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## Streams: Futures in Sequence

So far in this chapter, we’ve mostly stuck to individual futures. The one big exception was the async channel we used. Recall how we used the receiver for our async channel earlier in this chapter in the “Message Passing” section. The async `recv` method produces a sequence of items over time. This is an instance of a much more general pattern known as a stream.

We saw a sequence of items back in Chapter 13, when we looked at the `Iterator` trait in The Iterator Trait and the `next` Method section, but there are two differences between iterators and the async channel receiver. The first difference is time: iterators are synchronous, while the channel receiver is asynchronous. The second is the API. When working directly with `Iterator`, we call its synchronous `next` method. With the `trpl::Receiver` stream in particular, we called an asynchronous `recv` method instead. Otherwise, these APIs feel very similar, and that similarity isn’t a coincidence. A stream is like an asynchronous form of iteration. Whereas the `trpl::Receiver` specifically waits to receive messages, though, the general-purpose stream API is much broader: it provides the next item the way `Iterator` does, but asynchronously.

The similarity between iterators and streams in Rust means we can actually create a stream from any iterator. As with an iterator, we can work with a stream by calling its `next` method and then awaiting the output, as in Listing 17-30.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    fn main() {
        trpl::run(async {
            let values = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
            let iter = values.iter().map(|n| n * 2);
            let mut stream = trpl::stream_from_iter(iter);

            while let Some(value) = stream.next().await {
                println!("The value was: {value}");
            }
        });
    }

Listing 17-30: Creating a stream from an iterator and printing its values

We start with an array of numbers, which we convert to an iterator and then call `map` on to double all the values. Then we convert the iterator into a stream using the `trpl::stream_from_iter` function. Next, we loop over the items in the stream as they arrive with the `while let` loop.

Unfortunately, when we try to run the code, it doesn’t compile, but instead it reports that there’s no `next` method available:

    error[E0599]: no method named `next` found for struct `Iter` in the current scope
      --> src/main.rs:10:40
       |
    10 |         while let Some(value) = stream.next().await {
       |                                        ^^^^
       |
       = note: the full type name has been written to 'file:///projects/async-await/target/debug/deps/async_await-575db3dd3197d257.long-type-14490787947592691573.txt'
       = note: consider using `--verbose` to print the full type name to the console
       = help: items from traits can only be used if the trait is in scope
    help: the following traits which provide `next` are implemented but not in scope; perhaps you want to import one of them
       |
    1  + use crate::trpl::StreamExt;
       |
    1  + use futures_util::stream::stream::StreamExt;
       |
    1  + use std::iter::Iterator;
       |
    1  + use std::str::pattern::Searcher;
       |
    help: there is a method `try_next` with a similar name
       |
    10 |         while let Some(value) = stream.try_next().await {
       |                                        ~~~~~~~~

As this output explains, the reason for the compiler error is that we need the right trait in scope to be able to use the `next` method. Given our discussion so far, you might reasonably expect that trait to be `Stream`, but it’s actually `StreamExt`. Short for extension, `Ext` is a common pattern in the Rust community for extending one trait with another.

We’ll explain the `Stream` and `StreamExt` traits in a bit more detail at the end of the chapter, but for now all you need to know is that the `Stream` trait defines a low-level interface that effectively combines the `Iterator` and `Future` traits. `StreamExt` supplies a higher-level set of APIs on top of `Stream`, including the `next` method as well as other utility methods similar to those provided by the `Iterator` trait. `Stream` and `StreamExt` are not yet part of Rust’s standard library, but most ecosystem crates use the same definition.

The fix to the compiler error is to add a `use` statement for `trpl::StreamExt`, as in Listing 17-31.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use trpl::StreamExt;

    fn main() {
        trpl::run(async {
            let values = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
            let iter = values.iter().map(|n| n * 2);
            let mut stream = trpl::stream_from_iter(iter);

            while let Some(value) = stream.next().await {
                println!("The value was: {value}");
            }
        });
    }

Listing 17-31: Successfully using an iterator as the basis for a stream

With all those pieces put together, this code works the way we want! What’s more, now that we have `StreamExt` in scope, we can use all of its utility methods, just as with iterators. For example, in Listing 17-32, we use the `filter` method to filter out everything but multiples of three and five.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use trpl::StreamExt;

    fn main() {
        trpl::run(async {
            let values = 1..101;
            let iter = values.map(|n| n * 2);
            let stream = trpl::stream_from_iter(iter);

            let mut filtered =
                stream.filter(|value| value % 3 == 0 || value % 5 == 0);

            while let Some(value) = filtered.next().await {
                println!("The value was: {value}");
            }
        });
    }

Listing 17-32: Filtering a stream with the `StreamExt::filter` method

Of course, this isn’t very interesting, since we could do the same with normal iterators and without any async at all. Let’s look at what we can do that is unique to streams.

### Composing Streams

Many concepts are naturally represented as streams: items becoming available in a queue, chunks of data being pulled incrementally from the filesystem when the full data set is too large for the computer’s memory, or data arriving over the network over time. Because streams are futures, we can use them with any other kind of future and combine them in interesting ways. For example, we can batch up events to avoid triggering too many network calls, set timeouts on sequences of long-running operations, or throttle user interface events to avoid doing needless work.

Let’s start by building a little stream of messages as a stand-in for a stream of data we might see from a WebSocket or another real-time communication protocol, as shown in Listing 17-33.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let mut messages = get_messages();

            while let Some(message) = messages.next().await {
                println!("{message}");
            }
        });
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
        for message in messages {
            tx.send(format!("Message: '{message}'")).unwrap();
        }

        ReceiverStream::new(rx)
    }

Listing 17-33: Using the `rx` receiver as a `ReceiverStream`

First, we create a function called `get_messages` that returns `impl Stream<Item = String>`. For its implementation, we create an async channel, loop over the first 10 letters of the English alphabet, and send them across the channel.

We also use a new type: `ReceiverStream`, which converts the `rx` receiver from the `trpl::channel` into a `Stream` with a `next` method. Back in `main`, we use a `while let` loop to print all the messages from the stream.

When we run this code, we get exactly the results we would expect:

    Message: 'a'
    Message: 'b'
    Message: 'c'
    Message: 'd'
    Message: 'e'
    Message: 'f'
    Message: 'g'
    Message: 'h'
    Message: 'i'
    Message: 'j'

Again, we could do this with the regular `Receiver` API or even the regular `Iterator` API, though, so let’s add a feature that requires streams: adding a timeout that applies to every item in the stream, and a delay on the items we emit, as shown in Listing 17-34.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{pin::pin, time::Duration};
    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let mut messages =
                pin!(get_messages().timeout(Duration::from_millis(200)));

            while let Some(result) = messages.next().await {
                match result {
                    Ok(message) => println!("{message}"),
                    Err(reason) => eprintln!("Problem: {reason:?}"),
                }
            }
        })
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
        for message in messages {
            tx.send(format!("Message: '{message}'")).unwrap();
        }

        ReceiverStream::new(rx)
    }

Listing 17-34: Using the `StreamExt::timeout` method to set a time limit on the items in a stream

We start by adding a timeout to the stream with the `timeout` method, which comes from the `StreamExt` trait. Then we update the body of the `while let` loop, because the stream now returns a `Result`. The `Ok` variant indicates a message arrived in time; the `Err` variant indicates that the timeout elapsed before any message arrived. We `match` on that result and either print the message when we receive it successfully or print a notice about the timeout. Finally, notice that we pin the messages after applying the timeout to them, because the timeout helper produces a stream that needs to be pinned to be polled.

However, because there are no delays between messages, this timeout does not change the behavior of the program. Let’s add a variable delay to the messages we send, as shown in Listing 17-35.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{pin::pin, time::Duration};

    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let mut messages =
                pin!(get_messages().timeout(Duration::from_millis(200)));

            while let Some(result) = messages.next().await {
                match result {
                    Ok(message) => println!("{message}"),
                    Err(reason) => eprintln!("Problem: {reason:?}"),
                }
            }
        })
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
            for (index, message) in messages.into_iter().enumerate() {
                let time_to_sleep = if index % 2 == 0 { 100 } else { 300 };
                trpl::sleep(Duration::from_millis(time_to_sleep)).await;

                tx.send(format!("Message: '{message}'")).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

Listing 17-35: Sending messages through `tx` with an async delay without making `get_messages` an async function

In `get_messages`, we use the `enumerate` iterator method with the `messages` array so that we can get the index of each item we’re sending along with the item itself. Then we apply a 100-millisecond delay to even-index items and a 300-millisecond delay to odd-index items to simulate the different delays we might see from a stream of messages in the real world. Because our timeout is for 200 milliseconds, this should affect half of the messages.

To sleep between messages in the `get_messages` function without blocking, we need to use async. However, we can’t make `get_messages` itself into an async function, because then we’d return a `Future<Output = Stream<Item = String>>` instead of a `Stream<Item = String>>`. The caller would have to await `get_messages` itself to get access to the stream. But remember: everything in a given future happens linearly; concurrency happens between futures. Awaiting `get_messages` would require it to send all the messages, including the sleep delay between each message, before returning the receiver stream. As a result, the timeout would be useless. There would be no delays in the stream itself; they would all happen before the stream was even available.

Instead, we leave `get_messages` as a regular function that returns a stream, and we spawn a task to handle the async `sleep` calls.

Note: Calling `spawn_task` in this way works because we already set up our runtime; had we not, it would cause a panic. Other implementations choose different tradeoffs: they might spawn a new runtime and avoid the panic but end up with a bit of extra overhead, or they may simply not provide a standalone way to spawn tasks without reference to a runtime. Make sure you know what tradeoff your runtime has chosen and write your code accordingly!

Now our code has a much more interesting result. Between every other pair of messages, a `Problem: Elapsed(())` error.

    Message: 'a'
    Problem: Elapsed(())
    Message: 'b'
    Message: 'c'
    Problem: Elapsed(())
    Message: 'd'
    Message: 'e'
    Problem: Elapsed(())
    Message: 'f'
    Message: 'g'
    Problem: Elapsed(())
    Message: 'h'
    Message: 'i'
    Problem: Elapsed(())
    Message: 'j'

The timeout doesn’t prevent the messages from arriving in the end. We still get all of the original messages, because our channel is unbounded: it can hold as many messages as we can fit in memory. If the message doesn’t arrive before the timeout, our stream handler will account for that, but when it polls the stream again, the message may now have arrived.

You can get different behavior if needed by using other kinds of channels or other kinds of streams more generally. Let’s see one of those in practice by combining a stream of time intervals with this stream of messages.

### Merging Streams

First, let’s create another stream, which will emit an item every millisecond if we let it run directly. For simplicity, we can use the `sleep` function to send a message on a delay and combine it with the same approach we used in `get_messages` of creating a stream from a channel. The difference is that this time, we’re going to send back the count of intervals that have elapsed, so the return type will be `impl Stream<Item = u32>`, and we can call the function `get_intervals` (see Listing 17-36).

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{pin::pin, time::Duration};

    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let mut messages =
                pin!(get_messages().timeout(Duration::from_millis(200)));

            while let Some(result) = messages.next().await {
                match result {
                    Ok(message) => println!("{message}"),
                    Err(reason) => eprintln!("Problem: {reason:?}"),
                }
            }
        })
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
            for (index, message) in messages.into_iter().enumerate() {
                let time_to_sleep = if index % 2 == 0 { 100 } else { 300 };
                trpl::sleep(Duration::from_millis(time_to_sleep)).await;

                tx.send(format!("Message: '{message}'")).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

    fn get_intervals() -> impl Stream<Item = u32> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let mut count = 0;
            loop {
                trpl::sleep(Duration::from_millis(1)).await;
                count += 1;
                tx.send(count).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

Listing 17-36: Creating a stream with a counter that will be emitted once every millisecond

We start by defining a `count` in the task. (We could define it outside the task, too, but it’s clearer to limit the scope of any given variable.) Then we create an infinite loop. Each iteration of the loop asynchronously sleeps for one millisecond, increments the count, and then sends it over the channel. Because this is all wrapped in the task created by `spawn_task`, all of it—including the infinite loop—will get cleaned up along with the runtime.

This kind of infinite loop, which ends only when the whole runtime gets torn down, is fairly common in async Rust: many programs need to keep running indefinitely. With async, this doesn’t block anything else, as long as there is at least one await point in each iteration through the loop.

Now, back in our main function’s async block, we can attempt to merge the `messages` and `intervals` streams, as shown in Listing 17-37.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{pin::pin, time::Duration};

    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let messages = get_messages().timeout(Duration::from_millis(200));
            let intervals = get_intervals();
            let merged = messages.merge(intervals);

            while let Some(result) = merged.next().await {
                match result {
                    Ok(message) => println!("{message}"),
                    Err(reason) => eprintln!("Problem: {reason:?}"),
                }
            }
        })
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
            for (index, message) in messages.into_iter().enumerate() {
                let time_to_sleep = if index % 2 == 0 { 100 } else { 300 };
                trpl::sleep(Duration::from_millis(time_to_sleep)).await;

                tx.send(format!("Message: '{message}'")).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

    fn get_intervals() -> impl Stream<Item = u32> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let mut count = 0;
            loop {
                trpl::sleep(Duration::from_millis(1)).await;
                count += 1;
                tx.send(count).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

Listing 17-37: Attempting to merge the `messages` and `intervals` streams

We start by calling `get_intervals`. Then we merge the `messages` and `intervals` streams with the `merge` method, which combines multiple streams into one stream that produces items from any of the source streams as soon as the items are available, without imposing any particular ordering. Finally, we loop over that combined stream instead of over `messages`.

At this point, neither `messages` nor `intervals` needs to be pinned or mutable, because both will be combined into the single `merged` stream. However, this call to `merge` doesn’t compile! (Neither does the `next` call in the `while let` loop, but we’ll come back to that.) This is because the two streams have different types. The `messages` stream has the type `Timeout<impl Stream<Item = String>>`, where `Timeout` is the type that implements `Stream` for a `timeout` call. The `intervals` stream has the type `impl Stream<Item = u32>`. To merge these two streams, we need to transform one of them to match the other. We’ll rework the intervals stream, because messages is already in the basic format we want and has to handle timeout errors (see Listing 17-38).

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{pin::pin, time::Duration};

    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let messages = get_messages().timeout(Duration::from_millis(200));
            let intervals = get_intervals()
                .map(|count| format!("Interval: {count}"))
                .timeout(Duration::from_secs(10));
            let merged = messages.merge(intervals);
            let mut stream = pin!(merged);

            while let Some(result) = stream.next().await {
                match result {
                    Ok(message) => println!("{message}"),
                    Err(reason) => eprintln!("Problem: {reason:?}"),
                }
            }
        })
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
            for (index, message) in messages.into_iter().enumerate() {
                let time_to_sleep = if index % 2 == 0 { 100 } else { 300 };
                trpl::sleep(Duration::from_millis(time_to_sleep)).await;

                tx.send(format!("Message: '{message}'")).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

    fn get_intervals() -> impl Stream<Item = u32> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let mut count = 0;
            loop {
                trpl::sleep(Duration::from_millis(1)).await;
                count += 1;
                tx.send(count).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

Listing 17-38: Aligning the type of the the `intervals` stream with the type of the `messages` stream

First, we can use the `map` helper method to transform the `intervals` into a string. Second, we need to match the `Timeout` from `messages`. Because we don’t actually want a timeout for `intervals`, though, we can just create a timeout which is longer than the other durations we are using. Here, we create a 10-second timeout with `Duration::from_secs(10)`. Finally, we need to make `stream` mutable, so that the `while let` loop’s `next` calls can iterate through the stream, and pin it so that it’s safe to do so. That gets us almost to where we need to be. Everything type checks. If you run this, though, there will be two problems. First, it will never stop! You’ll need to stop it with ctrl-c. Second, the messages from the English alphabet will be buried in the midst of all the interval counter messages:

    --snip--
    Interval: 38
    Interval: 39
    Interval: 40
    Message: 'a'
    Interval: 41
    Interval: 42
    Interval: 43
    --snip--

Listing 17-39 shows one way to solve these last two problems.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use std::{pin::pin, time::Duration};

    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let messages = get_messages().timeout(Duration::from_millis(200));
            let intervals = get_intervals()
                .map(|count| format!("Interval: {count}"))
                .throttle(Duration::from_millis(100))
                .timeout(Duration::from_secs(10));
            let merged = messages.merge(intervals).take(20);
            let mut stream = pin!(merged);

            while let Some(result) = stream.next().await {
                match result {
                    Ok(message) => println!("{message}"),
                    Err(reason) => eprintln!("Problem: {reason:?}"),
                }
            }
        })
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
            for (index, message) in messages.into_iter().enumerate() {
                let time_to_sleep = if index % 2 == 0 { 100 } else { 300 };
                trpl::sleep(Duration::from_millis(time_to_sleep)).await;

                tx.send(format!("Message: '{message}'")).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

    fn get_intervals() -> impl Stream<Item = u32> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let mut count = 0;
            loop {
                trpl::sleep(Duration::from_millis(1)).await;
                count += 1;
                tx.send(count).unwrap();
            }
        });

        ReceiverStream::new(rx)
    }

Listing 17-39: Using `throttle` and `take` to manage the merged streams

First, we use the `throttle` method on the `intervals` stream so that it doesn’t overwhelm the `messages` stream. Throttling is a way of limiting the rate at which a function will be called—or, in this case, how often the stream will be polled. Once every 100 milliseconds should do, because that’s roughly how often our messages arrive.

To limit the number of items we will accept from a stream, we apply the `take` method to the `merged` stream, because we want to limit the final output, not just one stream or the other.

Now when we run the program, it stops after pulling 20 items from the stream, and the intervals don’t overwhelm the messages. We also don’t get `Interval: 100` or `Interval: 200` or so on, but instead get `Interval: 1`, `Interval: 2`, and so on—even though we have a source stream that can produce an event every millisecond. That’s because the `throttle` call produces a new stream that wraps the original stream so that the original stream gets polled only at the throttle rate, not its own “native” rate. We don’t have a bunch of unhandled interval messages we’re choosing to ignore. Instead, we never produce those interval messages in the first place! This is the inherent “laziness” of Rust’s futures at work again, allowing us to choose our performance characteristics.

    Interval: 1
    Message: 'a'
    Interval: 2
    Interval: 3
    Problem: Elapsed(())
    Interval: 4
    Message: 'b'
    Interval: 5
    Message: 'c'
    Interval: 6
    Interval: 7
    Problem: Elapsed(())
    Interval: 8
    Message: 'd'
    Interval: 9
    Message: 'e'
    Interval: 10
    Interval: 11
    Problem: Elapsed(())
    Interval: 12

There’s one last thing we need to handle: errors! With both of these channel-based streams, the `send` calls could fail when the other side of the channel closes—and that’s just a matter of how the runtime executes the futures that make up the stream. Up until now, we’ve ignored this possibility by calling `unwrap`, but in a well-behaved app, we should explicitly handle the error, at minimum by ending the loop so we don’t try to send any more messages. Listing 17-40 shows a simple error strategy: print the issue and then `break` from the loops.

    extern crate trpl; // required for mdbook test

    use std::{pin::pin, time::Duration};

    use trpl::{ReceiverStream, Stream, StreamExt};

    fn main() {
        trpl::run(async {
            let messages = get_messages().timeout(Duration::from_millis(200));
            let intervals = get_intervals()
                .map(|count| format!("Interval #{count}"))
                .throttle(Duration::from_millis(500))
                .timeout(Duration::from_secs(10));
            let merged = messages.merge(intervals).take(20);
            let mut stream = pin!(merged);

            while let Some(result) = stream.next().await {
                match result {
                    Ok(item) => println!("{item}"),
                    Err(reason) => eprintln!("Problem: {reason:?}"),
                }
            }
        });
    }

    fn get_messages() -> impl Stream<Item = String> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let messages = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];

            for (index, message) in messages.into_iter().enumerate() {
                let time_to_sleep = if index % 2 == 0 { 100 } else { 300 };
                trpl::sleep(Duration::from_millis(time_to_sleep)).await;

                if let Err(send_error) = tx.send(format!("Message: '{message}'")) {
                    eprintln!("Cannot send message '{message}': {send_error}");
                    break;
                }
            }
        });

        ReceiverStream::new(rx)
    }

    fn get_intervals() -> impl Stream<Item = u32> {
        let (tx, rx) = trpl::channel();

        trpl::spawn_task(async move {
            let mut count = 0;
            loop {
                trpl::sleep(Duration::from_millis(1)).await;
                count += 1;

                if let Err(send_error) = tx.send(count) {
                    eprintln!("Could not send interval {count}: {send_error}");
                    break;
                };
            }
        });

        ReceiverStream::new(rx)
    }

Listing 17-40: Handling errors and shutting down the loops

As usual, the correct way to handle a message send error will vary; just make sure you have a strategy.

Now that we’ve seen a bunch of async in practice, let’s take a step back and dig into a few of the details of how `Future`, `Stream`, and the other key traits Rust uses to make async work.