Decoding generics with phantom types - swift

Question

I am trying to define a Currency type that would prevent numeric and alphabetic currency codes from getting mixed up:
public protocol ISO4217Type {}
public enum ISO4217Alpha: ISO4217Type {}
public enum ISO4217Num: ISO4217Type {}
public struct Currency<T: ISO4217Type> {
public let value: String
}
extension Currency where T == ISO4217Alpha {
public init?(value: String) {
let isLetter = CharacterSet.letters.contains
guard value.unicodeScalars.all(isLetter) else { return nil }
self.value = value
}
}
extension Currency where T == ISO4217Num {
public init?(value: String) {
let isDigit = CharacterSet.decimalDigits.contains
guard value.unicodeScalars.all(isDigit) else { return nil }
self.value = value
}
}
This works great. Now, is it possible to add a Codable conformance that would throw a decoding error when trying to decode a currency code with the wrong payload? (For example, decoding USD as a numeric currency code.)

Answer 1

The key revelation was that it’s possible to customize the behaviour using static functions on the phantom type:
public protocol ISO4217Type {
static func isValidCode(_ code: String) -> Bool
}
public enum ISO4217Alpha: ISO4217Type {
public static func isValidCode(_ code: String) -> Bool {
let isLetter = CharacterSet.letters.contains
return code.unicodeScalars.all(isLetter)
}
}
public enum ISO4217Num: ISO4217Type {
public static func isValidCode(_ code: String) -> Bool {
let isDigit = CharacterSet.decimalDigits.contains
return code.unicodeScalars.all(isDigit)
}
}
public struct Currency<T: ISO4217Type> {
public let value: String
private init(uncheckedValue value: String) {
self.value = value
}
public init?(value: String) {
guard T.isValidCode(value) else { return nil }
self.value = value
}
}
extension Currency: Codable {
public func encode(to encoder: Encoder) throws {
var c = encoder.singleValueContainer()
try c.encode(value)
}
public init(from decoder: Decoder) throws {
let c = try decoder.singleValueContainer()
let value = try c.decode(String.self)
guard T.isValidCode(value) else {
throw DecodingError.dataCorruptedError(in: c,
debugDescription: "Invalid \(type(of: T.self)) code")
}
self.init(uncheckedValue: value)
}
}