struct Cacher<T, U, V>
    where T: Fn(V) -> U
{
    calculation: T,
    value: Option<U>,
    arg: V,
}

impl<T, U: Copy, V> Cacher<T, U, V>
    where T: Fn(V) -> U
{
    fn new(calculation: T, arg: V) -> Cacher<T, U, V> {
        Cacher {
            calculation,
            value: None,
            arg
        }
    }

    fn value(&mut self, arg: V) -> U {
        match self.value {
            Some(v) => v,
            None => {
                let v = (self.calculation)(arg);
                self.value = Some(v);
                v
            },
        }
    }
}