24 days of Rust - working with JSON

Important note: this article is outdated! Go to http://zsiciarz.github.io/24daysofrust/ for a recent version of all of 24 days of Rust articles. The blogpost here is kept as it is for historical reasons.

JSON is a workhorse data format of the modern Web. Originating from the JavaScript world, it gained a lot of traction and at the moment it's usually the first choice of a Web developer for a data interchange format. Not only Web - once JavaScript-only, JSON support is now ubiquitous. A lot of languages ship with JSON parsers in the standard libraries, and when it's not the case, surely someone has already built a third party library. In case of Rust, JSON support comes out of the box in the serialize::json module.

Note: in this article I deliberately do not focus on the Web, APIs, requests and so on. I mentioned JSON in a previous post about hyper but now I don't care where did the JSON-encoded data come from or what to do with it later. Here I'm just going to show you a few practical tips - Rust has already a quite extensive documentation on the subject.

Slightly offtopic: I'm writing this blogpost while getting ready to coach a group of fantastic women at Django Girls in Łódź, Poland. If you're a woman interested in learning programming (or know such girls) check out if there's a Django Girls workshop near you! The workshops focus on basic Python, Django and web technologies, but the general idea is to get the attendees genuinely interested in programming and empowered to create.

That said, now back to JSON and Rust.


A lot of Rust types serialize to JSON just as you would expect. Note that encode() immutably borrows its argument:

extern crate serialize;

use serialize::json;

fn main() {
    println!("{}", json::encode(&42i));
    println!("{}", json::encode(&vec!["to", "be", "or", "not", "to", "be"]));
    println!("{}", json::encode(&Some(true)));        

Option<T> maps to the encoding of value itself if it is a Some(value) while None maps to null.

Automatic (de)serialization

In my earlier article on CSV I mentioned the Encodable and Decodable traits. Here's an example with a nested struct:

use serialize::{Decodable, Encodable, json};

#[deriving(Decodable, Encodable)]
struct Photo {
    url: String,
    dimensions: (uint, uint),

#[deriving(Decodable, Encodable)]
struct User {
    name: String,
    post_count: uint,
    likes_burgers: bool,
    avatar: Option<Photo>,

let user = User {
    name: "Zbyszek".to_string(),
    post_count: 100u,
    likes_burgers: true,
    avatar: Some(Photo {
        url: "http://lorempixel.com/160/160/".to_string(),
        dimensions: (160u, 160u),
println!("{}", json::encode(&user));
// {"name":"Zbyszek","post_count":100,"likes_burgers":true,"avatar":{"url":"http://lorempixel.com/160/160/","dimensions":[160,160]}}

Pretty printing

The json::encode() doesn't care for readability of it's output. Although the JSON it emits is correct and machine-readable, there are no newlines or indents making it hard for a human to debug. Pretty-printing is a little bit more complex than just one function call, but not too complicated:

use serialize::json::PrettyEncoder;

let mut buffer: Vec<u8> = Vec::new();
    let mut encoder = PrettyEncoder::new(&mut buffer);
    user.encode(&mut encoder).ok().expect("JSON encode error");
let encoded = String::from_utf8(buffer).unwrap();
println!("{}", encoded);

You'll need a PrettyEncoder which requires a Writer. Fortunately for us, Vec<u8> (a vector of bytes) implements this trait. Note the explicit scope in the code above. The encoder mutably borrows the buffer and if I didn't do the scope trick, compilation would fail on the from_utf8 call. (You can't immutably borrow a value which is mutably borrowed at that time.) So in the code above mutable borrow ends when the encoder goes out of scope.

let decoded: User = json::decode(incoming_request).unwrap();
println!("My name is {} and I {} burgers",
    decoded.name, if decoded.likes_burgers { "love" } else { "don't like" });

As you cen see, decoding is also pretty easy. But what happens if we don't know all of the fields in advance? We can use another function in the json module - from_str(). The difference between from_str() and decode() is that the latter may return some struct implementing Decodable while the former returns a Json value. This type has a few methods of its own, including find(). See the example below:

let new_request = "{\"id\":64,\"title\":\"24days\",\"stats\":{\"pageviews\":1500}}";
if let Ok(request_json) = json::from_str(new_request) {
    if let Some(ref stats) = request_json.find("stats") {
        if let Some(ref pageviews) = stats.find("pageviews") {
            println!("Pageviews: {}", pageviews);

We're using the if let language construct which often simplifies pattern matches where we care for only one branch and do nothing if the expression doesn't match. However, at the moment it is hidden behind a feature gate, so you'll need to add a #![feature(if_let)] line in your crate root.

The json! macro

There's one more thing I wanted to show you today. You can embed JSON-like literals directly in your Rust code with the help of the json_macros crate. This is a compiler extension that allows for some nice syntactic sugar like below:

extern crate json_macros;

let config = json!({
    "hostname": "localhost",
    "port": 6543,
    "allowed_methods": ["get", "post"],
    "limits": {
        "bandwidth": (500 * 16u),
        "rate": null
println!("Configuration: {}", config);
println!("Bandwidth: {}", config.search("bandwidth").unwrap());

See also

  • JSON serialization in Rust, part 1 and part 2 - worth reading for an in-depth look on how to implement Encodable/Decodable yourself

Code examples in this article were built with rustc 0.13.0-nightly and json_macros 0.0.4.

The header photo (taken by me) shows Plac Unii shopping centre and office tower in Warsaw, Poland.