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Some people (including me) felt how async was added was worrying. Adding ".await" means when I teach Rust we now need to say "x.y is member access. Unless y is

Some people (including me) felt how async was added was worrying. Adding ".await" means when I teach Rust we now need to say "x.y is member access. Unless y is 'await'. Then it's something totally different". That's the type of strange rule that C++ has, and if you gather enough of them, languages become very hard to teach. However, async seems to be the only big language feature which effected many things which has been added.

cedws 5y ago
Yeah, the await syntax is just bizarre. One of the worst eyesores of any languages I've personally encountered. An await macro or function would have sufficed but because Rust is still mainly a hipster language with a hipster community, they had to choose a hipster syntax after years of bikeshedding about how to make it absolutely perfect.
- zozbot234 5y ago
  Prefix syntax has drawbacks that were constantly pointed out in that long discussion. In particular, it doesn't compose well in practical scenarios. .await just 'flows' better and in a more intuituve way.
  - Gwypaas 5y ago
    Completely agree. I've used it at work now for about a year and the postfix notation flows perfectly with more intricate usages of futures. Especially with the sense that you construct one, then decide when to await it, or await a collection or whatever. Following the rust style of immutable variables and chaining function calls. You can even await the same future multiple times in a select statement keeping it executing a bit further each time until it returns, if you do it by mutable reference. Really quite magic. Now we just need async closures and some nice async style iterators. Something like FuturesOrdered or even FuturesUnordered in an inline iterator style allowing efficient nice composability. Without any of the pitfalls and gotchas those two currently have. I think that might be what throws many people off? The regular style of method chaining and having the cold futures just be a dead piece of memory you can pass around until actually passed to the runtime?
- steveklabnik 5y ago
  > An await macro or function would have sufficed We tried, and it literally did not suffice.
  - c7DJTLrn 5y ago
    Works in other languages fine.
    - steveklabnik 5y ago
      Different languages have different semantics. The challenges here are related to the ways Rust works specifically, as well as its goals.
zozbot234 5y ago
.await is a postfix keyword - it effectively means "crazy control-flow (monadic) magic is happening here". "?" for failure handling is another bit of postfix syntax, that means something very similar. A good IDE will show it with a different highlight, so that it will never be confused for a struct member or method call. This stuff was discussed by the community for a long time - bikeshedding about syntax is a well-known "trap" in programming language design. It has turned out to be (IMHO) reasonably intuitive, and preferable to the alternatives.
- dgellow 5y ago
  Isn't "?" just a sugar for something like the following? That's how I think about it in my mind at least. // with "?" let v = x?; // without "?" let v = match x { Ok(o) => Ok(o), Err(e) => return Err(e), // exit current scope due to the return }.unwrap(); That feels like something that could be done with a macro.
  - zozbot234 5y ago
    Yes, that's control flow magic. It was prototyped with a macro, but failure handling comes up all the time in practical programming, and it's important to make it as ergonomic as possible.
    - dgellow 5y ago
      I see. I think reading your initial comment I took issue with the hyperbole "crazy control-flow (monadic) magic is happening here". But yes it is a hidden control flow, and taking in account hidden control flows is inherently more complicated (or at least has a steeper learning curve).
  - estebank 5y ago
    It's more like let v = match x { Ok(o) => o, Err(e) => return Err(e.into()), }; > That feels like something that could be done with a macro. Indeed. It used to be a macro called try!.
dgellow 5y ago
That's a fair point. I haven't touched async yet but I've seen lot of discussions regarding "await" and that seems to create a good amount of confusion. Still we are multiple magnitude away from the level of complexity C++ has.

cedws 5y ago

Yeah, the await syntax is just bizarre. One of the worst eyesores of any languages I've personally encountered. An await macro or function would have sufficed but because Rust is still mainly a hipster language with a hipster community, they had to choose a hipster syntax after years of bikeshedding about how to make it absolutely perfect.

zozbot234 5y ago
Prefix syntax has drawbacks that were constantly pointed out in that long discussion. In particular, it doesn't compose well in practical scenarios. .await just 'flows' better and in a more intuituve way.
- Gwypaas 5y ago
  Completely agree. I've used it at work now for about a year and the postfix notation flows perfectly with more intricate usages of futures. Especially with the sense that you construct one, then decide when to await it, or await a collection or whatever. Following the rust style of immutable variables and chaining function calls. You can even await the same future multiple times in a select statement keeping it executing a bit further each time until it returns, if you do it by mutable reference. Really quite magic. Now we just need async closures and some nice async style iterators. Something like FuturesOrdered or even FuturesUnordered in an inline iterator style allowing efficient nice composability. Without any of the pitfalls and gotchas those two currently have. I think that might be what throws many people off? The regular style of method chaining and having the cold futures just be a dead piece of memory you can pass around until actually passed to the runtime?
steveklabnik 5y ago
> An await macro or function would have sufficed We tried, and it literally did not suffice.
- c7DJTLrn 5y ago
  Works in other languages fine.
  - steveklabnik 5y ago
    Different languages have different semantics. The challenges here are related to the ways Rust works specifically, as well as its goals.

zozbot234 5y ago

.await is a postfix keyword - it effectively means "crazy control-flow (monadic) magic is happening here". "?" for failure handling is another bit of postfix syntax, that means something very similar. A good IDE will show it with a different highlight, so that it will never be confused for a struct member or method call. This stuff was discussed by the community for a long time - bikeshedding about syntax is a well-known "trap" in programming language design. It has turned out to be (IMHO) reasonably intuitive, and preferable to the alternatives.

dgellow 5y ago
Isn't "?" just a sugar for something like the following? That's how I think about it in my mind at least. // with "?" let v = x?; // without "?" let v = match x { Ok(o) => Ok(o), Err(e) => return Err(e), // exit current scope due to the return }.unwrap(); That feels like something that could be done with a macro.
- zozbot234 5y ago
  Yes, that's control flow magic. It was prototyped with a macro, but failure handling comes up all the time in practical programming, and it's important to make it as ergonomic as possible.
  - dgellow 5y ago
    I see. I think reading your initial comment I took issue with the hyperbole "crazy control-flow (monadic) magic is happening here". But yes it is a hidden control flow, and taking in account hidden control flows is inherently more complicated (or at least has a steeper learning curve).
- estebank 5y ago
  It's more like let v = match x { Ok(o) => o, Err(e) => return Err(e.into()), }; > That feels like something that could be done with a macro. Indeed. It used to be a macro called try!.

dgellow 5y ago

That's a fair point. I haven't touched async yet but I've seen lot of discussions regarding "await" and that seems to create a good amount of confusion. Still we are multiple magnitude away from the level of complexity C++ has.