Module std::stream

Composable asynchronous iteration.

If futures are asynchronous values, then streams are asynchronous iterators. If you’ve found yourself with an asynchronous collection of some kind, and needed to perform an operation on the elements of said collection, you’ll quickly run into ‘streams’. Streams are heavily used in idiomatic asynchronous Rust code, so it’s worth becoming familiar with them.

Before explaining more, let’s talk about how this module is structured:

Organization

This module is largely organized by type:

• Traits are the core portion: these traits define what kind of streams exist and what you can do with them. The methods of these traits are worth putting some extra study time into.
• Functions provide some helpful ways to create some basic streams.
• Structs are often the return types of the various methods on this module’s traits. You’ll usually want to look at the method that creates the struct, rather than the struct itself. For more detail about why, see ‘Implementing Stream’.

That’s it! Let’s dig into streams.

Stream

The heart and soul of this module is the Stream trait. The core of Stream looks like this:

trait Stream {
    type Item;
    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>;
}

Unlike Iterator, Stream makes a distinction between the poll_next method which is used when implementing a Stream, and a (to-be-implemented) next method which is used when consuming a stream. Consumers of Stream only need to consider next, which when called, returns a future which yields Option<Stream::Item>.

The future returned by next will yield Some(Item) as long as there are elements, and once they’ve all been exhausted, will yield None to indicate that iteration is finished. If we’re waiting on something asynchronous to resolve, the future will wait until the stream is ready to yield again.

Individual streams may choose to resume iteration, and so calling next again may or may not eventually yield Some(Item) again at some point.

Stream’s full definition includes a number of other methods as well, but they are default methods, built on top of poll_next, and so you get them for free.

Implementing Stream

Creating a stream of your own involves two steps: creating a struct to hold the stream’s state, and then implementing Stream for that struct.

Let’s make a stream named Counter which counts from 1 to 5:

#![feature(async_stream)]

// First, the struct:

/// A stream which counts from one to five
struct Counter {
    count: usize,
}

// we want our count to start at one, so let's add a new() method to help.
// This isn't strictly necessary, but is convenient. Note that we start
// `count` at zero, we'll see why in `poll_next()`'s implementation below.
impl Counter {
    fn new() -> Counter {
        Counter { count: 0 }
    }
}

// Then, we implement `Stream` for our `Counter`:

impl Stream for Counter {
    // we will be counting with usize
    type Item = usize;

    // poll_next() is the only required method
    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        // Increment our count. This is why we started at zero.
        self.count += 1;

        // Check to see if we've finished counting or not.
        if self.count < 6 {
            Poll::Ready(Some(self.count))
        } else {
            Poll::Ready(None)
        }
    }
}

Laziness

Streams are lazy. This means that just creating a stream doesn’t do a whole lot. Nothing really happens until you call next. This is sometimes a source of confusion when creating a stream solely for its side effects. The compiler will warn us about this kind of behavior:

warning: unused result that must be used: streams do nothing unless polled

Structs

Traits

Functions