On 2010 Apr 5, at 10:29 AM, Per Vognsen wrote:
In languages like Python and Smalltalk that conflate value and
identity, you need nasty tricks like abusing the garbage collector's
object graph traverser to find all references to a given object (e.g.
gc.get_referrers() in Python) and then rewire those references on the
fly.
Not the point. If it were, how could I do:
#{ #{ 1 2 3 } #{ 4 10 100 } }
without having to use refs to the interior sets?
Even in clojure I can have values containing other values (not refs),
but only if I build acyclic graphs and so long as I build them from
the leaves "up."
Immutability is orthogonal to reference-ness.
There is nothing "wrong" with having immutable cyclic graphs of
values. (modulo JVM GC details, I'm not an expert)
STM fixes (among other things) the non-composible-ness of locks.
STM provides the right tools (the A and I of ACI propertties) to
permit making cyclic values.
That they can't be done now is not an inherent issue of value vs.
identity, just a limitation of the current implementation.
Wait, don't transients (http://clojure.org/transients) solve this
problem? I'm not sure, but it seems that if persistent! took multiple
values it could... Hmmm...
-Doug
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