Try Operator

Runtime errors like connection-refused or file-not-found are handled with the Result type, but matching this type on every call can be cumbersome. The try-operator ? is used to return errors to the caller. It lets you turn the common

  1. match some_expression {
  2.     Ok(value) => value,
  3.     Err(err) => return Err(err),
  4. }

into the much simpler

  1. some_expression?

We can use this to simplify our error handling code:

  1. use std::io::Read;
  2. use std::{fs, io};
  4. fn read_username(path: &str) -> Result<String, io::Error> {
  5.     let username_file_result = fs::File::open(path);
  6.     let mut username_file = match username_file_result {
  7.         Ok(file) => file,
  8.         Err(err) => return Err(err),
  9.     };
  11.     let mut username = String::new();
  12.     match username_file.read_to_string(&mut username) {
  13.         Ok(_) => Ok(username),
  14.         Err(err) => Err(err),
  15.     }
  16. }
  18. fn main() {
  19.     //fs::write("config.dat", "alice").unwrap();
  20.     let username = read_username("config.dat");
  21.     println!("username or error: {username:?}");
  22. }

This slide should take about 5 minutes.

Simplify the read_username function to use ?.

Key points:

  • The username variable can be either Ok(string) or Err(error).
  • Use the fs::write call to test out the different scenarios: no file, empty file, file with username.
  • Note that main can return a Result<(), E> as long as it implements std::process::Termination. In practice, this means that E implements Debug. The executable will print the Err variant and return a nonzero exit status on error.