TOON Format for Rust

Token-Oriented Object Notation (TOON) is a compact, human-readable format designed for passing structured data to Large Language Models with significantly reduced token usage.

This crate provides the official, spec-compliant Rust implementation of TOON v2.0 with v1.5 optional features, offering both a library (toon-format) and a full-featured command-line tool (toon).

Quick Example

JSON (16 tokens, 40 bytes):

{
  "users": [
    { "id": 1, "name": "Alice" },
    { "id": 2, "name": "Bob" }
  ]
}

TOON (13 tokens, 28 bytes) - 18.75% token savings:

users[2]{id,name}:
  1,Alice
  2,Bob

Features

• Generic API: Works with any Serialize/Deserialize type - custom structs, enums, JSON values, and more
• Spec-Compliant: Fully compliant with TOON Specification v2.0
• v1.5 Optional Features: Key folding and path expansion
• Safe & Performant: Built with safe, fast Rust
• Powerful CLI: Full-featured command-line tool
• Strict Validation: Enforces all spec rules (configurable)
• Well-Tested: Comprehensive test suite with unit tests, spec fixtures, and real-world scenarios

Installation

As a Library

cargo add toon-format

As a CLI Tool

cargo install toon-format

---

Library Usage

Basic Encode & Decode

The encode and decode functions work with any type implementing Serialize/Deserialize:

With custom structs:

use serde::{Serialize, Deserialize};
use toon_format::{encode_default, decode_default};

#[derive(Serialize, Deserialize, Debug, PartialEq)]
struct User {
    name: String,
    age: u32,
    email: String,
}

fn main() -> Result<(), toon_format::ToonError> {
    let user = User {
        name: "Alice".to_string(),
        age: 30,
        email: "alice@example.com".to_string(),
    };

    // Encode to TOON
    let toon = encode_default(&user)?;
    println!("{}", toon);
    // Output:
    // name: Alice
    // age: 30
    // email: alice@example.com

    // Decode back to struct
    let decoded: User = decode_default(&toon)?;
    assert_eq!(user, decoded);

    Ok(())
}

With JSON values:

use serde_json::{json, Value};
use toon_format::{encode_default, decode_default};

fn main() -> Result<(), toon_format::ToonError> {
    let data = json!({
        "users": [
            {"id": 1, "name": "Alice"},
            {"id": 2, "name": "Bob"}
        ]
    });

    // Encode to TOON
    let toon_str = encode_default(&data)?;
    println!("{}", toon_str);
    // Output:
    // users[2]{id,name}:
    //   1,Alice
    //   2,Bob

    // Decode back to JSON
    let decoded: Value = decode_default(&toon_str)?;
    assert_eq!(decoded, data);
    
    Ok(())
}

---

API Reference

Encoding

encode<T: Serialize>(&value, &options) -> Result<String, ToonError>

Encode any serializable type to TOON format. Works with custom structs, enums, collections, and serde_json::Value.

use toon_format::{encode, EncodeOptions, Delimiter, Indent};
use serde_json::json;

let data = json!({"items": ["a", "b", "c"]});

// Default encoding
let toon = encode(&data, &EncodeOptions::default())?;
// items[3]: a,b,c

// Custom delimiter
let opts = EncodeOptions::new()
    .with_delimiter(Delimiter::Pipe);
let toon = encode(&data, &opts)?;
// items[3|]: a|b|c

// Custom indentation
let opts = EncodeOptions::new()
    .with_indent(Indent::Spaces(4));
let toon = encode(&data, &opts)?;

EncodeOptions

Method	Description	Default
with_delimiter(d)	Set delimiter: Comma, Tab, or Pipe	Comma
with_indent(i)	Set indentation (spaces only)	Spaces(2)
with_spaces(n)	Shorthand for Indent::Spaces(n)	2
with_key_folding(mode)	Enable key folding (v1.5)	Off
with_flatten_depth(n)	Set max folding depth	usize::MAX

Decoding

decode<T: Deserialize>(&input, &options) -> Result<T, ToonError>

Decode TOON format into any deserializable type. Works with custom structs, enums, collections, and serde_json::Value.

With custom structs:

use serde::Deserialize;
use toon_format::{decode, DecodeOptions};

#[derive(Deserialize)]
struct Config {
    host: String,
    port: u16,
}

let toon = "host: localhost\nport: 8080";
let config: Config = decode(toon, &DecodeOptions::default())?;

With JSON values:

use serde_json::Value;
use toon_format::{decode, DecodeOptions};

let toon = "name: Alice\nage: 30";

// Default (strict) decode
let json: Value = decode(toon, &DecodeOptions::default())?;

// Non-strict mode (relaxed validation)
let opts = DecodeOptions::new().with_strict(false);
let json: Value = decode(toon, &opts)?;

// Disable type coercion
let opts = DecodeOptions::new().with_coerce_types(false);
let json: Value = decode("active: true", &opts)?;
// With coercion: {"active": true}
// Without: {"active": "true"}

Helper functions:

• encode_default<T>(&value) - Encode with default options
• decode_default<T>(&input) - Decode with default options

DecodeOptions

Method	Description	Default
with_strict(b)	Enable strict validation	true
with_coerce_types(b)	Auto-convert strings to types	true
with_expand_paths(mode)	Enable path expansion (v1.5)	Off

---

v1.5 Features

Key Folding (Encoder)

New in v1.5: Collapse single-key object chains into dotted paths to reduce tokens.

Standard nesting:

data:
  metadata:
    items[2]: a,b

With key folding:

data.metadata.items[2]: a,b

Example:

use serde_json::json;
use toon_format::{encode, EncodeOptions, KeyFoldingMode};

let data = json!({
    "data": {
        "metadata": {
            "items": ["a", "b"]
        }
    }
});

// Enable key folding
let opts = EncodeOptions::new()
    .with_key_folding(KeyFoldingMode::Safe);

let toon = encode(&data, &opts)?;
// Output: data.metadata.items[2]: a,b

With Depth Control

let data = json!({"a": {"b": {"c": {"d": 1}}}});

// Fold only 2 levels
let opts = EncodeOptions::new()
    .with_key_folding(KeyFoldingMode::Safe)
    .with_flatten_depth(2);

let toon = encode(&data, &opts)?;
// Output:
// a.b:
//   c:
//     d: 1

Safety Features

Key folding only applies when:

• All segments are valid identifiers (a-z, A-Z, 0-9, _)
• Each level contains exactly one key
• No collision with sibling literal keys
• Within the specified flatten_depth

Keys like full-name, user.email (if quoted), or numeric keys won't be folded.

Path Expansion (Decoder)

New in v1.5: Automatically expand dotted keys into nested objects.

Compact input:

a.b.c: 1
a.b.d: 2
a.e: 3

Expanded output:

{
  "a": {
    "b": {
      "c": 1,
      "d": 2
    },
    "e": 3
  }
}

Example:

use serde_json::Value;
use toon_format::{decode, DecodeOptions, PathExpansionMode};

let toon = "a.b.c: 1\na.b.d: 2";

// Enable path expansion
let opts = DecodeOptions::new()
    .with_expand_paths(PathExpansionMode::Safe);

let json: Value = decode(toon, &opts)?;
// {"a": {"b": {"c": 1, "d": 2}}}

Round-Trip Example:

use serde_json::{json, Value};
use toon_format::{encode, decode, EncodeOptions, DecodeOptions, KeyFoldingMode, PathExpansionMode};

let original = json!({
    "user": {
        "profile": {
            "name": "Alice"
        }
    }
});

// Encode with folding
let encode_opts = EncodeOptions::new()
    .with_key_folding(KeyFoldingMode::Safe);
let toon = encode(&original, &encode_opts)?;
// Output: "user.profile.name: Alice"

// Decode with expansion
let decode_opts = DecodeOptions::new()
    .with_expand_paths(PathExpansionMode::Safe);
let restored: Value = decode(&toon, &decode_opts)?;

assert_eq!(restored, original); // Perfect round-trip!

Quoted Keys Remain Literal:

use serde_json::Value;
use toon_format::{decode, DecodeOptions, PathExpansionMode};

let toon = r#"a.b: 1
"c.d": 2"#;

let opts = DecodeOptions::new()
    .with_expand_paths(PathExpansionMode::Safe);
let json: Value = decode(toon, &opts)?;
// {
//   "a": {"b": 1},
//   "c.d": 2        <- quoted key preserved
// }

---

CLI Usage

Basic Commands

# Auto-detect from extension
toon data.json        # Encode
toon data.toon        # Decode

# Force mode
toon -e data.txt      # Force encode
toon -d output.txt    # Force decode

# Pipe from stdin
cat data.json | toon
echo '{"name": "Alice"}' | toon -e

Encode Options

# Custom delimiter
toon data.json --delimiter pipe
toon data.json --delimiter tab

# Custom indentation
toon data.json --indent 4

# Key folding (v1.5)
toon data.json --fold-keys
toon data.json --fold-keys --flatten-depth 2

# Show statistics
toon data.json --stats

Decode Options

# Pretty-print JSON
toon data.toon --json-indent 2

# Relaxed validation
toon data.toon --no-strict

# Disable type coercion
toon data.toon --no-coerce

# Path expansion (v1.5)
toon data.toon --expand-paths

Full Example

$ echo '{"data":{"meta":{"items":["x","y"]}}}' | toon --fold-keys --stats

data.meta.items[2]: x,y

Stats:
+--------------+------+------+---------+
| Metric       | JSON | TOON | Savings |
+======================================+
| Tokens       | 13   | 8    | 38.46%  |
|--------------+------+------+---------|
| Size (bytes) | 38   | 23   | 39.47%  |
+--------------+------+------+---------+

---

Testing

The library includes a comprehensive test suite covering core functionality, edge cases, spec compliance, and real-world scenarios.

# Run all tests
cargo test

# Run specific test suites
cargo test --test spec_fixtures
cargo test --lib

# With output
cargo test -- --nocapture

Error Handling

All operations return Result<T, ToonError> with descriptive error messages:

use serde_json::Value;
use toon_format::{decode_strict, ToonError};

match decode_strict::<Value>("items[3]: a,b") {
    Ok(value) => println!("Success: {:?}", value),
    Err(ToonError::LengthMismatch { expected, found, .. }) => {
        eprintln!("Array length mismatch: expected {}, found {}", expected, found);
    }
    Err(e) => eprintln!("Error: {}", e),
}

Error Types

• ParseError - Syntax errors with line/column info
• LengthMismatch - Array length doesn't match header
• TypeMismatch - Unexpected value type
• InvalidStructure - Malformed TOON structure
• SerializationError / DeserializationError - Conversion failures

---

Examples

Run with cargo run --example examples to see all examples:

• structs.rs - Custom struct serialization
• tabular.rs - Tabular array formatting
• arrays.rs - Various array formats
• arrays_of_arrays.rs - Nested arrays
• objects.rs - Object encoding
• mixed_arrays.rs - Mixed-type arrays
• delimiters.rs - Custom delimiters
• round_trip.rs - Encode/decode round-trips
• decode_strict.rs - Strict validation
• empty_and_root.rs - Edge cases

---

Resources

• 📖 TOON Specification v2.0
• 📦 Crates.io Package
• 📚 API Documentation
• 🔧 Main Repository (JS/TS)
• 🎯 Benchmarks & Performance

---

Contributing

Contributions are welcome! Please see CONTRIBUTING.md for guidelines.

Development

# Clone the repository
git clone https://github.com/your-org/toon-rust.git
cd toon-rust

# Run tests
cargo test --all

# Run lints
cargo clippy -- -D warnings

# Format code
cargo fmt

# Build docs
cargo doc --open

---

License

MIT License © 2025-PRESENT Johann Schopplich and Shreyas K S