Scripts loading Scripts Dynamically in Lua

Question

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))
end

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.lua

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)
    test:increaseCardId()
    print(self.id.. " new cardID") --the cardID will have increased by 1

    return self
end

return Card

Here is the sample script being loaded:

SampleScript02.lua

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
end

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

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.

Answer 1

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
end

return Card

The same things apply to your second block of code.