Luau Type System

When to Load

Load for Luau types: annotations, generics, unions, narrowing, sealed/unsealed tables, strictness (--!strict vs --!nonstrict), module type exports, metatable-backed object typing, and new-solver features (keyof, setmetatable<T, M>, type functions, read members). For syntax, use roblox-luau-core; for OOP/async, roblox-luau-patterns.

Quick Reference

Strictness: --!strict for maintained code, --!nonstrict while transitioning, --!nocheck only for legacy or generated code. Directives and project settings select the mode; never assume one global default.

New solver gate: keyof, rawkeyof, setmetatable<T, M>, type functions, and read members need the new type solver. It is on by default for nocheck/nonstrict projects; strict projects need Workspace.UseNewLuauTypeSolver = Enabled (Scripting category). Errors here usually mean the wrong solver, not wrong syntax.

Inference philosophy: Infer first; annotate boundaries (params, returns, exports). Don't annotate every local.

Sealed vs unsealed tables: An empty local t = {} stays open to new fields; annotating or passing it seals it, so later additions error. Build tables fully before annotating.

Unions: local id: string | number is a union; prefer tagged unions (type State<T> = {kind:"loading"} | {kind:"ready", value:T}) and discriminate on kind to narrow.

Narrowing: typeof(v) == "string" narrows primitives, instance:IsA("BasePart") narrows Instances, and assert(v, "msg") narrows away nil. Discriminant fields narrow tagged unions; see full reference for worked examples.

Generics: Use when input→output type matters: function first<T>(list: {T}): T?, or type Result<T> = {success: boolean, value: T?}. Never replace with any.

Type exports: export type Foo = {...} at module boundaries; consumers use Types.Foo.

Object typing: typeof(setmetatable({} :: CounterData, Counter)) types precise self; the new solver adds setmetatable<T, M> directly.

Casts (::): For narrowing overly generic inference, never for hiding errors.

Trust boundaries: Annotations contract with the compiler, not with runtime data. Remotes, DataStores, HttpService, and attributes still need checks; inside a trusted boundary, let types carry the load.

Key mistakes: any propagation in nonstrict, sealing too early, unions without discriminants, annotating every local, deleting new-solver syntax without checking the solver setting.

Full reference: see references/full.md