Aatch

struct OneShot<S, T> (S, ~fn (S) -> T);

impl<S, T> OneShot<S,T> {
    fn run(~self) -> T {
        let (s, f) = match self {
            ~OneShot(s, f) => (s,f)
        };

        f(s)
    }

Aatch

macro_rules! handle_element(
    ($tag:expr, $string:expr, $ctor:ident, [], $type_id:expr) => ( () );
    ($tag:expr, $string:expr, $ctor:ident, [ $($field:ident),* ], $type_id:expr, ) => (
        if eq_slice($tag, $string) {
            let _element = ~$ctor {
                parent: Element::new($type_id, ($tag).to_str()),
                $(
                    $field: None
                ),*
            };