Only produes empty value if next is ever a 'None'
@@ -14,11 +14,11 @@ use crate::task::{Context, Poll};
#[derive(Debug)]
pub struct Successors<F, Fut, T>
where
- Fut: Future<Output = T>,
+ Fut: Future<Output = Option<T>>,
{
successor: F,
future: Option<Fut>,
- next: T,
+ next: Option<T>,
_marker: PhantomData<Fut>,
}
@@ -33,9 +33,9 @@ where
/// use async_std::prelude::*;
/// use async_std::stream;
///
-/// let s = stream::successors(22, |val| {
+/// let s = stream::successors(Some(22), |val| {
/// async move {
-/// val + 1
+/// Some(val + 1)
/// }
/// });
///
@@ -43,14 +43,25 @@ where
/// assert_eq!(s.next().await, Some(23));
/// assert_eq!(s.next().await, Some(24));
/// assert_eq!(s.next().await, Some(25));
+///
+///
+///let never = stream::successors(None, |val: usize| {
+/// async move {
+/// Some(val + 1)
+/// }
+/// });
+///
+/// pin_utils::pin_mut!(never);
+/// assert_eq!(never.next().await, None);
+/// assert_eq!(never.next().await, None);
/// #
/// # }) }
///
/// ```
-pub fn successors<F, Fut, T>(start: T, func: F) -> Successors<F, Fut, T>
+pub fn successors<F, Fut, T>(start: Option<T>, func: F) -> Successors<F, Fut, T>
where
F: FnMut(T) -> Fut,
- Fut: Future<Output = T>,
+ Fut: Future<Output = Option<T>>,
T: Copy,
{
Successors {
@@ -64,26 +75,30 @@ where
impl<F, Fut, T> Successors<F, Fut, T>
where
F: FnMut(T) -> Fut,
- Fut: Future<Output = T>,
+ Fut: Future<Output = Option<T>>,
T: Copy,
{
pin_utils::unsafe_unpinned!(successor: F);
- pin_utils::unsafe_unpinned!(next: T);
+ pin_utils::unsafe_unpinned!(next: Option<T>);
pin_utils::unsafe_pinned!(future: Option<Fut>);
}
impl<F, Fut, T> Stream for Successors<F, Fut, T>
where
- Fut: Future<Output = T>,
+ Fut: Future<Output = Option<T>>,
F: FnMut(T) -> Fut,
T: Copy,
{
type Item = T;
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+ if self.next.is_none() {
+ return Poll::Ready(None);
+ }
+
match &self.future {
None => {
- let x = self.next;
+ let x = self.next.unwrap();
let fut = (self.as_mut().successor())(x);
self.as_mut().future().set(Some(fut));
}
@@ -93,6 +108,6 @@ where
let next = futures_core::ready!(self.as_mut().future().as_pin_mut().unwrap().poll(cx));
*self.as_mut().next() = next;
self.as_mut().future().set(None);
- Poll::Ready(Some(next))
+ Poll::Ready(next)
}
}
