json.lua: Allow encoding null values

A similar proposal was already rejected upstream:
https://github.com/rxi/json.lua/pull/6

src/json/README.md

@@ -34,8 +34,7 @@ json.decode('[1,2,3,{"x":10}]') -- Returns { 1, 2, 3, { x = 10 } }
 * Trying to encode values which are unrepresentable in JSON will never result
   in type conversion or other magic: sparse arrays, tables with mixed key types
   or invalid numbers (NaN, -inf, inf) will raise an error
-* `null` values contained within an array or object are converted to `nil` and
+* `null` values are converted to the special value `json.null`
-  are therefore lost upon decoding
 * *Pretty* encoding is not supported, `json.encode()` only encodes to a compact
   format
 

src/json/json.lua

@@ -24,6 +24,9 @@
 
 local json = { _version = "0.1.2" }
 
+-- unique placeholder for "null"
+json.null = { _ = "nil" }
+
 -------------------------------------------------------------------------------
 -- Encode
 -------------------------------------------------------------------------------
@@ -122,6 +125,10 @@ local type_func_map = {
 
 
 encode = function(val, stack)
+  if val == json.null then
+    return encode_nil(val)
+  end
+
   local t = type(val)
   local f = type_func_map[t]
   if f then
@@ -158,7 +165,7 @@ local literals      = create_set("true", "false", "null")
 local literal_map = {
   [ "true"  ] = true,
   [ "false" ] = false,
-  [ "null"  ] = nil,
+  [ "null"  ] = json.null,
 }
 
 

src/json/test/test.lua

@@ -243,3 +243,16 @@ test("encode escape", function()
     assert( res == v, fmt("'%s' was not escaped properly", k) )
   end
 end)
+
+test("encode null", function()
+  local t = {
+    a = "foo",
+    b = json.null,
+    c = 42,
+  }
+  local res = json.decode( json.encode(t) )
+  for k, v in pairs(t) do
+    assert( equal( v, res[k] ) )
+  end
+  assert( equal( json.null, res.b ) )
+end)