Conditional type checking in Luau?

[darkelementallord]

I’m still not very confident with Luau, and had a question. Is it possible to assign the type properly in this case?

type TestTable = {['Hello']: string};

local function Test(a: number): (boolean, TestTable?)
	if (a > 0) then
		return true, {['Hello'] = 'world'};
	else
		return false, nil;
	end
end

local function OtherTest()
	local Success, Output = Test(2);
	if (Success) then
		print(Output.Hello) -- "Value of type 'TestTable?' could be nil"
	end
end

Right now, if “Success” is true, then the Output MUST be the table, but the type checker still thinks it could be nil and give a type warning. Is there a correct way of handling this situation? Or some way of incorporating conditions in the type definitions?

[VegetationBush (FartedWayTooHard)]

Unfortunately, this does not exist at the moment. But there’s a feature request you can show support toward:

Ability to change function return type based on argument value

As a Roblox developer, it is currently impossible to change a function’s return type based on an argument value. local function getProductInfo(infoType: Enum.InfoType): (ProductInfo | AssetInfo | GamePassInfo) -- return product info end This function currently returns either a ProductInfo, AssetInfo, or GamePassInfo, but the type checker doesn’t really know which one it actually is, even though it will always be directly determined by the InfoType passed to the function. Ideally it should be …

But, there is a funky workaround you can try:

Implement tagged tuples

agreed that this would heavily improve typechecking, currently the only workaround would be to intersect the function’s type but it just feels really clunky local function _() error('uke9emc9wowk') end::(() -> false) & (() -> (true, number)) this feels like it’d be a better fit for an rfc for luau itself though, see GitHub - luau-lang/rfcs: RFCs for Luau evolution

[darkelementallord]

Ah that’s frustrating. Thanks for responding.