// Say we're writing a game where you can buy items with tokens. All items cost // 5 tokens, and whenever you purchase items there is a processing fee of 1 // token. A player of the game will type in how many items they want to buy, and // the `total_cost` function will calculate the total cost of the items. Since // the player typed in the quantity, we get it as a string. They might have // typed anything, not just numbers! // // Right now, this function isn't handling the error case at all. What we want // to do is: If we call the `total_cost` function on a string that is not a // number, that function will return a `ParseIntError`. In that case, we want to // immediately return that error from our function and not try to multiply and // add. // // There are at least two ways to implement this that are both correct. But one // is a lot shorter! use std::num::ParseIntError; fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> { let processing_fee = 1; let cost_per_item = 5; // TODO: Handle the error case as described above. let qty = item_quantity.parse::<i32>(); Ok(qty * cost_per_item + processing_fee) } fn main() { // You can optionally experiment here. } #[cfg(test)] mod tests { use super::*; use std::num::IntErrorKind; #[test] fn item_quantity_is_a_valid_number() { assert_eq!(total_cost("34"), Ok(171)); } #[test] fn item_quantity_is_an_invalid_number() { assert_eq!( total_cost("beep boop").unwrap_err().kind(), &IntErrorKind::InvalidDigit, ); } }