Combine: WithPrevious

Publisher+WithPrevious.swift

extension Publisher {
    public typealias Pair<T> = (previous: T?, current: T)

    public func withPrevious() -> AnyPublisher<Pair<Output>, Failure> {
        return scan(nil) { previous, current -> Pair<Output>? in
            return Pair(previous: previous?.current, current: current)
        }
        .compactMap { $0 }
        .eraseToAnyPublisher()
    }
}