rfcs/docs/rawget-type-operator.md

rawget type operator

Summary

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

Motivation

Currently, rawget is a built-in runtime operator in the language (rawget in Luau Global functions) that is missing a corresponding type operator that captures its behavior. This RFC seeks to address this hole in the type system by providing a builtin type operator for rawget that will allow Luau developers to express more accurate types in their code:

local prop: rawget<typeof(someExpr), "someProp"> = rawget(someExpr, "someProp")

Design

The functionality of the rawget type operator behaves the same as its runtime counterpart. Namely, it provide a way to look up a specific property of a table 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'

Note: rawget type operator does not work on class types because they do not have direct fields that can be looked up without invoking its metamethod.

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, this additional complexity is manageable because the language already incorporates such complexities at the runtime level. Developers are already familiar with concepts like rawget, so this does not add much new complexity.

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.