Simple Mixins

Part of the Object Oriented Programming section of Coddy's Lua journey — lesson 51 of 70.

Composition lets you put objects inside other objects. But what if you want to share behavior across unrelated classes?

A Player and a MenuItem might both need a :describe() method, but they don't share a parent class. This is where mixins come in.

A mixin is simply a table containing functions. You "mix" these functions into a class by copying them over:

local Printable = {}

function Printable:describe()
    print("Object: " .. tostring(self))
end

function Printable:log(message)
    print("[LOG] " .. message)
end

To apply this mixin to a class, you copy each function from the mixin table into the class table:

local function applyMixin(class, mixin)
    for key, value in pairs(mixin) do
        class[key] = value
    end
end

local Player = {}
Player.__index = Player

applyMixin(Player, Printable)  -- Now Player has :describe() and :log()

After applying the mixin, any Player instance can call those methods as if they were defined directly on the class:

local p = setmetatable({}, Player)
p:log("Player created")  -- [LOG] Player created

Unlike inheritance, mixins don't create a parent-child relationship. You're simply copying functions from one table to another.

This makes mixins perfect for sharing utility behaviors—like logging, serialization, or description methods—across classes that otherwise have nothing in common.

Challenge

Easy

Let's build a tagging system using mixins! You'll create a Taggable mixin that can be applied to any class, giving objects the ability to manage a collection of tags—useful for categorizing items, posts, or any entity that needs labels.

You'll organize your code across three files:

Taggable.lua: Create a mixin table containing two methods:
- :addTag(tag) — adds a tag to self.tags (assumes the table exists)
- :listTags() — prints all tags in the format Tags: tag1, tag2, tag3 (or Tags: none if empty)
Article.lua: Create an Article class with a constructor :new(title) that stores the title and initializes an empty tags table. Apply the Taggable mixin to give Article instances tagging capabilities. Include a :getTitle() method that returns the article's title.
main.lua: Write the applyMixin helper function here (you'll use it in Article.lua via require). Then read inputs for an article title and two tags. Create an Article, add both tags to it, print the title, and list all tags.

You will receive three inputs:

The article title (a string)
The first tag to add (a string)
The second tag to add (a string)

Your output should display the article's title followed by its tags:

Title: {title}
Tags: {tag1}, {tag2}

For example, if the inputs are Lua Basics, programming, and tutorial, the output should be:

Title: Lua Basics
Tags: programming, tutorial

Remember: the mixin methods work because Article instances have a tags table that the mixin methods can operate on. The mixin doesn't create this table—your Article constructor does. The mixin just provides the shared behavior!

Cheat sheet

A mixin is a table containing functions that can be shared across unrelated classes without creating a parent-child relationship.

To create a mixin, define a table with methods:

local Printable = {}

function Printable:describe()
    print("Object: " .. tostring(self))
end

function Printable:log(message)
    print("[LOG] " .. message)
end

To apply a mixin to a class, copy its functions into the class table:

local function applyMixin(class, mixin)
    for key, value in pairs(mixin) do
        class[key] = value
    end
end

local Player = {}
Player.__index = Player

applyMixin(Player, Printable)

After applying the mixin, instances can call the mixin methods:

local p = setmetatable({}, Player)
p:log("Player created")  -- [LOG] Player created

Mixins are ideal for sharing utility behaviors like logging, serialization, or tagging across classes that don't share inheritance.

Try it yourself

-- Main file for the tagging system
local Taggable = require('Taggable')
local Article = require('Article')

-- TODO: Implement the applyMixin function
-- This function should copy all methods from a mixin table to a target table
function applyMixin(target, mixin)
    -- TODO: Loop through the mixin and copy each method to target
end

-- Read inputs
local title = io.read()
local tag1 = io.read()
local tag2 = io.read()

-- TODO: Create an Article with the given title

-- TODO: Add both tags to the article

-- TODO: Print the title in format: Title: {title}

-- TODO: List all tags using the listTags method