资源说明：Extensible Data Notation

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).

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