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    use {Future, Poll, Async};

    /// Future for the `map` combinator, changing the type of a future.
    ///
    /// This is created by the `Future::map` method.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct Map<A, F> where A: Future {
        future: A,
        f: Option<F>,
    }

    pub fn new<A, F>(future: A, f: F) -> Map<A, F>
        where A: Future,
    {
        Map {
            future: future,
            f: Some(f),
        }
    }

    impl<U, A, F> Future for Map<A, F>
        where A: Future,
              F: FnOnce(A::Item) -> U,
    {
        type Item = U;
        type Error = A::Error;

        fn poll(&mut self) -> Poll<U, A::Error> {
            let e = match self.future.poll() {
                Ok(Async::NotReady) => return Ok(Async::NotReady),
                Ok(Async::Ready(e)) => Ok(e),
                Err(e) => Err(e),
            };
            e.map(self.f.take().expect("cannot poll Map twice"))
             .map(Async::Ready)
        }
    }

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