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> In fact, I think that the current state of async Rust is the best implementation of async in any language. Hahahaha hard disagree. Last year I implemented th

> In fact, I think that the current state of async Rust is the best implementation of async in any language. Hahahaha hard disagree. Last year I implemented the braid protocol (a custom streaming protocol using HTTP) in javascript in less than an hour and about 30 lines of code. Then I spent 2 weeks trying to do the same thing in rust - writing hundreds of lines of code in the process and I couldn't get it to work. Eventually I gave up. I got it working recently - but only by borrowing some wild tricks from reading the source code of tokio, that I never would have thought of on my own. > To get Pin stuff out of the way: it is indeed more complicated than it could be (because reverse compatibility etc), but when was the last time you needed to write a poll implementation manually? Last week, while writing a simple networked database application. Again I needed to produce an async stream, and thats impossible using async fn.

dgroshev 2y ago
> Then I spent 2 weeks trying to do the same thing in rust… Eventually I gave up. In my experience, that kind of difference boils down to a combination of three things. - Comparing apples and oranges. For example, Box makes pinning trivial (you can just move in and out of Pin no problem), but oftentimes people new to Rust try to prematurely optimise and eliminate a single pointer lookup. If that's the case, were you really writing the same thing in JS and in Rust? - An extension to the previous point, the behaviour is usually different. What would happen in your JS implementation if two streams were awaited concurrently, one received a message, and the other had to be cancelled? What if one threw an exception? In Rust, you're forced to think about those things from the start. In JS, you're coding the happy path. - Trying to reproduce the exact same architecture even if it's awkward of inefficient. For example, it's really really easy to use a stream wrapper [1] to produce a stream from a channel, but then the architecture gets very different. > Again I needed to produce an async stream, and thats impossible using async fn I strongly recommend a channel instead. There's also async_stream [2], but channels are simpler and cleaner. Over two years of writing embedded, web, and CLI rust I didn't have to write a raw future once. [1] https://docs.rs/tokio-stream/latest/tokio_stream/wrappers/index.html https://docs.rs/tokio-stream/latest/tokio_stream/wrappers/in... [2] https://docs.rs/async-stream/latest/async_stream/ https://docs.rs/async-stream/latest/async_stream/
- josephg 2y ago
  > - Comparing apples and oranges. For example, Box makes pinning trivial (you can just move in and out of Pin no problem), but oftentimes people new to Rust try to prematurely optimise and eliminate a single pointer lookup. If that's the case, were you really writing the same thing in JS and in Rust? Pointer lookups are cheap-ish, but allocating can be extremely expensive if you do it everywhere. I've seen plenty of lazy, allocation & clone heavy rust code end up running much slower than the equivalent javascript. I assume for this reason. But in this case, I couldn't get it working even when putting Box<> all over the place. > What would happen in your JS implementation if two streams were awaited concurrently, one received a message, and the other had to be cancelled? What if one threw an exception? In Rust, you're forced to think about those things from the start. In JS, you're coding the happy path. I implemented error handling in the javascript code. That was easy - since async generators in javascript support try-catch. Javascript doesn't support concurrent execution - so that problem doesn't exist there. Did multithreading contribute to javascript being easier to write than rust? Who cares? I had a problem to solve, and javascript made that trivial. Rust made it a total nightmare. I didn't know about the stream wrappers when I started coding this up. That was how I eventually found an the answer to this problem: I read that code then adapted their approach. And by the way, have you read the code in those wrappers? Its wild how they glue manual Future implementations and async functions together (with some clever Boxes) to make it work. It blew my mind how complex this code needs to be in order for it to work at all. > Over two years of writing embedded, web, and CLI rust I didn't have to write a raw future once. I'm happy for you, and I wish I had the same experience. Streams are bread and butter for my work (CRDTs, distributed systems and collaborative editing). And at this rate? Proper support for streams in rust is probably a decade away.
  - dgroshev 2y ago
    Every single JS future is boxed. Moreover, they aren't just boxed, they are often backed by a hashmap (which may or may not be optimised away by the JIT). Elaborate allocation-free async is not an apple-to-apples comparison. JS does support concurrent execution, Promise.all is an example. Without it, JS async would make little sense. The problem very much exists there, and try-catch is only a surface-level answer. As you can see here [1], the interaction of cancellation and async in JS is at least just as (or more) complex than in Rust. By the way, multithreading has little to do with Pin. I presume you're thinking of Send bounds. "To work at all" is very dismissive. Those wrappers are complex, but very well abstracted, well defined, and robust, the complexity is essential. Again, look at [1], JS async is hardly less complex, but also much more vague and ill-defined. [1]: https://github.com/whatwg/streams/issues/1255 https://github.com/whatwg/streams/issues/1255

dgroshev 2y ago

> Then I spent 2 weeks trying to do the same thing in rust… Eventually I gave up. In my experience, that kind of difference boils down to a combination of three things. - Comparing apples and oranges. For example, Box makes pinning trivial (you can just move in and out of Pin no problem), but oftentimes people new to Rust try to prematurely optimise and eliminate a single pointer lookup. If that's the case, were you really writing the same thing in JS and in Rust? - An extension to the previous point, the behaviour is usually different. What would happen in your JS implementation if two streams were awaited concurrently, one received a message, and the other had to be cancelled? What if one threw an exception? In Rust, you're forced to think about those things from the start. In JS, you're coding the happy path. - Trying to reproduce the exact same architecture even if it's awkward of inefficient. For example, it's really really easy to use a stream wrapper [1] to produce a stream from a channel, but then the architecture gets very different. > Again I needed to produce an async stream, and thats impossible using async fn I strongly recommend a channel instead. There's also async_stream [2], but channels are simpler and cleaner. Over two years of writing embedded, web, and CLI rust I didn't have to write a raw future once. [1] https://docs.rs/tokio-stream/latest/tokio_stream/wrappers/index.html https://docs.rs/tokio-stream/latest/tokio_stream/wrappers/in... [2] https://docs.rs/async-stream/latest/async_stream/ https://docs.rs/async-stream/latest/async_stream/

josephg 2y ago
> - Comparing apples and oranges. For example, Box makes pinning trivial (you can just move in and out of Pin no problem), but oftentimes people new to Rust try to prematurely optimise and eliminate a single pointer lookup. If that's the case, were you really writing the same thing in JS and in Rust? Pointer lookups are cheap-ish, but allocating can be extremely expensive if you do it everywhere. I've seen plenty of lazy, allocation & clone heavy rust code end up running much slower than the equivalent javascript. I assume for this reason. But in this case, I couldn't get it working even when putting Box<> all over the place. > What would happen in your JS implementation if two streams were awaited concurrently, one received a message, and the other had to be cancelled? What if one threw an exception? In Rust, you're forced to think about those things from the start. In JS, you're coding the happy path. I implemented error handling in the javascript code. That was easy - since async generators in javascript support try-catch. Javascript doesn't support concurrent execution - so that problem doesn't exist there. Did multithreading contribute to javascript being easier to write than rust? Who cares? I had a problem to solve, and javascript made that trivial. Rust made it a total nightmare. I didn't know about the stream wrappers when I started coding this up. That was how I eventually found an the answer to this problem: I read that code then adapted their approach. And by the way, have you read the code in those wrappers? Its wild how they glue manual Future implementations and async functions together (with some clever Boxes) to make it work. It blew my mind how complex this code needs to be in order for it to work at all. > Over two years of writing embedded, web, and CLI rust I didn't have to write a raw future once. I'm happy for you, and I wish I had the same experience. Streams are bread and butter for my work (CRDTs, distributed systems and collaborative editing). And at this rate? Proper support for streams in rust is probably a decade away.
- dgroshev 2y ago
  Every single JS future is boxed. Moreover, they aren't just boxed, they are often backed by a hashmap (which may or may not be optimised away by the JIT). Elaborate allocation-free async is not an apple-to-apples comparison. JS does support concurrent execution, Promise.all is an example. Without it, JS async would make little sense. The problem very much exists there, and try-catch is only a surface-level answer. As you can see here [1], the interaction of cancellation and async in JS is at least just as (or more) complex than in Rust. By the way, multithreading has little to do with Pin. I presume you're thinking of Send bounds. "To work at all" is very dismissive. Those wrappers are complex, but very well abstracted, well defined, and robust, the complexity is essential. Again, look at [1], JS async is hardly less complex, but also much more vague and ill-defined. [1]: https://github.com/whatwg/streams/issues/1255 https://github.com/whatwg/streams/issues/1255