rustlings/error_handling/errors3.rs

// errors3.rs
// This is a program that is trying to use a completed version of the
// `total_cost` function from the previous exercise. It's not working though--
// we can't use the `?` operator in the `main()` function! Why not?
// What should we do instead? Scroll for hints!

use std::num::ParseIntError;

fn main() {
    let mut tokens = 100;
    let pretend_user_input = "8";

    let cost = total_cost(pretend_user_input)?;

    if cost > tokens {
        println!("You can't afford that many!");
    } else {
        tokens -= cost;
        println!("You now have {} tokens.", tokens);
    }
}

pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
    let processing_fee = 1;
    let cost_per_item = 5;
    let qty = item_quantity.parse::<i32>()?;

    Ok(qty * cost_per_item + processing_fee)
}


















// Since the `?` operator returns an `Err` early if the thing it's trying to
// do fails, you can only use the `?` operator in functions that have a
// `Result` as their return type.

// Hence the error that you get if you run this code is:

// ```
// error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
// ```

// So we have to use another way of handling a `Result` within `main`.

// Decide what we should do if `pretend_user_input` has a string value that does
// not parse to an integer, and implement that instead of using the `?`
// operator.