how do I write custom types to a CSV writer?

[stevenliebregt]

What version of the csv crate are you using?

1.2.1

Briefly describe the question, bug or feature request.

I'm trying to use write_record with an array of enum instances that have an inner value. I'm running into the problem that those enums can not implement AsRef<[u8]> because they can have integers as well.

Include a complete program demonstrating a problem.

For example:

pub enum TagValue {
    String(String),
    Boolean(bool),
    Integer(i128),
    Empty
}

impl AsRef<[u8]> for TagValue {
    fn as_ref(&self) -> &[u8] {
        match self {
            // String works
            TagValue::String(value) => value.as_ref(),
            // But I have no idea how to do the other types,
            // int for example `to_be_bytes()` doesn't work because I can't return references
            // to the result of that since it's owned by this function
            _ => todo!()
        }
    }
}

// The calling code

let output_file = File::create(&self.output_path).unwrap(); 
let writer = BufWriter::new(output_file);
let gzip_encoder = GzEncoder::new(writer, Compression::default());

let mut tsv_writer = csv::WriterBuilder::new()
    .delimiter(TSV_DELIMITER)
    .from_writer(gzip_encoder);

let tags = vec![TagValue::String("test".into()), TagValue::Integer(1234), TagValue::Boolean(true), TagValue::Empty];

tsv_writer.write_record(tags).unwrap();

What is the observed behavior of the code above?

Returns a value referencing data owned by this function in the AsRef<[u8]> implementation.

What is the expected or desired behavior of the code above?

I hoped I could return a reference to the byte representations of those values.

[BurntSushi]

This is probably a good example of the XY Problem. That is, it looks like the actual problem you're trying to solve is, "how do I conveniently convert my custom data types to CSV data." But here, you're asking about how to implement AsRef in a way that doesn't result in returning a reference. The answer to the latter question is that you can't. write_record is only for types that are known to be cheaply convertible to &[u8] via the AsRef trait. If you don't have that, then you can't use write_record.

In your case, TagValue can almost implement AsRef<[u8]>. If you really want to go that route, then I'd suggest converting TagValue into a new type that does the necessary conversions, and then implement AsRef<[u8]> for that new type.

Otherwise, more generally, the answer to the question of "how do I convert my custom data types to CSV" is to use Serde via csv::Writer::serialize.