Unit): R? {
    val context = ResultExtractor<T, R>(value)
    return try {
        context.body()
        context.extract()
    } catch (e: CaseOf.Interrupt) {
        null
    }
}

fun <T, R> caseOf(value: T, body: CaseOf<T, R>.() -> Unit, orElse: () -> R): R {
    return caseOf(value, body) ?: orElse()
}