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//! Combinators for the [`Future`] trait.
//!
//! # Examples
//!
//! ```
//! use futures_lite::future;
//!
//! # spin_on::spin_on(async {
//! for step in 0..3 {
//! println!("step {}", step);
//!
//! // Give other tasks a chance to run.
//! future::yield_now().await;
//! }
//! # });
//! ```
#[cfg(all(not(feature = "std"), feature = "alloc"))]
extern crate alloc;
#[doc(no_inline)]
pub use core::future::{pending, ready, Future, Pending, Ready};
use core::fmt;
use core::pin::Pin;
use pin_project_lite::pin_project;
#[cfg(feature = "std")]
use std::{
any::Any,
panic::{catch_unwind, AssertUnwindSafe, UnwindSafe},
};
#[cfg(feature = "race")]
use fastrand::Rng;
#[cfg(all(not(feature = "std"), feature = "alloc"))]
use alloc::boxed::Box;
use core::task::{Context, Poll};
/// Blocks the current thread on a future.
///
/// # Examples
///
/// ```
/// use futures_lite::future;
///
/// let val = future::block_on(async {
/// 1 + 2
/// });
///
/// assert_eq!(val, 3);
/// ```
#[cfg(feature = "std")]
pub fn block_on<T>(future: impl Future<Output = T>) -> T {
use core::cell::RefCell;
use core::task::Waker;
use parking::Parker;
// Pin the future on the stack.
crate::pin!(future);
// Creates a parker and an associated waker that unparks it.
fn parker_and_waker() -> (Parker, Waker) {
let parker = Parker::new();
let unparker = parker.unparker();
let waker = Waker::from(unparker);
(parker, waker)
}
thread_local! {
// Cached parker and waker for efficiency.
static CACHE: RefCell<(Parker, Waker)> = RefCell::new(parker_and_waker());
}
CACHE.with(|cache| {
// Try grabbing the cached parker and waker.
let tmp_cached;
let tmp_fresh;
let (parker, waker) = match cache.try_borrow_mut() {
Ok(cache) => {
// Use the cached parker and waker.
tmp_cached = cache;
&*tmp_cached
}
Err(_) => {
// Looks like this is a recursive `block_on()` call.
// Create a fresh parker and waker.
tmp_fresh = parker_and_waker();
&tmp_fresh
}
};
let cx = &mut Context::from_waker(waker);
// Keep polling until the future is ready.
loop {
match future.as_mut().poll(cx) {
Poll::Ready(output) => return output,
Poll::Pending => parker.park(),
}
}
})
}
/// Polls a future just once and returns an [`Option`] with the result.
///
/// # Examples
///
/// ```
/// use futures_lite::future;
///
/// # spin_on::spin_on(async {
/// assert_eq!(future::poll_once(future::pending::<()>()).await, None);
/// assert_eq!(future::poll_once(future::ready(42)).await, Some(42));
/// # })
/// ```
pub fn poll_once<T, F>(f: F) -> PollOnce<F>
where
F: Future<Output = T>,
{
PollOnce { f }
}
pin_project! {
/// Future for the [`poll_once()`] function.
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct PollOnce<F> {
#[pin]
f: F,
}
}
impl<F> fmt::Debug for PollOnce<F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("PollOnce").finish()
}
}
impl<T, F> Future for PollOnce<F>
where
F: Future<Output = T>,
{
type Output = Option<T>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match self.project().f.poll(cx) {
Poll::Ready(t) => Poll::Ready(Some(t)),
Poll::Pending => Poll::Ready(None),
}
}
}
/// Creates a future from a function returning [`Poll`].
///
/// # Examples
///
/// ```
/// use futures_lite::future;
/// use std::task::{Context, Poll};
///
/// # spin_on::spin_on(async {
/// fn f(_: &mut Context<'_>) -> Poll<i32> {
/// Poll::Ready(7)
/// }
///
/// assert_eq!(future::poll_fn(f).await, 7);
/// # })
/// ```
pub fn poll_fn<T, F>(f: F) -> PollFn<F>
where
F: FnMut(&mut Context<'_>) -> Poll<T>,
{
PollFn { f }
}
pin_project! {
/// Future for the [`poll_fn()`] function.
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct PollFn<F> {
f: F,
}
}
impl<F> fmt::Debug for PollFn<F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("PollFn").finish()
}
}
impl<T, F> Future for PollFn<F>
where
F: FnMut(&mut Context<'_>) -> Poll<T>,
{
type Output = T;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
let this = self.project();
(this.f)(cx)
}
}
/// Wakes the current task and returns [`Poll::Pending`] once.
///
/// This function is useful when we want to cooperatively give time to the task scheduler. It is
/// generally a good idea to yield inside loops because that way we make sure long-running tasks
/// don't prevent other tasks from running.
///
/// # Examples
///
/// ```
/// use futures_lite::future;
///
/// # spin_on::spin_on(async {
/// future::yield_now().await;
/// # })
/// ```
pub fn yield_now() -> YieldNow {
YieldNow(false)
}
/// Future for the [`yield_now()`] function.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct YieldNow(bool);
impl Future for YieldNow {
type Output = ();
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
if !self.0 {
self.0 = true;
cx.waker().wake_by_ref();
Poll::Pending
} else {
Poll::Ready(())
}
}
}
/// Joins two futures, waiting for both to complete.
///
/// # Examples
///
/// ```
/// use futures_lite::future;
///
/// # spin_on::spin_on(async {
/// let a = async { 1 };
/// let b = async { 2 };
///
/// assert_eq!(future::zip(a, b).await, (1, 2));
/// # })
/// ```
pub fn zip<F1, F2>(future1: F1, future2: F2) -> Zip<F1, F2>
where
F1: Future,
F2: Future,
{
Zip {
future1,
future2,
output1: None,
output2: None,
}
}
pin_project! {
/// Future for the [`zip()`] function.
#[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>,
}
}
/// Extracts the contents of two options and zips them, handling `(Some(_), None)` cases
fn take_zip_from_parts<T1, T2>(o1: &mut Option<T1>, o2: &mut Option<T2>) -> Poll<(T1, T2)> {
match (o1.take(), o2.take()) {
(Some(t1), Some(t2)) => Poll::Ready((t1, t2)),
(o1x, o2x) => {
*o1 = o1x;
*o2 = o2x;
Poll::Pending
}
}
}
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);
}
}
take_zip_from_parts(this.output1, this.output2)
}
}
/// Joins two fallible futures, waiting for both to complete or one of them to error.
///
/// # Examples
///
/// ```
/// use futures_lite::future;
///
/// # spin_on::spin_on(async {
/// let a = async { Ok::<i32, i32>(1) };
/// let b = async { Err::<i32, i32>(2) };
///
/// assert_eq!(future::try_zip(a, b).await, Err(2));
/// # })
/// ```
pub fn try_zip<T1, T2, E, F1, F2>(future1: F1, future2: F2) -> TryZip<F1, T1, F2, T2>
where
F1: Future<Output = Result<T1, E>>,
F2: Future<Output = Result<T2, E>>,
{
TryZip {
future1,
future2,
output1: None,
output2: None,
}
}
pin_project! {
/// Future for the [`try_zip()`] function.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct TryZip<F1, T1, F2, T2> {
#[pin]
future1: F1,
output1: Option<T1>,
#[pin]
future2: F2,
output2: Option<T2>,
}
}
impl<T1, T2, E, F1, F2> Future for TryZip<F1, T1, F2, T2>
where
F1: Future<Output = Result<T1, E>>,
F2: Future<Output = Result<T2, E>>,
{
type Output = Result<(T1, T2), E>;
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) {
match out {
Ok(t) => *this.output1 = Some(t),
Err(err) => return Poll::Ready(Err(err)),
}
}
}
if this.output2.is_none() {
if let Poll::Ready(out) = this.future2.poll(cx) {
match out {
Ok(t) => *this.output2 = Some(t),
Err(err) => return Poll::Ready(Err(err)),
}
}
}
take_zip_from_parts(this.output1, this.output2).map(Ok)
}
}
/// Returns the result of the future that completes first, preferring `future1` if both are ready.
///
/// If you need to treat the two futures fairly without a preference for either, use the [`race()`]
/// function or the [`FutureExt::race()`] method.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{self, pending, ready};
///
/// # spin_on::spin_on(async {
/// assert_eq!(future::or(ready(1), pending()).await, 1);
/// assert_eq!(future::or(pending(), ready(2)).await, 2);
///
/// // The first future wins.
/// assert_eq!(future::or(ready(1), ready(2)).await, 1);
/// # })
/// ```
pub fn or<T, F1, F2>(future1: F1, future2: F2) -> Or<F1, F2>
where
F1: Future<Output = T>,
F2: Future<Output = T>,
{
Or { future1, future2 }
}
pin_project! {
/// Future for the [`or()`] function and the [`FutureExt::or()`] method.
#[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<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) {
return Poll::Ready(t);
}
if let Poll::Ready(t) = this.future2.poll(cx) {
return Poll::Ready(t);
}
Poll::Pending
}
}
/// Returns the result of the future that completes first, with no preference if both are ready.
///
/// Each time [`Race`] is polled, the two inner futures are polled in random order. Therefore, no
/// future takes precedence over the other if both can complete at the same time.
///
/// If you have preference for one of the futures, use the [`or()`] function or the
/// [`FutureExt::or()`] method.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{self, pending, ready};
///
/// # spin_on::spin_on(async {
/// assert_eq!(future::race(ready(1), pending()).await, 1);
/// assert_eq!(future::race(pending(), ready(2)).await, 2);
///
/// // One of the two futures is randomly chosen as the winner.
/// let res = future::race(ready(1), ready(2)).await;
/// # })
/// ```
#[cfg(all(feature = "race", feature = "std"))]
pub fn race<T, F1, F2>(future1: F1, future2: F2) -> Race<F1, F2>
where
F1: Future<Output = T>,
F2: Future<Output = T>,
{
Race {
future1,
future2,
rng: Rng::new(),
}
}
/// Race two futures but with a predefined random seed.
///
/// This function is identical to [`race`], but instead of using a random seed from a thread-local
/// RNG, it allows the user to provide a seed. It is useful for when you already have a source of
/// randomness available, or if you want to use a fixed seed.
///
/// See documentation of the [`race`] function for features and caveats.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{self, pending, ready};
///
/// // A fixed seed is used, so the result is deterministic.
/// const SEED: u64 = 0x42;
///
/// # spin_on::spin_on(async {
/// assert_eq!(future::race_with_seed(ready(1), pending(), SEED).await, 1);
/// assert_eq!(future::race_with_seed(pending(), ready(2), SEED).await, 2);
///
/// // One of the two futures is randomly chosen as the winner.
/// let res = future::race_with_seed(ready(1), ready(2), SEED).await;
/// # })
/// ```
#[cfg(feature = "race")]
pub fn race_with_seed<T, F1, F2>(future1: F1, future2: F2, seed: u64) -> Race<F1, F2>
where
F1: Future<Output = T>,
F2: Future<Output = T>,
{
Race {
future1,
future2,
rng: Rng::with_seed(seed),
}
}
#[cfg(feature = "race")]
pin_project! {
/// Future for the [`race()`] function and the [`FutureExt::race()`] method.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct Race<F1, F2> {
#[pin]
future1: F1,
#[pin]
future2: F2,
rng: Rng,
}
}
#[cfg(feature = "race")]
impl<T, F1, F2> Future for Race<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 this.rng.bool() {
if let Poll::Ready(t) = this.future1.poll(cx) {
return Poll::Ready(t);
}
if let Poll::Ready(t) = this.future2.poll(cx) {
return Poll::Ready(t);
}
} else {
if let Poll::Ready(t) = this.future2.poll(cx) {
return Poll::Ready(t);
}
if let Poll::Ready(t) = this.future1.poll(cx) {
return Poll::Ready(t);
}
}
Poll::Pending
}
}
#[cfg(feature = "std")]
pin_project! {
/// Future for the [`FutureExt::catch_unwind()`] method.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct CatchUnwind<F> {
#[pin]
inner: F,
}
}
#[cfg(feature = "std")]
impl<F: Future + UnwindSafe> Future for CatchUnwind<F> {
type Output = Result<F::Output, Box<dyn Any + Send>>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let this = self.project();
catch_unwind(AssertUnwindSafe(|| this.inner.poll(cx)))?.map(Ok)
}
}
/// Type alias for `Pin<Box<dyn Future<Output = T> + Send + 'static>>`.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{self, FutureExt};
///
/// // These two lines are equivalent:
/// let f1: future::Boxed<i32> = async { 1 + 2 }.boxed();
/// let f2: future::Boxed<i32> = Box::pin(async { 1 + 2 });
/// ```
#[cfg(feature = "alloc")]
pub type Boxed<T> = Pin<Box<dyn Future<Output = T> + Send + 'static>>;
/// Type alias for `Pin<Box<dyn Future<Output = T> + 'static>>`.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{self, FutureExt};
///
/// // These two lines are equivalent:
/// let f1: future::BoxedLocal<i32> = async { 1 + 2 }.boxed_local();
/// let f2: future::BoxedLocal<i32> = Box::pin(async { 1 + 2 });
/// ```
#[cfg(feature = "alloc")]
pub type BoxedLocal<T> = Pin<Box<dyn Future<Output = T> + 'static>>;
/// Extension trait for [`Future`].
pub trait FutureExt: Future {
/// A convenience for calling [`Future::poll()`] on `!`[`Unpin`] types.
fn poll(&mut self, cx: &mut Context<'_>) -> Poll<Self::Output>
where
Self: Unpin,
{
Future::poll(Pin::new(self), cx)
}
/// Returns the result of `self` or `other` future, preferring `self` if both are ready.
///
/// If you need to treat the two futures fairly without a preference for either, use the
/// [`race()`] function or the [`FutureExt::race()`] method.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{pending, ready, FutureExt};
///
/// # spin_on::spin_on(async {
/// assert_eq!(ready(1).or(pending()).await, 1);
/// assert_eq!(pending().or(ready(2)).await, 2);
///
/// // The first future wins.
/// assert_eq!(ready(1).or(ready(2)).await, 1);
/// # })
/// ```
fn or<F>(self, other: F) -> Or<Self, F>
where
Self: Sized,
F: Future<Output = Self::Output>,
{
Or {
future1: self,
future2: other,
}
}
/// Returns the result of `self` or `other` future, with no preference if both are ready.
///
/// Each time [`Race`] is polled, the two inner futures are polled in random order. Therefore,
/// no future takes precedence over the other if both can complete at the same time.
///
/// If you have preference for one of the futures, use the [`or()`] function or the
/// [`FutureExt::or()`] method.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{pending, ready, FutureExt};
///
/// # spin_on::spin_on(async {
/// assert_eq!(ready(1).race(pending()).await, 1);
/// assert_eq!(pending().race(ready(2)).await, 2);
///
/// // One of the two futures is randomly chosen as the winner.
/// let res = ready(1).race(ready(2)).await;
/// # })
/// ```
#[cfg(all(feature = "std", feature = "race"))]
fn race<F>(self, other: F) -> Race<Self, F>
where
Self: Sized,
F: Future<Output = Self::Output>,
{
Race {
future1: self,
future2: other,
rng: Rng::new(),
}
}
/// Catches panics while polling the future.
///
/// # Examples
///
/// ```
/// use futures_lite::future::FutureExt;
///
/// # spin_on::spin_on(async {
/// let fut1 = async {}.catch_unwind();
/// let fut2 = async { panic!() }.catch_unwind();
///
/// assert!(fut1.await.is_ok());
/// assert!(fut2.await.is_err());
/// # })
/// ```
#[cfg(feature = "std")]
fn catch_unwind(self) -> CatchUnwind<Self>
where
Self: Sized + UnwindSafe,
{
CatchUnwind { inner: self }
}
/// Boxes the future and changes its type to `dyn Future + Send + 'a`.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{self, FutureExt};
///
/// # spin_on::spin_on(async {
/// let a = future::ready('a');
/// let b = future::pending();
///
/// // Futures of different types can be stored in
/// // the same collection when they are boxed:
/// let futures = vec![a.boxed(), b.boxed()];
/// # })
/// ```
#[cfg(feature = "alloc")]
fn boxed<'a>(self) -> Pin<Box<dyn Future<Output = Self::Output> + Send + 'a>>
where
Self: Sized + Send + 'a,
{
Box::pin(self)
}
/// Boxes the future and changes its type to `dyn Future + 'a`.
///
/// # Examples
///
/// ```
/// use futures_lite::future::{self, FutureExt};
///
/// # spin_on::spin_on(async {
/// let a = future::ready('a');
/// let b = future::pending();
///
/// // Futures of different types can be stored in
/// // the same collection when they are boxed:
/// let futures = vec![a.boxed_local(), b.boxed_local()];
/// # })
/// ```
#[cfg(feature = "alloc")]
fn boxed_local<'a>(self) -> Pin<Box<dyn Future<Output = Self::Output> + 'a>>
where
Self: Sized + 'a,
{
Box::pin(self)
}
}
impl<F: Future + ?Sized> FutureExt for F {}