product.swift

let s1 = [1, 2]
let s2 = [3, 4, 5]

func product <Sequence1: Sequence, Sequence2: Sequence> (_ s1: Sequence1, _ s2: Sequence2) -> AnySequence<(Sequence1.Element, Sequence2.Element)> {
  return AnySequence(
    s1.flatMap { e1 in
      s2.map { e2 in
        print(e1, e2)
        return (e1, e2)
      }
    }
  )
}

for (e1, e2) in product(s1, s2) {
  if e1 == 1 && e2 == 3 {
    break
  }
}