Ki

Mutability

Mutability is a core concept in Ki; its safety features depend upon controlling the mutability of memory. For all of its responsibility however, mutability is quite simple.

Everything Is Immutable

Everything in Ki starts life immutable:

fn main() {
    var charlie: Person("Charlie", 33)  # Immutable
    var hillary: Person("Hillary", 68)  # Immutable
    var barack: Person("Barack", 55) # Immutable
}

If you want to change something, you need a mutable reference:

fn main() {
    var charlie: Person("Charlie", 33)  # Immutable
    var hillary: Person("Hillary", 68)  # Immutable
    var barack: Person("Barack", 55) # Immutable

    with (&charlie! as p) {
        p.age++
    }
}

But the core purpose of any program is to mutate memory, and wrapping each mutation in a with block is intolerably tedious. Therefore, mutations will create an implicit mutable reference and use it:

fn main() {
    var charlie: Person("Charlie", 33)  # Immutable
    var hillary: Person("Hillary", 68)  # Immutable
    var barack: Person("Barack", 55) # Immutable

    charlie.age++ # Really looks like (&charlie!).age++
}

Opting in to mutability instead of opting out provides numerous safety guarantees and opens up several avenues for optimization, and combined with references it's easy to manage as well.