Lua Guide

Lua Guide

Applies to: Lua 5.4+, LuaJIT 2.1, Neovim Plugins, Love2D, Embedded Scripting

Core Principles

1. Tables Are Everything: Arrays, maps, objects, modules, and namespaces -- master them
2. Local by Default: Always declare variables local; globals are a performance and correctness hazard
3. Explicit Error Handling: Use pcall/xpcall for recoverable errors; error() for programmer mistakes
4. Minimal Metatables: Use metatables for genuine OOP needs, not as decoration on simple data
5. Embed-Friendly Design: Lua exists to be embedded; keep the host/script boundary clean and narrow

Guardrails

Code Style

• Use local for every variable and function unless it must be global
• Naming: snake_case for variables/functions, PascalCase for class-like tables, UPPER_SNAKE_CASE for constants
• Indent with 2 spaces; one statement per line; avoid semicolons
• Use [[ ... ]] long strings for multi-line text and SQL/HTML templates
• Prefer #tbl over table.getn() for sequence length

Tables

• Arrays are 1-based; for i = 1, #arr not for i = 0, #arr - 1
• Use ipairs for sequential iteration, pairs for hash-map iteration
• Do not mix array indices and string keys in the same table (undefined # behavior)
• Use table.insert / table.remove for array ops; avoid manual index gaps
• Freeze config tables by setting a __newindex metamethod that errors

Error Handling

• Use pcall(fn, ...) to catch errors; xpcall(fn, handler, ...) for tracebacks
• Return nil, err_msg from functions that can fail (idiomatic two-value return)
• Reserve error("msg", level) for violated preconditions (programmer errors)
• Never silently swallow errors; always log or propagate

local function read_config(path)
  local f, err = io.open(path, "r")
  if not f then return nil, "cannot open config: " .. err end
  local content = f:read("*a")
  f:close()
  return content
end

local ok, result = xpcall(dangerous_operation, debug.traceback)
if not ok then log.error("failed: %s", result) end

Performance

• Localize hot functions: local insert = table.insert
• Avoid closures inside hot loops (allocates every iteration)
• Use table.concat instead of .. concatenation in loops
• LuaJIT: avoid pairs() in hot paths (not JIT-compiled); prefer arrays with ipairs
• LuaJIT: use FFI (ffi.new, ffi.cast) for C struct access instead of Lua tables

Embedding

• Keep the Lua-to-host API surface small (<20 registered functions)
• Validate all arguments from Lua in C/host bindings
• Set memory limits via lua_setallocf or lua_gc configuration
• Use debug.sethook instruction-count hooks for untrusted scripts

Key Patterns

Module Pattern

local M = {}
local TIMEOUT_MS = 5000

local function validate(data)
  assert(type(data) == "table", "expected table, got " .. type(data))
  assert(data.name, "missing required field: name")
end

function M.process(data)
  validate(data)
  return { status = "ok", name = data.name }
end

return M

OOP via Metatables

local Animal = {}
Animal.__index = Animal

function Animal.new(name, sound)
  return setmetatable({ name = name, sound = sound }, Animal)
end

function Animal:speak()
  return string.format("%s says %s", self.name, self.sound)
end

-- Inheritance
local Dog = setmetatable({}, { __index = Animal })
Dog.__index = Dog

function Dog.new(name)
  return setmetatable(Animal.new(name, "woof"), Dog)
end

function Dog:fetch(item)
  return string.format("%s fetches the %s", self.name, item)
end

Coroutines

local function producer(items)
  return coroutine.wrap(function()
    for _, item in ipairs(items) do
      coroutine.yield(item)
    end
  end)
end

local function filter(predicate, iter)
  return coroutine.wrap(function()
    for item in iter do
      if predicate(item) then coroutine.yield(item) end
    end
  end)
end

local nums = producer({ 1, 2, 3, 4, 5, 6 })
local evens = filter(function(n) return n % 2 == 0 end, nums)
for v in evens do print(v) end  --> 2, 4, 6

Custom Iterator

local function range(start, stop, step)
  step = step or 1
  local i = start - step
  return function()
    i = i + step
    if i <= stop then return i end
  end
end

for n in range(1, 10, 2) do print(n) end  --> 1, 3, 5, 7, 9

Neovim Lua API

local api, keymap = vim.api, vim.keymap
local M = {}

function M.setup(opts)
  opts = vim.tbl_deep_extend("force", { enabled = true, width = 80 }, opts or {})
  if not opts.enabled then return end

  local group = api.nvim_create_augroup("MyPlugin", { clear = true })
  api.nvim_create_autocmd("BufWritePre", {
    group = group, pattern = "*.lua",
    callback = function(ev)
      local lines = api.nvim_buf_get_lines(ev.buf, 0, -1, false)
      for i, line in ipairs(lines) do lines[i] = line:gsub("%s+$", "") end
      api.nvim_buf_set_lines(ev.buf, 0, -1, false, lines)
    end,
  })

  keymap.set("n", "<leader>mp", function()
    vim.notify("MyPlugin activated", vim.log.levels.INFO)
  end, { desc = "Activate MyPlugin" })
end

return M

Testing

Busted (Recommended)

local mymodule = require("mymodule")

describe("mymodule.process", function()
  it("returns ok for valid input", function()
    local result = mymodule.process({ name = "test" })
    assert.are.equal("ok", result.status)
  end)

  it("raises on missing name", function()
    assert.has_error(function() mymodule.process({}) end, "missing required field: name")
  end)
end)

Testing Standards

• Test files: spec/*_spec.lua (busted) or test_*.lua (luaunit)
• Test names describe behavior: it("returns nil when file not found")
• Coverage: >80% for library modules, >60% overall
• Test edge cases: nil, empty tables, boundary values, type mismatches
• Run: busted --verbose

Tooling

Luacheck

-- .luacheckrc
std = "lua54+busted"          -- or "luajit+busted"
globals = { "vim" }           -- for Neovim plugins
max_line_length = 120
max_cyclomatic_complexity = 10

StyLua

# stylua.toml
column_width = 100
indent_type = "Spaces"
indent_width = 2
quote_style = "AutoPreferDouble"
call_parentheses = "Always"

Essential Commands

lua myfile.lua                # Run Lua script
luajit myfile.lua             # Run with LuaJIT
busted --verbose              # Run tests
luacheck .                    # Lint
stylua .                      # Format
luarocks install busted       # Install test framework
luarocks install luacheck     # Install linter

References

For detailed patterns and examples, see:

• references/patterns.md -- OOP via metatables, module patterns, coroutine pipelines

External References

• Lua 5.4 Reference Manual
• Programming in Lua (4th ed)
• LuaJIT Documentation
• LuaJIT FFI Tutorial
• Neovim Lua Guide
• Busted Testing Framework
• Luacheck Linter
• StyLua Formatter
• Love2D Wiki
• Lua Style Guide