# Function futures_util::stream::unfold [−] [src]

    pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut> where
        F: FnMut(T) -> Fut,
        Fut: Future<Output = Option<(Item, T)>>,

Creates a `Stream` from a seed and a closure returning a `Future`.

This function is the dual for the `Stream::fold()` adapter: while `Stream::fold()` reduces a `Stream` to one single value, `unfold()` creates a `Stream` from a seed value.

`unfold()` will call the provided closure with the provided seed, then wait for the returned `Future` to complete with `(a, b)`. It will then yield the value `a`, and use `b` as the next internal state.

If the closure returns `None` instead of `Some(Future)`, then the `unfold()` will stop producing items and return `Poll::Ready(None)` in future calls to `poll()`.

This function can typically be used when wanting to go from the “world of futures” to the “world of streams”: the provided closure can build a `Future` using other library functions working on futures, and `unfold()` will turn it into a `Stream` by repeating the operation.

# Example

    use futures::stream::{self, StreamExt};

    let stream = stream::unfold(0, |state| async move {
        if state <= 2 {
            let next_state = state + 1;
            let yielded = state  * 2;
            Some((yielded, next_state))
        } else {
            None
        }
    });

    let result = stream.collect::<Vec<i32>>().await;
    assert_eq!(result, vec![0, 2, 4]);