Cross-joining two Swift arrays

CrossJoin.swift

extension Array
{
    func crossJoin<E, R>(
        array:  [E],
        joiner: (t: T, e: E) -> R?)
        -> [R]
    {
        return arrayCrossJoin(self, array, joiner)
    }
}

/**
Executes the `joiner` closure for every combination
of elements between `aArray` and `bArray` and returns
the resulting objects in the order they were created.
*/
func arrayCrossJoin<A, B, R>(
    aArray: [A],
    bArray: [B],
    joiner: (a: A, b: B) -> R?)
    -> [R]
{
    var results = [R]()
    for a in aArray
    {
        for b in bArray
        {
            if let result = joiner(a: a, b: b)
            {
                results.append(result)
            }
        }
    }
    return results
}

let letters = ["A", "B", "C"]
let numbers = [1, 2]
let dictionaries = letters.crossJoin(numbers) { [$0: $1] }

println(dictionaries)
// Prints: [[A: 1], [A: 2], [B: 1], [B: 2], [C: 1], [C: 2]]