Non-injective functions and URL slugs - Haskell

-- | Make a non-injective function injective
mkInjective :: [a] -> (a -> b) -> a -> b
mkInjective domain f = undefined

-- | Make a non-injective function injective
mkInjective :: Ord b => [a] -> (a -> b) -> a -> (b, Maybe a)
mkInjective domain f =
  let image = map f domain
      nonInjectiveImage = Set.fromList $ dups image
   in \a ->
        let b = f a
         in if Set.member b nonInjectiveImage
              then (b, Just a)
              else (b, Nothing)
  where
    dups = Map.keys . Map.filter (> 1) . Map.fromListWith (+) . fmap (,1 :: Int)