On 05.12.2013 03:15, Bill Myers wrote:
The reason I introduced COW when RC > 1 is that it allows persistent
data structures to be mutated in place if there aren't extra
references, just like non-persistent data structures.
That's a great idea that an might obviate the need to have a separate
`Builder` type (as in .Net [1]) or explicit `transients` (as in Clojure
[2]).
However, one has to be careful to "lock" the whole path when working in
a graph structure in a concurrent setting, so no other task/thread
changes a node further up, while the first task does changes to the some
node further down. This can happen, when relying on the reference count
alone because there can be N Arc-references + M borrowed references to
some memory box---and the reference count will only be N. Conceptually,
the `get()` method in Arc also should increase the reference count for
the lifetime of the borrowed reference. For safe memory management, this
is not necessary because the compiler will ensure that no borrowed
reference outlives all memory owners, but for counting who has access to
the memory contents, the reference count alone is inadequate.
This should not be hard to mitigate manually but it's a potential source
for some nasty race conditions ;)
-Michael
[1] https://msdn.microsoft.com/en-us/library/dn385366(v=vs.110).aspx
[2] http://clojure.org/transients
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