Typechecking question

[ExercitusMortem (Exercitus)]

Why can’t I create a type like this?
And how could I do it, alternatively?

type array_of_three = {number,number,number}

[12345koip]

It’s likely you can’t create a type like that because of duplicate types within the specifier table. Luau doesn’t let you specify numerical bounds to the type annnotation (which sucks).

You’ll need to use a generic type and enforce the 3-number limit at runtime. Or, if you really want to do something like this, you can use string number keys, but I wouldn’t recommend it.

type tableOfThree = { --not recommended
    ["1"]: number,
    ["2"]: number,
    ["3"]: number
}

--recommended
type tableOfThree = {number}
--or
type tableOfThree = {[number]: number}

--now enforce at runtime
local obj: tableOfThree = setmetatable({}, {["__newindex"] = function(self: tableOfThree, key: number, value: any)
    if type(key) ~= "number" then
        error(`Expected number key when adding to table, got {type(key)}`, 2)
    end

    if rawlen(self) < 3 then
        rawset(self, key, value)
    else
        error("Cannot add another key to table; table is at max size.", 2)
    end
end})

obj[1] = 3
obj[2] = "hi"
obj[3] = true
obj[4] = false --enforced by metamethod; error thrown