Add support for `mark` extension for highlighted text.

+ Extensions: Add `Ext_mark` extension.
  This is not part of the pandoc extensions by default.
+ Markdown reader: parse `==..==` if `mark` extension enabled.
+ Markdown writer: support `mark` extension.
+ Docx writer: render Span with class `mark` as highlighted.
  Currently yellow is hardcoded.
+ LaTeX writer: support highlighted text for Span with class `mark`.
+ RST writer: use special `mark` role for Span with class `mark`.
+ Update MANUAL.txt.

Closes #7743.

7 files changed, 37 insertions, 4 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index eb1a50d72..fed4a61dd 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -140,7 +140,7 @@ When using LaTeX, the following packages need to be available
 `--listings` option is used), [`fancyvrb`], [`longtable`],
 [`booktabs`], [`graphicx`] (if the document
 contains images), [`hyperref`], [`xcolor`],
-[`ulem`], [`geometry`] (with the `geometry` variable set),
+[`soul`], [`geometry`] (with the `geometry` variable set),
 [`setspace`] (with `linestretch`), and
 [`babel`] (with `lang`).  If `CJKmainfont` is set, [`xeCJK`]
 is needed.  The use of `xelatex` or `lualatex` as
@@ -191,7 +191,7 @@ footnotes in tables).
 [`polyglossia`]: https://ctan.org/pkg/polyglossia
 [`prince`]: https://www.princexml.com/
 [`setspace`]: https://ctan.org/pkg/setspace
-[`ulem`]: https://ctan.org/pkg/ulem
+[`soul`]: https://ctan.org/pkg/soul
 [`unicode-math`]: https://ctan.org/pkg/unicode-math
 [`upquote`]: https://ctan.org/pkg/upquote
 [`weasyprint`]: https://weasyprint.org
@@ -5577,6 +5577,13 @@ be rewritten relative to the file containing the link
 reference definition, not the file containing the reference link
 or image itself, if these differ.
 
+#### Extension: `mark` ####
+
+To highlight out a section of text, begin and end it with
+with `==`. Thus, for example,
+
+    This ==is deleted text.==
+
 #### Extension: `attributes` ####
 
 Allows attributes to be attached to any inline or block-level
diff --git a/src/Text/Pandoc/Extensions.hs b/src/Text/Pandoc/Extensions.hs
index 1751295ad..efe4ed879 100644
--- a/src/Text/Pandoc/Extensions.hs
+++ b/src/Text/Pandoc/Extensions.hs
@@ -95,6 +95,7 @@ data Extension =
     | Ext_link_attributes         -- ^ link and image attributes
     | Ext_lists_without_preceding_blankline -- ^ Allow lists without preceding blank
     | Ext_literate_haskell    -- ^ Enable literate Haskell conventions
+    | Ext_mark                -- ^ Enable ==mark== syntax to highlight text
     | Ext_markdown_attribute      -- ^ Interpret text inside HTML as markdown iff
                                   --   container has attribute 'markdown'
     | Ext_markdown_in_html_blocks -- ^ Interpret as markdown inside HTML blocks
@@ -485,6 +486,7 @@ getAllExtensions f = universalExtensions <> getAll f
        , Ext_mmd_title_block
        , Ext_abbreviations
        , Ext_autolink_bare_uris
+       , Ext_mark
        , Ext_mmd_link_attributes
        , Ext_mmd_header_identifiers
        , Ext_compact_definition_lists
diff --git a/src/Text/Pandoc/Readers/Markdown.hs b/src/Text/Pandoc/Readers/Markdown.hs
index 42df8b985..5efc424ce 100644
--- a/src/Text/Pandoc/Readers/Markdown.hs
+++ b/src/Text/Pandoc/Readers/Markdown.hs
@@ -1509,6 +1509,7 @@ inline = do
      '!'     -> image
      '$'     -> math
      '~'     -> strikeout <|> subscript
+     '='     -> mark
      '<'     -> autoLink <|> spanHtml <|> rawHtmlInline <|> ltSign
      '\\'    -> math <|> escapedNewline <|> escapedChar <|> rawLaTeXInline'
      '@'     -> cite <|> exampleRef
@@ -1698,6 +1699,13 @@ strikeout = fmap B.strikeout <$>
                         >> notFollowedBy (char '~')
           strikeEnd   = try $ string "~~"
 
+mark :: PandocMonad m => MarkdownParser m (F Inlines)
+mark = fmap (B.spanWith ("",["mark"],[])) <$>
+ (guardEnabled Ext_mark >> inlinesBetween markStart markEnd)
+    where markStart = string "==" >> lookAhead nonspaceChar
+                        >> notFollowedBy (char '=')
+          markEnd   = try $ string "=="
+
 superscript :: PandocMonad m => MarkdownParser m (F Inlines)
 superscript = do
   fmap B.superscript <$> try (do
diff --git a/src/Text/Pandoc/Writers/Docx.hs b/src/Text/Pandoc/Writers/Docx.hs
index 9abe04b7a..3da9a000f 100644
--- a/src/Text/Pandoc/Writers/Docx.hs
+++ b/src/Text/Pandoc/Writers/Docx.hs
@@ -1114,6 +1114,9 @@ inlineToOpenXML' _ (Str str) =
   map Elem <$> formattedString str
 inlineToOpenXML' opts Space = inlineToOpenXML opts (Str " ")
 inlineToOpenXML' opts SoftBreak = inlineToOpenXML opts (Str " ")
+inlineToOpenXML' opts (Span ("",["mark"],[]) ils) =
+  withTextProp (mknode "w:highlight" [("w:val","yellow")] ()) $
+    inlinesToOpenXML opts ils
 inlineToOpenXML' opts (Span ("",["csl-block"],[]) ils) =
   inlinesToOpenXML opts ils
 inlineToOpenXML' opts (Span ("",["csl-left-margin"],[]) ils) =
diff --git a/src/Text/Pandoc/Writers/LaTeX.hs b/src/Text/Pandoc/Writers/LaTeX.hs
index d82838e0d..6a402e1c5 100644
--- a/src/Text/Pandoc/Writers/LaTeX.hs
+++ b/src/Text/Pandoc/Writers/LaTeX.hs
@@ -735,6 +735,9 @@ inlineListToLaTeX lst = hcat <$>
 inlineToLaTeX :: PandocMonad m
               => Inline    -- ^ Inline to convert
               -> LW m (Doc Text)
+inlineToLaTeX (Span ("",["mark"],[]) lst) = do
+  modify $ \st -> st{ stStrikeout = True } -- this gives us the soul package
+  inCmd "hl" <$> inlineListToLaTeX lst
 inlineToLaTeX (Span (id',classes,kvs) ils) = do
   linkAnchor <- hypertarget False id' empty
   lang <- toLang $ lookup "lang" kvs
diff --git a/src/Text/Pandoc/Writers/Markdown/Inline.hs b/src/Text/Pandoc/Writers/Markdown/Inline.hs
index a956692d7..96f8ab055 100644
--- a/src/Text/Pandoc/Writers/Markdown/Inline.hs
+++ b/src/Text/Pandoc/Writers/Markdown/Inline.hs
@@ -85,6 +85,10 @@ escapeText opts = T.pack . go' . T.unpack
     | isAlphaNum c = '\\' : go (c:cs)
     | otherwise = '\\':'\\': go cs
   go ('!':'[':cs) = '\\':'!':'[': go cs
+  go ('=':'=':cs)
+    | isEnabled Ext_mark opts = '\\':'=':go ('=':cs)
+  go ('~':'~':cs)
+    | isEnabled Ext_strikeout opts = '\\':'~':go ('~':cs)
   go (c:cs) =
     case c of
        '[' -> '\\':c:go cs
@@ -98,8 +102,7 @@ escapeText opts = T.pack . go' . T.unpack
            | otherwise -> "&lt;" ++ go cs
        '|' | isEnabled Ext_pipe_tables opts -> '\\':'|':go cs
        '^' | isEnabled Ext_superscript opts -> '\\':'^':go cs
-       '~' | isEnabled Ext_subscript opts ||
-             isEnabled Ext_strikeout opts -> '\\':'~':go cs
+       '~' | isEnabled Ext_subscript opts -> '\\':'~':go cs
        '$' | isEnabled Ext_tex_math_dollars opts -> '\\':'$':go cs
        '\'' | isEnabled Ext_smart opts -> '\\':'\'':go cs
        '"' | isEnabled Ext_smart opts -> '\\':'"':go cs
@@ -344,6 +347,10 @@ inlineToMarkdown opts (Span ("",["emoji"],kvs) [Str s]) =
        Just emojiname | isEnabled Ext_emoji opts ->
             return $ ":" <> literal emojiname <> ":"
        _ -> inlineToMarkdown opts (Str s)
+inlineToMarkdown opts (Span ("",["mark"],[]) ils)
+  | isEnabled Ext_mark opts
+    = do contents <- inlineListToMarkdown opts ils
+         return $ "==" <> contents <> "=="
 inlineToMarkdown opts (Span attrs ils) = do
   variant <- asks envVariant
   contents <- inlineListToMarkdown opts ils
diff --git a/src/Text/Pandoc/Writers/RST.hs b/src/Text/Pandoc/Writers/RST.hs
index 6c166f279..2ddf8a3e0 100644
--- a/src/Text/Pandoc/Writers/RST.hs
+++ b/src/Text/Pandoc/Writers/RST.hs
@@ -744,6 +744,9 @@ writeInlines lst =
 
 -- | Convert Pandoc inline element to RST.
 inlineToRST :: PandocMonad m => Inline -> RST m (Doc Text)
+inlineToRST (Span ("",["mark"],[]) ils) = do
+  contents <- writeInlines ils
+  return $ ":mark:`" <> contents <> "`"
 inlineToRST (Span (_,_,kvs) ils) = do
   contents <- writeInlines ils
   return $