pub fn unfold<T, F, Fut, It>(init: T, f: F) -> Unfold<T, F, Fut>
Expand description
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 Ok(Async::Ready(None))
in future
calls to poll()
.
In case of error generated by the returned Future
, the error will be
returned by the Stream
. The Stream
will then yield
Ok(Async::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::prelude::*;
use futures::stream;
use futures::future;
use futures_executor::block_on;
let mut stream = stream::unfold(0, |state| {
if state <= 2 {
let next_state = state + 1;
let yielded = state * 2;
let fut = future::ok::<_, u32>((yielded, next_state));
Some(fut)
} else {
None
}
});
let result = block_on(stream.collect());
assert_eq!(result, Ok(vec![0, 2, 4]));