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

Brendan Eich brendan at mozilla.org
Tue Jun 19 10:50:33 PDT 2012

Another problem with your alternative: either it breaks a refactoring 

Let <==> be equivalence for a program fragment, and <!=> be 
inequivalence. Then we have in JS today extended with do expressions 
(and gensym via $tmp):

foo.bar() <==>  do {let $tmp = foo.bar; $tmp.call(foo)}

Now use ?. used instead of dot. Either the equivalence breaks:

foo?.bar() <!=>  do {let $tmp = foo?.bar; $tmp.call(foo)}

Or else your alternative leaks Reference type values into the language, 
observably, because if

foo?.bar() <==>  do {let $tmp = foo?.bar; ($tmp == null) ? void 0 : 


var fun = foo?.bar;

should not throw if fun is null or undefined, but there's nothing in the 
fun() expression to signal this to the implementation or readers. This 
fun would have as its value a new kind of MaybeReference type. We do not 
want this sub-alternative.


Brendan Eich wrote:
> Brendan Eich wrote:
>> Allen Wirfs-Brock wrote:
>>> I still stand by my alternative semantics as being a way to address 
>>> the more important use case without have ?()
>> No, the basis cases differ and that is a bug in your alternative.
>> foo?.bar means take bar from foo or undefined if foo == null.
>> foo?.bar() means take bar from foo and throw if foo == null else try 
>> to invoke the result if typeof "function", else undefined.
>> This is an off-by-one bug.
> It also uses ?. to mean . and magically applies the ? to the later ().
> Or if the semantics suppresses a TypeError both for the "take bar" and 
> "invoke it" parts, then it's mixing two things in one under syntax 
> located in the wrong place for the second suppression. Mainly, 
> suppression should be precise and at the point of application -- where 
> the dot goes if getting or setting, where the left paren goes if 
> invoking.
> Also, if you agree that the ?( case in CS is less significant, then we 
> shouldn't add it in haste or mangle ?. semantics to try to cover it.
> /be
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