Rust: Enums to Wrap Multiple Errors

Re:Rust: Enums to Wrap Multiple Errors by nagisa on Friday October 08, 2021 @02:42PM (#28802597)

`thiserror` definitely does allow it, e.g.

    #[derive(thiserror::Error, Debug)]
    pub enum Error {
        #[error("Cannot open `{1}`")]
        OpenFile(#[source] std::io::Error, PathBuf)
        #[error("Cannot read file contents")]
        ReadFileContents(#[source] std::io::Error)
        #[error("Cannot parse the configuration file")]
        ParseConfig(#[source] serde::Error)
    }

    fn open_config(path: &Path) -> Result<Config, Error> {
        let mut file = File::open(path).map_err(|e| Error::OpenFile(e, path.to_owned()))?;
        let mut data = Vec::with_capacity(1024);
        file.read_to_end(&mut data).map_err(Error::ReadFileContents)?;
        parse_config(&data).map_err(Error::ParseConfig)
    }

EDIT: adjusted to add an example of adding path to the error message.

[ Parent | on HN ]

Re:Rust: Enums to Wrap Multiple Errors by shepmaster on Friday October 08, 2021 @02:53PM (#28802729)

I should have worded myself better, apologies.

With that example, since you use `Result::map_err` and specify the specific variant, you aren't really using anything from thiserror at the site of the `?`, correct?

Most usages I have seen, people use `#[from]`, which would end up having conflicting implementations. Is there a reason you didn't use `#[from]` for `ParseConfig`?

[ Parent | on HN ]

Re:Rust: Enums to Wrap Multiple Errors by nagisa on Friday October 08, 2021 @03:44PM (#28803337)

Ah, I see.

In short, I consider `From::from` implementations for errors to be an anti-pattern. It is super easy to become lax about adding context with these implementations in place. Especially as code is modified in the future.

I describe the approach that I use for errors in a detail in an article (https://kazlauskas.me/entries/errors.html) that has already been linked elsewhere in the thread. It is, as far as I can tell, pretty much equivalent to what `snafu` makes users to do.

[ Parent | on HN ]