Implementing zip3 in Swift

zip3.swift

struct Zip3Iterator
<
    A: IteratorProtocol,
    B: IteratorProtocol,
    C: IteratorProtocol
>: IteratorProtocol {
    
    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(), let b = second.next(), let c = third.next() {
            return (a, b, c)
        }
        return nil
    }
}

func zip<A: Sequence, B: Sequence, C: Sequence>(a: A, b: B, c: C) -> IteratorSequence<Zip3Iterator<A.Iterator, B.Iterator, C.Iterator>> {
    return IteratorSequence(Zip3Iterator(a.makeIterator(), b.makeIterator(), c.makeIterator()))
}

// Use the zip
let iterator: Zip3Iterator = zip([1, 2, 3], ["test", "foo", "bar"], [2.2, 3.14, 5, 22])

// Make an array from it
let result: Array<(Int,String,Double)> = Array(iterator)

// Iterate over it
for triplet in iterator {
    print("\(triplet.0) is related to \(triplet.1) and \(triplet.2)")
}

Updated for Swift 4 compat