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//! Minimal and reusable non-blocking I/O layer //! //! The ultimate goal of this crate is *code reuse*. With this crate you can //! write *core* I/O APIs that can then be adapted to operate in either blocking //! or non-blocking manner. Furthermore those APIs are not tied to a particular //! asynchronous model and can be adapted to work with the `futures` model or //! with the `async` / `await` model. //! //! # Core idea //! //! The [`WouldBlock`](enum.Error.html) error variant signals that the operation //! can't be completed *right now* and would need to block to complete. //! [`WouldBlock`](enum.Error.html) is a special error in the sense that's not //! *fatal*; the operation can still be completed by retrying again later. //! //! [`nb::Result`](type.Result.html) is based on the API of //! [`std::io::Result`](https://doc.rust-lang.org/std/io/type.Result.html), //! which has a `WouldBlock` variant in its //! [`ErrorKind`](https://doc.rust-lang.org/std/io/enum.ErrorKind.html). //! //! We can map [`WouldBlock`](enum.Error.html) to different blocking and //! non-blocking models: //! //! - In blocking mode: [`WouldBlock`](enum.Error.html) means try again right //! now (i.e. busy wait) //! - In `futures` mode: [`WouldBlock`](enum.Error.html) means //! [`Async::NotReady`](https://docs.rs/futures) //! - In `await` mode: [`WouldBlock`](enum.Error.html) means `yield` //! (suspend the generator) //! //! # How to use this crate //! //! Application specific errors can be put inside the `Other` variant in the //! [`nb::Error`](enum.Error.html) enum. //! //! So in your API instead of returning `Result<T, MyError>` return //! `nb::Result<T, MyError>` //! //! ``` //! enum MyError { //! ThisError, //! ThatError, //! // .. //! } //! //! // This is a blocking function, so it returns a normal `Result` //! fn before() -> Result<(), MyError> { //! // .. //! # Ok(()) //! } //! //! // This is now a potentially (read: *non*) blocking function so it returns `nb::Result` //! // instead of blocking //! fn after() -> nb::Result<(), MyError> { //! // .. //! # Ok(()) //! } //! ``` //! //! You can use the *never type* (`!`) to signal that some API has no fatal //! errors but may block: //! //! ``` //! #![feature(never_type)] //! //! // This returns `Ok(())` or `Err(nb::Error::WouldBlock)` //! fn maybe_blocking_api() -> nb::Result<(), !> { //! // .. //! # Ok(()) //! } //! ``` //! //! Once your API uses [`nb::Result`](type.Result.html) you can leverage the //! [`block!`], [`try_nb!`] and [`await!`] macros to adapt it for blocking //! operation, or for non-blocking operation with `futures` or `await`. //! //! **NOTE** Currently, both `try_nb!` and `await!` are feature gated behind the `unstable` Cargo //! feature. //! //! [`block!`]: macro.block.html //! [`try_nb!`]: macro.try_nb.html //! [`await!`]: macro.await.html //! //! # Examples //! //! ## A Core I/O API //! //! Imagine the code (crate) below represents a Hardware Abstraction Layer for some microcontroller //! (or microcontroller family). //! //! *In this and the following examples let's assume for simplicity that peripherals are treated //! as global singletons and that no preemption is possible (i.e. interrupts are disabled).* //! //! ``` //! #![feature(never_type)] //! //! // This is the `hal` crate //! // Note that it doesn't depend on the `futures` crate //! //! extern crate nb; //! //! /// An LED //! pub struct Led; //! //! impl Led { //! pub fn off(&self) { //! // .. //! } //! pub fn on(&self) { //! // .. //! } //! } //! //! /// Serial interface //! pub struct Serial; //! pub enum Error { //! Overrun, //! // .. //! } //! //! impl Serial { //! /// Reads a single byte from the serial interface //! pub fn read(&self) -> nb::Result<u8, Error> { //! // .. //! # Ok(0) //! } //! //! /// Writes a single byte to the serial interface //! pub fn write(&self, byte: u8) -> nb::Result<(), Error> { //! // .. //! # Ok(()) //! } //! } //! //! /// A timer used for timeouts //! pub struct Timer; //! //! impl Timer { //! /// Waits until the timer times out //! pub fn wait(&self) -> nb::Result<(), !> { //! //^ NOTE the `!` indicates that this operation can block but has no //! // other form of error //! //! // .. //! # Ok(()) //! } //! } //! ``` //! //! ## Blocking mode //! //! Turn on an LED for one second and *then* loops back serial data. //! //! ``` //! # #![feature(never_type)] //! #[macro_use(block)] //! extern crate nb; //! //! use hal::{Led, Serial, Timer}; //! //! fn main() { //! // Turn the LED on for one second //! Led.on(); //! block!(Timer.wait()).unwrap(); // NOTE(unwrap) E = ! //! Led.off(); //! //! // Serial interface loopback //! # return; //! loop { //! let byte = block!(Serial.read()).unwrap(); //! block!(Serial.write(byte)).unwrap(); //! } //! } //! //! # mod hal { //! # use nb; //! # pub struct Led; //! # impl Led { //! # pub fn off(&self) {} //! # pub fn on(&self) {} //! # } //! # pub struct Serial; //! # impl Serial { //! # pub fn read(&self) -> nb::Result<u8, ()> { Ok(0) } //! # pub fn write(&self, _: u8) -> nb::Result<(), ()> { Ok(()) } //! # } //! # pub struct Timer; //! # impl Timer { //! # pub fn wait(&self) -> nb::Result<(), !> { Ok(()) } //! # } //! # } //! ``` //! //! ## `futures` //! //! Blinks an LED every second *and* loops back serial data. Both tasks run //! concurrently. //! //! ``` //! #![feature(conservative_impl_trait)] //! #![feature(never_type)] //! //! extern crate futures; //! #[macro_use(try_nb)] //! extern crate nb; //! //! use futures::{Async, Future}; //! use futures::future::{self, Loop}; //! use hal::{Error, Led, Serial, Timer}; //! //! /// `futures` version of `Timer.wait` //! /// //! /// This returns a future that must be polled to completion //! fn wait() -> impl Future<Item = (), Error = !> { //! future::poll_fn(|| { //! Ok(Async::Ready(try_nb!(Timer.wait()))) //! }) //! } //! //! /// `futures` version of `Serial.read` //! /// //! /// This returns a future that must be polled to completion //! fn read() -> impl Future<Item = u8, Error = Error> { //! future::poll_fn(|| { //! Ok(Async::Ready(try_nb!(Serial.read()))) //! }) //! } //! //! /// `futures` version of `Serial.write` //! /// //! /// This returns a future that must be polled to completion //! fn write(byte: u8) -> impl Future<Item = (), Error = Error> { //! future::poll_fn(move || { //! Ok(Async::Ready(try_nb!(Serial.write(byte)))) //! }) //! } //! //! fn main() { //! // Tasks //! let mut blinky = future::loop_fn::<_, (), _, _>(true, |state| { //! wait().map(move |_| { //! if state { //! Led.on(); //! } else { //! Led.off(); //! } //! //! Loop::Continue(!state) //! }) //! }); //! //! let mut loopback = future::loop_fn::<_, (), _, _>((), |_| { //! read().and_then(|byte| { //! write(byte) //! }).map(|_| { //! Loop::Continue(()) //! }) //! }); //! //! // Event loop //! loop { //! blinky.poll().unwrap(); // NOTE(unwrap) E = ! //! loopback.poll().unwrap(); //! # break //! } //! } //! //! # mod hal { //! # use nb; //! # pub struct Led; //! # impl Led { //! # pub fn off(&self) {panic!()} //! # pub fn on(&self) {} //! # } //! # #[derive(Debug)] //! # pub enum Error {} //! # pub struct Serial; //! # impl Serial { //! # pub fn read(&self) -> nb::Result<u8, Error> { Err(nb::Error::WouldBlock) } //! # pub fn write(&self, _: u8) -> nb::Result<(), Error> { Err(nb::Error::WouldBlock) } //! # } //! # pub struct Timer; //! # impl Timer { //! # pub fn wait(&self) -> nb::Result<(), !> { Err(nb::Error::WouldBlock) } //! # } //! # } //! ``` //! //! ## `await!` //! //! This is equivalent to the `futures` example but with much less boilerplate. //! //! ``` //! #![feature(generator_trait)] //! #![feature(generators)] //! #![feature(never_type)] //! //! #[macro_use(await)] //! extern crate nb; //! //! use std::ops::Generator; //! //! use hal::{Led, Serial, Timer}; //! //! fn main() { //! // Tasks //! let mut blinky = || { //! let mut state = false; //! loop { //! // `await!` means suspend / yield instead of blocking //! await!(Timer.wait()).unwrap(); // NOTE(unwrap) E = ! //! //! state = !state; //! //! if state { //! Led.on(); //! } else { //! Led.off(); //! } //! } //! }; //! //! let mut loopback = || { //! loop { //! let byte = await!(Serial.read()).unwrap(); //! await!(Serial.write(byte)).unwrap(); //! } //! }; //! //! // Event loop //! loop { //! blinky.resume(); //! loopback.resume(); //! # break //! } //! } //! //! # mod hal { //! # use nb; //! # pub struct Led; //! # impl Led { //! # pub fn off(&self) {} //! # pub fn on(&self) {} //! # } //! # pub struct Serial; //! # impl Serial { //! # pub fn read(&self) -> nb::Result<u8, ()> { Err(nb::Error::WouldBlock) } //! # pub fn write(&self, _: u8) -> nb::Result<(), ()> { Err(nb::Error::WouldBlock) } //! # } //! # pub struct Timer; //! # impl Timer { //! # pub fn wait(&self) -> nb::Result<(), !> { Err(nb::Error::WouldBlock) } //! # } //! # } //! ``` #![no_std] #![deny(warnings)] use core::fmt; /// A non-blocking result pub type Result<T, E> = ::core::result::Result<T, Error<E>>; /// A non-blocking error /// /// The main use of this enum is to add a `WouldBlock` variant to an existing /// error enum. #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)] pub enum Error<E> { /// A different kind of error Other(E), /// This operation requires blocking behavior to complete WouldBlock, } impl<E> fmt::Debug for Error<E> where E: fmt::Debug, { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { match *self { Error::Other(ref e) => fmt::Debug::fmt(e, f), Error::WouldBlock => f.write_str("WouldBlock"), } } } /// Await operation (*won't work until the language gains support for /// generators*) /// /// This macro evaluates the expression `$e` *cooperatively* yielding control /// back to the (generator) caller whenever `$e` evaluates to /// `Error::WouldBlock`. /// /// # Requirements /// /// This macro must be called within a generator body. /// /// # Input /// /// An expression `$e` that evaluates to `nb::Result<T, E>` /// /// # Output /// /// - `Ok(t)` if `$e` evaluates to `Ok(t)` /// - `Err(e)` if `$e` evaluates to `Err(nb::Error::Other(e))` #[cfg(feature = "unstable")] #[macro_export] macro_rules! await { ($e:expr) => { loop { #[allow(unreachable_patterns)] match $e { Err($crate::Error::Other(e)) => { #[allow(unreachable_code)] break Err(e) }, Err($crate::Error::WouldBlock) => {}, // yield (see below) Ok(x) => break Ok(x), } yield } } } /// Turns the non-blocking expression `$e` into a blocking operation. /// /// This is accomplished by continuously calling the expression `$e` until it no /// longer returns `Error::WouldBlock` /// /// # Input /// /// An expression `$e` that evaluates to `nb::Result<T, E>` /// /// # Output /// /// - `Ok(t)` if `$e` evaluates to `Ok(t)` /// - `Err(e)` if `$e` evaluates to `Err(nb::Error::Other(e))` #[macro_export] macro_rules! block { ($e:expr) => { loop { #[allow(unreachable_patterns)] match $e { Err($crate::Error::Other(e)) => { #[allow(unreachable_code)] break Err(e) }, Err($crate::Error::WouldBlock) => {}, Ok(x) => break Ok(x), } } } } /// Future adapter /// /// This is a *try* operation from a `nb::Result` to a `futures::Poll` /// /// # Requirements /// /// This macro must be called within a function / closure that has signature /// `fn(..) -> futures::Poll<T, E>`. /// /// This macro requires that the [`futures`] crate is in the root of the crate. /// /// [`futures`]: https://crates.io/crates/futures /// /// # Input /// /// An expression `$e` that evaluates to `nb::Result<T, E>` /// /// # Early return /// /// - `Ok(Async::NotReady)` if `$e` evaluates to `Err(nb::Error::WouldBlock)` /// - `Err(e)` if `$e` evaluates to `Err(nb::Error::Other(e))` /// /// # Output /// /// `t` if `$e` evaluates to `Ok(t)` #[cfg(feature = "unstable")] #[macro_export] macro_rules! try_nb { ($e:expr) => { match $e { Err($crate::Error::Other(e)) => return Err(e), Err($crate::Error::WouldBlock) => { return Ok(::futures::Async::NotReady) }, Ok(x) => x, } } }