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/*! [![](https://docs.rs/state_machine_future/badge.svg)](https://docs.rs/state_machine_future/) [![](https://img.shields.io/crates/v/state_machine_future.svg)](https://crates.io/crates/state_machine_future) [![](https://img.shields.io/crates/d/state_machine_future.png)](https://crates.io/crates/state_machine_future) [![Build Status](https://travis-ci.org/fitzgen/state_machine_future.png?branch=master)](https://travis-ci.org/fitzgen/state_machine_future) Easily create type-safe `Future`s from state machines — without the boilerplate. `state_machine_future` type checks state machines and their state transitions, and then generates `Future` implementations and typestate<sup>[0][]</sup> boilerplate for you. * [Introduction](#introduction) * [Guide](#guide) * [Example](#example) * [Attributes](#attributes) * [Macro](#macro) * [Features](#features) * [License](#license) * [Contribution](#contribution) ## Introduction Most of the time, using `Future` combinators like `map` and `then` are a great way to describe an asynchronous computation. Other times, the most natural way to describe the process at hand is a state machine. When writing state machines in Rust, we want to *leverage the type system to enforce that only valid state transitions may occur*. To do that, we want *typestates*<sup>[0][]</sup>: types that represents each state in the state machine, and methods whose signatures only permit valid state transitions. But we *also* need an `enum` of every possible state, so we can treat the whole state machine as a single entity, and implement `Future` for it. But this is getting to be a *lot* of boilerplate... Enter `#[derive(StateMachineFuture)]`. With `#[derive(StateMachineFuture)]`, we describe the states and the possible transitions between them, and then the custom derive generates: * A typestate for each state in the state machine. * A type for the whole state machine that implements `Future`. * A concrete `start` method that constructs the state machine `Future` for you, initialized to its start state. * A state transition polling trait, with a `poll_zee_choo` method for each non-final state `ZeeChoo`. This trait describes the state machine's valid transitions, and its methods are called by `Future::poll`. Then, all *we* need to do is implement the generated state transition polling trait. Additionally, `#[derive(StateMachineFuture)]` will statically prevent against some footguns that can arise when writing state machines: * Every state is reachable from the start state: *there are no useless states.* * *There are no states which cannot reach a final state*. These states would otherwise lead to infinite loops. * *All state transitions are valid.* Attempting to make an invalid state transition fails to type check, thanks to the generated typestates. ## Guide Describe the state machine's states with an `enum` and add `#[derive(StateMachineFuture)]` to it: ```ignore #[derive(StateMachineFuture)] enum MyStateMachine { // ... } ``` There must be one **start** state, which is the initial state upon construction; one **ready** state, which corresponds to `Future::Item`; and one **error** state, which corresponds to `Future::Error`. ```ignore #[derive(StateMachineFuture)] enum MyStateMachine { #[state_machine_future(start)] Start, // ... #[state_machine_future(ready)] Ready(MyItem), #[state_machine_future(error)] Error(MyError), } ``` Any other variants of the `enum` are intermediate states. We define which state-to-state transitions are valid with `#[state_machine_future(transitions(...))]`. This attribute annotates a state variant, and lists which other states can be transitioned to immediately after this state. A final state (either **ready** or **error**) must be reachable from every intermediate state and the **start** state. Final states are not allowed to have transitions. ```ignore #[derive(StateMachineFuture)] enum MyStateMachine { #[state_machine_future(start, transitions(Intermediate))] Start, #[state_machine_future(transitions(Start, Ready))] Intermediate { x: usize, y: usize }, #[state_machine_future(ready)] Ready(MyItem), #[state_machine_future(error)] Error(MyError), } ``` From this state machine description, the custom derive generates boilerplate for us. For each state, the custom derive creates: * A typestate for the state. The type's name matches the variant name, for example the `Intermediate` state variant's typestate is also named `Intermediate`. The kind of struct type generated matches the variant kind: a unit-style variant results in a unit struct, a tuple-style variant results in a tuple struct, and a struct-style variant results in a normal struct with fields. | State `enum` Variant | Generated Typestate | | ------------------------------------------------- | ------------------------------ | | `enum StateMachine { MyState, ... }` | `struct MyState;` | | `enum StateMachine { MyState(bool, usize), ... }` | `struct MyState(bool, usize);` | | `enum StateMachine { MyState { x: usize }, ... }` | `struct MyState { x: usize };` | * An `enum` for the possible states that can come after this state. This `enum` is named `AfterX` where `X` is the state's name. There is also a `From<Y>` implementation for each `Y` state that can be transitioned to after `X`. For example, the `Intermediate` state would get: ```ignore enum AfterIntermediate { Start(Start), Ready(Ready), } impl From<Start> for AfterIntermediate { // ... } impl From<Ready> for AfterIntermediate { // ... } ``` Next, for the state machine as a whole, the custom derive generates: * A state machine `Future` type, which is essentially an `enum` of all the different typestates. This type is named `BlahFuture` where `Blah` is the name of the state machine description `enum`. In this example, where the state machine description is named `MyStateMachine`, the generated state machine future type would be named `MyStateMachineFuture`. * A polling trait, `PollBordle` where `Bordle` is this state machine description's name. For each non-final state `TootWasabi`, this trait has a method, `poll_toot_wasabi`, which is like `Future::poll` but specialized to the current state. Each method takes conditional ownership of its state (via [`RentToOwn`][rent_to_own]) and returns a `futures::Poll<AfterThisState, Error>` where `Error` is the state machine's error type. This signature *does not allow invalid state transitions*, which makes attempting an illegal state transition fail to type check. Here is the `MyStateMachine`'s polling trait, for example: ```ignore trait PollMyStateMachine { fn poll_start<'a>( start: &'a mut RentToOwn<'a, Start>, ) -> Poll<AfterStart, Error>; fn poll_intermediate<'a>( intermediate: &'a mut RentToOwn<'a, Intermediate>, ) -> Poll<AfterIntermediate, Error>; } ``` * An implementation of `Future` for that type. This implementation dispatches to the appropriate polling trait method depending on what state the future is in: * If the `Future` is in the `Start` state, then it uses `<MyStateMachine as PollMyStateMachine>::poll_start`. * If it is in the `Intermediate` state, then it uses `<MyStateMachine as PollMyStateMachine>::poll_intermediate`. * Etc... * A concrete `start` method for the description type (so `MyStateMachine::start` in this example) which constructs a new state machine `Future` type in its **start** state for you. This method has a parameter for each field in the **start** state variant. | Start `enum` Variant | Generated `start` Method | | ------------------------------- | ------------------------------------------------------------------- | | `MyStart,` | `fn start() -> MyStateMachineFuture { ... }` | | `MyStart(bool, usize),` | `fn start(arg0: bool, arg1: usize) -> MyStateMachineFuture { ... }` | | `MyStart { x: char, y: bool },` | `fn start(x: char, y: bool) -> MyStateMachineFuture { ... }` | Given all those generated types and traits, all we have to do is `impl PollBlah for Blah` for our state machine `Blah`. ```ignore impl PollMyStateMachine for MyStateMachine { fn poll_start<'a>( start: &'a mut RentToOwn<'a, Start> ) -> Poll<AfterStart, MyError> { // Call `try_ready!(start.inner.poll())` with any inner futures here. // // If we're ready to transition states, then we should return // `Ok(Async::Ready(AfterStart))`. If we are not ready to transition // states, return `Ok(Async::NotReady)`. If we encounter an error, // return `Err(...)`. } fn poll_intermediate<'a>( intermediate: &'a mut RentToOwn<'a, Intermediate> ) -> Poll<AfterIntermediate, MyError> { // Same deal as above... } } ``` That's it! ## Example Here is an example of a simple turn-based game played by two players over HTTP. ``` #[macro_use] extern crate state_machine_future; #[macro_use] extern crate futures; use futures::{Async, Future, Poll}; use state_machine_future::RentToOwn; # pub struct Player; # impl Player { # fn request_turn(&self) -> HttpTurnFuture { unimplemented!() } # } # pub struct HttpError; # pub struct HttpInvitationFuture; # impl Future for HttpInvitationFuture { # type Item = (); # type Error = HttpError; # fn poll(&mut self) -> Poll<(), HttpError> { # unimplemented!() # } # } # pub struct HttpTurnFuture; # impl Future for HttpTurnFuture { # type Item = (); # type Error = HttpError; # fn poll(&mut self) -> Poll<(), HttpError> { # unimplemented!() # } # } # fn process_turn(_: ()) -> Option<GameResult> { unimplemented!() } # fn main() {} /// The result of a game. pub struct GameResult { winner: Player, loser: Player, } /// Some kind of simple turn based game. /// /// ```text /// Invite /// | /// | /// | accept invitation /// | /// | /// V /// WaitingForTurn --------+ /// | ^ | /// | | | receive turn /// | | | /// | +-------------+ /// game concludes | /// | /// | /// | /// V /// Finished /// ``` #[derive(StateMachineFuture)] enum Game { /// The game begins with an invitation to play from one player to another. /// /// Once the invited player accepts the invitation over HTTP, then we will /// switch states into playing the game, waiting to recieve each turn. #[state_machine_future(start, transitions(WaitingForTurn))] Invite { invitation: HttpInvitationFuture, from: Player, to: Player, }, // We are waiting on a turn. // // Upon receiving it, if the game is now complete, then we go to the // `Finished` state. Otherwise, we give the other player a turn. #[state_machine_future(transitions(WaitingForTurn, Finished))] WaitingForTurn { turn: HttpTurnFuture, active: Player, idle: Player, }, // The game is finished with a `GameResult`. // // The `GameResult` becomes the `Future::Item`. #[state_machine_future(ready)] Finished(GameResult), // Any state transition can implicitly go to this error state if we get an // `HttpError` while waiting on a turn or invitation acceptance. // // This `HttpError` is used as the `Future::Error`. #[state_machine_future(error)] Error(HttpError), } // Now, we implement the generated state transition polling trait for our state // machine description type. impl PollGame for Game { fn poll_invite<'a>( invite: &'a mut RentToOwn<'a, Invite> ) -> Poll<AfterInvite, HttpError> { // See if the invitation has been accepted. If not, this will early // return with `Ok(Async::NotReady)` or propagate any HTTP errors. try_ready!(invite.invitation.poll()); // We're ready to transition into the `WaitingForTurn` state, so take // ownership of the `Invite` and then construct and return the new // state. let invite = invite.take(); let waiting = WaitingForTurn { turn: invite.from.request_turn(), active: invite.from, idle: invite.to, }; transition!(waiting) } fn poll_waiting_for_turn<'a>( waiting: &'a mut RentToOwn<'a, WaitingForTurn> ) -> Poll<AfterWaitingForTurn, HttpError> { // See if the next turn has arrived over HTTP. Again, this will early // return `Ok(Async::NotReady)` if the turn hasn't arrived yet, and // propagate any HTTP errors that we might encounter. let turn = try_ready!(waiting.turn.poll()); // Ok, we have a new turn. Take ownership of the `WaitingForTurn` state, // process the turn and if the game is over, then transition to the // `Finished` state, otherwise swap which player we need a new turn from // and request the turn over HTTP. let waiting = waiting.take(); if let Some(game_result) = process_turn(turn) { transition!(Finished(game_result)) } else { let next_waiting = WaitingForTurn { turn: waiting.idle.request_turn(), active: waiting.idle, idle: waiting.active, }; Ok(Async::Ready(next_waiting.into())) } } } # struct TokioHandle; # impl TokioHandle { fn spawn<T>(&self, _: T) {} } # fn get_some_player() -> Player { unimplemented!() } # fn get_another_player() -> Player { unimplemented!() } # fn invite(_: &Player, _: &Player) -> HttpInvitationFuture { unimplemented!() } // To spawn a new `Game` as a `Future` on whatever executor we're using (for // example `tokio`), we use `Game::start` to construct the `Future` in its start // state and then pass it to the executor. fn spawn_game(handle: TokioHandle) { let from = get_some_player(); let to = get_another_player(); let invitation = invite(&from, &to); let future = Game::start(invitation, from, to); handle.spawn(future) } ``` ## Attributes This is a list of all of the attributes used by `state_machine_future`: * `#[derive(StateMachineFuture)]`: Placed on an `enum` that describes a state machine. * `#[state_machine_future(derive(Clone, Debug, ...))]`: Placed on the `enum` that describes the state machine. This attribute describes which `#[derive(...)]`s to place on the generated `Future` type. * `#[state_machine_future(start)]`: Used on a variant of the state machine description `enum`. There must be exactly one variant with this attribute. This describes the initial starting state. The generated `start` method has a parameter for each field in this variant. * `#[state_machine_future(ready)]`: Used on a variant of the state machine description `enum`. There must be exactly one variant with this attribute. It must be a tuple-style variant with one field, for example `Ready(MyItemType)`. The generated `Future` implementation uses the field's type as `Future::Item`. * `#[state_machine_future(error)]`: Used on a variant of the state machine description `enum`. There must be exactly one variant with this attribute. It must be a tuple-style variant with one field, for example `Error(MyError)`. The generated `Future` implementation uses the field's type as `Future::Error`. * `#[state_machine_future(transitions(OtherState, AnotherState, ...))]`: Used on a variant of the state machine description `enum`. Describes the states that this one can transition to. ## Macro An auxiliary macro is provided that helps reducing boilerplate code for state transitions. So, the following code: ```Ok(Ready(NextState(1).into()))``` Can be reduced to: ```transition!(NextState(1))``` ## Features Here are the `cargo` features that you can enable: * `debug_code_generation`: Prints the code generated by `#[derive(StateMachineFuture)]` to `stdout` for debugging purposes. ## License Licensed under either of * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0) * [MIT license](http://opensource.org/licenses/MIT) at your option. ## Contribution See [CONTRIBUTING.md](https://github.com/fitzgen/state_machine_future/blob/master/CONTRIBUTING.md) for hacking. 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. [0]: https://en.wikipedia.org/wiki/Typestate_analysis [rent_to_own]: https://crates.io/crates/rent_to_own */ #![deny(missing_docs)] #![deny(missing_debug_implementations)] extern crate futures; extern crate rent_to_own; // Re-export the custom derive. This allows people to depend only on this crate // directly, not both this one and `derive_state_machine_future`. #[allow(unused_imports)] #[macro_use] extern crate derive_state_machine_future; pub use derive_state_machine_future::*; mod compile_fail_tests; #[macro_use] mod transition; /// Re-export of `rent_to_own::RentToOwn`. pub type RentToOwn<'a, T> = rent_to_own::RentToOwn<'a, T>; /// A trait that links an `enum` with `#[derive(StateMachineFuture)]` to its /// generated type that implements `Future`. pub trait StateMachineFuture { /// The generated `Future` type for this state machine. type Future: futures::Future; }