Demo error handling with Result and question mark

In this example we set up the read_files function to return a Result enum that can contain either nothing (()) or a "boxed error". Don't worry about the details of this for now. Suffice to say that the read_files function itself now is expected to return a Result.

That means in the main function now we should check, using match, if it was a success (returning an OK() that contains nothing represented by an empty pair of parentheses, in which case we do nothing, or if it was an Err() in which case we print the error message.

Inside the read_files function we could remove all the match expressions and added a question-mark ? at the end of the read_to_string function calls. This operator will make Rust check if the returned value is Ok(). If it is then the expression will return the actual content without the Ok() wrapper. So in this case the variable content will already contain the content of the file. If the returned value is Err() then Rust will arrange for the read_files function to immediately return this same error wrapped in Err().

Basically if there was an error it passes up on the call-stack, if it was successful then it peals off the OK() wrapper and gives us the value we were looking for.

fn main() {
    match read_files() {
        Ok(()) => (),
        Err(err) => {
            println!("Error: {err}");
        }
    }
}

fn read_files() -> Result<(), Box<dyn std::error::Error>> {
    let content = std::fs::read_to_string("other.md")?;
    println!("content length: {}", content.len());
    println!();

    let content = std::fs::read_to_string("src/main.rs")?;
    println!("content length: {}", content.len());

    Ok(())
}

Running the code will result in this output:

Error: No such file or directory (os error 2)

This is different from the previous one as we also changed the semantics of the code. So I might need a better example to demonstrate this, but I think you get the idea of how Rust handles errors and how we can eliminate a lot of the boilerplate code.

• Result
• dyn
• Box