#[derive(Debug)]
struct Server {
    host: String,
    target: String,
    port: i32,
    timeout: i32,
}

impl Default for Server  {
    fn default() -> Self {
        Self {
            host: "0.0.0.0".to_string(),
            target: "8.8.8.8".to_string(),
            port: 0,
            timeout: 1000,
        }
    }
}

trait Builder {
    type Config : Default;

    fn build(v: &[BuilderFn<Self::Config>]) -> Self::Config {
        let mut ret = Default::default();
        for c in v {
            c(&mut ret)
        }
        ret
    }
}

type BuilderFn<T> = Box<dyn Fn(&mut T) -> ()>;


impl Builder for Server {
    type Config = Self;
}

fn host<S>(s: S) -> BuilderFn<Server>
where
    S: Into<String>,
{
    let host = s.into();
    Box::new(move |config: &mut Server| config.host = host.clone())
}

fn target<S>(s: S) -> BuilderFn<Server>
where
    S: Into<String>,
{
    let target = s.into();
    Box::new(move |config: &mut Server| config.target = target.clone())
}

fn port(n: i32) -> BuilderFn<Server> {
    Box::new(move |config: &mut Server| config.port = n)
}

fn timeout(n: i32) -> BuilderFn<Server> {
    Box::new(move |config: &mut Server| config.timeout = n)
}

fn main() {
    let server = Server::build(&[
        port(25),
        host("localhost"),
        target("example.com"),
        timeout(2000),
    ]);
    println!("server  = {:?}", server);
}