Trait async_std::stream::DoubleEndedStream

pub trait DoubleEndedStream: Stream {
    fn poll_next_back(
        self: Pin<&mut Self>, 
        cx: &mut Context<'_>
    ) -> Poll<Option<Self::Item>>;

    fn next_back(&mut self) -> NextBackFuture<'_, Self>
    where
        Self: Unpin,
    { ... }
    fn nth_back(&mut self, n: usize) -> NthBackFuture<'_, Self>
    where
        Self: Unpin + Sized,
    { ... }
    fn rfind<P>(&mut self, p: P) -> RFindFuture<'_, Self, P>
    where
        Self: Unpin + Sized,
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn rfold<B, F>(self, accum: B, f: F) -> RFoldFuture<Self, F, B>
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    { ... }
    fn try_rfold<B, F, E>(self, accum: B, f: F) -> TryRFoldFuture<Self, F, B>
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> Result<B, E>,
    { ... }
}

A stream able to yield elements from both ends.

Something that implements DoubleEndedStream has one extra capability over something that implements Stream: the ability to also take Items from the back, as well as the front.

Required methods

fn poll_next_back(
    self: Pin<&mut Self>,
    cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>

Attempts to receive the next item from the back of the stream.

There are several possible return values:

• Poll::Pending means this stream’s next_back value is not ready yet.
• Poll::Ready(None) means this stream has been exhausted.
• Poll::Ready(Some(item)) means item was received out of the stream.

Examples

use std::pin::Pin;

use async_std::prelude::*;
use async_std::stream;
use async_std::task::{Context, Poll};

fn increment(
    s: impl DoubleEndedStream<Item = i32> + Unpin,
) -> impl DoubleEndedStream<Item = i32> + Unpin {
    struct Increment<S>(S);

    impl<S: DoubleEndedStream<Item = i32> + Unpin> Stream for Increment<S> {
        type Item = S::Item;

        fn poll_next(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            match Pin::new(&mut self.0).poll_next(cx) {
                Poll::Pending => Poll::Pending,
                Poll::Ready(None) => Poll::Ready(None),
                Poll::Ready(Some(item)) => Poll::Ready(Some(item + 1)),
            }
        }
    }

    impl<S: DoubleEndedStream<Item = i32> + Unpin> DoubleEndedStream for Increment<S> {
        fn poll_next_back(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            match Pin::new(&mut self.0).poll_next_back(cx) {
                Poll::Pending => Poll::Pending,
                Poll::Ready(None) => Poll::Ready(None),
                Poll::Ready(Some(item)) => Poll::Ready(Some(item + 1)),
            }
        }
    }

    Increment(s)
}

let mut s = increment(stream::once(7));

assert_eq!(s.next_back().await, Some(8));
assert_eq!(s.next_back().await, None);

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Provided methods

fn next_back(&mut self) -> NextBackFuture<'_, Self> where
    Self: Unpin, 

Advances the stream and returns the next value.

Returns None when iteration is finished. Individual stream implementations may choose to resume iteration, and so calling next() again may or may not eventually start returning more values.

Examples

use async_std::prelude::*;
use async_std::stream;

let mut s = stream::from_iter(vec![7u8]);

assert_eq!(s.next_back().await, Some(7));
assert_eq!(s.next_back().await, None);

fn nth_back(&mut self, n: usize) -> NthBackFuture<'_, Self> where
    Self: Unpin + Sized, 

Returns the nth element from the back of the stream.

Examples

Basic usage:

use async_std::prelude::*;
use async_std::stream;

let mut s = stream::from_iter(vec![1u8, 2, 3, 4, 5]);

let second = s.nth_back(1).await;
assert_eq!(second, Some(4));

fn rfind<P>(&mut self, p: P) -> RFindFuture<'_, Self, P> where
    Self: Unpin + Sized,
    P: FnMut(&Self::Item) -> bool, 

Returns the the frist element from the right that matches the predicate.

Examples

Basic usage:

use async_std::prelude::*;
use async_std::stream;

let mut s = stream::from_iter(vec![1u8, 2, 3, 4, 5]);

let second = s.rfind(|v| v % 2 == 0).await;
assert_eq!(second, Some(4));

fn rfold<B, F>(self, accum: B, f: F) -> RFoldFuture<Self, F, B> where
    Self: Sized,
    F: FnMut(B, Self::Item) -> B, 

Examples

Basic usage:

use async_std::prelude::*;
use async_std::stream;

let s = stream::from_iter(vec![1u8, 2, 3, 4, 5]);

let second = s.rfold(0, |acc, v| v + acc).await;

assert_eq!(second, 15);

fn try_rfold<B, F, E>(self, accum: B, f: F) -> TryRFoldFuture<Self, F, B> where
    Self: Sized,
    F: FnMut(B, Self::Item) -> Result<B, E>, 

A combinator that applies a function as long as it returns successfully, producing a single, final value. Immediately returns the error when the function returns unsuccessfully.

Examples

Basic usage:

use async_std::prelude::*;
use async_std::stream;

let s = stream::from_iter(vec![1u8, 2, 3, 4, 5]);
let sum = s.try_rfold(0, |acc, v| {
    if (acc+v) % 2 == 1 {
        Ok(v+3)
    } else {
        Err("fail")
    }
}).await;

assert_eq!(sum, Err("fail"));

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Implementors

impl<T> DoubleEndedStream for Once<T>

fn poll_next_back(
    self: Pin<&mut Self>,
    _cx: &mut Context<'_>
) -> Poll<Option<Self::Item>>

impl<T> DoubleEndedStream for Pending<T>

fn poll_next_back(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>>

impl<T: DoubleEndedIterator> DoubleEndedStream for FromIter<T>

fn poll_next_back(
    self: Pin<&mut Self>,
    _cx: &mut Context<'_>
) -> Poll<Option<T::Item>>

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