[PATCH] json: Add support for encoding structs

[Vibhav Pant <vibhavp@gmail.com>]

The following patch to json.el adds support for encoding record types
declared via cl-defstruct. It also allows the user to change the json
key for a struct field by using the `:json' keyword while declaring
a slot.

An example of the new json-encode:
ELISP> (cl-defstruct foo (f1 "foo") (f2 "bar" :json "field2"))
foo
ELISP> (json-encode (make-foo))
"{\"f1\":\"foo\",\"field2\":\"bar\"}"


---
diff --git a/lisp/json.el b/lisp/json.el
index 049c9b1951..cc09b46b7c 100644
--- a/lisp/json.el
+++ b/lisp/json.el
@@ -53,6 +53,7 @@
 ;;; Code:

 (require 'map)
+(require 'cl-lib)

 ;; Parameters

@@ -549,6 +550,33 @@ json-encode-hash-table
                 ""
               json--encoding-current-indentation))))

+;; Struct encoding
+(defun json-encode-struct (struct)
+  "Return a JSON representation of STRUCT."
+  (let* ((struct-type (type-of struct))
+         (slots-info (cdr (cl-struct-slot-info struct-type))))
+    (format "{%s%s}"
+            (json-join
+             (json--with-indentation
+              (mapcar #'(lambda (slot)
+                          (let* ((slot-name (car slot))
+                                 (opts (cddr slot))
+                                 (key (or (plist-get opts :json) slot-name)))
+                            (format (if json-encoding-pretty-print
+                                        "%s%s: %s"
+                                      "%s%s:%s")
+                                    json--encoding-current-indentation
+                                    (json-encode-key key)
+                                    (json-encode
+                                     (cl-struct-slot-value
struct-type slot-name
+                                                           struct)))))
+                      slots-info))
+             json-encoding-separator)
+            (if (or (not json-encoding-pretty-print)
+                    json-encoding-lisp-style-closings)
+                ""
+              json--encoding-current-indentation))))
+
 ;; List encoding (including alists and plists)

 (defun json-encode-alist (alist)
@@ -721,6 +749,7 @@ json-encode
         ((arrayp object)       (json-encode-array object))
         ((hash-table-p object) (json-encode-hash-table object))
         ((listp object)        (json-encode-list object))
+        ((cl-struct-p object)  (json-encode-struct object))
         (t                     (signal 'json-error (list object)))))

 ;; Pretty printing


-- 
Vibhav Pant
vibhavp@gmail.com