# HG changeset patch
# User jbe
# Date 1406771133 -7200
# Node ID 004d2d50419e619307196825fde1a710d3c1f58a
# Parent  88b613f5bc22fcbf5b7044357470af8418433ce1
Allow direct usage of json.null values in JSON library (for writing, not reading)

diff -r 88b613f5bc22 -r 004d2d50419e libraries/json/json.c
--- a/libraries/json/json.c	Thu Jul 31 03:19:09 2014 +0200
+++ b/libraries/json/json.c	Thu Jul 31 03:45:33 2014 +0200
@@ -36,6 +36,12 @@
   JSON_REGENT arraymt;    // metatable for JSON arrays
 } json_registry;
 
+// returns the string "<JSON null marker>":
+static int json_nullmark_tostring(lua_State *L) {
+  lua_pushliteral(L, "<JSON null marker>");
+  return 1;
+}
+
 // marks a Lua table as JSON object or JSON array:
 // (returns its modified argument or a new table if argument is nil)
 static int json_mark(lua_State *L, JSON_REGPOINTER mt) {
@@ -739,6 +745,10 @@
   int needsep = 0;
   lua_Integer idx;
   lua_settop(L, 1);
+  if (json_isnullmark(L, 1)) {
+    lua_pushnil(L);
+    lua_replace(L, 1);
+  }
   switch (lua_type(L, 1)) {
   case LUA_TNIL:
     lua_pushliteral(L, "null");
@@ -889,6 +899,12 @@
   {NULL, NULL}
 };
 
+// metamethods for JSON null marker:
+static const struct luaL_Reg json_nullmark_metamethods[] = {
+  {"__tostring", json_nullmark_tostring},
+  {NULL, NULL}
+};
+
 // initializes json library:
 int luaopen_json(lua_State *L) {
   // empty stack:
@@ -918,7 +934,12 @@
   lua_rawset(L, -3);
   lua_setmetatable(L, -2);
   json_regstore(L, shadowtbl);
-  // return library module stored on lowest stack position:
-  lua_settop(L, 1);
+  // set metatable of null marker and make it available through library module:
+  json_pushnullmark(L);
+  lua_newtable(L);
+  luaL_setfuncs(L, json_nullmark_metamethods, 0);
+  lua_setmetatable(L, -2);
+  lua_setfield(L, 1, "null");
+  // return library module (that's expected on top of stack):
   return 1;
 }