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]);