Lua filter: allow natural access to meta elements

Meta elements that are treated as lua tables (i.e. MetaList,
MetaInlines, MetaBlocks, and MetaMap), are no longer wrapped in an
additional table but simply marked via a metatable.  This allows
treating those meta values just like normal tables, while still making
empty elements of those values distinguishable.

data/pandoc.lua

@@ -166,35 +166,40 @@ end
 -- @function MetaList
 -- @tparam {MetaValue,...} meta_values list of meta values
 
---- Meta boolean
--- @function MetaBool
--- @tparam boolean bool boolean value
-
 --- Meta map
 -- @function MetaMap
 -- @tparam table a string-index map of meta values
-
---- Meta string
--- @function MetaString
--- @tparam string str string value
 M.meta_value_types = {
   "MetaBlocks",
-  "MetaBool",
   "MetaInlines",
   "MetaList",
   "MetaMap",
-  "MetaString"
 }
 for i = 1, #M.meta_value_types do
   M[M.meta_value_types[i]] = M.MetaValue:create_constructor(
     M.meta_value_types[i],
     function(content)
-      return {c = content}
-    end,
-    "content"
+      return content
+    end
   )
 end
 
+--- Creates string to be used in meta data.
+-- Does nothing, lua strings are meta strings.
+-- @function MetaString
+-- @tparam string str string value
+function M.MetaString(str)
+  return str
+end
+
+--- Creates boolean to be used in meta data.
+-- Does nothing, lua booleans are meta booleans.
+-- @function MetaBool
+-- @tparam boolean bool boolean value
+function M.MetaBool(bool)
+  return bool
+end
+
 ------------------------------------------------------------------------
 -- Blocks
 -- @section Block

src/Text/Pandoc/Lua/StackInstances.hs

@@ -140,7 +140,7 @@ peekMetaValue :: LuaState -> Int -> IO (Maybe MetaValue)
 peekMetaValue lua idx = do
   -- Get the contents of an AST element.
   let elementContent :: StackValue a => IO (Maybe a)
-      elementContent = getTable lua idx "c"
+      elementContent = peek lua idx
   luatype <- ltype lua idx
   case luatype of
     TBOOLEAN -> fmap MetaBool <$> peek lua idx