引入 ?

    1. panic!，不过我们已经决定要尽可能避免 panic 了。
    2. 返回它，因为 Err 就意味着它已经不能被处理了。
    1. use std::num::ParseIntError;
    3. fn multiply(first_number_str: &str, second_number_str: &str) -> Result<i32, ParseIntError> {
    4.     let first_number = try!(first_number_str.parse::<i32>());
    6.     Ok(first_number * second_number)
    9. fn print(result: Result<i32, ParseIntError>) {
    10.     match result {
    11.         Ok(n)  => println!("n is {}", n),
    12.     }
    13. }
    15. fn main() {
    16.     print(multiply("10", "2"));
    17.     print(multiply("t", "2"));
    18. }