Crate futures_concurrency[][src]

Expand description

Concurrency extensions for Future.


use futures_lite::future::block_on;
use std::future::ready;
use futures_concurrency::prelude::*;

fn main() {
    block_on(async {
        // Await multiple similarly-typed futures.
        let fut = [ready(1u8), ready(2u8), ready(3u8)].join();
        let [a, b, c] = fut.await;
        println!("{} {} {}", a, b, c);

        // Await multiple differently-typed futures.
        let fut = (ready(1u8), ready("hello"), ready(3u8)).join();
        let (a, b, c) = fut.await;
        println!("{} {} {}", a, b, c);

        // It even works with vectors of futures, providing an alternative
        // to futures-rs' `join_all`.
        let numbers = std::iter::repeat(12u8).take(6);
        let fut: Vec<_> =;
        println!("{:?}", fut.join().await);


The following traits have been implemented.

  • Join
  • TryJoin
  • Race
  • TryRace

Base Futures Concurrency

Often it’s desireable to await multiple futures as if it was a single future. The join family of operations converts multiple futures into a single future that returns all of their outputs. The race family of operations converts multiple future into a single future that returns the first output.

For operating on futures the following functions can be used:

NameReturn signatureWhen does it return?
Join(T1, T2)Wait for all to complete
RaceTReturn on first value

Fallible Futures Concurrency

For operating on futures that return Result additional try_ variants of the functions mentioned before can be used. These functions are aware of Result, and will behave slightly differently from their base variants.

In the case of try_join, if any of the futures returns Err all futures are dropped and an error is returned. This is referred to as “short-circuiting”.

In the case of try_race, instead of returning the first future that completes it returns the first future that successfully completes. This means try_race will keep going until any one of the futures returns Ok, or all futures have returned Err.

However sometimes it can be useful to use the base variants of the functions even on futures that return Result. Here is an overview of operations that work on Result, and their respective semantics:

NameReturn signatureWhen does it return?
Join(Result<T, E>, Result<T, E>)Wait for all to complete
TryJoinResult<(T1, T2), E>Return on first Err, wait for all to complete
RaceResult<T, E>Return on first value
Try_raceResult<T, E>Return on first Ok, reject on last Err


Implementations for the Array type.

The futures concurrency prelude.

Implementations for the tuple type.

Implementations for the Vec type.


Wait for multiple futures to complete.