Re[Haskell-cafe] [2]: memoization
Alberto G. Corona
agocorona at gmail.com
Thu Sep 10 14:51:09 EDT 2009
key x= unsafePerformIO $ makeStableName x >>= return . hashStableName
sorry
2009/9/10 Alberto G. Corona <agocorona at gmail.com>
> instead o that you can use a key such is:
> key :: a -> Int
> key = unsafePerformIO . hashStableName . makeStableName
>
> that is defined for any kind of data
>
> then, a unique key for the pair f x could be:
>
> key1 f x=(key f , key x)
>
>
> However my experience is that ocassionally gives different hashes for the
> same object, so maybe a few registers will be duplicated.
>
>
>
> 2009/9/10 <mf-hcafe-15c311f0c at etc-network.de>
>
>
>> On Thu, Sep 10, 2009 at 05:23:26AM -0700, staafmeister wrote:
>> > To: haskell-cafe at haskell.org
>> > From: staafmeister <g.c.stavenga at 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
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>>
>
>
>
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