use futures::{prelude::*, future, stream};
use tokio::timer::Delay;
use std::time::{Duration, Instant};

fn upload(val: u32) -> impl Future<Item = u32, Error = &'static str> {
    println!("uploading {}", val);

    // simulating 100ms upload time
    Delay::new(Instant::now() + Duration::from_millis(100))
        .map_err(|_| "delay failed")
        .and_then(move |_| {
            if val == 20 {
                future::err("val failed")
            } else {
                future::ok(val)
            }
        })
}

fn do_stuffs(vals: impl Iterator<Item = u32>) -> impl Future<Item = (), Error = &'static str> {
    stream::iter_ok(vals)
        .and_then(upload)
        .for_each(|val| {
            println!("Done uploading {}", val);
            Ok(())
        })
}

fn main() {
    let uploads = do_stuffs(0..42);

    tokio::run(uploads);
}