On 27 December 2012 21:08, David Herman <dher...@mozilla.com> wrote:
> On Dec 27, 2012, at 1:23 AM, Andreas Rossberg <rossb...@google.com> wrote:
> >> var x;
> >> if (...) { x = ... }
> >> if (x === undefined) { ... }
> >>
> >> If you want to use let instead, the === if-condition will throw. You
> would instead have to write:
> >>
> >> let x = undefined;
> >> if (...) { x = ... }
> >> if (x === undefined) { ... }
> >
> > That is not actually true, because AFAICT, "let x" was always understood
> to be equivalent to "let x = undefined".
>
> Well that's TDZ-UBI. It *is* true for TDZ-RBA. Maybe I was the only person
> who thought that was a plausible semantics being considered, but my claim
> (P => Q) is true. Your argument is ~P. Anyway, one way or another hopefully
> everyone agrees that TDZ-RBA is a non-starter.
>
Even with TDZ-RBA you can have that meaning for "let x" (and that semantics
would be closest to 'var'). What TDZ-RBA gives you, then, is the
possibility to also assign to x _before_ the declaration.
But anyway, I think we agree that this is not a desirable semantics, so it
doesn't really matter.
> It's true that with TDZ, there is a difference between the two forms
> above, but that is irrelevant, because that difference can only be observed
> for erroneous programs (i.e. where the first version throws, because 'x' is
> used by 'stmt').
>
> Can you prove this? (Informally is fine, of course!) I mean, can you prove
> that it can only affect buggy programs?
>
Well, I think it's fairly obvious. Clearly, once the
assignment/initialization "x = e" has been (successfully) executed, there
is no observable difference in the remainder of the program. Before that
(including while evaluating e itself), accessing x always leads to a TDZ
exception in the first form. So the only way it can not throw is if stmt
and e do not access x, in which case the both forms are equivalent.
/Andreas
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