How to use generic type packs in a table merging function

[Uhsleep91]

Hello, I have a question about the Luau type check system. I created a function to merge two tables together and the returned table’s type is an intersection of the two argument tables

function TableUtils.Merge<T, U>(TableA: T, TableB: U): T & U
    -- Code to merge tables
end

local A = {
    ValueA = 10
}

local B = {
    ValueB = 20
}

local C = TableUtils.Merge(A, B) -- Intellisense will show C.ValueA and C.ValueB

This worked fine, but now I want to create a merge function that allows for any amount of tables to be passed in. This is the best I could come up with but I can’t figure out how to intersect the returning type.

function TableUtils.Merge<T...>(...: T...): T...
    -- Code to merge tables
end

Intellisense seems to only show the first table’s values in the returned table instead of all of them. Does anyone know how I can change TableUtils.Merge to accept any amount of tables and have the returned table be an intersection of all the passed arguments?

[Xxforlife (Nazuh)]

Currently it’s not 100% possible to have dynamic function return types (I say not 100% possible because there are cases like this). I’m pretty sure the built-in functions that have this functionality have it hardcoded.

I also came across this, which further explains this topic.

[Uhsleep91]

@Xxforlife

Expects all arguments to be of the same type.

I believe ...T expects all arguments to be the same type and not T.... From Type checking - Luau, there’s an excerpt that talks about it.

Keep in mind that ...T is a variadic type pack (many elements of the same type T ), while U... is a generic type pack that can contain zero or more types and they don’t have to be the same.

That first link you posted has given me an idea though. I suppose I could just hardcode the types and have any arguments past the first two be optional.

function TableUtils.Merge<T,U,V,W>(TableA: T, TableB:U, TableC: V?, TableD: W?): T & U & V & W
    -- Code to merge tables
end

It’s way more verbose than I would’ve ideally liked but eh, it works I guess lol. In any case, thanks for looking into this and posting the links

[Xxforlife (Nazuh)]

I believe ...T expects all arguments to be the same type and not T... . From Type checking - Luau, there’s an excerpt that talks about it.

Apologies for that, I’ll update my comment for future viewers.

It’s way more verbose than I would’ve ideally liked but eh, it works I guess lol.

Yeah I definitely agree with you there.

In any case, thanks for looking into this and posting the links

Ofc