Defining an Error Type

Introduction

In this lab, we are looking at how to define a custom error type in Rust and the key characteristics that make a good error type, such as representing different errors with the same type, providing clear error messages, easy comparison with other types, and the ability to hold information about the error. We also examine an example code that demonstrates the implementation of a custom error type and its usage in error handling scenarios.

Note: If the lab does not specify a file name, you can use any file name you want. For example, you can use main.rs, compile and run it with rustc main.rs && ./main.

Skills Graph

Defining an error type

Sometimes it simplifies the code to mask all of the different errors with a single type of error. We'll show this with a custom error.

Rust allows us to define our own error types. In general, a "good" error type:

• Represents different errors with the same type
• Presents nice error messages to the user
• Is easy to compare with other types
  • Good: Err(EmptyVec)
  • Bad: Err("Please use a vector with at least one element".to_owned())
• Can hold information about the error
  • Good: Err(BadChar(c, position))
  • Bad: Err("+ cannot be used here".to_owned())
• Composes well with other errors

use std::fmt;

type Result<T> = std::result::Result<T, DoubleError>;

// Define our error types. These may be customized for our error handling cases.
// Now we will be able to write our own errors, defer to an underlying error
// implementation, or do something in between.
#[derive(Debug, Clone)]
struct DoubleError;

// Generation of an error is completely separate from how it is displayed.
// There's no need to be concerned about cluttering complex logic with the display style.
//
// Note that we don't store any extra info about the errors. This means we can't state
// which string failed to parse without modifying our types to carry that information.
impl fmt::Display for DoubleError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "invalid first item to double")
    }
}

fn double_first(vec: Vec<&str>) -> Result<i32> {
    vec.first()
        // Change the error to our new type.
        .ok_or(DoubleError)
        .and_then(|s| {
            s.parse::<i32>()
                // Update to the new error type here also.
                .map_err(|_| DoubleError)
                .map(|i| 2 * i)
        })
}

fn print(result: Result<i32>) {
    match result {
        Ok(n) => println!("The first doubled is {}", n),
        Err(e) => println!("Error: {}", e),
    }
}

fn main() {
    let numbers = vec!["42", "93", "18"];
    let empty = vec![];
    let strings = vec!["tofu", "93", "18"];

    print(double_first(numbers));
    print(double_first(empty));
    print(double_first(strings));
}

Summary

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