#[derive(Debug)]
pub struct Token {
    data: u32,
}

pub trait TokenHandler {
    fn handle(self, token: &mut Token);
}

impl<F: FnOnce(&mut Token)> TokenHandler for F {
    fn handle(self, token: &mut Token) {
        self(token)
    }
}

// example generic caller
fn handle_with<H: TokenHandler>(token: &mut Token, handler: H) {
    handler.handle(token);
}

fn main() {
    let mut token = Token { data: 10 };

    let mut seen_tokens = 0;

    handle_with(&mut token, |t: &mut Token| {
        // example of modifying token
        t.data *= 2;
        // example of modifying internal state (for closures - scope)
        seen_tokens += 1;
    });

    println!("{:?} ({})", token, seen_tokens);
}