# try_fold.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.13/src/stream/try_stream/try_fold.rs`.
Keywords: rust, rustlang, rust-lang

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    use core::fmt;
    use core::pin::Pin;
    use futures_core::future::{FusedFuture, Future, TryFuture};
    use futures_core::ready;
    use futures_core::stream::TryStream;
    use futures_core::task::{Context, Poll};
    use pin_project_lite::pin_project;

    pin_project! {
        /// Future for the [`try_fold`](super::TryStreamExt::try_fold) method.
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        pub struct TryFold<St, Fut, T, F> {
            #[pin]
            stream: St,
            f: F,
            accum: Option<T>,
            #[pin]
            future: Option<Fut>,
        }
    }

    impl<St, Fut, T, F> fmt::Debug for TryFold<St, Fut, T, F>
    where
        St: fmt::Debug,
        Fut: fmt::Debug,
        T: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("TryFold")
                .field("stream", &self.stream)
                .field("accum", &self.accum)
                .field("future", &self.future)
                .finish()
        }
    }

    impl<St, Fut, T, F> TryFold<St, Fut, T, F>
    where St: TryStream,
          F: FnMut(T, St::Ok) -> Fut,
          Fut: TryFuture<Ok = T, Error = St::Error>,
    {
        pub(super) fn new(stream: St, f: F, t: T) -> Self {
            Self {
                stream,
                f,
                accum: Some(t),
                future: None,
            }
        }
    }

    impl<St, Fut, T, F> FusedFuture for TryFold<St, Fut, T, F>
        where St: TryStream,
              F: FnMut(T, St::Ok) -> Fut,
              Fut: TryFuture<Ok = T, Error = St::Error>,
    {
        fn is_terminated(&self) -> bool {
            self.accum.is_none() && self.future.is_none()
        }
    }

    impl<St, Fut, T, F> Future for TryFold<St, Fut, T, F>
        where St: TryStream,
              F: FnMut(T, St::Ok) -> Fut,
              Fut: TryFuture<Ok = T, Error = St::Error>,
    {
        type Output = Result<T, St::Error>;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            let mut this = self.project();

            Poll::Ready(loop {
                if let Some(fut) = this.future.as_mut().as_pin_mut() {
                    // we're currently processing a future to produce a new accum value
                    let res = ready!(fut.try_poll(cx));
                    this.future.set(None);
                    match res {
                        Ok(a) => *this.accum = Some(a),
                        Err(e) => break Err(e),
                    }
                } else if this.accum.is_some() {
                    // we're waiting on a new item from the stream
                    let res = ready!(this.stream.as_mut().try_poll_next(cx));
                    let a = this.accum.take().unwrap();
                    match res {
                        Some(Ok(item)) => this.future.set(Some((this.f)(a, item))),
                        Some(Err(e)) => break Err(e),
                        None => break Ok(a),
                    }
                } else {
                    panic!("Fold polled after completion")
                }
            })
        }
    }