Lua: support extensions in custom writers

Custom writers can define the extensions that they support via the
global `writer_extensions`. The variable's value must be a table with
all supported extensions as keys, and their default status as values.

E.g., the below specifies that the writer support the extensions `smart`
and `sourcepos`, but only the `smart` extension is enabled by default:

    writer_extensions = {
      smart = true,
      sourcepos = false,
    }

1 files changed, 33 insertions, 7 deletions

diff --git a/pandoc-lua-engine/test/Tests/Lua/Writer.hs b/pandoc-lua-engine/test/Tests/Lua/Writer.hs
index 18d4e700d..8b6e82816 100644
--- a/pandoc-lua-engine/test/Tests/Lua/Writer.hs
+++ b/pandoc-lua-engine/test/Tests/Lua/Writer.hs
@@ -1,12 +1,10 @@
 {-# LANGUAGE LambdaCase        #-}
+{-# LANGUAGE OverloadedStrings #-}
 {- |
 Module      : Tests.Lua.Writer
 Copyright   : © 2019-2022 Albert Krewinkel
 License     : GNU GPL, version 2 or above
-
 Maintainer  : Albert Krewinkel <albert@zeitkraut.de>
-Stability   : alpha
-Portability : portable
 
 Tests for custom Lua writers.
 -}
@@ -14,13 +12,19 @@ module Tests.Lua.Writer (tests) where
 
 import Data.Default (Default (def))
 import Text.Pandoc.Class (runIOorExplode, readFileStrict)
+import Text.Pandoc.Extensions (Extension (..))
+import Text.Pandoc.Format (ExtensionsDiff (..), FlavoredFormat (..),
+                           applyExtensionsDiff)
 import Text.Pandoc.Lua (writeCustom)
+import Text.Pandoc.Options (WriterOptions (..))
 import Text.Pandoc.Readers (readNative)
 import Text.Pandoc.Writers (Writer (ByteStringWriter, TextWriter))
 import Test.Tasty (TestTree)
 import Test.Tasty.Golden (goldenVsString)
+import Test.Tasty.HUnit (testCase, (@?=))
 
 import qualified Data.ByteString.Lazy as BL
+import qualified Text.Pandoc.Builder as B
 import qualified Text.Pandoc.UTF8 as UTF8
 
 tests :: [TestTree]
@@ -31,7 +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))
 
@@ -41,7 +45,7 @@ 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))
 
@@ -49,7 +53,29 @@ tests =
     "bytestring.bin"
     (runIOorExplode $ do
         txt <- writeCustom "bytestring.lua" >>= \case
-          ByteStringWriter f -> f def mempty
-          _                  -> error "Expected a bytestring writer"
+          (ByteStringWriter f, _) -> f def mempty
+          _                       -> error "Expected a bytestring writer"
         pure 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
+            exts <- applyExtensionsDiff extsConf format
+            write def{writerExtensions = exts} (B.doc mempty)
+          _                        -> error "Expected a text writer"
+      result @?= "smart extension is enabled;\ncitations extension is disabled\n"
+  , testCase "modified extensions" $ do
+      let ediff = ExtensionsDiff
+            { extsToEnable = [Ext_citations]
+            , extsToDisable = []
+            }
+      let format = FlavoredFormat "extensions.lua" ediff
+      result <- runIOorExplode $ writeCustom "extensions.lua" >>= \case
+          (TextWriter write, extsConf) -> do
+            exts <- applyExtensionsDiff extsConf format
+            write def{writerExtensions = exts} (B.doc mempty)
+          _                        -> error "Expected a text writer"
+      result @?= "smart extension is enabled;\ncitations extension is enabled\n"
   ]