[Haskell-cafe] Memoizing partially evaluated computations.

[Sebastiaan Visser]

On Mar 18, 2009, at 12:06 PM, minh thu wrote:

> 2009/3/18 Sebastiaan Visser <sfvisser at cs.uu.nl>:
>> Suppose I have a list of IO computations that depends on a few very  
>> time
>> consuming pure operations. The pure operations are not dependent on  
>> the real
>> world:
>>
>>> computation :: [IO Int]
>>> computation = [
>>>     smallIOfunc timeConsumingPureOperation0
>>>   , smallIOfunc timeConsumingPureOperation1
>>>   , smallIOfunc timeConsumingPureOperation2
>>>   , smallIOfunc timeConsumingPureOperation3
>>>   ]
>>>   where smallIOfunc a = print a >> return a
>>
>> In my main function I would like to repeatedly print the values
>>
>>> main = forever $
>>>   sequence_ (map (>>=print) computation)
>>
>> When I do this, all the time consuming operations will be  
>> reevaluated every
>> run of the main loop. Is there a any (simple or smart) way to  
>> prevent the
>> garbage collector from cleaning up the fully evaluated thunks  
>> inside my
>> computation? As if it were something like this:
>>
>>> computation :: [IO Int]
>>> computation = [smallIOfunc 42, smallIOfunc 34385, smallIOfunc 3,
>>> smallIOfunc 55]
>>
>> Of course I could plugin some kind of Int memoizer inside my  
>> computation,
>> but I do not really have the control to change things `deep' inside  
>> the
>> code. I want to have some form of snapshot of a list of partially  
>> evaluated
>> IO computations...
>>
>> Any suggestions?
>
> Hi,
>
> If timeConsumingPureOperation is pure, the problem is thus not  
> related to IO,
> and your question remains the same : how to memoize  
> timeConsumingPureOperation
> for some arguments. Since you want to repeatidly call main, it seems  
> a good idea
> to wrap your pure operation in a memoizing CAF (and give the wrapped  
> version to
> smalIOFuncf).

The problem is that the `timeConsumingPureOperation' is somewhere very  
deep inside my code at a point I cannot alter. Like this:

 > -- This I can change:
 > myIOCode = forever (deepLibraryCode >>= print)
 >
 > -- This I cannot change:
 > deepLibraryCode :: IO Int
 > deepLibraryCode = makeIOfunctionFrom timeConsumingPureOperation

The separation between the make `makeIOfunctionFrom' and  
`timeConsumingPureOperation' might not even be that clear as in my  
example.

That is why I am looking for some high level way of memoizing.

> You can here : http://www.haskell.org/haskellwiki/Memoization
>
> HTH,
> Thu