SE-0295: Codable synthesis for enums with associated values

Evolution Proposal Reviews
19

Idea: Could the proposal perhaps include sample synthesized code that would be generated?

For instance (and this only works with the limitations that I suggested in the post above):

enum Value: Codable {
  case noValues
  case allLabelled(a: String, b: String)
  case singleUnlabelled(String)
}

Could have the following conformance synthesized:

  enum CodingKeys: String, CodingKey {
    case noValues
    case allLabelled
    case singleUnlabelled
  }

  enum AllLabelledCodingKeys: String, CodingKey {
    case a
    case b
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    if container.contains(.noValues) {
      self = .noValues
    } else if container.contains(.singleUnlabelled) {
      self = try .singleUnlabelled(container.decode(String.self, forKey: .singleUnlabelled))
    } else if container.contains(.allLabelled) {
      let nested = try container.nestedContainer(keyedBy: AllLabelledCodingKeys.self, forKey: .allLabelled)
      self = try .allLabelled(
        a: nested.decode(String.self, forKey: .a),
        b: nested.decode(String.self, forKey: .b)
      )
    } else {
      throw ..
    }

  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    switch self {
    case .noValues:
      try container.encode(true, forKey: .noValues)
    case .allLabelled(let a, let b):
      var nested = container.nestedContainer(keyedBy: AllLabelledCodingKeys.self, forKey: .allLabelled)
      try nested.encode(a, forKey: .a)
      try nested.encode(b, forKey: .b)
    case .singleUnlabelled(let value):
      try container.encode(value, forKey: .singleUnlabelled)
    }
  }
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