Luau Recap: March 2023

How the time flies! The team has been busy since the last November Luau Recap working on some large updates that are coming in the future, but before those arrive, we have some improvements that you can already use!

[Cross-posted to the Roblox Developer Forum.]

Improved type refinements

Type refinements handle constraints placed on variables inside conditional blocks.

In the following example, while variable a is declared to have type number?, inside the if block we know that it cannot be nil:

local function f(a: number?)
    if a ~= nil then
        a *= 2 -- no type errors
    end
    ...
end

One limitation we had previously is that after a conditional block, refinements were discarded.

But there are cases where if is used to exit the function early, making the following code essentially act as a hidden else block.

We now correctly preserve such refinements and you should be able to remove assert function calls that were only used to get rid of false positive errors about types being nil.

local function f(x: string?)
    if not x then return end

    -- x is a 'string' here
end

Throwing calls like error() or assert(false) instead of a return statement are also recognized.

local function f(x: string?)
    if not x then error('first argument is nil') end

    -- x is 'string' here
end

Existing complex refinements like type/typeof, tagged union checks and other are expected to work as expected.

Marking table.getn/foreach/foreachi as deprecated

table.getn, table.foreach and table.foreachi were deprecated in Lua 5.1 that Luau is based on, and removed in Lua 5.2.

table.getn(x) is equivalent to rawlen(x) when ‘x’ is a table; when ‘x’ is not a table, table.getn produces an error.

It’s difficult to imagine code where table.getn(x) is better than either #x (idiomatic) or rawlen(x) (fully compatible replacement).

table.getn is also slower than both alternatives and was marked as deprecated.

table.foreach is equivalent to a for .. pairs loop; table.foreachi is equivalent to a for .. ipairs loop; both may also be replaced by generalized iteration.

Both functions are significantly slower than equivalent for loop replacements, are more restrictive because the function can’t yield.

Because both functions bring no value over other library or language alternatives, they were marked deprecated as well.

You may have noticed linter warnings about places where these functions are used. For compatibility, these functions are not going to be removed.

Autocomplete improvements

When table key type is defined to be a union of string singletons, those keys can now autocomplete in locations marked as ‘^’:

type Direction = "north" | "south" | "east" | "west"

local a: {[Direction]: boolean} = {[^] = true}
local b: {[Direction]: boolean} = {["^"]}
local b: {[Direction]: boolean} = {^}

We also fixed incorrect and incomplete suggestions inside the header of if, for and while statements.

Runtime improvements

On the runtime side, we added multiple optimizations.

table.sort is now ~4.1x faster (when not using a predicate) and ~2.1x faster when using a simple predicate.

We also have ideas on how improve the sorting performance in the future.

math.floor, math.ceil and math.round now use specialized processor instructions. We have measured ~7-9% speedup in math benchmarks that heavily used those functions.

A small improvement was made to builtin library function calls, getting a 1-2% improvement in code that contains a lot of fastcalls.

Finally, a fix was made to table array part resizing that brings large improvement to performance of large tables filled as an array, but at an offset (for example, starting at 10000 instead of 1).

Aside from performance, a correctness issue was fixed in multi-assignment expressions.

arr[1], n = n, n - 1

In this example, n - 1 was assigned to n before n was assigned to arr[1]. This issue has now been fixed.

Analysis improvements

Multiple changes were made to improve error messages and type presentation.

• Table type strings are now shown with newlines, to make them easier to read
• Fixed unions of nil types displaying as a single ? character
• “Type pack A cannot be converted to B” error is not reported instead of a cryptic “Failed to unify type packs”
• Improved error message for value count mismatch in assignments like local a, b = 2

You may have seen error messages like Type 'string' cannot be converted to 'string?' even though usually it is valid to assign local s: string? = 'hello' because string is a sub-type of string?.

This is true in what is called Covariant use contexts, but doesn’t hold in Invariant use contexts, like in the example below:

local a: { x: Model }
local b: { x: Instance } = a -- Type 'Model' could not be converted into 'Instance' in an invariant context

In this example, while Model is a sub-type of Instance and can be used where Instance is required.

The same is not true for a table field because when using table b, b.x can be assigned an Instance that is not a Model. When b is an alias to a, this assignment is not compatible with a’s type annotation.

---

Some other light changes to type inference include:

• string.match and string.gmatch are now defined to return optional values as match is not guaranteed at runtime
• Added an error when unrelated types are compared with ==/~=
• Fixed issues where variable after typeof(x) == 'table' could not have been used as a table

Thanks

A very special thanks to all of our open source contributors:

• niansa/tuxifan
• B. Gibbons
• Epix
• Harold Cindy
• Qualadore