Fix error_handling examples to use the ? operator

error_handling/errors2.rs

@@ -66,7 +66,7 @@ mod tests {
 // 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 `try!` macro that does pretty much what you would make that match statement
+// 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/error-handling.html#the-try-macro
+// https://doc.rust-lang.org/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-
-// and give it a `try!`
+// and give it a try!

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
+// Since the `?` operator 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
+// 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:
+// error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
-// expected `()`,
-// found `std::result::Result<_, _>`
 // ```
 
-// which is saying that the expected return type of the `main` function is
+// So we have to use another way of handling a `Result` within `main`.
-// 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`.
 
 // 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!`
+// not parse to an integer, and implement that instead of using the `?`
-// macro.
+// operator.

error_handling/errorsn.rs

@@ -115,21 +115,21 @@ impl error::Error for CreationError {
 
 // Next hint: There are three places in `read_and_validate` that we call a
 // function that returns a `Result` (that is, the functions might fail).
-// Wrap those calls in a `try!` macro call so that we return immediately from
+// Apply the `?` operator on those calls so that we return immediately from
 // `read_and_validate` if those function calls fail.
 
-// Another hint: under the hood, the `try!` macro calls `From::from`
+// Another hint: under the hood, the `?` operator calls `From::from`
 // on the error value to convert it to a boxed trait object, a Box<error::Error>,
 // which is polymorphic-- that means that lots of different kinds of errors
 // can be returned from the same function because all errors act the same
 // since they all implement the `error::Error` trait.
 // Check out this section of the book:
-// https://doc.rust-lang.org/stable/book/error-handling.html#standard-library-traits-used-for-error-handling
+// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-
 
-// Another another hint: Note that because the `try!` macro returns
+// Another another hint: Note that because the `?` operator returns
 // the *unwrapped* value in the `Ok` case, if we want to return a `Result` from
 // `read_and_validate` for *its* success case, we'll have to rewrap a value
-// that we got from the return value of a `try!` call in an `Ok`-- this will
+// that we got from the return value of a `?`ed call in an `Ok`-- this will
 // look like `Ok(something)`.
 
 // Another another another hint: `Result`s must be "used", that is, you'll