Implementing zip3 in Swift

zip3.swift

struct Zip3Generator
<
    A: GeneratorType,
    B: GeneratorType,
    C: GeneratorType
>: GeneratorType {
    
    private var first: A
    private var second: B
    private var third: C
    
    private var index = 0
    
    init(_ first: A, _ second: B, _ third: C) {
        self.first = first
        self.second = second
        self.third = third
    }
    
    mutating func next() -> (A.Element, B.Element, C.Element)? {
        if let a = first.next(), b = second.next(), c = third.next() {
            return (a, b, c)
        }
        return nil
    }
}

func zip<A: SequenceType, B: SequenceType, C: SequenceType>(a: A, b: B, c: C) -> GeneratorSequence<Zip3Generator<A.Generator, B.Generator, C.Generator>> {
    return GeneratorSequence(Zip3Generator(a.generate(), b.generate(), c.generate()))
}

let x = Array(zip([1, 2, 3], ["test", "foo", "bar"], [2.2, 3.14, 5, 22]))