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    use {Future, IntoFuture, Poll};
    use super::chain::Chain;

    /// Future for the `then` combinator, chaining computations on the end of
    /// another future regardless of its outcome.
    ///
    /// This is created by the `Future::then` method.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct Then<A, B, F> where A: Future, B: IntoFuture {
        state: Chain<A, B::Future, F>,
    }

    pub fn new<A, B, F>(future: A, f: F) -> Then<A, B, F>
        where A: Future,
              B: IntoFuture,
    {
        Then {
            state: Chain::new(future, f),
        }
    }

    impl<A, B, F> Future for Then<A, B, F>
        where A: Future,
              B: IntoFuture,
              F: FnOnce(Result<A::Item, A::Error>) -> B,
    {
        type Item = B::Item;
        type Error = B::Error;

        fn poll(&mut self) -> Poll<B::Item, B::Error> {
            self.state.poll(|a, f| {
                Ok(Err(f(a).into_future()))
            })
        }
    }

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