Lua filter: fall-back to global filters when none is returned

The implicitly defined global filter (i.e. all element filtering
functions defined in the global lua environment) is used if no filter is
returned from a lua script. This allows to just write top-level
functions in order to define a lua filter. E.g

    function Emph(elem) return pandoc.Strong(elem.content) end

data/pandoc.lua

@@ -803,7 +803,7 @@ end
 function M.global_filter()
   local res = {}
   for k, v in pairs(_G) do
-    if M.Inline.constructor[k] or M.Block.constructor[k] or M.Block.constructors[k] or k == "Doc" then
+    if M.Inline.constructor[k] or M.Block.constructor[k] or k == "Doc" then
       res[k] = v
     end
   end

src/Text/Pandoc/Lua.hs

@@ -54,13 +54,17 @@ runLuaFilter filterPath args pd = liftIO $ do
   -- store module in global "pandoc"
   pushPandocModule lua
   Lua.setglobal lua "pandoc"
+  top <- Lua.gettop lua
   status <- Lua.loadfile lua filterPath
   if (status /= 0)
     then do
       Just luaErrMsg <- Lua.peek lua 1
       error luaErrMsg
     else do
-      Lua.call lua 0 1
+      Lua.call lua 0 Lua.multret
+      newtop <- Lua.gettop lua
+      -- Use the implicitly defined global filter if nothing was returned
+      when (newtop - top < 1) $ pushGlobalFilter lua
       Just luaFilters <- Lua.peek lua (-1)
       Lua.push lua args
       Lua.setglobal lua "PandocParameters"
@@ -68,6 +72,13 @@ runLuaFilter filterPath args pd = liftIO $ do
       Lua.close lua
       return doc
 
+pushGlobalFilter :: LuaState -> IO ()
+pushGlobalFilter lua =
+  Lua.newtable lua
+  *> Lua.getglobal2 lua "pandoc.global_filter"
+  *> Lua.call lua 0 1
+  *> Lua.rawseti lua (-2) 1
+
 runAll :: [LuaFilter] -> Pandoc -> IO Pandoc
 runAll [] = return
 runAll (x:xs) = walkMWithLuaFilter x >=> runAll xs

test/Tests/Lua.hs

@@ -47,6 +47,12 @@ tests =
       (doc . para $ str "Hey!" <> linebreak <> str "What's up?")
       (doc . para $ str "Hello," <> space <> str "World!")
 
+  , testCase "implicit doc filter" $
+    assertFilterConversion "Document contains 'Hello, World!'"
+      "implicit-doc-filter.lua"
+      (doc . plain $ linebreak)
+      (doc . para $ str "Hello," <> space <> str "World!")
+
   , testCase "parse raw markdown blocks" $
     assertFilterConversion "raw markdown block is converted"
       "markdown-reader.lua"

test/lua/implicit-doc-filter.lua

@@ -0,0 +1,6 @@
+function Doc (doc)
+  local meta = {}
+  local hello = { pandoc.Str "Hello,", pandoc.Space(), pandoc.Str "World!" }
+  local blocks = { pandoc.Para(hello) }
+  return pandoc.Doc(blocks, meta)
+end