Crate futures_async_stream

Async stream API experiment that may be introduced as a language feature in the future.

This crate provides useful features for streams, using async_await and unstable generators.

#[for_await]

Processes streams using a for loop.

This is a reimplement of futures-await's #[async] for loops for futures 0.3 and is an experimental implementation of the idea listed as the next step of async/await.

#![feature(proc_macro_hygiene, stmt_expr_attributes)]

use futures::stream::Stream;
use futures_async_stream::for_await;

async fn collect(stream: impl Stream<Item = i32>) -> Vec<i32> {
    let mut vec = Vec::new();
    #[for_await]
    for value in stream {
        vec.push(value);
    }
    vec
}

value has the Item type of the stream passed in. Note that async for loops can only be used inside of async functions, closures, blocks, #[stream] functions and stream_block! macros.

#[stream]

Creates streams via generators.

This is a reimplement of futures-await's #[stream] for futures 0.3 and is an experimental implementation of the idea listed as the next step of async/await.

#![feature(generators)]

use futures::stream::Stream;
use futures_async_stream::stream;

// Returns a stream of i32
#[stream(item = i32)]
async fn foo(stream: impl Stream<Item = String>) {
    // `for_await` is built into `stream`. If you use `for_await` only in `stream`, there is no need to import `for_await`.
    #[for_await]
    for x in stream {
        yield x.parse().unwrap();
    }
}

#[stream] on async fn must have an item type specified via item = some::Path and the values output from the stream must be yielded via the yield expression.

#[stream] can also be used on async blocks:

#![feature(generators, proc_macro_hygiene, stmt_expr_attributes)]

use futures::stream::Stream;
use futures_async_stream::stream;

fn foo() -> impl Stream<Item = i32> {
    #[stream]
    async move {
        for i in 0..10 {
            yield i;
        }
    }
}

Note that #[stream] on async block does not require the item argument, but it may require additional type annotations.

Using async stream functions in traits

You can use async stream functions in traits by passing boxed or boxed_local as an argument.

#![feature(generators)]

use futures_async_stream::stream;

trait Foo {
    #[stream(boxed, item = u32)]
    async fn method(&mut self);
}

struct Bar(u32);

impl Foo for Bar {
    #[stream(boxed, item = u32)]
    async fn method(&mut self) {
        while self.0 < u32::max_value() {
            self.0 += 1;
            yield self.0;
        }
    }
}

A async stream function that received a boxed argument is converted to a function that returns Pin<Box<dyn Stream<Item = item> + Send + 'lifetime>>. If you passed boxed_local instead of boxed, async stream function returns a non-threadsafe stream (Pin<Box<dyn Stream<Item = item> + 'lifetime>>).

#![feature(generators)]

use futures::stream::Stream;
use futures_async_stream::stream;
use std::pin::Pin;

// The trait itself can be defined without unstable features.
trait Foo {
    fn method(&mut self) -> Pin<Box<dyn Stream<Item = u32> + Send + '_>>;
}

struct Bar(u32);

impl Foo for Bar {
    #[stream(boxed, item = u32)]
    async fn method(&mut self) {
        while self.0 < u32::max_value() {
            self.0 += 1;
            yield self.0;
        }
    }
}

#[try_stream]

? operator can be used with the #[try_stream]. The Item of the returned stream is Result with Ok being the value yielded and Err the error type returned by ? operator or return Err(...).

#![feature(generators)]

use futures::stream::Stream;
use futures_async_stream::try_stream;

#[try_stream(ok = i32, error = Box<dyn std::error::Error + Send + Sync>)]
async fn foo(stream: impl Stream<Item = String>) {
    #[for_await]
    for x in stream {
        yield x.parse()?;
    }
}

How to write the equivalent code without this API?

#[for_await]

You can write this by combining while let loop, .await, pin_mut macro, and StreamExt::next() method:

use futures::{
    pin_mut,
    stream::{Stream, StreamExt},
};

async fn collect(stream: impl Stream<Item = i32>) -> Vec<i32> {
    let mut vec = Vec::new();
    pin_mut!(stream);
    while let Some(value) = stream.next().await {
        vec.push(value);
    }
    vec
}

#[stream]

You can write this by manually implementing the combinator:

use futures::{
    ready,
    stream::Stream,
    task::{Context, Poll},
};
use pin_project::pin_project;
use std::pin::Pin;

fn foo<S>(stream: S) -> impl Stream<Item = i32>
where
    S: Stream<Item = String>,
{
    Foo { stream }
}

#[pin_project]
struct Foo<S> {
    #[pin]
    stream: S,
}

impl<S> Stream for Foo<S>
where
    S: Stream<Item = String>,
{
    type Item = i32;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        if let Some(x) = ready!(self.project().stream.poll_next(cx)) {
            Poll::Ready(Some(x.parse().unwrap()))
        } else {
            Poll::Ready(None)
        }
    }
}

Macros

stream_block	Creates streams via generators. This is equivalent to #[stream] on async blocks.
try_stream_block	Creates streams via generators. This is equivalent to #[try_stream] on async blocks.

Attribute Macros

for_await	Processes streams using a for loop.
stream	Creates streams via generators.
try_stream	Creates streams via generators.