#json #parser


A crate for parsing JSON into Rust types

3 stable releases

1.1.0 Sep 13, 2022
1.0.1 Feb 21, 2022
1.0.0 Feb 20, 2022

#455 in Parser implementations

GPL-3.0 license



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Parson is a Rust crate that parses a JSON string so that the JSON data can be used in another Rust project


My interest in Rust is growing rapidly due to my boredom of TypeScript and Web Development. Making a Parson in Rust can possibly teach me a lot more about Rust than I currently know. Learning how to make a parser will also teach me about the steps to make a Lexer and a Parser. This is a stepping stone to making my own JavaScript interpreter, my Rust dream project.


  • JSON Parsing into Rust types
    • A JSON String parses into a Rust owned string
    • A JSON Number parses into a Rust f64
    • A JSON Boolean parses into a Rust bool
    • A JSON Null is not parsable into Rust since there is no Null value
  • Error Messages
    • Parser will let you know on what line and column the parsing error occured on


Add this to your Rust project's Cargo.toml file

parson = "<version number>"


Since the JSONValue struct implements FromStr, you can parse it from a string directly into a JSONValue. If an error occurs in the parsing, you will get back a JSONError

use parson::JSONValue;

fn main() {
    let json = r#"{ "key": "value" }"#;
    let json_value = json.parse::<JSONValue>().expect("JSON parse failed");
	println!("{}", json_value);

The JSONValue struct

The JSONValue struct holds data contianing either of the following

JSONValue has methods to get the value of each type:

JSONValue also has methods to check if the value is of each type

  • is_string(): bool
  • is_number(): bool
  • is_boolean(): bool
  • is_null(): bool
  • is_array(): bool
  • is_object(): bool

JSONValue has a method which returns an enum of the type of value it holds

let json_value = /* result of parsing a string */

match json_value.get_type() {
	JSONType::String(json_string) => println!("{}", json_string.get_string()),
	JSONType::Number(json_number) => println!("{}", json_number.get_number()),
	JSONType::Boolean(json_boolean) => println!("{}", json_boolean.get_boolean()),
	JSONType::Null(json_null) => println!("Null"),
	JSONType::Array(json_array) => println!("{} items", json_array.len()),
	JSONType::Object(json_object) => println!("{} pairs", json_object.len()),

The JSONString struct

The JSONString struct holds data about a string in your input json as a String. The JSONString implementation has 1 method to get the data within it.

  • get_string(): String

The JSONNumber struct

The JSONNumber struct holds data about a number in your input json as an f64. The JSONNumber implementation has 1 method to get the data within it.

  • get_number(): f64

The JSONBoolean struct

The JSONBoolean struct holds data about a boolean in your input json as a bool. The JSONBoolean implementation has 1 method to get the data within it.

  • get_boolean(): bool

The JSONNull struct

The JSONNull struct is a representation of a null value in your input json. The JSONNull implementation has no methods to get the data within it since Rust has no way to represent Null values.

The JSONArray struct

The JSONArray struct holds data about an array in your input json as a Vec<JSONValue>. The JSONArray implementation has 3 methods to get the data within it.

The JSONObject struct

The JSONObject struct holds data about an object in your input json as a IndexMap<String, JSONValue>, which is a HashMap that maintains the order data was added to it. The JSONObject implementation has 3 methods to get the data within it.

The JSONError struct

The JSONError struct holds the message of an error that happened when parsing your input json.

JSONError has 1 method to get the error message within it.

  • get_message(): String


I also learnt how to make a basic JSON Parser from here. I understood and improvised the way the author wrote the Lexer and Parsers to make Parson more bug free.

Built with

  • Rust
    • indexmap