Either + Decodable.swift

enum Either<Left, Right> {
    case left(Left)
    case right(Right)
}

extension Either: Decodable where Left: Decodable, Right: Decodable {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        do {
            self = .left(try container.decode(Left.self))
        } catch {
            self = .right(try container.decode(Right.self))
            throw error
        }
    }
}