better example

main.rs

use std::env;
use std::io;
use std::fs::File;
use std::process::exit;

enum ProgramErr {
  UnableToOpenFile(io::Error)
}

fn main() {
  let path = env::args().nth(1).unwrap_or("/usr/share/dict/words".to_owned());
  if let Err(why) = open_input(&path) {
    match why {
      ProgramErr::UnableToOpenFile(reason) => {
        let _ = writeln!(io::stderr(), "program: unable to open {}: {}", path, reason);
      }
    }
    exit(1);
  }
}

fn open_input(path: &str) -> Result<(), ProgramErr> {
  if path == "-" {
    let stdout = io::stdout();
    do_something_with(stdout.lock())
  } else {
    do_something_with(File::open(path).map_err(ProgramErr::UnableToOpenFile)?);
  };
  
  Ok(())
}

fn do_something_with<I: Read>(input_source: I) {
  // do something
}