Implement stream::poll_fn

Author: ishitatsuyuki

src/stream/poll_fn.rs

@@ -0,0 +1,49 @@
+//! Definition of the `PollFn` combinator
+
+use {Stream, Poll};
+
+/// 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
+///
+/// ```
+/// use futures::stream::poll_fn;
+/// use futures::{Async, Poll};
+///
+/// let mut counter = 1usize;
+///
+/// let read_stream = poll_fn(move || -> Poll<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() -> Poll<T, E>,
+{
+    PollFn { inner: f }
+}
+
+impl<T, E, F> Stream for PollFn<F>
+where
+    F: FnMut() -> Poll<Option<T>, E>,
+{
+    type Item = T;
+    type Error = E;
+
+    fn poll(&mut self) -> Poll<Option<T>, E> {
+        (self.inner)()
+    }
+}