Lua: Support built-in default templates for custom writers

Custom writers can define a default template via a global `Template`
function; the data directory is no longer searched for a default
template.

Writer authors can restore the old lookup behavior with

``` lua
Template = function ()
  local template
  return template.compile(template.default(PANDOC_SCRIPT_FILE))
end
```

1 files changed, 24 insertions, 7 deletions

diff --git a/pandoc-lua-engine/src/Text/Pandoc/Lua/Writer.hs b/pandoc-lua-engine/src/Text/Pandoc/Lua/Writer.hs
index a9a044fe6..eeec9b6af 100644
--- a/pandoc-lua-engine/src/Text/Pandoc/Lua/Writer.hs
+++ b/pandoc-lua-engine/src/Text/Pandoc/Lua/Writer.hs
@@ -21,21 +21,24 @@ import Control.Exception
 import Control.Monad ((<=<))
 import Data.Default (def)
 import Data.Maybe (fromMaybe)
+import Data.Text (Text)
 import HsLua
 import HsLua.Core.Run (newGCManagedState, withGCManagedState)
 import Control.Monad.IO.Class (MonadIO)
 import Text.Pandoc.Class (PandocMonad, findFileWithDataFallback)
-import Text.Pandoc.Error (PandocError)
+import Text.Pandoc.Error (PandocError (..))
 import Text.Pandoc.Format (ExtensionsConfig (..))
 import Text.Pandoc.Lua.Global (Global (..), setGlobals)
 import Text.Pandoc.Lua.Init (runLuaWith)
 import Text.Pandoc.Lua.Marshal.Format (peekExtensionsConfig)
+import Text.Pandoc.Lua.Marshal.Template (peekTemplate)
+import Text.Pandoc.Templates (Template)
 import Text.Pandoc.Writers (Writer (..))
 import qualified Text.Pandoc.Lua.Writer.Classic as Classic
 
 -- | Convert Pandoc to custom markup.
 writeCustom :: (PandocMonad m, MonadIO m)
-            => FilePath -> m (Writer m, ExtensionsConfig)
+            => FilePath -> m (Writer m, ExtensionsConfig, m (Template Text))
 writeCustom luaFile = do
   luaState <- liftIO newGCManagedState
   luaFile' <- fromMaybe luaFile <$> findFileWithDataFallback "writers" luaFile
@@ -56,25 +59,39 @@ writeCustom luaFile = do
           pushName x
           rawget (nth 2) <* remove (nth 2) -- remove global table
 
-    let writerField = "PANDOC Writer function"
+    let writerField = "Pandoc Writer function"
 
     extsConf <- rawgetglobal "writer_extensions" >>= \case
-      TypeNil   -> pure $ ExtensionsConfig mempty mempty
+      TypeNil   -> ExtensionsConfig mempty mempty <$ pop 1
       _         -> forcePeek $ peekExtensionsConfig top `lastly` pop 1
 
+    -- Store template function in registry
+    let templateField = "Pandoc Writer Template"
+    rawgetglobal "Template" *> setfield registryindex templateField
+
+    let getTemplate = liftIO $ withGCManagedState @PandocError luaState $ do
+          getfield registryindex templateField >>= \case
+            TypeNil   -> failLua $ "No default template for writer; " <>
+                         "the global variable Template is undefined."
+            _ -> do
+              callTrace 0 1
+              forcePeek $ peekTemplate top `lastly` pop 1
+
+    let addProperties = (, extsConf, getTemplate)
+
     rawgetglobal "Writer" >>= \case
       TypeNil -> rawgetglobal "ByteStringWriter" >>= \case
         TypeNil -> do
           -- Neither `Writer` nor `BinaryWriter` are defined. Try to
           -- use the file as a classic writer.
           pop 1  -- remove nil
-          pure $ (,extsConf) . TextWriter $ \opts doc ->
+          pure $ addProperties . TextWriter $ \opts doc ->
             liftIO $ withGCManagedState luaState $ do
               Classic.runCustom @PandocError opts doc
         _ -> do
           -- Binary writer. Writer function is on top of the stack.
           setfield registryindex writerField
-          pure $ (,extsConf) . ByteStringWriter $ \opts doc ->
+          pure $ addProperties . ByteStringWriter $ \opts doc ->
             -- Call writer with document and writer options as arguments.
             liftIO $ withGCManagedState luaState $ do
               getfield registryindex writerField
@@ -85,7 +102,7 @@ writeCustom luaFile = do
       _ -> do
         -- New-type text writer. Writer function is on top of the stack.
         setfield registryindex writerField
-        pure $ (,extsConf) . TextWriter $ \opts doc ->
+        pure $ addProperties . TextWriter $ \opts doc ->
           liftIO $ withGCManagedState luaState $ do
             getfield registryindex writerField
             push doc