CodeUtility
Making your developer's life easier!
Structs, Enums and Traits

Rust Error Handling: Result and ?

Errors are values in Rust. Return Result, handle it with match, and pass errors on with ?.

  • Intermediate
  • 3 min read
  • 1 runnable example
What you'll learn
  • Ok and Err
  • The ? operator
  • Your own error messages
Run code →
parse_and_double("abc")
  1. 1text.parse()fails
  2. 2?returns early
  3. 3Err(...)the caller gets the error

Success or error, as a value

Rust has no exceptions. A function that can fail returns Result<T, E>, which is either Ok(value) or Err(error). The caller decides what to do with the error.

The ? after parse returns the error to the caller at once when parsing fails; otherwise it gives the number. divide uses a String as its error type and returns Err for a zero divisor. match prints the value or the error message.

Key points

  1. Result<T, E> is Ok(value) or Err(error).
  2. ? returns the error to the caller at once.
  3. The caller decides what to do.

Runnable Rust example

Rust
Run code →
main.rs
use std::num::ParseIntError;

fn parse_and_double(text: &str) -> Result<i32, ParseIntError> {
    let number: i32 = text.trim().parse()?; // ? returns the error early
    Ok(number * 2)
}

fn divide(a: i32, b: i32) -> Result<i32, String> {
    if b == 0 {
        return Err(String::from("cannot divide by zero"));
    }
    Ok(a / b)
}

fn main() {
    for input in ["21", "abc"] {
        match parse_and_double(input) {
            Ok(value) => println!("{input} -> {value}"),
            Err(error) => println!("{input} -> error: {error}"),
        }
    }

    println!("{:?}", divide(10, 2));
    println!("{:?}", divide(1, 0));
}
Get a Run button for this example Result of running the code
21 -> 42
abc -> error: invalid digit found in string
Ok(5)
Err("cannot divide by zero")

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

Finished this lesson?

Mark it as learned to track your progress.

Next: Bank Account with Result →

Practice with a challenge

Use what this lesson covers to solve a real problem. Each challenge runs in your browser with instant tests and an AI review.

All challenges