SION, a serialization format a little more expressive than JSON

This page introduces and describes SION, a data serialization format that addresses the shortcomings of JSON, arguably the most popular data serialization format today.

Because JSON is not expressive enough

SION is a data serialization format:

Named after "Swift Interchangeable Object Notation". As JSON is originated from a {ECMA,Java}Script literal, SION is originated from a Swift literal.
- but like JSON, it should not be considered acronym. It is a textual data format of its own and independent of programming languages.
It can serialize anything JSON can. Plus
- support Data - binary blobs
- support Date
non-String keys in Dictionary
- Int and Double distinctively, not Number.
  - Therefore you can exchange 64-bit integers losslessly.
No {} needed for Dictionary (aka Map, Hash, Object…).
- [ and ] delimit both Array and Dictionary. What makes a Dictionary a Dictionary is : between keys and values, not a different pair of brackets.
Double can be expressed both in decimal and C99 Hexadecimal floating-point notation.
NaN and ±Infinity are valid.
- This is necessary for really interchangeable format. Suppose you want to send calculation results with errors.
// comment is allowed!. // up to newline.
- The lack of comment support of JSON definitely makes it suck as a configuration file format.
Roughly equivalent to MsgPack in terms of capability.
- MsgPack is a binary serialization while SION is a text serialization.

Below is a table of a few notable serialization formats and capabilities.

Type	SION	MsgPack	JSON	Property List	Comment
`Nil`	✔︎	✔︎	✔︎	❌	plist: .binary only
`Bool`	✔︎	✔︎	✔︎	✔︎
`Int`	✔︎	✔︎	❌	✔︎	64bit
`Double`	✔︎	✔︎	✔︎	✔︎	JSON's Number
`String`	✔︎	✔︎	✔︎	✔︎	utf-8 encoded
`Data`	✔︎	✔︎	❌	✔︎	binary blob
`Date`	✔︎	✔︎	❌	✔︎	.timeIntervalSince1970 in `Double`
`[Self]`	✔︎	✔︎	✔︎	✔︎	aka Array
`[String:Self]`	✔︎	✔︎	✔︎	✔︎	aka Object, Map…
`[Self:Self]`	✔︎	✔︎	❌	❌	non-`String` keys
`Ext`	✔︎	✔︎	❌	❌	msgpack extension

One bracket to rule them all

Unlike JSON, SION needs no {} for Dictionary (aka Map, Hash, Object…). Both Array and Dictionary are delimited by [ and ]. They are distinguished not by brackets but by what is inside — key : value pairs make it a Dictionary:

[ "zero", "one", "two" ]        // an Array
[ 0:"zero", 1:"one", 2:"two" ]  // a Dictionary
[]                              // an empty Array
[:]                             // an empty Dictionary

This is not just aesthetics:

Simpler syntax. One kind of bracket for collections means fewer tokens, fewer delimiter-mismatch errors, and a smaller grammar.
Collections are one concept. A Dictionary is just a collection whose elements are keyed — : is what says so. An Array is arguably a Dictionary implicitly keyed by 0, 1, 2… as a matter of fact ["zero", "one", "two"] and [0:"zero", 1:"one", 2:"two"] are semantically equivalent.
A natural consequence of non-String keys. JSON's {} carries the implicit promise that keys are strings. Once any value can be a key, that distinction loses its reason to exist.

The only price is [:], the empty Dictionary, since [] is taken by the empty Array. A fair trade for a whole class of brackets.

Implementations

Swift
- swift-sion by dankogai
Go
- go-sion by mattn
ECMAScript (aka JavaScript)
- js-sion by dankogai
Rust
- sion-rs by pandaman64
Python 3
- py-sion by kamimura
Perl 5
- p5-sion by dankogai
Raku
- raku-sion by dankogai

Syntax

ANTLR4(EBNF)
- sion-antlr4 by kamimura

Examples

Below is an example of data encoded in SION.

[
    "array": [
        nil,
        true,
        1,      // Int in decimal
        1.0,    // Double in decimal
        "one",
        [1],
        ["one" : 1.0]
    ],
    "bool": true,
    "data": .Data("R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7"),
    "date": .Date(0x0p+0),
    "dictionary": [
        "array" : [],
        "bool" : false,
        "double" : 0.0,
        "int" : 0,
        "nil" : nil,
        "object" : [:],
        "string" : ""
    ],
    "double": 0x1.518f5c28f5c29p+5, // double in hex
    "ext": .Ext("1NTU"),            // 0xd4,0xd4,0xd4
    "int": -42,                     // Int in decimal
    "nil": nil,
    "string": "漢字、カタカナ、ひらがなの入ったstring😇",
    "url": "https://github.com/dankogai/"
]

As you notice,

comments are allowed - // up to newline.
non-String keys are allowed. Any data conforming to SION can be a {Dictionary,Map,Object} key.
no {} in sight. The whole document is a Dictionary, delimited by [ and ] like everything else.

And below is an example of JSON-compatible SION…

[
    "array" : [
        nil,
        true,
        1,    // Int in decimal
        1.0,  // Double in decimal
        "one",
        [1],
        ["one" : 1.0]
    ],
    "bool" : true,
    "dictionary" : [
        "array" : [],
        "bool" : false,
        "double" : 0.0,
        "int" : 0,
        "nil" : nil,
        "object" : [:],
        "string" : ""
    ],
    "double" : 42.195,
    "int" : -42,
    "nil" : nil,
    "string" : "漢字、カタカナ、ひらがなの入ったstring😇",
    "url" : "https://github.com/dankogai/"
]

…which turns into a JSON below.

{
    "array" : [
        null,
        true,
        1,
        1.0,
        "one",
        [1],
        {"one": 1.0}
    ],
    "bool" : true,
    "dictionary": {
        "array": [],
        "bool": false,
        "double": 0.0,
        "int": 0,
        "nil": null,
        "object": {},
        "string": ""
    },
    "double": 42.195,
    "int": -42,
    "nil": null,
    "string": "漢字、カタカナ、ひらがなの入ったstring😇",
    "url": "https://github.com/dankogai/"
}

Note the {}s are back, along with the comments gone.