Total price in three sauces...

total_price.rs

    fn total_price(&self, shopping_list: &[&str]) -> Option<f32> {
        let mut total = 0.0;
        for item in shopping_list {
            if self.price(item) == None {
                return None;
            }
            total += self.price(item).unwrap();
        }
        Some(total)
    }
    
    fn total_price(&self, shopping_list: &[&str]) -> Option<f32> {
        shopping_list.iter().fold(Some(0.0), |acc, &item| {
            match self.price(item) {
                Some(x) => Some(x + acc.unwrap()),
                None    => None,
            }
        });
    }

    fn total_price(&self, shopping_list: &[&str]) -> Option<f32> {
        shopping_list.iter().fold(Some(0.0), |acc, &item| {
            acc.and_then(|a| self.price(item).map(|p| p + a) )
        })
    }