Fix pretty-printing empty objects as null

* lisp/json.el (json-pretty-print): Force distinction between empty
  objects and null.
(json-encode-list): Remove responsibility to print "null" as this
value is not a list.
(json-encode): Give higher precedence to lists so that an empty list
is printed as an empty object, not as "null".

* test/lisp/json-tests.el (test-json-encode): Add many tests to check
  the behavior of pretty-printing.

1 files changed, 4 insertions, 3 deletions

diff --git a/lisp/json.el b/lisp/json.el
index d374f452e6b..cd95ec28327 100644
--- a/lisp/json.el
+++ b/lisp/json.el
@@ -609,8 +609,7 @@ Please see the documentation of `json-object-type' and `json-key-type'."
   "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))
+  (cond ((json-alist-p list) (json-encode-alist list))
         ((json-plist-p list) (json-encode-plist list))
         ((listp list)        (json-encode-array list))
         (t
@@ -723,12 +722,12 @@ Advances point just past JSON object."
         ((stringp object)      (json-encode-string object))
         ((keywordp object)     (json-encode-string
                                 (substring (symbol-name object) 1)))
+        ((listp object)        (json-encode-list object))
         ((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)))))
 
 ;; Pretty printing
@@ -743,6 +742,8 @@ Advances point just past JSON object."
   (interactive "r")
   (atomic-change-group
     (let ((json-encoding-pretty-print t)
+          ;; Distinguish an empty objects from 'null'
+          (json-null :json-null)
           ;; Ensure that ordering is maintained
           (json-object-type 'alist)
           (txt (delete-and-extract-region begin end)))