replaceEmpty/replaceNil/replaceError with publisher parameter

[yhkaplan]

I was looking at for something like RxSwift's ifEmpty(switchTo:), and there appeared to be a solution in
rxswift-to-combine-cheatsheet, but current versions of Combine do not offer replaceEmpty(with publisher: Publisher). I'm thinking something like the below would work, but what do you think? If it's okay, I'll go ahead and make a pull request.

public extension Publisher where Output: Collection {
    func replaceEmpty(with publisher: AnyPublisher<Output, Failure>) -> AnyPublisher<Output, Failure> {
        flatMap { (output: Output) -> AnyPublisher<Output, Failure> in
            if output.isEmpty { return publisher }
            return Just(output)
                .setFailureType(to: Failure.self)
                .eraseToAnyPublisher()
        }
        .eraseToAnyPublisher()
    }
}

[freak4pc]

Hey @yhkaplan, I think you've misunderstood how ifEmpty(switchTo:) works.

It isn't meant to work on empty/non-empty collections, but on empty Observables.

That means, a.ifEmpty(switchTo: b) will switch into the second observable if a completed without emitting anything, see DefaultIfEmpty in ReactiveX: http://reactivex.io/documentation/operators/defaultifempty.html

It should be easy enough to make this operator, but your implementation is of something else :)