protocol Container { associatedtype Element mutating func append(_ item: Element) var count: Int { get } subscript(i: Int) -> Element { get } } struct IntStack: Container { private var items: [Int] = [] mutating func append(_ item: Int) { items.append(item) } var count: Int { items.count } subscript(i: Int) -> Int { items[i] } } func allItemsMatch<C1: Container, C2: Container>(_ c1: C1, _ c2: C2) -> Bool where C1.Element == C2.Element, C1.Element: Equatable { guard c1.count == c2.count else { return false } for i in 0..<c1.count { if c1[i] != c2[i] { return false } } return true } var s = IntStack() s.append(1); s.append(2) print(allItemsMatch(s, s))