Cartesian power (non-lazy)

CartesianPower.swift

import Foundation

infix operator ^^

func ^^<A>(_ values: [A], _ exponent: Int) -> [[A]]? {
    guard exponent >= 0 else { return nil }
    func product(prod: [[A]], xs: [A]) -> [[A]] {
        return prod.flatMap { p in xs.map { x in p + [x] } }
    }
    let factors = [[A]](repeating: values, count: exponent)
    return factors.reduce([[]], product)
}

/*
[1, 2, 3] ^^ -1
[1, 2, 3] ^^ 0
[1, 2, 3] ^^ 3
[Int]() ^^ 2
*/