error_handling.rb

require "json"
require "net/http"

class Client
  class NotificationsResponse
    attr_reader :error

    def initialize(&block)
      @error = false
      @notifications = begin
        yield
      rescue Errno::ECONNREFUSED => error
        @error = error
        {} # sensible default
      end
    end

    def ok?
      @ok_predicate_checked = true
      @error == false
    end

    def notifications
      unless @ok_predicate_checked
        raise "ok? must be checked prior to accessing response data"
      end

      @notifications
    end
  end

  def notifications
    NotificationsResponse.new do
      request = Net::HTTP::Get.new("/")
      http = Net::HTTP.new("localhost", 9999)
      response = http.request(request)
      JSON.parse(response.body)
    end
  end
end

client = Client.new
response = client.notifications
# response.notifications would raise error because ok? was not checked