Re: [Haskell-cafe] Memoizing partially evaluated computations.

18 Mar 2009

      On Mar 18, 2009, at 12:06 PM, minh thu wrote:
...
2009/3/18 Sebastiaan Visser :
...
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