CallbackHell.swift

self.downloadUser() { user in
  self.downloadFavouriteMovies(for: user) { movies in
    self.downloadRatings(for: movies) { ratings in
      self.downloadFriends(for: users) { friends in
        self.downloadMovies(for: friends] { friendMovies in
          self.presentProfile(
            for: user, 
            movies: movies, 
            ratings: ratings, 
            friends: friends,
            movies: movies
          )
        }
      }
    }
  }
}