Rust Error Handling

An error is an unexpected behavior or event in a program that will produce an unwanted output.

In Rust, errors are of two categories:

• Unrecoverable Errors
• Recoverable Errors

Unrecoverable Errors in Rust

Unrecoverable errors are errors from which a program stops its execution. As the name suggests, we cannot recover from unrecoverable errors.

These errors are known as panic and can be triggered explicitly by calling the panic! macro.

Let's look at an example that uses the panic! macro.

Example 1: Rust Unrecoverable Errors with panic! Macro

fn main() {
    println!("Hello, World!");

    // Explicitly exit the program with an unrecoverable error
    panic!("Crash");
}

Output

Hello, World!
thread 'main' panicked at 'Crash', src/main.rs:5:5

Here, the call to the panic! macro causes an unrecoverable error.

thread 'main' panicked at 'Crash', src/main.rs:5:5

Notice that the program still runs the expressions above panic! macro. We can still see Hello, World! printed to the screen before the error message.

The panic! macro takes in an error message as an argument.

Example 2: Rust Unrecoverable Errors

Unrecoverable errors are also triggered by taking an action that might cause our code to panic. For example, accessing an array past its index will cause a panic.

fn main() {
    let numbers = [1, 2 ,3];

    println!("unknown index value = {}", numbers[3]);
}

Error

error: this operation will panic at runtime
 --> src/main.rs:4:42
  |
4 |     println!("unknown index value = {}", numbers[3]);
  |                                          ^^^^^^^^^^ index out of bounds: the length is 3 but the index is 3
  |

Here, Rust stops us from compiling the program because it knows the operation will panic at runtime.

The array numbers does not have a value at index 3 i.e. numbers[3].

Recoverable Errors

Recoverable errors are errors that won't stop a program from executing. Most errors are recoverable, and we can easily take action based on the type of error.

For example, if you try to open a file that doesn't exist, you can create the file instead of stopping the execution of the program or exiting the program with a panic.

Let's look at an example.

use std::fs::File;

fn main() {
    let data_result = File::open("data.txt");

    // using match for Result type
    let data_file = match data_result {
        Ok(file) => file,
        Err(error) => panic!("Problem opening the data file: {:?}", error),
    };

    println!("Data file", data_file);
}

If the data.txt file exists, the output is:

Data file: File { fd: 3, path: "/playground/data.txt", read: true, write: false }

If the data.txt file doesn't exist, the output is:

thread 'main' panicked at 'Problem opening the data file: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/main.rs:8:23

The Result Enum

In the above example, the return type of the File::open('data.txt') is a Result<T, E>.

The Result<T, E> type returns either a value or an error in Rust. It is an enum type with two possible variants.

• Ok(T) → operation succeeded with value T
• Err(E) → operation failed with an error E

Here, T and E are generic types. To know more about generics or generic types, visit Rust Generics.

The most basic way to see whether a Result enum has a value or error is to use pattern matching with a match expression.

// data_file is a Result<T, E>
match data_result {
    Ok(file) => file,
    Err(error) => panic!("Problem opening the data file: {:?}", error),
 };

When the result is Ok, this code will return the file, and when the result is Err, it will return a panic!.

To learn more about pattern matching, visit Rust Pattern Matching.

The Option Enum

The Option type or Option<T> type is an enum type just like Result with two possible variants.

• None → to indicate failure with no value
• Some(T) → a value with type T

Let's look at an example,

fn main() {
    let text = "Hello, World!";
    
    let character_option = text.chars().nth(15);
    
    // using match for Option type
    let character = match character_option {
        None => "empty".to_string(),
        Some(c) => c.to_string()
    };
    
    println!("Character at index 15 is {}", character);
}

Output

Character at index 15 is empty

Here, the method text.chars().nth(15) returns an Option<String>. So, to get the value out of the Option, we use a match expression.

In the example above, the 15th index of the string text doesn't exist. Thus, the Option type returns a None which matches the "empty" string.

None => "empty".to_string() 

If we were to get the 11th index of the string text, the Option enum would return Some(c), where c is the character in the 11th index.

Let's update the above example to find out the 11th index in the string.

fn main() {
    let text = "Hello, World!";
    
    let character_option = text.chars().nth(11);
    
    // using match for Option type
    let character = match character_option {
        None => "empty".to_string(),
        Some(c) => c.to_string()
    };
    
    println!("Character at index 11 is {}", character);
}

Output

Character at index 11 is d

Difference between Result and Option enum in Rust

Option enum can return None, which can indicate failure.

However, sometimes it is essential to express why an operation failed. Thus, we have the Result enum, which gives us the Err with the reason behind the failure of the operation.

In short,

• Option is about Some or None (value or no value)
• Result is about Ok or Err (result or error result)