either.swift

import Foundation

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

// Works only using Swift 4.1
extension Either: Codable where A: Codable, B: Codable {    
    enum CodingKeys: CodingKey {
        case left
        case right
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        switch self {
        case .left(let value):
            try container.encode(value, forKey: .left)
        case .right(let value):
            try container.encode(value, forKey: .right)
        }
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        do {
            let leftValue =  try container.decode(A.self, forKey: .left)
            self = .left(leftValue)
        } catch {
            let rightValue =  try container.decode(B.self, forKey: .right)
            self = .right(rightValue)
        }
    }
}

let values: [Either<String,Int>] = [
    .left("Hi"),
    .right(42)
]

let encoder = JSONEncoder()
let data = try! encoder.encode(values)
let str = String(data: data, encoding: .utf8)
print(str)

let decoder = JSONDecoder()
let result = try! decoder.decode([Either<String,Int>].self, from: data)