eWhen.kt

fun <T: Comparable<T>> eWhen(target: T, tester: Tester<T>.() -> Unit) {
    val test = Tester(target)
    test.tester()

    test.filterList.firstOrNull { it.filter }?.invoke() ?: return
}

class Tester<T: Comparable<T>>(val it: T) {
    val filterList = mutableListOf<Filter>()

    class Filter(val filter: Boolean, val block: () -> Unit) {
        operator fun invoke() = block()
    }

    fun lt(arg: T, block: () -> Unit) = filterList.add(Filter(it < arg, block))
    fun gt(arg: T, block: () -> Unit) = filterList.add(Filter(it > arg, block))
    fun ge(arg: T, block: () -> Unit) = filterList.add(Filter(it >= arg, block))
    fun le(arg: T, block: () -> Unit) = filterList.add(Filter(it <= arg, block))
    fun eq(arg: T, block: () -> Unit) = filterList.add(Filter(it == arg, block))
    fun ne(arg: T, block: () -> Unit) = filterList.add(Filter(it != arg, block))
    fun ct(arg: Collection<T>, block: () -> Unit) = filterList.add(Filter(it in arg, block))
    fun nc(arg: Collection<T>, block: () -> Unit) = filterList.add(Filter(it !in arg, block))
}

//Demo
fun main(args: Array<String>) {
    eWhen(1) {
        lt(5) { println("fuck") }
        gt(10) { println("fuck 2")}
    }

    eWhen("fu") {
        ct("fuck") {println("fuck again")}
    }
}