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

Brendan Eich brendan at mozilla.org
Tue Jun 19 15:43:43 PDT 2012

Allen Wirfs-Brock wrote:
> On Jun 19, 2012, at 10:50 AM, Brendan Eich wrote:
>> Another problem with your alternative: either it breaks a refactoring equivalence.
>> 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)}
> Why is it important that this equivalence holds for . and ?.

Equivalences are not sacred but they are informative and sometimes 
normative (as in, a refactoring norm exists in the world).

>    We already have other places using references where similar refactoring aren't equivalent:
>       typeof foo<!=>  do(let $tmp = foo /*Reference error if foo unresolvable) */; typeof $tmp)

That's right, and that is considered a botch not to imitate.The fact 
that typeof x == "function" testing is so long-winded is indeed one of 
the motivations for ?(.

> In particular, I'm pretty sure that this refactoring hazard is not going to be as significant as the
>        foo?.bar()   //oops I'm really think foo?.bar?()
> hazard

CoffeeScript has years of use, and this is not something that has caused 
confusion (ask @jashkenas, see github issues).


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