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Thiago Massari Guedes
Thiago Massari Guedes

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Quick tip: Type of tokio spawn return

#rust #tokio #programming

When I was implementing the metrics task using tokio, I wanted to save the result JoinHandle in a struct and I saw the type being displayed by the IDE: JoinHandle<?>

What does it mean?

When I looked the definition of the function spawn, that's the code:

pub fn spawn<F>(future: F) -> JoinHandle<F::Output> where F: Future + Send + 'static, F::Output: Send + 'static, { // ...
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In the Future trait, F::Output refers to the return value of the input function.

Now, the piece of code I have is a long running task. That is the piece of code.

let receiver_task = tokio::spawn(async move { println!("Starting metrics receiver"); while let Some(event) = rx.recv().await { if let Err(e) = metrics.add(&event.post_name, &event.origin) { error!("Error writing access metric for {}: {}", &event.post_name, e); } else { debug!("Metric event written for {}", &event.post_name); } } });
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As this function returns nothing, the type is Unit, hence for this lambda I can declare my struct as:

pub struct MetricHandler { receiver_task: JoinHandle<()>, //... }
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And now you can do:

let receiver_task = tokio::spawn(async move { // ... MetricHandler { receiver_task, // ... }
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