FB10233455.md

Add primary associated types to Publisher protocol

FB10233455

Sometimes it's necessary to construct a large, complex publisher and return it from a function. Currently you either need to explicitly return that type from the function, which is difficult and breaks compilation with any change to the publisher:

func onAppearEffects() -> Publishers.Zip<...> {
  fetchUser()
    .zip(with: fetchRespositories())
    .zip(with: fetchFavorites())
}

Or you have to erase the publisher:

func onAppearEffects() -> AnyPublisher<Action, Never> {
  fetchUser()
    .zip(with: fetchRespositories())
    .zip(with: fetchFavorites())
    .eraseToAnyPublisher()
}

If Publisher had primary associated types then you could use some Publisher:

func onAppearEffects() -> some Publisher<Action, Never> {
  fetchUser()
    .zip(with: fetchRespositories())
    .zip(with: fetchFavorites())
}