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

[Brendan Eich]

Herby Vojčík wrote:
> Let's allow foo.? to denote "soft foo", so you get:
>
> foo.?.bar    // foo == null ? undefined : foo.bar
> foo.?(bar)    // foo == null ? undefined : foo(bar)
> foo.?[bar]    // foo == null ? undefined : foo[bar]

?. works as well for these and the main (by use-frequency, so far) form 
matches CS. If we must have extra dots in the lesser-used forms, I'd put 
the dot in the middle to keep the question-mark on the left.

> but maybe also more esoteric uses, like:
>
> for (let key in foo.?)    // for (let key in (foo == null ? {} : foo))

That is even more of an abuse of dot, and it does not play well with ?: 
ternaries and the proposed ?? operator.

   foo.??bar:baz

would be legal by maximal munch, parsed as

   (foo.?)?bar:baz.

Beyond this objection, the need for foo.? instead of foo != null is not 
strong.

Worse, you show magic defaulting to {} in the for-in case, but the 
default result for anything like a suffix-? expression in CS is a 
boolean boolean result:

$ cat /tmp/w.coffee
lhs = foo?

$ ./bin/coffee -p /tmp/w.coffee
(function() {
   var lhs;

   lhs = typeof foo !== "undefined" && foo !== null;

}).call(this);

> foo.?        // foo == null ? undefined : foo // sort-of clearing

See above.

> foo.? = bar    // foo == null ? undefined : (foo = bar)
>         // this semantics is logical, just don't know
>         // what would be the use case...
> foo.?.baz = bar    // foo == null ? undefined : (foo.baz = bar)
>         // this is usable
>

These don't win over ?. in the proposal.

/be