[Haskell-cafe] HANSEI in Haskell?

Antoine Latter aslatter at gmail.com
Fri Feb 25 06:48:01 CET 2011

On Thu, Feb 24, 2011 at 10:15 PM, Chung-chieh Shan
<ccshan at cs.rutgers.edu> wrote:
> On 2011-02-24T16:20:46-0600, Antoine Latter wrote:
>> On Thu, Feb 24, 2011 at 12:57 PM, Chung-chieh Shan wrote:
>> > 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
> That's promising!  The lock I have in mind would probably be a map from
> Int to weak pointers.  I'm unfamiliar with System.Mem.Weak so I'll have
> to study this code further.  What is "addFinalizer lock (finalize wk)"
> for?  It seems wk doesn't have any finalizers to run.

I was confused by GHC weak references for some time.

GHC Weak references have the following parts:

* a key
* a value
* finalizers attached to the key

Setting up a weak reference to a value ties the value's liveness to
the key's liveness (even if the weak reference itself is dead).

Calling 'finalize' on the weak reference breaks the link between the
weak reference, and runs the finalizers attached to the key.

So here, we attach a finalizer to the "Lock" structure which then
breaks the link between the key and the value (this doesn't break the
link from the weak ref to the value, it simply stops the key from
keeping the value alive).

This might not be entirely safe in the face of some optimizations -
finalizers on ForeignPtrs and MVars are odd, to work around other
oddities that I don't yet grasp.


> Thanks!
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