Support shortcut reference links in markdown writer

Issue #1977

Most markdown processors support the [shortcut format] for reference links.
Pandoc's markdown reader parsed this shortcuts unoptionally.
Pandoc's markdown writer (with --reference-links option) never shortcutted links.

This commit adds an extension `shortcut_reference_links`. The extension is
enabled by default for those markdown flavors that support reading shortcut
reference links, namely:

    - pandoc
    - strict pandoc
    - github flavoured
    - PHPmarkdown

If extension is enabled, reader parses the shortcuts in the same way as
it preveously did. Otherwise it would parse them as normal text.

If extension is enabled, writer outputs shortcut reference links unless
doing so would cause problems (see test cases in `tests/Tests/Writers/Markdown.hs`).

3 files changed, 63 insertions, 30 deletions

diff --git a/src/Text/Pandoc/Options.hs b/src/Text/Pandoc/Options.hs
index 24e31fbb6..48803d36f 100644
--- a/src/Text/Pandoc/Options.hs
+++ b/src/Text/Pandoc/Options.hs
@@ -109,6 +109,7 @@ data Extension =
     | Ext_implicit_header_references -- ^ Implicit reference links for headers
     | Ext_line_blocks         -- ^ RST style line blocks
     | Ext_epub_html_exts      -- ^ Recognise the EPUB extended version of HTML
+    | Ext_shortcut_reference_links -- ^ Shortcut reference links
     deriving (Show, Read, Enum, Eq, Ord, Bounded)
 
 pandocExtensions :: Set Extension
@@ -151,6 +152,7 @@ pandocExtensions = Set.fromList
   , Ext_header_attributes
   , Ext_implicit_header_references
   , Ext_line_blocks
+  , Ext_shortcut_reference_links
   ]
 
 phpMarkdownExtraExtensions :: Set Extension
@@ -164,6 +166,7 @@ phpMarkdownExtraExtensions = Set.fromList
   , Ext_intraword_underscores
   , Ext_header_attributes
   , Ext_abbreviations
+  , Ext_shortcut_reference_links
   ]
 
 githubMarkdownExtensions :: Set Extension
@@ -180,6 +183,7 @@ githubMarkdownExtensions = Set.fromList
   , Ext_strikeout
   , Ext_hard_line_breaks
   , Ext_lists_without_preceding_blankline
+  , Ext_shortcut_reference_links
   ]
 
 multimarkdownExtensions :: Set Extension
@@ -202,7 +206,9 @@ multimarkdownExtensions = Set.fromList
 
 strictExtensions :: Set Extension
 strictExtensions = Set.fromList
-  [ Ext_raw_html ]
+  [ Ext_raw_html
+  , Ext_shortcut_reference_links
+  ]
 
 data ReaderOptions = ReaderOptions{
          readerExtensions      :: Set Extension  -- ^ Syntax extensions
diff --git a/src/Text/Pandoc/Readers/Markdown.hs b/src/Text/Pandoc/Readers/Markdown.hs
index 910936b34..9e6ad0e13 100644
--- a/src/Text/Pandoc/Readers/Markdown.hs
+++ b/src/Text/Pandoc/Readers/Markdown.hs
@@ -1679,6 +1679,7 @@ referenceLink constructor (lab, raw) = do
       lookAhead (try (spnl >> normalCite >> return (mempty, "")))
       <|>
       try (spnl >> reference)
+  when (raw' == "") $ guardEnabled Ext_shortcut_reference_links
   let labIsRef = raw' == "" || raw' == "[]"
   let key = toKey $ if labIsRef then raw else raw'
   parsedRaw <- parseFromString (mconcat <$> many inline) raw'
diff --git a/src/Text/Pandoc/Writers/Markdown.hs b/src/Text/Pandoc/Writers/Markdown.hs
index 662373209..15c6d9fb5 100644
--- a/src/Text/Pandoc/Writers/Markdown.hs
+++ b/src/Text/Pandoc/Writers/Markdown.hs
@@ -57,14 +57,15 @@ import qualified Data.Text as T
 
 type Notes = [[Block]]
 type Refs = [([Inline], Target)]
-data WriterState = WriterState { stNotes  :: Notes
-                               , stRefs   :: Refs
-                               , stInList :: Bool
-                               , stIds    :: [String]
-                               , stPlain  :: Bool }
+data WriterState = WriterState { stNotes           :: Notes
+                               , stRefs            :: Refs
+                               , stRefShortcutable :: Bool
+                               , stInList          :: Bool
+                               , stIds             :: [String]
+                               , stPlain           :: Bool }
 instance Default WriterState
-  where def = WriterState{ stNotes = [], stRefs = [], stInList = False,
-                           stIds = [], stPlain = False }
+  where def = WriterState{ stNotes = [], stRefs = [], stRefShortcutable = True,
+                           stInList = False, stIds = [], stPlain = False }
 
 -- | Convert Pandoc to Markdown.
 writeMarkdown :: WriterOptions -> Pandoc -> String
@@ -690,27 +691,47 @@ getReference label (src, tit) = do
 inlineListToMarkdown :: WriterOptions -> [Inline] -> State WriterState Doc
 inlineListToMarkdown opts lst = do
   inlist <- gets stInList
-  mapM (inlineToMarkdown opts)
-     (if inlist then avoidBadWraps lst else lst) >>= return . cat
-  where avoidBadWraps [] = []
-        avoidBadWraps (Space:Str ('>':cs):xs) =
-          Str (' ':'>':cs) : avoidBadWraps xs
-        avoidBadWraps (Space:Str [c]:[])
-          | c `elem` "-*+" = Str [' ', c] : []
-        avoidBadWraps (Space:Str [c]:Space:xs)
-          | c `elem` "-*+" = Str [' ', c] : Space : avoidBadWraps xs
-        avoidBadWraps (Space:Str cs:Space:xs)
-          | isOrderedListMarker cs = Str (' ':cs) : Space : avoidBadWraps xs
-        avoidBadWraps (Space:Str cs:[])
-          | isOrderedListMarker cs = Str (' ':cs) : []
-        avoidBadWraps (x:xs) = x : avoidBadWraps xs
-        isOrderedListMarker xs = endsWithListPunct xs &&
-                      isRight (runParserT (anyOrderedListMarker >> eof)
-                               defaultParserState "" xs)
-        endsWithListPunct xs = case reverse xs of
-                                     '.':_ -> True
-                                     ')':_ -> True
-                                     _     -> False
+  go (if inlist then avoidBadWrapsInList lst else lst)
+  where go [] = return empty
+        go (i:is) = case i of
+            (Link _ _) -> case is of
+                -- If a link is followed by another link or '[' we don't shortcut
+                (Link _ _):_                  -> unshortcutable
+                Space:(Link _ _):_            -> unshortcutable
+                Space:(Str('[':_)):_          -> unshortcutable
+                Space:(RawInline _ ('[':_)):_ -> unshortcutable
+                Space:(Cite _ _):_            -> unshortcutable
+                (Cite _ _):_                  -> unshortcutable
+                Str ('[':_):_                 -> unshortcutable
+                (RawInline _ ('[':_)):_       -> unshortcutable
+                (RawInline _ (' ':'[':_)):_   -> unshortcutable
+                _                             -> shortcutable
+            _ -> shortcutable
+          where shortcutable = liftM2 (<>) (inlineToMarkdown opts i) (go is)
+                unshortcutable = do
+                    iMark <- withState (\s -> s { stRefShortcutable = False })
+                                       (inlineToMarkdown opts i)
+                    modify (\s -> s {stRefShortcutable = True })
+                    fmap (iMark <>) (go is)
+
+avoidBadWrapsInList :: [Inline] -> [Inline]
+avoidBadWrapsInList [] = []
+avoidBadWrapsInList (Space:Str ('>':cs):xs) =
+  Str (' ':'>':cs) : avoidBadWrapsInList xs
+avoidBadWrapsInList (Space:Str [c]:[])
+  | c `elem` "-*+" = Str [' ', c] : []
+avoidBadWrapsInList (Space:Str [c]:Space:xs)
+  | c `elem` "-*+" = Str [' ', c] : Space : avoidBadWrapsInList xs
+avoidBadWrapsInList (Space:Str cs:Space:xs)
+  | isOrderedListMarker cs = Str (' ':cs) : Space : avoidBadWrapsInList xs
+avoidBadWrapsInList (Space:Str cs:[])
+  | isOrderedListMarker cs = Str (' ':cs) : []
+avoidBadWrapsInList (x:xs) = x : avoidBadWrapsInList xs
+
+isOrderedListMarker :: String -> Bool
+isOrderedListMarker xs = (last xs `elem` ".)") &&
+              isRight (runParserT (anyOrderedListMarker >> eof)
+                       defaultParserState "" xs)
 
 isRight :: Either a b -> Bool
 isRight (Right _) = True
@@ -873,6 +894,9 @@ inlineToMarkdown opts (Link txt (src, tit)) = do
                       [Str s] | escapeURI s == srcSuffix -> True
                       _                                  -> False
   let useRefLinks = writerReferenceLinks opts && not useAuto
+  shortcutable <- gets stRefShortcutable
+  let useShortcutRefLinks = shortcutable &&
+                            isEnabled Ext_shortcut_reference_links opts
   ref <- if useRefLinks then getReference txt (src, tit) else return []
   reftext <- inlineListToMarkdown opts ref
   return $ if useAuto
@@ -882,7 +906,9 @@ inlineToMarkdown opts (Link txt (src, tit)) = do
               else if useRefLinks
                       then let first  = "[" <> linktext <> "]"
                                second = if txt == ref
-                                           then "[]"
+                                           then if useShortcutRefLinks
+                                                   then ""
+                                                   else "[]"
                                            else "[" <> reftext <> "]"
                            in  first <> second
                       else if plain