Luau — Fast Embeddable Scripting Language Based on Lua

Introduction

Luau (pronounced "Lu-ow") started as an internal fork of Lua 5.1 at Roblox to support millions of developers building games on the platform. It adds a gradual type system, better performance, and modern language features while maintaining backward compatibility with standard Lua.

What Luau Does

• Provides a Lua-compatible scripting language with gradual typing
• Performs type inference to catch errors without requiring annotations
• Compiles to a custom bytecode with a fast interpreter and JIT compiler
• Embeds easily into C/C++ host applications via a simple API
• Supports sandboxing for safe execution of untrusted scripts

Architecture Overview

Luau consists of a compiler that produces bytecode, a bytecode interpreter, and a native code generator for x64 and ARM64. The type checker runs as a separate analysis pass and supports both strict and nonstrict modes. The runtime is designed for low-latency environments, with incremental garbage collection tuned for interactive applications.

Self-Hosting & Configuration

• Build from source using CMake on Linux, macOS, or Windows
• Embed by linking the Luau library and initializing a Lua state
• Use luau_compile() to compile source to bytecode ahead of time
• Enable the type checker with --!strict or --!nonstrict directives
• Configure sandboxing by restricting available standard library functions

Key Features

• Gradual type system with inference eliminates common runtime errors
• Native code generation (JIT) for x64 and ARM64 platforms
• Backward compatible with Lua 5.1 syntax and semantics
• Incremental garbage collector tuned for real-time applications
• String interpolation, compound assignment, and other syntax improvements over Lua

Comparison with Similar Tools

• Lua 5.4 — Standard Lua lacks a type system and JIT; Luau adds both while staying compatible
• LuaJIT — Faster JIT for Lua 5.1 but no type system; Luau trades peak throughput for type safety
• Teal — Typed Lua that transpiles to Lua; Luau is a native implementation with its own runtime
• MoonScript — CoffeeScript-like syntax for Lua; Luau extends Lua syntax without changing its feel

FAQ

Q: Is Luau only for Roblox development? A: No. Luau is open source and can be embedded in any C/C++ application. It is used outside Roblox in game engines and other projects.

Q: Can I run existing Lua 5.1 scripts in Luau? A: Most Lua 5.1 code runs without changes. Some differences exist in string patterns and coroutine behavior.

Q: Does Luau support LuaRocks packages? A: Not directly. LuaRocks packages targeting the C Lua API need adaptation for the Luau API differences.

Q: How does the gradual type system work? A: Type annotations are optional. Luau infers types where possible and only reports errors where it can prove a type mismatch. Strict mode requires all types to be resolvable.

Sources

• https://github.com/luau-lang/luau
• https://luau.org/