Support custom null and false objects when parsing JSON

* doc/lispref/text.texi (Parsing JSON): Describe new :null-object
and :false-object kwargs to json-parse-string and
json-parse-buffer.

* src/json.c
(struct json_configuration): New type.
(json_to_lisp): Accept a struct json_configuration* param.
(json_parse_args): Rename from json_parse_object_type.
(Fjson_parse_string): Rework docstring.
(Fjson_parse_string, Fjson_parse_buffer): Update call to
json_to_lisp.
(syms_of_json): Two new syms, QCnull_object and QCfalse_object.

* test/src/json-tests.el
(json-parse-with-custom-null-and-false-objects): New test.

3 files changed, 141 insertions, 69 deletions

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index 2c5b5a1b42e..5b94580827f 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -5008,9 +5008,10 @@ Specifically:
 @itemize
 
 @item
-JSON has a couple of keywords: @code{null}, @code{false}, and
-@code{true}.  These are represented in Lisp using the keywords
-@code{:null}, @code{:false}, and @code{t}, respectively.
+JSON uses three keywords: @code{true}, @code{null}, @code{false}.
+@code{true} is represented by the symbol @code{t}.  By default, the
+remaining two are represented, respectively, by the symbols
+@code{:null} and @code{:false}.
 
 @item
 JSON only has floating-point numbers.  They can represent both Lisp
@@ -5062,14 +5063,6 @@ JSON.  The subobjects within these top-level values can be of any
 type.  Likewise, the parsing functions will only return vectors,
 hashtables, alists, and plists.
 
-  The parsing functions accept keyword arguments.  Currently only one
-keyword argument, @code{:object-type}, is recognized; its value
-decides which Lisp object to use for representing the key-value
-mappings of a JSON object.  It can be either @code{hash-table}, the
-default, to make hashtables with strings as keys, @code{alist} to use
-alists with symbols as keys or @code{plist} to use plists with keyword
-symbols as keys.
-
 @defun json-serialize object
 This function returns a new Lisp string which contains the JSON
 representation of @var{object}.
@@ -5080,16 +5073,38 @@ This function inserts the JSON representation of @var{object} into the
 current buffer before point.
 @end defun
 
-@defun json-parse-string string &key (object-type @code{hash-table})
+@defun json-parse-string string &rest args
 This function parses the JSON value in @var{string}, which must be a
-Lisp string.
+Lisp string.  The argument @var{args} is a list of keyword/argument
+pairs.  The following keywords are accepted:
+
+@itemize
+
+@item @code{:object-type}
+The value decides which Lisp object to use for representing the
+key-value mappings of a JSON object.  It can be either
+@code{hash-table}, the default, to make hashtables with strings as
+keys; @code{alist} to use alists with symbols as keys; or @code{plist}
+to use plists with keyword symbols as keys.
+
+@item @code{:null-object}
+The value decides which Lisp object to use to represent the JSON
+keyword @code{null}.  It defaults to the lisp symbol @code{:null}.
+
+@item @code{:false-object}
+The value decides which Lisp object to use to represent the JSON
+keyword @code{false}.  It defaults to the lisp symbol @code{:false}.
+
+@end itemize
+
 @end defun
 
-@defun json-parse-buffer &key (object-type @code{hash-table})
+@defun json-parse-buffer &rest args
 This function reads the next JSON value from the current buffer,
 starting at point.  It moves point to the position immediately after
 the value if a value could be read and converted to Lisp; otherwise it
-doesn't move point.
+doesn't move point.  @var{args} is interpreted as in
+@code{json-parse-string}.
 @end defun
 
 
diff --git a/src/json.c b/src/json.c
index c28e14d63c6..e86ef237d03 100644
--- a/src/json.c
+++ b/src/json.c
@@ -7,7 +7,7 @@ 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.
+nyour 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
@@ -502,7 +502,7 @@ and plists are converted to JSON objects.  Hashtable keys must be
 strings without embedded null characters and must be unique within
 each object.  Alist and plist keys must be symbols; if a key is
 duplicate, the first instance is used.  */)
-  (Lisp_Object object)
+     (Lisp_Object object)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
 
@@ -579,10 +579,10 @@ json_insert_callback (const char *buffer, size_t size, void *data)
 
 DEFUN ("json-insert", Fjson_insert, Sjson_insert, 1, 1, NULL,
        doc: /* Insert the JSON representation of OBJECT before point.
-This is the same as (insert (json-serialize OBJECT)), but potentially
-faster.  See the function `json-serialize' for allowed values of
-OBJECT.  */)
-  (Lisp_Object object)
+               This is the same as (insert (json-serialize OBJECT)), but potentially
+               faster.  See the function `json-serialize' for allowed values of
+               OBJECT.  */)
+     (Lisp_Object object)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
 
@@ -621,22 +621,28 @@ OBJECT.  */)
 }
 
 enum json_object_type {
-  json_object_hashtable,
-  json_object_alist,
-  json_object_plist
+                       json_object_hashtable,
+                       json_object_alist,
+                       json_object_plist
+};
+
+struct json_configuration {
+  enum json_object_type object_type;
+  Lisp_Object null_object;
+  Lisp_Object false_object;
 };
 
 /* Convert a JSON object to a Lisp object.  */
 
 static _GL_ARG_NONNULL ((1)) Lisp_Object
-json_to_lisp (json_t *json, enum json_object_type object_type)
+json_to_lisp (json_t *json, struct json_configuration *conf)
 {
   switch (json_typeof (json))
     {
     case JSON_NULL:
-      return QCnull;
+      return conf->null_object;
     case JSON_FALSE:
-      return QCfalse;
+      return conf->false_object;
     case JSON_TRUE:
       return Qt;
     case JSON_INTEGER:
@@ -644,9 +650,9 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
          otherwise.  This loses precision for integers with large
          magnitude; however, such integers tend to be nonportable
          anyway because many JSON implementations use only 64-bit
-         floating-point numbers with 53 mantissa bits.  See
-         https://tools.ietf.org/html/rfc7159#section-6 for some
-         discussion.  */
+                      floating-point numbers with 53 mantissa bits.  See
+                      https://tools.ietf.org/html/rfc7159#section-6 for some
+      discussion.  */
       return make_fixnum_or_float (json_integer_value (json));
     case JSON_REAL:
       return make_float (json_real_value (json));
@@ -663,7 +669,7 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
         Lisp_Object result = Fmake_vector (make_natnum (size), Qunbound);
         for (ptrdiff_t i = 0; i < size; ++i)
           ASET (result, i,
-                json_to_lisp (json_array_get (json, i), object_type));
+                json_to_lisp (json_array_get (json, i), conf));
         --lisp_eval_depth;
         return result;
       }
@@ -672,7 +678,7 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
         if (++lisp_eval_depth > max_lisp_eval_depth)
           xsignal0 (Qjson_object_too_deep);
         Lisp_Object result;
-        switch (object_type)
+        switch (conf->object_type)
           {
           case json_object_hashtable:
             {
@@ -692,7 +698,7 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
                   /* Keys in JSON objects are unique, so the key can't
                      be present yet.  */
                   eassert (i < 0);
-                  hash_put (h, key, json_to_lisp (value, object_type), hash);
+                  hash_put (h, key, json_to_lisp (value, conf), hash);
                 }
               break;
             }
@@ -705,7 +711,7 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
                 {
                   Lisp_Object key = Fintern (json_build_string (key_str), Qnil);
                   result
-                    = Fcons (Fcons (key, json_to_lisp (value, object_type)),
+                    = Fcons (Fcons (key, json_to_lisp (value, conf)),
                              result);
                 }
               result = Fnreverse (result);
@@ -727,7 +733,7 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
                   /* Build the plist as value-key since we're going to
                      reverse it in the end.*/
                   result = Fcons (key, result);
-                  result = Fcons (json_to_lisp (value, object_type), result);
+                  result = Fcons (json_to_lisp (value, conf), result);
                   SAFE_FREE ();
                 }
               result = Fnreverse (result);
@@ -745,47 +751,66 @@ json_to_lisp (json_t *json, enum json_object_type object_type)
   emacs_abort ();
 }
 
-static enum json_object_type
-json_parse_object_type (ptrdiff_t nargs, Lisp_Object *args)
-{
-  switch (nargs)
-    {
-    case 0:
-      return json_object_hashtable;
-    case 2:
+static void
+json_parse_args (ptrdiff_t nargs,
+                 Lisp_Object *args,
+                 struct json_configuration *conf)
+{
+  if ((nargs % 2) != 0)
+    wrong_type_argument (Qplistp, Flist (nargs, args));
+
+  /* Start from the back so keyword values appearing
+     first take precedence. */
+  for (ptrdiff_t i = nargs; i > 0; i -= 2) {
+    Lisp_Object key = args[i - 2];
+    Lisp_Object value = args[i - 1];
+    if (EQ (key, QCobject_type))
       {
-        Lisp_Object key = args[0];
-        Lisp_Object value = args[1];
-        if (!EQ (key, QCobject_type))
-          wrong_choice (list1 (QCobject_type), key);
         if (EQ (value, Qhash_table))
-          return json_object_hashtable;
+          conf->object_type = json_object_hashtable;
         else if (EQ (value, Qalist))
-          return json_object_alist;
+          conf->object_type = json_object_alist;
         else if (EQ (value, Qplist))
-          return json_object_plist;
+          conf->object_type = json_object_plist;
         else
           wrong_choice (list3 (Qhash_table, Qalist, Qplist), value);
       }
-    default:
-      wrong_type_argument (Qplistp, Flist (nargs, args));
-    }
+    else if (EQ (key, QCnull_object))
+      conf->null_object = value;
+    else if (EQ (key, QCfalse_object))
+      conf->false_object = value;
+    else
+      wrong_choice (list3 (QCobject_type,
+                           QCnull_object,
+                           QCfalse_object),
+                    value);
+  }
 }
 
 DEFUN ("json-parse-string", Fjson_parse_string, Sjson_parse_string, 1, MANY,
        NULL,
        doc: /* Parse the JSON STRING into a Lisp object.
+
 This is essentially the reverse operation of `json-serialize', which
 see.  The returned object will be a vector, hashtable, alist, or
-plist.  Its elements will be `:null', `:false', t, numbers, strings,
-or further vectors, hashtables, alists, or plists.  If there are
-duplicate keys in an object, all but the last one are ignored.  If
-STRING doesn't contain a valid JSON object, an error of type
-`json-parse-error' is signaled.  The keyword argument `:object-type'
-specifies which Lisp type is used to represent objects; it can be
-`hash-table', `alist' or `plist'.
-usage: (json-parse-string STRING &key (OBJECT-TYPE \\='hash-table)) */)
-  (ptrdiff_t nargs, Lisp_Object *args)
+plist.  Its elements will be the JSON null value, the JSON false
+value, t, numbers, strings, or further vectors, hashtables, alists, or
+plists.  If there are duplicate keys in an object, all but the last
+one are ignored.  If STRING doesn't contain a valid JSON object, an
+error of type `json-parse-error' is signaled.  The arguments ARGS are
+a list of keyword/argument pairs:
+
+The keyword argument `:object-type' specifies which Lisp type is used
+to represent objects; it can be `hash-table', `alist' or `plist'.
+
+The keyword argument `:null-object' specifies which object to use
+to represent a JSON null value.  It defaults to `:null'.
+
+The keyword argument `:false-object' specifies which object to use to
+represent a JSON false value.  It defaults to `:false'.
+
+usage: (json-parse-string STRING &rest args) */)
+     (ptrdiff_t nargs, Lisp_Object *args)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
 
@@ -807,8 +832,8 @@ usage: (json-parse-string STRING &key (OBJECT-TYPE \\='hash-table)) */)
   Lisp_Object string = args[0];
   Lisp_Object encoded = json_encode (string);
   check_string_without_embedded_nulls (encoded);
-  enum json_object_type object_type
-    = json_parse_object_type (nargs - 1, args + 1);
+  struct json_configuration conf = {json_object_hashtable, QCnull, QCfalse};
+  json_parse_args (nargs - 1, args + 1, &conf);
 
   json_error_t error;
   json_t *object = json_loads (SSDATA (encoded), 0, &error);
@@ -819,7 +844,7 @@ usage: (json-parse-string STRING &key (OBJECT-TYPE \\='hash-table)) */)
   if (object != NULL)
     record_unwind_protect_ptr (json_release_object, object);
 
-  return unbind_to (count, json_to_lisp (object, object_type));
+  return unbind_to (count, json_to_lisp (object, &conf));
 }
 
 struct json_read_buffer_data
@@ -857,8 +882,8 @@ DEFUN ("json-parse-buffer", Fjson_parse_buffer, Sjson_parse_buffer,
 This is similar to `json-parse-string', which see.  Move point after
 the end of the object if parsing was successful.  On error, point is
 not moved.
-usage: (json-parse-buffer &key (OBJECT-TYPE \\='hash-table))  */)
-  (ptrdiff_t nargs, Lisp_Object *args)
+usage: (json-parse-buffer &rest args)  */)
+     (ptrdiff_t nargs, Lisp_Object *args)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
 
@@ -877,7 +902,8 @@ usage: (json-parse-buffer &key (OBJECT-TYPE \\='hash-table))  */)
     }
 #endif
 
-  enum json_object_type object_type = json_parse_object_type (nargs, args);
+  struct json_configuration conf = {json_object_hashtable, QCnull, QCfalse};
+  json_parse_args (nargs, args, &conf);
 
   ptrdiff_t point = PT_BYTE;
   struct json_read_buffer_data data = {.point = point};
@@ -892,7 +918,7 @@ usage: (json-parse-buffer &key (OBJECT-TYPE \\='hash-table))  */)
   record_unwind_protect_ptr (json_release_object, object);
 
   /* Convert and then move point only if everything succeeded.  */
-  Lisp_Object lisp = json_to_lisp (object, object_type);
+  Lisp_Object lisp = json_to_lisp (object, &conf);
 
   /* Adjust point by how much we just read.  */
   point += error.position;
@@ -955,6 +981,8 @@ syms_of_json (void)
   Fput (Qjson_parse_string, Qside_effect_free, Qt);
 
   DEFSYM (QCobject_type, ":object-type");
+  DEFSYM (QCnull_object, ":null-object");
+  DEFSYM (QCfalse_object, ":false-object");
   DEFSYM (Qalist, "alist");
   DEFSYM (Qplist, "plist");
 
diff --git a/test/src/json-tests.el b/test/src/json-tests.el
index 7a193545b1a..918b2336d0f 100644
--- a/test/src/json-tests.el
+++ b/test/src/json-tests.el
@@ -209,6 +209,35 @@ Test with both unibyte and multibyte strings."
     (should-not (bobp))
     (should (looking-at-p (rx " [456]" eos)))))
 
+(ert-deftest json-parse-with-custom-null-and-false-objects ()
+  (let ((input
+         "{ \"abc\" : [1, 2, true], \"def\" : null, \"abc\" : [9, false] }\n"))
+    (should (equal (json-parse-string input
+                                      :object-type 'plist
+                                      :null-object :json-null
+                                      :false-object :json-false)
+                   '(:abc [9 :json-false] :def :json-null)))
+    (should (equal (json-parse-string input
+                                      :object-type 'plist
+                                      :false-object :json-false)
+                   '(:abc [9 :json-false] :def :null)))
+    (should (equal (json-parse-string input
+                                      :object-type 'alist
+                                      :null-object :zilch)
+                   '((abc . [9 :false]) (def . :zilch))))
+    (should (equal (json-parse-string input
+                                      :object-type 'alist
+                                      :false-object nil
+                                      :null-object nil)
+                   '((abc . [9 nil]) (def))))
+    (let* ((thingy '(1 2 3))
+           (retval (json-parse-string input
+                                      :object-type 'alist
+                                      :false-object thingy
+                                      :null-object nil)))
+      (should (equal retval `((abc . [9 ,thingy]) (def))))
+      (should (eq (elt (cdr (car retval)) 1) thingy)))))
+
 (ert-deftest json-insert/signal ()
   (skip-unless (fboundp 'json-insert))
   (with-temp-buffer