On 4 January 2012 21:52, Brendan Eich <bren...@mozilla.com> wrote:
> On Jan 4, 2012, at 4:23 AM, Andreas Rossberg wrote:
>
>> - Despite the superficial "fewer modes" mantra, it actually doesn't
>> make the language simpler, but more complicated: instead of defining
>> the semantics for Harmony features only for strict-mode programs, we
>> now have to define many features for both modes (and mixed uses of
>> both modes). I.e., there are more syntactic and semantic combinations
>> to worry about.
>
> Yes, this is a trade-off. It probably helps developers migrate, provided they
> choose sane combinations. For example, Allen (private correspondence)
> wondered if a function using the arguments object is evolved to have
> destructuring parameters:
>
> function f({a,b}, c) { arguments[0] = {a:3,b:4}; return [a, b] }
> f({a:1,b:2}, 3);
>
> I say anyone wanting deep aliasing (f returns [3, 4]) should suffer
> disappointment. I doubt developers want such deep aliasing, and no
> implementor does.
Whoa, yeah, please let's not go there.
>> - All this together (new features in old mode,
>
> Nope. ;-)
>
>
>> implicit opt-in,
>
> Yup.
>
>
>> explicit opt-in)
>
> No -- "use strict"; the string literal expression-statement is meaningless
> pre-ES5, ES5-strict in ES5, and redundant in Harmony code.
You said earlier that we best rely on the meta tag for the script
toplevel, which isn't exactly a No. ;)
> Now what do you say?
Sorry, I still think that a growing set of subtle implicit rules for
activating subtle semantic changes on a fine-grained level is far more
confusing (and error-prone!) than helpful. In all sorts of ways.
I'm also concerned about the 3 and a half language modes that might
result. With Dave's original proposal at least, the only opt-in was on
module level. That precluded a number of highly undesirable
combinations, e.g. extended mode nested into a "with" statement. Allen
gave a couple of good examples we also ran into when trying to
implement block scoping liberally in V8 -- in the end, we concluded
that it doesn't make sense to allow opting into extended mode locally.
/Andreas
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