Le 07/07/2011 15:42, Andreas Rossberg a écrit :
On 7 July 2011 15:09, David Bruant<david.bru...@labri.fr> wrote:
Yes, that's what I would propose, too. It's just a bit ugly that we
have to do that in two places now.
Three if counting the "enumerate" trap for for-in loops. Regardless of
ugliness, it's necessary. keys and enumerate are derived traps. They have a
default implementation for developer convenience, however, developers could
decide to reimplement the trap and the proxy engine implementation have to
enforce types anyway. Each trap has to be guarded independently.
Derived traps as showed are written in JS for expository purposes. Engines
will be free to optimize as they wish internally as long as the observed
behavior is the same.
True, but optimizing that actually is more tricky than you might
think, since in general it would change the semantics if an engine
decided to call toString only once. It has to make sure that none of
the names are objects, or at least none of their toString methods was
modified and they are all free of side effects.
Interesting.
However, I'm not sure side-effects are a problem.
-----
var o = {a:1, toString:function(){o.b = 12; return 'a'; }};
console.log(o[o], o.b); // 1, 12 on Firefox 5
-----
Here, o[o] triggers a side effect and that sound like the normal behavior.
I agree that performance is an issue, but as you say below, programmers
returning objects as property names are aware of the performance issue.
Specifically, I think that type inference engines can
be of a great help in ensuring that types are correct without having to pay
the price of looking at every single element independently.
I don't think that the type checks are the biggest cost. Doing the
actual conversion several times for those cases where the type is
_not_ string is potentially much more expensive.
I guess it's fine if programmers suffer for returning objects as
property names. But something like integers might be a valid use case.
Is there a solution at all to that problem? If we want to enforce the
invariant, we have to pay the cost. I don't see a workaround... or
maybe, instead of returning an array with names, the traps could return
a proxy that lazily coerces types on getting? :-p
David
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