nojson

A flexible Rust JSON library with no dependencies and no macros.

nojson is a flexible and ergonomic JSON library for Rust that offers a balance between the type-safety of Rust and the dynamic nature of JSON. Unlike serde, which typically requires one-to-one mapping between Rust types and JSON structures (or other serialization formats), nojson provides a toolbox approach that allows you to leverage both type-level programming and imperative code flexibility.

Features

• No strict one-to-one type mapping required - Mix type-level programming with imperative flexibility as needed
• Clean parsing error messages with position information for better debugging
• Customizable validation - Add application-specific validation rules with proper error context
• Flexible formatting options including pretty-printing with customizable indentation
• Low-level access to the JSON structure when needed
• High-level conveniences for common JSON operations

Core Design Principles

• A toolbox rather than a monolithic framework
• Gain the benefits of both type-level programming and imperative code
• Easy to add custom validations with rich error context
• Error messages that precisely indicate the problematic position in the JSON text

Getting Started

Parsing JSON with Strong Typing

The Json<T> wrapper allows parsing JSON text into Rust types that implement the FromRawJsonValue trait:

use nojson::Json;

fn main() -> Result<(), nojson::JsonParseError> {
    // Parse a JSON array into a typed Rust array
    let text = "[1, null, 2]";
    let value: Json<[Option<u32>; 3]> = text.parse()?;
    assert_eq!(value.0, [Some(1), None, Some(2)]);
    Ok(())
}

Generating JSON

The DisplayJson trait allows converting Rust types to JSON:

use nojson::Json;

// Generate a JSON array from a Rust array
let value = [Some(1), None, Some(2)];
assert_eq!(Json(value).to_string(), "[1,null,2]");

In-place JSON Generation with Formatting

The json() function provides a convenient way to generate JSON with custom formatting:

use nojson::json;

// Compact JSON
let compact = json(|f| f.value([1, 2, 3]));
assert_eq!(compact.to_string(), "[1,2,3]");

// Pretty-printed JSON with custom indentation
let pretty = json(|f| {
    f.set_indent_size(2);
    f.set_spacing(true);
    f.array(|f| {
        f.element(1)?;
        f.element(2)?;
        f.element(3)
    })
});

assert_eq!(
    format!("\n{}", pretty),
    r#"
[
  1,
  2,
  3
]"#
);

Custom Types

Implementing DisplayJson and FromRawJsonValue for your own types:

use nojson::{DisplayJson, FromRawJsonValue, Json, JsonFormatter, JsonParseError, RawJsonValue};

struct Person {
    name: String,
    age: u32,
}

impl DisplayJson for Person {
    fn fmt(&self, f: &mut JsonFormatter<'_, '_>) -> std::fmt::Result {
        f.object(|f| {
            f.member("name", &self.name)?;
            f.member("age", self.age)
        })
    }
}

impl<'text> FromRawJsonValue<'text> for Person {
    fn from_raw_json_value(value: RawJsonValue<'text, '_>) -> Result<Self, JsonParseError> {
        let ([name, age], []) = value.to_fixed_object(["name", "age"], [])?;
        Ok(Person {
            name: name.try_to()?,
            age: age.try_to()?,
        })
    }
}

fn main() -> Result<(), JsonParseError> {
    // Parse JSON to Person
    let json_text = r#"{"name":"Alice","age":30}"#;
    let person: Json<Person> = json_text.parse()?;

    // Generate JSON from Person
    assert_eq!(Json(&person.0).to_string(), json_text);

    Ok(())
}

Advanced Features

Custom Validations

You can add custom validations using JsonParseError::invalid_value():

use nojson::{JsonParseError, RawJson, RawJsonValue};

fn parse_positive_number(text: &str) -> Result<u32, JsonParseError> {
    let json = RawJson::parse(text)?;
    let raw_value = json.value();

    let num: u32 = raw_value.as_number_str()?
        .parse()
        .map_err(|e| JsonParseError::invalid_value(raw_value, e))?;

    if num == 0 {
        return Err(JsonParseError::invalid_value(
            raw_value,
            "Expected a positive number, got 0"
        ));
    }

    Ok(num)
}

Error Handling with Context

Rich error information helps with debugging:

use nojson::{JsonParseError, RawJson};

let text = r#"{"invalid": 123e++}"#;
let result = RawJson::parse(text);

if let Err(error) = result {
    println!("Error: {}", error);

    // Get line and column information
    if let Some((line, column)) = error.get_line_and_column_numbers(text) {
        println!("At line {}, column {}", line, column);
    }

    // Get the full line with the error
    if let Some(line_text) = error.get_line(text) {
        println!("Line content: {}", line_text);
    }
}