is inheritence possible?

[SDFTDusername]

i was wondering if it is possible to have userdata that inherits another userdata with the luau feature.
i've already tried setting __index to return the super class. i also tried returning the metatable of the super, though i wasn't able to due to the metatable being wrapped around UserDataMetatable.

my goal is to have two types named Instance and WorldInstance, where Instance has a field named Name and WorldInstance has a field named Position. the user would then be able to access the Name field from an instance of WorldInstance. would that be possible?

[khvzak]

You can do something like:

struct Instance(String);

impl UserData for Instance {
    fn register(registry: &mut UserDataRegistry<Self>) {
        registry.add_field_method_get("Name", |_, this| Ok(this.0.clone()));
    }
}

struct WorldInstance {
    parent: AnyUserData,
    position: f32,
}

impl UserData for WorldInstance {
    fn register(registry: &mut UserDataRegistry<Self>) {
        registry.add_field_method_get("Position", |_, this| Ok(this.position));

        registry.add_meta_method("__index", |_, this, key: String| {
            use mlua::ObjectLike as _;
            this.parent.get::<Value>(key)
        });
    }
}

let instance = lua.create_userdata(Instance("example".to_string()))?;
lua.globals().set("Instance", &instance)?;
lua.globals().set(
    "WorldInstance",
    lua.create_userdata(WorldInstance {
        parent: instance,
        position: 1.23,
    })?,
)?;

[SDFTDusername]

that does work, though the only issue that i have is that if the key doesn't exist, it would return nil instead of giving me an error. this also makes it unknown if its a field with a value of nil, or if the field doesn't exist.
since i can't find any method for checking if a key exists, is there another way make it return an error for when the parent field doesn't exist?

[khvzak]

if the key doesn't exist, it would return nil instead of giving me an error

you sure? Accessing WorldInstance.NonExistent raise an exception runtime error: attempt to get an unknown field 'NonExistent'.

This can happen though for static fields. In Lua nil and non-existent is the same (for example you cannot have a table with a key set to nil).