java Antlr 4 语法文件合集

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 f