I have been working with Lua lately and I have discovered that it is capable of doing some very interesting things. I have created a proof of concept that allows one Lua script to load another Lua script and use the loaded script to modify itself by passing itself as an argument to the creation function of the loaded script. The basic example here is from a simple card game.

In this specific example, I have a Card "object" which is really just a LuaTable. At the top of the Player.lua script I have:

local card = require "Card"

Then I create a card "object" by calling:

function Player:addSpecificCardToDeck(cardSuit, cardType, cardId)
    table.insert(self.deck, card:new(cardSuit, cardType, cardId))

The Card.lua script loaded by require contains code that loads another script during its creation. Here is the Card.lua script. Please ignore cardSuit in this example as that code is incomplete and I have omitted it.


Card = {} -- the table representing the class, which will double as the metatable for the instances
Card.__index = Card -- failed table lookups on the instances should fallback to the class table, to get methods

local cardList = require "Cardlist"

-- syntax equivalent to "MyClass.new = function..."
function Card:new(cardSuit, cardType, cardId)

    local self = setmetatable({}, Card)

    self.id = cardId
    self.type = cardType

    --this is where another script is loaded
    print(self.id .. " cardID")
    --getAbilityScriptsForId returns the appropriate string
    self.testScript = require (cardList:getAbilityScriptsForId(2)) --2 is the proof of concept script being called
    test = self.testScript:new(self)
    print(self.id.. " new cardID") --the cardID will have increased by 1

    return self

return Card

Here is the sample script being loaded:


SampleScript02 = {} -- the table representing the class, which will double as the metatable for the instances
SampleScript02.__index = SampleScript02 -- failed table lookups on the instances should fallback to the class table, to get methods

-- syntax equivalent to "MyClass.new = function..."
function SampleScript02:new(inputCard)

    self.card = inputCard

    return self

function SampleScript02:increaseCardId()
    self.card.id = self.card.id + 1

return SampleScript02

As stated in the comments, the self.id of the script that loads the sample script will be incremented by 1. In theory, a script could pass itself into any potential script that modifies it. When a table is passed as the argument to a function in Lua, changes made to it inside that function apply to the original table. In contrast, when values such as integer or boolean are passed into a function, a copy is made instead.

What I am looking for is whether I am following the best practices for Lua, and also how to avoid any potential pitfalls that the language presents. Am I using require properly? I am still learning the language, so any tips on syntax are also much appreciated.

  • 1
    \$\begingroup\$ I won't post this as an answer because it's just a link but you could use pcall as a way to catch errors that may occur when loading the external files \$\endgroup\$
    – Dan
    Aug 21 '14 at 20:37

On the first line, Card = {} should be local Card = {}, otherwise Card becomes a global, breaking encapsulation. You might want to consider using strict.lua or another method of "locking" the global table to prevent mistakes like this.

Having Card double as the metatable for card instances is pretty unusual. It will work, but now Card and each of its instances are polluted with an __index property, and this could be avoided simply by writing setmetatable({}, { __index = Card }) instead of setmetatable({}, Card), which is probably what people would expect to see anyway.

You declare the function as Card:new; it has an implicit self argument because of the colon. But you never use that argument; instead, you shadow it with a local variable named self. You probably wanted to do something like local instance = setmetatable({}, self), although as mentioned it would probably be better not to have Card double as the metatable for instances, so instead you could use local instance = setmetatable({}, { __index = self }).

local Card = {}

function Card:new(cardSuit, cardType, cardId)
    local instance = setmetatable({}, { __index = self })

    instance.id = cardId
    instance.type = cardType

    -- ...

    return instance

return Card

The same things apply to your second block of code.

  • \$\begingroup\$ Thanks for the feedback. To clarify, when I do local self = setmetatable, is conflicting with the original self in any way? \$\endgroup\$
    – bazola
    Aug 21 '14 at 19:48
  • 1
    \$\begingroup\$ local self = selfmetatable will cause self to be come shadowed in the scope in which local self was declared, however you will be fine outside of the method. I would recommend against it though because semantically self is known to refer to the current object (in this case, the Card table) in which this function is being invoked on \$\endgroup\$
    – Dan
    Aug 21 '14 at 20:35
  • \$\begingroup\$ Thing is, when I change it to instance, I have to change all the method calls elsewhere from self:whatever() to instance:whatever() and reference instance.property in those functions. Is this still the preferred way to do it? Thanks for your response. \$\endgroup\$
    – bazola
    Aug 21 '14 at 20:40
  • \$\begingroup\$ Ah I see your point now. I would say that you really should keep using instance as self should always refer to the owner of the current scope - it's just semantics, though. If it's local it's only going to affect your code. \$\endgroup\$
    – Dan
    Aug 21 '14 at 20:41

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