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    use core::marker::PhantomData;

    use {Poll, Async, StartSend, AsyncSink};
    use sink::Sink;
    use stream::Stream;

    /// Sink for the `Sink::with_flat_map` combinator, chaining a computation that returns an iterator
    /// to run prior to pushing a value into the underlying sink
    #[derive(Debug)]
    #[must_use = "sinks do nothing unless polled"]
    pub struct WithFlatMap<S, U, F, St>
    where
        S: Sink,
        F: FnMut(U) -> St,
        St: Stream<Item = S::SinkItem, Error=S::SinkError>,
    {
        sink: S,
        f: F,
        stream: Option<St>,
        buffer: Option<S::SinkItem>,
        _phantom: PhantomData<fn(U)>,
    }

    pub fn new<S, U, F, St>(sink: S, f: F) -> WithFlatMap<S, U, F, St>
    where
        S: Sink,
        F: FnMut(U) -> St,
        St: Stream<Item = S::SinkItem, Error=S::SinkError>,
    {
        WithFlatMap {
            sink: sink,
            f: f,
            stream: None,
            buffer: None,
            _phantom: PhantomData,
        }
    }

    impl<S, U, F, St> WithFlatMap<S, U, F, St>
    where
        S: Sink,
        F: FnMut(U) -> St,
        St: Stream<Item = S::SinkItem, Error=S::SinkError>,
    {
        /// Get a shared reference to the inner sink.
        pub fn get_ref(&self) -> &S {
            &self.sink
        }

        /// Get a mutable reference to the inner sink.
        pub fn get_mut(&mut self) -> &mut S {
            &mut self.sink
        }

        /// Consumes this combinator, returning the underlying sink.
        ///
        /// Note that this may discard intermediate state of this combinator, so
        /// care should be taken to avoid losing resources when this is called.
        pub fn into_inner(self) -> S {
            self.sink
        }

        fn try_empty_stream(&mut self) -> Poll<(), S::SinkError> {
            if let Some(x) = self.buffer.take() {
                if let AsyncSink::NotReady(x) = self.sink.start_send(x)? {
                    self.buffer = Some(x);
                    return Ok(Async::NotReady);
                }
            }
            if let Some(mut stream) = self.stream.take() {
                while let Some(x) = try_ready!(stream.poll()) {
                    if let AsyncSink::NotReady(x) = self.sink.start_send(x)? {
                        self.stream = Some(stream);
                        self.buffer = Some(x);
                        return Ok(Async::NotReady);
                    }
                }
            }
            Ok(Async::Ready(()))
        }
    }

    impl<S, U, F, St> Stream for WithFlatMap<S, U, F, St>
    where
        S: Stream + Sink,
        F: FnMut(U) -> St,
        St: Stream<Item = S::SinkItem, Error=S::SinkError>,
    {
        type Item = S::Item;
        type Error = S::Error;
        fn poll(&mut self) -> Poll<Option<S::Item>, S::Error> {
            self.sink.poll()
        }
    }

    impl<S, U, F, St> Sink for WithFlatMap<S, U, F, St>
    where
        S: Sink,
        F: FnMut(U) -> St,
        St: Stream<Item = S::SinkItem, Error=S::SinkError>,
    {
        type SinkItem = U;
        type SinkError = S::SinkError;
        fn start_send(&mut self, i: Self::SinkItem) -> StartSend<Self::SinkItem, Self::SinkError> {
            if self.try_empty_stream()?.is_not_ready() {
                return Ok(AsyncSink::NotReady(i));
            }
            assert!(self.stream.is_none());
            self.stream = Some((self.f)(i));
            self.try_empty_stream()?;
            Ok(AsyncSink::Ready)
        }
        fn poll_complete(&mut self) -> Poll<(), Self::SinkError> {
            if self.try_empty_stream()?.is_not_ready() {
                return Ok(Async::NotReady);
            }
            self.sink.poll_complete()
        }
        fn close(&mut self) -> Poll<(), Self::SinkError> {
            if self.try_empty_stream()?.is_not_ready() {
                return Ok(Async::NotReady);
            }
            assert!(self.stream.is_none());
            self.sink.close()
        }
    }

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