Add `empty_paragraphs` extension.

* Deprecate `--strip-empty-paragraphs` option.  Instead we now
  use an `empty_paragraphs` extension that can be enabled on
  the reader or writer.  By default, disabled.

* Add `Ext_empty_paragraphs` constructor to `Extension`.

* Revert "Docx reader: don't strip out empty paragraphs."
  This reverts commit d6c58eb836f033a48955796de4d9ffb3b30e297b.

* Implement `empty_paragraphs` extension in docx reader and writer,
  opendocument writer, html reader and writer.

* Add tests for `empty_paragraphs` extension.

3 files changed, 27 insertions, 20 deletions

diff --git a/src/Text/Pandoc/Writers/Docx.hs b/src/Text/Pandoc/Writers/Docx.hs
index f80c2b59a..c9eaaf838 100644
--- a/src/Text/Pandoc/Writers/Docx.hs
+++ b/src/Text/Pandoc/Writers/Docx.hs
@@ -922,19 +922,22 @@ blockToOpenXML' opts (Para [Image attr alt (src,'f':'i':'g':':':tit)]) = do
   captionNode <- withParaProp (pCustomStyle "ImageCaption")
                  $ blockToOpenXML opts (Para alt)
   return $ mknode "w:p" [] (paraProps ++ contents) : captionNode
-blockToOpenXML' opts (Para lst) = do
-  isFirstPara <- gets stFirstPara
-  paraProps <- getParaProps $ case lst of
-                               [Math DisplayMath _] -> True
-                               _                    -> False
-  bodyTextStyle <- pStyleM "Body Text"
-  let paraProps' = case paraProps of
-        [] | isFirstPara -> [mknode "w:pPr" [] [pCustomStyle "FirstParagraph"]]
-        []               -> [mknode "w:pPr" [] [bodyTextStyle]]
-        ps               -> ps
-  modify $ \s -> s { stFirstPara = False }
-  contents <- inlinesToOpenXML opts lst
-  return [mknode "w:p" [] (paraProps' ++ contents)]
+blockToOpenXML' opts (Para lst)
+  | null lst && not (isEnabled Ext_empty_paragraphs opts) = return []
+  | otherwise = do
+      isFirstPara <- gets stFirstPara
+      paraProps <- getParaProps $ case lst of
+                                   [Math DisplayMath _] -> True
+                                   _                    -> False
+      bodyTextStyle <- pStyleM "Body Text"
+      let paraProps' = case paraProps of
+            [] | isFirstPara -> [mknode "w:pPr" []
+                                [pCustomStyle "FirstParagraph"]]
+            []               -> [mknode "w:pPr" [] [bodyTextStyle]]
+            ps               -> ps
+      modify $ \s -> s { stFirstPara = False }
+      contents <- inlinesToOpenXML opts lst
+      return [mknode "w:p" [] (paraProps' ++ contents)]
 blockToOpenXML' opts (LineBlock lns) = blockToOpenXML opts $ linesToPara lns
 blockToOpenXML' _ b@(RawBlock format str)
   | format == Format "openxml" = return [ x | Elem x <- parseXML str ]
diff --git a/src/Text/Pandoc/Writers/HTML.hs b/src/Text/Pandoc/Writers/HTML.hs
index 756bc3fd8..f25bbadfb 100644
--- a/src/Text/Pandoc/Writers/HTML.hs
+++ b/src/Text/Pandoc/Writers/HTML.hs
@@ -56,7 +56,7 @@ import qualified Data.Text.Lazy as TL
 import Network.HTTP (urlEncode)
 import Network.URI (URI (..), parseURIReference, unEscapeString)
 import Numeric (showHex)
-import Text.Blaze.Internal (customLeaf)
+import Text.Blaze.Internal (customLeaf, MarkupM(Empty))
 import Text.Blaze.Html hiding (contents)
 import Text.Pandoc.Definition
 import Text.Pandoc.Highlighting (formatHtmlBlock, formatHtmlInline, highlight,
@@ -658,6 +658,7 @@ blockToHtml opts (Para [Image attr txt (s,'f':'i':'g':':':tit)]) =
   figure opts attr txt (s,tit)
 blockToHtml opts (Para lst)
   | isEmptyRaw lst = return mempty
+  | null lst && not (isEnabled Ext_empty_paragraphs opts) = return mempty
   | otherwise = do
       contents <- inlineListToHtml opts lst
       return $ H.p contents
@@ -902,8 +903,7 @@ tableItemToHtml opts tag' align' item = do
   let tag'' = if null alignStr
                  then tag'
                  else tag' ! attribs
-  return $ (
-              tag'' contents) >> nl opts
+  return $ tag'' contents >> nl opts
 
 toListItems :: WriterOptions -> [Html] -> [Html]
 toListItems opts items = map (toListItem opts) items ++ [nl opts]
@@ -911,9 +911,13 @@ toListItems opts items = map (toListItem opts) items ++ [nl opts]
 toListItem :: WriterOptions -> Html -> Html
 toListItem opts item = nl opts >> H.li item
 
-blockListToHtml :: PandocMonad m => WriterOptions -> [Block] -> StateT WriterState m Html
+blockListToHtml :: PandocMonad m
+                => WriterOptions -> [Block] -> StateT WriterState m Html
 blockListToHtml opts lst =
-  (mconcat . intersperse (nl opts)) <$> mapM (blockToHtml opts) lst
+  (mconcat . intersperse (nl opts) . filter nonempty)
+    <$> mapM (blockToHtml opts) lst
+  where nonempty (Empty _) = False
+        nonempty _         = True
 
 -- | Convert list of Pandoc inline elements to HTML.
 inlineListToHtml :: PandocMonad m => WriterOptions -> [Inline] -> StateT WriterState m Html
diff --git a/src/Text/Pandoc/Writers/OpenDocument.hs b/src/Text/Pandoc/Writers/OpenDocument.hs
index 702349636..8aa19dbb5 100644
--- a/src/Text/Pandoc/Writers/OpenDocument.hs
+++ b/src/Text/Pandoc/Writers/OpenDocument.hs
@@ -130,7 +130,6 @@ setFirstPara :: PandocMonad m => OD m ()
 setFirstPara =  modify $  \s -> s { stFirstPara = True }
 
 inParagraphTags :: PandocMonad m => Doc -> OD m Doc
-inParagraphTags d | isEmpty d = return empty
 inParagraphTags d = do
   b <- gets stFirstPara
   a <- if b
@@ -323,7 +322,8 @@ blockToOpenDocument o bs
                                   else inParagraphTags =<< inlinesToOpenDocument o b
     | Para [Image attr c (s,'f':'i':'g':':':t)] <- bs
                              = figure attr c s t
-    | Para           b <- bs = if null b
+    | Para           b <- bs = if null b &&
+                                    not (isEnabled Ext_empty_paragraphs o)
                                   then return empty
                                   else inParagraphTags =<< inlinesToOpenDocument o b
     | LineBlock      b <- bs = blockToOpenDocument o $ linesToPara b