Fix error_handling examples to use the ? operator
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@ -66,7 +66,7 @@ mod tests {
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// One way to handle this is using a `match` statement on
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// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
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// `Err(something)`. This pattern is very common in Rust, though, so there's
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// a `try!` macro that does pretty much what you would make that match statement
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// a `?` operator that does pretty much what you would make that match statement
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// do for you! Take a look at this section of the Error Handling chapter:
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// https://doc.rust-lang.org/stable/book/error-handling.html#the-try-macro
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// and give it a `try!`
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// https://doc.rust-lang.org/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-
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// and give it a try!
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@ -1,7 +1,7 @@
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// errors3.rs
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// This is a program that is trying to use a completed version of the
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// `total_cost` function from the previous exercise. It's not working though--
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// we can't call the `try!` macro in the `main()` function! Why not?
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// we can't use the `?` operator in the `main()` function! Why not?
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// What should we do instead? Scroll for hints!
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use std::num::ParseIntError;
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@ -10,7 +10,7 @@ fn main() {
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let mut tokens = 100;
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let pretend_user_input = "8";
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let cost = try!(total_cost(pretend_user_input));
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let cost = total_cost(pretend_user_input)?;
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if cost > tokens {
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println!("You can't afford that many!");
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@ -23,7 +23,7 @@ fn main() {
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pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
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let processing_fee = 1;
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let cost_per_item = 5;
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let qty = try!(item_quantity.parse::<i32>());
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let qty = item_quantity.parse::<i32>()?;
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Ok(qty * cost_per_item + processing_fee)
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}
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@ -45,23 +45,18 @@ pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
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// Since the `try!` macro returns an `Err` early if the thing it's trying to
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// do fails, you can only use the `try!` macro in functions that have a
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// Since the `?` operator returns an `Err` early if the thing it's trying to
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// do fails, you can only use the `?` operator in functions that have a
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// `Result` as their return type.
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// The error that you get if you run this code is:
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// Hence the error that you get if you run this code is:
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// ```
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// error: mismatched types:
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// expected `()`,
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// found `std::result::Result<_, _>`
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// error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
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// ```
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// which is saying that the expected return type of the `main` function is
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// the empty tuple, but we tried to return a `Result`-- and that's happening
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// in the implementation of `try!`. The `main` function never has a return type,
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// so we have to use another way of handling a `Result` within `main`.
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// So we have to use another way of handling a `Result` within `main`.
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// Decide what we should do if `pretend_user_input` has a string value that does
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// not parse to an integer, and implement that instead of calling the `try!`
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// macro.
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// not parse to an integer, and implement that instead of using the `?`
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// operator.
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@ -115,21 +115,21 @@ impl error::Error for CreationError {
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// Next hint: There are three places in `read_and_validate` that we call a
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// function that returns a `Result` (that is, the functions might fail).
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// Wrap those calls in a `try!` macro call so that we return immediately from
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// Apply the `?` operator on those calls so that we return immediately from
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// `read_and_validate` if those function calls fail.
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// Another hint: under the hood, the `try!` macro calls `From::from`
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// Another hint: under the hood, the `?` operator calls `From::from`
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// on the error value to convert it to a boxed trait object, a Box<error::Error>,
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// which is polymorphic-- that means that lots of different kinds of errors
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// can be returned from the same function because all errors act the same
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// since they all implement the `error::Error` trait.
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// Check out this section of the book:
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// https://doc.rust-lang.org/stable/book/error-handling.html#standard-library-traits-used-for-error-handling
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// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-
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// Another another hint: Note that because the `try!` macro returns
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// Another another hint: Note that because the `?` operator returns
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// the *unwrapped* value in the `Ok` case, if we want to return a `Result` from
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// `read_and_validate` for *its* success case, we'll have to rewrap a value
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// that we got from the return value of a `try!` call in an `Ok`-- this will
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// that we got from the return value of a `?`ed call in an `Ok`-- this will
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// look like `Ok(something)`.
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// Another another another hint: `Result`s must be "used", that is, you'll
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