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

rust,ignore

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
match some_expression {
    Ok(value) => value,
    Err(err) => return Err(err),
}

into the much simpler

rust,ignore

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
some_expression?

We can use this to simplify our error handling code:

rust,editable

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
use std::io::Read;
use std::{fs, io};

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:?}");
}

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.

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