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//! ![license: MIT](https://img.shields.io/github/license/boltlabs-inc/dialectic) //! [![crates.io](https://img.shields.io/crates/v/dialectic)](https://crates.io/crates/dialectic) //! [![docs.rs documentation](https://docs.rs/dialectic/badge.svg)](https://docs.rs/dialectic) //! //! > **dialectic (noun):** The process of arriving at the truth by stating a thesis, developing a //! > contradictory antithesis, and combining them into a coherent synthesis. //! > //! > **dialectic (crate):** Transport-polymorphic session types for asynchronous Rust. //! //! When two concurrent processes communicate, it's good to give their messages *types*, which //! ensure every message is of an expected form. //! //! - **Conventional types** merely describe **what is valid** to communicate. //! - **Session types** describe **when it is valid** to communicate, and **in what manner**. //! //! This crate provides a generic wrapper around almost any type of asynchronous channel that adds //! compile-time guarantees that a specified *session protocol* will not be violated by any code //! using the channel. Such a wrapped channel: //! //! - has **almost no runtime cost** in time or memory; //! - is **built on `async`/`.await`** to allow integration with Rust's powerful `async` ecosystem; //! - gracefully handles runtime protocol violations, introducing **no panics**; //! - allows for **full duplex concurrent communication**, if specified in its type, while //! preserving all the same session-type safety guarantees; and //! - can even implement **context free sessions**, a more general form of session type than //! supported by most other session typing libraries. //! //! Together, these make Dialectic ideal for writing networked services that need to ensure **high //! levels of availability** and **complex protocol correctness properties** in the real world, //! where protocols might be violated and connections might be dropped. //! //! <!-- snip --> //! //! # What now? //! //! - If you are **new to session types** you might consider starting with the **[tutorial-style //! tour of the crate](tutorial)**. //! - If you're **familiar with session types**, you might jump to the **[quick //! reference](#quick-reference)**, then read more in the [`types`](crate::types) module and the //! documentation for [`Chan`](crate::Chan). //! - If you want to **integrate your own channel type** with Dialectic, you need to implement the //! [`Transmit`] and [`Receive`] traits from the [`backend`] module. //! - Or, you can **[dive into the reference documentation](#modules)**... //! //! # Quick reference //! //! The **[tutorial]** covers all the constructs necessary to write session-typed programs with //! Dialectic. A quick summary: //! //! - To make a pair of dual [`Chan`]s for a session type `P`: [`let (c1, c2) = P::channel(|| //! {...})`](NewSession::channel) with some closure that builds a unidirectional underlying //! channel. //! - To wrap an existing sender `tx` and receiver `rx` in a single [`Chan`] for `P`: [`let c = //! P::wrap(tx, rx)`](NewSession::wrap). //! - Backend transports suitable for being wrapped in a [`Chan`] are provided in [`backend`], along //! with the [`Transmit`] and [`Receive`] traits necessary to implement your own. //! //! Once you've got a channel, here's what you can do: //! //! | Session Type (`S`) | Channel Operation(s) (on a channel `c: Chan<_, _, S, _>`) | Dual Type (`S::Dual`) | //! | :----------- | :------------------- | :-------- | //! | [`Send<T, P = Done>`](Send) | Given some `t: T`, returns a new `c`:<br>[`let c = c.send(t).await?;`](CanonicalChan::send) | [`Recv<T, P::Dual>`](Recv) | //! | [`Recv<T, P = Done>`](Recv) | Returns some `t: T` and a new `c`:<br>[`let (t, c) = c.recv().await?;`](CanonicalChan::recv) | [`Send<T, P::Dual>`](Send) | //! | [`Choose<Choices>`](Choose) | Given some `_N` < the length of `Choices`, returns a new `c`:<br>[`let c = c.choose(_N).await?;`](CanonicalChan::choose) | [`Offer<Choices::Dual>`](Offer) | //! | [`Offer<Choices>`](Offer) | Given a set of labeled branches `_N => ...` in ascending order, exactly one for each option in the tuple `Choices`, returns a new `c` whose type each branch must match:<br>[`let c = offer!(c => { _0 => ..., _1 => ..., ... });`](offer!) | [`Choose<Choices::Dual>`](Choose) | //! | [`Split<P, Q>`](Split) | Given a closure evaluating the session types `P` (send-only) and `Q` (receive-only) each to `Done` (potentially concurrently), returns a result and a channel for `Done`:<br>[<code>let (t, c) = c.split(|c| async move { ... }).await?;</code>](CanonicalChan::split) | [`Split<Q::Dual, P::Dual>`](Split) | //! | [`Loop<P>`](Loop) | Whatever operations are available for `P` | [`Loop<P::Dual>`](Loop) | //! | [`Continue<N = Z>`](Continue) | Whatever operations are available for the start of the `N`th-innermost [`Loop`] | [`Continue<N>`](Continue) | //! | [`Break<N = Z>`](Break) | • If exiting the *outermost* [`Loop`]: Returns the underlying [`Transmit`]/[`Receive`] ends: [`let (tx, rx) = c.close();`](CanonicalChan::close)<br> • If exiting an *inner* [`Loop`]: Whatever operations are available for the start of the `(N + 1)`th-innermost [`Loop`] | [`Break<N>`](Break) | //! | [`Seq<P, Q>`](Seq) | Given a closure evaluating the session type `P` to `Done`, returns a result and a channel for the type `Q`:<br>[<code>let (t, c) = c.seq(|c| async move { ... }).await?;</code>](CanonicalChan::seq) | [`Seq<P::Dual, Q::Dual>`](Seq) | //! | [`Done`] | • If *outside* a [`Loop`]: Returns the underlying [`Transmit`]/[`Receive`] ends: [`let (tx, rx) = c.close();`](CanonicalChan::close)<br> • If *inside* a [`Loop`], equivalent to [`Continue`]: whatever operations are available for the start of the innermost [`Loop`] | [`Done`] | [`c.close()`](CanonicalChan::close) | #![recursion_limit = "256"] #![allow(clippy::type_complexity)] #![warn(missing_docs)] #![warn(missing_copy_implementations, missing_debug_implementations)] #![warn(unused_qualifications, unused_results)] #![warn(future_incompatible)] #![warn(unused)] #![forbid(broken_intra_doc_links)] #![cfg_attr(docsrs, feature(doc_cfg))] use std::{ marker::{self, PhantomData}, pin::Pin, }; #[macro_use] extern crate derivative; use crate::backend::*; use futures::Future; /// The prelude module for quickly getting started with Dialectic. /// /// This module is designed to be imported as `use dialectic::prelude::*;`, which brings into scope /// all the bits and pieces you need to start writing programs with Dialectic. pub mod prelude { #[doc(no_inline)] pub use crate::backend::{Choice, Receive, Transmit}; #[doc(no_inline)] pub use crate::new_session::NewSession; pub use crate::tuple::{List, Tuple}; pub use crate::types::unary::constants::*; pub use crate::types::unary::types::*; pub use crate::types::unary::{LessThan, Unary, S, Z}; pub use crate::types::*; pub use crate::{ canonical::{Branches, CanonicalChan}, offer, Chan, IncompleteHalf, SessionIncomplete, }; #[doc(no_inline)] pub use call_by::{CallBy, CallingConvention, Mut, Ref, Val}; } pub mod backend; pub mod tutorial; pub mod types; mod new_session; pub use new_session::NewSession; pub mod canonical; #[doc(inline)] pub use canonical::Branches; use prelude::*; /// A bidirectional communications channel using the session type `P` over the connections `Tx` and /// `Rx`. /// /// **[See the documentation for `CanonicalChan` for available methods and trait /// implementations.](CanonicalChan)** /// /// **Important: always write this type synonym ([`Chan`]) in type signatures, not️ /// [`CanonicalChan`] directly.** This is because the [`Chan`] type synonym canonicalizes its /// session type argument, which means it can be used more flexibly. The details: /// /// # Technical notes on canonicity: TL;DR: always write `Chan` /// /// In Dialectic, operations are liberally available on [`Chan`]s wherever they make /// sense. For instance, it's valid to call [`send`](CanonicalChan::send) on a channel which was /// created using the session type `Loop<Send<String>>`, even though the type does not literally /// begin with `Send<String>`. /// /// A `CanonicalChan` always has a session type which is syntactically a real action: it will never /// be [`Loop`], [`Continue`], or [`Break`] (or, when inside a [`Loop`], it will never be [`Done`]). /// Every action available on a [`CanonicalChan`] "fast-forwards" through such control operators, /// yielding a [`CanonicalChan`] that corresponds to the next real action available. /// /// While this design means greater flexibility and concision in writing session-typed code, it can /// be confusing in the case where you want to explicitly write out the session type of a channel, /// because the automatic canonicalization can mean a [`Chan`] does not have the type /// you might think it does. /// /// ⚠️ **The problem:** Suppose you wanted to explicitly annotate the type of a new channel: /// /// ```compile_fail /// # use dialectic::prelude::*; /// # use dialectic::backend::mpsc; /// use dialectic::canonical::CanonicalChan; /// /// type P = Loop<Send<String>>; /// let (c1, c2): (CanonicalChan<_, _, P, ()>, _) = P::channel(mpsc::unbounded_channel); /// ``` /// /// This fails to typecheck, returning several errors (abridged for clarity): /// /// ```text /// error[E0271]: type mismatch resolving `<Loop<Send<String>> as Actionable>::Action == Loop<Send<String>>` /// = note: expected struct `Loop<Send<_>>` /// found struct `Send<_>` /// /// error[E0271]: type mismatch resolving `<Loop<Send<String>> as Actionable>::Env == ()` /// = note: expected unit type `()` /// found type `(Send<String>, ())` /// ``` /// /// These errors indicate that the returned [`CanonicalChan`] from `P::channel` *does not* have the /// session type `P` and the initial empty environment `E = ()`, as annotated. Instead, it has the /// session type and environment corresponding to the *inside* of the `Loop`, which are the /// *canonical* session type and environment for `P`. /// /// 💡 **Do this instead:** When annotating the types of channels, prefer the type synonym /// [`Chan`], which computes the correct [`CanonicalChan`] type for a given (possibly /// non-canonical) session type. Using [`Chan`]instead of [`CanonicalChan`], we can /// correctly annotate a newly created channel of any session type: /// /// ``` /// # use dialectic::prelude::*; /// # use dialectic::backend::mpsc; /// # /// type P = Loop<Send<String>>; /// let (c1, c2): (Chan<_, _, P>, Chan<_, _, <P as Session>::Dual>) = /// P::channel(mpsc::unbounded_channel); /// ``` pub type Chan<Tx, Rx, P, E = ()> = CanonicalChan<Tx, Rx, <P as Actionable<E>>::Action, <P as Actionable<E>>::Env>; /// Offer a set of different protocols, allowing the other side of the channel to choose with which /// one to proceed. This macro only works in a `Try` context, i.e. somewhere the `?` operator would /// make sense to use. /// /// # Notes /// /// - You must specify exactly as many branches as there are options in the type of the /// [`Offer`](crate::types::Offer) to which this expression corresponds, and they must be in the /// same order as the choices are in the tuple [`Offer`](crate::types::Offer)ed. /// - In the body of each branch, the identifier for the channel is rebound to have the session type /// corresponding to that branch. /// - To use `offer!` as an expression, ensure the type of every branch matches. /// /// # Examples /// /// ``` /// use dialectic::prelude::*; /// use dialectic::backend::mpsc; /// /// # #[tokio::main] /// # async fn main() -> Result<(), Box<dyn std::error::Error>> { /// type GiveOrTake = Choose<(Send<i64>, Recv<String>)>; /// /// let (c1, c2) = GiveOrTake::channel(|| mpsc::channel(1)); /// /// // Spawn a thread to offer a choice /// let t1 = tokio::spawn(async move { /// offer!(c2 => { /// _0 => { c2.recv().await?; }, /// _1 => { c2.send("Hello!".to_string()).await?; }, /// }); /// Ok::<_, mpsc::Error>(()) /// }); /// /// // Choose to send an integer /// c1.choose(_0).await?.send(42).await?; /// /// // Wait for the offering thread to finish /// t1.await??; /// # Ok(()) /// # } /// ``` #[macro_export] macro_rules! offer { ( $chan:ident => { $($t:tt)* } ) => ( { match $crate::canonical::CanonicalChan::offer($chan).await { Ok(b) => $crate::offer!{@branches b, $chan, $crate::types::unary::Z, $($t)* }, Err(e) => Err(e)?, } } ); ( @branches $branch:ident, $chan:ident, $n:ty, $(,)? ) => ( $crate::Branches::empty_case($branch), ); ( @branches $branch:ident, $chan:ident, $n:ty, $label:expr => $code:expr $(,)? ) => ( match $crate::Branches::case($branch) { std::result::Result::Ok($chan) => { let _: $n = $label; $code }, std::result::Result::Err($branch) => $crate::Branches::empty_case($branch), } ); ( @branches $branch:ident, $chan:ident, $n:ty, $label:expr => $code:expr, $($t:tt)+ ) => ( match $crate::Branches::case($branch) { std::result::Result::Ok($chan) => { let _: $n = $label; $code }, std::result::Result::Err($branch) => { $crate::offer!{@branches $branch, $chan, $crate::types::unary::S<$n>, $($t)+ } }, } ); } /// A placeholder for a missing [`Transmit`] or [`Receive`] end of a connection. /// /// When using [`split`](CanonicalChan::split), the resultant two channels can only send or only /// receive, respectively. This is reflected at the type level by the presence of [`Unavailable`] on /// the type of the connection which *is not* present for each part of the split, and [`Available`] /// on the type of the connection which *is*. #[derive(Debug)] pub struct Unavailable<T>(PhantomData<T>); impl<T> Unavailable<T> { /// Make a new `Unavailable`. fn new() -> Self { Unavailable(PhantomData) } } /// An available [`Transmit`] or [`Receive`] end of a connection. /// /// When using [`split`](CanonicalChan::split), the resultant two channels can only send or only /// receive, respectively. This is reflected at the type level by the presence of [`Available`] on /// the type of the connection which *is* present for each part of the split, and [`Unavailable`] on /// the type of the connection which *is not*. /// /// Whenever `C` implements [`Transmit`] or [`Receive`], so does `Available<C>`. #[derive(Debug, Clone, Copy, PartialEq, PartialOrd, Eq, Hash, Default)] pub struct Available<C>(C); impl<C> Available<C> { /// Retrieve the inner `C` connection. pub fn into_inner(self) -> C { self.0 } } impl<C> AsRef<C> for Available<C> { fn as_ref(&self) -> &C { &self.0 } } impl<C> AsMut<C> for Available<C> { fn as_mut(&mut self) -> &mut C { &mut self.0 } } impl<T, Convention: CallingConvention, C> Transmit<T, Convention> for Available<C> where C: Transmit<T, Convention>, { type Error = C::Error; fn send<'a, 'async_lifetime>( &'async_lifetime mut self, message: <T as CallBy<'a, Convention>>::Type, ) -> Pin<Box<dyn Future<Output = Result<(), Self::Error>> + marker::Send + 'async_lifetime>> where T: CallBy<'a, Convention>, <T as CallBy<'a, Convention>>::Type: marker::Send, 'a: 'async_lifetime, { self.0.send(message) } } impl<T, C> Receive<T> for Available<C> where C: Receive<T>, { type Error = C::Error; fn recv<'async_lifetime>( &'async_lifetime mut self, ) -> Pin<Box<dyn Future<Output = Result<T, Self::Error>> + marker::Send + 'async_lifetime>> { self.0.recv() } } /// The error returned when a closure which is expected to complete a channel's session fails to /// finish the session of the channel it is given. /// /// This error can arise either if the channel is dropped *before* its session is completed, or if /// it is stored somewhere and is dropped *after* the closure's future is finished. The best way to /// ensure this error does not occur is to call [`close`](CanonicalChan::close) on the channel, /// which statically ensures it is dropped exactly when the session is complete. #[derive(Derivative)] #[derivative(Debug(bound = ""))] pub enum SessionIncomplete<Tx, Rx> { /// Both the sending half `Tx` and the receiving half `Rx` did not complete the session /// correctly. BothHalves { /// The incomplete sending half: [`Unfinished`](IncompleteHalf::Unfinished) if dropped /// before the end of the session, [`Unclosed`](IncompleteHalf::Unclosed) if not dropped /// after the end of the session. tx: IncompleteHalf<Tx>, /// The incomplete receiving half: [`Unfinished`](IncompleteHalf::Unfinished) if dropped /// before the end of the session, [`Unclosed`](IncompleteHalf::Unclosed) if not dropped /// after the end of the session. rx: IncompleteHalf<Rx>, }, /// Only the sending half `Tx` did not complete the session correctly, but the receiving half /// `Rx` did complete it correctly. TxHalf { /// The incomplete sending half: [`Unfinished`](IncompleteHalf::Unfinished) if dropped /// before the end of the session, [`Unclosed`](IncompleteHalf::Unclosed) if not dropped /// after the end of the session. tx: IncompleteHalf<Tx>, /// The receiving half, whose session was completed. #[derivative(Debug = "ignore")] rx: Rx, }, /// Only the receiving half `Rx` did not complete the session correctly, but the sending half /// `Tx` did complete it correctly. RxHalf { /// The sending half, whose session was completed. #[derivative(Debug = "ignore")] tx: Tx, /// The incomplete receiving half: [`Unfinished`](IncompleteHalf::Unfinished) if dropped /// before the end of the session, [`Unclosed`](IncompleteHalf::Unclosed) if not dropped /// after the end of the session. rx: IncompleteHalf<Rx>, }, } /// A representation of what has gone wrong when a connection half `Tx` or `Rx` is incomplete. #[derive(Derivative)] #[derivative(Debug(bound = ""))] pub enum IncompleteHalf<T> { /// The underlying channel was dropped before the session was `Done`. Unfinished(#[derivative(Debug = "ignore")] T), /// The underlying channel was not dropped or [`close`](CanonicalChan::close)d after the session /// was `Done`. Unclosed, } impl<T> std::error::Error for IncompleteHalf<T> {} impl<T> std::fmt::Display for IncompleteHalf<T> { fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { write!(f, "incomplete session or sub-session: channel half ")?; write!( f, "{}", match self { IncompleteHalf::Unfinished(_) => "was dropped before the session was `Done`", IncompleteHalf::Unclosed => "was not closed after the session was `Done`", } ) } } impl<Tx, Rx> SessionIncomplete<Tx, Rx> { /// Extract the send and receive halves `Tx` and `Rx`, if they are present, from this /// `SessionIncomplete` error. pub fn into_halves( self, ) -> ( Result<Tx, IncompleteHalf<Tx>>, Result<Rx, IncompleteHalf<Rx>>, ) { match self { SessionIncomplete::BothHalves { tx, rx } => (Err(tx), Err(rx)), SessionIncomplete::TxHalf { tx, rx } => (Err(tx), Ok(rx)), SessionIncomplete::RxHalf { tx, rx } => (Ok(tx), Err(rx)), } } } impl<Tx, Rx> std::error::Error for SessionIncomplete<Tx, Rx> {} impl<Tx, Rx> std::fmt::Display for SessionIncomplete<Tx, Rx> { fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { use IncompleteHalf::*; write!(f, "incomplete session or sub-session: channel")?; let reason = match self { SessionIncomplete::BothHalves { tx, rx } => match (tx, rx) { (Unclosed, Unclosed) => " was not closed after the session was `Done`", (Unclosed, Unfinished(_)) => { "'s sending half was not closed after the session was `Done` \ and its receiving half was dropped before the session was `Done`" } (Unfinished(_), Unclosed) => { "'s sending half was dropped before the session was `Done` \ and its receiving half was not closed after the session was `Done`" } (Unfinished(_), Unfinished(_)) => " was dropped before the session was `Done`", }, SessionIncomplete::TxHalf { tx, .. } => match tx { Unfinished(_) => "'s sending half was dropped before the session was `Done`", Unclosed => "'s sending half was not closed after the session was `Done`", }, SessionIncomplete::RxHalf { rx, .. } => match rx { Unfinished(_) => "'s receiving half was dropped before the session was `Done`", Unclosed => "'s receiving half was not closed after the session was `Done`", }, }; write!(f, "{}", reason) } }