futures_micro/lib.rs
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//! A very small, no-std compatible toolbox of async utilities.
#![no_std]
#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
pub mod prelude;
#[doc(no_inline)]
pub use core::future::Future;
#[doc(no_inline)]
pub use core::pin::Pin;
#[doc(no_inline)]
pub use core::task::{Context, Poll, Waker};
use core::fmt;
use pin_project_lite::pin_project;
// ---------- futures using the poll api -----------
/// Creates a future from a function returning [`Poll`].
///
/// # Examples
///
/// ```
/// use futures_lite::future::block_on;
/// use futures_micro::poll_fn;
/// use std::task::{Context, Poll};
///
/// # block_on(async {
/// fn f(_ctx: &mut Context<'_>) -> Poll<i32> {
/// Poll::Ready(7)
/// }
///
/// assert_eq!(poll_fn(f).await, 7);
/// # })
/// ```
#[inline(always)]
pub fn poll_fn<F, T>(inner: F) -> PollFn<F>
where
F: FnMut(&mut Context<'_>) -> Poll<T>,
{
PollFn { inner }
}
pin_project! {
/// Future for the [`poll_fn()`] function.
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct PollFn<F> {
inner: F,
}
}
impl<F> fmt::Debug for PollFn<F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("PollFn").finish()
}
}
impl<F, T> Future for PollFn<F>
where
F: FnMut(&mut Context<'_>) -> Poll<T>,
{
type Output = T;
fn poll(self: Pin<&mut Self>, ctx: &mut Context<'_>) -> Poll<T> {
let this = self.project();
(this.inner)(ctx)
}
}
pin_project! {
/// Returns the result of `left` or `right` future, preferring `left` if both are ready.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct Or<F1, F2> {
#[pin]
future1: F1,
#[pin]
future2: F2,
}
}
impl<F1, F2> Or<F1, F2>
where
F1: Future,
F2: Future,
{
/// Returns the result of `left` or `right` future, preferring `left` if both are ready.
pub fn new(future1: F1, future2: F2) -> Self {
Or { future1, future2 }
}
}
impl<T, F1, F2> Future for Or<F1, F2>
where
F1: Future<Output = T>,
F2: Future<Output = T>,
{
type Output = T;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
if let Poll::Ready(t) = this.future1.poll(cx) {
Poll::Ready(t)
} else if let Poll::Ready(t) = this.future2.poll(cx) {
Poll::Ready(t)
} else {
Poll::Pending
}
}
}
pin_project! {
/// Waits for two [`Future`]s to complete, returning both results.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct Zip<F1, F2>
where
F1: Future,
F2: Future,
{
#[pin]
future1: F1,
output1: Option<F1::Output>,
#[pin]
future2: F2,
output2: Option<F2::Output>,
}
}
impl<F1, F2> Zip<F1, F2>
where
F1: Future,
F2: Future,
{
/// Zips two futures, waiting for both to complete.
///
/// # Examples
///
/// ```
/// use futures_micro::Zip;
///
/// # futures_lite::future::block_on(async {
/// let a = async { 1 };
/// let b = async { 2 };
///
/// assert_eq!(Zip::new(a, b).await, (1, 2));
/// # })
/// ```
pub fn new(future1: F1, future2: F2) -> Self {
Zip {
future1,
future2,
output1: None,
output2: None,
}
}
}
impl<F1, F2> Future for Zip<F1, F2>
where
F1: Future,
F2: Future,
{
type Output = (F1::Output, F2::Output);
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
if this.output1.is_none() {
if let Poll::Ready(out) = this.future1.poll(cx) {
*this.output1 = Some(out);
}
}
if this.output2.is_none() {
if let Poll::Ready(out) = this.future2.poll(cx) {
*this.output2 = Some(out);
}
}
if this.output1.is_some() && this.output2.is_some() {
Poll::Ready((this.output1.take().unwrap(), this.output2.take().unwrap()))
} else {
Poll::Pending
}
}
}
/// Get the [`Waker`] inside an async fn where you aren't supposed to
/// have it.
///
/// This is a low level primitive for implementing more complex
/// patterns while avoiding the [`Poll`] API.
///
/// # Examples
///
/// ```
/// use futures_micro::{sleep, waker};
///
/// # futures_lite::future::block_on(async {
/// let waker = waker().await;
/// assert_eq!(async { waker.wake(); sleep().await; 1 }.await, 1)
/// # })
/// ```
pub fn waker() -> impl Future<Output = Waker> {
poll_fn(|ctx| Poll::Ready(ctx.waker().clone()))
}
/// Goes to sleep until woken by its [`Waker`] being called.
///
/// This is a low level primitive for implementing more complex
/// patterns while avoiding the [`Poll`] API.
///
/// # Examples
///
/// ```
/// use futures_micro::{sleep, waker};
///
/// # futures_lite::future::block_on(async {
/// let waker = waker().await;
/// assert_eq!(async { waker.wake(); sleep().await; 1 }.await, 1)
/// # })
/// ```
pub fn sleep() -> impl Future<Output = ()> {
let mut done = false;
poll_fn(move |_| {
if done {
Poll::Ready(())
} else {
done = true;
Poll::Pending
}
})
}
/// Pushes itself to the back of the executor queue so some other
/// tasks can do some work.
pub fn yield_once() -> impl Future<Output = ()> {
let mut done = false;
poll_fn(move |ctx| {
if done {
Poll::Ready(())
} else {
done = true;
ctx.waker().wake_by_ref();
Poll::Pending }
})
}
// --------- MACROS ---------
// Helper for `or!`
#[doc(hidden)]
#[macro_export]
macro_rules! __internal_fold_with {
($func:path, $e:expr) => { $e };
($func:path, $e:expr, $($es:expr),+) => {
$func($e, $crate::__internal_fold_with!($func, $($es),+))
};
}
/// Polls arbitrarily many futures, returning the first ready value.
///
/// All futures must have the same output type. Left biased when more
/// than one Future is ready at the same time.
#[macro_export]
macro_rules! or {
($($es:expr),+$(,)?) => { $crate::__internal_fold_with!($crate::Or::new, $($es),+) };
}
/// Pins a variable of type `T` on the stack and rebinds it as `Pin<&mut T>`.
///
/// ```
/// use futures_micro::*;
/// use std::fmt::Debug;
/// use std::time::Instant;
///
/// // Inspects each invocation of `Future::poll()`.
/// async fn inspect<T: Debug>(f: impl Future<Output = T>) -> T {
/// pin!(f);
/// poll_fn(|cx| dbg!(f.as_mut().poll(cx))).await
/// }
///
/// # futures_lite::future::block_on(async {
/// let f = async { 1 + 2 };
/// inspect(f).await;
/// # })
/// ```
#[macro_export]
macro_rules! pin {
($($x:ident),* $(,)?) => {
$(
let mut $x = $x;
#[allow(unused_mut)]
let mut $x = unsafe {
core::pin::Pin::new_unchecked(&mut $x)
};
)*
}
}
/// Unwraps `Poll<T>` or returns [`Pending`][`Poll::Pending`].
///
/// # Examples
///
/// ```
/// use futures_micro::*;
///
/// // Polls two futures and sums their results.
/// fn poll_sum(
/// cx: &mut Context<'_>,
/// mut a: impl Future<Output = i32> + Unpin,
/// mut b: impl Future<Output = i32> + Unpin,
/// ) -> Poll<i32> {
/// let x = ready!(Pin::new(&mut a).poll(cx));
/// let y = ready!(Pin::new(&mut b).poll(cx));
/// Poll::Ready(x + y)
/// }
/// ```
#[macro_export]
macro_rules! ready {
($e:expr $(,)?) => {
match $e {
core::task::Poll::Ready(t) => t,
t @ core::task::Poll::Pending => return t,
}
};
}
/// Zips arbitrarily many futures, waiting for all to complete.
///
/// # Examples
///
/// ```
/// use futures_micro::zip;
///
/// # futures_lite::future::block_on(async {
/// let a = async { 1 };
/// let b = async { 2 };
///
/// assert_eq!(zip!(a, b).await, (1, 2));
/// # })
/// ```
#[macro_export]
macro_rules! zip {
($($es:expr),+ $(,)?) => {{
let mut zips = $crate::__internal_fold_with!($crate::Zip::new, $($es),+);
$crate::poll_fn(move |ctx| {
use ::core::pin::Pin;
use ::core::task::Poll;
let zips = unsafe { Pin::new_unchecked(&mut zips) };
if let Poll::Ready(val) = ::core::future::Future::poll(zips, ctx) {
Poll::Ready($crate::zip!(@flatten; ; val; $($es),+))
} else {
Poll::Pending
}
})
}};
(@flatten; $($prev:expr,)*; $tuple:expr; $e:expr) => {
($($prev,)* $tuple)
};
(@flatten; $($prev:expr,)*; $tuple:expr; $e:expr, $($es:expr),+) => {
$crate::zip!(@flatten; $($prev,)* $tuple.0,; $tuple.1; $($es),+)
};
}