First part of an IF system in Lua

if1.lua

-- Object utilities
----------------------------------------

function dispatch(obj, message_name)
   local m = getmetatable(obj)
   if not m then return nil end
   
   for _, operations in ipairs(m.behaviors) do
	  if operations[message_name] then return operations[message_name] end
   end
   
   return nil
end

function become(obj, operations)
   local m = getmetatable(obj)
   
   if not m then -- No metatable yet; we'll add one
	  m = {behaviors={}, __index=dispatch}
	  setmetatable(obj, m)
   end
   
   table.insert(m.behaviors, 1, operations)

   return obj
end

function new(type, obj)
   become(obj, type)
   return obj
end

-- Room functions
----------------------------------------

Room = {}

function Room.print_description(room)
   print(room.name .. "\n\n" .. room.description)
end

function Room.room_in_dir(room, dir)
   local name = room.exits[dir]

   if name then return ROOMS[name]
   else return nil end
end

-- All the rooms
----------------------------------------

ROOMS = {
   Kitchen = new(Room, {
					name = "Kitchen",
					description = "A simple kitchen.",
					exits = { w = "LivingRoom" }
				 }),

   LivingRoom = new(Room, {
					   name = "Living Room",
					   description = "A well-appointed but cluttered living room",
					   exits = { e = "Kitchen", s = "Bedroom" }
					}),

   Bedroom = new(Room, {
					name = "Bedroom",
					description = "A comfortable bedroom with a well-thumbed book collection",
					exits = { n = "LivingRoom" }
				 })
}

-- Game functions
----------------------------------------

Game = {}

function Game.input(game, command)
   for _, rule in ipairs(game.globals) do
	  local message = rule(game, command)
	  if message then
		 return message
	  end
   end

   return "Sorry, I didn't understand that"
end

function Game.add_global(game, rule)
   table.insert(game.globals, 1, rule)
end

-- The actual game object
----------------------------------------

game = new(Game, {
			  current_room = ROOMS.Bedroom,
			  globals = {}
		   })

game:add_global(function(game, command)
				   if command.verb == "walk" then
					  local new_room = game.current_room:room_in_dir(command.subject)
					  if new_room then
						 game.current_room = new_room
						 return new_room.name .. "\n\n" .. new_room.description
					  else return "There is no exit that way" end
				   end
				end)