Fix error_handling examples to use the ? operator

This commit is contained in:
Hynek Schlawack 2018-03-06 15:57:41 +01:00
parent 65bedf2d90
commit 3182b4d9ec
4 changed files with 21 additions and 26 deletions

File diff suppressed because one or more lines are too long

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@ -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
// and give it a `try!`
// 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!

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@ -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.

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@ -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