Typechecking with metamethods (__call)

  1. What do you want to achieve?
    Get the luau type engine to recognize callable tables which implement the __call metamethod while using --!strict mode

  2. What is the issue?
    I cannot figure out how to get the luau type engine to properly typehint a table with a metatable implementing a call method, particularly allowing the editor to autocomplete for a predefined type

  3. What solutions have you tried so far?

  • Searching on devforum turned up some results on how to implement typechecking with the typical OOP idiom (employing metatables), however, I did not find any results on the __call method specifically
  • The Luau typechecking docs make barely any mention of metatables, and don’t seem to be of much help
  • Trying to cast a table implementing __call to a callable (...any) -> someCustomType, as well as other solutions such as type tbl = typeof(setmetatable({}, {__call = ...

Below is a shortened version of what my code looks like:
Note: I have already solved this issue in --!nonstrict mode by casting the table into a callable, im asking if its possible to also allow the engine to typehint this in --!strict

--!strict
--inside a modulescript
type someCustomType = {
	spawnNew: (parent: Instance, location: Vector3 | CFrame) -> nil
}
return setmetatable({
	_id = game:GetService("HttpService"):GenerateGUID(false),
}, {
	__call = function(self, ...): someCustomType
		--create an "object" here
		local object = {}
		function object.spawnNew(parent: Instance, location: Vector3 | CFrame)
			--clone some instance and spawn it into the world
		end
		return object :: someCustomType
	end,
})

--!strict
--inside a script
local constructor = require(modulescript)
local createdObject = constructor()

--the luau engine does not think constructor can be called, nor does it infer a type for createdObject
createdObject.spawnNew(workspace, Vector3.new(0, 0, 0)) --does not typehint

(this is my first post on devforum so excuse me if I did something wrong)

5 Likes