rustlings/old_curriculum/error_handling/errors3.rs

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2018-02-22 06:09:53 +00:00
// errors3.rs
2016-06-21 14:40:32 +00:00
// 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 call the `try!` macro 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 = try!(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 = try!(item_quantity.parse::<i32>());
Ok(qty * cost_per_item + processing_fee)
}
// Since the `try!` macro returns an `Err` early if the thing it's trying to
// do fails, you can only use the `try!` macro in functions that have a
// `Result` as their return type.
// The error that you get if you run this code is:
// ```
// error: mismatched types:
// expected `()`,
// found `std::result::Result<_, _>`
// ```
// which is saying that the expected return type of the `main` function is
// the empty tuple, but we tried to return a `Result`-- and that's happening
// in the implementation of `try!`. The `main` function never has a return type,
// 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 calling the `try!`
// macro.