cfg_unstable! {
mod delay;
mod flatten;
mod race;
mod try_race;
mod join;
mod try_join;
use std::time::Duration;
use delay::DelayFuture;
use flatten::FlattenFuture;
use crate::future::IntoFuture;
use race::Race;
use try_race::TryRace;
use join::Join;
use try_join::TryJoin;
}
extension_trait! {
use std::pin::Pin;
use std::ops::{Deref, DerefMut};
use crate::task::{Context, Poll};
#[doc = r#"
A future represents an asynchronous computation.
A future is a value that may not have finished computing yet. This kind of
"asynchronous value" makes it possible for a thread to continue doing useful
work while it waits for the value to become available.
The [provided methods] do not really exist in the trait itself, but they become
available when [`FutureExt`] from the [prelude] is imported:
```
# #[allow(unused_imports)]
use async_std::prelude::*;
```
# The `poll` method
The core method of future, `poll`, *attempts* to resolve the future into a
final value. This method does not block if the value is not ready. Instead,
the current task is scheduled to be woken up when it's possible to make
further progress by `poll`ing again. The `context` passed to the `poll`
method can provide a [`Waker`], which is a handle for waking up the current
task.
When using a future, you generally won't call `poll` directly, but instead
`.await` the value.
[`Waker`]: ../task/struct.Waker.html
[provided methods]: #provided-methods
[`FutureExt`]: ../prelude/trait.FutureExt.html
[prelude]: ../prelude/index.html
"#]
pub trait Future {
#[doc = r#"
The type of value produced on completion.
"#]
type Output;
#[doc = r#"
Attempt to resolve the future to a final value, registering
the current task for wakeup if the value is not yet available.
# Return value
This function returns:
- [`Poll::Pending`] if the future is not ready yet
- [`Poll::Ready(val)`] with the result `val` of this future if it
finished successfully.
Once a future has finished, clients should not `poll` it again.
When a future is not ready yet, `poll` returns `Poll::Pending` and
stores a clone of the [`Waker`] copied from the current [`Context`].
This [`Waker`] is then woken once the future can make progress.
For example, a future waiting for a socket to become
readable would call `.clone()` on the [`Waker`] and store it.
When a signal arrives elsewhere indicating that the socket is readable,
[`Waker::wake`] is called and the socket future's task is awoken.
Once a task has been woken up, it should attempt to `poll` the future
again, which may or may not produce a final value.
Note that on multiple calls to `poll`, only the [`Waker`] from the
[`Context`] passed to the most recent call should be scheduled to
receive a wakeup.
# Runtime characteristics
Futures alone are *inert*; they must be *actively* `poll`ed to make
progress, meaning that each time the current task is woken up, it should
actively re-`poll` pending futures that it still has an interest in.
The `poll` function is not called repeatedly in a tight loop -- instead,
it should only be called when the future indicates that it is ready to
make progress (by calling `wake()`). If you're familiar with the
`poll(2)` or `select(2)` syscalls on Unix it's worth noting that futures
typically do *not* suffer the same problems of "all wakeups must poll
all events"; they are more like `epoll(4)`.
An implementation of `poll` should strive to return quickly, and should
not block. Returning quickly prevents unnecessarily clogging up
threads or event loops. If it is known ahead of time that a call to
`poll` may end up taking awhile, the work should be offloaded to a
thread pool (or something similar) to ensure that `poll` can return
quickly.
# Panics
Once a future has completed (returned `Ready` from `poll`), calling its
`poll` method again may panic, block forever, or cause other kinds of
problems; the `Future` trait places no requirements on the effects of
such a call. However, as the `poll` method is not marked `unsafe`,
Rust's usual rules apply: calls must never cause undefined behavior
(memory corruption, incorrect use of `unsafe` functions, or the like),
regardless of the future's state.
[`Poll::Pending`]: ../task/enum.Poll.html#variant.Pending
[`Poll::Ready(val)`]: ../task/enum.Poll.html#variant.Ready
[`Context`]: ../task/struct.Context.html
[`Waker`]: ../task/struct.Waker.html
[`Waker::wake`]: ../task/struct.Waker.html#method.wake
"#]
fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output>;
}
#[doc = r#"
Extension methods for [`Future`].
[`Future`]: ../future/trait.Future.html
"#]
pub trait FutureExt: std::future::Future {
#[cfg(all(feature = "default", feature = "unstable"))]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
fn delay(self, dur: Duration) -> impl Future<Output = Self::Output> [DelayFuture<Self>]
where
Self: Sized,
{
DelayFuture::new(self, dur)
}
#[cfg(feature = "unstable")]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
fn flatten(
self,
) -> impl Future<Output = <Self::Output as IntoFuture>::Output>
[FlattenFuture<Self, <Self::Output as IntoFuture>::Future>]
where
Self: Sized,
<Self as Future>::Output: IntoFuture,
{
FlattenFuture::new(self)
}
#[doc = r#"
Waits for one of two similarly-typed futures to complete.
Awaits multiple futures simultaneously, returning the output of the
first future that completes.
This function will return a new future which awaits for either one of both
futures to complete. If multiple futures are completed at the same time,
resolution will occur in the order that they have been passed.
Note that this function consumes all futures passed, and once a future is
completed, all other futures are dropped.
# Examples
```
# async_std::task::block_on(async {
use async_std::prelude::*;
use async_std::future;
let a = future::pending();
let b = future::ready(1u8);
let c = future::ready(2u8);
let f = a.race(b).race(c);
assert_eq!(f.await, 1u8);
# });
```
"#]
#[cfg(feature = "unstable")]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
fn race<F>(
self,
other: F,
) -> impl Future<Output = <Self as std::future::Future>::Output> [Race<Self, F>]
where
Self: std::future::Future + Sized,
F: std::future::Future<Output = <Self as std::future::Future>::Output>,
{
Race::new(self, other)
}
#[doc = r#"
Waits for one of two similarly-typed fallible futures to complete.
Awaits multiple futures simultaneously, returning all results once complete.
`try_race` is similar to [`race`], but keeps going if a future
resolved to an error until all futures have been resolved. In which case
an error is returned.
The ordering of which value is yielded when two futures resolve
simultaneously is intentionally left unspecified.
[`race`]: #method.race
# Examples
```
# fn main() -> std::io::Result<()> { async_std::task::block_on(async {
#
use async_std::prelude::*;
use async_std::future;
use std::io::{Error, ErrorKind};
let a = future::pending::<Result<_, Error>>();
let b = future::ready(Err(Error::from(ErrorKind::Other)));
let c = future::ready(Ok(1u8));
let f = a.try_race(b).try_race(c);
assert_eq!(f.await?, 1u8);
#
# Ok(()) }) }
```
"#]
#[cfg(feature = "unstable")]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
fn try_race<F, T, E>(
self,
other: F
) -> impl Future<Output = <Self as std::future::Future>::Output> [TryRace<Self, F>]
where
Self: std::future::Future<Output = Result<T, E>> + Sized,
F: std::future::Future<Output = <Self as std::future::Future>::Output>,
{
TryRace::new(self, other)
}
#[doc = r#"
Waits for two similarly-typed futures to complete.
Awaits multiple futures simultaneously, returning the output of the
futures once both complete.
This function returns a new future which polls both futures
concurrently.
# Examples
```
# async_std::task::block_on(async {
use async_std::prelude::*;
use async_std::future;
let a = future::ready(1u8);
let b = future::ready(2u16);
let f = a.join(b);
assert_eq!(f.await, (1u8, 2u16));
# });
```
"#]
#[cfg(any(feature = "unstable", feature = "docs"))]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
fn join<F>(
self,
other: F
) -> impl Future<Output = (<Self as std::future::Future>::Output, <F as std::future::Future>::Output)> [Join<Self, F>]
where
Self: std::future::Future + Sized,
F: std::future::Future,
{
Join::new(self, other)
}
#[doc = r#"
Waits for two similarly-typed fallible futures to complete.
Awaits multiple futures simultaneously, returning all results once
complete.
`try_join` is similar to [`join`], but returns an error immediately
if a future resolves to an error.
[`join`]: #method.join
# Examples
```
# fn main() -> std::io::Result<()> { async_std::task::block_on(async {
#
use async_std::prelude::*;
use async_std::future;
let a = future::ready(Err::<u8, &str>("Error"));
let b = future::ready(Ok(1u8));
let f = a.try_join(b);
assert_eq!(f.await, Err("Error"));
let a = future::ready(Ok::<u8, String>(1u8));
let b = future::ready(Ok::<u16, String>(2u16));
let f = a.try_join(b);
assert_eq!(f.await, Ok((1u8, 2u16)));
#
# Ok(()) }) }
```
"#]
#[cfg(any(feature = "unstable", feature = "docs"))]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
fn try_join<F, A, B, E>(
self,
other: F
) -> impl Future<Output = Result<(A, B), E>> [TryJoin<Self, F>]
where
Self: std::future::Future<Output = Result<A, E>> + Sized,
F: std::future::Future<Output = Result<B, E>>,
{
TryJoin::new(self, other)
}
}
impl<F: Future + Unpin + ?Sized> Future for Box<F> {
type Output = F::Output;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
unreachable!("this impl only appears in the rendered docs")
}
}
impl<F: Future + Unpin + ?Sized> Future for &mut F {
type Output = F::Output;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
unreachable!("this impl only appears in the rendered docs")
}
}
impl<P> Future for Pin<P>
where
P: DerefMut + Unpin,
<P as Deref>::Target: Future,
{
type Output = <<P as Deref>::Target as Future>::Output;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
unreachable!("this impl only appears in the rendered docs")
}
}
impl<F: Future> Future for std::panic::AssertUnwindSafe<F> {
type Output = F::Output;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
unreachable!("this impl only appears in the rendered docs")
}
}
}