Constants and Atoms

Another way to build state machines is by combining other state machines. Aper ships with some built-in state machines which provide common patterns for managing state.

The simplest state machine is a Constant. It's a state machine whose transition has no exposed constructor, and therefore which can never be modified once it's created. It takes an initial value, and then stubbornly keeps that state for the rest of its life.

use aper::data_structures::Constant;

fn main() {
    // Construct a constant representing an i64.
    let int_constant = Constant::new(5);
    assert_eq!(&5, int_constant.value());

    // Construct a constant representing a String.
    let string_constant = Constant::new("Hi Aper".to_string());

    assert_eq!("Hi Aper", string_constant.value().as_str());
}

An Atom is similar to a Constant, except that it has a transition called ReplaceAtom.

It represents a value that can only be changed by replacing it entirely.

use aper::data_structures::Atom;
use aper::StateMachine;

fn main() {
    let mut atom = Atom::new(5);

    // Construct a new `ReplaceAtom` transition.
    let transition = atom.replace(6);

    // Remember, calling `.replace` does not actually change any
    // state -- only a call to `.apply` can do that.
    assert_eq!(&5, atom.value());
    
    atom.apply(transition);

    // Now the change takes effect.
    assert_eq!(&6, atom.value());
}