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//! Definition of the `PollFn` combinator use futures_core::{Stream, Poll}; use futures_core::task; /// A stream which adapts a function returning `Poll`. /// /// Created by the `poll_fn` function. #[derive(Debug)] #[must_use = "streams do nothing unless polled"] pub struct PollFn<F> { inner: F, } /// Creates a new stream wrapping around a function returning `Poll`. /// /// Polling the returned stream delegates to the wrapped function. /// /// # Examples /// /// ``` /// # extern crate futures; /// use futures::prelude::*; /// use futures::stream::poll_fn; /// /// # fn main() { /// let mut counter = 1usize; /// /// let read_stream = poll_fn(move |_| -> Poll<Option<String>, std::io::Error> { /// if counter == 0 { return Ok(Async::Ready(None)); } /// counter -= 1; /// Ok(Async::Ready(Some("Hello, World!".to_owned()))) /// }); /// # } /// ``` pub fn poll_fn<T, E, F>(f: F) -> PollFn<F> where F: FnMut(&mut task::Context) -> Poll<Option<T>, E>, { PollFn { inner: f } } impl<T, E, F> Stream for PollFn<F> where F: FnMut(&mut task::Context) -> Poll<Option<T>, E>, { type Item = T; type Error = E; fn poll_next(&mut self, cx: &mut task::Context) -> Poll<Option<T>, E> { (self.inner)(cx) } }