A framework for building finite state machines in Rust
The rust-fsm
crate provides a simple and universal framework for building
state machines in Rust with minimum effort.
The essential part of this crate is the StateMachineImpl
trait. This trait
allows a developer to provide a strict state machine definition, e.g.
specify its:
- An input alphabet - a set of entities that the state machine takes as inputs and performs state transitions based on them.
- Possible states - a set of states this machine could be in.
- An output alphabet - a set of entities that the state machine may output as results of its work.
- A transition function - a function that changes the state of the state machine based on its current state and the provided input.
- An output function - a function that outputs something from the output alphabet based on the current state and the provided inputs.
- The initial state of the machine.
Note that on the implementation level such abstraction allows build any type of state machines:
- A classical state machine by providing only an input alphabet, a set of states and a transition function.
- A Mealy machine by providing all entities listed above.
- A Moore machine by providing an output function that do not depend on the provided inputs.
Features
This library has the feature named std
which is enabled by default. You
may want to import this library as
rust-fsm = { version = "0.6", default-features = false, features = ["dsl"] }
to use it in a no_std
environment. This only affects error types (the Error
trait is only available in std
).
The DSL implementation re-export is gated by the feature named dsl
which is
also enabled by default.
Use
Initially this library was designed to build an easy to use DSL for defining
state machines on top of it. Using the DSL will require to connect an
additional crate rust-fsm-dsl
(this is due to limitation of the procedural
macros system).
Using the DSL for defining state machines
The DSL is parsed by the state_machine
macro. Here is a little example.
use rust_fsm::*;
state_machine! {
derive(Debug)
repr_c(true)
CircuitBreaker(Closed)
Closed(Unsuccessful) => Open [SetupTimer],
Open(TimerTriggered) => HalfOpen,
HalfOpen => {
Successful => Closed,
Unsuccessful => Open [SetupTimer],
}
}
This code sample:
- Defines a state machine called
CircuitBreaker
; - Derives the
Debug
trait for it (thederive
section is optional); - Adds repr(C) support to generated code for better FFI compatability
(the
repr_c
section is optional and defaults to false); - Sets the initial state of this state machine to
Closed
; - Defines state transitions. For example: on receiving the
Successful
input when in theHalfOpen
state, the machine must move to theClosed
state; - Defines outputs. For example: on receiving
Unsuccessful
in theClosed
state, the machine must outputSetupTimer
.
This state machine can be used as follows:
// Initialize the state machine. The state is `Closed` now.
let mut machine: StateMachine<CircuitBreaker> = StateMachine::new();
// Consume the `Successful` input. No state transition is performed.
let _ = machine.consume(&CircuitBreakerInput::Successful);
// Consume the `Unsuccesful` input. The machine is moved to the `Open`
// state. The output is `SetupTimer`.
let output = machine.consume(&CircuitBreakerInput::Unsuccessful).unwrap();
// Check the output
if let Some(CircuitBreakerOutput::SetupTimer) = output {
// Set up the timer...
}
// Check the state
if let CircuitBreakerState::Open = machine.state() {
// Do something...
}
As you can see, the following entities are generated:
- An empty structure
CircuitBreaker
that implements theStateMachineImpl
trait. - Enums
CircuitBreakerState
,CircuitBreakerInput
andCircuitBreakerOutput
that represent the state, the input alphabet and the output alphabet respectively.
Note that if there is no outputs in the specification, the output alphabet
is set to ()
. The set of states and the input alphabet must be non-empty
sets.
Visibility
You can specify visibility like this:
state_machine! {
pub CircuitBreaker(Closed)
Closed(Unsuccessful) => Open [SetupTimer],
Open(TimerTriggered) => HalfOpen,
HalfOpen => {
Successful => Closed,
Unsuccessful => Open [SetupTimer],
}
}
Note that the default visibility is private just like for any structure. The specified visibility will apply to all structures and enums generated by the macro.
Without DSL
The state_machine
macro has limited capabilities (for example, a state
cannot carry any additional data), so in certain complex cases a user might
want to write a more complex state machine by hand.
All you need to do to build a state machine is to implement the
StateMachineImpl
trait and use it in conjuctions with some of the provided
wrappers (for now there is only StateMachine
).
You can see an example of the Circuit Breaker state machine in the project repository.