A passthrough operator for Combine Publishers

Publisher+passthrough.swift

extension Publisher {
    @discardableResult
    func passthrough(_ closure: @escaping (Result<Output, Failure>) -> Void) -> AnyPublisher<Output, Failure> {
        map { output in
            closure(.success(output))
            return output
        }
        .mapError { error in
            closure(.failure(error))
            return error
        }
        .eraseToAnyPublisher()
    }
}