Example.swift

func requestTopPosts(completion: @escaping (Result<RedditListingResult<RedditPostServerModel>, RedditError>) -> Void) {
      let request = URLRequest(url: Constants.topPostsAbsoluteURL)
      networkService.perform(request: request) {
          completion(
              $0.flatMap(ifSuccess: self.verifyServerResponse, ifFailure: self.networkErrorToResult)
                .flatMap(ifSuccess: self.parsePostModels, ifFailure: liftError)
          )
      }
  }