HTML writer: add task-list class to ul if all elements are task list items.

This will allow styling unordered task lists in a way that omits
the bullet.

src/Text/Pandoc/Writers/HTML.hs

@@ -370,6 +370,13 @@ defList :: PandocMonad m
         => WriterOptions -> [Html] -> StateT WriterState m Html
 defList opts items = toList H.dl opts (items ++ [nl opts])
 
+isTaskListItem :: [Block] -> Bool
+isTaskListItem (Plain (Str "☐":Space:_):_) = True
+isTaskListItem (Plain (Str "☒":Space:_):_) = True
+isTaskListItem (Para  (Str "☐":Space:_):_) = True
+isTaskListItem (Para  (Str "☒":Space:_):_) = True
+isTaskListItem _                           = False
+
 listItemToHtml :: PandocMonad m
                => WriterOptions -> [Block] -> StateT WriterState m Html
 listItemToHtml opts bls
@@ -819,7 +826,9 @@ blockToHtml opts (Header level attr@(_,classes,_) lst) = do
               _ -> H.p ! A.class_ "heading" $ contents'
 blockToHtml opts (BulletList lst) = do
   contents <- mapM (listItemToHtml opts) lst
-  unordList opts contents
+  let isTaskList = not (null lst) && all isTaskListItem lst
+  (if isTaskList then (! A.class_ "task-list") else id) <$>
+    unordList opts contents
 blockToHtml opts (OrderedList (startnum, numstyle, _) lst) = do
   contents <- mapM (listItemToHtml opts) lst
   html5 <- gets stHtml5

test/command/tasklist.md

@@ -5,7 +5,7 @@ tests adapted from <https://github.github.com/gfm/#task-list-items-extension->
 - [ ] foo
 - [x] bar
 ^D
-<ul>
+<ul class="task-list">
 <li><input type="checkbox" disabled="" />
 foo</li>
 <li><input type="checkbox" disabled="" checked="" />
@@ -21,9 +21,9 @@ bar</li>
   - [x] baz
 - [ ] bim
 ^D
-<ul>
+<ul class="task-list">
 <li><input type="checkbox" disabled="" checked="" />
-foo<ul>
+foo<ul class="task-list">
 <li><input type="checkbox" disabled="" />
 bar</li>
 <li><input type="checkbox" disabled="" checked="" />
@@ -73,7 +73,7 @@ ordered unchecked</li>
 ordered checked</li>
 </ol>
 <p>paragraph</p>
-<ul>
+<ul class="task-list">
 <li><p><input type="checkbox" disabled="" />
 list item with a</p><p>second paragraph</p></li>
 <li><p><input type="checkbox" disabled="" checked="" />