`rawget` type operator

Summary

This RFC proposes the addition on one type operator, rawget, which can be used to look up a specific property of another type without invoking the __index metamethod.

Motivation

Given that rawget is a built-in runtime operator in the language (rawget Lua Globals), it feels natural for there to be a version of this as a type operator. As such, the main motivation behind this feature is to close out holes in Luau's type families and allow Luau developers to be more expressive writing typed code:

local prop: rawget<someTy, someProp> = rawget(someTy, someProp)

Design

The functionality of rawget type operator behaves the same as its runtime pair: provide a way to look up a specific property of a type without invoking the __index metamethod.

local var1 = {
  property = "Hello"
}

local var2 = setmetatable({ }, { __index = var1 })
type doesntExist = rawget<typeof(var2), "property"> -- reduces to an error

local var3 = setmetatable({ property = 1 }, { __index = var1 })
type doesExist = rawget<typeof(var3), "property"> -- doesExist = number

Error messages would be consistent with those of the index type operator:

type doesntExist = rawget<typeof(var2), "property">  -- Error message: Property '"property"' does not exist on type 'var2'

local key = "property"
type age = rawget<Person, key> -- Error message: Second argument to rawget<Person, _> is not a valid index type; Unknown type 'key'

The implementation effort for this type operator is very minimal. Since the rawget type operator functions similarly to the index type operator, we can reuse the functions already used to implement the index type operator.

Drawbacks

There aren't appreciable drawbacks. One possible drawback is the increase in the size of the codebase, which may complicate maintenance. However, this is not expected to be a major drawback since the code for index type operators already exists. By reusing the code, it is even fair to say that there will be less than 50 lines of code needed to implement this type operator.

Another drawback can come from the extra knowledge a user may need in order to use this type operator. For example, users will need to know the inner workings of types and how different operations interact with them (for instance, how index interacts with metatables that have the __index metamethod). However, there are documentations that outline each interactions, and thus this drawback does not seem to pose an issue.

Alternatives

An alternative to the rawget type operator is to enable users to modify the behavior of the existing index type operator to control its recursive nature. While this approach offers flexibility, it requires additional design considerations and complicates both usage and implementation. However, the introduction of "user-defined type functions" is planned for the near future. This will allow users to create their own type operators, making this alternative design feasible for developers to implement.

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rawget type operator