Dynamic Error Types

Sometimes we want to allow any type of error to be returned without writing our own enum covering all the different possibilities. std::error::Error makes this easy.

  1. use std::fs::{self, File};
  2. use std::io::Read;
  3. use thiserror::Error;
  4. use std::error::Error;
  6. #[derive(Clone, Debug, Eq, Error, PartialEq)]
  7. #[error("Found no username in {0}")]
  8. struct EmptyUsernameError(String);
  10. fn read_username(path: &str) -> Result<String, Box<dyn Error>> {
  11.     let mut username = String::with_capacity(100);
  12.     File::open(path)?.read_to_string(&mut username)?;
  13.     if username.is_empty() {
  14.         return Err(EmptyUsernameError(String::from(path)).into());
  15.     }
  16.     Ok(username)
  17. }
  19. fn main() {
  20.     //fs::write("config.dat", "").unwrap();
  21.     match read_username("config.dat") {
  22.         Ok(username) => println!("Username: {username}"),
  23.         Err(err)     => println!("Error: {err}"),
  24.     }
  25. }

This saves on code, but gives up the ability to cleanly handle different error cases differently in the program. As such it’s generally not a good idea to use Box<dyn Error> in the public API of a library, but it can be a good option in a program where you just want to display the error message somewhere.