none C. Zangl
Internet-Draft May 23, 2016
Intended status: Standards Track
Expires: November 24, 2016

The Human JSON (Hjson) Configuration Format


Human JSON (Hjson) is a configuration file format based on the JavaScript Object Notation [RFC7159]. Its focus is to provide a reasonable alternative to plain JSON configs that are hard to edit and do not allow comments.

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Table of Contents

1. Introduction

Human JSON (Hjson) is a configuration file format. It is a superset of the JavaScript Object Notation [RFC7159].

Hjson uses the same four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays) as JSON.

A string is a sequence of zero or more Unicode characters [UNICODE]. Note that this citation references the latest version of Unicode rather than a specific release. It is not expected that future changes in the UNICODE specification will impact the syntax of Hjson.

An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

An array is an ordered sequence of zero or more values.

The terms “object” and “array” come from the conventions of JavaScript.

Hjson’s design goals were

Hjson SHOULD be used for data whose primary purpose is to be viewed or edited by a human. For example configuration, resource files or debug data dumps.

Hjson SHOULD NOT be used in other cases, like transfer protocols.

1.1. Syntax compared to JSON

Hjson is a superset of JSON. Its syntax allows you to

Because the punctuator characters {}[],: are used to define the structure of the Hjson text, you need to use quotes

For examples see Section 14.1.

1.2. Conventions Used in This Document

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].

The grammatical rules in this document are to be interpreted as described in [RFC5234] except where they are specifically marked as [SABNF].

SABNF is used to express grammars that are not possible in ABNF, like the block-comment, and to define terminators, like ql-end, that allow us to exclude certain sequences. These constructs allow us to generate parsers and to verify the grammar against test cases. For the general understanding of the language they can be ignored.

1.3. Specifications of Hjson

This document describes Hjson and registers the media type “application/hjson”.

2. Hjson Grammar

A Hjson text is a sequence of tokens. The set of tokens includes seven structural characters, comments, strings, numbers, and three literal names.

A Hjson text is either a serialized value or a root object.

   Hjson-text = ws-c ( root-object / value ) ws-c

These are the seven structural characters:

   begin-array      = %x5B     ; [ left square bracket

   begin-object     = %x7B     ; { left curly bracket

   end-array        = %x5D     ; ] right square bracket

   end-object       = %x7D     ; } right curly bracket

   name-separator   = %x3A     ; : colon

   comma-separator  = %x2C     ; , comma

   lf-separator     = lf       ; Line feed or New line

Insignificant whitespace and comments are allowed before or after any of the first six structural characters.

   ws-c  = *( comment / ws )

   ws    = *( space / tab / lf / cr )

   space = %x20    ; Space
   tab   = %x09    ; Horizontal tab
   lf    = %x0A    ; Line feed or New line
   cr    = %x0D    ; Carriage return

Comments can be specified as line or block comments.

   comment = line-comment / block-comment

   line-comment  = ( %x23 /        ; # hash
                     %x2F.2F )     ; // slash + slash
                   *( tab / cr / %x20-10FFFF ) ; until lf

   block-comment = start-block-comment
                   *( !end-block-comment anychar )

   start-block-comment = %x2F.2A   ; /* slash + star

   end-block-comment = %x2A.2F     ; */ star + slash

   anychar = tab / cr / lf / %x20-10FFFF

   ; SABNF: block-comment requies the use of the
   ;        ! operator to allow * and / in the text
   ;        while stopping at */

3. Values

A Hjson value MUST be an object, array, number, or string, or one of the following three literal names:

   false = %x66.61.6C.73.65   ; false

   null  = %x6E.75.6C.6C      ; null

   true  = %x74.72.75.65      ; true

The literal names MUST be lowercase. No other literal names are allowed.

   value = literal / object / array / number / string

   literal = ( false / null / true ) !literal-end

   literal-end =
            *( space / tab )
            ( %x21-22 / %x24-2B / %x2D-2E / %x30-5A /
              %x5C / %x5E-7A / %x7C / %x7E-10FFFF )
            ; exclude #/,[]{}

   ; SABNF: define literal-end to prevent matches
   ;        that are actually a ql-string
   ;        (like "true blue")

4. Value Separators

Values can be separated directly by a comma or indirectly by one or more linefeeds.

   value-separator =  ( ws-c comma-separator ws-c ) /
                      ( *( comment /
                           *(space / tab / cr) )
                        lf-separator ws-c )

Note that while Hjson allows the use of lf as a separator, the cr character is generally ignored. This should not be an issue as all modern operating systems use either lf or cr+lf as their line terminator.

5. Objects

An object structure is represented as a pair of curly brackets surrounding zero or more name/value pairs (or members). A single colon comes after each name, separating the name from the value. A comma separates a value from the next name unless the value is followed by a linefeed, in which case the comma is optional. Trailing separators are allowed. The names within an object SHOULD be unique.

   object = begin-object
            [ member *( value-separator member ) [value-separator] ]

   member = name ws-c name-separator ws-c value

If the Hjson text defines an object it does not have to include the braces at the root level:

   root-object =
            *( value-separator member ) [value-separator]

A name can be specified as a JSON string (with quotes) or as a name without quotes.

   name = json-string / keyname

   keyname =  1*non-punctuator-char

   non-punctuator-char = %x21-2B / %x2D-39 / %x3B-5A /
                         %x5C / %x5E-7A / %x7C / %x7E-10FFFF
   ; any non-punctuator character (excludes ,:[]{} and ws)

For interoperability issues regarding the uniquness of names and object member ordering, see [RFC7159] Section 4.

6. Arrays

An array structure is represented as square brackets surrounding zero or more values (or elements). Elements are separated by commas or linefeeds. Trailing commas are allowed.

   array =  begin-array
            [ value *( value-separator value ) [value-separator ] ]

There is no requirement that the values in an array be of the same type.

7. Numbers

The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed.

A fraction part is a decimal point followed by one or more digits.

An exponent part begins with the letter E in upper or lower case, which may be followed by a plus or minus sign. The E and optional sign are followed by one or more digits.

Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

   number = [ minus ] int [ frac ] [ exp ] !num-end

   decimal-point = %x2E       ; .

   digit = %x30-39            ; 0-9

   digit1-9 = %x31-39         ; 1-9

   e = %x65 / %x45            ; e E

   exp = e [ minus / plus ] 1*digit

   frac = decimal-point 1*digit

   int = zero / ( digit1-9 *digit )

   minus = %x2D               ; -

   plus = %x2B                ; +

   zero = %x30                ; 0

   num-end =  *( space / tab )
              ( %x21-22 / %x24-2B / %x2D-2E / %x30-5A /
              %x5C / %x5E-7A / %x7C / %x7E-10FFFF )
              ; exclude #/,[]{}

   ; SABNF: define num-end to prevent matches
   ;        that are actually a ql-string
   ;        (like "1 minute")

For limits on the range and precision of numbers see [RFC7159] Section 6.

8. Strings

Hjson allows strings to be represented in three different formats.

   string = json-string / ml-string / ql-string

8.1. JSON Strings

The representation of JSON strings is similar to conventions used in the C family of programming languages. A string begins and ends with quotation marks. All Unicode characters may be placed within the quotation marks, except for the characters that must be escaped: quotation mark, reverse solidus, and the control characters (U+0000 through U+001F).

Any character may be escaped. If the character is in the Basic Multilingual Plane (U+0000 through U+FFFF), then it may be represented as a six-character sequence: a reverse solidus, followed by the lowercase letter u, followed by four hexadecimal digits that encode the character’s code point. The hexadecimal letters A though F can be upper or lower case. So, for example, a string containing only a single reverse solidus character may be represented as “\u005C”.

Alternatively, there are two-character sequence escape representations of some popular characters. So, for example, a string containing only a single reverse solidus character may be represented more compactly as “\”.

To escape an extended character that is not in the Basic Multilingual Plane, the character is represented as a 12-character sequence, encoding the UTF-16 surrogate pair. So, for example, a string containing only the G clef character (U+1D11E) may be represented as “\uD834\uDD1E”.

   json-string = quotation-mark *char quotation-mark

   char = unescaped /
       escape (
           %x22 /          ; "    quotation mark  U+0022
           %x5C /          ; \    reverse solidus U+005C
           %x2F /          ; /    solidus         U+002F
           %x62 /          ; b    backspace       U+0008
           %x66 /          ; f    form feed       U+000C
           %x6E /          ; n    line feed       U+000A
           %x72 /          ; r    carriage return U+000D
           %x74 /          ; t    tab             U+0009
           %x75 4hexdig )  ; uXXXX                U+XXXX

   hexdig = %x30-39 / "A" / "B" / "C" / "D" / "E" / "F"

   escape = %x5C              ; \

   quotation-mark = %x22      ; "

   unescaped = %x20-21 / %x23-5B / %x5D-10FFFF

8.2. Quoteless Strings

Hjson allows strings to be represented only by their content without quotes. A quoteless string may contain any character except the control characters (U+0000 through U+001F), excluding trailing whitespace. It cannot start with a punctuator character (,:[]{}). It ends at the first linefeed. Strings are taken as is without escapes.

   ql-string = non-punctuator-char
               *( !ql-end ql-char )

   ql-end    = *( space / tab / cr ) lf

   ql-char   = space / tab / %x21-10FFFF

   ; SABNF: define ql-end to exclude trailing
   ;        whitespace from the ql-string

A Hjson parser must still detect values (number, true, false or null) and parse them correctly. For example

Not allowing escapes inside the quoteless string allows you to specify HTML fragments, regular expressions or Windows style paths as-is, keeping them readable.

8.3. Multiline Strings

A multiline string begins and ends with three single quotes.

   ml-string =
      *( !ml-seq ( ml-char / ml-ignore ) )

   ml-seq = "'''"

   ml-char = space / tab / lf / %x21-10FFFF

   ml-ignore = cr

   ; SABNF: ml-string requies the use of the
   ;        ! operator to allow ' in the text
   ;        while stopping at '''

The carrige return character is generally ignored to make the parser platform independent.

Whitespace handling cannot be expressed in the ABNF grammar and is thus defined as:

9. String and Character Issues

9.1. Character Encoding

Hjson text SHALL be encoded in UTF-8. Since Hjson parsers can read JSON they may be able to read texts in other encodings (such as UTF-16 and UTF-32). Hjson texts that are encoded in UTF-8 are interoperable in the sense that they will be read successfully by the maximum number of implementations.

Implementations MUST NOT add a byte order mark to the beginning of a Hjson text. In the interests of interoperability, implementations that parse Hjson texts MAY ignore the presence of a byte order mark rather than treating it as an error.

9.2. Unicode Characters

See [RFC7159] Section 8.2.

9.3. String Comparison

See [RFC7159] Section 8.3.

10. Parsers

A Hjson parser transforms a Hjson text into another representation. A Hjson parser MUST accept all texts that conform to the Hjson grammar. A Hjson parser MAY accept non-Hjson forms or extensions.

An implementation may set limits on the size of texts that it accepts. An implementation may set limits on the maximum depth of nesting. An implementation may set limits on the range and precision of numbers. An implementation may set limits on the length and character contents of strings.

11. Generators

A Hjson generator produces Hjson text. The resulting text MUST strictly conform to the Hjson grammar.

12. IANA Considerations

The MIME media type for Hjson text is application/hjson.

Type name: application

Subtype name: hjson

Required parameters: n/a

Optional parameters: n/a

Encoding considerations: binary

Security considerations: See XXXX, Section 13.

Interoperability considerations: Described in XXXX

Published specification: XXXX

Applications that use this media type: Hjson has been used to provide configuration data for applications written in all of these programming languages: C#, Go, Java, JavaScript, PHP and Python

Additional information: Magic number(s): n/a File extension(s): .hjson Macintosh file type code(s): TEXT

Person & email address to contact for further information: IESG

Intended usage: COMMON

Restrictions on usage: none

Author: Christian Zangl

Change controller: IESG

Note: No “charset” parameter is defined for this registration. Adding one really has no effect on compliant recipients.

13. Security Considerations

Unlike JSON, Hjson is not a subset of Javascript and cannot be parsed using “eval()”.

14. Examples

This is a Hjson object:

   # comments are useful

   # specify rate in requests/second
   rate: 1000

   #  you may also use
   // c style
   /* or block
      comments */

   # key names do not need to be placed in
   # quotes unless they contain a punctuator
   # character {}[],:
   key: 1

   # strings may also omit quotes if they do
   # not start with a punctuator
   text: look ma, no quotes!

   # quoteless strings do not use escapes
   # and end at the LF/newline

   # commas are optional
     one: 1
     two: 2

   # trailing commas are allowed
     one: 1,
     two: 2,

   # multiline string
     JSON I love you.
     But you strangle my expression.
     This is so much better.

   # Hjson is a superset of JSON so you
   # may use any valid JSON syntax:
   favNumbers: [ 1, 2, 3, 6, 42 ]

14.1. JSON vs Hjson

Example of a document processor configuration, first in JSON:

     "header": "The Foo Manual\nCopyright Bar Inc.",
     "source": {
       "include": [
       "includePattern": ".+\\.foo(doc)?$",
       "excludePattern": "(^|\\/|\\\\)_"
     "templates": {
       "cleverLinks": false,
       "monospaceLinks": false

and in Hjson:

   # define the header using a multiline string
     The Foo Manual
     Copyright Bar Inc.
     # generate documentation for these paths
     # include/exclude files
     # notice that no escapes are required
     includePattern: .+\.foo(doc)?$
     excludePattern: (^|\/|\\)_
   templates: {
     # trailing commas are allowed
     cleverLinks: false,
     monospaceLinks: false,

Example of a npm dependency configuration in JSON:

     "dependencies": {
       "foo": "2.0.1",
       "bar": "*",
       "til": "~1.2.1",
       "elf": "^1.2.3"

and in Hjson:

       // match version exactly
       foo: 2.0.1
       // * matches any version
       bar: *
       // approximately, allows patch-level changes
       til: ~1.2.1
       // compatible with major version
       elf: ^1.2.3

15. Contributors

This document is based on [RFC4627] and [RFC7159] which were written by Douglas Crockford and Tim Bray. This document was constructed by extending it with the elements of the Hjson syntax.

16. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4627] Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, DOI 10.17487/RFC4627, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014.
[SABNF] Thomas, L., "Superset Augmented Backus-Naur Form", version 2.2.1, n.d..
[UNICODE] The Unicode Consortium, "The Unicode Standard", n.d..

Author's Address

Christian Zangl EMail: