Clojure-style trampoline using boxing

trampoline.rs

#![feature(unboxed_closures)]
use Bounce::{Process, Result};

fn main() {
    let x: int = from_str(std::os::args()[1].as_slice()).unwrap();
    let result = Process(box move || is_even(x)).execute();

    println!("{}", if result { "even" } else { "odd" });
}

fn is_even(n: int) -> Bounce<bool> {
    match n {
        0i => Result(true),
        n  => Process(box move || is_odd(n - 1))
    }
}

fn is_odd(n: int) -> Bounce<bool> {
    match n {
        0i => Result(false),
        n  => Process(box move || is_even(n - 1))
    }
}

enum Bounce<R> {
    Process(Box<Fn() -> Bounce<R> + 'static>),
    Result(R),
}

impl<R> Bounce<R> {
    fn execute(mut self) -> R {
        loop {
            match self {
                Process(exec) => { self = exec.call(()); },
                Result(value) => { return value; }
            }
        }
    }
}