Handling Errors with Box in Rust

Introduction

In this lab, the code demonstrates how to use the Box type to preserve original errors by wrapping them, allowing for dynamic error handling, and the Std library's From trait helps in converting any type that implements the Error trait into the trait object Box<Error>. It includes an example of converting and handling errors using Box with a custom error type.

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

Boxing errors

A way to write simple code while preserving the original errors is to Box them. The drawback is that the underlying error type is only known at runtime and not statically determined.

The stdlib helps in boxing our errors by having Box implement conversion from any type that implements the Error trait into the trait object Box<Error>, via From.

use std::error;
use std::fmt;

// Change the alias to `Box<error::Error>`.
type Result<T> = std::result::Result<T, Box<dyn error::Error>>;

#[derive(Debug, Clone)]
struct EmptyVec;

impl fmt::Display for EmptyVec {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "invalid first item to double")
    }
}

impl error::Error for EmptyVec {}

fn double_first(vec: Vec<&str>) -> Result<i32> {
    vec.first()
        .ok_or_else(|| EmptyVec.into()) // Converts to Box
        .and_then(|s| {
            s.parse::<i32>()
                .map_err(|e| e.into()) // Converts to Box
                .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

Congratulations! You have completed the Boxing Errors lab. You can practice more labs in LabEx to improve your skills.