Add WeakMap shim about es6-shim HOT 12 CLOSED

I presume you know of Gonzola's implementation

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yep, I've looked at it. seems to be OK.

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I think it'd be best not to have this, so people just use Maps and realize what they're getting into, instead of using WeakMaps under the perhaps-mistaken impression that the shim could be weak.

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@DomenicDenicola the shim can be weak. Look at Gonzola's implementation, it adhere's to the weak garbage collection semantics

See blog article

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I've tested WeakMap in node.js, it does proper GC. I've created 100000 objects (and weak map with them) and counted ram usage of node process. Then i've deleted objects and RAM usage has dropped to expected level.

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I admit to not spending the necessary time to entirely understanding Gonzola's implementation, but it looks similar in spirit to Mark Miller's, which according to the comments has the flaw that

However, the values associated with a key in some map are retained so long as that key is retained and those associations are not overridden.

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https://gist.github.com/1638059

I re-implemented my own flavour using gonzola's implementation as a base.

@DomenicDenicola I don't quite understand what's wrong with that. The values are retained as long as the key is retained. And as soon as the key is gone the values are gone too. That's the desired behaviour, right?

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@Raynos no, the desired behavior is that neither keys nor values are held strongly. See the membrane example in Miller's comments.

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So guys what do you finally think -- should es6-shim use @Raynos implementation of WeakMap even when it hold values strongly?

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I vote no: people should use Maps.

The main use case for including it would be for situations where freeing the memory would be nice, if possible, but not necessary. In those cases people should do global.WeakMap = global.Map, knowing full well what they're getting into, or use a separate library. (@Raynos, perhaps you should publish a WeaklyKeyedMap repo we can refer people to?)

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@dominic, pd.Name is a subset of weaklykeyedmap. It's more efficient and abstractions can be build on top of it. I don't intent to use a WeakMap because a Name is better.

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I agree -- we should provide only 100% working shims. Mimicking API isn't good. I'll add a note to readme on weakmaps.

Thanks everyone.

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