[Haskell-cafe] Explicit garbage collection

[Miguel Mitrofanov]

On 6 Jan 2010, at 23:21, Edward Kmett wrote:

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

That's what I do now. But I want to minimize the network traffic, so I  
want referenced values to be garbage collected as soon as possible -  
and I couldn't find anything except System.Mem.performIO to do the job  
- which is a bit too global for me.

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

Not necessary. What I'm imagining is that there is essentially only  
one way to access the value stored in the reference - with readRef.  
So, if there isn't any chance that readRef would be called, the value  
can be garbage collected; "isStillNeeded" function only needs the  
reference, not the value.

Well, yeah, that's kinda like weak references.

>
> 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
>
> I'm not aware of anything out-of-the-box that does what you want,  
> though.
>
> -- Dan
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