Welcome to the Luau Recap!
Luau is our new language that you can read more about at https://luau-lang.org.
Generic functions
The big news this month is that generic functions are back!
Luau has always supported type inference for generic functions, for example:
type Point<X,Y> = { x: X, y: Y }
function swap(p)
return { x = p.y, y = p.x }
end
local p : Point<number, string> = swap({ x = "hi", y = 37 })
local q : Point<boolean, string> = swap({ x = "hi", y = true })
but up until now, there’s been no way to write the type of swap , since Luau didn’t have type parameters to functions (just regular old data parameters). Well, now you can:
function swap<X, Y>(p : Point<X, Y>): Point<Y, X>
return { x = p.y, y = p.x }
end
Generic functions can be used in function declarations, and function types too, for example
type Swapper = { swap : <X, Y>(Point<X, Y>) -> Point<Y, X> }
People may remember that back in April we announced generic functions, but then had to disable them. That was because DataBrain discovered a nasty interaction between typeof and generics, which meant that it was possible to write code that needed nested generic functions, which weren’t supported back then.
Well, now we do support nested generic functions, so you can write code like
function mkPoint(x)
return function(y)
return { x = x, y = y }
end
end
and have Luau infer a type where a generic function returns a generic function
function mkPoint<X>(x : X) : <Y>(Y) -> Point<X,Y>
return function<Y>(y : Y) : Point<X,Y>
return { x = x, y = y }
end
end
For people who like jargon, Luau now supports Rank N Types , where previously it only supported Rank 1 Types.
Bidirectional typechecking
Up until now, Luau has used bottom-up typechecking. For example, for a function call f(x) we first find the type of f (say it’s (T)->U ) and the type for x (say it’s V ), make sure that V is a subtype of T , so the type of f(x) is U .
This works in many cases, but has problems with examples like registering callback event handlers. In code like
part.Touched:Connect(function (other) ... end)
if we try to typecheck this bottom-up, we have a problem because we don’t know the type of other when we typecheck the body of the function.
What we want in this case is a mix of bottom-up and top-down typechecking. In this case, from the type of part.Touched:Connect we know that other must have type BasePart .
This mix of top-down and bottom-up typechecking is called bidirectional typechecking , and means that tools like type-directed autocomplete can provide better suggestions.
Editor features
We have made some improvements to the Luau-powered autocomplete beta feature in Roblox Studio:
- We no longer give autocomplete suggestions for client-only APIs in server-side scripts, or vice versa.
- For table literals with known shape, we provide autocomplete suggestions for properties.
- We provide autocomplete suggestions for
Player.PlayerGui. - Keywords such as
thenandelseare autocompleted better. - Autocompletion is disabled inside a comment span (a comment starting
--[[).
Typechecking improvements
In other typechecking news:
- The Luau constraint resolver can now refine the operands of equality expressions.
- Luau type guard refinements now support more arbitrary cases, for instance
typeof(foo) ~= "Instance"eliminates anything not a subclass ofInstance. - We fixed some crashes caused by use-after-free during type inference.
- We do a better job of tracking updates when script is moved inside the data model.
- We fixed one of the ways that recursive types could cause free types to leak.
- We improved the way that
returnstatements interact with mutually recursive function declarations. - We improved parser recovery from code which looks like a function call (but isn’t) such as
local x = y
(expr)[smth] = z
- We consistently report parse errors before type errors.
- We display more types as
*unknown*rather than as an internal type name likeerror####. - Luau now infers the result of
Instance:Clone()much more accurately.
Performance improvements
-
Vector3.newconstructor has been optimized and is now ~2x faster - A previously implemented optimization for table size prediction has been enhanced to predict final table size when
setmetatableis used, such aslocal self = setmetatable({}, Klass) - Method calls for user-specified objects have been optimized and are now 2-4% faster
-
debug.tracebackis now 1.5x faster, althoughdebug.infois likely still superior for performance-conscious code - Creating table literals with explicit numeric indices, such as
{ [1] = 42 }, is now noticeably faster, although list-style construction is still recommended.
Other improvements
- The existing ‘TableLiteral’ lint now flags cases when table literals have duplicate numeric indices, such as
{ [1] = 1, [1] = 2 }
