On Thu, Sep 10, 2009 at 5:46 PM, Luke Palmer
However, because the body of cache didn't depend on f, we can use lambda calculus rules to lift the let outside the lambda. So your transformation is completely valid... And yet, the original code works, and Bulat's equivalent code does not (in fact you can make a segfault using it).
I wouldn't dare say the original code is "correct" though, since a valid transformation can break it. Compilers do valid transformations.
O unsafePerformIO, how I love thee...
Right, which is why you should write it like this: memoIO :: Ord a => (a -> b) -> IO (a -> IO b) memoIO f = do cache <- newIORef M.empty return $ \x -> do m <- readIORef cache case M.lookup x m of Just y -> return y Nothing -> do let res = f x writeIORef cache $ M.insert x res m return res memo :: Ord a => (a -> b) -> (a -> b) memo f = unsafePerformIO $ do fmemo <- memoIO f return (unsafePerformIO . fmemo) I don't think there is any valid transformation that breaks this, since the compiler can't lift anything through unsafePerformIO. Am I mistaken? -- ryan