A set of traits and proc-macros involved in making and using generators to create AsyncIterators, Futures and Iterators

Asynchronous example

// a couple required nightly features
#![feature(generators, generator_trait, never_type, into_future, async_iterator)]

use jenner::generator;
use std::async_iter::AsyncIterator;
use std::time::{Instant, Duration};

/// Creates a stream that yields u32s that countdown from 5 to 0.
/// Waiting 0.2s between each (1s total)
#[generator]
#[yields(u32)]
async fn countdown() {
    yield 5;
    for i in (0..5).rev() {
        tokio::time::sleep(Duration::from_millis(200)).await;
        yield i;
    }
}

/// Iterates over the provided stream, printing the value and
/// pushing it to a vec that is returned
#[generator]
async fn collect(input: impl AsyncIterator<Item = u32>) -> Vec<u32> {
    let mut v = vec![];

    for i in input {
        println!("{:?}", i);
        v.push(i)
    }.await; // special syntax to consume a stream

    v
}

#[tokio::main]
async fn main() {
    let start = Instant::now();

    // countdown() is a valid stream
    // collect(...) is a valid future
    let v = collect(countdown()).await;
    assert_eq!(v, vec![5, 4, 3, 2, 1, 0]);

    assert!(start.elapsed() > Duration::from_millis(200 * 5));
}

Synchronous example

#![feature(generators)]

use jenner::generator;

#[generator]
#[yields(usize)]
fn fibonacii() {
    use std::mem;

    let mut a = 0;
    let mut b = 1;
    loop {
        yield a;

        mem::swap(&mut a, &mut b);
        b += a;
    }
}

fn main() {
    // fibonacii() is a valid `Iterator<Item = usize>`
    let v: Vec<_> = fibonacii().take(10).collect();
    assert_eq!(v, vec![0, 1, 1, 2, 3, 5, 8, 13, 21, 34]);
}