Crate futures

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Abstractions for asynchronous programming.

This crate provides a number of core abstractions for writing asynchronous code:

  • Futures are single eventual values produced by asynchronous computations. Some programming languages (e.g. JavaScript) call this concept “promise”.
  • Streams represent a series of values produced asynchronously.
  • Sinks provide support for asynchronous writing of data.
  • Executors are responsible for running asynchronous tasks.

The crate also contains abstractions for asynchronous I/O and cross-task communication.

Underlying all of this is the task system, which is a form of lightweight threading. Large asynchronous computations are built up using futures, streams and sinks, and then spawned as independent tasks that are run to completion, but do not block the thread running them.

The following example describes how the task system context is built and used within macros and keywords such as async and await!.

fn main() {
    let pool = ThreadPool::new().expect("Failed to build pool");
    let (tx, rx) = mpsc::unbounded::<i32>();

    // Create a future by an async block, where async is responsible for an
    // implementation of Future. At this point no executor has been provided
    // to this future, so it will not be running.
    let fut_values = async {
        // Create another async block, again where the Future implementation
        // is generated by async. Since this is inside of a parent async block,
        // it will be provided with the executor of the parent block when the parent
        // block is executed.
        // This executor chaining is done by Future::poll whose second argument
        // is a std::task::Context. This represents our executor, and the Future
        // implemented by this async block can be polled using the parent async
        // block's executor.
        let fut_tx_result = async move {
            (0..100).for_each(|v| {
                tx.unbounded_send(v).expect("Failed to send");

        // Use the provided thread pool to spawn the generated future
        // responsible for transmission

        let fut_values = rx
            .map(|v| v * 2)

        // Use the executor provided to this async block to wait for the
        // future to complete.

    // Actually execute the above future, which will invoke Future::poll and
    // subsequently chain appropriate Future::poll and methods needing executors
    // to drive all futures. Eventually fut_values will be driven to completion.
    let values: Vec<i32> = executor::block_on(fut_values);

    println!("Values={:?}", values);

The majority of examples and code snippets in this crate assume that they are inside an async block as written above.



  • Asynchronous channels.
  • Built-in executors and related tools.
  • Asynchronous values.
  • Asynchronous I/O.
  • Futures-powered synchronization primitives.
  • This module contains the Never type.
  • A “prelude” for crates using the futures crate.
  • Asynchronous sinks.
  • Asynchronous streams.
  • Tools for working with tasks.


  • Polls multiple futures simultaneously, returning a tuple of all results once complete.
  • A macro which yields to the event loop once.
  • Pins a value on the stack.
  • A macro which returns the result of polling a future once within the current async context.
  • Extracts the successful type of a Poll<T>.
  • Polls multiple futures and streams simultaneously, executing the branch for the future that finishes first. If multiple futures are ready, one will be pseudo-randomly selected at runtime. Futures directly passed to select! must be Unpin and implement FusedFuture.
  • Polls multiple futures and streams simultaneously, executing the branch for the future that finishes first. Unlike select!, if multiple futures are ready, one will be selected in order of declaration. Futures directly passed to select_biased! must be Unpin and implement FusedFuture.
  • Combines several streams, all producing the same Item type, into one stream. This is similar to select_all but does not require the streams to all be the same type. It also keeps the streams inline, and does not require Box<dyn Stream>s to be allocated. Streams passed to this macro must be Unpin.
  • Polls multiple futures simultaneously, resolving to a Result containing either a tuple of the successful outputs or an error.