Function unfold 

pub fn unfold<T, F, R, Item, E>(init: T, function: F) -> Unfold<T, F, R>
where
    F: FnMut(T, Item) -> R,
    R: Future<Output = Result<T, E>>,

Create a sink from a function which processes one item at a time.

Examples

use core::pin::pin;

use futures::sink;
use futures::sink::SinkExt;

let unfold = sink::unfold(0, |mut sum, i: i32| {
    async move {
        sum += i;
        eprintln!("{}", i);
        Ok::<_, futures::never::Never>(sum)
    }
});
let mut unfold = pin!(unfold);
unfold.send(5).await?;