1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134

use core::fmt;
use core::pin::Pin;
use futures_core::future::TryFuture;
use futures_core::stream::Stream;
use futures_core::task::{Context, Poll};
use pin_utils::{unsafe_pinned, unsafe_unpinned};

/// Creates a `TryStream` from a seed and a closure returning a `TryFuture`.
///
/// This function is the dual for the `TryStream::try_fold()` adapter: while
/// `TryStream::try_fold()` reduces a `TryStream` to one single value,
/// `try_unfold()` creates a `TryStream` from a seed value.
///
/// `try_unfold()` will call the provided closure with the provided seed, then
/// wait for the returned `TryFuture` 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(TryFuture)`, then the
/// `try_unfold()` will stop producing items and return `Poll::Ready(None)` in
/// future calls to `poll()`.
///
/// In case of error generated by the returned `TryFuture`, the error will be
/// returned by the `TryStream`. The `TryStream` will then yield
/// `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
/// `TryFuture` using other library functions working on futures, and
/// `try_unfold()` will turn it into a `TryStream` by repeating the operation.
///
/// # Example
///
/// ```
/// # #[derive(Debug, PartialEq)]
/// # struct SomeError;
/// # futures::executor::block_on(async {
/// use futures::stream::{self, TryStreamExt};
///
/// let stream = stream::try_unfold(0, |state| async move {
///     if state < 0 {
///         return Err(SomeError);
///     }
///
///     if state <= 2 {
///         let next_state = state + 1;
///         let yielded = state * 2;
///         Ok(Some((yielded, next_state)))
///     } else {
///         Ok(None)
///     }
/// });
///
/// let result: Result<Vec<i32>, _> = stream.try_collect().await;
/// assert_eq!(result, Ok(vec![0, 2, 4]));
/// # });
/// ```
pub fn try_unfold<T, F, Fut, Item>(init: T, f: F) -> TryUnfold<T, F, Fut>
where
    F: FnMut(T) -> Fut,
    Fut: TryFuture<Ok = Option<(Item, T)>>,
{
    TryUnfold {
        f,
        state: Some(init),
        fut: None,
    }
}

/// Stream for the [`try_unfold`] function.
#[must_use = "streams do nothing unless polled"]
pub struct TryUnfold<T, F, Fut> {
    f: F,
    state: Option<T>,
    fut: Option<Fut>,
}

impl<T, F, Fut: Unpin> Unpin for TryUnfold<T, F, Fut> {}

impl<T, F, Fut> fmt::Debug for TryUnfold<T, F, Fut>
where
    T: fmt::Debug,
    Fut: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TryUnfold")
            .field("state", &self.state)
            .field("fut", &self.fut)
            .finish()
    }
}

impl<T, F, Fut> TryUnfold<T, F, Fut> {
    unsafe_unpinned!(f: F);
    unsafe_unpinned!(state: Option<T>);
    unsafe_pinned!(fut: Option<Fut>);
}

impl<T, F, Fut, Item> Stream for TryUnfold<T, F, Fut>
where
    F: FnMut(T) -> Fut,
    Fut: TryFuture<Ok = Option<(Item, T)>>,
{
    type Item = Result<Item, Fut::Error>;

    fn poll_next(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Item, Fut::Error>>> {
        if let Some(state) = self.as_mut().state().take() {
            let fut = (self.as_mut().f())(state);
            self.as_mut().fut().set(Some(fut));
        }

        match self.as_mut().fut().as_pin_mut() {
            None => {
                // The future previously errored
                Poll::Ready(None)
            }
            Some(fut) => {
                let step = ready!(fut.try_poll(cx));
                self.as_mut().fut().set(None);

                match step {
                    Ok(Some((item, next_state))) => {
                        *self.as_mut().state() = Some(next_state);
                        Poll::Ready(Some(Ok(item)))
                    }
                    Ok(None) => Poll::Ready(None),
                    Err(e) => Poll::Ready(Some(Err(e))),
                }
            }
        }
    }
}