foreachWithSelector.kt

trait SelectResult<T> {
    val take: Boolean
    val value: T

    class object {
        fun take<T>(value: T) = object : SelectResult<T> {
            override val value: T = value
            override val take: Boolean = true
        }
        fun skip<T>() = object : SelectResult<T> {
            override val value: T
                get() = throw UnsupportedOperationException("Cannot get value from skip object")
            override val take: Boolean = false
        }
    }
}

val <T> indexed: (T) -> SelectResult<Pair<Int, T>>
    get() {
        var index = 0
        return { SelectResult.take(index++ to it) }
    }

val <T : Any> notNull: (T?) -> SelectResult<T>
    get() = {
        if (it != null) SelectResult.take(it) else SelectResult.skip()
    }

inline fun <T, R> Iterable<T>.foreach(selector: (T) -> SelectResult<R>, body: (R) -> Unit) {
    for (element in this) {
        val selected = selector(element)
        if (selected.take)
            body(selected.value)
    }
}

fun fn(items: List<String>) {
    items.foreach<String, String>(notNull) { value ->
        println("$value")
    }
}