!Re: proposal: Object Members

[Isiah Meadows]

The reason private symbols are appropriate for Node's use case is
because it's conceptually a mixin, not a simple key/value map with
various utility functions (and weak map lookup is slower than property
access). JSDOM uses a similar utility [1] as a sort of mixin.

Keep in mind, I'm specifically *against* the abuse of weak maps for
private state that's conceptually (in an abstract sense, not runtime)
part of an object. Weak maps make sense when the weak map is the
dictionary conceptually (think: caching). But if conceptually, the
object is the dictionary, putting it in a weak map is giving the
engine the wrong info - properties have inline caches and heavy
optimization, but you can't do the same for weak maps in the other
direction without literally implementing them as properties. (I would
*love* to be proven wrong here, BTW.)

Let me draw a quick comparison: When do you use a map/set with string
keys, and when do you use an object instead?

- Both are functionally equivalent, but engines use *very* different
algorithms for each one.
- I can almost guarantee you don't use maps when object properties work.

One last thing: how would you hope to deal with module-internal data
stored on arbitrary objects, using any means other than private
symbols or something similar? To clarify, I'm talking of opaque object
structs [2], not simply classes. (BTW, that one is easier to manage as
a struct rather than a class, because of how many "methods" there are
operating on the state.)

[1]: https://github.com/jsdom/js-symbol-tree
[2]: https://github.com/isiahmeadows/enigma/blob/master/src/parser.ts

-----

Isiah Meadows
contact at isiahmeadows.com
www.isiahmeadows.com


On Mon, Jul 30, 2018 at 9:00 PM, Ranando King <kingmph at gmail.com> wrote:
> I meant to say if the object passed to the 3rd party function.....
>
>
> On Mon, Jul 30, 2018 at 7:59 PM Ranando King <kingmph at gmail.com> wrote:
>>
>> Just that use case alone is problematic. If the 3rd party function is not
>> extensible, then the new private data should not be allowed. If the library
>> cannot function without storing that data, then the function will have no
>> choice but to fall back to WeakMaps which don't care if the key is not
>> extensible. So why not just stick with WeakMaps for that case? And if that's
>> the case, then there would be little need for so open a means of defining
>> private field names. The proposal I'm offering offers the room to extend it
>> in the future to support everything else you might look for from your
>> private symbols idea.... unless you think I missed something.
>>
>> On Mon, Jul 30, 2018 at 7:26 PM Isiah Meadows <isiahmeadows at gmail.com>
>> wrote:
>>>
>>> That is one supported use case, yes. But that isn't the only use case
>>> this supports. It can still extend to traditional private class data,
>>> too.
>>>
>>> -----
>>>
>>> Isiah Meadows
>>> contact at isiahmeadows.com
>>> www.isiahmeadows.com
>>>
>>>
>>> On Mon, Jul 30, 2018 at 8:04 PM, Ranando King <kingmph at gmail.com> wrote:
>>> > So you're wanting the ability for a 3rd-party function to be able to
>>> > store
>>> > data private to that library on an object it didn't create, and that
>>> > only
>>> > that library can access?
>>> >
>>> > On Mon, Jul 30, 2018 at 6:36 PM Isiah Meadows <isiahmeadows at gmail.com>
>>> > wrote:
>>> >>
>>> >> First, my private symbols are properly *private*. The only
>>> >> "unexpected" thing that could happen is making an object larger
>>> >> memory-wise, which engines already have to be equipped to handle now
>>> >> (libraries aren't always well-behaved, and like to occasionally add
>>> >> expando properties to builtins and DOM elements). About the only thing
>>> >> most people would care about is in the debugger.
>>> >>
>>> >> Second, I had things like this in mind with supporting expando
>>> >> properties:
>>> >>
>>> >> https://github.com/nodejs/node/blob/ae4fde8bc883686def5badfb324236320669e8f4/lib/internal/linkedlist.js
>>> >>
>>> >> In that case, the Node.js people made it a pseudo-mixin rather than an
>>> >> actual type for performance reasons - there's fewer object allocations
>>> >> and they needed that.
>>> >>
>>> >> So I've considered the expando problem, and I disagree about it being
>>> >> a problem at all.
>>> >>
>>> >> -----
>>> >>
>>> >> Isiah Meadows
>>> >> contact at isiahmeadows.com
>>> >> www.isiahmeadows.com
>>> >>
>>> >>
>>> >> On Mon, Jul 30, 2018 at 6:35 PM, Waldemar Horwat <waldemar at google.com>
>>> >> wrote:
>>> >> > On 07/29/2018 04:37 PM, Isiah Meadows wrote:
>>> >> >>
>>> >> >> BTW, I came up with an alternate proposal for privacy altogether:
>>> >> >> https://github.com/tc39/proposal-class-fields/issues/115
>>> >> >>
>>> >> >> TL;DR: private symbols that proxies can't see and that can't be
>>> >> >> enumerated.
>>> >> >
>>> >> >
>>> >> > Aside from syntax, the main semantic difference I see between this
>>> >> > alternative and the main one is that this alternative defines
>>> >> > private
>>> >> > fields
>>> >> > as expandos, creating opportunities for mischief by attaching them
>>> >> > to
>>> >> > unexpected objects.  Aside from privacy, one of the things the
>>> >> > private
>>> >> > fields proposal gives you is consistency among multiple private
>>> >> > fields
>>> >> > on
>>> >> > the same object.  In the rare cases where you don't want that, you
>>> >> > could
>>> >> > use
>>> >> > weak maps.
>>> >> >
>>> >> >     Waldemar
>>> >> _______________________________________________
>>> >> es-discuss mailing list
>>> >> es-discuss at mozilla.org
>>> >> https://mail.mozilla.org/listinfo/es-discuss