I have an asynchronous Stream and I'd like to get the first value out of it. How can I do so?
use futures::Stream; // 0.3.5
async fn example<T>(s: impl Stream<Item = T>) -> Option<T> {
todo!("What goes here?")
}
You can use StreamExt::next:
use futures::{Stream, StreamExt}; // 0.3.5
async fn example<T>(mut s: impl Stream<Item = T> + Unpin) -> Option<T> {
s.next().await
}
You can use StreamExt::into_future:
use futures::{FutureExt, Stream, StreamExt}; // 0.3.5
async fn example<T>(s: impl Stream<Item = T> + Unpin) -> Option<T> {
s.into_future().map(|(v, _)| v).await
}
In rare cases, you may want to use future::poll_fn to have complete control:
use futures::{future, task::Poll, Stream, StreamExt}; // 0.3.5
async fn example<T>(mut s: impl Stream<Item = T> + Unpin) -> Option<T> {
future::poll_fn(|ctx| {
// Could use methods like `Poll::map` or
// the `?` operator instead of a `match`.
match s.poll_next_unpin(ctx) {
Poll::Ready(v) => {
// Do any special logic here
Poll::Ready(v)
}
Poll::Pending => Poll::Pending,
}
})
.await
}
See also:
How to convert a Future into a Stream?
More broadly
If you wanted to act upon all the values in the stream, producing a single value, you can use StreamExt::fold:
use futures::{Stream, StreamExt}; // 0.3.5
async fn example(s: impl Stream + Unpin) -> usize {
s.fold(0, |st, _| async move { st + 1 }).await
}
If you wanted to act upon all the values in the stream without producing a value, you can use StreamExt::for_each:
use futures::{Stream, StreamExt}; // 0.3.5
async fn example<I: std::fmt::Debug>(s: impl Stream<Item = I> + Unpin) {
s.for_each(|i| async {
dbg!(i);
})
.await;
}
See also:
How to select between a future and stream in Rust?
Unpin
These example all require that the incoming Stream implement Unpin. You could also pin the stream yourself via Box::pin or the pin_mut! macro.
See also:
What are the use cases of the newly proposed Pin type?
Related
I am trying to use hyper to grab the content of an HTML page and would like to synchronously return the output of a future. I realized I could have picked a better example since synchronous HTTP requests already exist, but I am more interested in understanding whether we could return a value from an async calculation.
extern crate futures;
extern crate hyper;
extern crate hyper_tls;
extern crate tokio;
use futures::{future, Future, Stream};
use hyper::Client;
use hyper::Uri;
use hyper_tls::HttpsConnector;
use std::str;
fn scrap() -> Result<String, String> {
let scraped_content = future::lazy(|| {
let https = HttpsConnector::new(4).unwrap();
let client = Client::builder().build::<_, hyper::Body>(https);
client
.get("https://hyper.rs".parse::<Uri>().unwrap())
.and_then(|res| {
res.into_body().concat2().and_then(|body| {
let s_body: String = str::from_utf8(&body).unwrap().to_string();
futures::future::ok(s_body)
})
}).map_err(|err| format!("Error scraping web page: {:?}", &err))
});
scraped_content.wait()
}
fn read() {
let scraped_content = future::lazy(|| {
let https = HttpsConnector::new(4).unwrap();
let client = Client::builder().build::<_, hyper::Body>(https);
client
.get("https://hyper.rs".parse::<Uri>().unwrap())
.and_then(|res| {
res.into_body().concat2().and_then(|body| {
let s_body: String = str::from_utf8(&body).unwrap().to_string();
println!("Reading body: {}", s_body);
Ok(())
})
}).map_err(|err| {
println!("Error reading webpage: {:?}", &err);
})
});
tokio::run(scraped_content);
}
fn main() {
read();
let content = scrap();
println!("Content = {:?}", &content);
}
The example compiles and the call to read() succeeds, but the call to scrap() panics with the following error message:
Content = Err("Error scraping web page: Error { kind: Execute, cause: None }")
I understand that I failed to launch the task properly before calling .wait() on the future but I couldn't find how to properly do it, assuming it's even possible.
Standard library futures
Let's use this as our minimal, reproducible example:
async fn example() -> i32 {
42
}
Call executor::block_on:
use futures::executor; // 0.3.1
fn main() {
let v = executor::block_on(example());
println!("{}", v);
}
Tokio
Use the tokio::main attribute on any function (not just main!) to convert it from an asynchronous function to a synchronous one:
use tokio; // 0.3.5
#[tokio::main]
async fn main() {
let v = example().await;
println!("{}", v);
}
tokio::main is a macro that transforms this
#[tokio::main]
async fn main() {}
Into this:
fn main() {
tokio::runtime::Builder::new_multi_thread()
.enable_all()
.build()
.unwrap()
.block_on(async { {} })
}
This uses Runtime::block_on under the hood, so you can also write this as:
use tokio::runtime::Runtime; // 0.3.5
fn main() {
let v = Runtime::new().unwrap().block_on(example());
println!("{}", v);
}
For tests, you can use tokio::test.
async-std
Use the async_std::main attribute on the main function to convert it from an asynchronous function to a synchronous one:
use async_std; // 1.6.5, features = ["attributes"]
#[async_std::main]
async fn main() {
let v = example().await;
println!("{}", v);
}
For tests, you can use async_std::test.
Futures 0.1
Let's use this as our minimal, reproducible example:
use futures::{future, Future}; // 0.1.27
fn example() -> impl Future<Item = i32, Error = ()> {
future::ok(42)
}
For simple cases, you only need to call wait:
fn main() {
let s = example().wait();
println!("{:?}", s);
}
However, this comes with a pretty severe warning:
This method is not appropriate to call on event loops or similar I/O situations because it will prevent the event loop from making progress (this blocks the thread). This method should only be called when it's guaranteed that the blocking work associated with this future will be completed by another thread.
Tokio
If you are using Tokio 0.1, you should use Tokio's Runtime::block_on:
use tokio; // 0.1.21
fn main() {
let mut runtime = tokio::runtime::Runtime::new().expect("Unable to create a runtime");
let s = runtime.block_on(example());
println!("{:?}", s);
}
If you peek in the implementation of block_on, it actually sends the future's result down a channel and then calls wait on that channel! This is fine because Tokio guarantees to run the future to completion.
See also:
How can I efficiently extract the first element of a futures::Stream in a blocking manner?
As this is the top result that come up in search engines by the query "How to call async from sync in Rust", I decided to share my solution here. I think it might be useful.
As #Shepmaster mentioned, back in version 0.1 futures crate had beautiful method .wait() that could be used to call an async function from a sync one. This must-have method, however, was removed from later versions of the crate.
Luckily, it's not that hard to re-implement it:
trait Block {
fn wait(self) -> <Self as futures::Future>::Output
where Self: Sized, Self: futures::Future
{
futures::executor::block_on(self)
}
}
impl<F,T> Block for F
where F: futures::Future<Output = T>
{}
After that, you can just do following:
async fn example() -> i32 {
42
}
fn main() {
let s = example().wait();
println!("{:?}", s);
}
Beware that this comes with all the caveats of original .wait() explained in the #Shepmaster's answer.
This works for me using tokio:
tokio::runtime::Runtime::new()?.block_on(fooAsyncFunction())?;
Recursive calls with a retry count embedded into the argument is good for the mental model of the project because you don't have to keep thinking about the state of the object. The retry_count is passed in every call. Here's a simple implementation:
use std::future::Future;
use futures::future::{BoxFuture, FutureExt};
fn do_or_fail() -> std::result::Result<(),()> {
Ok(())
}
fn do_something<'a>(
retry_count: u32
) -> BoxFuture<'a, std::result::Result<(), ()>> {
async move {
match do_or_fail() {
Ok(_) => Ok(()),
Err(_) => do_something(retry_count -1).await
}
}.boxed()
}
fn main() {
do_something(3);
}
Playground
The problem is that this requires a dynamic allocation at every call so it can return the BoxFuture. This is because of how async is implemented in Rust. It generates a state machine for every await call so the return type is not Sized.
What would be good ways to overcome dynamic allocation in recursive async calls?
While it technically doesn't overcome the problem (it just hides it), I can recommend the crate async_recursion.
https://crates.io/crates/async-recursion
use async_recursion::async_recursion;
use tokio;
fn do_or_fail() -> std::result::Result<(), ()> {
Ok(())
}
#[async_recursion]
async fn do_something(retry_count: u32) -> std::result::Result<(), ()> {
match do_or_fail() {
Ok(_) => Ok(()),
Err(_) => do_something(retry_count - 1).await,
}
}
#[tokio::main]
async fn main() {
do_something(3).await.unwrap()
}
I don't think you actually can overcome dynamic allocation in async calls, that's just how async works in general.
I have a struct Test I want to implement std::future::Future that would poll function:
use std::{
future::Future,
pin::Pin,
task::{Context, Poll},
};
struct Test;
impl Test {
async fn function(&mut self) {}
}
impl Future for Test {
type Output = ();
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
match self.function() {
Poll::Pending => Poll::Pending,
Poll::Ready(_) => Poll::Ready(()),
}
}
}
That didn't work:
error[E0308]: mismatched types
--> src/lib.rs:17:13
|
10 | async fn function(&mut self) {}
| - the `Output` of this `async fn`'s expected opaque type
...
17 | Poll::Pending => Poll::Pending,
| ^^^^^^^^^^^^^ expected opaque type, found enum `Poll`
|
= note: expected opaque type `impl Future`
found enum `Poll<_>`
error[E0308]: mismatched types
--> src/lib.rs:18:13
|
10 | async fn function(&mut self) {}
| - the `Output` of this `async fn`'s expected opaque type
...
18 | Poll::Ready(_) => Poll::Ready(()),
| ^^^^^^^^^^^^^^ expected opaque type, found enum `Poll`
|
= note: expected opaque type `impl Future`
found enum `Poll<_>`
I understand that function must be called once, the returned Future must be stored somewhere in the struct, and then the saved future must be polled. I tried this:
struct Test(Option<Box<Pin<dyn Future<Output = ()>>>>);
impl Test {
async fn function(&mut self) {}
fn new() -> Self {
let mut s = Self(None);
s.0 = Some(Box::pin(s.function()));
s
}
}
That also didn't work:
error[E0277]: the size for values of type `(dyn Future<Output = ()> + 'static)` cannot be known at compilation time
--> src/lib.rs:7:13
|
7 | struct Test(Option<Box<Pin<dyn Future<Output = ()>>>>);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `Sized` is not implemented for `(dyn Future<Output = ()> + 'static)`
After I call function() I have taken a &mut reference of Test, because of that I can't change the Test variable, and therefore can't store the returned Future inside the Test.
I did get an unsafe solution (inspired by this)
struct Test<'a>(Option<BoxFuture<'a, ()>>);
impl Test<'_> {
async fn function(&mut self) {
println!("I'm alive!");
}
fn new() -> Self {
let mut s = Self(None);
s.0 = Some(unsafe { &mut *(&mut s as *mut Self) }.function().boxed());
s
}
}
impl Future for Test<'_> {
type Output = ();
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
self.0.as_mut().unwrap().poll_unpin(cx)
}
}
I hope that there is another way.
Though there are times when you may want to do things similar to what you're trying to accomplish here, they are a rarity. So most people reading this, maybe even OP, may wish to restructure such that struct state and data used for a single async execution are different objects.
To answer your question, yes it is somewhat possible. Unless you want to absolutely resort to unsafe code you will need to use Mutex and Arc. All fields you wish to manipulate inside the async fn will have to be wrapped inside a Mutex and the function itself will accept an Arc<Self>.
I must stress, however, that this is not a beautiful solution and you probably don't want to do this. Depending on your specific case your solution may vary, but my guess of what OP is trying to accomplish while using Streams would be better solved by something similar to this gist that I wrote.
use std::{
future::Future,
pin::Pin,
sync::{Arc, Mutex},
};
struct Test {
state: Mutex<Option<Pin<Box<dyn Future<Output = ()>>>>>,
// if available use your async library's Mutex to `.await` locks on `buffer` instead
buffer: Mutex<Vec<u8>>,
}
impl Test {
async fn function(self: Arc<Self>) {
for i in 0..16u8 {
let data: Vec<u8> = vec![i]; // = fs::read(&format("file-{}.txt", i)).await.unwrap();
let mut buflock = self.buffer.lock().unwrap();
buflock.extend_from_slice(&data);
}
}
pub fn new() -> Arc<Self> {
let s = Arc::new(Self {
state: Default::default(),
buffer: Default::default(),
});
{
// start by trying to aquire a lock to the Mutex of the Box
let mut lock = s.state.lock().unwrap();
// create boxed future
let b = Box::pin(s.clone().function());
// insert value into the mutex
*lock = Some(b);
} // block causes the lock to be released
s
}
}
impl Future for Test {
type Output = ();
fn poll(
self: std::pin::Pin<&mut Self>,
ctx: &mut std::task::Context<'_>,
) -> std::task::Poll<<Self as std::future::Future>::Output> {
let mut lock = self.state.lock().unwrap();
let fut: &mut Pin<Box<dyn Future<Output = ()>>> = lock.as_mut().unwrap();
Future::poll(fut.as_mut(), ctx)
}
}
I'm not sure what you want to achieve and why, but I suspect that you're trying to implement Future for Test based on some ancient tutorial or misunderstanding and just overcomplicating things.
You don't have to implement Future manually. An async function
async fn function(...) {...}
is really just syntax sugar translated behind the scenes into something like
fn function(...) -> Future<()> {...}
All you have to do is to use the result of the function the same way as any future, e.g. use await on it or call block a reactor until it's finished. E.g. based on your first version, you can simply call:
let mut test = Test{};
test.function().await;
UPDATE1
Based on your descriptions I still think you're trying to overcomplicate this minimal working snippet without the need to manually implement Future for anything:
async fn asyncio() { println!("Doing async IO"); }
struct Test {
count: u32,
}
impl Test {
async fn function(&mut self) {
asyncio().await;
self.count += 1;
}
}
#[tokio::main]
async fn main() {
let mut test = Test{count: 0};
test.function().await;
println!("Count: {}", test.count);
}
I would like to replicate the behavior and ergonomics of taking a closure/function as an argument much like map does: iterator.map(|x| ...).
I've noticed that some library code allows passing in async functionality, but this method doesn't allow me to pass in arguments:
pub fn spawn<F, T>(future: F) -> JoinHandle<T>
where
F: Future<Output = T> + Send + 'static,
T: Send + 'static,
spawn(async { foo().await });
I'm hoping to do one of the following:
iterator.map(async |x| {...});
async fn a(x: _) {}
iterator.map(a)
async functions are effectively desugared as returning impl Future. Once you know that, it's a matter of combining existing Rust techniques to accept a function / closure, resulting in a function with two generic types:
use std::future::Future;
async fn example<F, Fut>(f: F)
where
F: FnOnce(i32, i32) -> Fut,
Fut: Future<Output = bool>,
{
f(1, 2).await;
}
This can also be written as
use std::future::Future;
async fn example<Fut>(f: impl FnOnce(i32, i32) -> Fut)
where
Fut: Future<Output = bool>,
{
f(1, 2).await;
}
How do you pass a Rust function as a parameter?
What is the concrete type of a future returned from `async fn`?
What is the purpose of async/await in Rust?
How can I store an async function in a struct and call it from a struct instance?
What is the difference between `|_| async move {}` and `async move |_| {}`
The async |...| expr closure syntax is available on the nightly channel enabling the feature async_closure.
#![feature(async_closure)]
use futures::future;
use futures::Future;
use tokio;
pub struct Bar;
impl Bar {
pub fn map<F, T>(&self, f: F)
where
F: Fn(i32) -> T,
T: Future<Output = Result<i32, i32>> + Send + 'static,
{
tokio::spawn(f(1));
}
}
async fn foo(x: i32) -> Result<i32, i32> {
println!("running foo");
future::ok::<i32, i32>(x).await
}
#[tokio::main]
async fn main() {
let bar = Bar;
let x = 1;
bar.map(foo);
bar.map(async move |x| {
println!("hello from async closure.");
future::ok::<i32, i32>(x).await
});
}
See the 2394-async_await RFC for more detalis
I'm trying to use async_std to receive UDP datagrams from the network.
There is a UdpSocket that implements async recv_from, this method returns a future but I need a async_std::stream::Stream that gives a stream of UDP datagrams because it is a better abstraction.
I've found tokio::net::UdpFramed that does exactly what I need but it is not available in current versions of tokio.
Generally speaking the question is how do I convert Futures from a given async function into a Stream?
For a single item, use FutureExt::into_stream:
use futures::prelude::*; // 0.3.1
fn outer() -> impl Stream<Item = i32> {
inner().into_stream()
}
async fn inner() -> i32 {
42
}
For a stream from a number of futures generated by a closure, use stream::unfold:
use futures::prelude::*; // 0.3.1
fn outer() -> impl Stream<Item = i32> {
stream::unfold((), |()| async { Some((inner().await, ())) })
}
async fn inner() -> i32 {
42
}
In your case, you can use stream::unfold:
use async_std::{io, net::UdpSocket}; // 1.4.0, features = ["attributes"]
use futures::prelude::*; // 0.3.1
fn read_many(s: UdpSocket) -> impl Stream<Item = io::Result<Vec<u8>>> {
stream::unfold(s, |s| {
async {
let data = read_one(&s).await;
Some((data, s))
}
})
}
async fn read_one(s: &UdpSocket) -> io::Result<Vec<u8>> {
let mut data = vec![0; 1024];
let (len, _) = s.recv_from(&mut data).await?;
data.truncate(len);
Ok(data)
}
#[async_std::main]
async fn main() -> io::Result<()> {
let s = UdpSocket::bind("0.0.0.0:9876").await?;
read_many(s)
.for_each(|d| {
async {
match d {
Ok(d) => match std::str::from_utf8(&d) {
Ok(s) => println!("{}", s),
Err(_) => println!("{:x?}", d),
},
Err(e) => eprintln!("Error: {}", e),
}
}
})
.await;
Ok(())
}
Generally speaking the question is how do I convert Futures from a given async function into a Stream?
There is FutureExt::into_stream, but don't let the name fool you; it is not a good fit for your situation.
There is a UdpSocket that implements async recv_from, this method returns a future but I need a async_std::stream::Stream that gives a stream of UDP datagrams because it is a better abstraction.
It is not necessarily a better abstraction here.
Specifically, async-std's UdpSocket::recv_from returns a future that has output type of (usize, SocketAddr) — the size of the data received and the peer address. If you were to use into_stream to convert it to a stream, it would give you just that, not the data received.
I've found tokio::net::UdpFramed that does exactly what I need but it is not available in current versions of tokio.
It has been moved to tokio-util crate. Unfortunately, you can't (easily) use that either. It requires a tokio::net::UdpSocket, which is not the same as async_std::net::UdpSocket.
You can, of course, use futures utility functions such as futures::stream::poll_fn or futures::stream::unfold to give UdpSocket::recv_from a futures::stream::Stream facade, but then what will you do with that? If you end up calling StreamExt::next to poll a value out of it, you could have used recv_from directly.
It is only necessary to reach for Stream if some API you are using requires a Stream input, such as rusoto:
Is it possible to create a Stream from a File rather than loading the file contents into memory?