Lua language guardrails, patterns, and best practices for AI-assisted development.
Use when working with Lua files (.lua), or when the user mentions Lua/LuaJIT/Neovim/Love2D.
Provides table patterns, metatable guidelines, coroutine usage,
and embedding conventions specific to this project's coding standards.
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Lua language guardrails, patterns, and best practices for AI-assisted development.
Use when working with Lua files (.lua), or when the user mentions Lua/LuaJIT/Neovim/Love2D.
Provides table patterns, metatable guidelines, coroutine usage,
and embedding conventions specific to this project's coding standards.
Tables Are Everything: Arrays, maps, objects, modules, and namespaces -- master them
Local by Default: Always declare variables local; globals are a performance and correctness hazard
Explicit Error Handling: Use pcall/xpcall for recoverable errors; error() for programmer mistakes
Minimal Metatables: Use metatables for genuine OOP needs, not as decoration on simple data
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
localfunctionread_config(path)local f, err = io.open(path, "r")
ifnot f thenreturnnil, "cannot open config: " .. err endlocal content = f:read("*a")
f:close()
return content
endlocal ok, result = xpcall(dangerous_operation, debug.traceback)
ifnot ok thenlog.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 = 5000localfunctionvalidate(data)assert(type(data) == "table", "expected table, got " .. type(data))
assert(data.name, "missing required field: name")
endfunctionM.process(data)
validate(data)
return { status = "ok", name = data.name }
endreturn M
OOP via Metatables
local Animal = {}
Animal.__index = Animal
functionAnimal.new(name, sound)returnsetmetatable({ name = name, sound = sound }, Animal)
endfunctionAnimal:speak()returnstring.format("%s says %s", self.name, self.sound)
end-- Inheritancelocal Dog = setmetatable({}, { __index = Animal })
Dog.__index = Dog
functionDog.new(name)returnsetmetatable(Animal.new(name, "woof"), Dog)
endfunctionDog:fetch(item)returnstring.format("%s fetches the %s", self.name, item)
end
Coroutines
localfunctionproducer(items)returncoroutine.wrap(function()for _, item inipairs(items) docoroutine.yield(item)
endend)
endlocalfunctionfilter(predicate, iter)returncoroutine.wrap(function()for item in iter doif predicate(item) thencoroutine.yield(item) endendend)
endlocal nums = producer({ 1, 2, 3, 4, 5, 6 })
local evens = filter(function(n)return n % 2 == 0end, nums)
for v in evens doprint(v) end--> 2, 4, 6
Custom Iterator
localfunctionrange(start, stop, step)
step = step or1local i = start - step
returnfunction()
i = i + step
if i <= stop thenreturn i endendendfor n in range(1, 10, 2) doprint(n) end--> 1, 3, 5, 7, 9
Neovim Lua API
local api, keymap = vim.api, vim.keymap
local M = {}
functionM.setup(opts)
opts = vim.tbl_deep_extend("force", { enabled = true, width = 80 }, opts or {})
ifnot opts.enabled thenreturnendlocal group = api.nvim_create_augroup("MyPlugin", { clear = true })
api.nvim_create_autocmd("BufWritePre", {
group = group, pattern = "*.lua",
callback = function(ev)locallines = api.nvim_buf_get_lines(ev.buf, 0, -1, false)
for i, line inipairs(lines) dolines[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" })
endreturn 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
