December 06
On Thursday, 26 November 2020 at 14:18:55 UTC, Jacob Carlborg wrote:
> On Thursday, 26 November 2020 at 09:49:51 UTC, Ola Fosheim Grostad wrote:
>
>> Isnt await just syntactic sugar for a promise/future?
>
> No, not really, but it's used together with promises or they're used under the hood.
>
>> If you can provide an example of what await should expand into then I might be able to add it to my experimental D compiler and you could then write a DIP.
>
> Async/await is basically a coroutine. It allows a function to be suspended and resumed. Here's an example of how the code could look like in D:
>
> async int foo()
> {
>     await sleep(1.seconds); // simulate work that takes long time
>     writeln("foo");
>     return 1;
> }
>
> async int bar()
> {
>     writeln("bar");
>     return 2;
> }
>
> async void main()
> {
>     auto a = foo();
>     auto b = bar();
>
>     writeln("main");
>     writeln(await a + await b);
> }
>
> The output of the above would be:
>
> bar
> main
> foo
> 3
>
> The control flow is something like this:
>
> 1. `foo` is called
> 2. `foo` is awaiting for `sleep`
> 3. `sleep` returns a promise which is not fulfilled
> 4. This causes `foo` to suspend (return) and control comes back to `main`
> 5. `bar` is called
> 6. `bar` has no suspension points (awaits) so this is more or less a synchronous function
> 7. "bar" is printed
> 8. `bar` returns a fulfilled promise with the value `2`
> 9. Control comes back to `main`
> 10. "main" is printed
> 11. `main` awaits `a`. If the `a` promise is not fulfilled at this point `main` is suspended
> 12. `foo` is resumed at some point in the future
> 13. "foo" is printed
> 14. `foo` returns a fulfilled promise with the value `1`
> 15. `main` is now resumed since `a` has been fulfilled
> 16. `main` awaits `b`
> 17. `b` is already fulfilled, which means `main` doesn't have to suspend
> 18. The addition is performed
> 19. "3" is printed
>
> I'm not entirely sure on when `foo` will be resumed. Probably decided by a scheduler in the runtime.
>
> To have the full experience of async/await, an event loop in the runtime is necessary and non-blocking IO.
>
> When a function is suspended, all its state (stack and registers) need to be saved. When it's later resumed, the state need to be restored. I'm not entirely sure how async/await functions are different from fibers, which already exists in the D runtime.
>
> --
> /Jacob Carlborg

Yes! await is very good.
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