Crate sm

💋 SM – a static State Machine library

SM aims to be a safe, fast and simple macro-based state machine library.

• safe — the type system, move semantics and exhaustive pattern matching prevent you from mis-using your state machines

• fast — near-zero runtime overhead, all validation is done at compile-time

• simple — one declarative macro, control-flow only, no business logic attached

Using this library, you declaratively define your state machines as as set of states, connected via transitions, triggered by events. You can query the current state of the machine, or pattern match all possible states.

The implementation ensures a zero-sized abstraction that uses Rust's type-system and ownership model to guarantee valid transitions between states using events, and makes sure previous states are no longer accessible after transitioning away to another state. Rust validates correct usage of the state machine at compile-time, no runtime checking occurs when using the library.

The library exposes the sm! macro, which allows you to declaratively build the state machine.

Examples

Quick Example

#[macro_use] extern crate sm;

sm! {
    Lock {
        States { Locked, Unlocked, Broken }

        TurnKey {
            Locked => Unlocked
            Unlocked => Locked
        }

        Break {
            Locked => Broken
            Unlocked => Broken
        }
    }
}

fn main() {
    use Lock::*;
    let sm = Machine::new(Locked);
    let sm = sm.transition(TurnKey);

    assert_eq!(sm.state(), Unlocked);
}

Descriptive Example

The below example explains step-by-step how to create a new state machine using the provided macro, and then how to use the created machine in your code by querying states, and transitioning between states by triggering events.

Declaring a new State Machine

First, we import the macro from the crate:

#[macro_use] extern crate sm;

Next, we initiate the macro declaration:

sm! {

Then, provide a name for the machine, and declare its states:

    Lock {
        States { Locked, Unlocked, Broken }

Finally, we declare one or more events and the associated transitions:

        TurnKey {
            Locked => Unlocked
            Unlocked => Locked
        }

        Break {
            Locked => Broken
            Unlocked => Broken
        }
    }
}

And we're done. We've defined our state machine structure, and the valid transitions, and can now use this state machine in our code.

Using your State Machine

You can initialise the machine as follows:

let sm = Lock::Machine::new(Lock::Locked);

We can make this a bit less verbose by bringing our machine into scope:

use Lock::*;
let sm = Machine::new(Locked);

We've initialised our machine in the Locked state. You can get the current state of the machine by sending the state() method to the machine:

let state = sm.state();
assert_eq!(state, Locked);

While you can use sm.state() with conditional branching to execute your code based on the current state, this can be a bit tedious, it's less idiomatic, and it prevents you from using one extra compile-time validation tool in our toolbox: using Rust's exhaustive pattern matching requirement to ensure you've covered all possible state variants in your business logic.

While sm.state() returns the state as a unit-like struct (which itself is a ZST, or Zero Sized Type), you can use the sm.as_enum() method to get the state machine wrapped in an enum type.

Using the enum type and pattern matching, you are able to do the following:

match sm.as_enum() {
    States::Locked(m) => assert_eq!(m.state(), Locked),
    States::Unlocked(m) => assert_eq!(m.state(), Unlocked),
    States::Broken(m) =>  assert_eq!(m.state(), Broken),
}

The compiler won't be satisfied until you've either exhausted all possible enum variants, or you explicitly opt-out of matching all variants, either way, you can be much more confident that your code won't break if you add a new state down the road, but forget to add it to a pattern match somewhere deep inside your code-base.

Finally, as per our declaration, we can transition this machine to the Unlocked state by sending the TurnKey event:

let sm = sm.transition(TurnKey);
assert_eq!(sm.state(), Unlocked);

A word about Type-Safety and Ownership

It's important to realise that we've consumed the original machine in the above example, and got a newly initialised machine back in the Unlocked state.

This allows us to safely use the machine without having to worry about multiple readers using the machine in different states.

All these checks are applied on compile-time, so the following example would fail to compile:

let sm2 = sm.transition(TurnKey);
assert_eq!(sm.state(), Locked);

This fails with the following compilation error:

error[E0382]: use of moved value: `sm`
  --> src/lib.rs:140:12
   |
14 | let sm2 = sm.transition(TurnKey);
   |           -- value moved here
15 | assert_eq!(sm.state(), Locked);
   |            ^^ value used here after move
   |
   = note: move occurs because `sm` has type `Lock::Machine<Lock::Locked>`, which does not implement the `Copy` trait

Similarly, we cannot execute undefined transitions, these are also caught by the compiler:

let sm = sm.transition(TurnKey);
assert_eq!(sm.state(), Broken);

This fails with the following compilation error:

error[E0599]: no method named `transition` found for type `Lock::Machine<Lock::Broken>` in the current scope
  --> src/lib.rs:246:13
   |
3  | / sm! {
4  | |    Lock { Locked, Unlocked, Broken }
5  | |    TurnKey {
6  | |        Locked => Unlocked
...  |
13 | |    }
14 | | }
   | |_- method `transition` not found for this
...
19 |   let sm = sm.transition(TurnKey);
   |               ^^^^^^^^^^
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `transition`, perhaps you need to implement it:
           candidate #1: `Lock::Transition`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

The error message is not great (and can potentially be improved in the future), but any error telling you transition is not implemented, or the passed in event type is invalid is an indication that you are trying to execute an illegal state transition.

The End 👋

And that's it! There's nothing else to it, except a declarative – and easy to read – state machine construction macro, and a type-safe and ownership-focused way of dealing with states and transitions, without any runtime overhead.

Go forth and transition!

Macros

sm	Generate the declaratively described state machine diagram.

Traits

AsEnum	AsEnum provides the method to convert a state machine instance to an enum type.
Event	Event is a custom marker trait that allows unit-like structs to be used as states in a state machine.
Machine	Machine provides the method required to query a state machine for its current state.
State	State is a custom marker trait that allows unit-like structs to be used as states in a state machine.
Transition	Transition provides the method required to transition from one state to another.