right let's try this one again

1 files changed, 72 insertions, 0 deletions

diff --git a/exercises/error_handling/errors2.rs b/exercises/error_handling/errors2.rs
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+++ b/exercises/error_handling/errors2.rs
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+// errors2.rs
+// 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 number of tokens.
+// Since the player typed in the quantity, though, we get it as a string-- and
+// they might have typed anything, not just numbers!
+
+// Right now, this function isn't handling the error case at all (and isn't
+// handling the success case properly either). What we want to do is:
+// if we call the `parse` function on a string that is not a number, that
+// function will return a `ParseIntError`, and 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! Scroll down for hints to both ways.
+
+use std::num::ParseIntError;
+
+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)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[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().to_string(),
+            "invalid digit found in string"
+        );
+    }
+}
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+// One way to handle this is using a `match` statement on
+// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
+// `Err(something)`. This pattern is very common in Rust, though, so there's
+// a `?` operator that does pretty much what you would make that match statement
+// do for you! Take a look at this section of the Error Handling chapter:
+// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
+// and give it a try!