json-scanner

A minimalist JSON parser that reports positions.

I want to parse JSON, manipulate the results, and then (if necessary) emit errors that point to locations in the JSON input. toml and toml-edit do this for TOML, so why not JSON? Well, serde_json doesn't support reporting locations (except on parse errors). sonic_rs doesn't seem to support it either (at least, I couldn't figure out how). The two best options I could find were json-spanned-value and json-syntax, but they both require making a trip through a dynamically typed intermediate representation and I didn't want to do that.

So here's my quick and dirty addition to the zoo of poorly-maintained JSON parsers on crates.io. It's a pull-based streaming JSON parser (delivering events instead of structured data) that reports positions. That's all it does. Maybe at some point it can gain a serde-ish wrapper like the toml crate has with serde_spanned.

let mut parser = Parser::new(br#"{ "foo": "bar" }"#);
assert_eq!(
  parser.next_event().unwrap().unwrap(),
  SpannedEvent {
    start: 0,
    end: 1,
    event: Event::BeginObject,
  }
);

assert_eq!(
  parser.next_event().unwrap().unwrap(),
  SpannedEvent {
    start: 2,
    end: 7,
    event: Event::String("foo".into()),
  }
);

assert_eq!(
  parser.next_event().unwrap().unwrap(),
  SpannedEvent {
    start: 9,
    end: 14,
    event: Event::String("bar".into()),
  }
);

assert_eq!(
  parser.next_event().unwrap().unwrap(),
  SpannedEvent {
    start: 15,
    end: 16,
    event: Event::EndObject,
  }
);

How does it handle numbers?

It doesn't. It validates them, but leaves them as &str. You get to decide how to represent them.

How does it handle strings?

They must be valid UTF-8, and escaped UTF-16 surrogates must be properly paired. There's no "tolerant" parsing mode, sorry.

Does it work?

At least a little. I lifted a bunch of test cases from json-syntax, which I believe were mostly lifted from JSONTestSuite.