Wonder

An Erlang/Elixir inspired actor library for Rust

Requirements

• stable/nightly Rust

Quick Start

Define your actor

pub struct MyActor;

Implement the GenServer trait

impl GenServer for MyActor {
    type T = MyMessage;
    type S = ();
    type E = MyError;

    fn init(&self, _tx: &ActorSender<Self::T>, state: &mut Self::S) -> InitResult<Self::E> {
      // initialization implementation
      Ok(None)
    }

    // Implement overrides for default implementations for additional callbacks
}

Start your actor

use wonder::actor;

fn main() {
    let actor = actor::Builder::new(MyActor).start(()).unwrap();
}

GenServer Trait

To create a GenServer actor you need to create a struct (perhaps a unit-like struct?) and implement the GenServer trait.

struct MyActor;

Associated Types

To implement the GenServer trait you need to define 3 associated types

1. T - A public enum type for messages to be sent to and from the GenServer.
2. S - The state which the actor will own and maintain. () can be provided in the case of an actor who will manage no state.
3. E - A public enum type for errors to be returned by your GenServer.

struct MyState {
    pub initialized: bool,
}

#[derive(Debug)]
enum MyError {
    DirtyState,
}

#[derive(Debug)]
enum MyMessage {
    State(bool),
    GetState,
    SetState(bool),
}

impl GenServer for MyActor {
    type T = MyMessage;
    type S = MyState;
    // type S = (); // no state
    type E = MyError;

    // ... callbacks
}

note: It is required for both the custom error and message enums to derive the Debug trait.

Callbacks

The GenServer trait exposes 4 callbacks; one of which is required while the remaining three have default implementations making them optional to implement.

init/3 -> InitResult<E>

analogous to GenServer:init/1

Handles initialization of the newly created actor. As with proccess initialization in Elixir/Erlang, this is a blocking call. This function will be called once while the actor is starting.

The init function must returns an InitResult which is either Ok(Option<u64>) or Err(E) where E is your custom error type. The optional u64 is the timeout value (in milliseconds) for the actor.

impl GenServer for MyActor {
    type T = MyMessage;
    type S = MyState;
    type E = MyError;

    fn init(&self, atx: &ActorSender<Self::T>, state: &mut Self::S) -> InitResult<Self::E> {
        // perform some initialization
        Ok(None)
    }
}

Parameters

• atx - Channel sender for the running actor. This is equivalent to sending to "self" in Elixir/Erlang.
• state - A mutable reference to the state that the running actor owns.

note: InitResult is analogous to the return tuple for the init/1 callback in Elixir; ie. {:ok, state} or {:stop, reason}

handle_call/5 -> HandleResult<T> (optional)

analogous to GenServer:handle_call/3

Handles synchronous messages sent to the running actor.

impl GenServer for MyActor {
    type T = MyMessage;
    type S = MyState;
    type E = MyError;

    // ... other callbacks ...

    fn handle_call(&self, msg: Self::T, tx: &ActorSender<Self::T>,
        atx: &ActorSender<Self::T>, state: &mut Self::S) -> HandleResult<Self::T> {
        match msg {
            MyMessage::GetState => HandleResult::Reply(MyMessage::State(state.initialized), None),
            MyMessage::SetState(value) => {
                state.initialized = value;
                HandleResult::Reply(MyMessage::Ok, None)
            }
            _ => HandleResult::Stop(StopReason::Fatal(String::from("unhandled call")), None),
        }
    }
}

Parameters

• tx - Channel sender for the caller who started the actor. This is equivalent to sending to the caller's PID in Elixir/Erlang.
• atx - Same as init/3.
• state - Same as init/3.

note: HandleResult is analogous to the return tuple for the handle_call/3 callback in Elixir/Erlang.

handle_cast/4 -> HandleResult<T> (optional)

analogous to GenServer:handle_cast/2

Handles asynchronous messages sent to the running actor.

impl GenServer for MyActor {
    type T = MyMessage;
    type S = MyState;
    type E = MyError;

    // ... other callbacks ...

    fn handle_cast(&self, msg: Self::T, atx: &ActorSender<Self::T>, state: &mut Self::S) -> HandleResult<Self::T> {
        match msg {
            MyMessage::SetState(value) => {
                state.initialized = value;
                HandleResult::NoReply(None)
            },
            _ => HandleResult::NoReply(None)
        }
    }
}

Parameters

• atx - Same as init/3
• state - Same as init/3

note: HandleResult is analogous to the return tuple for the handle_cast/2 callback in Elixir/Erlang.

handle_timeout/3 -> HandleResult<T> (optional)

analogous to matching handle_info(:timeout, _state) in Elixir

Called when a timeout is set and the required amount of time has elapsed. A timeout can be set by including a timeout value in a HandleResult or InitResult returned by one of the GenServer callbacks.

The timeout can be used for various reason, but a great example is a pattern to perform late initialization. If your actor has a long running initialization period you can timeout immediately and perform initialization within the handle_timeout callback.

impl GenServer for MyActor {
    type T = MyMessage;
    type S = MyState;
    type E = MyError;

    fn init(&self, atx: &ActorSender<Self::T>, state: &mut Self::S) -> InitResult<Self::E> {
        Ok(Some(0))
    }

    fn handle_timeout(&self, atx: &ActorSender<Self::T>, state: &mut Self::S) -> HandleResult<Self::T> {
        // long running function for late initialization
        HandleResult::NoReply(None)
    }
}

Parameters

• atx - Same as init/3
• state - Same as init/3

note: HandleResult is analogous to the return tuple for the handle_info/2 callback in Elixir/Erlang.

Authors

Jamie Winsor (jamie@vialstudios.com)