Idiomatic way to create n-ary cartesian product (combinations of several sets of parameters)

Question

To create all possible combinations of two sets of parameters and perform an action on them, you can do:

setOf(foo, bar, baz).forEach { a ->
    setOf(0,          


        

Answer 1

Here's another way to do it with an arbitrary combiner function:

fun  product(xs: Collection, ys: Collection, combiner: (T, U) -> V): Collection =
    xs.flatMap { x ->
        ys.map { y ->
            combiner(x, y)
        }
    }

operator fun  Set.times(ys: Set): Set> =
    product(this, ys) { x, y ->
        if (x is Set<*>) x + y // keeps the result flat
        else setOf(x, y)
    }.toSet()

operator fun  List.times(ys: List): List> =
    product(this, ys) { x, y ->
        if (x is List<*>) x + y // keeps the result flat
        else listOf(x, y)
    }.toList()

// then you can do

listOf(1, 2, 3) * listOf(true, false)

// or

setOf(1, 2, 3) * setOf(true, false)

// you can also use product's combiner to create arbitrary result objects, e.g. data classes