Positional Associated Types

Positional Associated Types

Proposal

Summary and problem statement

It should be possible to abbreviate Future<Output=T> as Future<T>.

Motivation, use-cases, and solution sketches

Many of the most-used traits in the rust programming language specify a single associated type:

• Iterator<Item=T>
• Future<Output=T>
• FromStr<Err=T>
• Deref<Target=T>
• Stream<Item=T>
• and so on

The impl Trait syntax is intended to make instances of such traits easy to pass and return. But this can be quite verbose in practice, because each of these traits must be written with the associated type explicitly spelled out:

fn deltas(iter: impl Iterator<Item=f32>) -> impl Iterator<Item=f32> {
    iter.array_windows().map(|[a, b]| b - a) 
}

Worse, it may not be clear to incoming Rust programmers why they have to write Iterator<Item=f32> instead of Iterator<f32>. After all, the generics of traits like From<T> and types like Option<T> are not named. It can take many months for the difference between generic parameters and associated types to click, and in that time users probably guess at random which they should use for any given trait.

It is worth investigating whether these common cases can be abbreviated:

pub trait Future {
    #[positional]
    type Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}

impl Logger {
    pub fn log_outcome(task: impl Future<String>) {
        ...
    }
}

This should also apply to the dyn Future<T> and F: Future<T> syntaxes.

The Iterator<T> and Future<T> traits would almost certainly be the first to have positional associated types (PATs) stabilized. But the feature could also be beneficial for some traits with multiple associated types:

pub trait Try: FromResidual<Self::Residual> {
    #[positional]
    type Output;

    #[positional]
    type Residual;

    fn from_output(output: Self::Output) -> Self;
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output>;
}

impl Logger {
    pub fn log_outcome(task: impl Future<impl Try<impl Display, impl Display>>) {
        ...
    }
}

Note this proposal would not immediately include any of the following use cases:

// PATs in trait implementations 
impl Iterator<f32> for MyIterator { }

// Mixing type parameters and PATs
trait MyTrait<T> {
    #[positional]
    type Extra;
}

// Named type parameters
fn get(&self, key: &impl Borrow<Borrowed=str>) { }

Links and related work

• Discussion in the async foundations Zulip stream
• The Fut alias trait
• Rules for positional and keyword arguments to Python functions
• Typescript’s debate over named type parameters

Initial people involved

• Owner: Sam Sartor
• Liaison: Josh Triplett

What happens now?

This issue is part of the lang-team initiative process. Once this issue is filed, a Zulip topic will be opened for discussion, and the lang-team will review open proposals in its weekly triage meetings. You should receive feedback within a week or two.

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[nikomatsakis]

While I have felt the pain of wishing for a shorthand like Iterator<u32>, I am concerned that this change is going to make the distinction between associated types and generic parameters harder for people to understand. I would like to think it’s possible for us to empirically evaluate this to some degree, though I also do feel that prototyping and using it for a while is a good way to gain experience.

(That said, I am also skeptical of the motivation, or at least one motivation I have heard: specifically, I don’t think that having generators have a return type like -> impl Iterator<Item = u32> is necessarily a good idea, given the precedent set by async fn (and, in fact, generators share the property that they capture all their arguments, which to me was a crucial reason for async fn to have the type they do).)

I don’t see this concern as blocking exploration, but I wanted to note it down, and I would expect to spend some time discussing it before we moved to any sort of RFC here.

[pnkfelix]

Summary of concerns gathered from lang team meeting on 2021-11-16 and from the zulip thread:

• sem. ver issue: supporting PAT on a trait (via the given syntaxTrait<Assoc>) implies it is a breaking change to ever add a defaulted generic param.
  • Concern: So, even though PAT’s are opt-in, there is a huge back compat promise. How much use is this expected to get, and does that justify a language change (as opposed to some alternative solution; some are outlined below)
• the proposal says it is unclear to incoming Rust programmers why they have to write Iterator<Item=f32> instead of Iterator<f32>, and attempts to address that by unifying the two syntaxes.
  • Concern: We should not assume that conflating two different things, or hiding the difference between them, improves learning curves.
  • Counterpoint: Forcing people to pay attention to a difference like this is not necessarily a win either.
• Concern (admittedly vague): in future scenarios, the distinction between generics and associated types may become important in new ways.

However, it is important to stress that that this is a proposal to solve a problem, not a commitment to a specific solution. The syntax could change, which might address some of the concerns above. In that spirit, here is a list of alternative solutions suggested in Zulip thread:

• trait aliases: def’n site: trait Fut<T> = Future<Output=T>;; use site: fn foo() -> impl Fut<u32>
• name elision for sole associated type without default: use site: fn foo() -> impl Future<_=u32> (no change to def’n site, it remains trait Future { type Output; }).
• output type in input position of trait definition: def’n site: trait Future<O == Self::Output> { type Output; }; use site: fn foo() -> impl Future<u32>