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

Aymeric Vitte vitteaymeric at gmail.com
Mon Jun 18 04:13:51 PDT 2012


This is related to what I was trying to figure out in the "more fun with 
undefined" thread, maybe it is wrong or have too many impact but I was 
about to suggest :

8.9.1 GetValue (V)
...
5. If IsUnresolvableReference(V), return undefined

11.2.1 Property Accessors
Runtime Semantics: Evaluation
...
3. If baseValue is an abrupt completion or undefined, return baseValue.

Why not, instead of adding "?" operator ?

Le 18/06/2012 07:11, Brendan Eich a écrit :
> Sorry, meant to start a new thread for:
>
> http://wiki.ecmascript.org/doku.php?id=strawman:existential_operator
>
> As the Syntax section hints, we can't also adopt CoffeeScript's ?( 
> variant, which enables foo.bar?(args, go, here).baz and the like. The 
> C syntax heritage prevails.
>
> /be
>
> Brendan Eich wrote:
>> David Herman wrote:
>>> On Jun 15, 2012, at 5:57 PM, satyr wrote:
>>>
>>>> On Sat, Jun 16, 2012 at 4:33 AM, David Herman <dherman at mozilla.com 
>>>> <mailto:dherman at mozilla.com>> wrote:
>>>>
>>>>     As for null, I can see how there's confusion about whether to use
>>>>     null vs undefined, and so I can see why CoffeeScript would just
>>>>     try to blur the distinction between them.
>>>>
>>>>
>>>> Not just for blurring. Rejecting `null` is essential for 
>>>> CoffeeScript's "existence" due to `?.`, the soak/safe access operator.
>>>
>>> I think you could make a case for ?. defaulting for both but ?? 
>>> defaulting only undefined. The case goes something like this:
>>>
>>> - The purpose of ?? is to provide a default value when no value was 
>>> provided. The way to say "no value" in JavaScript is undefined.
>>>
>>> - The purpose of ?. is to fail soft when doing a property lookup. 
>>> Both null and undefined throw when doing a property lookup.
>>
>> Agreed. This is one choice, it's plausible because of the distinction 
>> between defaulting (which requires intentional passing of a "please 
>> default" sentinel value, or not passing a trailing actual argument) 
>> and soaking up null-or-undefined.
>>
>> Yes, we could make ?? and ??= do the same for null as for undefined. 
>> I'm not sure that's the right choice, but it's a choice. For 
>> foo.bar?.baz, though, the clearer choice is to avoid throwing, which 
>> means evaluating to undefined if foo.bar is missing (evaluates to 
>> undefined) *or* has a value not coercible to object type (null or 
>> undefined). See
>>
>> http://wiki.ecmascript.org/doku.php?id=strawman:existential_operator
>>
>> /be
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