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//! Definition of the `LoopFn` combinator, implementing `Future` loops.
use {Async, Future, IntoFuture, Poll};
/// The status of a `loop_fn` loop.
#[derive(Debug)]
pub enum Loop<T, S> {
/// Indicates that the loop has completed with output `T`.
Break(T),
/// Indicates that the loop function should be called again with input
/// state `S`.
Continue(S),
}
/// A future implementing a tail-recursive loop.
///
/// Created by the `loop_fn` function.
#[derive(Debug)]
pub struct LoopFn<A, F> where A: IntoFuture {
future: A::Future,
func: F,
}
/// Creates a new future implementing a tail-recursive loop.
///
/// The loop function is immediately called with `initial_state` and should
/// return a value that can be converted to a future. On successful completion,
/// this future should output a `Loop<T, S>` to indicate the status of the
/// loop.
///
/// `Loop::Break(T)` halts the loop and completes the future with output `T`.
///
/// `Loop::Continue(S)` reinvokes the loop function with state `S`. The returned
/// future will be subsequently polled for a new `Loop<T, S>` value.
///
/// # Examples
///
/// ```
/// use futures::future::{ok, loop_fn, Future, FutureResult, Loop};
/// use std::io::Error;
///
/// struct Client {
/// ping_count: u8,
/// }
///
/// impl Client {
/// fn new() -> Self {
/// Client { ping_count: 0 }
/// }
///
/// fn send_ping(self) -> FutureResult<Self, Error> {
/// ok(Client { ping_count: self.ping_count + 1 })
/// }
///
/// fn receive_pong(self) -> FutureResult<(Self, bool), Error> {
/// let done = self.ping_count >= 5;
/// ok((self, done))
/// }
/// }
///
/// let ping_til_done = loop_fn(Client::new(), |client| {
/// client.send_ping()
/// .and_then(|client| client.receive_pong())
/// .and_then(|(client, done)| {
/// if done {
/// Ok(Loop::Break(client))
/// } else {
/// Ok(Loop::Continue(client))
/// }
/// })
/// });
/// ```
pub fn loop_fn<S, T, A, F>(initial_state: S, mut func: F) -> LoopFn<A, F>
where F: FnMut(S) -> A,
A: IntoFuture<Item = Loop<T, S>>,
{
LoopFn {
future: func(initial_state).into_future(),
func: func,
}
}
impl<S, T, A, F> Future for LoopFn<A, F>
where F: FnMut(S) -> A,
A: IntoFuture<Item = Loop<T, S>>,
{
type Item = T;
type Error = A::Error;
fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
loop {
match try_ready!(self.future.poll()) {
Loop::Break(x) => return Ok(Async::Ready(x)),
Loop::Continue(s) => self.future = (self.func)(s).into_future(),
}
}
}
}
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