json-streams is a Common Lisp library for reading and writing JSON text.

Homepage: https://github.com/copyleft/json-streams


  • Designed to used as a building block for more high level libraries.
  • No dependencies.
  • Easy to use streaming API.
  • Validates input and output.
  • Unambiguous mapping of datatypes between lisp and JSON.
  • Streaming design, can proccess JSON text of unlimted size.
  • Proper handling of Unicode.

High level interface

Streaming interface for output


(with-json-output (nil :key-encoder #'string-downcase :indent t)
    (json-output-member :first-name "John")
    (json-output-member :last-name "Smith")
    (with-json-member :is-alive (json-output-boolean t))
    (json-output-member :age 25)
    (with-json-member :address
      (json-output-alist '((:street-address . "21 2nd Street")
                           (:city . "New York")
                           (:state . "NY")
                           (:postal-code . "10021-3100"))))
    (with-json-member :phone-numbers
        (json-output-plist '(:type "home"
                             :number "212 555-1234"))
        (json-output-plist '(:type "office"
                             :number "646 555-4567"))
        (json-output-plist '(:type "mobile"
                             :number "123 456-7890"))))
    (json-output-member :children #())
    (with-json-member :spouse (json-output-null))))
JSON-STREAMS:WITH-JSON-OUTPUT (&optional target &rest options) &body body

The options are explained under MAKE-JSON-OUTPUT-STREAM. Target must be a character stream or NIL. If target is NIL output is written to a string that is returned, otherwise the values of body is returned.

The following functions and macros can only be used inside the body of WITH-JSON-OUTPUT:

  • WITH-JSON-ARRAY - Use any of the other functions/macros in the body to output elements of the array.
  • WITH-JSON-MEMBER - The body must contain one call to one output function/macros to output the value.
  • JSON-OUTPUT-MEMBER - Outputs the member value using JSON-OUTPUT-VALUE.
  • JSON-OUTPUT-VALUE - string, number, sequence or hash-table.
  • JSON-OUTPUT-BOOLEAN - NIL outputs false, anything else outputs true.
  • JSON-OUTPUT-NULL - Always outputs null. No arguments.
  • JSON-OUTPUT-ALIST - Outputs an assoc list as a JSON object using JSON-OUTPUT-MEMBER.
  • JSON-OUTPUT-PLIST - Outputs a propretry list as JSON object using JSON-OUTPUT-MEMBER.

Parsing and writing using an unambigious s-expression notation

The high level interface uses the following datatype mapping:

JSON Common Lisp
true T
false NIL
null :NULL
string string
number integer, float or ratio
array (:ARRAY ... )
object (:OBJECT (key . value) ... )

The options are explained under MAKE-JSON-INPUT-STREAM and MAKE-JSON-OUTPUT-STREAM.

JSON-STREAMS:JSON-PARSE source &rest options
=> json, position

Parses a single JSON text from source. The second value is the position where parsing stopped.

=> jsons

Parses zero or more JSON texts from source. Returns a list of JSON values.

JSON-STREAMS:JSON-STRINGIFY value &optional target &rest options

Stringifies value into JSON text. If target is not provided or NIL, returns a string with the JSON text. Otherwise target must be a character stream.

JSON-STREAMS:JSON-STRINGIFY-MULTIPLE values &optional target &rest options

Stringifies multiple json values.

The streaming interface

See parse.lisp and stringify.lisp for examples of how to use the streaming interface.

JSON-STREAMS:MAKE-JSON-INPUT-STREAM source &key start end close-stream multiple duplicate-key-check use-ratios max-exponent raw-strings
=> json-input-stream
  • source - A character stream or a string. When source is a string, start and end are bounding index designators of string.
  • use-ratios - Default false. When false, numbers that can not be represented as integers are returned as double-floats. Then true, these numbers are returned as ratios.
  • max-exponent - Default 325. The maximum (positive and negative) exponent accepted in numbers. Setting to a high number opens up for a easy to use denial-of-service attack.
  • raw-strings - Default false. When true, strings are returned as (:STRING numbers*), where numbers represent Unicode code points. For Unicode escape sequences in strings (), no UTF-16 validation or decoding is done.
  • key-encoder - Default NIL. A function taking one argument. This used by the higher lever functions to transform the keys of objects into strings.
JSON-STREAMS:MAKE-JSON-OUTPUT-STREAM stream &key close-stream multiple duplicate-key-check indent escape-non-ascii
=> json-input-stream
  • stream - A character stream.
  • indent - Default false. When true, the JSON text is broken into lines and indented.
  • escape-non-ascii - Default false. When true, any character outside the ASCII character set will be escaped using the syntax.

Common options

  • close-stream - Default false. When true, the source stream will be closed by JSON-CLOSE.
  • multiple - Default false. When true, only a single JSON text (object or array) is expected in the stream. When T, zero or more JSON texts can be read from or written to stream.
  • duplicate-key-check - Default true. When true, an error will be signaled when duplicate keys are detected in an object. Setting this to false avoids the added overhead of checking for duplicate keys.
JSON-STREAMS:JSON-READ json-input-stream
=> token

Parses JSON text from the underlying stream and returns the next token.

JSON-STREAMS:JSON-WRITE token json-output-stream
=> token

Outputs JSON text to underlying stream based on the given token.

JSON-STREAMS:JSON-CLOSE json-stream &key abort

Closes the json-stream. Applications should always call JSON-CLOSE when finished with a JSON stream. This ensures that all syntax errors are detected. The value of abort is forwarded to CL:CLOSE of the underlying stream (if close-stream was true) and JSON syntax checking is suppressed.

JSON-STREAMS:WITH-OPEN-JSON-STREAM (var json-stream) &body

Binds var to json-stream, executes body, and ensures that JSON-CLOSE is called on json-stream.


The possible tokens for input and output are

  • :NULL
  • :FALSE
  • :TRUE
  • A number
  • A string or (:STRING integer*)
  • :EOF

After a :BEGIN-OBJECT is returned, JSON-READ will alternate between returning keys and values of the object, until :END-OBJECT is returned instead of a key. Keys are always returned as strings. Values can be anything, except :END-OBJECT, :END-ARRAY and :EOF. Thus some kind of recursive or stack based algorithm is neccearry.

Handling of numbers

While JSON standard doesn't define any limits on the size of numbers, many progamming languages does. This library assumes the limits of JavaScript. In JavaScript there is only one number type, a 64-bit IEEE 754 double precision float. This has the following implications:

The syntax doesn't matter: 20, 20.0 or 2e1 are all the same number, the integer 20.

The integer range is +/- (expt 10 53). Numbers outside this range can't be represented accurately. This library will refuse to write an integer outside this range. If you need to output such large numbers, convert them to double-float or string.

At input the handling of numbers outside the integer range depends on syntax. If the number contains a decimal point or an exponent it will be returned as a double-float. Otherwise an error will be signalled. The option :USE-RATIOS disables this check.

Thomas Bakketun <thomas.bakketun@copyleft.no>
GNU General Public Licence 3.0