luau/docs/_posts/2022-11-30-luau-recap-november-2022.md

layout	title
single	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.]

Error messages

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
myfunction(123)
           ~~~

We now highlight this:

function myfunction(a: number, b:number) end
myfunction(123)
~~~~~~~~~~

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
}

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
        --...
    end
end

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)'."

Analysis improvements

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

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
end