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//! Definition of the `SelectOk` combinator, finding the first successful future
//! in a list.
use std::mem;
use std::prelude::v1::*;
use {Future, IntoFuture, Poll, Async};
/// Future for the `select_ok` combinator, waiting for one of any of a list of
/// futures to successfully complete. Unlike `select_all`, this future ignores all
/// but the last error, if there are any.
///
/// This is created by the `select_ok` function.
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
pub struct SelectOk<A> where A: Future {
inner: Vec<A>,
}
/// Creates a new future which will select the first successful future over a list of futures.
///
/// The returned future will wait for any future within `iter` to be ready and Ok. Unlike
/// `select_all`, this will only return the first successful completion, or the last
/// failure. This is useful in contexts where any success is desired and failures
/// are ignored, unless all the futures fail.
///
/// # Panics
///
/// This function will panic if the iterator specified contains no items.
pub fn select_ok<I>(iter: I) -> SelectOk<<I::Item as IntoFuture>::Future>
where I: IntoIterator,
I::Item: IntoFuture,
{
let ret = SelectOk {
inner: iter.into_iter()
.map(|a| a.into_future())
.collect(),
};
assert!(ret.inner.len() > 0);
ret
}
impl<A> Future for SelectOk<A> where A: Future {
type Item = (A::Item, Vec<A>);
type Error = A::Error;
fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
// loop until we've either exhausted all errors, a success was hit, or nothing is ready
loop {
let item = self.inner.iter_mut().enumerate().filter_map(|(i, f)| {
match f.poll() {
Ok(Async::NotReady) => None,
Ok(Async::Ready(e)) => Some((i, Ok(e))),
Err(e) => Some((i, Err(e))),
}
}).next();
match item {
Some((idx, res)) => {
// always remove Ok or Err, if it's not the last Err continue looping
drop(self.inner.remove(idx));
match res {
Ok(e) => {
let rest = mem::replace(&mut self.inner, Vec::new());
return Ok(Async::Ready((e, rest)))
},
Err(e) => {
if self.inner.is_empty() {
return Err(e)
}
},
}
}
None => {
// based on the filter above, nothing is ready, return
return Ok(Async::NotReady)
},
}
}
}
}