toon-encode

Minimal TOON encoder for Rust — Token-Oriented Object Notation for LLM output.

TOON is a compact, human-readable encoding of JSON data that reduces token consumption by 30-50% for list-heavy responses. Field names are declared once in tabular headers instead of repeated on every row.

Usage

use serde::Serialize;

#[derive(Serialize)]
struct Item { name: String, value: i32 }

let items = vec![
    Item { name: "alpha".into(), value: 1 },
    Item { name: "beta".into(), value: 2 },
];

// From any Serialize type
let toon = toon_encode::to_toon_string(&items).unwrap();
// Output:
// [2]{name,value}:
//   alpha,1
//   beta,2

You can also encode a serde_json::Value directly:

let value = serde_json::json!({
    "status": "ok",
    "items": [
        {"file": "main.rs", "kind": "fn", "lines": 25},
        {"file": "lib.rs", "kind": "mod", "lines": 10},
    ]
});

let toon = toon_encode::encode_toon(&value, 0);
// Output:
// status: ok
// items:
//   [2]{file,kind,lines}:
//     main.rs,fn,25
//     lib.rs,mod,10

When TOON helps

TOON is most effective for flat tabular arrays — uniform objects with only primitive fields:

For nested or content-heavy responses, TOON may be larger than JSON.

Encoding rules

• Objects: YAML-like key: value with 2-space indentation
• Uniform arrays of objects: Tabular [N]{field1,field2,...}: header + CSV rows
• Primitive arrays: List with - value per line
• Strings: Quoted when containing ,, :, ", \, [, ], {, }, \n, \r, \t, or starting with -

API

/// Encode any Serialize type as TOON.
pub fn to_toon_string<T: serde::Serialize>(value: &T) -> Result<String, serde_json::Error>;

/// Encode a serde_json::Value as TOON at the given indentation depth.
pub fn encode_toon(value: &serde_json::Value, depth: usize) -> String;

License

MIT