How to convert Box<dyn Error> or should I do this another way?

1

Hi, Im continue encapsulate HTTP client base on reqwest. Now I wanna return Result<Option<Record>, Box<dyn Error>> type for different API with a common error type Box<dyn Error>. Everything is ok until Im tring to return some API error message:

async fn find_record(
    client: &Client,
    base_url: &Url,
    username: &str,
    token: &str,
    domain: &str,
    record_host: Option<&str>,
) -> Result<Option<Record>, Box<dyn Error>> {
    let url = base_url.join(&format!("{}/records", domain))?;

    let response = client
        .get(url)
        .basic_auth(username, Some(token))
        .send()
        .await?;

    println!("GET {} {}", response.url(), response.status());

    if let Err(e) = response.error_for_status_ref() {
        let api_error = response.json::<ApiError>().await?;

        // Can I combine the two lines into one? I have tried turbofish but it is not working.
        let ret: Box<dyn Error> =
            format!("API response error: {}", error(api_error.message)).into();
        return Err(ret);
    }

  // more code ...
}

Q1: What's the right way to implement the code I commented?

THX for any help.

Here is the error message I meet when I trying to use turbofish:

error[E0107]: wrong number of type arguments: expected 0, found 1
  --> src/dns_provider/name_com.rs:88:83
   |
88 |         return format!("API response error: {}", error(api_error.message)).into::<Box<dyn Error>>()
   |                                                                                   ^^^^^^^^^^^^^^ unexpected type argument

error[E0277]: the trait bound `std::result::Result<std::option::Option<dns_provider::name_com::Record>, std::boxed::Box<dyn std::error::Error>>: std::convert::From<std::string::String>` is not satisfied
  --> src/dns_provider/name_com.rs:88:76
   |
88 |         return format!("API response error: {}", error(api_error.message)).into::<Box<dyn Error>>()
   |                                                                            ^^^^ the trait `std::convert::From<std::string::String>` is not implemented for `std::result::Result<std::option::Option<dns_provider::name_com::Record>, std::boxed::Box<dyn std::error::Error>>`
   |
   = note: required because of the requirements on the impl of `std::convert::Into<std::result::Result<std::option::Option<dns_provider::name_com::Record>, std::boxed::Box<dyn std::error::Error>>>` for `std::string::String`

After reading it again I find it seems just because into() do not accept type arguments. :sweat_smile:

2

This works for me:

async fn ret_str_err() -> Result<(), Box<dyn std::error::Error>> {
    Err(format!("").into())
}

What's the error message you get?

3

You don't tell us what is wrong with it, but my guess is that you're getting an error that talks about Send or Sync. Change the error type to Box<dyn Error + Send + Sync> if so.

1 Like
4

Instead of return Err(ret) you could do Err(format!("yadayada'))?. The ? operator has built-in conversion already.

Clippy may complain that it's a bad style to use stand-alone Err()? used instead of a return. You can try to refactor this code to use .map_err() and .and_then() and such, instead of if let Err, so that the whole thing remains a single expression with a single ? operator.

1 Like
5

Sorry for forgeting paste the error message, I have put it at the end of post.
And really thanks for your help!

6

Thanks for your answer, this is just what I want! The ? operator can always suprise me. :joy:

Well, I found that it seems hard to use .map_err() here, because I need let api_error = response.json::<ApiError>().await?; to get the error message from API response. If I write .map_error() like this:

response.error_for_status_ref().map_err(|err| {
        let api_error = response.json::<ApiError>().await?;
        format!("API response error: {}", error(api_error.message))
    })?;

then I will get another error:

error[E0277]: the `?` operator can only be used in a closure that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/dns_provider/name_com.rs:86:25
   |
85 |       response.error_for_status_ref().map_err(|err| {
   |  _____________________________________________-
86 | |         let api_error = response.json::<ApiError>().await?;
   | |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a closure that returns `std::string::String`
87 | |         format!("API response error: {}", error(api_error.message))
88 | |     })?;
   | |_____- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `std::ops::Try` is not implemented for `std::string::String`
   = note: required by `std::ops::Try::from_error`
7

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