f.hs

longestSuffix :: Domain -> DomTrie -> Maybe Domain
longestSuffix (Domain labels) table =
    Domain <$> go (reverse labels) table [] Nothing
  where
    go [] (DomTrie has _) ts r | has       = Just ts
                               | otherwise = r
    go (x:xs) (DomTrie has table) t r
      = let t' = x:t in
       case M.lookup x table of
         Just sub | has       -> go xs sub t' (Just t')
                  | otherwise -> go xs sub t' r
         Nothing  -> r