Codable Either

Either.swift

enum Either<A, B> {
    case left(A)
    case right(B)
}

extension Either: Decodable where A: Decodable, B: Decodable {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        if let a = try? container.decode(A.self) {
            self = .left(a)
        } else if let b = try? container.decode(B.self) {
            self = .right(b)
        } else {
            fatalError("Must be decoded into \(A.self) or \(B.self)")
        }
    }
}

extension Either: Encodable where A: Encodable, B: Encodable {
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        switch self {
        case .left(let a):
            try container.encode(a)
        case .right(let b):
            try container.encode(b)
        }
    }
}

// Example model

// You can directly decode `Foo` when the value of `bar` is either a `String` or `Bar` object.
struct Foo: Codable {
    let bar: Either<String, Bar>
}

struct Bar: Codable {
    let name: String
}