A fast, small, safe, gradually typed embeddable scripting language derived from Lua

Why Luau?
Type checking


Luau is based on Lua 5.1, and as such incorporates all features of 5.1, except for ones that had to be taken out due to sandboxing limitations. Because of backwards compatibility constraints, we don’t remove features deprecated by later versions (e.g. we still support getfenv/setfenv). Later Lua versions introduce new features into the language and new libraries/functions.

Our overall goal is to incorporate features from the later versions of Lua when it makes sense for us to do so - the motivations behind some newer features are unclear or don’t apply to the domain Luau is used in, and many features carry costs that don’t always make sense to pay. The rest of this document describes the status of all features of Lua 5.2 and beyond, with the following classification:

Please note that all of these decisions are not final, they are just our current stance. In some cases evolution of our VM may make a feature that was previously impractical to support due to performance complications feasible. In some cases a feature that didn’t have a strong use case gains one, so we can implement it.

Implementation limits

Luau has certain limitations around the number of local variables, registers, upvalues, constants and instructions. These limits are often different from the limits imposed by various versions of Lua, and are documented here without promising that future versions will adhere to these. Note that writing code that is close to any of these limits is dangerous because this code may become invalid as our codegen evolves.

Note that Lua 5.3 has a larger upvalue limit (255) and a larger constant limit (2^26); existing Luau limits are likely sufficient for reasonable use cases.

Lua 5.1

Since several features were removed from Lua 5.1 for sandboxing reasons, this table lists them for completeness.

feature notes
tail calls removed to simplify implementation and make debugging and stack traces easier
io, os, package and debug library note that some functions in os/debug are still present
loadfile, dofile removed for sandboxing, no direct file access
loadstring bytecode and string.dump exposing bytecode is dangerous for sandboxing reasons

Sandboxing challenges are covered in the dedicated section.

Lua 5.2

feature status notes
yieldable pcall and metamethods ✔️/❌ pcall/xpcall supports yielding but metamethods don’t
ephemeron tables this complicates the garbage collector esp. for large weak tables
emergency garbage collector Luau runs in environments where handling memory exhaustion in emergency situations is not tenable
goto statement this complicates the compiler due to handling of locals and doesn’t address a significant need
finalizers for tables no __gc support due to sandboxing and performance/complexity
no more fenv for threads or functions 😞 we love this, but it breaks compatibility
tables honor the __len metamethod performance implications, no strong use cases
hex and \z escapes in strings ✔️  
support for hexadecimal floats 🤷‍♀️ no strong use cases
order metamethods work for different types no strong use cases and more complicated semantics + compat
empty statement 🤷‍♀️ less useful in Lua than in JS/C#/C/C++
break statement may appear in the middle of a block 🤷‍♀️ we’d like to do it for return/continue as well but there be dragons
arguments for function called through xpcall ✔️  
optional base in math.log ✔️  
optional separator in string.rep 🤷‍♀️ no real use cases
new metamethods __pairs and __ipairs would like to reevaluate iteration design long term
frontier patterns ✔️  
%g in patterns ✔️  
\0 in patterns ✔️  
bit32 library ✔️  

Two things that are important to call out here are various new metamethods for tables and yielding in metamethods. In both cases, there are performance implications to supporting this - our implementation is very highly tuned for performance, so any changes that affect the core fundamentals of how Lua works have a price. To support yielding in metamethods we’d need to make the core of the VM more involved, since almost every single “interesting” opcode would need to learn how to be resumable - which also complicates future JIT/AOT story. Metamethods in general are important for extensibility, but very challenging to deal with in implementation, so we err on the side of not supporting any new metamethods unless a strong need arises.

For __pairs/__ipairs, we aren’t sure that this is the right design choice - self-iterating tables via __iter are very appealing, and if we can resolve some challenges with array iteration order, that would make the language more accessible so we may go that route instead.

Ephemeron tables may be implemented at some point since they do have valid uses and they make weak tables semantically cleaner, however the cleanup mechanism for these is expensive and complicated, and as such this can only be considered after the pending GC rework is complete.

Lua 5.3

feature status notes
\u escapes in strings ✔️  
integers (64-bit by default) backwards compatibility and performance implications
bitwise operators bit32 library covers this
basic utf-8 support ✔️ we include utf8 library and other UTF8 features
functions for packing and unpacking values (string.pack/unpack/packsize) ✔️  
floor division no strong use cases, syntax overlaps with C comments
ipairs and the table library respect metamethods no strong use cases, performance implications
new function table.move ✔️  
collectgarbage("count") now returns only one result ✔️  
coroutine.isyieldable ✔️  

It’s important to highlight integer support and bitwise operators. For Luau, it’s rare that a full 64-bit integer type is necessary - double-precision types support integers up to 2^53 (in Lua which is used in embedded space, integers may be more appealing in environments without a native 64-bit FPU). However, there’s a lot of value in having a single number type, both from performance perspective and for consistency. Notably, Lua doesn’t handle integer overflow properly, so using integers also carries compatibility implications.

If integers are taken out of the equation, bitwise operators make much less sense; additionally, bit32 library is more fully featured (includes commonly used operations such as rotates and arithmetic shift; bit extraction/replacement is also more readable). Adding operators along with metamethods for all of them increases complexity, which means this feature isn’t worth it on the balance.

Floor division is less harmful, but it’s used rarely enough that math.floor(a/b) seems like an adequate replacement; additionally, // is a comment in C-derived languages and we may decide to adopt it in addition to -- at some point.

Lua 5.4

feature status notes
new generational mode for garbage collection 🔜 we’re working on gc optimizations and generational mode is on our radar
to-be-closed variables the syntax is ugly and inconsistent with how we’d like to do attributes long-term; no strong use cases in our domain
const variables while there’s some demand for const variables, we’d never adopt this syntax
new implementation for math.random ✔️ our RNG is based on PCG, unlike Lua 5.4 which uses Xoroshiro
optional init argument to string.gmatch 🤷‍♀️ no strong use cases
new functions lua_resetthread and coroutine.close not useful without to-be-closed variables
coersions string-to-number moved to the string library 😞 we love this, but it breaks compatibility
new format %p in string.format 🤷‍♀️ no strong use cases
utf8 library accepts codepoints up to 2^31 🤷‍♀️ no strong use cases
The use of the __lt metamethod to emulate __le has been removed 😞 breaks compatibility and doesn’t seem very interesting otherwise
When finalizing objects, Lua will call __gc metamethods that are not functions no __gc support due to sandboxing and performance/complexity
The function print calls __tostring instead of tostring to format its arguments. 🔜  
By default, the decoding functions in the utf8 library do not accept surrogates. 😞 breaks compatibility and doesn’t seem very interesting otherwise

Lua has a beautiful syntax and frankly we’re disappointed in the <const>/<toclose> which takes away from that beauty. Taking syntax aside, <toclose> isn’t very useful in Luau - its dominant use case is for code that works with external resources like files or sockets, but we don’t provide such APIs - and has a very large complexity cost, evidences by a lot of bug fixes since the initial implementation in 5.4 work versions. <const> in Luau doesn’t matter for performance - our multi-pass compiler is already able to analyze the usage of the variable to know if it’s modified or not and extract all performance gains from it - so the only use here is for code readability, where the <const> syntax is… suboptimal.

If we do end up introducing const variables, it would be through a const var = value syntax, which is backwards compatible through a context-sensitive keyword similar to type.