comprehensive-rust/src/error-handling/try-operator.md

Propagating Errors with ?

The try-operator ? is used to return errors to the caller. It lets you turn the common

match some_expression {
    Ok(value) => value,
    Err(err) => return Err(err),
}

into the much simpler

some_expression?

We can use this to simplify our error handling code:

use std::{fs, io};
use std::io::Read;

fn read_username(path: &str) -> Result<String, io::Error> {
    let username_file_result = fs::File::open(path);
    let mut username_file = match username_file_result {
        Ok(file) => file,
        Err(err) => return Err(err),
    };

    let mut username = String::new();
    match username_file.read_to_string(&mut username) {
        Ok(_) => Ok(username),
        Err(err) => Err(err),
    }
}

fn main() {
    //fs::write("config.dat", "alice").unwrap();
    let username = read_username("config.dat");
    println!("username or error: {username:?}");
}

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.
• The return type of the function has to be compatible with the nested functions it calls. For instance, a function returning a Result<T, Err> can only apply the ? operator on a function returning a Result<AnyT, Err>. It cannot apply the ? operator on a function returning an Option<AnyT> or Result<T, OtherErr> unless OtherErr implements From<Err>. Reciprocally, a function returning an Option<T> can only apply the ? operator on a function returning an Option<AnyT>.
  • You can convert incompatible types into one another with the different Option and Result methods such as Option::ok_or, Result::ok, Result::err.