Lua: Set `pandoc.List` as default metatable for JSON lists

Lists created by `pandoc.json.decode` now behave like lists generated
via `pandoc.List`. This also ensures that `pandoc.List` tables are
encoded as JSON arrays when passed to `pandoc.json.encode`.

Fixes: #9834

2 files changed, 20 insertions, 7 deletions

diff --git a/pandoc-lua-engine/src/Text/Pandoc/Lua/Init.hs b/pandoc-lua-engine/src/Text/Pandoc/Lua/Init.hs
index e0dd830b2..1e2b628fd 100644
--- a/pandoc-lua-engine/src/Text/Pandoc/Lua/Init.hs
+++ b/pandoc-lua-engine/src/Text/Pandoc/Lua/Init.hs
@@ -28,7 +28,7 @@ import Text.Pandoc.Data (readDataFile)
 import Text.Pandoc.Error (PandocError (PandocLuaError))
 import Text.Pandoc.Logging (LogMessage (ScriptingWarning))
 import Text.Pandoc.Lua.Global (Global (..), setGlobals)
-import Text.Pandoc.Lua.Marshal.List (newListMetatable, pushListModule)
+import Text.Pandoc.Lua.Marshal.List (pushPandocList, pushListModule)
 import Text.Pandoc.Lua.PandocLua (PandocLua (..), liftPandocLua)
 import Text.Pandoc.Lua.SourcePos (luaSourcePos)
 import qualified Data.ByteString.Char8 as Char8
@@ -120,8 +120,8 @@ initLuaState :: PandocLua ()
 initLuaState = do
   liftPandocLua Lua.openlibs
   setWarnFunction
-  initJsonMetatable
   initPandocModule
+  initJsonMetatable
   installLpegSearcher
   setGlobalModules
   loadInitScript "init.lua"
@@ -210,7 +210,9 @@ initLuaState = do
 -- from/via JSON arrays.
 initJsonMetatable :: PandocLua ()
 initJsonMetatable = liftPandocLua $ do
-  newListMetatable HsLua.Aeson.jsonarray (pure ())
+  pushPandocList (const pushnil) []
+  getmetatable top
+  setfield registryindex HsLua.Aeson.jsonarray
   Lua.pop 1
 
 -- | Evaluate a @'PandocLua'@ computation, running all contained Lua
diff --git a/pandoc-lua-engine/test/lua/module/pandoc-json.lua b/pandoc-lua-engine/test/lua/module/pandoc-json.lua
index 46400d572..2b0adadca 100644
--- a/pandoc-lua-engine/test/lua/module/pandoc-json.lua
+++ b/pandoc-lua-engine/test/lua/module/pandoc-json.lua
@@ -1,6 +1,7 @@
 --
 -- Tests for the system module
 --
+local pandoc = require 'pandoc'
 local json = require 'pandoc.json'
 local tasty = require 'tasty'
 
@@ -33,19 +34,26 @@ return {
       local obj = setmetatable(
         {title = 23},
         {
-          __tojson = function (obj)
+          __tojson = function (_)
             return '"Nichts ist so wie es scheint"'
           end
         }
       )
       assert.are_same(json.encode(obj), [["Nichts ist so wie es scheint"]])
     end),
-    test('Inline (Space)', function ()
+    test('pandoc.List', function ()
+      local list = pandoc.List {'foo', 'bar', 'baz'}
       assert.are_equal(
-        json.encode(pandoc.Space()),
-        '{"t":"Space"}'
+        json.encode(list),
+        '["foo","bar","baz"]'
       )
     end),
+    test('Inline (Space)', function ()
+           assert.are_equal(
+             json.encode(pandoc.Space()),
+             '{"t":"Space"}'
+           )
+    end),
     test('Block (HorizontalRule)', function ()
       assert.are_equal(
         json.encode(pandoc.HorizontalRule()),
@@ -85,6 +93,9 @@ return {
     test('object', function ()
       assert.are_same(json.decode '{"a":5}', {a = 5})
     end),
+    test('list of strings', function ()
+      assert.are_equal(json.decode '["foo", "bar"]', pandoc.List{"foo", "bar"})
+    end),
     test('Inline (Space)', function ()
       assert.are_equal(json.decode '{"t":"Space"}', pandoc.Space())
     end),