toon/docs/reference/syntax-cheatsheet.md

# Syntax Cheatsheet

Quick reference for mapping JSON to TOON format. For rigorous, normative syntax rules and edge cases, see the [Specification](/reference/spec).

## Objects

::: code-group

```json [JSON]
{
  "id": 1,
  "name": "Ada"
}
```

```yaml [TOON]
id: 1
name: Ada
```

:::

## Nested Objects

::: code-group

```json [JSON]
{
  "user": {
    "id": 1,
    "name": "Ada"
  }
}
```

```yaml [TOON]
user:
  id: 1
  name: Ada
```

:::

## Primitive Arrays

::: code-group

```json [JSON]
{
  "tags": ["foo", "bar", "baz"]
}
```

```yaml [TOON]
tags[3]: foo,bar,baz
```

:::

## Tabular Arrays

::: code-group

```json [JSON]
{
  "items": [
    { "id": 1, "qty": 5 },
    { "id": 2, "qty": 3 }
  ]
}
```

```yaml [TOON]
items[2]{id,qty}:
  1,5
  2,3
```

:::

## Mixed / Non-Uniform Arrays

::: code-group

```json [JSON]
{
  "items": [1, { "a": 1 }, "x"]
}
```

```yaml [TOON]
items[3]:
  - 1
  - a: 1
  - x
```

:::

> [!NOTE]
> When a list-item object has a tabular array as its first field, the tabular header appears on the hyphen line. Rows are indented two levels deeper than the hyphen, and other fields are indented one level deeper. This is the canonical encoding for this pattern.

::: code-group

```yaml [Multi-field object]
items[1]:
  - users[2]{id,name}:
      1,Ada
      2,Bob
    status: active
```

```yaml [Single-field object]
items[1]:
  - users[2]{id,name}:
      1,Ada
      2,Bob
```

:::

## Arrays of Arrays

::: code-group

```json [JSON]
{
  "pairs": [[1, 2], [3, 4]]
}
```

```yaml [TOON]
pairs[2]:
  - [2]: 1,2
  - [2]: 3,4
```

:::

## Root Arrays

::: code-group

```json [JSON]
["x", "y", "z"]
```

```yaml [TOON]
[3]: x,y,z
```

:::

## Empty Containers

::: code-group

```json [Empty Object]
{}
```

```yaml [Empty Object]
(empty output)
```

:::

::: code-group

```json [Empty Array]
{
  "items": []
}
```

```yaml [Empty Array]
items[0]:
```

:::

## Quoting Special Cases

### Strings That Look Like Literals

::: code-group

```json [JSON]
{
  "version": "123",
  "enabled": "true"
}
```

```yaml [TOON]
version: "123"
enabled: "true"
```

:::

These strings must be quoted because they look like numbers/booleans.

### Strings with Active Delimiter

::: code-group

```json [JSON]
{
  "note": "hello, world"
}
```

```yaml [TOON]
note: "hello, world"
```

:::

Strings containing the active delimiter (comma by default) must be quoted.

### Strings with Leading/Trailing Spaces

::: code-group

```json [JSON]
{
  "message": " padded "
}
```

```yaml [TOON]
message: " padded "
```

:::

### Empty String

::: code-group

```json [JSON]
{
  "name": ""
}
```

```yaml [TOON]
name: ""
```

:::

## Quoting Rules Summary

Strings **must** be quoted if they:

- Are empty (`""`)
- Have leading or trailing whitespace
- Equal `true`, `false`, or `null` (case-sensitive)
- Look like numbers (e.g., `"42"`, `"-3.14"`, `"1e-6"`, `"05"`)
- Contain special characters: `:`, `"`, `\`, `[`, `]`, `{`, `}`, newline, tab, carriage return
- Contain the active delimiter (comma by default, or tab/pipe if declared in header)
- Equal `"-"` or start with `"-"` followed by any character

Otherwise, strings can be unquoted. Unicode and emoji are safe:

```yaml
message: Hello 世界 👋
note: This has inner spaces
```

## Escape Sequences

Only five escape sequences are valid in quoted strings:

| Character | Escape |
|-----------|--------|
| Backslash (`\`) | `\\` |
| Double quote (`"`) | `\"` |
| Newline | `\n` |
| Carriage return | `\r` |
| Tab | `\t` |

All other escapes (e.g., `\x`, `\u`) are invalid.

## Array Headers

### Basic Header

```
key[N]:
```

- `N` = array length
- Default delimiter: comma

### Tabular Header

```
key[N]{field1,field2,field3}:
```

- `N` = array length
- `{fields}` = column names
- Default delimiter: comma

### Alternative Delimiters

::: code-group

```yaml [Tab Delimiter]
items[2	]{id	name}:
  1	Alice
  2	Bob
```

```yaml [Pipe Delimiter]
items[2|]{id|name}:
  1|Alice
  2|Bob
```

:::

The delimiter symbol appears inside the brackets and braces.

## Key Folding (Optional)

Standard nesting:

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

With key folding (`keyFolding: 'safe'`):

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

See [Format Overview – Key Folding](/guide/format-overview#key-folding-optional) for details.

## Type Conversions

| Input | Output |
|-------|--------|
| Finite number | Canonical decimal (no exponent, no trailing zeros) |
| `NaN`, `Infinity`, `-Infinity` | `null` |
| `BigInt` (safe range) | Number |
| `BigInt` (out of range) | Quoted decimal string |
| `Date` | ISO string (quoted) |
| `undefined`, `function`, `symbol` | `null` |