# Function std::future::poll_fn

1.64.0 · source ·

    pub fn poll_fn<T, F>(f: F) -> PollFn<F> ⓘwhere
        F: FnMut(&mut Context<'_>) -> Poll<T>,

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!

Run

This 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 the `pin!` macro work, thereby avoiding any need for the `unsafe` `Pin::new_unchecked(&mut fut)` constructor.