pub fn poll_fn<T, F>(f: F) -> PollFn<F> ⓘ
Expand description
Creates a future that wraps a function returning Poll
.
Polling the future delegates to the wrapped function. If the returned future is pinned, then the captured environment of the wrapped function is also pinned in-place, so as long as the closure does not move out of its captures it can soundly create pinned references to them.
§Examples
use core::future::poll_fn;
use std::task::{Context, Poll};
fn read_line(_cx: &mut Context<'_>) -> Poll<String> {
Poll::Ready("Hello, World!".into())
}
let read_future = poll_fn(read_line);
assert_eq!(read_future.await, "Hello, World!".to_owned());
Run§Capturing a pinned state
Example of a closure wrapping inner futures:
use core::future::{self, Future};
use core::task::Poll;
/// Resolves to the first future that completes. In the event of a tie, `a` wins.
fn naive_select<T>(
a: impl Future<Output = T>,
b: impl Future<Output = T>,
) -> impl Future<Output = T>
{
let (mut a, mut b) = (Box::pin(a), Box::pin(b));
future::poll_fn(move |cx| {
if let Poll::Ready(r) = a.as_mut().poll(cx) {
Poll::Ready(r)
} else if let Poll::Ready(r) = b.as_mut().poll(cx) {
Poll::Ready(r)
} else {
Poll::Pending
}
})
}
let a = async { 42 };
let b = future::pending();
let v = naive_select(a, b).await;
assert_eq!(v, 42);
let a = future::pending();
let b = async { 27 };
let v = naive_select(a, b).await;
assert_eq!(v, 27);
let a = async { 42 };
let b = async { 27 };
let v = naive_select(a, b).await;
assert_eq!(v, 42); // biased towards `a` in case of tie!
RunThis time without Box::pin
ning:
use core::future::{self, Future};
use core::pin::pin;
use core::task::Poll;
/// Resolves to the first future that completes. In the event of a tie, `a` wins.
fn naive_select<T>(
a: impl Future<Output = T>,
b: impl Future<Output = T>,
) -> impl Future<Output = T>
{
async {
let (mut a, mut b) = (pin!(a), pin!(b));
future::poll_fn(move |cx| {
if let Poll::Ready(r) = a.as_mut().poll(cx) {
Poll::Ready(r)
} else if let Poll::Ready(r) = b.as_mut().poll(cx) {
Poll::Ready(r)
} else {
Poll::Pending
}
}).await
}
}
let a = async { 42 };
let b = future::pending();
let v = naive_select(a, b).await;
assert_eq!(v, 42);
Run- Notice how, by virtue of being in an
async
context, we have been able to make thepin!
macro work, thereby avoiding any need for theunsafe
Pin::new_unchecked(&mut fut)
constructor.