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]));