direct-product-2.kt

fun <E, F, ES : Collection<E>, FS : Collection<F>> directProduct(es: ES, fs: FS): Set<Pair<E, F>> =
        fs.flatMap { f -> es.map { e -> e to f } }.toSet()

fun <A> flatten(pair: Pair<*, A>): List<A> {
    val xs = mutableListOf(pair.second)
    var x: Any? = pair.first
    while (x is Pair<*, *>) {
        xs.add(0, x.second as A)
        x = x.first
    }
    xs.add(0, x as A)
    return xs
}

// directProduct(setOf(1, 2, 3), setOf('a', 'b'))
// or
// listOf(setOf(1, 2, 3), setOf('a', 'b')).reduce(::directProduct)

// listOf(setOf(1, 2, 3), setOf('a', 'b'), setOf('X', 'Y', 'Z')).reduce(::directProduct)
//        .map { flatten(it as Pair<*, *>) }