There are two implementations of the JSON parser. The first one is the hand-rolled version I wrote myself while the second is written using the nom-rs library.

This project is for my understanding of how to implement basic tokenization and parsing, as well as how to use a parser combinator library like nom.

pub enum Number<'a> {
    Float(&'a str),
    Int(&'a str),
}

pub enum JsonValue<'a> {
    Bool(bool),
    String(&'a str),
    Number(Number<'a>),
    Null,
    Array(Vec<JsonValue<'a>>),
    Object(Vec<(&'a str, JsonValue<'a>)>),
}

The data structure for a JSON document is quite straightforward. A Vec of pairs is used to represent JSON objects instead of a HashMap in order to preserve the key-value order of the object. A JSON number can be in multiple formats such as 123, 1.23, 1e8 etc. so that is also taken into account in the nom parser. The hand-rolled parser, however, only supports integer numbers.

use json_parser_rs;

fn main() {
    let input = r#"{"foo": "bar"}"#;
    let parsed_json_default = parse_json(&input); // hand-rolled parser
    let parsed_json_nom = nom_json::parse_json(&input); // parser written with nom
    println!("{:?}", parsed_json_default); // Object([("foo", String("bar"))])
    println!("{:?}", parsed_json_nom); // Object([("foo", String("bar"))])
}

# using stdin
cat '{"foo":"bar"}' | json_parser_rs
# passing in a file
json_parser_rs foo.json
# compact mode
json_parser_rs -c foo.json
# custom indent spaces (number of spaces each level of indentation has, default is 2 spaces)
json_parser_rs --indent-spaces 4 foo.json

The parser/formatter is very basic and definitely does not implement the entire JSON spec. String escapes has also not been implemented.