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

    pub fn try_unfold<T, F, Fut, Item>(init: T, f: F) -> TryUnfold<T, F, Fut> where
        F: FnMut(T) -> Fut,
        Fut: TryFuture<Ok = Option<(Item, T)>>,

Creates a `TryStream` from a seed and a closure returning a `TryFuture`.

This function is the dual for the `TryStream::try_fold()` adapter: while `TryStream::try_fold()` reduces a `TryStream` to one single value, `try_unfold()` creates a `TryStream` from a seed value.

`try_unfold()` will call the provided closure with the provided seed, then wait for the returned `TryFuture` 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(TryFuture)`, then the `try_unfold()` will stop producing items and return `Poll::Ready(None)` in future calls to `poll()`.

In case of error generated by the returned `TryFuture`, the error will be returned by the `TryStream`. The `TryStream` will then yield `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 `TryFuture` using other library functions working on futures, and `try_unfold()` will turn it into a `TryStream` by repeating the operation.

# Example

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

    let stream = stream::try_unfold(0, |state| async move {
        if state < 0 {
            return Err(SomeError);
        }

        if state <= 2 {
            let next_state = state + 1;
            let yielded = state * 2;
            Ok(Some((yielded, next_state)))
        } else {
            Ok(None)
        }
    });

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