use std::sync::Arc;
use std::cell::RefCell;

struct Test {
    pub cache: RefCell<Option<Arc<String>>>,
}

impl Test {
    pub fn new() -> Self {
        Test {
            cache: RefCell::new(None)
        }
    }

    pub fn test(&self) -> Arc<String> {
        let mut cache = self.cache.borrow_mut();
        let s = cache.get_or_insert_with(|| {
            Arc::new("a".to_string())
        });

        Arc::make_mut(s).push_str(".");

        s.clone()
    }
}


fn main() {
    let t = Test::new();
    for _ in 1..3 {
        println!("{}", t.test());
    }

    // It is possible to keep old value around, but it means that
    // Arc::make_mut will have to make a copy.
    let a = t.test();
    let b = t.test();
    println!("{} {}", a, b);
}