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Comment by dom96 | Hacker News Reader

It's exciting to see so many systems programming languages implement async/await.

For a further comparison, Nim works in the same way as Zig when it comes to eager evaluation. Something that I'm particularly proud of when it comes to Nim's async/await implementation is that everything, right down to the macro which defines what `await` means, is implemented in the standard library. The compiler only implements the coroutines. This means that the language isn't bloated by this extra feature and makes it much easier for developers to implement their own async/await.

I believe there was talk to do the same in Rust, but for some reason the developers decided to implement it in the compiler instead.

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We did implement it out of tree at first. [0] Because Rust requires annotations on macros to distinguish them from built in syntax, they are less ergonomic to use. In general, we believe in adopting a path of beginning as an out-of-tree macro, and then deciding to elevate that to first class syntax if its clear that they will be extremely commonly used.

[0]: https://github.com/alexcrichton/futures-await

As another comparison, Dart started out delaying execution of async functions and then switched to eager in Dart 2:

https://groups.google.com/forum/#!topic/flutter-dev/3R9qhjNG...

Async blocks require a keyword to be unambiguous with a struct constructor.

Can you clarify what youre referring to with "decided to implement it in the compiler instead", be it from this blog post or elsewhere?

Clearly you need coroutines in the compiler, and the way Rust treats ownership means pinning is needed in some way (I've not been following closely so don't know if this is in libstd or a language-level feature), but the rest (according to this post) seems to be going in libstd so you can write your own if you want.

Pinning is almost entirely a library feature; it does use the unstable “auto traits” functionality of the language, so it’s a bit special in that regard. But it’s provided by the standard library.

> Can you clarify what youre referring to with "decided to implement it in the compiler instead"

I'm referring specifically to the async/await syntax. Coroutines probably need to be implemented in the compiler, sure, but the actual async/await can be implemented with metaprogramming.

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