On 9/11/17 5:36 AM, Matthew Robb wrote:
> I think it's irrelevant if internally VMs are not too happy. VMs are
there to solve our problems, not vice-versa ;-)
This ^ is very important for everyone to get on board with.
Regardless the cost should be negligible as the shape is only changing
at the point of delayed init. This will cause, for example V8, to deop
the object and have to build a new hidden class but only the one time.
I guess it would potentially be interesting to support an own property
that when undefined would delegate up the proto chain.
(I don't know, but) I would expect it to be worse than this. The shape
is changing at the point of delayed init, which means that if an engine
is associating the possible set of shapes with the constructor (or some
other form of allocation site + mandatory initialization), then that
site will produce multiple shapes. All code using such objects, if they
ever see both shapes, will have to handle them both. Even worse, if you
have several of these delayed init properties and you end up lazily
initializing them in different orders (which seems relatively easy to
do), then the internal slot offsets will vary.
You don't need to bend over backwards to make things easy for the VMs,
but you don't want to be mean to them either. :-)
Not to mention that the observable property iteration order will vary.
On Mon, Sep 11, 2017 at 7:09 AM, Andrea Giammarchi
<[email protected] <mailto:[email protected]>> wrote:
Hi Peter.
Unless you have a faster way to do lazy property assignment, I
think it's irrelevant if internally VMs are not too happy. VMs are
there to solve our problems, not vice-versa ;-)
Regards
On Mon, Sep 11, 2017 at 11:54 AM, peter miller
<[email protected] <mailto:[email protected]>> wrote:
Hi Andrea,
```
class CaseLazy {
get bar() {
var value = Math.random();
Object.defineProperty(this, 'bar', {value});
return value;
}
}
```
Doesn't this count as redefining the shape of the object? Or
are the compilers fine with it?
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