尝试从元组中提取值时，编译器引发的预期模式错误

Question

我正在尝试根据收到的用户输入提取状态信息。但是，编译器在提到的地方会抛出预期的模式。请帮帮我。

let rules = [
state.Rest : [(trigger.dialup, state.Connecting)],
state.Connecting : [(trigger.hungup, state.Rest), (trigger.wait, state.Connected)],
state.Connected : [(trigger.placeonhold, state.OnHold), (trigger.endcall, state.Rest)],
state.OnHold : [(trigger.unhold, state.Connected)]
]

func sumupState() {

    var S = state.Rest

    while true {

    for i in 0 ..< rules[S]!.count {
        let (t, _ ) = rules[S]![i]
        print("\(i) \(t)")
    }

    if let ch = Int(readLine()!) {
        let (_, s) = rules[S]![ch] //Erroroccurs "Expected Pattern"
        S = s
    }
  }
}