10 releases

0.4.1 Aug 13, 2021
0.4.0 Mar 21, 2020
0.3.0 Dec 20, 2019
0.2.4 Oct 3, 2019
0.1.1 Feb 8, 2019

#328 in Simulation

34 downloads per month
Used in rust-rpg-toolkit

Apache-2.0 OR MIT

180 lines

#mode Build Status Gitter

A simple and effective state machine library, written in idiomatic Rust.

What is mode?

This library provides three main types, Automaton, Mode, and Family, that facilitate the creation of finite state machines. An Automaton can be used to quickly create a state machine over a Family of states, where each state is an object that implements the Mode trait.


  • mode supports creating several different kinds of state machine:
    1. Simple state machines, where each state is a separate enum value and transitions are handled externally.
    2. More complex state machines, where each state is a separate struct that implements some common dyn Trait, and the responsibility for transitioning to the next Mode is delegated to the current Mode implementation.
    3. Data-driven state machines, where all states are represented by the same concrete type with different input.
  • Function calls can be dispatched to the current Mode easily through the containing Automaton, via Deref coercion.
  • You have total control over which public interface is exposed for the current Mode outside of the Automaton.
  • A flexible transition system allows the next Mode in the state machine to steal state from the previous Mode when it transitions in.
  • Modes can be stored in-place or heap-allocated, i.e. stored in a Box<T>, Rc<T>, or Arc<T>.
  • The library itself uses zero allocations. Any and all allocations are controlled by you and passed into the Automaton.

Why use mode?

  • It's flexible. This library imposes very few restrictions on how you write and organize your code. All lifetimes, allocations, and conventions are in your total control.
  • It's well-documented. All public types have detailed documentation and examples, so getting up to speed is easy.
  • It's easy to digest. Barring examples and comments, the whole library clocks in at less than 200 lines of code. That means digging into the internals of mode to figure out how something works is effortless.
  • It's pure Rust. No macro magic. No convoluted attribute markup. Just traits, structs, and generics.
  • 100% safe, 100% stable. There are zero unsafe blocks in this library, and no features that require the nightly toolchain. That means mode is dependable and robust.


See the full list of releases on GitHub.

Upgrading to version 0.4

A lot has been streamlined in version 0.4, in an effort to make mode even easier to understand and use. If you're interested in upgrading your project from version 0.3 to 0.4 of mode, please see UPGRADING-v0.4.md for a step-by-step guide.


Please see docs.rs for detailed documentation.


use mode::{Automaton, Family, Mode};

// This meta-struct represents a group of all Modes that can be used with the same Automaton, i.e. all states in the
// same state machine. By implementing Family, we can specify the common interface that will be exposed for all states
// (type Base) and how the current state will be stored in the Automaton (type Mode). The important thing to note is
// that this struct will never be instantiated. It only exists to group a set of states (Modes) together.
struct ActivityFamily;

impl Family for ActivityFamily {
    // This is the public interface that will be exposed by the Automaton for all Modes in this Family.
    type Base = dyn Activity;

    // This is the type that will be stored in the Automaton and passed into the Automaton::next() function.
    type Mode = Box<dyn Activity>;

// This trait defines a common interface for all Modes in ActivityFamily.
trait Activity : Mode<Family = ActivityFamily> {
    fn update(self : Box<Self>) -> Box<dyn Activity>;

// Each state in the state machine implements both Activity (the Base type) and Mode.
struct Working {
    pub hours_worked : u32,

impl Mode for Working {
    type Family = ActivityFamily;

impl Activity for Working {
    // This function updates the Mode and allows it to swap another one in as current, when ready.
    fn update(mut self : Box<Self>) -> Box<dyn Activity> {
        println!("Work, work, work...");
        self.hours_worked += 1;

        if self.hours_worked == 4 || self.hours_worked >= 8 {
            // To swap to another Mode, we can return a new, boxed Mode with the same signature as this one. Note that
            // because this function consumes the input Box<Self>, we can freely move state out of this Mode and into
            // the new one that will be swapped in.
            println!("Time for {}!", if self.hours_worked == 4 { "lunch" } else { "dinner" });
            Box::new(Eating { hours_worked: self.hours_worked, calories_consumed: 0 })
        else { self } // Returning self means that this Mode should remain current.

struct Eating {
    pub hours_worked : u32,
    pub calories_consumed : u32,

impl Mode for Eating {
    type Family = ActivityFamily;

impl Activity for Eating {
    fn update(mut self : Box<Self>) -> Box<dyn Activity> {
        self.calories_consumed += 100;

        if self.calories_consumed >= 500 {
            if self.hours_worked >= 8 {
                println!("Time for bed!");
                Box::new(Sleeping { hours_rested: 0 })
            else {
                println!("Time to go back to work!");
                Box::new(Working { hours_worked: self.hours_worked })
        else { self }

struct Sleeping {
    pub hours_rested : u32,

impl Mode for Sleeping {
    type Family = ActivityFamily;

impl Activity for Sleeping {
    fn update(mut self : Box<Self>) -> Box<dyn Activity> {
        self.hours_rested += 1;

        if self.hours_rested >= 8 {
            println!("Time for breakfast!");
            Box::new(Eating { hours_worked: 0, calories_consumed: 0 })
        else { self }

fn main() {
    let mut person = ActivityFamily::automaton_with_mode(Box::new(Working { hours_worked: 0 }));
    for _age in 18..100 {
        // Update the current Mode and/or transition to another Mode, when the current Mode requests it.
        Automaton::next(&mut person, |current_mode| current_mode.update());


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

If you find bugs, please feel free to open an issue on GitHub! Otherwise, if you would like to propose changes to this library, feel free to send me a pull request or message me on the mode Gitter channel. I'll try to respond to these requests as quickly as I can.

No runtime deps