Luau Recap: November 2022

While the team is busy to bring some bigger things in the future, we have made some small improvements this month.

[Cross-posted to the Roblox Developer Forum.]

Analysis improvements

We have improved tagged union type refinements to only include unhandled type cases in the else branch of the if statement:

type Ok<T> = { tag: "ok", value: T }
type Err = { tag: "error", msg: string }
type Result<T> = Ok<T> | Err

function unwrap<T>(r: Result<T>): T?
    if r.tag == "ok" then
        return r.value
        -- Luau now understands that 'r' here can only be the 'Err' part
        return nil

For better inference, we updated the definition of Enum.SomeType:GetEnumItems() to return {Enum.SomeType} instead of common {EnumItem} and the return type of next function now includes the possibility of key being nil.

Finally, if you use and operator on non-boolean values, boolean type will no longer be added by the type inference:

local function f1(a: number?)
    -- 'x' is still a 'number?' and doesn't become 'boolean | number'
    local x = a and 5

Error message improvements

We now give an error when built-in types are being redefined:

type string = number -- Now an error: Redefinition of type 'string'

We also had a parse error missing in case you forgot your default type pack parameter value. We accepted the following code silently without raising an issue:

type Foo<T... = > = nil -- Now an error: Expected type, got '>'

Error about function argument count mismatch no longer points at the last argument, but instead at the function in question. So, instead of:

function myfunction(a: number, b:number) end

We now highlight this:

function myfunction(a: number, b:number) end

If you iterate over a table value that could also be nil, you get a better explanation in the error message:

local function f(t: {number}?)
    for i,v in t do -- Value of type {number}? could be nil

Previously it was Cannot call non-function {number}? which was confusing.

And speaking of confusing, some of you might have seen an error like Type 'string' could not be converted into 'string'.

This was caused by Luau having both a primitive type string and a table type coming from string library. Since the way you can get the type of the string library table is by using typeof(string), the updated error message will mirror that and report Type 'string' could not be converted into 'typeof(string)'.

Parsing now recovers with a more precise error message if you forget a comma in table constructor spanning multiple lines:

local t = {
    a = 1
    b = 2 -- Expected ',' after table constructor element
    c = 3 -- Expected ',' after table constructor element