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rustlings/solutions/13_error_handling/errors2.rs

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// 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!
//
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// 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.
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//
// There are at least two ways to implement this that are both correct. But one
// is a lot shorter!
use std::num::ParseIntError;
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#[allow(unused_variables)]
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fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1;
let cost_per_item = 5;
// Added `?` to propagate the error.
let qty = item_quantity.parse::<i32>()?;
// ^ added
// Equivalent to this verbose version:
let qty = match item_quantity.parse::<i32>() {
Ok(v) => v,
Err(e) => return Err(e),
};
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,
);
}
}