Deriving Error Enums
The thiserror crate is a popular way to create an error enum like we did on the previous page:
use std::{fs, io};
use std::io::Read;
use thiserror::Error;
#[derive(Debug, Error)]
enum ReadUsernameError {
#[error("Could not read: {0}")]
IoError(#[from] io::Error),
#[error("Found no username in {0}")]
EmptyUsername(String),
}
fn read_username(path: &str) -> Result<String, ReadUsernameError> {
let mut username = String::with_capacity(100);
fs::File::open(path)?.read_to_string(&mut username)?;
if username.is_empty() {
return Err(ReadUsernameError::EmptyUsername(String::from(path)));
}
Ok(username)
}
fn main() {
//fs::write("config.dat", "").unwrap();
match read_username("config.dat") {
Ok(username) => println!("Username: {username}"),
Err(err) => println!("Error: {err}"),
}
}
thiserror
’s derive macro automatically implements std::error::Error
, and optionally Display
(if the #[error(...)]
attributes are provided) and From
(if the #[from]
attribute is added). It also works for structs.
It doesn’t affect your public API, which makes it good for libraries.