use std::fmt::{Debug, Formatter};

struct Cache<T, E>
where
    T: Fn(E) -> E,
    E: Clone + Debug,
{
    query: T,
    value: Option<E>,
}

impl<T, E> Cache<T, E>
where
    T: Fn(E) -> E,
    E: Clone + Debug,
{
    fn new(query: T) -> Self {
        Self { query, value: None }
    }

    fn value(&mut self, arg: E) -> E {
        match &self.value {
            Some(v) => v.clone(),
            None => {
                let v = (self.query)(arg);
                self.value = Some(v.clone());
                v
            }
        }
    }
}

impl<T, E> Debug for Cache<T, E>
where
    T: Fn(E) -> E,
    E: Clone + Debug,
{
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        if let Some(v) = &self.value {
            writeln!(f, "{:?}", v.clone()).expect("TODO: panic message");
        }
        Ok(())
    }
}

fn main() {
    let mut cache = Cache::new(|item: i32| item * 2);
    println!("{:?}", cache);
    println!("{:?}", cache.value(3));
    println!("{:?}", cache);
}