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

[Allen Wirfs-Brock]

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 ?.  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)

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


> 
> 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 : $tmp.call(foo)}
> 
> then
> 
> var fun = foo?.bar;
> fun();
> 
> 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.

No, of course not.  You can't allow References to leak to the user value level!

Allen