Luau Core

luau-core

When to Use

Use this skill when the task is primarily about plain Luau language behavior:

• Writing or fixing pure Luau syntax.
• Choosing idiomatic control flow, variable scope, or function structure.
• Working with tables as arrays, dictionaries, or object-like values.
• Using the standard library correctly.
• Applying metatables for indexing, operators, iteration, or object-like patterns.
• Checking whether a Lua feature or idiom is compatible with Luau.

Do not use this skill when the task is mainly about:

• Luau type annotations, generics, inference, refinements, or strictness policy.
• Profiling, optimization strategy, or performance tradeoffs.
• Roblox engine APIs, services, events, classes, networking, data, or gameplay architecture.

Decision Rules

• Use this skill if the answer can stay within pure Luau syntax, semantics, idioms, and standard library usage.
• If the task centers on type aliases, annotations, generics, exported types, or analyzer behavior, hand off to luau-types.
• If the task centers on allocation patterns, hot loops, benchmarking, profiling, or optimization strategy, hand off to luau-performance.
• If the task depends on game, Instance, services, remotes, datastores, UI, physics, or other Roblox runtime concepts, use the appropriate roblox/* skill instead.
• If a request mixes core language work with out-of-scope topics, answer only the pure Luau portion and explicitly exclude the rest.
• If unsure, prefer the narrower interpretation and omit material that could overlap another skill.

Instructions

1. Treat Luau as Lua 5.1 plus selected Luau extensions. Do not assume Lua 5.2+ features unless Luau explicitly supports them. 2. Prefer local variables and local function declarations by default. Use globals only when the broader scope is required by the task. 3. Keep control flow direct. Prefer normal if, elseif, else, loops, break, and continue over clever boolean tricks. 4. Use Luau-specific syntax when it improves correctness or readability:

• if ... then ... else ... expressions instead of a and b or c ternary emulation.
• Compound assignment such as += when the left side should only be evaluated once.
• Generalized iteration for k, v in table do when iterating a table directly is the clearest form.

5. Model tables intentionally:

• Arrays for ordered numeric sequences.
• Dictionaries for keyed lookup.
• Metatable-backed records only when custom behavior materially improves the API.

6. Use : for method definitions and calls that conceptually pass self; use . for plain function access. 7. Prefer built-in library functions over handwritten helpers when the standard library already expresses the operation clearly. 8. Use metatables sparingly and explicitly. Define only the metamethods needed for the abstraction and avoid surprising behavior. 9. When discussing compatibility, call out unsupported assumptions early, such as goto, bitwise operators, integer-only reasoning, or full Lua standard library access. 10. Keep all examples host-agnostic and pure Luau. Do not rely on Roblox engine objects or services.

Using References

• Open references/getting-started-and-syntax.md for syntax forms, literals, assignments, expression forms, and Luau-only syntax additions.
• Open references/functions-tables-operators-scope-and-control-structures.md for the core execution model: functions, closures, tables, truthiness, iteration, and control flow.
• Open references/metatables-and-userdata-concepts.md for metamethod behavior, object-like table patterns, raw operations, and userdata boundaries.
• Open references/library-and-grammar-reference.md for standard-library selection and compact grammar reminders for statements and expressions.
• Open references/compatibility-notes.md when a request compares Luau with Lua or depends on version-specific behavior.
• Do not open or cite references outside this skill unless the task clearly crosses into another skill's scope.

Checklist

• The solution is valid Luau syntax.
• Scope is explicit and defaults to local.
• Control flow is clear and uses the simplest correct construct.
• Table usage is intentional: array, dictionary, or metatable-backed record.
• Iteration choice matches the data shape.
• Standard library usage is correct and simpler than a custom helper.
• Any metatable behavior is minimal, predictable, and documented in code if non-obvious.
• No deep type-system material is included.
• No performance-tuning guidance is included.
• No Roblox engine APIs or architecture advice is included.

Common Mistakes

• Treating 0 or "" as falsy. In Luau, only false and nil are falsy.
• Using a and b or c as a ternary replacement when b can be false or nil.
• Assuming arrays are zero-based instead of one-based.
• Mixing method and function syntax, such as defining with : and calling with ..
• Relying on dictionary iteration order.
• Forgetting that missing function arguments become nil and extra arguments are ignored.
• Using globals where locals or closures are the correct fit.
• Overusing metatables for simple data containers.
• Assuming newer Lua features like goto, bitwise operators, or integer semantics exist in Luau.

Examples

Use if expressions instead of boolean ternary tricks

local function labelScore(score)
    return if score >= 100 then "high" else "normal"
end

Prefer locals and straightforward control flow

local function firstPositive(values)
    for _, value in values do
        if value > 0 then
            return value
        end
    end

    return nil
end

Use tables intentionally

local queue = {}

queue[#queue + 1] = "a"
queue[#queue + 1] = "b"

local first = table.remove(queue, 1)

Use method syntax for object-like tables

local Counter = {}
Counter.__index = Counter

function Counter.new()
    return setmetatable({ value = 0 }, Counter)
end

function Counter:increment()
    self.value += 1
    return self.value
end

Add metatables only when behavior is deliberate

local Range = {}
Range.__index = Range

function Range.new(minimum, maximum)
    return setmetatable({ minimum = minimum, maximum = maximum }, Range)
end

function Range:contains(value)
    return value >= self.minimum and value <= self.maximum
end

function Range:__tostring()
    return string.format("[%d, %d]", self.minimum, self.maximum)
end

Compatibility Notes

Key Concepts

• Luau starts from Lua 5.1 behavior and adopts selected ideas from later Lua versions.
• Luau is intentionally not a full superset of newer Lua releases.
• Compatibility questions often matter more than syntax similarity.

Rules

• Assume Lua 5.1 baseline unless a Luau extension is known to exist.
• Do not assume goto, bitwise operators, integer-only arithmetic, or the full later-Lua library surface.
• Prefer bit32 over bitwise operators when bit manipulation is needed in pure Luau.
• Remember that only selected later-Lua features were adopted, such as floor division and some string escape improvements.
• Call out environment-sensitive libraries early; sandboxed hosts may remove or restrict them.

Patterns

Supported or notable Luau additions

• continue
• Compound assignment operators
• if expressions
• Generalized iteration and __iter
• Floor division //
• Extra string escape forms such as \x, \u{...}, and \z

Common unsupported assumptions

• goto
• Native bitwise operators
• Separate integer runtime type
• Guaranteed tail-call support
• Full io, package, loadfile, dofile, or unrestricted debug access

Examples

Prefer Luau if expressions over Lua idioms

local value = if input ~= nil then input else defaultValue

Prefer bit32 instead of operator syntax

local masked = bit32.band(0xFF, 0x0F)

Do not assume goto

local found = false

for _, value in values do
    if value == target then
        found = true
        break
    end
end

Functions, Tables, Operators, Scope, and Control Structures

Key Concepts

• Functions are first-class values and can be stored, returned, and passed around.
• Closures capture locals from outer scopes as upvalues.
• Tables serve as arrays, dictionaries, records, namespaces, and object-like instances.
• Logical operators return operands, not forced booleans.
• Generic table iteration order is not guaranteed.

Rules

• Prefer local function name(...) for named helpers.
• Use : when the function logically receives self; use . otherwise.
• Treat arrays and dictionaries as different shapes even though both are tables.
• Use ipairs for array-like traversal that should stop at the first hole.
• Use pairs or direct generalized iteration for dictionary-like traversal.
• Do not rely on order from dictionary traversal.
• Remember that extra function arguments are ignored and missing ones become nil.

Patterns

Closure over local state

local function makeCounter()
    local value = 0

    return function()
        value += 1
        return value
    end
end

Array operations

local values = { "a", "b" }
values[#values + 1] = "c"
table.insert(values, "d")
local removed = table.remove(values, 2)

Dictionary lookup with default

local counts = {}

local function increment(key)
    counts[key] = (counts[key] or 0) + 1
end

Method syntax

local Buffer = {}
Buffer.__index = Buffer

function Buffer.new()
    return setmetatable({ items = {} }, Buffer)
end

function Buffer:push(value)
    self.items[#self.items + 1] = value
end

Examples

Truthiness and control flow

local value = ""

if value then
    print("empty string is still truthy")
end

Numeric and generic loops

for i = 1, 3 do
    print(i)
end

for key, value in { a = 1, b = 2 } do
    print(key, value)
end

Avoid boolean ternary emulation

local result = if maybeValue ~= nil then maybeValue else "fallback"

Getting Started and Syntax

Key Concepts

• Luau is based on Lua 5.1 and adds selected language features instead of matching later Lua versions wholesale.
• Files usually use .luau, but the language rules here are about Luau itself, not any host runtime.
• Only false and nil are falsy.
• Luau has one numeric type at runtime: a double-precision number.

Rules

• Prefer local for variables and functions.
• Use one-based indexing for arrays.
• return, break, and continue end the current block path.
• continue must be the last statement in its block path.
• Compound assignments such as += are statements, not expressions.
• Prefer if ... then ... else ... expressions over a and b or c.
• Do not bring in type-system detail here beyond recognizing that type syntax exists; deep typing belongs elsewhere.

Patterns

Local function and local state

local function clamp(value, minimum, maximum)
    if value < minimum then
        return minimum
    elseif value > maximum then
        return maximum
    end

    return value
end

Literal forms and simple expressions

local decimal = 1_000_000
local hex = 0xFF
local binary = 0b1010
local text = "line 1\nline 2"
local merged = "a" .. "b"

if expression

local function sign(x)
    return if x < 0 then -1 elseif x > 0 then 1 else 0
end

Compound assignment

local counts = { apples = 1 }
counts.apples += 2

Examples

General syntax shape

local total = 0

for i = 1, 5 do
    total += i
end

print(total)

continue in a loop

for _, value in { -2, 0, 5 } do
    if value <= 0 then
        continue
    end

    print(value)
end

Library and Grammar Reference

Key Concepts

• Luau exposes a compact standard library centered on builtin types and utility globals.
• Hosts may restrict environment access; do not assume file-system or process libraries exist.
• The grammar for this skill focuses on runtime statements and expressions, not deep type syntax.

Rules

• Prefer library functions when they directly express the operation.
• Use assert, error, pcall, and xpcall for explicit error paths.
• Use table, string, and math helpers instead of manual reimplementation when they improve clarity.
• Use next, pairs, ipairs, and direct for ... in table do intentionally based on traversal semantics.
• Do not assume io, package, unrestricted os, or unrestricted debug are available.

Patterns

Common globals

• assert(value, message?)
• error(obj, level?)
• type(obj) and typeof(obj)
• pairs(table) and ipairs(table)
• pcall(fn, ...) and xpcall(fn, handler, ...)
• rawget, rawset, rawlen
• tostring, tonumber, select, unpack

Useful library choices

• table.insert, table.remove, table.sort, table.concat, table.move, table.unpack
• string.format, string.gsub, string.gmatch, string.find, string.sub
• math.abs, math.floor, math.ceil, math.max, math.min, math.random

Runtime grammar reminders

• Assignment: varlist = explist
• Compound assignment: var compoundop exp
• Local declaration: local bindinglist ['=' explist]
• Loops: while, repeat ... until, numeric for, generic for
• Conditionals: if ... then ... elseif ... else ... end
• Last statements: return, break, continue
• Expressions include literals, function calls, table constructors, unary/binary operators, and if expressions

Examples

Error handling

local ok, result = pcall(function()
    assert(#"abc" == 3, "unexpected length")
    return math.max(1, 4, 2)
end)

Library-focused table work

local values = { 3, 1, 2 }
table.sort(values)
local text = table.concat(values, ",")

String iteration

for word in string.gmatch("alpha beta gamma", "%S+") do
    print(word)
end

Metatables and Userdata Concepts

Key Concepts

• A metatable lets a table or userdata customize selected operations.
• Common metamethods include __index, __newindex, arithmetic operators, __tostring, __len, and __iter.
• rawget, rawset, and rawlen bypass metatable behavior.
• Userdata represents host-defined opaque values. In plain Luau discussion, treat userdata as external values with limited direct control.

Rules

• Reach for metatables only when normal tables are no longer expressive enough.
• Prefer __index for shared methods and fallback lookup.
• Protect metatables with __metatable only when hiding mutation is part of the design.
• Use raw* operations only when bypassing metamethods is intentional.
• Avoid inventing broad operator overloading for simple records.
• Treat userdata support as conceptual unless the host explicitly exposes it.

Patterns

Shared methods via __index

local Point = {}
Point.__index = Point

function Point.new(x, y)
    return setmetatable({ x = x, y = y }, Point)
end

function Point:magnitudeSquared()
    return self.x * self.x + self.y * self.y
end

Read fallback table

local defaults = { retries = 3 }
local options = setmetatable({}, { __index = defaults })

Custom iteration

local reversed = setmetatable({ 10, 20, 30 }, {
    __iter = function(t)
        local index = #t + 1

        return function()
            index -= 1

            if index > 0 then
                return index, t[index]
            end
        end
    end,
})

Examples

__tostring

local Box = {}
Box.__index = Box

function Box.new(width, height)
    return setmetatable({ width = width, height = height }, Box)
end

function Box:__tostring()
    return string.format("Box(%d, %d)", self.width, self.height)
end

Userdata boundary

local ok, value = pcall(function()
    return newproxy(true)
end)

if ok then
    local mt = getmetatable(value)
    mt.__tostring = function()
        return "opaque"
    end
end