On Thu, Sep 10, 2009 at 05:23:26AM -0700, staafmeister wrote:
> To: haskell-cafe@haskell.org
> From: staafmeister <g.c.stavenga@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