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Using Rune

Luau is the default scripting language. A project switches to Rune with one line in project.toml:

name = "my-game"
main_scene = "scenes/main.toml"
language = "rune"

One project, one language. Both backends can run in a process, but scripts within a project do not mix languages.

The script model​

A Rune script is a set of free functions taking the instance first. The host makes one instance per attached node and hands it back on every call; an object handed into a Rune function is mutated in place, so what a script writes to this is what the host reads next frame.

// scripts/spinner.rn
const SPEED = 2.0;

pub fn init(this) {
    this.angle = 0.0;
    this.direction = 1.0;
}

pub fn update(this, dt) {
    if input::just_pressed(input::KEY_SPACE) {
        this.direction = -this.direction;
    }
    this.angle += dt * SPEED * this.direction;
    this.node.set_rotation_euler(0.0, this.angle, 0.0);
}

The same bindings​

Both languages call the same engine bindings: subsystems declare them once against the language-neutral seam, and each backend adds only its own call sugar. The script API is identical in both.

LuauRune
self.node:position()this.node.position()
input.just_pressed(input.KEY_SPACE)input::just_pressed(input::KEY_SPACE)
physics.add_body(self.node, physics.BODY_DYNAMIC)physics::add_body(this.node, physics::BODY_DYNAMIC)

Hot reload​

Rune compiles to an immutable unit, so hot reload replaces the unit rather than swapping a class table: instances keep their state objects, and the next call resolves against the new code. A compile error keeps the previous unit running, the same as Luau.

Export​

Rune resolves module items while compiling, so balaur export validates scripts against the modules the project's plugins actually register — a script that calls a binding that does not exist fails at export, not at run time.

The complete example is examples/hello_rune in the repository.