rawget type functionThis RFC proposes the addition of a new type function, rawget, which can be used to look up a specific property of a table type without invoking the __index metamethod.
Currently, rawget is a built-in runtime operator in the language (rawget in Luau Global functions) that is missing a corresponding type function that captures its behavior. This RFC seeks to address this hole in the type system by providing a builtin type function for rawget that will allow Luau developers to express more accurate types in their code:
local prop: rawget<typeof(someExpr), "someProp"> = rawget(someExpr, "someProp")
The functionality of the rawget type function 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 function:
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 function 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 function is very minimal. Since the rawget type function functions similarly to the index type function, we can reuse the functions already used to implement the index type function.
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 functions 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 function.
Another drawback can come from the extra knowledge a user may need in order to use this type function. 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.
An alternative to the rawget type function is to enable users to modify the behavior of the existing index type function 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 functions, making this alternative design feasible for developers to implement.