JsonNode in JSON in Rust

Question

I ran through the standard JSON library of Rust http://doc.rust-lang.org/serialize/json/ and couldn't figure out what represents a node in it. In Java it's JsonNode . What's it in Rust? For example, how can I pass an argument of the type JsonNode in Rust?

Answer 1

Rust's "DOM" for JSON is defined by Json enum. For example, this JSON object:

{ "array": [1, 2, 3], "submap": { "bool": true, "string": "abcde" } }

is represented by this expression in Rust:

macro_rules! tree_map {
    ($($k:expr -> $v:expr),*) => ({
        let mut r = ::std::collections::TreeMap::new();
        $(r.insert($k, $v);)*
        r
    })
}

let data = json::Object(tree_map! {
    "array".to_string() -> json::List(vec![json::U64(1), json::U64(2), json::U64(3)]),
    "submap".to_string() -> json::Object(tree_map! {
        "bool".to_string() -> json::Boolean(true),
        "string".to_string() -> json::String("abcde".to_string())
    })
});

(try it here )

I've used custom map construction macro because unfortunately Rust standard library does not provide one (yet, I hope).

Json is just a regular enum, so you have to use pattern matching to extract values from it. Object contains an instance of TreeMap , so then you have to use its methods to inspect object structure:

if let json::Object(ref m) = data {
    if let Some(value) = m.find_with(|k| "submap".cmp(k)) {
        println!("Found value at 'submap' key: {}", value);
    } else {
        println!("'submap' key does not exist");
    }
} else {
    println!("data is not an object")
}

Update

Apparently, Json provides a lot of convenience methods, including find() , which will return Option<&Json> if the target is an Object which has corresponding key:

if let Some(value) = data.find("submap") {
    println!("Found value at 'submap' key: {}", value);
} else {
    println!("'submap' key does not exist or data is not an Object");
}

Thanks @ChrisMorgan for the finding.