memoize

memoize.hs

memoize2 :: (Eq a, Hashable a) => (a -> IO b) -> IO (a -> b)
memoize2 f = do
  ref <- newIORef HashMap.empty
  pure $ \x ->
    do mp <- readIORef ref
       case HashMap.lookup x mp of
         Nothing -> unsafePerformIO $ do
                       res <- f x
                       writeIORef ref $ HashMap.insert x res mp
                       pure res
         Just res -> pure res