Merge pull request #73 from hynek/master

Fix error_handling examples to use the ? operator
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liv 2018-05-22 20:50:55 +02:00 committed by GitHub
commit 2fa0fa17f9
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4 changed files with 21 additions and 26 deletions

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@ -66,7 +66,7 @@ mod tests {
// One way to handle this is using a `match` statement on // One way to handle this is using a `match` statement on
// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and // `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
// `Err(something)`. This pattern is very common in Rust, though, so there's // `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: // 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!

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@ -1,7 +1,7 @@
// errors3.rs // errors3.rs
// This is a program that is trying to use a completed version of the // 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-- // `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! // What should we do instead? Scroll for hints!
use std::num::ParseIntError; use std::num::ParseIntError;
@ -10,7 +10,7 @@ fn main() {
let mut tokens = 100; let mut tokens = 100;
let pretend_user_input = "8"; let pretend_user_input = "8";
let cost = try!(total_cost(pretend_user_input)); let cost = total_cost(pretend_user_input)?;
if cost > tokens { if cost > tokens {
println!("You can't afford that many!"); println!("You can't afford that many!");
@ -23,7 +23,7 @@ fn main() {
pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> { pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1; let processing_fee = 1;
let cost_per_item = 5; 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) 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. // `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 // 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.

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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 // 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). // 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. // `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>, // 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 // 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 // can be returned from the same function because all errors act the same
// since they all implement the `error::Error` trait. // since they all implement the `error::Error` trait.
// Check out this section of the book: // 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 // 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 // `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)`. // look like `Ok(something)`.
// Another another another hint: `Result`s must be "used", that is, you'll // Another another another hint: `Result`s must be "used", that is, you'll