# 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. ```rust,editable,compile_fail use std::fs; use std::io::Read; use thiserror::Error; use std::error::Error; #[derive(Clone, Debug, Eq, Error, PartialEq)] #[error("Found no username in {0}")] struct EmptyUsernameError(String); fn read_username(path: &str) -> Result> { let mut username = String::new(); fs::File::open(path)?.read_to_string(&mut username)?; if username.is_empty() { return Err(EmptyUsernameError(String::from(path)).into()); } Ok(username) } fn main() { //fs::write("config.dat", "").unwrap(); match read_username("config.dat") { Ok(username) => println!("Username: {username}"), Err(err) => println!("Error: {err}"), } } ```
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` 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.