Error Handling

Handle errors correctly in main

anyhow-badge cat-rust-patterns-badge thiserror-badge cat-rust-patterns-badge error-chain-badge cat-rust-patterns-badge

Error Strategies (2024)

As recommended in Rust by Example, Boxing errors is seen as an easy strategy for getting started.

#![allow(unused_variables)] fn main() { Box<dyn Error> }

To understand what kind of error handling may be required study Designing error types in Rust and consider thiserror for libraries or anyhow as a maintained error aggregation option.

#![allow(unused_variables)] fn main() { use thiserror::Error; #[derive(Error,Debug)] pub enum MultiError { #[error("🦀 got {0}")] ErrorClass(String), } }

Application authors can compose enums using anyhow can import the Result type from the crate to provide auto-Boxing behavior

use anyhow::Result; fn main() -> Result<()> { let my_string = "yellow".to_string(); let _my_int = my_string.parse::<i32>()?; Ok(()) }

Error Chain (2015-2018)

Handles error that occur when trying to open a file that does not exist. It is achieved by using error-chain, a library that takes care of a lot of boilerplate code needed in order to handle errors in Rust.

Io(std::io::Error) inside foreign_links allows automatic conversion from std::io::Error into error_chain! defined type implementing the Error trait.

The below recipe will tell how long the system has been running by opening the Unix file /proc/uptime and parse the content to get the first number. Returns uptime unless there is an error.

Other recipes in this book will hide the error-chain boilerplate, and can be seen by expanding the code with the ⤢ button.

use error_chain::error_chain; use std::fs::File; use std::io::Read; error_chain!{ foreign_links { Io(std::io::Error); ParseInt(::std::num::ParseIntError); } } fn read_uptime() -> Result<u64> { let mut uptime = String::new(); File::open("/proc/uptime")?.read_to_string(&mut uptime)?; Ok(uptime .split('.') .next() .ok_or("Cannot parse uptime data")? .parse()?) } fn main() { match read_uptime() { Ok(uptime) => println!("uptime: {} seconds", uptime), Err(err) => eprintln!("error: {}", err), }; }