DeVoN - Developers Value Notation

• A minimal, complete, language neutral notation for representing structured values.
• I wanted something as easy to read as yaml, as easy to parse as json, and a clean syntax like edn
• I choose not to use
  • xml, too verbose
  • json, only allows strings as keys, too verbose
  • yaml, too complex to parse
  • edn, more complicated due to its support for advanced types
  • hocon, more complicated due to its support for merging configurations

Goals

• minimal (nothing unnecessary, everything that exists, exists for a reason)
• language neutral (no dependence on implementation data types)
• human readable/writable (can pretty print)
• machine readable/writable (easy to parse and format)
• complete (can support any data structure)

Examples

Simple strings. Devon does not require a top level element, which makes it easy to append more devon elements to a file. An empty string must be represented as two single quotes. Spaces are only supported within a quoted string. To represent a single quote in a quoted string, you have to double them up.

Hello
World
''
'Hello, world!'
'Sean''s favorite notation'

An array of urls. Notice that quotes are not needed since none of the characters used here have special meaning in devon.

[
  http://example.com/document.txt#line=10,20
  http://example.com/foo.mp4#t=10,20
  http://example.com/bar.webm#t=40,80&xywh=160,120,320,240
]

An array of windows paths. Since devon has no escape characters, nothing special is needed to represent a backslash. Since devon uses whitespace to support formatting, quoting is needed to represent a space.

[
  'C:\Program Files'
  C:\Winnt
  C:\Winnt\System32
]

A map. The key is the group and artifact id. The value is an array of versions. Unlike other formats, keys are not restricted to strings only.

{
  {
    group org.joda
    artifact joda-convert
  }
  [
    1.7
    1.6
    1.5
  ]
  {
    group joda-time
    artifact joda-time
  }
  [
    2.7
    2.6
    2.5
  ]
}

A sample of what an HTTP PATCH request might look like. We are updating the price of the item with sku 123 to 499.99, and we are unsetting the seasonal discount by sending a null '()'

{
  sku 123
  price 499.99
  'seasonal discount' ()
}

Features

• no unnecessary punctuation
• the notation only gives special meaning to 11 characters (6 structural, 4 whitespace, and quote)
• whitespace outside of a quoted string is not significant
• language independent (no language specific words like "true", "false", "null")
• maps to common data structures
• support for dictionaries, arrays, null, and strings
• supports streaming (no top level element)

Intentionally not features

• no support for primitives other than strings
  • not the business of a notation to know about data types
  • instead, have the implementation marshall to and from strings
• no escaping
  • the notation maximizes the types of strings that can be naturally represented
  • implementations can marshall strings with an escaping policy in mind
• no comments
  • for simplicity, there is no separate set of rules for representing comments
  • you can build comments into your data structure, and have your consumer ignore them
  • if you do represent a comment this way, it benefits from the same structural expressiveness
• no references
  • you can designate a value as an id, and refer to it from another value

Overview

There are 5 aspects of this notation

unquoted string

• MUST NOT contain whitespace, single quote, or the symbols ()[]{}
• an empty string MUST be represented as a quoted string, like so: ''

quoted string

• starts and ends with a single quote, internal quotes must be doubled up
• an empty string MUST be represented as a quoted string, like so: ''

unit

• ()
• explicitly indicates no value
• sometimes referred to as 'null', 'unit', or 'void'
• useful for messages intended to un-set a value
• whitespace is not allowed between the '(' and ')'

sequence

• []
• this can be used to model tuples, ordered sequences, or unordered sets
• the transport layer MUST preserve order, although the consumer MAY decide order is unimportant

map

• {}
• a set of key-value pairs
• this can be used to model dictionaries (such as java maps), or records (such as java classes)
• the number of elements MUST be even
• duplicate keys SHOULD NOT be present
• the transport layer MUST preserve order, although the consumer MAY decide order is unimportant
• the transport layer MUST preserve duplicate keys, although in the case of key-value pairs with duplicate keys, the consumer MAY enact a rule for deciding which key-value pairs to keep and which to discard
• the consumer SHOULD ignore keys it does not understand, for forward compatibility

Entry Points

• If you want to use default settings DevonMarshallerWiring.Default
• If you want full control over configuration DevonMarshallerImpl

Maven

<dependency>
  <groupId>com.seanshubin.devon</groupId>
  <artifactId>devon-domain</artifactId>
  <version>1.0</version>
</dependency>

Specification

• Character Classes (Unicode addresses of the characters that can have special meaning)
• Tokens (Extended Backus–Naur Form for the tokenizer)
• Structure (Extended Backus–Naur Form for the parser)

Character Classes

single characters

alternatives

whitespace               = tab | newline | return | space
structural               = quote | open-paren | close-paren | open-bracket | close-bracket | open-brace | close-brace
structural-or-whitespace = structural | whitespace

negations (these do not match end of file)

unquoted-string-character  = not structural-or-whitespace
not-quote                  = not quote

Tokens

token                   = open-brace | close-brace | open-bracket | close-bracket | null | string | whitespace-block
null                    = open-paren close-paren
string                  = quoted-string | unquoted-string
quoted-string           = quote quoted-contents quote
unquoted-string         = unquoted-string-character { unquoted-string-character }
whitespace-block        = whitespace { whitespace }
quoted-contents         = { quoted-string-character }
quoted-string-character = not-quote | two-quotes
two-quotes              = quote quote

Structure

Ignore tokens of type "whitespace-block".

elements = { element }
element = map | array | string | null
map = open-brace pairs close-brace
array = open-bracket elements close-bracket
pairs = { pair }
pair = element element

Implementations

Implementations should, at a minimum, provide the following functionality, expressed here as the corresponding method signatures for this Scala reference implementation.

def charsToIterator(charIterator: Iterator[Char]): Iterator[Devon]
def toCompact(devon: Devon): String
def toPretty(devon: Devon): Seq[String]
def fromValue[T](value: T): Devon
def toValue[T](devon: Devon, theClass: Class[T]): T

The type 'Devon' corresponds to the abstract syntax tree.

sealed trait Devon
case object DevonNull extends Devon
case class DevonString(string: String) extends Devon
case class DevonArray(array: Seq[Devon]) extends Devon
case class DevonMap(map: Map[Devon, Devon]) extends Devon

The generic type 'T' corresponds to a Scala case class, primitive, map or sequence

Alternatives

Be sure you think about your needs. DeVoN was designed to be very good at a specific set of goals. It was not designed to be everything to everyone. Before choosing a notation, be sure to consider the advantages and disadvantages of your alternatives.

Here are some alternatives worth considering

• xml
• json
• yaml
• edn
• hocon