Markdown Syntax for Object Notation

This document provides an introduction to Markdown Syntax for Object Notation (MSON), a Markdown syntax compatible with describing JSON and JSON Schema.

What?

MSON is a plain-text, human and machine readable, description format for describing data structures in common markup formats such as JSON, XML or YAML.

What for?

The aim of this description format is to facilitate the discussion (and thus validation) of data structures. The format, being agnostic to the common markup formats, is well suited for "resource & representations" and "content negotiation" scenarios.

In addition, this format also offers (limited) serialization functionality.

Similar to the original Markdown to HTML (markup) conversion, MSON enables conversion to other markup formats.

Who & Why?

This format is being developed by Apiary as a part of the API Blueprint syntax to provide a means for description and validation of HTTP payloads and DRY, media-type agnostic, resource descriptions and to simplify content-negotiation.

NOTE: While this document focuses primarily on JSON and JSON Schema examples, the underlying specification will ultimately allow producing XML or YAML representations from MSON.

Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119.

Example 1

A simple object structure and its associated JSON expression.

MSON

- id: 1
- name: A green door
- price: 12.50
- tags: home, green

Rendered Markdown

• id: 1
• name: A green door
• price: 12.50
• tags: home, green

JSON

{
    "id": "1",
    "name": "A green door",
    "price": "12.50",
    "tags": [ "home", "green" ]
}

NOTE: By default, a Markdown list item is considered to be a string type.

Example 2

A Named Type with its associated JSON expression and JSON Schema.

MSON

# Product
A product from Acme's catalog

## Properties

- id: 1 (number, required) - The unique identifier for a product
- name: A green door (string, required) - Name of the product
- price: 12.50 (number, required)
- tags: home, green (array[string])

Rendered Markdown

Product

A product from Acme's catalog

Properties

• id: 1 (number, required) - The unique identifier for a product
• name: A green door (string, required) - Name of the product
• price: 12.50 (number, required)
• tags: home, green (array[string], fixed-type)

JSON

{
    "id": 1,
    "name": "A green door",
    "price": 12.50,
    "tags": [ "home", "green" ]
}

NOTE: The id and price sample values are numbers given the explicit declaration of their base type of number vs. the default of string as in Example 1.

JSON Schema

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "title": "Product",
    "description": "A product from Acme's catalog",
    "type": "object",
    "properties": {
        "id": {
            "description": "The unique identifier for a product",
            "type": "number"
        },
        "name": {
            "description": "Name of the product",
            "type": "string"
        },
        "price": {
            "type": "number"
        },
        "tags": {
            "type": "array",
            "items": {
                "type": "string"
            }
        }
    },
    "required": ["id", "name", "price"]
}

NOTE: This proposal covers only basic features of JSON Schema. At this moment, it is out of the scope of this syntax to support all the JSON Schema keywords (such as uniqueItems, exclusiveMinimum, etc.).

MSON Language Specification

The rest of this document covers some advanced syntax examples. Refer to the MSON Language Specification for the complete MSON Grammar Reference.

Quick Links

• MSON Language Specification
• Objects & Arrays
• Advanced Objects
• Advanced Arrays
• Escaping
• Multi-line Descriptions
• Variable Property Name
• Type Definition
• Referencing
• Mixins

Objects & Arrays

By default, a Markdown list item with a nested Markdown list is considered to be an object structure:

MSON

- address
    - street
    - city
    - state

JSON

{
    "address" : {
        "street": "",
        "city": "",
        "state": ""
    }
}

If a Markdown list's items are intended to be literals (represent array values), the type of the parent list item MUST be explicitly set to array:

MSON

- address (array)
    - street
    - city
    - state

Or, alternately:

- address: street, city, state (array)

In this latter case, using a comma-separated list of values, the type (array) is implied and thus MAY be omitted.

JSON

{
    "address": [ "street", "city", "state" ]
}

NOTE: The values "street", "city", and "state" are solely sample values of the address array in this example. No constraints are implied on the quantity or types of values in such an array other than that they MAY be string literals.

Advanced Objects

Non-uniform Property

A Property whose value can be of different types is defined by the enum structure type:

MSON

- tag (enum)
    - green (string)
    - (object)
        - tag_id: 1
        - label: green

Rendered Markdown

• tag (enum)
  • green (string)
  • (object)
    • tag_id: 1
    • label: green

JSON

{
    "tag": "green"
}

or

{
    "tag": {
        "tag_id": "1",
        "label": "green"
    }
}

NOTE: In an enum structure, in contrast to an array type structure, a value like "green" is a fully-qualified value of the enumeration vs. being a sample value.

Mutually Exclusive Properties

By default, all properties are optional and may or may not be included in the object. If there is a choice of mutually exclusive properties available MSON defines a One Of type:

MSON

- city
- One Of
    - state
    - province
- country

Rendered Markdown

• city
• One Of
  • province
  • state
• country

JSON

{
    "street": "",
    "province": "",
    "country": ""
}

or

{
    "street": "",
    "state": "",
    "country": ""
}

NOTE: Because an enum type MUST define a list of types vs. properties and by default an un-nested Markdown list defines properties of an implied object structure, the One Of type declaration MUST only be used to indicate mutually exclusive properties in an object structure.

Advanced Arrays

Array of Mixed Types

MSON

- tags (array)
    - hello (string)
    - 42 (number)

Rendered Markdown

• tags (array)
  • hello (string)
  • 42 (number)

JSON

{
    "tags": [ "hello", 42 ]
}

MSON

- (array)
    - (object)
        - name: snow (string)
        - description (string)
    - 42 (number)

Rendered Markdown

• (array)
  • (object)
    • name: snow (string)
    • description (string)
  • 42 (number)

JSON

[
    {
        "name": "snow",
        "description": ""
    },
    42
]

Array of Arrays

MSON

- (array)
    - 1, 2, 3, 4 (array[number])

Rendered Markdown

• (array)
  • 1, 2, 3, 4 (array[number])

JSON

[
    [ 1, 2, 3, 4 ]
]

Multi-line Descriptions

In the case where a one-liner description is not sufficient, a multi-paragraph text block is the way to go.

MSON

- id: 1 (number, required) - The unique identifier for a product
- name: A green door (string, required)

    Lorem ipsum dolor sit amet, consectetur adipiscing elit.

    Sed sed lacus a arcu vehicula ultricies sed vel nibh. Mauris id cursus felis.

    Interdum et malesuada fames ac ante ipsum primis in faucibus.

    - unus
    - duo
    - tres
    - quattuor

- price: 12.50 (number, required)
- tags: home, green (array)

For a multi-line description of a structure type, an Items, Members , or Properties keyword MUST be used to avoid conflict with potential list item values that are part of the description:

- tags (array)

    Lorem ipsum dolor sit amet, consectetur adipiscing elit.

    Sed sed lacus a arcu vehicula ultricies sed vel nibh. Mauris id cursus felis.

    Interdum et malesuada fames ac ante ipsum primis in faucibus.

    - unus
    - duo
    - tres
    - quattuor

    - Items
        - home
        - green

NOTE: Unus ... quattuor are considered part of the text block vs. defining items of the array.

Escaping

Markdown code span element syntax (` `) is used to escape reserved Keywords that should just be interpretted as literals as well as other constructs that may be erroneously interpreted by an MSON parser as meaningful.

For non-Keywords, a code span can be used for formatting Markdown as an aesthetic preference.

MSON

- listing (object)

    Our real estate listing has different properties available.

    - `Properties`
        - This one.
        - That one.

    - Properties
        - description (string)
        - date_listed (string)
        - `some:location`: local (string)

Rendered Markdown

• listing (object)

  Our real estate listing has different properties available.

  • Properties

    • This one.
    • That one.

  • Properties

    • description
    • date_listed
    • some:location: local (string)

NOTE: In this example, the first "Properties" string is part of the multi-line block description whereas the second defines the properties of the listing object.

Variable Property Name

Variable property name (key) is defined using italics. Note that a variable property cannot be required.

MSON

- _links
    - *self*
        - href: a URI

Rendered Markdown

• _links
  • self
    • href: a URI

JSON

{
    "_links": {
        "self": {
            "href": "..."
        },
        "users": {
            "href": "..."
        }
    }
}

Type Definition

Additional Named Types can be defined using a Markdown header:

MSON

# Address (object)
Description is here! Properties to follow.

## Properties
- street
- state
- zip

The same entity defined as an address property:

- address (object)

    Description is here! Properties to follow.

    - Properties
        - street
        - state
        - zip

The same address property referencing the previously Named type:

- address (Address)

Referencing

Anywhere a type is expected, a top-level MSON Named Type can be referenced.

MSON

# Address (object)
- street
- city
- state
- zip

# User (object)
- first_name
- last_name
- address (Address)

Rendered Markdown

Address (object)

• street
• city
• state
• zip

User (object)

• first_name
• last_name
• address (Address)

JSON

{
    "first_name": "",
    "last_name": "",
    "address": {
        "street": "",
        "city": "",
        "state": "",
        "zip": ""
    }
}

Mixins

To include (mixin) values or properties in another Named Type use the Include keyword followed by a reference to the
MSON Named Type.

MSON

# Address Object
- street
- city
- state
- zip

# User Object
- first_name
- last_name
- Include Address

Rendered Markdown

Address Object

• street
• city
• state
• zip

User Object

• first_name
• last_name
• Include Address

JSON

{
    "first_name": "",
    "last_name": "",
    "street": "",
    "city": "",
    "state": "",
    "zip": ""
}

NOTE: A mixin can only use a Named Type that is derived from the same type of structure including the mixin. Further, the parent structure inherits all the members of the included Named Type and maintains the order the members were originally defined.