extension_nested_type.swift

extension CreditCard {
    
    enum CardType {
        case visa, unknown
    }
    
    var type: CardType {
        guard number.count >= 1 else { return .unknown }
        let firstNumber = number[number.startIndex..<number.index(number.startIndex, offsetBy: 1)]
        switch firstNumber {
        case "4":
            return .visa
        default:
            return .unknown
        }
    }
    
}

let visaCard = CreditCard(number: "4111 0000 0000 0000", date: "03/05", ccv: "098")
print("card type: \(visaCard.type)")
// card type: visa