Syntax and the Parser

Janet 0.4.0 Documentation
Special Forms
String Library
Data Structures
Object Oriented Programming
Abstract Machine

A Janet program begins life as a text file, just a sequence of bytes like any other on your system. Janet source files should be UTF-8 or ASCII encoded. Before Janet can compile or run your program, it must transform your source code into a data structure. Janet is a lisp, which means it is homoiconic - code is data, so all of the facilities in the language for manipulating arrays, tuples, strings, and tables can be used for manipulating your source code as well.

But before janet code is represented as a data structure, it must be read, or parsed, by the janet parser. Called the reader in many other lisps, the parser is a machine that takes in plain text and outputs data structures which can be used by both the compiler and macros. In janet, it is a parser rather than a reader because there is no code execution at read time. This is safer and simpler, and also lets janet syntax serve as a robust data interchange format. While a parser is not extensible, in janet the philosophy is to extend the language via macros rather than reader macros.

Nil, True and False

Nil, true and false are all literals than can be entered as such in the parser.



Janet symbols are represented as a sequence of alphanumeric characters not starting with a digit or a colon. They can also contain the characters !, @, $, %, ^, &, *, -, _, +, =, |, ~, :, <, >, ., ?, \, /, as well as any Unicode codepoint not in the ASCII range.

By convention, most symbols should be all lower case and use dashes to connect words (sometimes called kebab case).

Symbols that come from another module often contain a forward slash that separates the name of the module from the name of the definition in the module



Janet keywords are like symbols that begin with the character :. However, they are used differently and treated by the compiler as a constant rather than a name for something. Keywords are used mostly for keys in tables and structs, or pieces of syntax in macros.



Janet numbers are represented by IEEE-754 floating point numbers. The syntax is similar to that of many other languages as well. Numbers can be written in base 10, with underscores used to separate digits into groups. A decimal point can be used for floating point numbers. Numbers can also be written in other bases by prefixing the number with the desired base and the character 'r'. For example, 16 can be written as 16, 1_6, 16r10, 4r100, or 0x10. The 0x prefix can be used for hexadecimal as it is so common. The radix must be themselves written in base 10, and can be any integer from 2 to 36. For any radix above 10, use the letters as digits (not case sensitive).



Strings in janet are surrounded by double quotes. Strings are 8bit clean, meaning they can contain any arbitrary sequence of bytes, including embedded 0s. To insert a double quote into a string itself, escape the double quote with a backslash. For unprintable characters, you can either use one of a few common escapes, use the \xHH escape to escape a single byte in hexidecimal. The supported escapes are:

Strings can also contain literal newline characters that will be ignored. This lets one define a multiline string that does not contain newline characters.

An alternative way of representing strings in janet is the long string, or the backquote delimited string. A string can also be define to start with a certain number of backquotes, and will end the same number of backquotes. Long strings do not contain escape sequences; all bytes will be parsed literally until ending delimiter is found. This is useful for defining multi-line strings with literal newline characters, unprintable characters, or strings that would otherwise require many escape sequences.

"This is a string."


Buffers are similar to strings except they are mutable data structures. Strings in janet cannot be mutated after being created, where a buffer can be changed after creation. The syntax for a buffer is the same as that for a string or long string, but the buffer must be prefixed with the '@' character.

@``Another buffer``


Tuples are a sequence of white space separated values surrounded by either parentheses or brackets. The parser considers any of the characters ASCII 32, \0, \f, \n, \r or \t to be white-space.

(do 1 2 3)
[do 1 2 3]

Square brackets indicate that a tuple will be used as a tuple literal rather than a function call, macro call, or special form. The parser will set a flag on a tuple if it has square brackets to let the compiler know to compile the tuple into a constructor. The programmer can check if a tuple has brackets via the tuple/type function.


Arrays are the same as tuples, but have a leading @ to indicate mutability.

@(:one :two :three)
@[:one :two :three]


Structs are represented by a sequence of white-space delimited key value pairs surrounded by curly braces. The sequence is defined as key1, value1, key2, value2, etc. There must be an even number of items between curly braces or the parser will signal a parse error. Any value can be a key or value. Using nil or NaN as a key, however, will drop that pair from the parsed struct. Using nil as value will drop that pair from the parsed struct

{:key1 "value1" :key2 :value2 :key3 3}
{(1 2 3) (4 5 6)}
{@[] @[]}
{1 2 3 4 5 6}


Table have the same syntax as structs, except they have the @ prefix to indicate that they are mutable.

@{:key1 "value1" :key2 :value2 :key3 3}
@{(1 2 3) (4 5 6)}
@{@[] @[]}
@{1 2 3 4 5 6}


Comments begin with a # character and continue until the end of the line. There are no multi-line comments.


Often called reader macros in other lisps, Janet provides several shorthand notations for some forms.


Shorthand for (quote x)


Shorthand for (splice x)


Shorthand for (quasiquote x)


Shorthand for (unquote x)

These shorthand notations can be combined in any order, allowing forms like ''x ((quote (quote x))), or ,;x ((unquote (splice x))).

Syntax Highlighting

For syntax highlighting, there is some preliminary vim syntax highlighting in janet.vim. Generic lisp syntax highlighting should, however, provide good results. One can also generate a janet.tmLanguage file for other programs with make grammar from the Janet source code.


For anyone looking for a more succinct description of the grammar, a PEG grammar for recognizing Janet source code is below. The PEG syntax is itself similar to EBNF. More info on the PEG syntax can be found in the documentation for the peg module.

(def grammar
  ~{:ws (set " \t\r\f\n\0")
    :readermac (set "';~,")
    :symchars (+ (range "09" "AZ" "az" "\x80\xFF") (set "!$%&*+-./:<?=>@^_|"))
    :token (some :symchars)
    :hex (range "09" "af" "AF")
    :escape (* "\\" (+ (set "ntrzf0\"\\e")
                       (* "x" :hex :hex)
                       (error (constant "bad hex escape"))))
    :comment (* "#" (any (if-not (+ "\n" -1) 1)))
    :symbol :token
    :keyword (* ":" (any :symchars))
    :constant (+ "true" "false" "nil")
    :bytes (* "\"" (any (+ :escape (if-not "\"" 1))) "\"")
    :string :bytes
    :buffer (* "@" :bytes)
    :long-bytes {:delim (some "`")
                 :open (capture :delim :n)
                 :close (cmt (* (not (> -1 "`")) (-> :n) ':delim) ,=)
                 :main (drop (* :open (any (if-not :close 1)) :close))}
    :long-string :long-bytes
    :long-buffer (* "@" :long-bytes)
    :number (cmt (<- :token) ,scan-number)
    :raw-value (+ :comment :constant :number :keyword
                  :string :buffer :long-string :long-buffer
                  :parray :barray :ptuple :btuple :struct :dict :symbol)
    :value (* (any (+ :ws :readermac)) :raw-value (any :ws))
    :root (any :value)
    :root2 (any (* :value :value))
    :ptuple (* "(" :root (+ ")" (error "")))
    :btuple (* "[" :root (+ "]" (error "")))
    :struct (* "{" :root2 (+ "}" (error "")))
    :parray (* "@" :ptuple)
    :barray (* "@" :btuple)
    :dict (* "@" :struct)
    :main :root})