Block_on error in rust1.39

help
1

block_on(learn_and_sing);
error[E0425]: cannot find function block_on in this scope
why?

2

Did you import block_on? Can you post a bit more code?

3

@CryZe Yes.

use std::futures::executor::block_on;
^^^^^^^ could not find futures in std
......
async fn learn_and_sing() {
learn_sone().await;
sing_song().await;
}
......
fn main() {
block_on(learn_and_sing());
^^^^^^^^ not found in this scope
}

Demo I saw in Asynchronous Programming in Rust。 Thank you。

4

I don't think block_on is in std (yet). You need to import that from the futures crate. So you can probably just remove the std:: from there.

5

Roughly speaking, the standard library has only an interface for Futures, but no implementation. You still need (and probably will always need) a 3rd party crate to implement the futures.

Rust the language only knows how to create futures, which by themselves don't do anything. You need a runtime like async-std or tokio to run the futures. That's not a bug or missing feature, but an intentional flexibility of the design, because there are different ways of executing futures with different trade-offs.

Also note that std::future has just been released very recently, so very few libraries (mostly alpha versions) have support for them.

6

Rust the language doesn't even really know how to create futures (other than trivial ones that complete instantly). What the async/await syntax gives is the ability to compose futures.

You need to bring in both a source of leaf-level non-trivial futures (e.g. timers, IO) and something that can run them; async-std and tokio both provide both of these things, but there are other crates like futures-intrusive that just provide leaf-futures which can run on any runtime.

7

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