Removal of WeakMap/WeakSet clear

Allen Wirfs-Brock allen at
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> wrote:
>> On Wed, Nov 26, 2014 at 11:09 AM, Allen Wirfs-Brock
>> <allen at> 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 [1]:
>>> (...) 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
>>> welcome.
>>> —Claude
>>> [1]:
>>> 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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