Rewrite leftmostLongest and leftmostLongestParse so that they don't rely

on 'head' which is a partial function

src/MdToHTML.hs

@@ -68,15 +68,21 @@ instance Monoid MdToken where
 
 -- ---------------
 -- Helpers
-leftmostLongest :: (Foldable t) => [(a, t b)] -> (a, t b)
+leftmostLongest :: (Foldable t) => [(a, t b)] -> Maybe (a, t b)
 leftmostLongest xs =
   let lastElem = last xs
       filteredLst = filter (\val -> length (snd val) == length (snd lastElem)) xs
-   in head filteredLst
+   in case filteredLst of
+        [] -> Nothing
+        (x : xs) -> Just x
 
 -- Get the first parse returned by readP_to_S that consumed the most input
-leftmostLongestParse :: ReadP a -> String -> (a, String)
-leftmostLongestParse parser input = leftmostLongest $ readP_to_S parser input
+leftmostLongestParse :: (Monoid a) => ReadP a -> String -> (a, String)
+leftmostLongestParse parser input =
+  let res = leftmostLongest $ readP_to_S parser input
+   in case res of
+        Nothing -> (mempty, mempty)
+        Just x -> x
 
 -- Parse if the string that's left matches the string comparator function
 lookaheadParse :: (String -> Bool) -> ReadP Char