On Thu, Sep 10, 2009 at 05:23:26AM -0700, staafmeister wrote: > To: haskell-cafe@haskell.org > From: staafmeister <g.c.stave...@uu.nl> > Date: Thu, 10 Sep 2009 05:23:26 -0700 (PDT) > Subject: Re: Re[Haskell-cafe] [2]: memoization > > > > Hi Bulat, > > > Bulat Ziganshin-2 wrote: > > > > Hello staafmeister, > > > > Thursday, September 10, 2009, 3:54:34 PM, you wrote: > > > >> What do you think about such a function? This function is > > > > a bit of refactoring > > > > -- "global variable" in haskell way > > cache = unsafePerformIO $ newIORef M.empty > > > > memo f x = unsafePerformIO$ 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 > > > > memo2 = curry . memo . uncurry > > > > This doesn't work and is exactly what I'm afraid the compiler is going to > do. Cache needs to > be associated with the function f. > > Otherwise one would get conflicts
then make the cache object store functions together with values. cache = unsafePerformIO $ newIORef M.empty memo f x = unsafePerformIO$ do m <- readIORef cache case M.lookup (mkKey f, x) m of Just y -> return y Nothing -> do let res = f x writeIORef cache $ M.insert (mkKey f, x) res m return res memo2 = curry . memo . uncurry This leaves mkKey. Since functions are neither Ord nor Show, you'd have to hack something together yourself. Perhaps an explicit argument to memo? memo :: (Ord a) => String -> (a -> b) -> a -> IO b memo fname f x = unsafePerformIO$ do m <- readIORef cache case M.lookup (fname, x) m of Just y -> return y Nothing -> do let res = f x writeIORef cache $ M.insert (fname, x) res m return res there is probably a better and more elegant solution, but this should at least work. right? matthias _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe