[Haskell-cafe] Weak pointers and referential transparency???

[Brian Hulley]

tpledger at ihug.co.nz wrote:
> Brian Hulley wrote:
> [...]
>>         Ref.write proxiesRef $! (weakProxy : proxies)
>
> (This is nothing to do with your main question, but the
> strict application looks unnecessary there.  Its right hand
> side is already a constructor cell.)

Yes, thanks for pointing this out.

>
> [...]
>> In other words, if the entry for the proxy
>> in the table stored in the Model dies, can
>> I be absolutely 100% sure that this means
>> the proxy no longer exists in any shape or
>> form in the running program?
>
> My reading of the semantics
> (http://haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html#4)
> is that you can be sure the proxy *object* is gone.

My problem is that I don't know what to make of the word "object" in the 
context of Haskell ie when can I be sure that a value is actually being 
represented as a pointer to a block of memory and not stored in registers or 
optimized out? Or is the compiler clever enough to preserve the concept of 
"object" despite such optimizations? I had been designing my Model/Proxy 
data types with the Java notion of "everything is a pointer to an object" 
but is this always correct relative to Haskell as a language or is it just a 
consequence of the current GHC implementation?

>
> As for referential transparency...
>
> Fan-in:
> If you create equivalent proxies in different calls to
> createProxy, it's possible that they'll end up referring to
> the same object (e.g. if the compiler or RTS does something
> fancy, or you use a memoised smart constructor).  So then a
> single live proxy object *could* protect many elements of
> your Weak Proxy list from scavenging.
>
> Fan-out:
> It would seem perverse to cry "referential transparency!"
> and spontaneously clone one of your proxy objects.  That
> *could* lead to deRefWeak returning Nothing while an
> *equivalent* Proxy object is still alive.  Might you run
> your program on an avant-garde distributed RTS?  ;-)

Someone else might even if I don't! ;-)

In the meantime I found a better solution to my original problem without 
needing to use weak pointers: I now make the proxy pull the changes from the 
model instead of making the model push them to all registered proxies, so 
there is no longer any need to have a table of registered proxies hence no 
need for weak pointers.

Thanks,
Brian.
-- 
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.

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