thiserror and anyhow

The thiserror and anyhow crates are widely used to simplify error handling.

• thiserror 經常在程式庫中使用，目的是建立可實作 From<T> 的自訂錯誤型別。
• anyhow 經常由應用程式使用，目的是協助函式中的錯誤處理機制，包括為錯誤加上背景資訊。

use anyhow::{bail, Context, Result};
use std::fs;
use std::io::Read;
use thiserror::Error;

#[derive(Clone, Debug, Eq, Error, PartialEq)]
#[error("Found no username in {0}")]
struct EmptyUsernameError(String);

fn read_username(path: &str) -> Result<String> {
    let mut username = String::with_capacity(100);
    fs::File::open(path)
        .with_context(|| format!("Failed to open {path}"))?
        .read_to_string(&mut username)
        .context("Failed to read")?;
    if username.is_empty() {
        bail!(EmptyUsernameError(path.to_string()));
    }
    Ok(username)
}

fn main() {
    //fs::write("config.dat", "").unwrap();
    match read_username("config.dat") {
        Ok(username) => println!("Username: {username}"),
        Err(err) => println!("Error: {err:?}"),
    }
}