Return error from async function to caller of synchronous function [duplicate]

Question

I am learning Tokio/futures and can't find a way to return the error up to the caller in the main function; is this possible?

My use case is for AWS Lambda with the runtime being synchronous and would like to return any errors from the async parts.

use futures::Future; // 0.1.26 use reqwest::r#async::Client; // 0.9.14 use reqwest::Error; // 0.1.18 use serde::Deserialize; use tokio; fn main() { let call = synchronous_function(); if let Err(e) = call { println!("{:?}", e); } } fn synchronous_function() -> Result<(), Error> { let fut = async_function() .and_then(|res| { println!("{:?}", res); Ok(()) }) .map_err(|_| ()); tokio::run(fut); Ok(()) } fn async_function() -> impl Future<Item = Json, Error = Error> { let client = Client::new(); client .get("https://jsonplaceholder.typicode.com/todos/1") .send() .map_err(Into::into) .and_then(|mut res| res.json().and_then(|j| Ok(j))) } #[derive(Debug, Deserialize)] struct Json { userId: u16, id: u16, title: String, completed: bool, }

Shepmaster · Accepted Answer · 2019-05-13 01:51:59Z

If you want to "return" something from tokio::run, you can use a communications channel that will transport your result back to the main thread, like so:

fn run_sync<T, E, F>(fut: F) -> Result<T, E> where T: Debug + Send + 'static, E: Debug + Send + 'static, F: Future<Item = T, Error = E> + Send + 'static, { let (sender, receiver) = tokio::sync::oneshot::channel(); let exec = fut.then(move |result| { sender.send(result).expect("Unable to send result"); Ok(()) }); tokio::run(exec); receiver.wait().expect("Receive error") }

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