[Haskell-cafe] Explicit garbage collection

[Edward Kmett]

You probably just want to hold onto weak references for your 'isStillNeeded'
checks.

Otherwise the isStillNeeded check itself will keep you from garbage
collecting!

http://cvs.haskell.org/Hugs/pages/libraries/base/System-Mem-Weak.html

-Edward Kmett

On Wed, Jan 6, 2010 at 9:39 AM, Miguel Mitrofanov <miguelimo38 at yandex.ru>wrote:

> I'll take a look at them.
>
> I want something like this:
>
> refMaybe b dflt ref = if b then readRef ref else return dflt
> refIgnore ref = return "blablabla"
> refFst ref =
>   do
>      (v, w) <- readRef ref
>      return v
> test =
>   do
>      a <- newRef "x"
>      b <- newRef 1
>      c <- newRef ('z', Just 0)
>      performLocalGC -- if necessary
>      x <- isStillNeeded a
>      y <- isStillNeeded b
>      z <- isStillNeeded c
>      u <- refMaybe y "t" a -- note that it wouldn't actually read "a",
>                            -- but it won't be known until runtime.
>      w <- refIgnore b
>      v <- refFst c
>      return (x, y, z)
>
> so that "run test" returns (True, False, True).
>
>
> Dan Doel wrote:
>
>> On Wednesday 06 January 2010 8:52:10 am Miguel Mitrofanov wrote:
>>
>>> Is there any kind of "ST" monad that allows to know if some STRef is no
>>>  longer needed?
>>>
>>> The problem is, I want to send some data to an external storage over a
>>>  network and get it back later, but I don't want to send unnecessary
>>> data.
>>>
>>> I've managed to do something like that with weak pointers,
>>>  System.Mem.performGC and unsafePerformIO, but it seems to me that
>>> invoking
>>>  GC every time is an overkill.
>>>
>>> Oh, and I'm ready to trade the purity of runST for that, if necessary.
>>>
>>
>> You may be able to use something like Oleg's Lightweight Monadic Regions
>> to get this effect. I suppose it depends somewhat on what qualifies a
>> reference as "no longer needed".
>>
>>  http://www.cs.rutgers.edu/~ccshan/capability/region-io.pdf<http://www.cs.rutgers.edu/%7Eccshan/capability/region-io.pdf>
>>
>> I'm not aware of anything out-of-the-box that does what you want, though.
>>
>> -- Dan
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>
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