Metatable keyword in type annotations

Summary

Introduce a new contextual keyword metatable to define the type of a metatable for a table.

Motivation

Today, the only way to attach a metatable to a table, one has to write typeof(setmetatable(t, mt)) in type annotation context. While supported, it's not the most obvious path for everyone and leaves a pretty critical gap between the syntax and the type checker for an important part of Lua.

Design

We already have the concept of metatables in the type inference engine, so this RFC will not get into the subtyping relationship or anything of the sort. The scope of this RFC is specifically to expose syntax to produce a type variable whose table has this metatable, and no more than that.

We want to introduce a new contextual keyword metatable where we can write metatable <type> inside table type annotations. It can show up anywhere in a table type next to properties or indexers. Just like indexers, only one metatable is permitted per table and it will be enforced at parse time.

By being able to write that a table has some metatable, we would be able to say that our data type has overloaded some operators. For example, our Vec3 where the + operator takes two Vec3 and produces a new Vec3:

type Vec3 = {
    x: number,
    y: number,
    z: number,
    metatable {
        __add: (self: Vec3, other: Vec3) -> Vec3
    }
}

Furthermore, the metatable syntax can also help with writing the type definitions for idiomatic OO code.

type Person = {
    name: string,
    metatable PersonImpl,
}

type PersonImpl = {
    __index: PersonImpl,
    new: (name: string) -> Person,
    say: (self: Person, message: string) -> ()
}

local Person = {} :: PersonImpl
Person.__index = Person

function Person.new(name)
    local person = {}
    person.name = name
    return setmetatable(person, Person)
end

function Person:say(message)
    print(self.name .. ": " .. message)
end

The reason why two type synonyms are required here is because every instance created by Person.new is of type Person, but the table Person is what implements the methods for each instance of Person. The self in each methods of the table Person is not PersonImpl because usually we want to call the methods of PersonImpl where a Person is passed as self.

We would also be able to define the getmetatable function without using C++.

declare function getmetatable<T>(tab: {metatable {__metatable: T}}): T
declare function getmetatable<MT>(tab: {metatable MT}): MT
declare function getmetatable(tab: any): nil

Drawbacks

The syntax is unfortunately very identical to properties, where the only difference between them is a single character, :. Observe: {metatable T} vs {metatable: T} where the former is a table with the metatable of type T, and the latter is a table with one property metatable of type T.

Alternatives

A few other alternative designs have been proposed in the past, but ultimately were decided against for various reasons.

1: `setmetatable()`

One option is to copy the design of typeof for this use case: setmetatable(<type>, <type>). However, we decided against it because it is inconsistent with typeof where it uses () to access the value namespace, whereas setmetatable would use () to access the type namespace.

Another issue is in the name itself, setmetatable, which reads like it will mutate the type variable to have the metatable. There are cases where it is incorrect to mutate the type variable, so we would also have to introduce withmetatable(<type>, <type>) along with setmetatable(<type>, <type>).

This option also does not allow a way to return the metatable for any given type variable. The only way to do so is to add yet another syntax, getmetatable(<type>).

That means we need 3 new syntax in total to support different use cases:

setmetatable(T, MT) where it is a side-effecting function that mutably adds the metatable MT to T.
withmetatable(T, MT) where it is a pure function that returns a copy of T with MT attached.
getmetatable(T) where it returns the metatable type of T if it exists, the type of __metatable field if it exists, or nil type otherwise.

2: `setmetatable<>`

Another option is to define a setmetatable<T, MT> type function from the C++ side, which would indeed bridge the gap, but all type functions are pure, so the name setmetatable is thus inaccurate. It is more accurately called withmetatable. If we were to opt for this option, we would rather name it withmetatable.

It still does not grant us any syntax to return the metatable of the type, or nil otherwise. We would need to define a getmetatable<T> type function from C++ again, but even that would be inaccurate in some edge cases (metatable has __metatable or is missing/not a table, so returns nil) and requires either conditional types or making this an instrinsic type.

This option would mean withmetatable<> and getmetatable<> are the first exported types in Luau's prelude, which is not something that we want to do at this time.

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