import Combine

protocol OptionalType {
    associatedtype Wrapped

    var optional: Wrapped? { get }
}

extension Optional: OptionalType {
    public var optional: Wrapped? { self }
}

extension Publisher where Output: OptionalType {
    func ignoreNil() -> AnyPublisher<Output.Wrapped, Failure> {
        flatMap { output -> AnyPublisher<Output.Wrapped, Failure> in
            guard let output = output.optional else {
                return Empty<Output.Wrapped, Failure>(completeImmediately: false).eraseToAnyPublisher()
            }
            return Just(output).setFailureType(to: Failure.self).eraseToAnyPublisher()
        }
        .eraseToAnyPublisher()
    }
}

extension Publisher {
    func combineWithPrevious() -> AnyPublisher<(Output, Output), Failure> {
        zip(dropFirst()).eraseToAnyPublisher()
    }
}