Recoverable Errors with Result - The Rust Programming Language

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## Recoverable Errors with `Result`

Most errors aren’t serious enough to require the program to stop entirely. Sometimes when a function fails, it’s for a reason that you can easily interpret and respond to. For example, if you try to open a file and that operation fails because the file doesn’t exist, you might want to create the file instead of terminating the process.

Recall from “Handling Potential Failure with `Result`” in Chapter 2 that the `Result` enum is defined as having two variants, `Ok` and `Err`, as follows:

    #![allow(unused)]
    fn main() {
    enum Result<T, E> {
        Ok(T),
        Err(E),
    }
    }

The `T` and `E` are generic type parameters: We’ll discuss generics in more detail in Chapter 10. What you need to know right now is that `T` represents the type of the value that will be returned in a success case within the `Ok` variant, and `E` represents the type of the error that will be returned in a failure case within the `Err` variant. Because `Result` has these generic type parameters, we can use the `Result` type and the functions defined on it in many different situations where the success value and error value we want to return may differ.

Let’s call a function that returns a `Result` value because the function could fail. In Listing 9-3, we try to open a file.

Filename: src/main.rs

    use std::fs::File;

    fn main() {
        let greeting_file_result = File::open("hello.txt");
    }

Listing 9-3: Opening a file

The return type of `File::open` is a `Result<T, E>`. The generic parameter `T` has been filled in by the implementation of `File::open` with the type of the success value, `std::fs::File`, which is a file handle. The type of `E` used in the error value is `std::io::Error`. This return type means the call to `File::open` might succeed and return a file handle that we can read from or write to. The function call also might fail: For example, the file might not exist, or we might not have permission to access the file. The `File::open` function needs to have a way to tell us whether it succeeded or failed and at the same time give us either the file handle or error information. This information is exactly what the `Result` enum conveys.

In the case where `File::open` succeeds, the value in the variable `greeting_file_result` will be an instance of `Ok` that contains a file handle. In the case where it fails, the value in `greeting_file_result` will be an instance of `Err` that contains more information about the kind of error that occurred.

We need to add to the code in Listing 9-3 to take different actions depending on the value `File::open` returns. Listing 9-4 shows one way to handle the `Result` using a basic tool, the `match` expression that we discussed in Chapter 6.

Filename: src/main.rs

    use std::fs::File;

    fn main() {
        let greeting_file_result = File::open("hello.txt");

        let greeting_file = match greeting_file_result {
            Ok(file) => file,
            Err(error) => panic!("Problem opening the file: {error:?}"),
        };
    }

Listing 9-4: Using a `match` expression to handle the `Result` variants that might be returned

Note that, like the `Option` enum, the `Result` enum and its variants have been brought into scope by the prelude, so we don’t need to specify `Result::` before the `Ok` and `Err` variants in the `match` arms.

When the result is `Ok`, this code will return the inner `file` value out of the `Ok` variant, and we then assign that file handle value to the variable `greeting_file`. After the `match`, we can use the file handle for reading or writing.

The other arm of the `match` handles the case where we get an `Err` value from `File::open`. In this example, we’ve chosen to call the `panic!` macro. If there’s no file named hello.txt in our current directory and we run this code, we’ll see the following output from the `panic!` macro:

    $ cargo run
       Compiling error-handling v0.1.0 (file:///projects/error-handling)
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.73s
         Running `target/debug/error-handling`

    thread 'main' panicked at src/main.rs:8:23:
    Problem opening the file: Os { code: 2, kind: NotFound, message: "No such file or directory" }
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

As usual, this output tells us exactly what has gone wrong.

### Matching on Different Errors

The code in Listing 9-4 will `panic!` no matter why `File::open` failed. However, we want to take different actions for different failure reasons. If `File::open` failed because the file doesn’t exist, we want to create the file and return the handle to the new file. If `File::open` failed for any other reason—for example, because we didn’t have permission to open the file—we still want the code to `panic!` in the same way it did in Listing 9-4. For this, we add an inner `match` expression, shown in Listing 9-5.

Filename: src/main.rs

    use std::fs::File;
    use std::io::ErrorKind;

    fn main() {
        let greeting_file_result = File::open("hello.txt");

        let greeting_file = match greeting_file_result {
            Ok(file) => file,
            Err(error) => match error.kind() {
                ErrorKind::NotFound => match File::create("hello.txt") {
                    Ok(fc) => fc,
                    Err(e) => panic!("Problem creating the file: {e:?}"),
                },
                _ => {
                    panic!("Problem opening the file: {error:?}");
                }
            },
        };
    }

Listing 9-5: Handling different kinds of errors in different ways

The type of the value that `File::open` returns inside the `Err` variant is `io::Error`, which is a struct provided by the standard library. This struct has a method, `kind`, that we can call to get an `io::ErrorKind` value. The enum `io::ErrorKind` is provided by the standard library and has variants representing the different kinds of errors that might result from an `io` operation. The variant we want to use is `ErrorKind::NotFound`, which indicates the file we’re trying to open doesn’t exist yet. So, we match on `greeting_file_result`, but we also have an inner match on `error.kind()`.

The condition we want to check in the inner match is whether the value returned by `error.kind()` is the `NotFound` variant of the `ErrorKind` enum. If it is, we try to create the file with `File::create`. However, because `File::create` could also fail, we need a second arm in the inner `match` expression. When the file can’t be created, a different error message is printed. The second arm of the outer `match` stays the same, so the program panics on any error besides the missing file error.

#### Alternatives to Using `match` with `Result<T, E>`

That’s a lot of `match`! The `match` expression is very useful but also very much a primitive. In Chapter 13, you’ll learn about closures, which are used with many of the methods defined on `Result<T, E>`. These methods can be more concise than using `match` when handling `Result<T, E>` values in your code.

For example, here’s another way to write the same logic as shown in Listing 9-5, this time using closures and the `unwrap_or_else` method:

    use std::fs::File;
    use std::io::ErrorKind;

    fn main() {
        let greeting_file = File::open("hello.txt").unwrap_or_else(|error| {
            if error.kind() == ErrorKind::NotFound {
                File::create("hello.txt").unwrap_or_else(|error| {
                    panic!("Problem creating the file: {error:?}");
                })
            } else {
                panic!("Problem opening the file: {error:?}");
            }
        });
    }

Although this code has the same behavior as Listing 9-5, it doesn’t contain any `match` expressions and is cleaner to read. Come back to this example after you’ve read Chapter 13 and look up the `unwrap_or_else` method in the standard library documentation. Many more of these methods can clean up huge, nested `match` expressions when you’re dealing with errors.

#### Shortcuts for Panic on Error

Using `match` works well enough, but it can be a bit verbose and doesn’t always communicate intent well. The `Result<T, E>` type has many helper methods defined on it to do various, more specific tasks. The `unwrap` method is a shortcut method implemented just like the `match` expression we wrote in Listing 9-4. If the `Result` value is the `Ok` variant, `unwrap` will return the value inside the `Ok`. If the `Result` is the `Err` variant, `unwrap` will call the `panic!` macro for us. Here is an example of `unwrap` in action:

Filename: src/main.rs

    use std::fs::File;

    fn main() {
        let greeting_file = File::open("hello.txt").unwrap();
    }

If we run this code without a hello.txt file, we’ll see an error message from the `panic!` call that the `unwrap` method makes:

    thread 'main' panicked at src/main.rs:4:49:
    called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Similarly, the `expect` method lets us also choose the `panic!` error message. Using `expect` instead of `unwrap` and providing good error messages can convey your intent and make tracking down the source of a panic easier. The syntax of `expect` looks like this:

Filename: src/main.rs

    use std::fs::File;

    fn main() {
        let greeting_file = File::open("hello.txt")
            .expect("hello.txt should be included in this project");
    }

We use `expect` in the same way as `unwrap`: to return the file handle or call the `panic!` macro. The error message used by `expect` in its call to `panic!` will be the parameter that we pass to `expect`, rather than the default `panic!` message that `unwrap` uses. Here’s what it looks like:

    thread 'main' panicked at src/main.rs:5:10:
    hello.txt should be included in this project: Os { code: 2, kind: NotFound, message: "No such file or directory" }

In production-quality code, most Rustaceans choose `expect` rather than `unwrap` and give more context about why the operation is expected to always succeed. That way, if your assumptions are ever proven wrong, you have more information to use in debugging.

### Propagating Errors

When a function’s implementation calls something that might fail, instead of handling the error within the function itself, you can return the error to the calling code so that it can decide what to do. This is known as propagating the error and gives more control to the calling code, where there might be more information or logic that dictates how the error should be handled than what you have available in the context of your code.

For example, Listing 9-6 shows a function that reads a username from a file. If the file doesn’t exist or can’t be read, this function will return those errors to the code that called the function.

Filename: src/main.rs

    #![allow(unused)]
    fn main() {
    use std::fs::File;
    use std::io::{self, Read};

    fn read_username_from_file() -> Result<String, io::Error> {
        let username_file_result = File::open("hello.txt");

        let mut username_file = match username_file_result {
            Ok(file) => file,
            Err(e) => return Err(e),
        };

        let mut username = String::new();

        match username_file.read_to_string(&mut username) {
            Ok(_) => Ok(username),
            Err(e) => Err(e),
        }
    }
    }

Listing 9-6: A function that returns errors to the calling code using `match`

This function can be written in a much shorter way, but we’re going to start by doing a lot of it manually in order to explore error handling; at the end, we’ll show the shorter way. Let’s look at the return type of the function first: `Result<String, io::Error>`. This means the function is returning a value of the type `Result<T, E>`, where the generic parameter `T` has been filled in with the concrete type `String` and the generic type `E` has been filled in with the concrete type `io::Error`.

If this function succeeds without any problems, the code that calls this function will receive an `Ok` value that holds a `String`—the `username` that this function read from the file. If this function encounters any problems, the calling code will receive an `Err` value that holds an instance of `io::Error` that contains more information about what the problems were. We chose `io::Error` as the return type of this function because that happens to be the type of the error value returned from both of the operations we’re calling in this function’s body that might fail: the `File::open` function and the `read_to_string` method.

The body of the function starts by calling the `File::open` function. Then, we handle the `Result` value with a `match` similar to the `match` in Listing 9-4. If `File::open` succeeds, the file handle in the pattern variable `file` becomes the value in the mutable variable `username_file` and the function continues. In the `Err` case, instead of calling `panic!`, we use the `return` keyword to return early out of the function entirely and pass the error value from `File::open`, now in the pattern variable `e`, back to the calling code as this function’s error value.

So, if we have a file handle in `username_file`, the function then creates a new `String` in variable `username` and calls the `read_to_string` method on the file handle in `username_file` to read the contents of the file into `username`. The `read_to_string` method also returns a `Result` because it might fail, even though `File::open` succeeded. So, we need another `match` to handle that `Result`: If `read_to_string` succeeds, then our function has succeeded, and we return the username from the file that’s now in `username` wrapped in an `Ok`. If `read_to_string` fails, we return the error value in the same way that we returned the error value in the `match` that handled the return value of `File::open`. However, we don’t need to explicitly say `return`, because this is the last expression in the function.

The code that calls this code will then handle getting either an `Ok` value that contains a username or an `Err` value that contains an `io::Error`. It’s up to the calling code to decide what to do with those values. If the calling code gets an `Err` value, it could call `panic!` and crash the program, use a default username, or look up the username from somewhere other than a file, for example. We don’t have enough information on what the calling code is actually trying to do, so we propagate all the success or error information upward for it to handle appropriately.

This pattern of propagating errors is so common in Rust that Rust provides the question mark operator `?` to make this easier.

#### The `?` Operator Shortcut

Listing 9-7 shows an implementation of `read_username_from_file` that has the same functionality as in Listing 9-6, but this implementation uses the `?` operator.

Filename: src/main.rs

    #![allow(unused)]
    fn main() {
    use std::fs::File;
    use std::io::{self, Read};

    fn read_username_from_file() -> Result<String, io::Error> {
        let mut username_file = File::open("hello.txt")?;
        let mut username = String::new();
        username_file.read_to_string(&mut username)?;
        Ok(username)
    }
    }

Listing 9-7: A function that returns errors to the calling code using the `?` operator

The `?` placed after a `Result` value is defined to work in almost the same way as the `match` expressions that we defined to handle the `Result` values in Listing 9-6. If the value of the `Result` is an `Ok`, the value inside the `Ok` will get returned from this expression, and the program will continue. If the value is an `Err`, the `Err` will be returned from the whole function as if we had used the `return` keyword so that the error value gets propagated to the calling code.

There is a difference between what the `match` expression from Listing 9-6 does and what the `?` operator does: Error values that have the `?` operator called on them go through the `from` function, defined in the `From` trait in the standard library, which is used to convert values from one type into another. When the `?` operator calls the `from` function, the error type received is converted into the error type defined in the return type of the current function. This is useful when a function returns one error type to represent all the ways a function might fail, even if parts might fail for many different reasons.

For example, we could change the `read_username_from_file` function in Listing 9-7 to return a custom error type named `OurError` that we define. If we also define `impl From<io::Error> for OurError` to construct an instance of `OurError` from an `io::Error`, then the `?` operator calls in the body of `read_username_from_file` will call `from` and convert the error types without needing to add any more code to the function.

In the context of Listing 9-7, the `?` at the end of the `File::open` call will return the value inside an `Ok` to the variable `username_file`. If an error occurs, the `?` operator will return early out of the whole function and give any `Err` value to the calling code. The same thing applies to the `?` at the end of the `read_to_string` call.

The `?` operator eliminates a lot of boilerplate and makes this function’s implementation simpler. We could even shorten this code further by chaining method calls immediately after the `?`, as shown in Listing 9-8.

Filename: src/main.rs

    #![allow(unused)]
    fn main() {
    use std::fs::File;
    use std::io::{self, Read};

    fn read_username_from_file() -> Result<String, io::Error> {
        let mut username = String::new();

        File::open("hello.txt")?.read_to_string(&mut username)?;

        Ok(username)
    }
    }

Listing 9-8: Chaining method calls after the `?` operator

We’ve moved the creation of the new `String` in `username` to the beginning of the function; that part hasn’t changed. Instead of creating a variable `username_file`, we’ve chained the call to `read_to_string` directly onto the result of `File::open("hello.txt")?`. We still have a `?` at the end of the `read_to_string` call, and we still return an `Ok` value containing `username` when both `File::open` and `read_to_string` succeed rather than returning errors. The functionality is again the same as in Listing 9-6 and Listing 9-7; this is just a different, more ergonomic way to write it.

Listing 9-9 shows a way to make this even shorter using `fs::read_to_string`.

Filename: src/main.rs

    #![allow(unused)]
    fn main() {
    use std::fs;
    use std::io;

    fn read_username_from_file() -> Result<String, io::Error> {
        fs::read_to_string("hello.txt")
    }
    }

Listing 9-9: Using `fs::read_to_string` instead of opening and then reading the file

Reading a file into a string is a fairly common operation, so the standard library provides the convenient `fs::read_to_string` function that opens the file, creates a new `String`, reads the contents of the file, puts the contents into that `String`, and returns it. Of course, using `fs::read_to_string` doesn’t give us the opportunity to explain all the error handling, so we did it the longer way first.

#### Where to Use the `?` Operator

The `?` operator can only be used in functions whose return type is compatible with the value the `?` is used on. This is because the `?` operator is defined to perform an early return of a value out of the function, in the same manner as the `match` expression we defined in Listing 9-6. In Listing 9-6, the `match` was using a `Result` value, and the early return arm returned an `Err(e)` value. The return type of the function has to be a `Result` so that it’s compatible with this `return`.

In Listing 9-10, let’s look at the error we’ll get if we use the `?` operator in a `main` function with a return type that is incompatible with the type of the value we use `?` on.

Filename: src/main.rs

    use std::fs::File;

    fn main() {
        let greeting_file = File::open("hello.txt")?;
    }

Listing 9-10: Attempting to use the `?` in the `main` function that returns `()` won’t compile.

This code opens a file, which might fail. The `?` operator follows the `Result` value returned by `File::open`, but this `main` function has the return type of `()`, not `Result`. When we compile this code, we get the following error message:

    $ cargo run
       Compiling error-handling v0.1.0 (file:///projects/error-handling)
    error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
     --> src/main.rs:4:48
      |
    3 | fn main() {
      | --------- this function should return `Result` or `Option` to accept `?`
    4 |     let greeting_file = File::open("hello.txt")?;
      |                                                ^ cannot use the `?` operator in a function that returns `()`
      |
    help: consider adding return type
      |
    3 ~ fn main() -> Result<(), Box<dyn std::error::Error>> {
    4 |     let greeting_file = File::open("hello.txt")?;
    5 +     Ok(())
      |

    For more information about this error, try `rustc --explain E0277`.
    error: could not compile `error-handling` (bin "error-handling") due to 1 previous error

This error points out that we’re only allowed to use the `?` operator in a function that returns `Result`, `Option`, or another type that implements `FromResidual`.

To fix the error, you have two choices. One choice is to change the return type of your function to be compatible with the value you’re using the `?` operator on as long as you have no restrictions preventing that. The other choice is to use a `match` or one of the `Result<T, E>` methods to handle the `Result<T, E>` in whatever way is appropriate.

The error message also mentioned that `?` can be used with `Option<T>` values as well. As with using `?` on `Result`, you can only use `?` on `Option` in a function that returns an `Option`. The behavior of the `?` operator when called on an `Option<T>` is similar to its behavior when called on a `Result<T, E>`: If the value is `None`, the `None` will be returned early from the function at that point. If the value is `Some`, the value inside the `Some` is the resultant value of the expression, and the function continues. Listing 9-11 has an example of a function that finds the last character of the first line in the given text.

    fn last_char_of_first_line(text: &str) -> Option<char> {
        text.lines().next()?.chars().last()
    }

    fn main() {
        assert_eq!(
            last_char_of_first_line("Hello, world\nHow are you today?"),
            Some('d')
        );

        assert_eq!(last_char_of_first_line(""), None);
        assert_eq!(last_char_of_first_line("\nhi"), None);
    }

Listing 9-11: Using the `?` operator on an `Option<T>` value

This function returns `Option<char>` because it’s possible that there is a character there, but it’s also possible that there isn’t. This code takes the `text` string slice argument and calls the `lines` method on it, which returns an iterator over the lines in the string. Because this function wants to examine the first line, it calls `next` on the iterator to get the first value from the iterator. If `text` is the empty string, this call to `next` will return `None`, in which case we use `?` to stop and return `None` from `last_char_of_first_line`. If `text` is not the empty string, `next` will return a `Some` value containing a string slice of the first line in `text`.

The `?` extracts the string slice, and we can call `chars` on that string slice to get an iterator of its characters. We’re interested in the last character in this first line, so we call `last` to return the last item in the iterator. This is an `Option` because it’s possible that the first line is the empty string; for example, if `text` starts with a blank line but has characters on other lines, as in `"\nhi"`. However, if there is a last character on the first line, it will be returned in the `Some` variant. The `?` operator in the middle gives us a concise way to express this logic, allowing us to implement the function in one line. If we couldn’t use the `?` operator on `Option`, we’d have to implement this logic using more method calls or a `match` expression.

Note that you can use the `?` operator on a `Result` in a function that returns `Result`, and you can use the `?` operator on an `Option` in a function that returns `Option`, but you can’t mix and match. The `?` operator won’t automatically convert a `Result` to an `Option` or vice versa; in those cases, you can use methods like the `ok` method on `Result` or the `ok_or` method on `Option` to do the conversion explicitly.

So far, all the `main` functions we’ve used return `()`. The `main` function is special because it’s the entry point and exit point of an executable program, and there are restrictions on what its return type can be for the program to behave as expected.

Luckily, `main` can also return a `Result<(), E>`. Listing 9-12 has the code from Listing 9-10, but we’ve changed the return type of `main` to be `Result<(), Box<dyn Error>>` and added a return value `Ok(())` to the end. This code will now compile.

Filename: src/main.rs

    use std::error::Error;
    use std::fs::File;

    fn main() -> Result<(), Box<dyn Error>> {
        let greeting_file = File::open("hello.txt")?;

        Ok(())
    }

Listing 9-12: Changing `main` to return `Result<(), E>` allows the use of the `?` operator on `Result` values.

The `Box<dyn Error>` type is a trait object, which we’ll talk about in “Using Trait Objects to Abstract over Shared Behavior” in Chapter 18. For now, you can read `Box<dyn Error>` to mean “any kind of error.” Using `?` on a `Result` value in a `main` function with the error type `Box<dyn Error>` is allowed because it allows any `Err` value to be returned early. Even though the body of this `main` function will only ever return errors of type `std::io::Error`, by specifying `Box<dyn Error>`, this signature will continue to be correct even if more code that returns other errors is added to the body of `main`.

When a `main` function returns a `Result<(), E>`, the executable will exit with a value of `0` if `main` returns `Ok(())` and will exit with a nonzero value if `main` returns an `Err` value. Executables written in C return integers when they exit: Programs that exit successfully return the integer `0`, and programs that error return some integer other than `0`. Rust also returns integers from executables to be compatible with this convention.

The `main` function may return any types that implement the `std::process::Termination` trait, which contains a function `report` that returns an `ExitCode`. Consult the standard library documentation for more information on implementing the `Termination` trait for your own types.

Now that we’ve discussed the details of calling `panic!` or returning `Result`, let’s return to the topic of how to decide which is appropriate to use in which cases.