Trait async_std::stream::StreamExt

pub trait StreamExt: Stream {
    fn next(&mut self) -> NextFuture<'_, Self>
    where
        Self: Unpin,
    { ... }
    fn take(self, n: usize) -> Take<Self>
    where
        Self: Sized,
    { ... }
    fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn throttle(self, d: Duration) -> Throttle<Self>
    where
        Self: Sized,
    { ... }
    fn step_by(self, step: usize) -> StepBy<Self>
    where
        Self: Sized,
    { ... }
    fn chain<U>(self, other: U) -> Chain<Self, U>
    where
        Self: Sized,
        U: Stream<Item = Self::Item> + Sized,
    { ... }
    fn cloned<'a, T>(self) -> Cloned<Self>
    where
        Self: Sized + Stream<Item = &'a T>,
        T: Clone + 'a,
    { ... }
    fn copied<'a, T>(self) -> Copied<Self>
    where
        Self: Sized + Stream<Item = &'a T>,
        T: Copy + 'a,
    { ... }
    fn cycle(self) -> Cycle<Self>
    where
        Self: Clone + Sized,
    { ... }
    fn enumerate(self) -> Enumerate<Self>
    where
        Self: Sized,
    { ... }
    fn delay(self, dur: Duration) -> Delay<Self>
    where
        Self: Sized,
    { ... }
    fn map<B, F>(self, f: F) -> Map<Self, F>
    where
        Self: Sized,
        F: FnMut(Self::Item) -> B,
    { ... }
    fn inspect<F>(self, f: F) -> Inspect<Self, F>
    where
        Self: Sized,
        F: FnMut(&Self::Item),
    { ... }
    fn last(self) -> LastFuture<Self, Self::Item>
    where
        Self: Sized,
    { ... }
    fn fuse(self) -> Fuse<Self>
    where
        Self: Sized,
    { ... }
    fn filter<P>(self, predicate: P) -> Filter<Self, P>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
    where
        Self: Sized,
        U: IntoStream,
        F: FnMut(Self::Item) -> U,
    { ... }
    fn flatten(self) -> Flatten<Self>
    where
        Self: Sized,
        Self::Item: IntoStream,
    { ... }
    fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
    where
        Self: Sized,
        F: FnMut(Self::Item) -> Option<B>,
    { ... }
    fn min_by_key<B, F>(self, key_by: F) -> MinByKeyFuture<Self, Self::Item, F>
    where
        Self: Sized,
        B: Ord,
        F: FnMut(&Self::Item) -> B,
    { ... }
    fn max_by_key<B, F>(self, key_by: F) -> MaxByKeyFuture<Self, Self::Item, F>
    where
        Self: Sized,
        B: Ord,
        F: FnMut(&Self::Item) -> B,
    { ... }
    fn min_by<F>(self, compare: F) -> MinByFuture<Self, F, Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    { ... }
    fn max(self) -> MaxFuture<Self, Self::Item>
    where
        Self: Sized,
        Self::Item: Ord,
    { ... }
    fn min(self) -> MinFuture<Self, Self::Item>
    where
        Self: Sized,
        Self::Item: Ord,
    { ... }
    fn max_by<F>(self, compare: F) -> MaxByFuture<Self, F, Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    { ... }
    fn nth(&mut self, n: usize) -> NthFuture<'_, Self>
    where
        Self: Unpin + Sized,
    { ... }
    fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F, Self::Item>
    where
        Self: Unpin + Sized,
        F: FnMut(Self::Item) -> bool,
    { ... }
    fn find<P>(&mut self, p: P) -> FindFuture<'_, Self, P>
    where
        Self: Unpin + Sized,
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn find_map<F, B>(&mut self, f: F) -> FindMapFuture<'_, Self, F>
    where
        Self: Unpin + Sized,
        F: FnMut(Self::Item) -> Option<B>,
    { ... }
    fn fold<B, F>(self, init: B, f: F) -> FoldFuture<Self, F, B>
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> B,
    { ... }
    fn partition<B, F>(self, f: F) -> PartitionFuture<Self, F, B>
    where
        Self: Sized,
        F: FnMut(&Self::Item) -> bool,
        B: Default + Extend<Self::Item>,
    { ... }
    fn for_each<F>(self, f: F) -> ForEachFuture<Self, F>
    where
        Self: Sized,
        F: FnMut(Self::Item),
    { ... }
    fn any<F>(&mut self, f: F) -> AnyFuture<'_, Self, F, Self::Item>
    where
        Self: Unpin + Sized,
        F: FnMut(Self::Item) -> bool,
    { ... }
    fn by_ref(&mut self) -> &mut Self { ... }
    fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
    where
        Self: Sized,
        F: FnMut(&mut St, Self::Item) -> Option<B>,
    { ... }
    fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
    where
        Self: Sized,
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn skip(self, n: usize) -> Skip<Self>
    where
        Self: Sized,
    { ... }
    fn timeout(self, dur: Duration) -> Timeout<Self>
    where
        Self: Stream + Sized,
    { ... }
    fn try_fold<B, F, T, E>(
        &mut self, 
        init: T, 
        f: F
    ) -> TryFoldFuture<'_, Self, F, T>
    where
        Self: Unpin + Sized,
        F: FnMut(B, Self::Item) -> Result<T, E>,
    { ... }
    fn try_for_each<F, E>(&mut self, f: F) -> TryForEachFuture<'_, Self, F>
    where
        Self: Unpin + Sized,
        F: FnMut(Self::Item) -> Result<(), E>,
    { ... }
    fn zip<U>(self, other: U) -> Zip<Self, U>
    where
        Self: Sized,
        U: Stream,
    { ... }
    fn unzip<A, B, FromA, FromB>(self) -> UnzipFuture<Self, FromA, FromB>
    where
        FromA: Default + Extend<A>,
        FromB: Default + Extend<B>,
        Self: Stream<Item = (A, B)> + Sized,
    { ... }
    fn collect<'a, B>(self) -> Pin<Box<dyn Future<Output = B> + Send + 'a>>
    where
        Self: Sized + 'a + Send,
        B: FromStream<Self::Item>,
        Self::Item: Send,
    { ... }
    fn merge<U>(self, other: U) -> Merge<Self, U>
    where
        Self: Sized,
        U: Stream<Item = Self::Item> + Sized,
    { ... }
    fn partial_cmp<S>(self, other: S) -> PartialCmpFuture<Self, S>
    where
        Self: Sized + Stream,
        S: Stream,
        Self::Item: PartialOrd<S::Item>,
    { ... }
    fn position<P>(&mut self, predicate: P) -> PositionFuture<'_, Self, P>
    where
        Self: Unpin + Sized,
        P: FnMut(Self::Item) -> bool,
    { ... }
    fn cmp<S>(self, other: S) -> CmpFuture<Self, S>
    where
        Self: Sized + Stream,
        S: Stream,
        Self::Item: Ord,
    { ... }
    fn count(self) -> CountFuture<Self>
    where
        Self: Sized,
    { ... }
    fn ne<S>(self, other: S) -> NeFuture<Self, S>
    where
        Self: Sized,
        S: Sized + Stream,
        Self::Item: PartialEq<S::Item>,
    { ... }
    fn ge<S>(self, other: S) -> GeFuture<Self, S>
    where
        Self: Sized + Stream,
        S: Stream,
        Self::Item: PartialOrd<S::Item>,
    { ... }
    fn eq<S>(self, other: S) -> EqFuture<Self, S>
    where
        Self: Sized + Stream,
        S: Sized + Stream,
        Self::Item: PartialEq<S::Item>,
    { ... }
    fn gt<S>(self, other: S) -> GtFuture<Self, S>
    where
        Self: Sized + Stream,
        S: Stream,
        Self::Item: PartialOrd<S::Item>,
    { ... }
    fn le<S>(self, other: S) -> LeFuture<Self, S>
    where
        Self: Sized + Stream,
        S: Stream,
        Self::Item: PartialOrd<S::Item>,
    { ... }
    fn lt<S>(self, other: S) -> LtFuture<Self, S>
    where
        Self: Sized + Stream,
        S: Stream,
        Self::Item: PartialOrd<S::Item>,
    { ... }
    fn sum<'a, S>(self) -> Pin<Box<dyn Future<Output = S> + 'a>>
    where
        Self: Sized + Stream<Item = S> + 'a,
        S: Sum<Self::Item>,
    { ... }
    fn product<'a, P>(self) -> Pin<Box<dyn Future<Output = P> + 'a>>
    where
        Self: Sized + Stream<Item = P> + 'a,
        P: Product,
    { ... }
}

Extension methods for Stream.

Provided methods

fn next(&mut self) -> NextFuture<'_, 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::once(7);

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

fn take(self, n: usize) -> Take<Self> where
    Self: Sized, 

Creates a stream that yields its first n elements.

Examples

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

let mut s = stream::repeat(9).take(3);

while let Some(v) = s.next().await {
    assert_eq!(v, 9);
}

fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> where
    Self: Sized,
    P: FnMut(&Self::Item) -> bool, 

Creates a stream that yields elements based on a predicate.

Examples

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

let s = stream::from_iter(vec![1, 2, 3, 4]);
let mut s = s.take_while(|x| x < &3 );

assert_eq!(s.next().await, Some(1));
assert_eq!(s.next().await, Some(2));
assert_eq!(s.next().await, None);

fn throttle(self, d: Duration) -> Throttle<Self> where
    Self: Sized, 

Limit the amount of items yielded per timeslice in a stream.

This stream does not drop any items, but will only limit the rate at which items pass through.

Examples

use async_std::prelude::*;
use async_std::stream;
use std::time::{Duration, Instant};

let start = Instant::now();

// emit value every 5 milliseconds
let s = stream::interval(Duration::from_millis(5)).take(2);

// throttle for 10 milliseconds
let mut s = s.throttle(Duration::from_millis(10));

s.next().await;
assert!(start.elapsed().as_millis() >= 5);

s.next().await;
assert!(start.elapsed().as_millis() >= 15);

s.next().await;
assert!(start.elapsed().as_millis() >= 25);

fn step_by(self, step: usize) -> StepBy<Self> where
    Self: Sized, 

Creates a stream that yields each stepth element.

Panics

This method will panic if the given step is 0.

Examples

Basic usage:

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

let s = stream::from_iter(vec![0u8, 1, 2, 3, 4]);
let mut stepped = s.step_by(2);

assert_eq!(stepped.next().await, Some(0));
assert_eq!(stepped.next().await, Some(2));
assert_eq!(stepped.next().await, Some(4));
assert_eq!(stepped.next().await, None);

fn chain<U>(self, other: U) -> Chain<Self, U> where
    Self: Sized,
    U: Stream<Item = Self::Item> + Sized, 

Takes two streams and creates a new stream over both in sequence.

Examples

Basic usage:

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

let first = stream::from_iter(vec![0u8, 1]);
let second = stream::from_iter(vec![2, 3]);
let mut c = first.chain(second);

assert_eq!(c.next().await, Some(0));
assert_eq!(c.next().await, Some(1));
assert_eq!(c.next().await, Some(2));
assert_eq!(c.next().await, Some(3));
assert_eq!(c.next().await, None);

fn cloned<'a, T>(self) -> Cloned<Self> where
    Self: Sized + Stream<Item = &'a T>,
    T: Clone + 'a, 

Creates an stream which copies all of its elements.

Examples

Basic usage:

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

let v = stream::from_iter(vec![&1, &2, &3]);

let mut v_cloned = v.cloned();

assert_eq!(v_cloned.next().await, Some(1));
assert_eq!(v_cloned.next().await, Some(2));
assert_eq!(v_cloned.next().await, Some(3));
assert_eq!(v_cloned.next().await, None);

fn copied<'a, T>(self) -> Copied<Self> where
    Self: Sized + Stream<Item = &'a T>,
    T: Copy + 'a, 

Creates an stream which copies all of its elements.

Examples

Basic usage:

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

let s = stream::from_iter(vec![&1, &2, &3]);
let mut s_copied  = s.copied();

assert_eq!(s_copied.next().await, Some(1));
assert_eq!(s_copied.next().await, Some(2));
assert_eq!(s_copied.next().await, Some(3));
assert_eq!(s_copied.next().await, None);

fn cycle(self) -> Cycle<Self> where
    Self: Clone + Sized, 

Creates a stream that yields the provided values infinitely and in order.

Examples

Basic usage:

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

let mut s = stream::once(7).cycle();

assert_eq!(s.next().await, Some(7));
assert_eq!(s.next().await, Some(7));
assert_eq!(s.next().await, Some(7));
assert_eq!(s.next().await, Some(7));
assert_eq!(s.next().await, Some(7));

fn enumerate(self) -> Enumerate<Self> where
    Self: Sized, 

Creates a stream that gives the current element’s count as well as the next value.

Overflow behaviour.

This combinator does no guarding against overflows.

Examples

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

let s = stream::from_iter(vec!['a', 'b', 'c']);
let mut s = s.enumerate();

assert_eq!(s.next().await, Some((0, 'a')));
assert_eq!(s.next().await, Some((1, 'b')));
assert_eq!(s.next().await, Some((2, 'c')));
assert_eq!(s.next().await, None);

fn delay(self, dur: Duration) -> Delay<Self> where
    Self: Sized, 

Creates a stream that is delayed before it starts yielding items.

Examples

use async_std::prelude::*;
use async_std::stream;
use std::time::{Duration, Instant};

let start = Instant::now();
let mut s = stream::from_iter(vec![0u8, 1, 2]).delay(Duration::from_millis(200));

assert_eq!(s.next().await, Some(0));
// The first time will take more than 200ms due to delay.
assert!(start.elapsed().as_millis() >= 200);

assert_eq!(s.next().await, Some(1));
// There will be no delay after the first time.
assert!(start.elapsed().as_millis() < 400);

assert_eq!(s.next().await, Some(2));
assert!(start.elapsed().as_millis() < 400);

assert_eq!(s.next().await, None);
assert!(start.elapsed().as_millis() < 400);

fn map<B, F>(self, f: F) -> Map<Self, F> where
    Self: Sized,
    F: FnMut(Self::Item) -> B, 

Takes a closure and creates a stream that calls that closure on every element of this stream.

Examples

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

let s = stream::from_iter(vec![1, 2, 3]);
let mut s = s.map(|x| 2 * x);

assert_eq!(s.next().await, Some(2));
assert_eq!(s.next().await, Some(4));
assert_eq!(s.next().await, Some(6));
assert_eq!(s.next().await, None);

fn inspect<F>(self, f: F) -> Inspect<Self, F> where
    Self: Sized,
    F: FnMut(&Self::Item), 

A combinator that does something with each element in the stream, passing the value on.

Examples

Basic usage:

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

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

let sum = s
   .inspect(|x| println!("about to filter {}", x))
   .filter(|x| x % 2 == 0)
   .inspect(|x| println!("made it through filter: {}", x))
   .fold(0, |sum, i| sum + i)
   .await;

assert_eq!(sum, 6);

fn last(self) -> LastFuture<Self, Self::Item> where
    Self: Sized, 

Returns the last element of the stream.

Examples

Basic usage:

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

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

let last  = s.last().await;
assert_eq!(last, Some(3));

An empty stream will return None:

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

let s = stream::empty::<()>();

let last  = s.last().await;
assert_eq!(last, None);

fn fuse(self) -> Fuse<Self> where
    Self: Sized, 

Creates a stream which ends after the first None.

After a stream returns None, future calls may or may not yield Some(T) again. fuse() adapts an iterator, ensuring that after a None is given, it will always return None forever.

Examples

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

let mut s = stream::once(1).fuse();
assert_eq!(s.next().await, Some(1));
assert_eq!(s.next().await, None);
assert_eq!(s.next().await, None);

fn filter<P>(self, predicate: P) -> Filter<Self, P> where
    Self: Sized,
    P: FnMut(&Self::Item) -> bool, 

Creates a stream that uses a predicate to determine if an element should be yielded.

Examples

Basic usage:

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

let s = stream::from_iter(vec![1, 2, 3, 4]);
let mut s = s.filter(|i| i % 2 == 0);

assert_eq!(s.next().await, Some(2));
assert_eq!(s.next().await, Some(4));
assert_eq!(s.next().await, None);

fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F> where
    Self: Sized,
    U: IntoStream,
    F: FnMut(Self::Item) -> U, 

Creates an stream that works like map, but flattens nested structure.

Examples

Basic usage:

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

let words = stream::from_iter(&["alpha", "beta", "gamma"]);

let merged: String = words
    .flat_map(|s| stream::from_iter(s.chars()))
    .collect().await;
    assert_eq!(merged, "alphabetagamma");

let d3 = stream::from_iter(&[[[1, 2], [3, 4]], [[5, 6], [7, 8]]]);
let d1: Vec<_> = d3
    .flat_map(|item| stream::from_iter(item))
    .flat_map(|item| stream::from_iter(item))
    .collect().await;

assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);

fn flatten(self) -> Flatten<Self> where
    Self: Sized,
    Self::Item: IntoStream, 

Creates an stream that flattens nested structure.

Examples

Basic usage:

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

let inner1 = stream::from_iter(vec![1u8,2,3]);
let inner2 = stream::from_iter(vec![4u8,5,6]);
let s  = stream::from_iter(vec![inner1, inner2]);

let v: Vec<_> = s.flatten().collect().await;

assert_eq!(v, vec![1,2,3,4,5,6]);

fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where
    Self: Sized,
    F: FnMut(Self::Item) -> Option<B>, 

Both filters and maps a stream.

Examples

Basic usage:

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

let s = stream::from_iter(vec!["1", "lol", "3", "NaN", "5"]);

let mut parsed = s.filter_map(|a| a.parse::<u32>().ok());

let one = parsed.next().await;
assert_eq!(one, Some(1));

let three = parsed.next().await;
assert_eq!(three, Some(3));

let five = parsed.next().await;
assert_eq!(five, Some(5));

let end = parsed.next().await;
assert_eq!(end, None);

fn min_by_key<B, F>(self, key_by: F) -> MinByKeyFuture<Self, Self::Item, F> where
    Self: Sized,
    B: Ord,
    F: FnMut(&Self::Item) -> B, 

Returns the element that gives the minimum value with respect to the specified key function. If several elements are equally minimum, the first element is returned. If the stream is empty, None is returned.

Examples

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

let s = stream::from_iter(vec![-1isize, 2, -3]);

let min = s.clone().min_by_key(|x| x.abs()).await;
assert_eq!(min, Some(-1));

let min = stream::empty::<isize>().min_by_key(|x| x.abs()).await;
assert_eq!(min, None);

fn max_by_key<B, F>(self, key_by: F) -> MaxByKeyFuture<Self, Self::Item, F> where
    Self: Sized,
    B: Ord,
    F: FnMut(&Self::Item) -> B, 

Returns the element that gives the maximum value with respect to the specified key function. If several elements are equally maximum, the first element is returned. If the stream is empty, None is returned.

Examples

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

let s = stream::from_iter(vec![-3_i32, 0, 1, 5, -10]);

let max = s.clone().max_by_key(|x| x.abs()).await;
assert_eq!(max, Some(-10));

let max = stream::empty::<isize>().max_by_key(|x| x.abs()).await;
assert_eq!(max, None);

fn min_by<F>(self, compare: F) -> MinByFuture<Self, F, Self::Item> where
    Self: Sized,
    F: FnMut(&Self::Item, &Self::Item) -> Ordering, 

Returns the element that gives the minimum value with respect to the specified comparison function. If several elements are equally minimum, the first element is returned. If the stream is empty, None is returned.

Examples

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

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

let min = s.clone().min_by(|x, y| x.cmp(y)).await;
assert_eq!(min, Some(1));

let min = s.min_by(|x, y| y.cmp(x)).await;
assert_eq!(min, Some(3));

let min = stream::empty::<u8>().min_by(|x, y| x.cmp(y)).await;
assert_eq!(min, None);

fn max(self) -> MaxFuture<Self, Self::Item> where
    Self: Sized,
    Self::Item: Ord, 

Returns the element that gives the maximum value. If several elements are equally maximum, the first element is returned. If the stream is empty, None is returned.

Examples

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

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

let max = s.clone().max().await;
assert_eq!(max, Some(3));

let max = stream::empty::<usize>().max().await;
assert_eq!(max, None);

fn min(self) -> MinFuture<Self, Self::Item> where
    Self: Sized,
    Self::Item: Ord, 

Returns the element that gives the minimum value. If several elements are equally minimum, the first element is returned. If the stream is empty, None is returned.

Examples

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

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

let min = s.clone().min().await;
assert_eq!(min, Some(1));

let min = stream::empty::<usize>().min().await;
assert_eq!(min, None);

fn max_by<F>(self, compare: F) -> MaxByFuture<Self, F, Self::Item> where
    Self: Sized,
    F: FnMut(&Self::Item, &Self::Item) -> Ordering, 

Returns the element that gives the maximum value with respect to the specified comparison function. If several elements are equally maximum, the first element is returned. If the stream is empty, None is returned.

Examples

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

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

let max = s.clone().max_by(|x, y| x.cmp(y)).await;
assert_eq!(max, Some(3));

let max = s.max_by(|x, y| y.cmp(x)).await;
assert_eq!(max, Some(1));

let max = stream::empty::<usize>().max_by(|x, y| x.cmp(y)).await;
assert_eq!(max, None);

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

Returns the nth element of the stream.

Examples

Basic usage:

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

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

let second = s.nth(1).await;
assert_eq!(second, Some(2));

Calling nth() multiple times:

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

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

let second = s.nth(0).await;
assert_eq!(second, Some(1));

let second = s.nth(0).await;
assert_eq!(second, Some(2));

Returning None if the stream finished before returning n elements:

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

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

let fourth = s.nth(4).await;
assert_eq!(fourth, None);

fn all<F>(&mut self, f: F) -> AllFuture<'_, Self, F, Self::Item> where
    Self: Unpin + Sized,
    F: FnMut(Self::Item) -> bool, 

Tests if every element of the stream matches a predicate.

all() takes a closure that returns true or false. It applies this closure to each element of the stream, and if they all return true, then so does all(). If any of them return false, it returns false.

all() is short-circuiting; in other words, it will stop processing as soon as it finds a false, given that no matter what else happens, the result will also be false.

An empty stream returns true.

Examples

Basic usage:

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

let mut s = stream::repeat::<u32>(42).take(3);
assert!(s.all(|x| x ==  42).await);

Empty stream:

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

let mut s = stream::empty::<u32>();
assert!(s.all(|_| false).await);

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

Searches for an element in a stream that satisfies a predicate.

Examples

Basic usage:

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

let mut s = stream::from_iter(vec![1u8, 2, 3]);
let res = s.find(|x| *x == 2).await;
assert_eq!(res, Some(2));

Resuming after a first find:

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

let mut s= stream::from_iter(vec![1, 2, 3]);
let res = s.find(|x| *x == 2).await;
assert_eq!(res, Some(2));

let next = s.next().await;
assert_eq!(next, Some(3));

fn find_map<F, B>(&mut self, f: F) -> FindMapFuture<'_, Self, F> where
    Self: Unpin + Sized,
    F: FnMut(Self::Item) -> Option<B>, 

Applies function to the elements of stream and returns the first non-none result.

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

let mut s = stream::from_iter(vec!["lol", "NaN", "2", "5"]);
let first_number = s.find_map(|s| s.parse().ok()).await;

assert_eq!(first_number, Some(2));

fn fold<B, F>(self, init: B, f: F) -> FoldFuture<Self, F, B> where
    Self: Sized,
    F: FnMut(B, Self::Item) -> B, 

A combinator that applies a function to every element in a stream producing a single, final value.

Examples

Basic usage:

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

let s = stream::from_iter(vec![1u8, 2, 3]);
let sum = s.fold(0, |acc, x| acc + x).await;

assert_eq!(sum, 6);

fn partition<B, F>(self, f: F) -> PartitionFuture<Self, F, B> where
    Self: Sized,
    F: FnMut(&Self::Item) -> bool,
    B: Default + Extend<Self::Item>, 

A combinator that applies a function to every element in a stream creating two collections from it.

Examples

Basic usage:

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

let (even, odd): (Vec<i32>, Vec<i32>) = stream::from_iter(vec![1, 2, 3])
    .partition(|&n| n % 2 == 0).await;

assert_eq!(even, vec![2]);
assert_eq!(odd, vec![1, 3]);

fn for_each<F>(self, f: F) -> ForEachFuture<Self, F> where
    Self: Sized,
    F: FnMut(Self::Item), 

Call a closure on each element of the stream.

Examples

use async_std::prelude::*;
use async_std::stream;
use std::sync::mpsc::channel;

let (tx, rx) = channel();

let s = stream::from_iter(vec![1usize, 2, 3]);
let sum = s.for_each(move |x| tx.clone().send(x).unwrap()).await;

let v: Vec<_> = rx.iter().collect();

assert_eq!(v, vec![1, 2, 3]);

fn any<F>(&mut self, f: F) -> AnyFuture<'_, Self, F, Self::Item> where
    Self: Unpin + Sized,
    F: FnMut(Self::Item) -> bool, 

Tests if any element of the stream matches a predicate.

any() takes a closure that returns true or false. It applies this closure to each element of the stream, and if any of them return true, then so does any(). If they all return false, it returns false.

any() is short-circuiting; in other words, it will stop processing as soon as it finds a true, given that no matter what else happens, the result will also be true.

An empty stream returns false.

Examples

Basic usage:

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

let mut s = stream::repeat::<u32>(42).take(3);
assert!(s.any(|x| x ==  42).await);

Empty stream:

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

let mut s = stream::empty::<u32>();
assert!(!s.any(|_| false).await);

fn by_ref(&mut self) -> &mut Self

Borrows an stream, rather than consuming it.

This is useful to allow applying stream adaptors while still retaining ownership of the original stream.

Examples

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

let a = vec![1isize, 2, 3];

let stream = stream::from_iter(a);

let sum: isize = stream.take(5).sum().await;

assert_eq!(sum, 6);

// if we try to use stream again, it won't work. The following line
// gives error: use of moved value: `stream`
// assert_eq!(stream.next(), None);

// let's try that again
let a = vec![1isize, 2, 3];

let mut stream = stream::from_iter(a);

// instead, we add in a .by_ref()
let sum: isize = stream.by_ref().take(2).sum().await;

assert_eq!(sum, 3);

// now this is just fine:
assert_eq!(stream.next().await, Some(3));
assert_eq!(stream.next().await, None);

fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F> where
    Self: Sized,
    F: FnMut(&mut St, Self::Item) -> Option<B>, 

A stream adaptor similar to fold that holds internal state and produces a new stream.

scan() takes two arguments: an initial value which seeds the internal state, and a closure with two arguments, the first being a mutable reference to the internal state and the second a stream element. The closure can assign to the internal state to share state between iterations.

On iteration, the closure will be applied to each element of the stream and the return value from the closure, an Option, is yielded by the stream.

Examples

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

let s = stream::from_iter(vec![1isize, 2, 3]);
let mut s = s.scan(1, |state, x| {
    *state = *state * x;
    Some(-*state)
});

assert_eq!(s.next().await, Some(-1));
assert_eq!(s.next().await, Some(-2));
assert_eq!(s.next().await, Some(-6));
assert_eq!(s.next().await, None);

fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> where
    Self: Sized,
    P: FnMut(&Self::Item) -> bool, 

Combinator that skips elements based on a predicate.

Takes a closure argument. It will call this closure on every element in the stream and ignore elements until it returns false.

After false is returned, SkipWhile’s job is over and all further elements in the strem are yielded.

Examples

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

let a = stream::from_iter(vec![-1i32, 0, 1]);
let mut s = a.skip_while(|x| x.is_negative());

assert_eq!(s.next().await, Some(0));
assert_eq!(s.next().await, Some(1));
assert_eq!(s.next().await, None);

fn skip(self, n: usize) -> Skip<Self> where
    Self: Sized, 

Creates a combinator that skips the first n elements.

Examples

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

let s = stream::from_iter(vec![1u8, 2, 3]);
let mut skipped = s.skip(2);

assert_eq!(skipped.next().await, Some(3));
assert_eq!(skipped.next().await, None);

fn timeout(self, dur: Duration) -> Timeout<Self> where
    Self: Stream + Sized, 

Await a stream or times out after a duration of time.

If you want to await an I/O future consider using io::timeout instead.

Examples

use std::time::Duration;

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

let mut s = stream::repeat(1).take(3).timeout(Duration::from_secs(1));

while let Some(v) = s.next().await {
    assert_eq!(v, Ok(1));
}

// when timeout
let mut s = stream::pending::<()>().timeout(Duration::from_millis(10));
match s.next().await {
    Some(item) => assert!(item.is_err()),
    None => panic!()
};

fn try_fold<B, F, T, E>(
    &mut self,
    init: T,
    f: F
) -> TryFoldFuture<'_, Self, F, T> where
    Self: Unpin + Sized,
    F: FnMut(B, Self::Item) -> Result<T, 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 mut s = stream::from_iter(vec![1usize, 2, 3]);
let sum = s.try_fold(0, |acc, v| {
    if (acc+v) % 2 == 1 {
        Ok(v+3)
    } else {
        Err("fail")
    }
}).await;

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

fn try_for_each<F, E>(&mut self, f: F) -> TryForEachFuture<'_, Self, F> where
    Self: Unpin + Sized,
    F: FnMut(Self::Item) -> Result<(), E>, 

Applies a falliable function to each element in a stream, stopping at first error and returning it.

Examples

use std::sync::mpsc::channel;
use async_std::prelude::*;
use async_std::stream;

let (tx, rx) = channel();

let mut s = stream::from_iter(vec![1u8, 2, 3]);
let s = s.try_for_each(|v| {
    if v % 2 == 1 {
        tx.clone().send(v).unwrap();
        Ok(())
    } else {
        Err("even")
    }
});

let res = s.await;
drop(tx);
let values: Vec<_> = rx.iter().collect();

assert_eq!(values, vec![1]);
assert_eq!(res, Err("even"));

fn zip<U>(self, other: U) -> Zip<Self, U> where
    Self: Sized,
    U: Stream, 

‘Zips up’ two streams into a single stream of pairs.

zip() returns a new stream that will iterate over two other streams, returning a tuple where the first element comes from the first stream, and the second element comes from the second stream.

In other words, it zips two streams together, into a single one.

If either stream returns None, poll_next from the zipped stream will return None. If the first stream returns None, zip will short-circuit and poll_next will not be called on the second stream.

Examples

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

let l = stream::from_iter(vec![1u8, 2, 3]);
let r = stream::from_iter(vec![4u8, 5, 6, 7]);
let mut s = l.zip(r);

assert_eq!(s.next().await, Some((1, 4)));
assert_eq!(s.next().await, Some((2, 5)));
assert_eq!(s.next().await, Some((3, 6)));
assert_eq!(s.next().await, None);

fn unzip<A, B, FromA, FromB>(self) -> UnzipFuture<Self, FromA, FromB> where
    FromA: Default + Extend<A>,
    FromB: Default + Extend<B>,
    Self: Stream<Item = (A, B)> + Sized, 

Converts an stream of pairs into a pair of containers.

unzip() consumes an entire stream of pairs, producing two collections: one from the left elements of the pairs, and one from the right elements.

This function is, in some sense, the opposite of zip.

Example

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

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

let (left, right): (Vec<_>, Vec<_>) = s.unzip().await;

assert_eq!(left, [1, 3]);
assert_eq!(right, [2, 4]);

fn collect<'a, B>(self) -> Pin<Box<dyn Future<Output = B> + Send + 'a>> where
    Self: Sized + 'a + Send,
    B: FromStream<Self::Item>,
    Self::Item: Send, 

Transforms a stream into a collection.

collect() can take anything streamable, and turn it into a relevant collection. This is one of the more powerful methods in the async standard library, used in a variety of contexts.

The most basic pattern in which collect() is used is to turn one collection into another. You take a collection, call into_stream on it, do a bunch of transformations, and then collect() at the end.

Because collect() is so general, it can cause problems with type inference. As such, collect() is one of the few times you’ll see the syntax affectionately known as the ‘turbofish’: ::<>. This helps the inference algorithm understand specifically which collection you’re trying to collect into.

Examples

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

let s = stream::repeat(9u8).take(3);
let buf: Vec<u8> = s.collect().await;

assert_eq!(buf, vec![9; 3]);

// You can also collect streams of Result values
// into any collection that implements FromStream
let s = stream::repeat(Ok(9)).take(3);
// We are using Vec here, but other collections
// are supported as well
let buf: Result<Vec<u8>, ()> = s.collect().await;

assert_eq!(buf, Ok(vec![9; 3]));

// The stream will stop on the first Err and
// return that instead
let s = stream::repeat(Err(5)).take(3);
let buf: Result<Vec<u8>, u8> = s.collect().await;

assert_eq!(buf, Err(5));

fn merge<U>(self, other: U) -> Merge<Self, U> where
    Self: Sized,
    U: Stream<Item = Self::Item> + Sized, 

Combines multiple streams into a single stream of all their outputs.

Items are yielded as soon as they’re received, and the stream continues yield until both streams have been exhausted. The output ordering between streams is not guaranteed.

Examples

use async_std::prelude::*;
use async_std::stream::{self, FromStream};

let a = stream::once(1u8);
let b = stream::once(2u8);
let c = stream::once(3u8);

let s = a.merge(b).merge(c);
let mut lst = Vec::from_stream(s).await;

lst.sort_unstable();
assert_eq!(&lst, &[1u8, 2u8, 3u8]);

fn partial_cmp<S>(self, other: S) -> PartialCmpFuture<Self, S> where
    Self: Sized + Stream,
    S: Stream,
    Self::Item: PartialOrd<S::Item>, 

Lexicographically compares the elements of this Stream with those of another.

Examples

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

use std::cmp::Ordering;

let s1 = stream::from_iter(vec![1]);
let s2 = stream::from_iter(vec![1, 2]);
let s3 = stream::from_iter(vec![1, 2, 3]);
let s4 = stream::from_iter(vec![1, 2, 4]);
assert_eq!(s1.clone().partial_cmp(s1.clone()).await, Some(Ordering::Equal));
assert_eq!(s1.clone().partial_cmp(s2.clone()).await, Some(Ordering::Less));
assert_eq!(s2.clone().partial_cmp(s1.clone()).await, Some(Ordering::Greater));
assert_eq!(s3.clone().partial_cmp(s4.clone()).await, Some(Ordering::Less));
assert_eq!(s4.clone().partial_cmp(s3.clone()).await, Some(Ordering::Greater));

fn position<P>(&mut self, predicate: P) -> PositionFuture<'_, Self, P> where
    Self: Unpin + Sized,
    P: FnMut(Self::Item) -> bool, 

Searches for an element in a Stream that satisfies a predicate, returning its index.

Examples

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

let s = stream::from_iter(vec![1usize, 2, 3]);
let res = s.clone().position(|x| x == 1).await;
assert_eq!(res, Some(0));

let res = s.clone().position(|x| x == 2).await;
assert_eq!(res, Some(1));

let res = s.clone().position(|x| x == 3).await;
assert_eq!(res, Some(2));

let res = s.clone().position(|x| x == 4).await;
assert_eq!(res, None);

fn cmp<S>(self, other: S) -> CmpFuture<Self, S> where
    Self: Sized + Stream,
    S: Stream,
    Self::Item: Ord, 

Lexicographically compares the elements of this Stream with those of another using ‘Ord’.

Examples

use async_std::prelude::*;
use async_std::stream;
use std::cmp::Ordering;

let s1 = stream::from_iter(vec![1]);
let s2 = stream::from_iter(vec![1, 2]);
let s3 = stream::from_iter(vec![1, 2, 3]);
let s4 = stream::from_iter(vec![1, 2, 4]);

assert_eq!(s1.clone().cmp(s1.clone()).await, Ordering::Equal);
assert_eq!(s1.clone().cmp(s2.clone()).await, Ordering::Less);
assert_eq!(s2.clone().cmp(s1.clone()).await, Ordering::Greater);
assert_eq!(s3.clone().cmp(s4.clone()).await, Ordering::Less);
assert_eq!(s4.clone().cmp(s3.clone()).await, Ordering::Greater);

fn count(self) -> CountFuture<Self> where
    Self: Sized, 

Counts the number of elements in the stream.

Examples

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

let s1 = stream::from_iter(vec![0]);
let s2 = stream::from_iter(vec![1, 2, 3]);

assert_eq!(s1.count().await, 1);
assert_eq!(s2.count().await, 3);

fn ne<S>(self, other: S) -> NeFuture<Self, S> where
    Self: Sized,
    S: Sized + Stream,
    Self::Item: PartialEq<S::Item>, 

Determines if the elements of this Stream are lexicographically not equal to those of another.

Examples

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

let single     = stream::from_iter(vec![1usize]);
let single_ne  = stream::from_iter(vec![10usize]);
let multi      = stream::from_iter(vec![1usize,2]);
let multi_ne   = stream::from_iter(vec![1usize,5]);

assert_eq!(single.clone().ne(single.clone()).await, false);
assert_eq!(single_ne.clone().ne(single.clone()).await, true);
assert_eq!(multi.clone().ne(single_ne.clone()).await, true);
assert_eq!(multi_ne.clone().ne(multi.clone()).await, true);

fn ge<S>(self, other: S) -> GeFuture<Self, S> where
    Self: Sized + Stream,
    S: Stream,
    Self::Item: PartialOrd<S::Item>, 

Determines if the elements of this Stream are lexicographically greater than or equal to those of another.

Examples

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

let single    = stream::from_iter(vec![1]);
let single_gt = stream::from_iter(vec![10]);
let multi     = stream::from_iter(vec![1,2]);
let multi_gt  = stream::from_iter(vec![1,5]);

assert_eq!(single.clone().ge(single.clone()).await, true);
assert_eq!(single_gt.clone().ge(single.clone()).await, true);
assert_eq!(multi.clone().ge(single_gt.clone()).await, false);
assert_eq!(multi_gt.clone().ge(multi.clone()).await, true);

fn eq<S>(self, other: S) -> EqFuture<Self, S> where
    Self: Sized + Stream,
    S: Sized + Stream,
    Self::Item: PartialEq<S::Item>, 

Determines if the elements of this Stream are lexicographically equal to those of another.

Examples

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

let single     = stream::from_iter(vec![1]);
let single_eq  = stream::from_iter(vec![10]);
let multi      = stream::from_iter(vec![1,2]);
let multi_eq   = stream::from_iter(vec![1,5]);

assert_eq!(single.clone().eq(single.clone()).await, true);
assert_eq!(single_eq.clone().eq(single.clone()).await, false);
assert_eq!(multi.clone().eq(single_eq.clone()).await, false);
assert_eq!(multi_eq.clone().eq(multi.clone()).await, false);

fn gt<S>(self, other: S) -> GtFuture<Self, S> where
    Self: Sized + Stream,
    S: Stream,
    Self::Item: PartialOrd<S::Item>, 

Determines if the elements of this Stream are lexicographically greater than those of another.

Examples

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

let single = stream::from_iter(vec![1]);
let single_gt = stream::from_iter(vec![10]);
let multi = stream::from_iter(vec![1,2]);
let multi_gt = stream::from_iter(vec![1,5]);

assert_eq!(single.clone().gt(single.clone()).await, false);
assert_eq!(single_gt.clone().gt(single.clone()).await, true);
assert_eq!(multi.clone().gt(single_gt.clone()).await, false);
assert_eq!(multi_gt.clone().gt(multi.clone()).await, true);

fn le<S>(self, other: S) -> LeFuture<Self, S> where
    Self: Sized + Stream,
    S: Stream,
    Self::Item: PartialOrd<S::Item>, 

Determines if the elements of this Stream are lexicographically less or equal to those of another.

Examples

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

let single = stream::from_iter(vec![1]);
let single_gt = stream::from_iter(vec![10]);
let multi = stream::from_iter(vec![1,2]);
let multi_gt = stream::from_iter(vec![1,5]);

assert_eq!(single.clone().le(single.clone()).await, true);
assert_eq!(single.clone().le(single_gt.clone()).await, true);
assert_eq!(multi.clone().le(single_gt.clone()).await, true);
assert_eq!(multi_gt.clone().le(multi.clone()).await, false);

fn lt<S>(self, other: S) -> LtFuture<Self, S> where
    Self: Sized + Stream,
    S: Stream,
    Self::Item: PartialOrd<S::Item>, 

Determines if the elements of this Stream are lexicographically less than those of another.

Examples

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

let single = stream::from_iter(vec![1]);
let single_gt = stream::from_iter(vec![10]);
let multi = stream::from_iter(vec![1,2]);
let multi_gt = stream::from_iter(vec![1,5]);

assert_eq!(single.clone().lt(single.clone()).await, false);
assert_eq!(single.clone().lt(single_gt.clone()).await, true);
assert_eq!(multi.clone().lt(single_gt.clone()).await, true);
assert_eq!(multi_gt.clone().lt(multi.clone()).await, false);

fn sum<'a, S>(self) -> Pin<Box<dyn Future<Output = S> + 'a>> where
    Self: Sized + Stream<Item = S> + 'a,
    S: Sum<Self::Item>, 

Sums the elements of a stream.

Takes each element, adds them together, and returns the result.

An empty streams returns the zero value of the type.

Panics

When calling sum() and a primitive integer type is being returned, this method will panic if the computation overflows and debug assertions are enabled.

Examples

Basic usage:

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

let s = stream::from_iter(vec![0u8, 1, 2, 3, 4]);
let sum: u8 = s.sum().await;

assert_eq!(sum, 10);

fn product<'a, P>(self) -> Pin<Box<dyn Future<Output = P> + 'a>> where
    Self: Sized + Stream<Item = P> + 'a,
    P: Product, 

Multiplies all elements of the stream.

An empty stream returns the one value of the type.

Panics

When calling product() and a primitive integer type is being returned, method will panic if the computation overflows and debug assertions are enabled.

Examples

This example calculates the factorial of n (i.e. the product of the numbers from 1 to n, inclusive):

async fn factorial(n: u32) -> u32 {
    use async_std::prelude::*;
    use async_std::stream;

    let s = stream::from_iter(1..=n);
    s.product().await
}

assert_eq!(factorial(0).await, 1);
assert_eq!(factorial(1).await, 1);
assert_eq!(factorial(5).await, 120);

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Implementors

impl<T: Stream + ?Sized> StreamExt for T

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