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, 17 insertions, 6 deletions

diff --git a/pandoc-lua-engine/test/Tests/Lua/Writer.hs b/pandoc-lua-engine/test/Tests/Lua/Writer.hs
index 8b6e82816..18274d124 100644
--- a/pandoc-lua-engine/test/Tests/Lua/Writer.hs
+++ b/pandoc-lua-engine/test/Tests/Lua/Writer.hs
@@ -35,7 +35,7 @@ tests =
         source <- UTF8.toText <$> readFileStrict "testsuite.native"
         doc <- readNative def source
         txt <- writeCustom "sample.lua" >>= \case
-          (TextWriter f, _) -> f def doc
+          (TextWriter f, _, _) -> f def doc
           _            -> error "Expected a text writer"
         pure $ BL.fromStrict (UTF8.fromText txt))
 
@@ -45,23 +45,34 @@ tests =
         source <- UTF8.toText <$> readFileStrict "tables.native"
         doc <- readNative def source
         txt <- writeCustom "sample.lua" >>= \case
-          (TextWriter f, _) -> f def doc
+          (TextWriter f, _, _) -> f def doc
           _            -> error "Expected a text writer"
         pure $ BL.fromStrict (UTF8.fromText txt))
 
-  , goldenVsString "tables testsuite"
+  , goldenVsString "bytestring writer"
     "bytestring.bin"
     (runIOorExplode $ do
         txt <- writeCustom "bytestring.lua" >>= \case
-          (ByteStringWriter f, _) -> f def mempty
+          (ByteStringWriter f, _, _) -> f def mempty
           _                       -> error "Expected a bytestring writer"
         pure txt)
 
+  , goldenVsString "template"
+    "writer-template.out.txt"
+    (runIOorExplode $ do
+        txt <- writeCustom "writer-template.lua" >>= \case
+          (TextWriter f, _, mt) -> do
+            template <- mt
+            let opts = def{ writerTemplate = Just template }
+            f opts (B.doc (B.plain (B.str "body goes here")))
+          _ -> error "Expected a text writer"
+        pure $ BL.fromStrict (UTF8.fromText txt))
+
   , testCase "preset extensions" $ do
       let ediff = ExtensionsDiff{extsToEnable = [], extsToDisable = []}
       let format = FlavoredFormat "extensions.lua" ediff
       result <- runIOorExplode $ writeCustom "extensions.lua" >>= \case
-          (TextWriter write, extsConf) -> do
+          (TextWriter write, extsConf, _) -> do
             exts <- applyExtensionsDiff extsConf format
             write def{writerExtensions = exts} (B.doc mempty)
           _                        -> error "Expected a text writer"
@@ -73,7 +84,7 @@ tests =
             }
       let format = FlavoredFormat "extensions.lua" ediff
       result <- runIOorExplode $ writeCustom "extensions.lua" >>= \case
-          (TextWriter write, extsConf) -> do
+          (TextWriter write, extsConf, _) -> do
             exts <- applyExtensionsDiff extsConf format
             write def{writerExtensions = exts} (B.doc mempty)
           _                        -> error "Expected a text writer"