Luau for loop type checking

[APointProven (Soupy)]

I’m sure I’m simply misunderstanding how type checking works but I have this function here:

local function collgroup(model: Model)
    for i : number, v : BasePart in pairs(model:GetDescendants()) do
        v.CollisionGroup = 'nocollision'
    end
end

My issue is that this script still allows for instances other than BaseParts to pass through the for loop.

I’m aware that I could easily just do:

local function collgroup(model: Model)
    for i : number, v : BasePart in pairs(model:GetDescendants()) do
        if v:IsA('BasePart') then
            v.CollisionGroup = 'nocollision'
        end
    end
end

but I want to at least understand how the type checking works.

Thanks!

[lumizk (luz)]

if that happens, then the type of v has to be Instance. It can only be BasePart if you are absolutely sure that all decendants of model are BaseBarts.

[MagmaBurnsV]

The descendants’ array has the type {Instance}, so instead of typing v with BasePart, you should use Instance.

Once you pass the if v:IsA("BasePart") part, the type for v will then be refined to BasePart inside that scope.

[jake_4543 (Jakey)]

Type checking doesn’t effect runtime, it just helps you navigate your code better. (And will soon help the compiler run faster)