Existential operator (was: ||= is much needed?)

Herby Vojčík herby at mailbox.sk
Thu Jun 21 12:37:22 PDT 2012



Brendan Eich wrote:
> Herby Vojčík wrote:
>> Adn that is precisely what I proposed under name
>> [[NullPatternObject]]. So all of (1), (3), (4) and (5) are void.
>
> Apologies, I missed that and saw only someone (perhaps it was Domenic)
> write ({}) as the default value, which is not the same.
>
>>> Such a Nil proxy still cannot masquerade as false, though, and that is
>>> required for suffix-? testing. An object that can convert to false
>>> awaits value objects
>>> (http://wiki.ecmascript.org/doku.php?id=strawman:value_objects) married
>>> with proxies to make value proxies.
>>
>> Such a [[NullPattternObject]] can ToBoolean to false. No problem I see.
>
> You're right we could hardcode a special case in ToBoolean ahead of
> value objects.
>
> To prevent this object from escaping into the language, we would need a
> special case in GetValue too. That makes this equvalent to Allen's
> conditional Reference (MaybeReference). Or were you thinking of letting
> this magic singleton esape into the language?

Definitely. It brings all pros of Null Pattern there (all of the uses I 
included in previous posts, automatically and consistently).

When it is explicit, users can begin to use it willingly. It may bring 
previously hard-to-get solutions.

> If so, I think you'll get serious push-back on TC39. Escaping raises the
> ante quite a bit. We have to be sure a falsy object won't confuse some
> code (SES code in particular) in a way that could lead to an exploit.

That I cannot envision... but Null Pattern object that produces itself 
for all operations ([[Get]], [[Call]], ...) should not be problematic.

> Apart from SES-focused concerns, letting the magic reference-like type
> escape exposes something ahead of schedule, before value objects in
> particular. If there's no need to let this thing escape, then we
> sholudn't. And in that case it's equivalent to MaybeReference.
>
> /be

Herby


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