errors2.rs (1841B)
1 // Say we're writing a game where you can buy items with tokens. All items cost 2 // 5 tokens, and whenever you purchase items there is a processing fee of 1 3 // token. A player of the game will type in how many items they want to buy, and 4 // the `total_cost` function will calculate the total cost of the items. Since 5 // the player typed in the quantity, we get it as a string. They might have 6 // typed anything, not just numbers! 7 // 8 // Right now, this function isn't handling the error case at all. What we want 9 // to do is: If we call the `total_cost` function on a string that is not a 10 // number, that function will return a `ParseIntError`. In that case, we want to 11 // immediately return that error from our function and not try to multiply and 12 // add. 13 // 14 // There are at least two ways to implement this that are both correct. But one 15 // is a lot shorter! 16 17 use std::num::ParseIntError; 18 19 #[allow(unused_variables, clippy::question_mark)] 20 fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> { 21 let processing_fee = 1; 22 let cost_per_item = 5; 23 24 // Added `?` to propagate the error. 25 let qty = item_quantity.parse::<i32>()?; 26 // ^ added 27 28 // Equivalent to this verbose version: 29 let qty = match item_quantity.parse::<i32>() { 30 Ok(v) => v, 31 Err(e) => return Err(e), 32 }; 33 34 Ok(qty * cost_per_item + processing_fee) 35 } 36 37 fn main() { 38 // You can optionally experiment here. 39 } 40 41 #[cfg(test)] 42 mod tests { 43 use super::*; 44 use std::num::IntErrorKind; 45 46 #[test] 47 fn item_quantity_is_a_valid_number() { 48 assert_eq!(total_cost("34"), Ok(171)); 49 } 50 51 #[test] 52 fn item_quantity_is_an_invalid_number() { 53 assert_eq!( 54 total_cost("beep boop").unwrap_err().kind(), 55 &IntErrorKind::InvalidDigit, 56 ); 57 } 58 }