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async-fn()-->-impl-Future.md

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Summary

Transition #[async] functions

from

being written with a Result return type that gets rewritten inside the macro to impl Future, e.g.

#[async]
fn fetch_rust_lang(client: hyper::Client) -> io::Result<String>
to

being written with an impl Future or similar return type directly, e.g.

#[async]
fn fetch_rust_lang(client: hyper::Client) -> impl Future<Item=String, Error=io::Error>

Also support return types of impl Stream, Box<Future>, impl StableFuture etc. (potentially any CoerceUnsized smart pointer if possible?).

Motivation

Tie the written signature more directly to the final signature

When browsing the documentation, then moving to the source code it is confusing to see different signatures. By specifying the expected signature directly this will be avoided.

Reduce friction for migrating to #[async]

If a library is planning on using #[async] in the future then they can prepare today by declaring all their functions -> impl Future and returning explicitly implemented futures. They will then simply have to add the #[async] attribute and rewrite the body, whereas today they would be forced to change the signature as well:

+#[async]
+fn fetch_rust_lang(client: hyper::Client) -> io::Result<String> {
-fn fetch_rust_lang(client: hyper::Client) -> impl Future<Item=String, Error=io::Error> {

Forward compatibility with nominal existential types

pub abstract type CountDown: Future<Item = (), Error = !>;

#[async]
pub fn count_down(count: Duration) -> CountDown {
    ...
}

Forward compatibility with impl Trait in trait

pub trait CountDown {
  fn start(&mut self, count: Duration) -> impl Future<Item = (), Error = !> + '_;
}

impl CountDown for Timer {
  #[async]
  fn start(&mut self, count: Duration) -> impl Future<Item = (), Error = !> + '_ {
    ...
  }
}

Remove necessity for #[async_stream] and associated macros.

It's possible to support all return types via just a single macro. This may still require arguments for boxing or pinning the return type, but it might be possible to automatically infer this from the return type as well.

More consistency with other languages

All existing statically typed async/await supporting languages use this form of signature. See Prior Art below for examples.

Guide-level explanation

Update all examples:

s/-> Result<(.+), (.+)>/-> impl Future<Item = \1, Error = \1>/

Warning, highly cribbed from the C# async programming docs, not suitable for direct inclusion in Rust docs.

The #[async] and await! macros are the heart of asynchronous programming in Rust. By using these two keywords you can easily and succinctly create asynchronous methods without the overhead of define creating structs and writing manual Future implementations.

The following example shows an asynchronous method. Almost everything in the code should look completely familiar to you. The comments call out the features that you add to create the asynchrony.

// Two things to note in this signature:
//  - The function has the `#[async]` macro applied to it.
//  - The return type is `impl Future`
// See below for the description of the `unpinned` argument.
#[async(unpinned)]
fn fetch_rust_lang(client: hyper::Client) -> impl Future<Item=String, Error=io::Error> {
    // hyper::Client::get returns a `Future<Item=Response>`. That means that
    // when you await the future you'll get a `Response`
    let responseTask = client.get("https://www.rust-lang.org");

    // You can do work here that doesn't rely on the response.
    //
    // Wether or not this will actually run in parallel depends on whether
    // `hyper::Client::get` spawned its work off onto a thread pool internally.
    do_something_else();

    // The `await!` macro suspends `fetch_rust_lang`
    //  - `fetch_rust_lang` can't continue until `responseTask` is complete
    //  - Meanwhile, the parent future that called `fetch_rust_lang` can perform
    //    other work of its own
    //  - Control will resume here once `responseTask` is complete
    //  - The `await!` macro then returns a `Result<Response, io::Error>`
    //    containing the result or error returned from `responseTask` (this is
    //    handled via the `?` operator here).
    let response = await!(responseTask)?;

    // If something goes wrong, you can return a `Result::Err` value to
    // shortcircuit this Future.
    if !response.status().is_success() {
        return Err(io::Error::new(io::ErrorKind::Other, "request failed"))
    }

    // `response.body().concat()` returns another `Future<Item=Vec<u8>>` which
    // will resolve with the complete body of the response once it's downloaded.
    let body = await!(response.body().concat())?;
    let string = String::from_utf8(body)?;

    // The return value must be a `Result` of the same `Item` and `Error` types
    // as the signature declares. Any methods that are awaiting
    // `fetch_rust_lang` will be able to resume with the returned value.
    Ok(string)
}

The #[async] macro defaults to creating potentially self-referential impl StableFuture, to instead create movable non-self-referential impl Future you must pass the unpinned argument. Similarly if you wish to create a boxed future you can use #[async(boxed)] fn foo() -> Box<Future>.

Reference-level explanation

WIP working implementation.

#[async] will not touch the provided signature in any way. Most of the generated code will be similar to what it is today. The futures-async-runtime will only contain 2 different wrapper types, one to support StableFuture/StableStream and one to support Future/Stream. The wrappers will implement Future or Stream (or the Stable variants of) based on the yield and return type of the generated closure.

If it's possible to detect a Box return type automatically then this will be done and any Box<...> return type will be boxed. Otherwise, the user will still have to pass a boxed argument as today when they specify Box<...> as a return type.

Drawbacks

The generators return type is not written down

While this changes the written return type, it doesn't affect what the user is allowed to return from the generator. This is still forced to be a Result of some kind (although, this could likely be extended to Try). This inconsistency between the functions declared return type and the function body's actual return is what prompted the original design of the macro.

impl Trait requires specifying all lifetimes

Because of how impl Trait and trait objects handle lifetimes, this interacts very poorly with lifetime elisions. Specifically, the return type only captures explicitly called out lifetimes, but the returned generator necessarily captures all lifetimes.

So say you have a method like this:

#[async]
fn foo(&self, arg: &str) -> Result<i32, io::Error>

You'd have to write it like this:

#[async]
fn foo<'a, 'b>(&'a self, arg: &'b str) -> impl Future<Item = i32, Error = io::Error> + 'a + 'b

The alternative would be to make lifetime elisions not work the same way the work with regular impl Trait return types, but that seems clearly out to me, since the whole idea of this change is to make the return type accurately reflect with the function returns.

-- @withoutboats comment

I argue this is not a true downside. This is just the current state of impl Trait, if this is a major issue for #[async] then it will be a major issue for other users of impl Trait and should be fixed at the source.

It also seems to be closer to the current thinking around lifetimes in signatures, along with the plan to support explicit elided lifetimes this is ensuring that reading the function signature will allow you to quickly determine the lifetime dependencies.

Rationale and alternatives

  • Why is this design the best in the space of possible designs?
  • What other designs have been considered and what is the rationale for not choosing them?
  • What is the impact of not doing this?

Prior art

More consistency with other languages

C#:

async Task<int> AccessTheWebAsync()
{
    HttpClient client = new HttpClient();
    string urlContents = await client.GetStringAsync("http://msdn.microsoft.com");
    return urlContents.Length;
}

TypeScript:

async function foo(): Promise<number> {
    await delay(100);
    return 5;
}

Dart:

If the expression being returned has type T, the function should have return type Future<T> (or a supertype thereof). Otherwise, a static warning is issued.

Hack:

async function curl_A(): Awaitable<string> {
  $x = await \HH\Asio\curl_exec("http://example.com/");
  return $x;
}

Unresolved questions

  • Is it possible to automatically detect Box return types?

    • This would allow removing the boxed argument to the #[async] macro.
    • Extending from this, could this then allow any CoerceUnsized smart pointer to be used?

  • Could generators be changed to not require a different syntax to support self-references?

    • For example if they just automatically don't implement Unpin if they have a self-reference then this could allow removing the unpinned argument to the #[async] macro and just rely on whether Unpin is implemented.

  • If it's not possible to remove the unpinned argument, should unpinned be the default, and what should the exact argument be (pinned, move, unpin, stable, ...)?