Fix error_handling examples to use the ? operator

error_handling/errors3.rs

@@ -1,7 +1,7 @@
 // errors3.rs
 // This is a program that is trying to use a completed version of the
 // `total_cost` function from the previous exercise. It's not working though--
-// we can't call the `try!` macro in the `main()` function! Why not?
+// we can't use the `?` operator in the `main()` function! Why not?
 // What should we do instead? Scroll for hints!
 
 use std::num::ParseIntError;
@@ -10,7 +10,7 @@ fn main() {
     let mut tokens = 100;
     let pretend_user_input = "8";
 
-    let cost = try!(total_cost(pretend_user_input));
+    let cost = total_cost(pretend_user_input)?;
 
     if cost > tokens {
         println!("You can't afford that many!");
@@ -23,7 +23,7 @@ fn main() {
 pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
     let processing_fee = 1;
     let cost_per_item = 5;
-    let qty = try!(item_quantity.parse::<i32>());
+    let qty = item_quantity.parse::<i32>()?;
 
     Ok(qty * cost_per_item + processing_fee)
 }
@@ -45,23 +45,18 @@ pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
 
 
 
-// Since the `try!` macro returns an `Err` early if the thing it's trying to
-// do fails, you can only use the `try!` macro in functions that have a
+// Since the `?` operator returns an `Err` early if the thing it's trying to
+// do fails, you can only use the `?` operator in functions that have a
 // `Result` as their return type.
 
-// The error that you get if you run this code is:
+// Hence the error that you get if you run this code is:
 
 // ```
-// error: mismatched types:
-// expected `()`,
-// found `std::result::Result<_, _>`
+// error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
 // ```
 
-// which is saying that the expected return type of the `main` function is
-// the empty tuple, but we tried to return a `Result`-- and that's happening
-// in the implementation of `try!`. The `main` function never has a return type,
-// so we have to use another way of handling a `Result` within `main`.
+// So we have to use another way of handling a `Result` within `main`.
 
 // Decide what we should do if `pretend_user_input` has a string value that does
-// not parse to an integer, and implement that instead of calling the `try!`
-// macro.
+// not parse to an integer, and implement that instead of using the `?`
+// operator.