Luau Type System

When to Load

Load for Luau annotations, inference, narrowing, strictness, exports, and solver features. Syntax: roblox-luau-core; lifecycle/scheduling: 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. On by default for nocheck/nonstrict projects; strict projects need Workspace.UseNewLuauTypeSolver = Enabled (Scripting). Errors here usually mean wrong solver, not wrong syntax.

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

Sealed vs unsealed tables: An unannotated local table stays open to new fields until annotated, returned from a function, or its scope exits; annotation seals it. Build tables fully before annotating; passing a table to a typed function does not seal it.

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, IsA, assertions, and tagged discriminants refine types; examples in full.md.

Missing keys: use { [K]: V? } when lookups may miss; narrow before use.

Closed vocabularies: prefer literal unions; never checks can flag missing branches. Keep requires statically resolvable. Practitioner guidance: full.md.

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 runtime data. Remotes, DataStores, HttpService, and attributes still need checks; inside a trusted boundary, let types carry the load.

Review: no hidden any, premature sealing, unchecked casts, or solver-blind rewrites.

Full reference: see references/full.md