rustlings/exercises/13_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!
// Why not? What should we do to fix it?
//
// Execute `rustlings hint errors3` or use the `hint` watch subcommand for a
// hint.

use std::num::ParseIntError;
use std::error;

// I could have just returned the ParseIntError from the total_cost function
// because we know the exact error that's being returned.

fn main() -> Result<(), Box<dyn error::Error>>{
    // Box <dyn error::Error> is a trait object.
    // Meaning, you would use this when you know an error can be returned but 
    // not necessarily what kind 
    
    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);
    }

    Ok(())
}

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)
}