Re: [Haskell-cafe] HANSEI in Haskell?

On Thu, Feb 24, 2011 at 12:57 PM, Chung-chieh Shan wrote:

Arnaud Clère wrote in article in gmane.comp.lang.haskell.cafe:

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On 24/02/2011 09:30, oleg@okmij.org wrote:

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The sort of laziness needed for non-deterministic programming calls for first-class store.  It can be emulated, albeit *imperfectly*,  > for example, as was described in the ICFP09 paper. What do you mean by "imperfectly"?

I think Oleg meant that, because the garbage collector only understands the native store and not our emulation of first-class stores, cells of a first-class store that are no longer referenced will nevertheless not be garbage-collected until the store itself is garbage-collected.  What we need is a way to tell the garbage collector that the store reference and the cell reference are both needed to access the data so the data can be garbage-collected as soon as *either* the store reference *or the cell reference* is.  (Analogy from the capabilities literature: the key and the lock are both needed to open the door so the door can be garbage-collected as soon as either the key or the lock is.)  Any thoughts on how to achieve that?

Here's a weak cell which is a candidate for GC as soon as either the cell or the key is lost: http://hpaste.org/44280/weak_ref_needing_both_halves The implementation is GHC specific, as far as I know. I don't know if it's exactly what you're looking for, but the idea could be adapted towards some other purpose. Antoine