Removal of WeakMap/WeakSet clear
allen at wirfs-brock.com
Wed Nov 26 13:28:57 PST 2014
On Nov 26, 2014, at 11:25 AM, Tab Atkins Jr. wrote:
> On Wed, Nov 26, 2014 at 11:23 AM, Mark S. Miller <erights at google.com> wrote:
>> On Wed, Nov 26, 2014 at 11:09 AM, Allen Wirfs-Brock
>> <allen at wirfs-brock.com> wrote:
>>> On Nov 26, 2014, at 10:22 AM, Claude Pache wrote:
>>> The root of the issue, is that WeakMaps are thought as a tool for two
>>> unrelated use cases. Quoting :
>>> (...) WeakMaps were motivate[d] by two distinct use cases. Soft fields and
>>> object-keyed caches.
>>> Now, in short, for the "soft fields" use case, a `.clear()` method is
>>> unwanted, but for the "object-keyed caches" use case, a `.clear()` method is
>>> : https://esdiscuss.org/topic/weakmap-clear-and-inverted-implementations
>>> Let's ignore security for the moment.
>>> If WeakMaps/WeakSets are not inspectable (via iteration) and do not have a
>>> clear operation, then the inverted implementation technique can be use used.
>>> This technique eliminates significant GS complexity.
>>> The ability to iterate over the elements of a WeakMap/Set would make
>>> implementation specific (and probably non-deterministic) GC timing
>>> observable to ES programs. Non-deterministic and observable implementation
>>> specific behavior is bad for interoperability.
>> Hi Allen, as much as I'd like to agree with this, I don't see the
>> observability-of-GC issue. Could you explain more? Thanks.
> Allen appears to be mixing in an iterability argument. This isn't
> germane to the .clear() discussion, unless you think of .clear() as
> being implemented by iterating and removing keys.
Right, it's the lack of iterability and clear that makes the inverted implementation feasible. We justify lack of iterability because it exposes GC.
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