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https://github.com/edn-format/edn

Extensible Data Notation
https://github.com/edn-format/edn

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Extensible Data Notation

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README 

        
edn

===



extensible data notation [eed-n]



# Rationale



**edn** is an extensible data notation. A superset of **edn** is used by Clojure to represent

programs, and it is used by Datomic and other applications as a data transfer format. This spec

describes **edn** in isolation from those and other specific use cases, to help facilitate

implementation of readers and writers in other languages, and for other uses.



**edn** supports a rich set of built-in elements, and the definition of extension elements in terms

of the others. Users of data formats without such facilities must rely on either convention or

context to convey elements not included in the base set. This greatly complicates application

logic, betraying the apparent simplicity of the format. **edn** is simple, yet powerful enough to

meet the demands of applications without convention or complex context-sensitive logic.



**edn** is a system for the conveyance of _values_. It is not a type system, and has no schemas.

Nor is it a system for representing objects - there are no reference types, nor should a consumer

have an expectation that two equivalent elements in some body of **edn** will yield distinct object

identities when read, unless a reader implementation goes out of its way to make such a promise.

Thus the resulting values should be considered immutable, and a reader implementation should yield

values that ensure this, to the extent possible.



**edn** is a set of definitions for acceptable _elements_. A use of **edn** might be a stream or

file containing elements, but it could be as small as the conveyance of a single element in e.g. an

HTTP query param.



There is no enclosing element at the top level. Thus **edn** is suitable for streaming and

interactive applications.



The base set of elements in **edn** is meant to cover the basic set of data structures common to

most programming languages. While **edn** specifies how those elements are formatted in text, it

does not dictate the representation that results on the consumer side. A well behaved reader

library should endeavor to map the elements to programming language types with similar semantics.



# Spec



Currently this specification is casual, as we gather feedback from implementors. A more rigorous

e.g. BNF will follow.



## General considerations



**edn** elements, streams and files should be encoded using [UTF-8](http://en.wikipedia.org/wiki/UTF-8).



Elements are generally separated by whitespace. Whitespace, other than within strings, is not

otherwise significant, nor need redundant whitespace be preserved during transmissions. Commas `,`

are also considered whitespace, other than within strings.



The delimiters `{ } ( ) [ ]` need not be separated from adjacent elements by whitespace.



### # dispatch character



Tokens beginning with `#` are reserved. The character following `#` determines the behavior. The

dispatches `#{` (sets), `#_` (discard), #alphabetic-char (tag) are defined below. `#` is not a

delimiter.



## Built-in elements



### nil



`nil` represents nil, null or nothing. It should be read as an object with similar meaning on the

target platform.



### booleans



`true` and `false` should be mapped to booleans.



If a platform has canonic values for true and false, it is a further semantic of booleans that all

instances of `true` yield that (identical) value, and similarly for `false`.



### strings



Strings are enclosed in `"double quotes"`. May span multiple lines. Standard C/Java escape

characters `\t, \r, \n, \\ and \"` are supported.



### characters



Characters are preceded by a backslash: `\c`, `\newline`, `\return`, `\space` and `\tab` yield the

corresponding characters. Unicode characters are represented with \uNNNN as in Java. Backslash cannot be

followed by whitespace.



### symbols



Symbols are used to represent identifiers, and should map to something other than strings, if

possible.



Symbols begin with a non-numeric character and can contain alphanumeric characters and `. * + ! - _ ?

$ % & = < >`. If `-`, `+` or `.` are the first character, the second character (if any) must be

non-numeric. Additionally, `: #` are allowed as constituent characters in symbols other than as the

first character.



`/` has special meaning in symbols. It can be used once only in the middle of a symbol to separate

the _prefix_ (often a namespace) from the _name_, e.g. `my-namespace/foo`. `/` by itself is a legal

symbol, but otherwise neither the _prefix_ nor the _name_ part can be empty when the symbol

contains `/`.



If a symbol has a _prefix_ and `/`, the following _name_ component should follow the

first-character restrictions for symbols as a whole. This is to avoid ambiguity in reading contexts

where prefixes might be presumed as implicitly included namespaces and elided thereafter.



### keywords



Keywords are identifiers that typically designate themselves. They are semantically akin to

enumeration values. Keywords follow the rules of symbols, except they can (and must) begin with `:`, e.g. `:fred` or `:my/fred`. If the target platform does not have a keyword type distinct

from a symbol type, the same type can be used without conflict, since the mandatory leading `:` of

keywords is disallowed for symbols. Per the symbol rules above, :/ and :/anything are not legal keywords.

A keyword cannot begin with ::



If the target platform supports some notion of interning, it is a further semantic of keywords that

all instances of the same keyword yield the identical object.



### integers



Integers consist of the digits `0` - `9`, optionally prefixed by `-` to indicate a negative number, or

(redundantly) by `+`. No integer other than 0 may begin with 0. 64-bit (signed integer) precision is

expected. An integer can have the suffix `N` to indicate that arbitrary precision is desired. -0 is a

valid integer not distinct from 0.



    integer

      int

      int N

    digit

      0-9

    int

      digit

      1-9 digits

      + digit

      + 1-9 digits

      - digit

      - 1-9 digits



### floating point numbers



64-bit (double) precision is expected.



    floating-point-number

      int M

      int frac

      int exp

      int frac exp

    digit

      0-9

    int

      digit

      1-9 digits

      + digit

      + 1-9 digits

      - digit

      - 1-9 digits

    frac

      . digits

    exp

      ex digits

    digits

      digit

      digit digits

    ex

      e

      e+

      e-

      E

      E+

      E-



In addition, a floating-point number may have the suffix `M` to indicate that exact precision is

desired.



### lists



A list is a sequence of values. Lists are represented by zero or more elements enclosed in

parentheses `()`. Note that lists can be heterogeneous.

 

    (a b 42)



### vectors



A vector is a sequence of values that supports random access. Vectors are represented by zero or

more elements enclosed in square brackets `[]`. Note that vectors can be heterogeneous.



    [a b 42]



### maps



A map is a collection of associations between keys and values. Maps are represented by zero or more

key and value pairs enclosed in curly braces `{}`. Each key should appear at most once. No

semantics should be associated with the order in which the pairs appear.



    {:a 1, "foo" :bar, [1 2 3] four}



Note that keys and values can be elements of any type. The use of commas above is optional, as they

are parsed as whitespace.



### sets



A set is a collection of unique values. Sets are represented by zero or more elements enclosed in

curly braces preceded by `#` `#{}`. No semantics should be associated with the order in which the

elements appear. Note that sets can be heterogeneous.



    #{a b [1 2 3]}



## tagged elements



**edn** supports extensibility through a simple mechanism. `#` followed immediately by a symbol

starting with an alphabetic character indicates that _that symbol_ is a **_tag_**. A tag indicates

the semantic interpretation of _the following element_. It is envisioned that a reader

implementation will allow clients to register handlers for specific tags. Upon encountering a tag,

the reader will first read the next element (which may itself be or comprise other tagged elements),

then pass the result to the corresponding handler for further interpretation, and the result of the

handler will be the data value yielded by the tag + tagged element, i.e. reading a tag and tagged

element yields one value. This value is the value to be returned to the program and is not further

interpreted as **edn** data by the reader.



This process will bottom out on elements either understood or built-in. 



Thus you can build new distinct readable elements out of (and only out of) other readable elements,

keeping extenders and extension consumers out of the text business.



The semantics of a tag, and the type and interpretation of the tagged element are defined by the

steward of the tag.



    #myapp/Person {:first "Fred" :last "Mertz"}



If a reader encounters a tag for which no handler is registered, the implementation can either

report an error, call a designated 'unknown element' handler, or create a well-known generic

representation that contains both the tag and the tagged element, as it sees fit. Note that the

non-error strategies allow for readers which are capable of reading any and all **edn**, in spite

of being unaware of the details of any extensions present.



### rules for tags



Tag symbols without a prefix are reserved by **edn** for built-ins defined using the tag system. 



User tags _**must**_ contain a prefix component, which must be owned by the user (e.g. trademark or

domain) or known unique in the communication context.



A tag _may_ specify more than one format for the tagged element, e.g. both a string and a vector

representation.



Tags themselves are not elements. It is an error to have a tag without a corresponding tagged

element.



## built-in tagged elements



### #inst "rfc-3339-format"



An instant in time. The tagged element is a string in

[RFC-3339](http://www.ietf.org/rfc/rfc3339.txt) format.



`#inst "1985-04-12T23:20:50.52Z"`



### #uuid "f81d4fae-7dec-11d0-a765-00a0c91e6bf6"



A [UUID](http://en.wikipedia.org/wiki/Universally_unique_identifier). The tagged element is a

canonical UUID string representation.



## comments



If a `;` character is encountered outside of a string, that character and all subsequent characters

to the next newline should be ignored.



## discard



`#` followed immediately by `_` is the discard sequence, indicating that the next element (whether

separated from `#_` by whitespace or not) should be read and discarded. Note that the next element

must still be a readable element. A reader should not call user-supplied tag handlers during the

processing of the element to be discarded.



    [a b #_foo 42] => [a b 42]



The discard sequence is not an element. It is an error to have a discard sequence without a

following element.



## equality



Sets and maps have requirements that their elements and keys respectively be unique, which requires

a mechanism for determining when 2 values are not unique (i.e. are equal).



nil, booleans, strings, characters, and symbols are equal to values of the same type with the same

**edn** representation.



integers and floating point numbers should be considered equal to values only of the same

magnitude, _type, and precision_. Comingling numeric types and precision in map/set key/elements,

or constituents therein, is not advised.



sequences (lists and vectors) are equal to other sequences whose count of elements is the same, and

for which each corresponding pair of elements (by ordinal) is equal.



sets are equal if they have the same count of elements and, for every element in one set, an equal

element is in the other.



maps are equal if they have the same number of entries, and for every key/value entry in one map an

equal key is present and mapped to an equal value in the other.



tagged elements must define their own equality semantics. #uuid elements are equal if their canonic

representations are equal. #inst elements are equal if their representation strings designate the

same timestamp per [RFC-3339](http://www.ietf.org/rfc/rfc3339.txt).