# futures-async-stream
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[crates-url]: https://crates.io/crates/futures-async-stream
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[docs-url]: https://docs.rs/futures-async-stream
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[license]: #license
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Async stream for Rust and the futures crate.
This crate provides useful features for streams, using `async_await` and unstable `generators`.
## Usage
Add this to your `Cargo.toml`:
```toml
[dependencies]
futures-async-stream = "0.2"
futures = "0.3"
```
The current futures-async-stream requires Rust nightly-2020-03-22 or later.
## `#[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](https://github.com/rust-lang/rfcs/blob/master/text/2394-async_await.md#for-await-and-processing-streams).
```rust
#![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](https://github.com/rust-lang/rfcs/blob/master/text/2394-async_await.md#generators-and-streams).
```rust
#![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:
```rust
#![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.
```rust
#![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>>`).
```rust
#![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(...)`.
```rust
#![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()?;
}
}
```
`#[try_stream]` can be used wherever `#[stream]` can be used.
## 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:
```rust
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:
```rust
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)
}
}
}
```
[futures-await]: https://github.com/alexcrichton/futures-await
## License
Licensed under either of [Apache License, Version 2.0](LICENSE-APACHE) or [MIT license](LICENSE-MIT) at your option.
### Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.