Thank you Brent for your effort. I will try this way.
By the way I thought that the protocol conformance with conditions were already
retained/adopted in Swift 4. You seem to say that it was postponed. It’s too
bad in my opinion. But thanks again for the info.
> On 18 Oct 2017, at 23:38, Brent Royal-Gordon <br...@architechies.com> wrote:
>
>> On Oct 17, 2017, at 10:00 AM, Thierry Passeron via swift-users
>> <swift-users@swift.org> wrote:
>>
>> Hi everyone,
>>
>> In the process of familiarising myself with Encodable/Decodable protocols I
>> was trying to apply it to the Range struct in order to persist a Range in
>> CoreData records. However, I seem to hit the wall with it and keep getting
>> errors. This happens in the Xcode 9.0.1 playground, not sure which swift
>> version is used if it’s 4 or 3.x but anyways, I get “Ambiguous reference to
>> member ‘encode(_:forKey:)’ on every encode/decode method calls.
>>
>> extension Range {
>> enum CodingKeys : String, CodingKey {
>> case upperBound
>> case lowerBound
>> }
>> }
>>
>> extension Range: Codable {
>>
>> public func encode(to encoder: Encoder) throws {
>> var container = encoder.container(keyedBy: CodingKeys.self)
>> try container.encode(upperBound, forKey: .upperBound)
>> try container.encode(lowerBound, forKey: .lowerBound)
>> }
>>
>> public init(from decoder: Decoder) throws {
>> let values = try decoder.container(keyedBy: CodingKeys.self)
>> self.upperBound = try values.decode(Bound.self, forKey: .upperBound)
>> self.lowerBound = try values.decode(Bound.self, forKey: .lowerBound)
>> }
>>
>> }
>>
>> How would one add Codable support to such a struct? I’m feeling it may
>> require a bit of “where” clauses in extension because of the Generic aspect
>> of this struct but I fail to make the compiler happy.
>
> What you'd *like* to be able to do is say that a Range is codable only when
> its bounds are also codable:
>
> extension Range: Codable where Bound: Codable {
> …
> }
>
> But this isn't currently supported in Swift. (The compiler team is working on
> it right now, and it'll probably be here in Swift 5, if not in 4.1.) So for
> the time being, you have to fake it dynamically. Unless you want to use
> private APIs, the easiest way is probably to use a dictionary:
>
> extension Range: Decodable /* where Bound: Decodable */ {
> public init(from decoder: Decoder) throws {
> let dict = try [String: Bound](from: decoder)
>
> guard let lower =
> dict[CodingKey.lowerBound.stringValue] else {
> throw DecodingError.valueNotFound(Bound.self,
> .init(codingPath: decoder.codingPath + [CodingKey.lowerBound],
> debugDescription: "lowerBound not found")
> }
> guard let lower =
> dict[CodingKey.upperBound.stringValue] else {
> throw DecodingError.valueNotFound(Bound.self,
> .init(codingPath: decoder.codingPath + [CodingKey.upperBound],
> debugDescription: "upperBound not found")
> }
>
> self.init(uncheckedBounds: (lower: lower, upper: upper))
> }
> }
>
> extension Range: Encodable /* where Bound: Encodable */ {
> public func encode(to encoder: Encoder) throws {
> try [CodingKey.lowerBound.stringValue: lowerBound,
> CodingKey.upperBound.stringValue: upperBound].encode(to: encoder)
> }
> }
>
> This should mimic the structure you'll eventually be able to generate with a
> keyed container; when Swift becomes able to do this properly, you can update
> the code to encode and decode directly.
>
> Hope this helps,
> --
> Brent Royal-Gordon
> Architechies
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