Markdown, CommonMark: add support for wiki links. [API change]

Adds the Markdown/CommonMark extensions `wikilinks_title_after_pipe` and
`wikilinks_title_before_pipe`. The former enables links of style `[[Name
of page|Title]]` and the latter `[[Title|Name of page]]`. Titles are
optional in both variants, so this works for both:
`[[https://example.org]]`, `[[Name of page]]`.

The writer is modified to render links with title `wikilink` as a
wikilink if a respective extension is enabled.

Pandoc will use `wikilinks_title_after_pipe` if both extensions are
enabled.

Closes: #2923

4 files changed, 40 insertions, 3 deletions

diff --git a/src/Text/Pandoc/Extensions.hs b/src/Text/Pandoc/Extensions.hs
index 525e96ac6..97ac8e77a 100644
--- a/src/Text/Pandoc/Extensions.hs
+++ b/src/Text/Pandoc/Extensions.hs
@@ -135,6 +135,10 @@ data Extension =
     | Ext_tex_math_dollars    -- ^ TeX math between $..$ or $$..$$
     | Ext_tex_math_double_backslash  -- ^ TeX math btw \\(..\\) \\[..\\]
     | Ext_tex_math_single_backslash  -- ^ TeX math btw \(..\) \[..\]
+    | Ext_wikilinks_title_after_pipe -- ^ Support wikilinks of style
+                                     -- [[target|title]]
+    | Ext_wikilinks_title_before_pipe  -- ^ Support wikilinks of style
+                                       -- [[title|target]]
     | Ext_xrefs_name          -- ^ Use xrefs with names
     | Ext_xrefs_number        -- ^ Use xrefs with numbers
     | Ext_yaml_metadata_block -- ^ YAML metadata block
@@ -505,6 +509,8 @@ getAllExtensions f = universalExtensions <> getAll f
        , Ext_literate_haskell
        , Ext_short_subsuperscripts
        , Ext_rebase_relative_paths
+       , Ext_wikilinks_title_after_pipe
+       , Ext_wikilinks_title_before_pipe
        ]
   getAll "markdown_strict"   = allMarkdownExtensions
   getAll "markdown_phpextra" = allMarkdownExtensions
@@ -557,6 +563,8 @@ getAllExtensions f = universalExtensions <> getAll f
     , Ext_implicit_header_references
     , Ext_attributes
     , Ext_sourcepos
+    , Ext_wikilinks_title_after_pipe
+    , Ext_wikilinks_title_before_pipe
     , Ext_yaml_metadata_block
     , Ext_rebase_relative_paths
     ]
diff --git a/src/Text/Pandoc/Readers/CommonMark.hs b/src/Text/Pandoc/Readers/CommonMark.hs
index b19637a3e..6f1e270ba 100644
--- a/src/Text/Pandoc/Readers/CommonMark.hs
+++ b/src/Text/Pandoc/Readers/CommonMark.hs
@@ -157,5 +157,9 @@ specFor opts = foldr ($) defaultSyntaxSpec exts
          [ (footnoteSpec <>) | isEnabled Ext_footnotes opts ] ++
          [ (definitionListSpec <>) | isEnabled Ext_definition_lists opts ] ++
          [ (taskListSpec <>) | isEnabled Ext_task_lists opts ] ++
+         [ (wikilinksSpec TitleAfterPipe <>)
+           | isEnabled Ext_wikilinks_title_after_pipe opts ] ++
+         [ (wikilinksSpec TitleBeforePipe <>)
+           | isEnabled Ext_wikilinks_title_before_pipe opts ] ++
          [ (rebaseRelativePathsSpec <>)
            | isEnabled Ext_rebase_relative_paths opts ]
diff --git a/src/Text/Pandoc/Readers/Markdown.hs b/src/Text/Pandoc/Readers/Markdown.hs
index 776eecd62..4369c0b2a 100644
--- a/src/Text/Pandoc/Readers/Markdown.hs
+++ b/src/Text/Pandoc/Readers/Markdown.hs
@@ -24,7 +24,6 @@ import Control.Monad
 import Control.Monad.Except (throwError)
 import Data.Bifunctor (second)
 import Data.Char (isAlphaNum, isPunctuation, isSpace)
-import Text.DocLayout (realLength)
 import Data.List (transpose, elemIndex, sortOn, foldl')
 import qualified Data.Map as M
 import Data.Maybe
@@ -35,6 +34,7 @@ import qualified Data.ByteString as BS
 import System.FilePath (addExtension, takeExtension, takeDirectory)
 import qualified System.FilePath.Windows as Windows
 import qualified System.FilePath.Posix as Posix
+import Text.DocLayout (realLength)
 import Text.HTML.TagSoup hiding (Row)
 import Text.Pandoc.Builder (Blocks, Inlines)
 import qualified Text.Pandoc.Builder as B
@@ -1516,7 +1516,7 @@ inline = do
      '_'     -> strongOrEmph
      '*'     -> strongOrEmph
      '^'     -> superscript <|> inlineNote -- in this order bc ^[link](/foo)^
-     '['     -> note <|> cite <|> bracketedSpan <|> link
+     '['     -> note <|> cite <|> bracketedSpan <|> wikilink <|> link
      '!'     -> image
      '$'     -> math
      '~'     -> strikeout <|> subscript
@@ -1831,6 +1831,21 @@ source = do
 linkTitle :: PandocMonad m => MarkdownParser m Text
 linkTitle = quotedTitle '"' <|> quotedTitle '\''
 
+wikilink :: PandocMonad m => MarkdownParser m (F Inlines)
+wikilink =
+  (guardEnabled Ext_wikilinks_title_after_pipe *> wikilink' swap) <|>
+  (guardEnabled Ext_wikilinks_title_before_pipe *> wikilink' id)
+  where
+    swap (a, b) = (b, a)
+    wikilink' order = try $ do
+      string "[["
+      notFollowedBy' (char '[')
+      raw <- many1TillChar (noneOf "\n\r\f\t") (try $ string "]]")
+      let (title, url) = case T.break (== '|') raw of
+            (before, "") -> (before, before)
+            (before, after) -> order (before, T.drop 1 after)
+      return . pure . B.link url "wikilink" $ B.str title
+
 link :: PandocMonad m => MarkdownParser m (F Inlines)
 link = try $ do
   st <- getState
diff --git a/src/Text/Pandoc/Writers/Markdown/Inline.hs b/src/Text/Pandoc/Writers/Markdown/Inline.hs
index 56970e706..e7be4fc21 100644
--- a/src/Text/Pandoc/Writers/Markdown/Inline.hs
+++ b/src/Text/Pandoc/Writers/Markdown/Inline.hs
@@ -651,6 +651,9 @@ inlineToMarkdown opts lnk@(Link attr@(ident,classes,kvs) txt (src, tit)) = do
                 case txt of
                       [Str s] | escapeURI s == srcSuffix -> True
                       _       -> False
+  let useWikilink = tit == "wikilink" &&
+                    (isEnabled Ext_wikilinks_title_after_pipe opts ||
+                     isEnabled Ext_wikilinks_title_before_pipe opts)
   let useRefLinks = writerReferenceLinks opts && not useAuto
   shortcutable <- asks envRefShortcutable
   let useShortcutRefLinks = shortcutable &&
@@ -667,7 +670,14 @@ inlineToMarkdown opts lnk@(Link attr@(ident,classes,kvs) txt (src, tit)) = do
       | otherwise ->  return $ result <> attrsToMarkua attributes
         where result = "[" <> linktext <> "](" <> (literal src) <> ")"
               attributes = addKeyValueToAttr attr ("title", tit)
-    _ | useAuto -> return $ "<" <> literal srcSuffix <> ">"
+    -- Use wikilinks where possible
+    _ | src == stringify txt && useWikilink ->
+        return $ "[[" <> literal (stringify txt) <> "]]"
+      | useAuto -> return $ "<" <> literal srcSuffix <> ">"
+      | useWikilink && isEnabled Ext_wikilinks_title_after_pipe opts -> return $
+        "[[" <> literal src <> "|" <> literal (stringify txt) <> "]]"
+      | useWikilink && isEnabled Ext_wikilinks_title_before_pipe opts -> return $
+        "[[" <> literal (stringify txt) <> "|" <> literal src <> "]]"
       | useRefLinks ->
            let first  = "[" <> linktext <> "]"
                second = if getKey linktext == getKey reftext