fn trampoline<In, Out, F>(init: In, mut func: F) -> Out
    where F: FnMut(In) -> Result<Out, In>
{
    let mut arg = init;
    loop {
        match func(arg) {
            Ok(out) => return out,
            Err(new) => arg = new,
        }
    }
}

fn main() {
    println!("{}", trampoline((5, 1), |(n, acc)| if n <= 1 { Ok(acc) } else { Err((n - 1, acc * n)) }));
}