Metamethod __type

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#1

So, I was reviewing the documentation, and I was looking to make my own library and create my own classes. However I ran into one issue:

local meta = {__type = "Custom Type!"};
local obj = setmetatable({}, meta);
print(type(obj));

Using type() on my class just returns table. I even tried implementing it the same way shown in the documentation.

local obj = {__type = "Custom Type!"};
local meta = {__index = obj};
print(type(setmetatable({}, meta)));

A friend of mine said that roblox doesn’t allow custom types. Why would they provide it in the documentation if we aren’t allowed to use it? Like a father buying 2 fishing poles and going fishing without his neglected son.

1 Like
#2

If you check the code carefuly you will notice that they don’t get the datatype with type function, they check it manually:

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And they never mention about the __type metamethod in the list of metamethods. So I don’t know what are oyu trying to prove here:

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#3

so how do I make it return my custom type with type or typeof? I’d like to also include roblox types and native types

#4

They probably use another method that doesn’t involve metamethods because otherwise it would be a bit problematic for many reasons.

Or if they do, they don’t read the ones from metatables that can be changed by user.

#5

You’ll have to wrap around it yourself.

local old_typeof = typeof

local function typeof(arg: any): string
    local mt = getmetatable(arg)

    if mt and mt.__type then
        return mt.__type
    end
    return old_typeof(arg)
end
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#6

Closest you can do to get that would be through type script:

type myType = {[string]: any}

local _type: myType = {
    Index = 1
}

print(type(_type)) --> table

But still wouldn’t be a custom datatype.

or do what @sjr04 did

#7

Why does it have to get the metatable of arg? Does this work if I pass anything besides a metatable?

#8

So that it can get the __type “metamethod”

#9

I thought this would be useful

Using @sjr04’s method
If you want to use custom typing, while protecting the majority of the metatable, you can do this:

local meta = {
	__metatable = {
		__type = 'fish'
	},
	__newindex = function(self, i, v)
		print(i, v)
		rawset(self, i, v)
	end,
	__index = function(self, i)
		print(i)
		return rawget(self, i)
	end
}

This way, whenever you get the metatable, it will return __metatable, and everything else will be protected; __newindex and __index cannot be accessed from the getmetatable global.

local obj = setmetatable({}, meta)

print(getmetatable(obj).__type) --"fish"

The typeof function that @sjr04 provided will work exactly the same as before.

You can protect __metatable as well, but it is somewhat hacky and I would only recommend it for objects that are frequently and publicly used:

local meta = {}
do
	local __metatable = {
		__type = 'fish'
	}

	local proxy = newproxy(true)
	local proxy_meta = getmetatable(proxy);
	proxy_meta.__index = function(self, i) return __metatable[i] end
	proxy_meta.__metatable = 'locked'

	meta.__metatable = proxy
end
1 Like