Lua Academy · Lesson

Module Patterns and Best Practices

Use local M = {} pattern and expose public API cleanly.

Module Patterns and Best Practices is a free Lua Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

The M = {} Pattern

The universal Lua module pattern: declare a local table, populate it, return it. All public symbols go in M; all private helpers are plain locals. This is clear, minimal, and works everywhere.

-- The canonical module pattern
local M = {}

-- Private helper (not exported)
local function validate(x)
  return type(x) == "number" and x >= 0
end

-- Public API
function M.sqrt(x)
  assert(validate(x), "expected non-negative number")
  return math.sqrt(x)
end

M.PI = math.pi

return M

Self-Referential Module

Inside a module, functions can call other module functions either by name (M.foo()) or as locals. Using locals is slightly faster; using M.foo() allows users to override M.foo and have the internal call use the override (monkey patching).

local M = {}

-- Option A: use M.foo inside (allows override)
function M.double(n) return M.multiply(n, 2) end
function M.multiply(a, b) return a * b end

-- Option B: use local function (faster, no override)
local function mul(a, b) return a * b end
function M.triple(n) return mul(n, 3) end

return M

Singleton Pattern

A module can act as a singleton: it has mutable internal state shared by all callers. Since require caches the module, all require("mod") calls get the same object with the same state.

-- config.lua (singleton)
local M = {}
local _config = {env="dev", logLevel="info"}

function M.set(key, val)
  _config[key] = val
end

function M.get(key)
  return _config[key]
end

function M.load(t)
  for k,v in pairs(t) do _config[k]=v end
end

return M

-- All callers share the same config:

Module as Namespace

Use a module purely as a namespace to avoid polluting the global table. Group related constants and utilities under one name, like a package in other languages.

-- constants.lua
local M = {
  HTTP = {
    OK=200, CREATED=201, NO_CONTENT=204,
    BAD_REQUEST=400, UNAUTHORIZED=401,
    NOT_FOUND=404, SERVER_ERROR=500,
  },
  COLORS = {RED="#FF0000", GREEN="#00FF00", BLUE="#0000FF"},
  MAX_RETRIES = 3,
  TIMEOUT_SEC = 30,
}
return M

-- local C = require("constants")
-- if status == C.HTTP.NOT_FOUND then ...

Factory Module

A module that exports a factory function instead of a plain table. The factory creates and returns new instances with their own private state. This is the class pattern at the module level.

-- logger.lua
local M = {}

function M.new(name, level)
  level = level or "info"
  local levels = {debug=1,info=2,warn=3,error=4}
  local self = {}
  
  function self.log(msgLevel, msg)
    if levels[msgLevel] >= levels[level] then
      print(string.format("[%s][%s] %s", name, msgLevel:upper(), msg))
    end
  end
  
  function self.info(msg)  self.log("info",  msg) end
  function self.warn(msg)  self.log("warn",  msg) end
  function self.error(msg) self.log("error", msg) end
  
  return self
end

return M

Module Init Function

Some modules require configuration before use. Provide a M.init(config) function that stores config in the module's private state. This enables dependency injection and testability.

-- db.lua
local M = {}
local pool = nil

function M.init(config)
  pool = {
    host = config.host or "localhost",
    port = config.port or 5432,
    connections = {},
  }
  print("DB initialized:", pool.host, pool.port)
end

function M.query(sql)
  assert(pool, "call db.init() first")
  -- ... execute query
  return {}
end

return M

Immutable Module

Prevent users from modifying the module API accidentally by using __newindex to block all writes. This is especially useful for library modules where accidental monkey-patching could break things.

local function freeze(t)
  return setmetatable({}, {
    __index = t,
    __newindex = function(_, k, _)
      error("module is read-only, cannot set: " .. tostring(k), 2)
    end
  })
end

local M = {}
function M.add(a, b) return a + b end
function M.sub(a, b) return a - b end

return freeze(M)

Documenting with LDoc

A common convention for documenting Lua modules is LDoc-style comments with --- prefix. While not enforced by the language, these comments enable documentation generation tools to produce API docs automatically.

--- A utility module for string operations.
-- @module stringutils
local M = {}

--- Trim leading and trailing whitespace.
-- @param s string The input string.
-- @return string The trimmed string.
function M.trim(s)
  return s:match("^%s*(.-)%s*$")
end

--- Count occurrences of a substring.
-- @param str string The string to search.
-- @param sub string The substring to count.
-- @return number Count of occurrences.
function M.count(str, sub)
  local _, n = str:gsub(sub, "")
  return n
end

return M

Testing Modules

Test a module by requiring it and exercising each function. Use a simple test runner or busted (the Lua testing framework). Keep tests in a separate file that mirrors the module's path.

-- test/test_stringutils.lua
local su = require("stringutils")

local function test(name, fn)
  local ok, err = pcall(fn)
  if ok then print("[PASS] " .. name)
  else   print("[FAIL] " .. name .. ": " .. err)
  end
end

test("trim removes spaces", function()
  assert(su.trim("  hello  ") == "hello")
end)

test("trim empty string", function()
  assert(su.trim("") == "")
end)

test("count occurrences", function()
  assert(su.count("banana", "a") == 3)
end)

Composing Modules

Complex systems compose multiple modules. A main entry point requires and wires together sub-modules. This separation of concerns keeps each module focused and independently testable.

-- app.lua (main entry point)
local config = require("config")
local db     = require("db")
local server = require("server")

-- Configure from environment
config.load({
  dbHost = os.getenv("DB_HOST") or "localhost",
  port   = tonumber(os.getenv("PORT")) or 8080,
})

-- Wire modules together
db.init({host=config.get("dbHost"), port=5432})
server.init({port=config.get("port"), db=db})
server.start()

Quick Check

What is the main purpose of the local M = {} ... return M module pattern?

Recap: Module Best Practices

Summary:

Always local M = {} ... return M
Private = file-level locals; Public = M fields
Singleton: require caches the module instance
Use factory functions for per-instance state
Freeze modules with __newindex to prevent modification
Test in separate files; document with --- comments

Frequently asked questions

Is the “Module Patterns and Best Practices” lesson free?

Yes — the full text of “Module Patterns and Best Practices” is free to read here on the web, and the Lua Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Lua Academy course, upgrade to CoddyKit PRO.

What will I learn in “Module Patterns and Best Practices”?

Use local M = {} pattern and expose public API cleanly. You practise Lua Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Module Patterns and Best Practices” lesson take?

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