[EMAIL PROTECTED] 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.
http://www.metamilk.com
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