Decoding 3rd party Codable data structures

[kamcma]

Was having trouble decoding into IdentifiedArray, a data structure from @pointfreeco. It conforms to Codable. Came up with the following minimal reproduction:

import IdentifiedCollections
import XCTest
import XMLCoder

let sourceXML: String = """
<?xml version="1.0" encoding="utf-8"?>
<team>
  <user>
    <id>1</id>
    <name>Bob</name>
  </user>
  <user>
    <id>2</id>
    <name>Alice</name>
  </user>
</team>
"""

struct User: Decodable, Identifiable {
  let id: Int
  let name: String
}

struct Team: Decodable {
  let users: [User]

  enum CodingKeys: String, CodingKey {
    case users = "user"
  }
}

struct IdentifiableTeam: Decodable {
  let users: IdentifiedArrayOf<User>

  enum CodingKeys: String, CodingKey {
    case users = "user"
  }
}

final class XMLCoderTests: XCTestCase {
  func testDecodeArray() throws {
    let decoder = XMLDecoder()
    guard let data = sourceXML.data(using: .utf8) else { XCTFail(); return }
    let team = try decoder.decode(Team.self, from: data)
    XCTAssertEqual(2, team.users.count)
  }

  func testDecodeIdentifiedArray() throws {
    let decoder = XMLDecoder()
    guard let data = sourceXML.data(using: .utf8) else { XCTFail(); return }
    let team = try decoder.decode(IdentifiableTeam.self, from: data) // ❌ throws
    XCTAssertEqual(2, team.users.count)
  }
}

I would expect both tests to pass. The vanilla array decoding does pass, but the IdentifiedArray decoding throws the following:

testDecodeIdentifiedArray(): failed: caught error: "typeMismatch(Swift.Dictionary<Swift.String, Any>, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "user", intValue: nil), XMLKey(stringValue: "0", intValue: 0), XMLKey(stringValue: "0", intValue: 0)], debugDescription: "Expected to decode Dictionary<String, Any> but found ChoiceBox instead.", underlyingError: nil))"

Had a little trouble understanding XMLCoder's internals, but did notice that it extends a few standard library data structures to conform to an internal protocol AnySequence. If I change my minimal reproduction like so:

-import XMLCoder
+@testable import XMLCoder

// ...

+extension IdentifiedArray: AnySequence where Element: Identifiable, ID == Element.ID { }

then both tests pass.

Are my findings correct that conformance to this internal protocol are necessary to decode arbitrary data structures from outside the standard library?

I think ideally this would not be the case, and the encoder/decoder implementations would support arbitrary Codable types generally. But I understand the maintainers of the library are not the original authors.

In the mean time, if conformance to AnySequence is required, what do the maintainers think about making it public?

[MaxDesiatov]

@kamcma I've renamed the protocol to XMLDecodableSequence and made it public in #244, does that resolve your issue?

[kamcma]

It does. Thanks.

[MaxDesiatov]

Great, I've tagged 0.14.0 which includes this change. Thanks for reporting the issue!