即使出现 function 错误，我该如何继续？

Question

I'm using reqwest and scraper to write a simple web scraper in rust. One section of my code is given a link "next_page" that may or may not be valid. I want to try to access the link, and if there's an error, print the error and continue to the next link. Here's what I tried to do:

let next_page_response = reqwest::get(next_page).await;
let next_page_response = match next_page_response {
    Ok(response) => response,
    Err(error) => println!("WARN: Problem getting the url '{}'. \
        The error was {:?}", next_page, error),
};

This code is wrapped in a loop, and next_page changes every iteration.

This doesn't work, rustc gives the error error[E0308]: 'match' arms have incompatible types . I suppose this makes sense, in the first arm the expression becomes a Response , whereas in the second arm it becomes a () . However, if I change println! to panic! , the code compiles.

Questions:

1. How can I acknowledge an error and then just continue?
2. Why does panic! work when println! doesn't?

Full code, for the curious .

Answer 1

As you allude to in the original post, the issue is that the return values of the two branches don't match. And as kmdreko mentions panic! works since it has a return type that can be used anywhere.

So if you want to avoid the panic you need to make the return values from each branch match.

One way is to have both arms return () . The code can be pretty simple if you put the processing of the successful response into the body of the match.

pub fn process_response(response: &Response) {
    // ...
}

for next_page in pages {
    let next_page_response = reqwest::get(next_page).await;
    match next_page_response {
        Ok(response) => process_response(response),
        Err(error) => println!("WARN: Problem getting the url '{}'. \
             The error was {:?}", next_page, error),
    };
}

An alternate is to have both arms return an Option that you use later. (In this case, it makes the code longer and uglier in my opinion, but there can be cases where it is useful). It could look something like this:

pub fn process_response(response: &Response) {
    // ...
}

for next_page in pages {
    let next_page_response = reqwest::get(next_page).await;
    let next_page_response = match next_page_response {
        Ok(response) => Some(response),
        Err(error) => {
             println!("WARN: Problem getting the url '{}'. \
                 The error was {:?}", next_page, error);
             None
        }
    };

    if let Some(response) = next_page_response {
        process_response(response)
    }
}