errors2 solution

exercises/13_error_handling/errors2.rs

@@ -2,16 +2,16 @@
 // 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, though, we get it as a string-- and they
-// might have typed anything, not just numbers!
+// 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 (and isn't
-// handling the success case properly either). What we want to do is: if we call
+// handling the success case properly either). 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`, and in that case, we want to immediately
-// return that error from our function and not try to multiply and add.
+// 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
+// There are at least two ways to implement this that are both correct. But one
 // is a lot shorter!
 
 use std::num::ParseIntError;
@@ -19,6 +19,8 @@ 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)
@@ -31,6 +33,7 @@ fn main() {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::num::IntErrorKind;
 
     #[test]
     fn item_quantity_is_a_valid_number() {
@@ -40,8 +43,8 @@ mod tests {
     #[test]
     fn item_quantity_is_an_invalid_number() {
         assert_eq!(
-            total_cost("beep boop").unwrap_err().to_string(),
-            "invalid digit found in string"
+            total_cost("beep boop").unwrap_err().kind(),
+            &IntErrorKind::InvalidDigit,
         );
     }
 }

rustlings-macros/info.toml

@@ -660,12 +660,11 @@ 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
+This pattern is very common in Rust, though, so there's the `?` 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/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
-and give it a try!"""
+Take a look at this section of the "Error Handling" chapter:
+https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator"""
 
 [[exercises]]
 name = "errors3"

solutions/13_error_handling/errors2.rs

@@ -1 +1,57 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+// 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 (and isn't
+// handling the success case properly either). 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;
+
+    // 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,
+        );
+    }
+}