use std::fmt;
use itertools::Itertools;

enum Object {
    Num(i32),
    List(Vec<Object>),
}

impl fmt::Display for Object {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Object::Num(x) => write!(f, "{}", x),
            Object::List(lst) => write!(f, "({})", lst.iter().join(" "))
        }
    }
}

fn main() {
    assert_eq!("()", Object::List(vec![]).to_string());
    assert_eq!("(42)", Object::List(vec![Object::Num(42)]).to_string());
    assert_eq!("(1 2 3)", Object::List((1..=3).map(Object::Num).collect()).to_string());
    assert_eq!(
        "((1) (2))",
        Object::List(vec![
            Object::List(vec![Object::Num(1)]),
            Object::List(vec![Object::Num(2)]),
        ]).to_string()
    )
}