a simple cat implementation with rust, plus some customized error handling

cat_copycat_with_rust.rs

// main.rs
use std::env;
use std::error::Error;
use std::fs;

use std::fmt;

#[derive(Debug)]
struct MyError(String);

impl fmt::Display for MyError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "application error: {}", self.0)
    }
}

impl Error for MyError {}

fn main() {
    let res = run();
    match res {
        Ok(_) => {}
        Err(e) => println!("oops: {}", e),
    }
}

fn run() -> std::result::Result<(), Box<dyn Error>> {
    let filename = &env::args().skip(1).next();
    if let Some(filename) = filename {
        let text = fs::read_to_string(filename)?;
        for line in text.lines() {
            println!("{}", line);
        }
        Ok(())
    } else {
        Err(Box::new(MyError("please provide a file".into())))
    }
}