struct Cache<T>
where
    T: Fn(u32) -> u32,
{
    calc: T,
    value: Option<u32>,
}

impl<T> Cache<T>
where
    T: Fn(u32) -> u32,
{
    fn new(calc: T) -> Cache<T> {
        Cache { calc, value: None }
    }

    fn value(&mut self, v: u32) -> u32 {
        match self.value {
            Some(v) => v,
            None => {
                // 必须加括号
                let v = (self.calc)(v);
                self.value = Some(v);
                v
            }
        }
    }
}

fn main() {
    let mut c = Cache::new(|x| x + 1);
    let v1 = c.value(1);

    println!("{}", v1);
    let v2 = c.value(2);
    println!("{}", v2);
}