Global lexical tier

Brendan Eich brendan at mozilla.org
Thu Sep 3 01:50:37 UTC 2015


Andreas Rossberg wrote:
> On 3 September 2015 at 01:58, Brendan Eich <brendan at mozilla.org 
> <mailto:brendan at mozilla.org>> wrote:
>
>     I was there, I just re-read and re-remembered. I do not agree with
>     Allen that some tiny needle was uniquely threaded. Rather, an
>     aesthetic preference for the new ES6 binding forms to have a
>     lexical contour of their own when used at top level prevailed.
>     This leads to problems, not all of which were known at the time --
>     but some problems were noted.
>
>     The REPL problem, where let z=z; makes a poison pill, could be
>     coped with by ad-hoc REPL deviations from the spec -- at some
>     cost. Let's set it aside.
>
>     The one-time change to a reference, from global object to lexical
>     shadowing binding, is a serious flaw. Yes, it could happen due to
>     explicit scope nesting, but the global scope is apparently
>     uniform. There's no explicit delimiter.
>
>
> I still maintain that a tower-of-nested-scopes model would have been 
> cleaner AND would have avoided both the shadowing issue and the REPL 
> restriction. A mutable scope that gets extended under your feet is 
> terrible, lexical or not.

I don't remember you overcoming the counterarguments about async scripts 
and event handlers in async-generated/written markup twisting the nested 
scopes unexpectedly.

> I also maintain that putting lexical bindings onto the global object 
> is not an option. It is incompatible with having a TDZ, unless one 
> wants to introduce TDZs for properties into the object model,

Nope!

> which I doubt anybody wants. Or unless one makes toplevel binding 
> semantics completely different (and insane), which I also hope nobody 
> wants (though I'm not so sure).

Something has to give. This seems least bad.

>     The implementors seem to be rebelling but I'm not trying to stir
>     up trouble. It would help if V8 did support let, etc. in sloppy
>     mode. Then we might see open rebellion among two or more implementors.
>
>
> AFAICS this issue is completely orthogonal to sloppy-vs-script, and 
> there are no new effects specific to sloppy mode. V8 already fully 
> implements the necessary semantics. It wasn't pretty, but there are 
> uglier things in ES6.

There's still user not implementor feedback to be gained by V8 
supporting top-level let/const/class in sloppy mode per spec, but fair 
enough.

> The main thing holding back sloppy let in V8 right now is the parsing 
> nonsense and extra look-ahead required, which turns out to be a major 
> pain for us (and FWIW, slows down the V8 parser by a couple of percent 
> with little hope for recovery :( ).

I thought we resolved this (non-simple parameter list in function makes 
"use strict"; directive prologue an early error). What's left?

/be


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