[−][src]Trait futures::prelude::TryStreamExt

pub trait TryStreamExt: TryStream {
    fn err_into<E>(self) -> ErrInto<Self, E>
    where
        Self::Error: Into<E>,
    { ... }

    fn map_ok<T, F>(self, f: F) -> MapOk<Self, F>
    where
        F: FnMut(Self::Ok) -> T,
    { ... }

    fn map_err<E, F>(self, f: F) -> MapErr<Self, F>
    where
        F: FnMut(Self::Error) -> E,
    { ... }

    fn into_stream(self) -> IntoStream<Self> { ... }

    fn try_next(&mut self) -> TryNext<Self>
    where
        Self: Unpin,
    { ... }

    fn try_for_each<Fut, F>(self, f: F) -> TryForEach<Self, Fut, F>
    where
        F: FnMut(Self::Ok) -> Fut,
        Fut: TryFuture<Ok = (), Error = Self::Error>,
    { ... }

    fn try_skip_while<Fut, F>(self, f: F) -> TrySkipWhile<Self, Fut, F>
    where
        F: FnMut(&Self::Ok) -> Fut,
        Fut: TryFuture<Ok = bool, Error = Self::Error>,
    { ... }

    fn try_for_each_concurrent<Fut, F, impl Into>>(
        self, 
        limit: impl Into>, 
        f: F
    ) -> TryForEachConcurrent<Self, Fut, F>
    where
        F: FnMut(Self::Ok) -> Fut,
        Fut: Future<Output = Result<(), Self::Error>>,
        impl Into>: Into<Option<usize>>,
    { ... }

    fn try_collect<C>(self) -> TryCollect<Self, C>
    where
        C: Default + Extend<Self::Ok>,
    { ... }

    fn try_filter_map<Fut, F, T>(self, f: F) -> TryFilterMap<Self, Fut, F>
    where
        F: FnMut(Self::Ok) -> Fut,
        Fut: TryFuture<Ok = Option<T>, Error = Self::Error>,
    { ... }

    fn try_fold<T, Fut, F>(self, init: T, f: F) -> TryFold<Self, Fut, T, F>
    where
        F: FnMut(T, Self::Ok) -> Fut,
        Fut: TryFuture<Ok = T, Error = Self::Error>,
    { ... }

    fn try_buffer_unordered(self, n: usize) -> TryBufferUnordered<Self>
    where
        Self::Ok: TryFuture,
        <Self::Ok as TryFuture>::Error == Self::Error,
    { ... }

    fn compat<Sp>(self, spawn: Sp) -> Compat<Self, Sp>
    where
        Self: Unpin,
        Sp: Spawn,
    { ... }
}

Adapters specific to Result-returning streams

Provided Methods

Wraps the current stream in a new stream which converts the error type into the one provided.

Examples

#![feature(async_await, await_macro)]
use futures::stream::{self, TryStreamExt};

let mut stream =
    stream::iter(vec![Ok(()), Err(5i32)])
        .err_into::<i64>();

assert_eq!(await!(stream.try_next()), Ok(Some(())));
assert_eq!(await!(stream.try_next()), Err(5i64));

Wraps the current stream in a new stream which maps the success value using the provided closure.

Examples

#![feature(async_await, await_macro)]
use futures::stream::{self, TryStreamExt};

let mut stream =
    stream::iter(vec![Ok(5), Err(0)])
        .map_ok(|x| x + 2);

assert_eq!(await!(stream.try_next()), Ok(Some(7)));
assert_eq!(await!(stream.try_next()), Err(0));

Wraps the current stream in a new stream which maps the error value using the provided closure.

Examples

#![feature(async_await, await_macro)]
use futures::stream::{self, TryStreamExt};

let mut stream =
    stream::iter(vec![Ok(5), Err(0)])
        .map_err(|x| x + 2);

assert_eq!(await!(stream.try_next()), Ok(Some(5)));
assert_eq!(await!(stream.try_next()), Err(2));

Wraps a TryStream into a type that implements Stream

TryStreams currently do not implement the Stream trait because of limitations of the compiler.

Examples

use futures::stream::{Stream, TryStream, TryStreamExt};

fn make_try_stream() -> impl TryStream<Ok = T, Error = E> { // ... }
fn take_stream(stream: impl Stream<Item = Result<T, E>>) { /* ... */ }

take_stream(make_try_stream().into_stream());

Creates a future that attempts to resolve the next item in the stream. If an error is encountered before the next item, the error is returned instead.

This is similar to the Stream::next combinator, but returns a Result<Option<T>, E> rather than an Option<Result<T, E>>, making for easy use with the ? operator.

Examples

#![feature(async_await, await_macro)]
use futures::stream::{self, TryStreamExt};

let mut stream = stream::iter(vec![Ok(()), Err(())]);

assert_eq!(await!(stream.try_next()), Ok(Some(())));
assert_eq!(await!(stream.try_next()), Err(()));

Attempts to run this stream to completion, executing the provided asynchronous closure for each element on the stream.

The provided closure will be called for each item this stream produces, yielding a future. That future will then be executed to completion before moving on to the next item.

The returned value is a Future where the Output type is Result<(), Self::Error>. If any of the intermediate futures or the stream returns an error, this future will return immediately with an error.

Examples

#![feature(async_await, await_macro)]
use futures::future;
use futures::stream::{self, TryStreamExt};

let mut x = 0i32;

{
    let fut = stream::repeat(Ok(1)).try_for_each(|item| {
        x += item;
        future::ready(if x == 3 { Err(()) } else { Ok(()) })
    });
    assert_eq!(await!(fut), Err(()));
}

assert_eq!(x, 3);

Skip elements on this stream while the provided asynchronous predicate resolves to true.

This function is similar to StreamExt::skip_while but exits early if an error occurs.

Examples

#![feature(async_await, await_macro)]
use futures::future;
use futures::stream::{self, TryStreamExt};

let stream = stream::iter(vec![Ok::<i32, i32>(1), Ok(3), Ok(2)]);
let mut stream = stream.try_skip_while(|x| future::ready(Ok(*x < 3)));

let output: Result<Vec<i32>, i32> = await!(stream.try_collect());
assert_eq!(output, Ok(vec![3, 2]));

Attempts to run this stream to completion, executing the provided asynchronous closure for each element on the stream concurrently as elements become available, exiting as soon as an error occurs.

This is similar to StreamExt::for_each_concurrent, but will resolve to an error immediately if the underlying stream or the provided closure return an error.

This method is only available when the std feature of this library is activated, and it is activated by default.

Examples

#![feature(async_await, await_macro)]
use futures::channel::oneshot;
use futures::stream::{self, StreamExt, TryStreamExt};

let (tx1, rx1) = oneshot::channel();
let (tx2, rx2) = oneshot::channel();
let (_tx3, rx3) = oneshot::channel();

let stream = stream::iter(vec![rx1, rx2, rx3]);
let fut = stream.map(Ok).try_for_each_concurrent(
    /* limit */ 2,
    async move |rx| {
        let res: Result<(), oneshot::Canceled> = await!(rx);
        res
    }
);

tx1.send(()).unwrap();
// Drop the second sender so that `rx2` resolves to `Canceled`.
drop(tx2);

// The final result is an error because the second future
// resulted in an error.
assert_eq!(Err(oneshot::Canceled), await!(fut));

Attempt to Collect all of the values of this stream into a vector, returning a future representing the result of that computation.

This combinator will collect all successful results of this stream and collect them into a Vec<Self::Item>. If an error happens then all collected elements will be dropped and the error will be returned.

The returned future will be resolved when the stream terminates.

This method is only available when the std feature of this library is activated, and it is activated by default.

Examples

#![feature(async_await, await_macro)]
use futures::channel::mpsc;
use futures::executor::block_on;
use futures::stream::TryStreamExt;
use std::thread;

let (mut tx, rx) = mpsc::unbounded();

thread::spawn(move || {
    for i in (1..=5) {
        tx.unbounded_send(Ok(i)).unwrap();
    }
    tx.unbounded_send(Err(6)).unwrap();
});

let output: Result<Vec<i32>, i32> = await!(rx.try_collect());
assert_eq!(output, Err(6));

Attempt to filter the values produced by this stream while simultaneously mapping them to a different type according to the provided asynchronous closure.

As values of this stream are made available, the provided function will be run on them. If the future returned by the predicate f resolves to Some(item) then the stream will yield the value item, but if it resolves to None then the next value will be produced.

All errors are passed through without filtering in this combinator.

Note that this function consumes the stream passed into it and returns a wrapped version of it, similar to the existing filter_map methods in the standard library.

Examples

#![feature(async_await, await_macro)]
use futures::executor::block_on;
use futures::future;
use futures::stream::{self, StreamExt, TryStreamExt};

let stream = stream::iter(vec![Ok(1i32), Ok(6i32), Err("error")]);
let mut halves = stream.try_filter_map(|x| {
    let ret = if x % 2 == 0 { Some(x / 2) } else { None };
    future::ready(Ok(ret))
});

assert_eq!(await!(halves.next()), Some(Ok(3)));
assert_eq!(await!(halves.next()), Some(Err("error")));

Attempt to execute an accumulating asynchronous computation over a stream, collecting all the values into one final result.

This combinator will accumulate all values returned by this stream according to the closure provided. The initial state is also provided to this method and then is returned again by each execution of the closure. Once the entire stream has been exhausted the returned future will resolve to this value.

This method is similar to fold, but will exit early if an error is encountered in either the stream or the provided closure.

Examples

#![feature(async_await, await_macro)]
use futures::future;
use futures::stream::{self, TryStreamExt};

let number_stream = stream::iter(vec![Ok::<i32, i32>(1), Ok(2)]);
let sum = number_stream.try_fold(0, |acc, x| future::ready(Ok(acc + x)));
assert_eq!(await!(sum), Ok(3));

let number_stream_with_err = stream::iter(vec![Ok::<i32, i32>(1), Err(2), Ok(1)]);
let sum = number_stream_with_err.try_fold(0, |acc, x| future::ready(Ok(acc + x)));
assert_eq!(await!(sum), Err(2));

Attempt to execute several futures from a stream concurrently.

This stream's Ok type must be a TryFuture with an Error type that matches the stream's Error type.

This adaptor will buffer up to n futures and then return their outputs in the order in which they complete. If the underlying stream returns an error, it will be immediately propagated.

The returned stream will be a stream of results, each containing either an error or a future's output. An error can be produced either by the underlying stream itself or by one of the futures it yielded.

This method is only available when the std feature of this library is activated, and it is activated by default.

Examples

Results are returned in the order of completion:

#![feature(async_await, await_macro)]
use futures::channel::oneshot;
use futures::stream::{self, StreamExt, TryStreamExt};

let (send_one, recv_one) = oneshot::channel();
let (send_two, recv_two) = oneshot::channel();

let stream_of_futures = stream::iter(vec![Ok(recv_one), Ok(recv_two)]);

let mut buffered = stream_of_futures.try_buffer_unordered(10);

send_two.send(2i32);
assert_eq!(await!(buffered.next()), Some(Ok(2i32)));

send_one.send(1i32);
assert_eq!(await!(buffered.next()), Some(Ok(1i32)));

assert_eq!(await!(buffered.next()), None);

Errors from the underlying stream itself are propagated:

#![feature(async_await, await_macro)]
use futures::channel::mpsc;
use futures::future;
use futures::stream::{StreamExt, TryStreamExt};

let (sink, stream_of_futures) = mpsc::unbounded();
let mut buffered = stream_of_futures.try_buffer_unordered(10);

sink.unbounded_send(Ok(future::ready(Ok(7i32))));
assert_eq!(await!(buffered.next()), Some(Ok(7i32)));

sink.unbounded_send(Err("error in the stream"));
assert_eq!(await!(buffered.next()), Some(Err("error in the stream")));

Wraps a TryStream into a stream compatible with libraries using futures 0.1 Stream. Requires the compat feature to be enabled.

#![feature(async_await, await_macro, futures_api)]
use futures::future::{FutureExt, TryFutureExt};
use futures::spawn;
use futures::compat::TokioDefaultSpawner;

let future03 = async {
    println!("Running on the pool");
    spawn!(async {
        println!("Spawned!");
    }).unwrap();
};

let future01 = future03
    .unit_error() // Make it a TryFuture
    .boxed()  // Make it Unpin
    .compat(TokioDefaultSpawner);

tokio::run(future01);

Implementors