# Pastebin Dio1MtKJ
memo' :: forall t a. Grouping t => (t -> a) -> IO (t -> IO a)
memo' t2a = do
  f <- getGroup (grouping @t) \(t, ref1) -> 
     let a = t2a t
     writeIORef ref1 a
     pure \(_, ref2) -> writeIORef ref2 a
  pure \t -> do
    ref <- newIORef undefined
    f t (t, ref)
    readIORef ref