async
/await
At a high level, async Rust code looks very much like "normal" sequential code:
use futures::executor::block_on; async fn count_to(count: i32) { for i in 1..=count { println!("Count is: {i}!"); } } async fn async_main(count: i32) { count_to(count).await; } fn main() { block_on(async_main(10)); }
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-
Note that this is a simplified example to show the syntax. There is no long running operation or any real concurrency in it!
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What is the return type of an async call?
- Use
let future: () = async_main(10);
inmain
to see the type.
- Use
-
The "async" keyword is syntactic sugar. The compiler replaces the return type with a future.
-
You cannot make
main
async, without additional instructions to the compiler on how to use the returned future. -
You need an executor to run async code.
block_on
blocks the current thread until the provided future has run to completion. -
.await
asynchronously waits for the completion of another operation. Unlikeblock_on
,.await
doesn't block the current thread. -
.await
can only be used inside anasync
function (or block; these are introduced later).