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;;; json.el --- JavaScript Object Notation parser / generator
;; Copyright (C) 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
;; Author: Edward O'Connor <>
;; Version: 1.2
;; Keywords: convenience
;; This file is part of GNU Emacs.
;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs. If not, see <>.
;;; Commentary:
;; This is a library for parsing and generating JSON (JavaScript Object
;; Notation).
;; Learn all about JSON here: <URL:>.
;; The user-serviceable entry points for the parser are the functions
;; `json-read' and `json-read-from-string'. The encoder has a single
;; entry point, `json-encode'.
;; Since there are several natural representations of key-value pair
;; mappings in elisp (alist, plist, hash-table), `json-read' allows you
;; to specify which you'd prefer (see `json-object-type' and
;; `json-array-type').
;; Similarly, since `false' and `null' are distinct in JSON, you can
;; distinguish them by binding `json-false' and `json-null' as desired.
;;; History:
;; 2006-03-11 - Initial version.
;; 2006-03-13 - Added JSON generation in addition to parsing. Various
;; other cleanups, bugfixes, and improvements.
;; 2006-12-29 - XEmacs support, from Aidan Kehoe <>.
;; 2008-02-21 - Installed in GNU Emacs.
;;; Code:
(eval-when-compile (require 'cl))
;; Compatibility code
(defalias 'json-encode-char0 'encode-char)
(defalias 'json-decode-char0 'decode-char)
;; Parameters
(defvar json-object-type 'alist
"Type to convert JSON objects to.
Must be one of `alist', `plist', or `hash-table'. Consider let-binding
this around your call to `json-read' instead of `setq'ing it.")
(defvar json-array-type 'vector
"Type to convert JSON arrays to.
Must be one of `vector' or `list'. Consider let-binding this around
your call to `json-read' instead of `setq'ing it.")
(defvar json-key-type nil
"Type to convert JSON keys to.
Must be one of `string', `symbol', `keyword', or nil.
If nil, `json-read' will guess the type based on the value of
If `json-object-type' is: nil will be interpreted as:
`hash-table' `string'
`alist' `symbol'
`plist' `keyword'
Note that values other than `string' might behave strangely for
Sufficiently Weird keys. Consider let-binding this around your call to
`json-read' instead of `setq'ing it.")
(defvar json-false :json-false
"Value to use when reading JSON `false'.
If this has the same value as `json-null', you might not be able to tell
the difference between `false' and `null'. Consider let-binding this
around your call to `json-read' instead of `setq'ing it.")
(defvar json-null nil
"Value to use when reading JSON `null'.
If this has the same value as `json-false', you might not be able to
tell the difference between `false' and `null'. Consider let-binding
this around your call to `json-read' instead of `setq'ing it.")
;;; Utilities
(defun json-join (strings separator)
(mapconcat 'identity strings separator))
(defun json-alist-p (list)
"Non-null if and only if LIST is an alist."
(or (null list)
(and (consp (car list))
(json-alist-p (cdr list)))))
(defun json-plist-p (list)
"Non-null if and only if LIST is a plist."
(or (null list)
(and (keywordp (car list))
(consp (cdr list))
(json-plist-p (cddr list)))))
;; Reader utilities
(defsubst json-advance (&optional n)
"Skip past the following N characters."
(forward-char n))
(defsubst json-peek ()
"Return the character at point."
(let ((char (char-after (point))))
(or char :json-eof)))
(defsubst json-pop ()
"Advance past the character at point, returning it."
(let ((char (json-peek)))
(if (eq char :json-eof)
(signal 'end-of-file nil)
(defun json-skip-whitespace ()
"Skip past the whitespace at point."
(skip-chars-forward "\t\r\n\f\b "))
;; Error conditions
(put 'json-error 'error-message "Unknown JSON error")
(put 'json-error 'error-conditions '(json-error error))
(put 'json-readtable-error 'error-message "JSON readtable error")
(put 'json-readtable-error 'error-conditions
'(json-readtable-error json-error error))
(put 'json-unknown-keyword 'error-message "Unrecognized keyword")
(put 'json-unknown-keyword 'error-conditions
'(json-unknown-keyword json-error error))
(put 'json-number-format 'error-message "Invalid number format")
(put 'json-number-format 'error-conditions
'(json-number-format json-error error))
(put 'json-string-escape 'error-message "Bad unicode escape")
(put 'json-string-escape 'error-conditions
'(json-string-escape json-error error))
(put 'json-string-format 'error-message "Bad string format")
(put 'json-string-format 'error-conditions
'(json-string-format json-error error))
(put 'json-object-format 'error-message "Bad JSON object")
(put 'json-object-format 'error-conditions
'(json-object-format json-error error))
;;; Keywords
(defvar json-keywords '("true" "false" "null")
"List of JSON keywords.")
;; Keyword parsing
(defun json-read-keyword (keyword)
"Read a JSON keyword at point.
KEYWORD is the keyword expected."
(unless (member keyword json-keywords)
(signal 'json-unknown-keyword (list keyword)))
(mapc (lambda (char)
(unless (char-equal char (json-peek))
(signal 'json-unknown-keyword
(list (save-excursion
(backward-word 1)
(thing-at-point 'word)))))
(unless (looking-at "\\(\\s-\\|[],}]\\|$\\)")
(signal 'json-unknown-keyword
(list (save-excursion
(backward-word 1)
(thing-at-point 'word)))))
(cond ((string-equal keyword "true") t)
((string-equal keyword "false") json-false)
((string-equal keyword "null") json-null)))
;; Keyword encoding
(defun json-encode-keyword (keyword)
"Encode KEYWORD as a JSON value."
(cond ((eq keyword t) "true")
((eq keyword json-false) "false")
((eq keyword json-null) "null")))
;;; Numbers
;; Number parsing
(defun json-read-number (&optional sign)
"Read the JSON number following point.
The optional SIGN argument is for internal use.
N.B.: Only numbers which can fit in Emacs Lisp's native number
representation will be parsed correctly."
;; If SIGN is non-nil, the number is explicitly signed.
(let ((number-regexp
(cond ((and (null sign) (char-equal (json-peek) ?-))
(- (json-read-number t)))
((and (null sign) (char-equal (json-peek) ?+))
(json-read-number t))
((and (looking-at number-regexp)
(or (match-beginning 1)
(match-beginning 2)))
(goto-char (match-end 0))
(string-to-number (match-string 0)))
(t (signal 'json-number-format (list (point)))))))
;; Number encoding
(defun json-encode-number (number)
"Return a JSON representation of NUMBER."
(format "%s" number))
;;; Strings
(defvar json-special-chars
'((?\" . ?\")
(?\\ . ?\\)
(?/ . ?/)
(?b . ?\b)
(?f . ?\f)
(?n . ?\n)
(?r . ?\r)
(?t . ?\t))
"Characters which are escaped in JSON, with their elisp counterparts.")
;; String parsing
(defun json-read-escaped-char ()
"Read the JSON string escaped character at point."
;; Skip over the '\'
(let* ((char (json-pop))
(special (assq char json-special-chars)))
(special (cdr special))
((not (eq char ?u)) char)
((looking-at "[0-9A-Fa-f][0-9A-Fa-f][0-9A-Fa-f][0-9A-Fa-f]")
(let ((hex (match-string 0)))
(json-advance 4)
(json-decode-char0 'ucs (string-to-number hex 16))))
(signal 'json-string-escape (list (point)))))))
(defun json-read-string ()
"Read the JSON string at point."
(unless (char-equal (json-peek) ?\")
(signal 'json-string-format (list "doesn't start with '\"'!")))
;; Skip over the '"'
(let ((characters '())
(char (json-peek)))
(while (not (char-equal char ?\"))
(push (if (char-equal char ?\\)
(setq char (json-peek)))
;; Skip over the '"'
(if characters
(apply 'string (nreverse characters))
;; String encoding
(defun json-encode-char (char)
"Encode CHAR as a JSON string."
(setq char (json-encode-char0 char 'ucs))
(let ((control-char (car (rassoc char json-special-chars))))
;; Special JSON character (\n, \r, etc.)
(format "\\%c" control-char))
;; ASCIIish printable character
((and (> char 31) (< char 161))
(format "%c" char))
;; Fallback: UCS code point in \uNNNN form
(format "\\u%04x" char)))))
(defun json-encode-string (string)
"Return a JSON representation of STRING."
(format "\"%s\"" (mapconcat 'json-encode-char string "")))
;;; JSON Objects
(defun json-new-object ()
"Create a new Elisp object corresponding to a JSON object.
Please see the documentation of `json-object-type'."
(cond ((eq json-object-type 'hash-table)
(make-hash-table :test 'equal))
(defun json-add-to-object (object key value)
"Add a new KEY -> VALUE association to OBJECT.
Returns the updated object, which you should save, e.g.:
(setq obj (json-add-to-object obj \"foo\" \"bar\"))
Please see the documentation of `json-object-type' and `json-key-type'."
(let ((json-key-type
(if (eq json-key-type nil)
(cdr (assq json-object-type '((hash-table . string)
(alist . symbol)
(plist . keyword))))
(setq key
(cond ((eq json-key-type 'string)
((eq json-key-type 'symbol)
(intern key))
((eq json-key-type 'keyword)
(intern (concat ":" key)))))
(cond ((eq json-object-type 'hash-table)
(puthash key value object)
((eq json-object-type 'alist)
(cons (cons key value) object))
((eq json-object-type 'plist)
(cons key (cons value object))))))
;; JSON object parsing
(defun json-read-object ()
"Read the JSON object at point."
;; Skip over the "{"
;; read key/value pairs until "}"
(let ((elements (json-new-object))
key value)
(while (not (char-equal (json-peek) ?}))
(setq key (json-read-string))
(if (char-equal (json-peek) ?:)
(signal 'json-object-format (list ":" (json-peek))))
(setq value (json-read))
(setq elements (json-add-to-object elements key value))
(unless (char-equal (json-peek) ?})
(if (char-equal (json-peek) ?,)
(signal 'json-object-format (list "," (json-peek))))))
;; Skip over the "}"
;; Hash table encoding
(defun json-encode-hash-table (hash-table)
"Return a JSON representation of HASH-TABLE."
(format "{%s}"
(let (r)
(lambda (k v)
(push (format "%s:%s"
(json-encode k)
(json-encode v))
", ")))
;; List encoding (including alists and plists)
(defun json-encode-alist (alist)
"Return a JSON representation of ALIST."
(format "{%s}"
(json-join (mapcar (lambda (cons)
(format "%s:%s"
(json-encode (car cons))
(json-encode (cdr cons))))
", ")))
(defun json-encode-plist (plist)
"Return a JSON representation of PLIST."
(let (result)
(while plist
(push (concat (json-encode (car plist))
(json-encode (cadr plist)))
(setq plist (cddr plist)))
(concat "{" (json-join (nreverse result) ", ") "}")))
(defun json-encode-list (list)
"Return a JSON representation of LIST.
Tries to DWIM: simple lists become JSON arrays, while alists and plists
become JSON objects."
(cond ((null list) "null")
((json-alist-p list) (json-encode-alist list))
((json-plist-p list) (json-encode-plist list))
((listp list) (json-encode-array list))
(signal 'json-error (list list)))))
;;; Arrays
;; Array parsing
(defun json-read-array ()
"Read the JSON array at point."
;; Skip over the "["
;; read values until "]"
(let (elements)
(while (not (char-equal (json-peek) ?\]))
(push (json-read) elements)
(unless (char-equal (json-peek) ?\])
(if (char-equal (json-peek) ?,)
(signal 'json-error (list 'bleah)))))
;; Skip over the "]"
(apply json-array-type (nreverse elements))))
;; Array encoding
(defun json-encode-array (array)
"Return a JSON representation of ARRAY."
(concat "[" (mapconcat 'json-encode array ", ") "]"))
;;; JSON reader.
(defvar json-readtable
(let ((table
'((?t json-read-keyword "true")
(?f json-read-keyword "false")
(?n json-read-keyword "null")
(?{ json-read-object)
(?\[ json-read-array)
(?\" json-read-string))))
(mapc (lambda (char)
(push (list char 'json-read-number) table))
'(?- ?+ ?. ?0 ?1 ?2 ?3 ?4 ?5 ?6 ?7 ?8 ?9))
"Readtable for JSON reader.")
(defun json-read ()
"Parse and return the JSON object following point.
Advances point just past JSON object."
(let ((char (json-peek)))
(if (not (eq char :json-eof))
(let ((record (cdr (assq char json-readtable))))
(if (functionp (car record))
(apply (car record) (cdr record))
(signal 'json-readtable-error record)))
(signal 'end-of-file nil))))
;; Syntactic sugar for the reader
(defun json-read-from-string (string)
"Read the JSON object contained in STRING and return it."
(insert string)
(goto-char (point-min))
(defun json-read-file (file)
"Read the first JSON object contained in FILE and return it."
(insert-file-contents file)
(goto-char (point-min))
;;; JSON encoder
(defun json-encode (object)
"Return a JSON representation of OBJECT as a string."
(cond ((memq object (list t json-null json-false))
(json-encode-keyword object))
((stringp object) (json-encode-string object))
((keywordp object) (json-encode-string
(substring (symbol-name object) 1)))
((symbolp object) (json-encode-string
(symbol-name object)))
((numberp object) (json-encode-number object))
((arrayp object) (json-encode-array object))
((hash-table-p object) (json-encode-hash-table object))
((listp object) (json-encode-list object))
(t (signal 'json-error (list object)))))
(provide 'json)
;; arch-tag: 15f6e4c8-b831-4172-8749-bbc680c50ea1
;;; json.el ends here