Le 16 janv. 2013 à 09:11, David Bruant <bruan...@gmail.com> a écrit :
> Hi,
>
> This is an idea naturally derived of all the current discussions about
> WeakMaps and Private symbols. The proposal is easily summarized by these
> lines of code:
>
> var wm = new WeakMap();
> var o = {};
> o[wm] = 12 // desugars to wm.set(o, 12)
> var a = o[wm]; // desugars to wm.get(o);
> wm in o // desugars to wm.has(o);
> delete o[wm] // desugars to wm.delete(o);
>
> <snip>
Hello,
I've just thought of a fundamental flow in the idea of using of weak maps
instead of private symbols, which I believe is enough for forcing to forget
that idea, at least on a language design level.
(Note: I have not reread all what have been posted on this list on the subject
since then, so I am not sure that someone has not already raised that
objection.)
Weak maps and private symbols have completely different goals, and conflating
the two features into one can (and, as I'll show, will) lead to conflict of
interest in designing the API. Weak maps are designed for better memory
management, while private symbols are designed for better encapsulation. The
key difference of philosophy between the two features is:
* In weak maps, you ***need not*** have access to the value if you don't have
access to the key.
* With private symbols used as property keys, you ***must not*** have access
to the property value if you don't have access to the symbol.
It is just a happenstance that, in our case "need not" mostly implies "do not".
The problem is that it is only "mostly". Indeed:
It is very reasonable to have a -clear() method on WeakMaps, which delete all
relations defined by the weak map, in situation where you know that you won't
need these mappings any more, but it would be impractical to either enumerate
all the keys, or to wipe all references to the weak map. Good for memory
management.
On the other hand, you certainly do not want a ".revoke()" method available on
every private symbols, which wipe all properties which use that key as private
symbol, even on objects to which you are not allowed to have access. Bad for
encapsulation.
There is no fundamental problem to use a feature for something very different
that it was not designed for. But before sanctioning such a use in a language
design, conflict of interest between the two intended use must be carefully
considered.
Claude
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