/// Define
extension Validator where T == String {
    /// Validates whether a `String` is a valid email address.
    public static var zipCode: Validator<T> {
        .init {
            guard
                let range = $0.range(of: zipCodeRegex, options: [.regularExpression]),
                range.lowerBound == $0.startIndex && range.upperBound == $0.endIndex
            else {
                return ValidatorResults.ZipCode(isValidZipCode: false)
            }
            return ValidatorResults.ZipCode(isValidZipCode: true)
        }
    }

    /// Validates whether a `String` is a valid two letter state.
    public static var usState: Validator<T> {
        .init {
            guard
                validStates.contains($0)
            else {
                return ValidatorResults.USState(isValidState: false)
            }
            return ValidatorResults.USState(isValidState: true)
        }
    }
}
extension ValidatorResults {
    /// `ValidatorResult` of a validator that validates whether a `String` is a valid zip code.
    public struct ZipCode {
        /// The input is a valid zip code
        public let isValidZipCode: Bool
    }

    /// `ValidatorResult` of a validator that validates whether a `String` is a valid two letter state.
    public struct USState {
        /// The input is a valid state
        public let isValidState: Bool
    }
}

extension ValidatorResults.ZipCode: ValidatorResult {
    public var isFailure: Bool {
        !self.isValidZipCode
    }

    public var successDescription: String? {
        "is a valid zip code"
    }

    public var failureDescription: String? {
        "is not a valid zip code"
    }
}

extension ValidatorResults.USState: ValidatorResult {
    public var isFailure: Bool {
        !self.isValidState
    }

    public var successDescription: String? {
        "is a valid state"
    }

    public var failureDescription: String? {
        "is not a valid state"
    }
}

private let zipCodeRegex: String = "^\\d{5}(?:[-\\s]\\d{4})?$"
private let validStates = [
"AK",
"AL",
"AR",
"AZ",
"CA",
"CO",
"CT",
"DC",
"DE",
"FL",
"GA",
"HI",
"IA",
"ID",
"IL",
"IN",
"KS",
"KY",
"LA",
"MA",
"MD",
"ME",
"MI",
"MN",
"MO",
"MS",
"MT",
"NE",
"NC",
"ND",
"NH",
"NJ",
"NM",
"NY",
"NV",
"OH",
"OK",
"OR",
"PA",
"RI",
"SC",
"SD",
"TN",
"TX",
"UT",
"VA",
"VT",
"WA",
"WI",
"WV",
"WY",
"AS",
"GU",
"MH",
"PR",
"UM",
]

/// Usage
extension CreateUserData: Validatable {
    public static func validations(_ validations: inout Validations) {
        // ...
        validations.add("addressZipCode", as: String.self, is: .zipCode, required: false)
        validations.add("addressState", as: String.self, is: .usState, required: false)
    }
}