责任链

-- ConcreteHandlers: a handler answers its own
-- request and returns nothing for the others
local function firefighter(code)
  if code == 1 then
    return "call firefighters"
  end
end

local function police(code)
  if code == 2 then
    return "call the police"
  end
end

local function ambulance(code)
  if code == 3 then
    return "call an ambulance"
  end
end

-- The chain is an array: the handlers
-- are asked in this order
local rescuers = {firefighter, police, ambulance}

-- HandleRequest(): the first answer ends the walk
local function help(code)
  for _, rescuer in ipairs(rescuers) do
    local answer = rescuer(code)
    if answer then
      return answer
    end
  end
  return "nobody handles code " .. code
end

-- Client
local answer1 = help(1)
-- answer1 = call firefighters
local answer2 = help(2)
-- answer2 = call the police
local answer3 = help(3)
-- answer3 = call an ambulance
local answer9 = help(9)
-- answer9 = nobody handles code 9

print("answer1 = " .. answer1)
print("answer2 = " .. answer2)
print("answer3 = " .. answer3)
print("answer9 = " .. answer9)