Futures Concurrency Notes: join and const-eval
— 2021-01-16

  1. what does joining futures do?
  2. what does join! return?
  3. consistent return types
  4. maybe const can help?
  5. what does that mean for adding futures concurrency to the stdlib?
  6. other considerations
  7. conclusion

Happy new year everyone! Today we're trying something new: less of a blog post, more of research notes. This is less of a "here's something I've concluded", and more of: "here's something I'm thinking about". Today's topic is: "How can we add Future::{try_}join and {try_}join! to the stdlib in a way that feels consistent?"

What does joining Futures do?

A Future in Rust is best though of as a "value which eventually becomes available". It's not specified when a value becomes available, so using .await allows us to wait for it until it's available.

Sometimes we want to wait on more than one future at the time: after all, when we're waiting on things, we can do other things in the mean time. And one way to do this is by calling join.

async-std exposes a Future::join method, and async-macros exposes a join! macro. An example joining two futures:

let a = future::ready(1u8);
let b = future::ready(2u8);
assert_eq!(join!(a, b).await, (1, 2));

let a = future::ready(1u8);
let b = future::ready(2u8);
assert_eq!(a.join(b).await, (1, 2));

However once we start joining more than two futures, the output types become different:

let a = future::ready(1u8);
let b = future::ready(2u8);
let c = future::ready(3u8);
assert_eq!(join!(a, b, c).await, (1, 2, 3));

let a = future::ready(1u8);
let b = future::ready(2u8);
let c = future::ready(3u8);
assert_eq!(a.join(b).join(c).await, (1, (2, 3)));

As you can see, each invocation of Future::join returns a tuple. But that means that chaining calls to it starts to nest tuples, which becomes hard to use. And it becomes more nested the more times you chain. Oh no!

In contrast, the join! macro dynamically grows the number of items returned in the tuple. This is possible because macros have loops, and can just write code -- so inside the macro we just expand the output to a tuple which is large enough to hold all of the outputs.

What does join! return?

The definition of async_macros::join! is fairly brief, so I'll just share it right here. The only detail missing is the defition of the MaybeDone type: it's a wrapper which can be awaited, and stores the output type of the future once it completes. We wait for all instances of MaybeDone to complete, and at the end we take all their values and return it from the future:

macro_rules! join {
    ($($fut:ident),* $(,)?) => { {
        async {
                // Move future into a local so that it is pinned in one place and
                // is no longer accessible by the end user.
                let mut $fut = $crate::MaybeDone::new($fut);
            $crate::utils::poll_fn(move |cx| {
                use $crate::utils::future::Future;
                use $crate::utils::task::Poll;
                use $crate::utils::pin::Pin;

                let mut all_done = true;
                    let fut = unsafe { Pin::new_unchecked(&mut $fut) };
                    all_done &= Future::poll(fut, cx).is_ready();
                if all_done {
                        unsafe { Pin::new_unchecked(&mut $fut) }.take().unwrap(),
                } else {
    } }

As you can see the outer-most value returned is an async {} block. This isn't a specific type, but can be referred to using impl Future. However the type returned by Future::join is a concrete Join future. This type can be addressed by name, and actually passed around.

However as we chain Future::join repeatedly, the resulting future's signature will look somewhat like: Join<Join<Join<Join<T>>>>. This is not great.

So on the one hand we have anonymous futures which can only be addressed through impl Future. And on the other hand we have deeply nested futures which are a pain to write by hand. Can we do better?

consistent return types

One thing I mentioned at the start but didn't dive in yet is the fact that we'd like to align the return types of join! and Future::join. Even if Future::join would only ever take one other future as an argument, being able to switch between the method and the macro without needing to change the signature of the returned types is a a huge bonus.

After having worked on async-std for the past two years where a lot of APIs use async fn, I'm now somewhat convinced that the stdlib should never do this. Which you can see reflected in APIs such as std::future::ready which now returns the concrete future std::future::Ready, whereas in async-std it was just an async fn.

Probably another point worth touching on is the futures-rs implementation of join!. This doesn't return any kind of future at all, wrapping the .await call within the macro instead. I feel somewhat strongly that .await calls shouldn't be hidden in code, but instead always be visible.

// Example of futures_rs::join!
let a = future::ready(1u8);
let b = future::ready(2u8);
assert_eq!(join!(a, b), (1, 2)); // is this sync or async?

This slight digression into futures-rs aside, I think if we were to add future::Future::join and future::join! functions to the stdlib, both should be returning concrete futures. And because they effectively do the same thing, we should make it so they both return the same Join type.

Maybe const can help?

So the question now becomes: "how can we do this?". And I think the answer for this is: "const tuples may be able to help".

So const tuples don't exist in Rust today yet. Not even on nightly. The only way to create variadic tuples is through macros like we've shown. However from talking to members of the const-eval WG const tuples are definitely on the roadmap, though it may take a while. However now that we're seeing a move to fund more people to work on the compiler, I'm hoping that this may be possible within a few years, which isn't that long in the grand scheme of things.

Given there's no proposal for const tuples, it's hard to write an example since I have no clue what the syntax for it will be. For N-length arrays the syntax is the following:

pub fn array_windows<const N: usize>(&self) -> ArrayWindows<'_, T, N>;

The const N: usize here is the length argument for the array of type T. The operations this function returns are on [T; N]. However tuples don't have a consistent type T; values contained within tuples are heterogenous. So a tuple of length N can contain N different types. I have no clue how this would be expressed in const contexts (if at all possible?).

So for now let's just pretend we can define N-length tuples inside function signatures, and cross our fingers that this makes enough sense that the idea comes across. Assuming something like that would work, I would expect Future::Join to be able to defined along these lines:

impl Future {
    /// Join with one other future.
    pub fn join<F: Future>(self, other: F) -> Join<(Self, F)>;

This signature tries to convey: this future holds at least two futures: Self, and another future we're joining with. For the join! macro we could fill out the types using code generation, populating the values of the tuple at compile time. Invoking it would yield the following return type:

let a = future::ready(1u8);
let b = future::ready(2u8);
let fut = join!(a, b);
// fut: Join<(Ready<Output = u8>, Ready<Output = u8>)>

let a = future::ready(1u8);
let b = future::ready(2u8);
let c = future::ready(3u8);
let fut = join!(a, b, c);
// fut: Join<(Ready<Output = u8>, Ready<Output = u8>, Ready<Output = u8>)>

Assuming once we have const tuples we'll also have const panic, we can guard against the case where zero futures are provided to Join. Or perhaps the signature should instead be:

Join<Self, (Other, Other2)>

The details are unclear, because well, I don't know how this should work in the future. Maybe there's a different feature a play here too: what if expressing this in signatures actually requires const-variadics or something. This may actually be relying on a variety of features I'm not tracking.

What does that mean for adding futures concurrency to the stdlib?

This post is rooted in research I was doing exactly to answer that question. join! and Future::join feel like they do exactly what they should, module some issues around their return types. Unfortunately however it seems the best solution would require a const-eval feature that doesn't even have an RFC yet.

Given I expect Rust to stick around for at least a few more decades, and how core this functionality is for async programming. I think it's actually worth waiting to implement these features correctly, rather than rushing to add them in the short term. Libraries such as async-std and async-macros can provide suitable solutions through user space in the interim.

In addition to that, there's one more feature in the language required before we can consider adding Future::join: namely, we need either #[cfg(accessible)], or #[cfg(version)]. This is currently a blocker to adding any method on the Future trait. Since the majority of the async ecosystem relies on Ext traits to implement missing functionality, adding a method of the same name to the stdlib would cause ambiguity. So in order to prevent accidental ecosystem breakage, libraries outside of std should gain the ability to detect whether a method has been implemented in the stdlib. Which is what accessible and version are for.

However one possibility may be that we add join! based on the async {} block implementation in the near term without adding Future::join as well. That would at least allow us to expose that funcionality from the stdlib, even if in some instances it may not be the most ergonomic.

Then later on, once we gain the ability to reason about tuples/variadics in const contexts, we can switch the return type to be Join, and add the Future::join method as well. That way we get a solution in the short term, but still do the right thing in the long term. This depends on async {} being forward compatible with returning a concrete future though. I'm not sure if this has been done before, and the lang team might need to weigh in on that.

edit (2020-01-17): As pointed out here by matthieum, if we had a variadic Join type, there's no reason we couldn't implement join on tuples directly.

// Tuple::join
let a = future::ready(1u8);
let b = future::ready(2u8);
let c = future::ready(3u8);
assert_eq!((a, b, c).await, (1, 2, 3));

// Array::join
let a = future::ready(1u8);
let b = future::ready(2u8);
let c = future::ready(3u8);
assert_eq!([a, b, c].await, [1, 2, 3]);

This would soft-deprecate the need to use future::join! and could probably also be extended to arrays and slices of futures too 1. The question is whether the same Join type could be used for all implementations, since it wouldn't return a tuple but an array or vec instead. This can probably only be answered once designs for the corresponding language features start.


join! on an array is effectively an instance of join_all!. In my designs I've mostly relegated join_all! as not being a primitive, instead favoring designs such as TaskGroup and ParallelStream for a collection of N futures since these more often than not will want to be run on a executor anyway. However wanting to join N futures is still nice to be able to do, and implementing Array::join may very well provide a way for us to do so.

Other considerations

Everything we've expressed here not only applies to future::Future::Join and future::join!. It applies to the try_join, race, and try_race variants as well. The async_std::future docs explain how these types cover the full range of awaiting futures.

Additionally the future::join variants only really work well when you know ahead of time how many futures you're going to be awaiting. If the number is dynamic, other constructs should be used. In a future post I may talk about TaskGroup, an adaptation of crossbeam::scope I'm working on, inspired by Swift's upcoming task proposal. But other constructs like parallel-stream and FuturesUnordered already exist as well.


In this post we've looked at what it would take to add Future::join and join! to the stdlib where both functions would return the same, named future. One plausible way to achieve this would be through const tuples (and possible const variadics, which may or may not be the same thing).

However it may be possible to add future::join! in the near term, and once Rust gains the appropriate language features add Future::join and upgrade join! to use the same Join future. This would enable adding the functionality in the near term, but still achieving the ideal design later on.

This post is a bit of an experiment: single draft, Saturday morning writing. I've been doing a lot of research into stabilizing async Rust paradigms recently, and figured I'd share some of the findings along the way. In part for my own reference. But also to communicate needs async Rust may have to members of other Rust teams.

Anyway, hope you enjoyed this, and hope you have a good weekend!

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