code:

trait Monoid {
    fn empty(&self) -> Self;
    fn append(&self, other: Self) -> Self;
}

impl Monoid for String {
    fn empty(&self) -> String {
        "".to_string()
    }
    
    fn append(&self, other: String) -> String {
        let new = self.clone();
        new + &other
    }
}

fn main(){
    //Concatenation
    let first: String = "Hello".to_string();
    let second: String = " World".to_string();
    println!("{}", first.append(second).append("!".to_string()));
}