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Curiously, will there be limitations to where it can be used? Eg, imagine a `foos.iter().map(|f| f.await).collect::<Vec<_>>()`? That seems crazy, think it'll b

Curiously, will there be limitations to where it can be used? Eg, imagine a `foos.iter().map(|f| f.await).collect::<Vec<_>>()`? That seems crazy, think it'll be possible?

comex 7y ago
Not as things are currently designed. This is similar to how you can’t use things like the `?` operator, break, return, etc. inside a lambda and expect it to affect the outer function: it doesn’t work because lambdas are treated as their own functions. Personally I think it would be cool to pursue an extension to lambdas that would allow some of those things to work, but I’m not a Rust team member or anything, just an interested observer.
- swsieber 7y ago
  Technically, `?` can work in lambda's if the return value is a `Result` - though it won't work like it's being called as part of a normal loop or whatever. That's largely mitigated by the combinators available on an interator of `Result`s. So I think we'll just need similar tooling for lambdas - perhaps an `async` modifier for them? That (I think) would lift await stuff up to `?` in terms of lambda support.
  - comex 7y ago
    Yep, and in fact async lambdas are already a thing on nightly. But, while I may just be nitpicking, `.map(async |f| f.await)` wouldn't do anything useful. Applied to an Iterator of Futures, it would be a no-op, kind of like (since you mentioned `?`) `.map(|x| Ok(x?)). Instead you'd probably want some combinator to turn it into whatever "Iterator but async" trait Rust eventually standardizes on – futures-rs has Stream for this purpose, but std doesn't have anything yet. That trait would have its own collect() method which would return a Future<Output=Vec<T>>, and you'd then await on that. Yeah, I guess I'm nitpicking.
    - swsieber 7y ago
      Thanks for the nitpick! I hadn't thought it through all the way. I was mostly thinking about doing async stuff with the inner stuff, but you're definitely right. I'm glad you commented because I've had to rethink it.

Curiously, will there be limitations to where it can be used? Eg, imagine a `foos.iter().map(|f| f.await).collect::<Vec<_>>()`? That seems crazy, think it'll be possible?

comex 7y ago

Not as things are currently designed. This is similar to how you can’t use things like the `?` operator, break, return, etc. inside a lambda and expect it to affect the outer function: it doesn’t work because lambdas are treated as their own functions. Personally I think it would be cool to pursue an extension to lambdas that would allow some of those things to work, but I’m not a Rust team member or anything, just an interested observer.

swsieber 7y ago
Technically, `?` can work in lambda's if the return value is a `Result` - though it won't work like it's being called as part of a normal loop or whatever. That's largely mitigated by the combinators available on an interator of `Result`s. So I think we'll just need similar tooling for lambdas - perhaps an `async` modifier for them? That (I think) would lift await stuff up to `?` in terms of lambda support.
- comex 7y ago
  Yep, and in fact async lambdas are already a thing on nightly. But, while I may just be nitpicking, `.map(async |f| f.await)` wouldn't do anything useful. Applied to an Iterator of Futures, it would be a no-op, kind of like (since you mentioned `?`) `.map(|x| Ok(x?)). Instead you'd probably want some combinator to turn it into whatever "Iterator but async" trait Rust eventually standardizes on – futures-rs has Stream for this purpose, but std doesn't have anything yet. That trait would have its own collect() method which would return a Future<Output=Vec<T>>, and you'd then await on that. Yeah, I guess I'm nitpicking.
  - swsieber 7y ago
    Thanks for the nitpick! I hadn't thought it through all the way. I was mostly thinking about doing async stuff with the inner stuff, but you're definitely right. I'm glad you commented because I've had to rethink it.