SomeAPI.rb

class SomeAPI
  def fetch_users
    prom = if only_need_to_fetch_one_page?
             client.fetch_page(1).then { |res| res.fetch('users') }
           else
             promises = Concurrent::Promise.zip(client.fetch_page(1), client.fetch_page(2))

             promises.then do |first_page, second_page|
               combine_results(first_page, second_page)
             end
           end

    prom.value
  end
  # ...
end