MultipleReceiversTypeClasses.kt

interface Monoid<T> {
    operator fun T.plus(other: T): T
    fun empty(): T
}

object StringMonoid : Monoid<String> {
    override fun String.plus(other: String): String = plus(other)
    override fun empty(): String = ""
}

/* works now perfectly */

fun <T> Monoid<T>.fold(list: List<T>): T = list.fold(empty()) { x, acc -> x.plus(acc)}

fun main(args: Array<String>) {
    with (StringMonoid) {
        println("Hello" + empty())
        println(fold(listOf("123", "456", "789")))
    }
}

/* 1000 times cooler, but no multiple receivers supported */

fun <T> Monoid<T>.List<T>.Xfold(): T = fold(empty()) { x, acc -> x.plus(acc)}

fun Xmain(args: Array<String>) {
    with (StringMonoid) {
        println("Hello" + empty())
        println(listOf("123", "456", "789").Xfold())
    }
}