# Pastebin 7zTQAoyz
memo' :: forall t a. Grouping t => (t -> a) -> IO (t -> IO a)
memo' t2a = do
  f <- getGroup (grouping @t) \ref1 -> do
     a <- readIORef ref1
     pure \ref2 -> writeIORef ref2 a
  pure \t -> do
    ref <- newIORef (t2a t) -- laziness to the rescue
    f t ref
    readIORef ref