sorting the wheat from the chaff

XML parsing in Rust

Did I think that XML was dead? Well, I was wrong: it isn't. XML is here to stay with us forever. Let's see how I parsed some XML with Rust.

The library that seems the right tool for the job is serde-xml-rs. Like the name suggests, it will parse an XML file and leverage Serde for the serialization.

Let's say we have this file (examples from the crate's documentation).

<Project name="my_project">
    <Item name="hello" source="world.rs" />

Let's parse it using an XML library that leverages serde into our own Struct:

#[derive(Debug, Deserialize)]
struct Project {
    pub name: String,
    #[serde(rename = "Item", default)]
    pub items: Vec<Item>,

Basic usage, load the whole XML file in memory and parse it:

let s = "project.xml";
let f = File::open(s).expect(&format!("Cannot open file {}", s));
let r = BufReader::new(f);
let prj: Project = serde_xml_rs::de::from_reader(r).unwrap();
println!("{:?}", prj);

Note: like noted in the Rust documentation, in this specific case using a buffered reader (std::io::BufReader) does not offer significant advantages.

The output will be:

$ cargo run
Project { name: "my_project", items: [Item { name: "hello", source: "world.rs" }] }

In my case I had a chunked body response received from a Hyper Future so I need to jump through more hoops (I suspect I can improve on this).

use serde_xml_rs::from_reader;

// Hyper::Chunk -> Bytes -> &str
let b = body.into_bytes();
let xml_str = str::from_utf8(&b).unwrap();

// deserialize the slice into a Project
let project: Project = serde_xml_rs::from_str(xml_str).unwrap();

Now, parsing with serde sometimes is boring. When the parsing fails error messages are not always useful.

Let's add Serde path to error to the recipe: a crate that tries to point you where exactly the parsing fails.


// create an instance of the deserializer suitable for serde_path_to_error
let jd = &mut serde_xml_rs::de::Deserializer::new(r);
let result: Result<Project, _> = serde_path_to_error::deserialize(jd);
if let Err(err) = result {
    let path = err.path().to_string();
    panic!("Could not deserialize at: {}", path);

So the final result would look like something like this:

let s = "project.xml";
let f = File::open(s).expect(&format!("Cannot open file {}", s));
let r = BufReader::new(f);
let jd = &mut serde_xml_rs::de::Deserializer::new_from_reader(r);
let result: Result<Project, _> = serde_path_to_error::deserialize(jd);

Warning: serde_xml has some limitations. For example one thing that bit me is that it does not parse XML tags with namespaces. That could be a serious limitation in some contexts.

§ Closing thoughts

Parsing XML is ugly because XML tried (and failed) to conquer the world. So it has been filled with all kind of extensions and shit.

I wouldn't say that Rust is the ideal tool for parsing XML, other languages (and libraries) can run circles around this tool I've tried, but what I gain here is an implicit and strict type-checking of the XML parsed.

In the end, after playing a bit with all this, I just dumped everything and used the rss crate, since all I had to do was parsing a RSS feed (lol).

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