TOON Format for Zig

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 is a Zig implementation of the official TOON specification.

Quick Example

const std = @import("std");
const toon = @import("toon");

pub fn main() !void {
    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .{};
    defer _ = gpa.deinit();
    const alloc = gpa.allocator();

    const src =
        \\users[2]{id,name,role}:
        \\  1,Alice,admin
        \\  2,Bob,user
    ;

    var result = try toon.parse(alloc, src, .{});
    defer result.deinit();
    // result.value is a std.json.Value

    const encoded = try toon.stringify(alloc, result.value, .{});
    defer alloc.free(encoded);
    // encoded == src
}

Features

• Spec-compliant with TOON v3.0; passes all 358 official conformance fixtures.
• Encoder (JSON value -> TOON) with tabular array detection, delimiter scoping (comma / tab / pipe), canonical number formatting, and deterministic quoting.
• Decoder (TOON -> JSON value) with strict-mode validation, indentation enforcement, root-form detection, and full quoted-string escape handling.
• v1.5 features: key folding (safe, with flattenDepth) on encode; path expansion (safe, deep-merge, conflict detection) on decode.
• No external dependencies beyond the Zig standard library.
• Arena-allocated decode output -- single deinit call frees the entire tree.

Installation

Requires Zig 0.15.2.

Add the dependency to your build.zig.zon:

.dependencies = .{
    .toon = .{
        .url = "https://github.com/LatentEvals/toon-zig/archive/refs/tags/v0.1.3.tar.gz",
        .hash = "toon-0.1.3-1wXXCUBCAQAszySx8OsCtALWeXjee4KNU9M7s0hH4WNN",
    },
},

Then in your build.zig:

const toon_dep = b.dependency("toon", .{
    .target = target,
    .optimize = optimize,
});
exe.root_module.addImport("toon", toon_dep.module("toon"));

For local development, you can also point at a path:

.toon = .{ .path = "../toon-zig" },

Library Usage

Decoding

const result = try toon.parse(alloc, input, .{});
defer result.deinit();

switch (result.value) {
    .object => |obj| {
        if (obj.get("name")) |name| {
            std.debug.print("name = {s}\n", .{name.string});
        }
    },
    else => {},
}

Encoding

var obj: std.json.ObjectMap = .init(alloc);
defer obj.deinit();
try obj.put("id", .{ .integer = 1 });
try obj.put("name", .{ .string = "Ada" });

const out = try toon.stringify(alloc, .{ .object = obj }, .{});
defer alloc.free(out);
// out == "id: 1\nname: Ada"

Round-tripping JSON

var parsed = try std.json.parseFromSlice(std.json.Value, alloc, json_text, .{});
defer parsed.deinit();

const toon_text = try toon.stringify(alloc, parsed.value, .{});
defer alloc.free(toon_text);

API Reference

toon.parse

pub fn parse(
    allocator: std.mem.Allocator,
    input: []const u8,
    options: DecodeOptions,
) DecodeError!ParseResult

Parses a TOON document into a std.json.Value. The returned ParseResult owns its memory via an internal arena -- call result.deinit() to free.

toon.stringify

pub fn stringify(
    allocator: std.mem.Allocator,
    value: std.json.Value,
    options: EncodeOptions,
) EncodeError![]u8

Encodes a JSON value to a freshly allocated TOON string. Caller owns the slice.

EncodeOptions

Field	Type	Default	Description
indent	u8	2	Spaces per indentation level.
delimiter	Delimiter	.comma	One of .comma, .tab, .pipe.
key_folding	KeyFolding	.off	Fold single-key chains into dotted paths.
flatten_depth	?u32	null	Max segments to fold (null = unlimited).

DecodeOptions

Field	Type	Default	Description
indent	u8	2	Expected spaces per indentation level.
strict	bool	true	Enforce spec §14 validation rules.
expand_paths	ExpandPaths	.off	Split dotted keys into nested objects.

v1.5 Features

Key Folding (Encode)

const opts: toon.EncodeOptions = .{ .key_folding = .safe };
// {"a": {"b": {"c": 1}}} -> "a.b.c: 1"

const partial: toon.EncodeOptions = .{ .key_folding = .safe, .flatten_depth = 2 };
// {"a": {"b": {"c": {"d": 1}}}} -> "a.b:\n  c:\n    d: 1"

Folding is collision-aware: chains are skipped when the resulting dotted key would clash with a literal sibling or a root-level dotted key.

Path Expansion (Decode)

const opts: toon.DecodeOptions = .{ .expand_paths = .safe };
// "a.b.c: 1" -> {"a": {"b": {"c": 1}}}
// Multiple dotted keys deep-merge; quoted dotted keys are preserved as literals.

In strict = true mode (default), conflicting paths raise error.ExpansionConflict. With strict = false, last-write-wins applies.

Error Handling

parse returns a tagged DecodeError. Common variants:

Error	Meaning
MissingColon	Key without a trailing :.
InvalidEscape	Unknown escape sequence in a quoted string.
UnterminatedString	Quoted string missing its closing ".
CountMismatch	Array element count differs from the declared [N].
FieldCountMismatch	Tabular row width differs from the field list.
InvalidIndentation	Indent not a multiple of indent (strict mode).
TabInIndentation	Tab character in indentation (strict mode).
BlankLineInArray	Blank line inside an array body (strict mode).
ExpansionConflict	Path-expansion deep-merge conflict (strict mode).

stringify can return allocator, writer, and formatting errors. Non-finite float inputs (NaN, +Infinity, -Infinity) are normalized to null to match TOON encoding normalization.

Testing

zig build test

This runs both the library's unit tests and the full language-agnostic conformance suite from the spec repository.

Resources

• TOON Specification (v3.0)
• Conformance Test Fixtures
• Other implementations
• Reference TypeScript implementation

License

MIT License

Copyright (c) 2026 Montana Flynn & LatentEvals

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.