Le 22/01/2013 11:47, Jason Orendorff a écrit :
On Mon, Jan 21, 2013 at 6:04 AM, David Bruant <bruan...@gmail.com
<mailto:bruan...@gmail.com>> wrote:
[...] WeakMap.prototype.clear questions the property that was true
before its adoption ("you can only modify a weakmap entry if you
have the key")
David, would you please elaborate your argument for this invariant?
This the first I've seen it stated.
An invariant can be a powerful thing. Still, I guess my default
position is that (1) the object-capabilities perspective is only one
view among many; (2) even looking at things with an eye for o-c
integrity and security, clearing a data structure seems like a
reasonable thing to allow, treating a reference to the data structure
itself as a sufficient capability. It's (2) that I would especially
like you to address.
I think Rick already suggested your (2), though phrased a bit
differently [1] (that was his #1). I answered [2]: "I thought more about
how I use weakmaps and [well-encapsulate my weakmaps so that I'm the
only holder] is a thing I do naturally indeed."
The problem may arise when you start sharing weakmaps around and some
use cases require you to [3].
Regarding your (1), I don't doubt the need to clear a data structure
since Allen explained a very compelling use case for that [3].
However, Mark showed an elegant way to implement .clear on top of
clear-less weakmaps and the class syntax [4][5] (reproducing here the
final version for clarity)
// note: implements the WeakMap API but does *not* extend WeakMap.
class WeakMapWithClear {
private let wrapped;
constructor() {
wrapped = new WeakMap();
}
get(key) => wrapped.get(key),
set(key, val) => wrapped.set(key, value),
has(key) => wrapped.has(key),
delete(key) => wrapped.delete(key),
clear() { wrapped = new WeakMap(); }
}
Now, the only thing that can differentiate both the native against this
version is performance I think. Allen seems to argue that a native
.clear would have better perf characteristics (related to GC). I still
fail to see why the difference would be significant (but I need to
re-read his recent posts about that).
In all likelihood, .clear is a method that is used sporadically. At
least, one needs to fill up the weakmap a bit before calling it, so I
don't think a marginal perf difference would matter.
As an implementor, what is your feeling about performance
characteristics of both the native and the class-based version?
David
[1] https://mail.mozilla.org/pipermail/es-discuss/2013-January/028353.html
[2] https://mail.mozilla.org/pipermail/es-discuss/2013-January/028357.html
[3] https://mail.mozilla.org/pipermail/es-discuss/2013-January/028380.html
[4] https://mail.mozilla.org/pipermail/es-discuss/2013-January/028370.html
[5] https://mail.mozilla.org/pipermail/es-discuss/2013-January/028371.html
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