Variables and memory

Variables in Rune are defined using the let keyword. In contrast to Rust, all variables in Rune are mutable and can be changed at any time.

pub fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
}

$> cargo run -- run scripts/book/variables/variables.rn
The value of x is: 5
The value of x is: 6

Rune is a memory safe language. Regardless of what you write in a Rune script, we maintain the same memory safety guarantees as safe Rust. This is accomplished through reference counting.

Unless a value is Copy, they are reference counted and can be used at multiple locations. This means that they have shared ownership. Every variable that points to that value therefore points to the same instance of that value. You can think of every nontrivial value being automatically wrapped in an Rc<RefCell<T>> if that helps you out.

This is not exactly what's going on. If you're interested to learn more, Rune uses a container called Shared<T> which is like an Rc<RefCell<T>>, but has a few more tricks.

We can see how this works by sharing and mutating one object across two variables:

pub fn main() {
    let object = #{ field: 1 };
    let object2 = object;
    println!("{}", object.field);
    object2.field = 2;

    // Note: we changed `object2`, but read out `object`
    println!("{}", object.field);
}

$> cargo run -- run scripts/book/variables/shared_ownership.rn
1
2

This can cause issues if we call an external function which expects to take ownership of its arguments. We say that functions like these move their argument, and if we try to use a variable which has been moved an error will be raised in the virtual machine.

Note: Below we use the drop function, which is a built-in function that will take its argument and free it.

pub fn main() {
    let object = #{ field: 1 };
    let object2 = object;
    println!("field: {}", object.field);
    drop(object2);
    println!("field: {}", object.field);
}

$> cargo run -- run scripts/book/variables/take_argument.rn
field: 1
== ! (cannot read, value is moved (at 14)) (469µs)
error: virtual machine error
  ┌─ scripts/book/variables/take_argument.rn:6:27
  │
6 │     println!("field: {}", object.field);
  │                           ^^^^^^^^^^^^ cannot read, value is moved

If you need to, you can test if a variable is still accessible for reading with is_readable, and for writing with is_writable. These are both imported in the prelude. An object which is writable is also movable, and can be provided to functions which need to move the value, like drop.

pub fn main() {
    let object = #{ field: 1 };
    let object2 = object;
    println!("field: {}", object.field);
    drop(object2);

    if is_readable(object) {
        println!("field: {}", object.field);
    } else {
        println!("object is no longer readable 😢");
    }
}

$> cargo run -- run scripts/book/variables/is_readable.rn
field: 1
object is no longer readable 😢