1use super::assert_stream;
2use core::fmt;
3use core::pin::Pin;
4use futures_core::future::TryFuture;
5use futures_core::ready;
6use futures_core::stream::Stream;
7use futures_core::task::{Context, Poll};
8use pin_project_lite::pin_project;
9
10/// Creates a `TryStream` from a seed and a closure returning a `TryFuture`.
11///
12/// This function is the dual for the `TryStream::try_fold()` adapter: while
13/// `TryStream::try_fold()` reduces a `TryStream` to one single value,
14/// `try_unfold()` creates a `TryStream` from a seed value.
15///
16/// `try_unfold()` will call the provided closure with the provided seed, then
17/// wait for the returned `TryFuture` to complete with `(a, b)`. It will then
18/// yield the value `a`, and use `b` as the next internal state.
19///
20/// If the closure returns `None` instead of `Some(TryFuture)`, then the
21/// `try_unfold()` will stop producing items and return `Poll::Ready(None)` in
22/// future calls to `poll()`.
23///
24/// In case of error generated by the returned `TryFuture`, the error will be
25/// returned by the `TryStream`. The `TryStream` will then yield
26/// `Poll::Ready(None)` in future calls to `poll()`.
27///
28/// This function can typically be used when wanting to go from the "world of
29/// futures" to the "world of streams": the provided closure can build a
30/// `TryFuture` using other library functions working on futures, and
31/// `try_unfold()` will turn it into a `TryStream` by repeating the operation.
32///
33/// # Example
34///
35/// ```
36/// # #[derive(Debug, PartialEq)]
37/// # struct SomeError;
38/// # futures::executor::block_on(async {
39/// use futures::stream::{self, TryStreamExt};
40///
41/// let stream = stream::try_unfold(0, |state| async move {
42/// if state < 0 {
43/// return Err(SomeError);
44/// }
45///
46/// if state <= 2 {
47/// let next_state = state + 1;
48/// let yielded = state * 2;
49/// Ok(Some((yielded, next_state)))
50/// } else {
51/// Ok(None)
52/// }
53/// });
54///
55/// let result: Result<Vec<i32>, _> = stream.try_collect().await;
56/// assert_eq!(result, Ok(vec![0, 2, 4]));
57/// # });
58/// ```
59pub fn try_unfold<T, F, Fut, Item>(init: T, f: F) -> TryUnfold<T, F, Fut>
60where
61 F: FnMut(T) -> Fut,
62 Fut: TryFuture<Ok = Option<(Item, T)>>,
63{
64 assert_stream::<Result<Item, Fut::Error>, _>(TryUnfold { f, state: Some(init), fut: None })
65}
66
67pin_project! {
68 /// Stream for the [`try_unfold`] function.
69 #[must_use = "streams do nothing unless polled"]
70 pub struct TryUnfold<T, F, Fut> {
71 f: F,
72 state: Option<T>,
73 #[pin]
74 fut: Option<Fut>,
75 }
76}
77
78impl<T, F, Fut> fmt::Debug for TryUnfold<T, F, Fut>
79where
80 T: fmt::Debug,
81 Fut: fmt::Debug,
82{
83 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
84 f.debug_struct("TryUnfold").field("state", &self.state).field("fut", &self.fut).finish()
85 }
86}
87
88impl<T, F, Fut, Item> Stream for TryUnfold<T, F, Fut>
89where
90 F: FnMut(T) -> Fut,
91 Fut: TryFuture<Ok = Option<(Item, T)>>,
92{
93 type Item = Result<Item, Fut::Error>;
94
95 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
96 let mut this = self.project();
97
98 if let Some(state) = this.state.take() {
99 this.fut.set(Some((this.f)(state)));
100 }
101
102 match this.fut.as_mut().as_pin_mut() {
103 None => {
104 // The future previously errored
105 Poll::Ready(None)
106 }
107 Some(future) => {
108 let step = ready!(future.try_poll(cx));
109 this.fut.set(None);
110
111 match step {
112 Ok(Some((item, next_state))) => {
113 *this.state = Some(next_state);
114 Poll::Ready(Some(Ok(item)))
115 }
116 Ok(None) => Poll::Ready(None),
117 Err(e) => Poll::Ready(Some(Err(e))),
118 }
119 }
120 }
121 }
122}
123