async_std/stream/
successors.rs

1use core::mem;
2use core::pin::Pin;
3
4use crate::stream::Stream;
5use crate::task::{Context, Poll};
6
7use pin_project_lite::pin_project;
8
9/// Creates a new stream where to produce each new element a closure is called with the previous
10/// value.
11///
12/// # Examples
13///
14/// ```
15/// # fn main() { async_std::task::block_on(async {
16/// #
17/// use async_std::prelude::*;
18/// use async_std::stream;
19///
20/// let mut s = stream::successors(Some(22), |&val| Some(val + 1));
21///
22/// assert_eq!(s.next().await, Some(22));
23/// assert_eq!(s.next().await, Some(23));
24/// assert_eq!(s.next().await, Some(24));
25/// assert_eq!(s.next().await, Some(25));
26///
27/// #
28/// # }) }
29/// ```
30#[cfg(feature = "unstable")]
31#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
32pub fn successors<F, T>(first: Option<T>, succ: F) -> Successors<F, T>
33where
34    F: FnMut(&T) -> Option<T>,
35{
36    Successors { succ, slot: first }
37}
38
39pin_project! {
40    /// A stream that yields elements by calling an async closure with the previous value as an
41    /// argument
42    ///
43    /// This stream is constructed by [`successors`] function
44    ///
45    /// [`successors`]: fn.successors.html
46    #[cfg(feature = "unstable")]
47    #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
48    #[derive(Debug)]
49    pub struct Successors<F, T>
50    where
51        F: FnMut(&T) -> Option<T>
52    {
53        succ: F,
54        slot: Option<T>,
55    }
56}
57
58impl<F, T> Stream for Successors<F, T>
59where
60    F: FnMut(&T) -> Option<T>,
61{
62    type Item = T;
63
64    fn poll_next(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
65        let this = self.project();
66
67        if this.slot.is_none() {
68            return Poll::Ready(None);
69        }
70
71        let mut next = (this.succ)(&this.slot.as_ref().unwrap());
72
73        // 'swapping' here means 'slot' will hold the next value and next will be th one from the previous iteration
74        mem::swap(this.slot, &mut next);
75        Poll::Ready(next)
76    }
77}
async_std/stream/successors.rs

async_std/stream/
successors.rs
