Custom Combine Operators (Swift)

New Combine operators do not need a hand-written Publisher conformance. Compose existing operators inside a Publisher extension and erase the type. Three operators cover the error handling most apps repeat.

replaceErrorOrEmpty

replaceEmpty and replaceError almost always travel together; fuse them.

extension Publisher {
  func replaceErrorOrEmpty(with output: Output) -> AnyPublisher<Output, Never> {
    self
      .replaceEmpty(with: output)
      .replaceError(with: output)
      .eraseToAnyPublisher()
  }
}

finishOnFail

Sometimes a failure should just end the stream silently, such as a remote refresh merged with cached data: the cache already covers the failure case.

extension Publisher {
  func finishOnFail() -> AnyPublisher<Output, Never> {
    self
      .catch { _ in Empty() }
      .eraseToAnyPublisher()
  }
}

Publishers.Merge(
  newsService.fetchCachedNews(),
  newsService.fetchRemoteNews().finishOnFail()
)
.map(Action.setNews)

catchResult

When the error belongs in app state, wrap value and failure into one Result so the publisher itself never fails.

extension Publisher {
  func catchResult() -> AnyPublisher<Result<Output, Failure>, Never> {
    self
      .map(Result.success)
      .catch { Just(Result.failure($0)) }
      .eraseToAnyPublisher()
  }
}

In a Redux-style store this is the natural shape for side effects: a reducer's effect must never fail, so a failure becomes an ordinary action carrying Result.failure.

case .fetchAuth:
  return environment.service
    .authorize()
    .catchResult()
    .map(Action.setAuth)
    .eraseToAnyPublisher()
case .setAuth(let result):
  state.auth = result
  return Empty().eraseToAnyPublisher()