excluding features from sloppy mode

Mark S. Miller erights at google.com
Sat Dec 29 17:31:36 PST 2012

On Sat, Dec 29, 2012 at 5:16 PM, Brendan Eich <brendan at mozilla.com> wrote:
> I'm prepared to give up on the ban of duplicate formals when destructuring
> parameters are present, if that will get rid of your objection to
> "micro-modes". I don't recall anything else like this case. We arrived at it
> mainly to simplify implementation in SpiderMonkey, but again: users do not
> notice because no one uses duplicate formals.
>>> Are arrow functions, syntax and definite semantics, a "micro-mode"? If
>>> not,
>>> why not? I suspect you are using a mental desugaring spec but there's no
>>> such spec. Allen has to deal with whether arrows have [[Construct]] (we
>>> decided no, because |this| is bound to outer |this|). Is that a
>>> "micro-mode"? I say no.
> Did you have a thought here? It may be we're arguing only about the
> "destructuring parameter bans duplicate parameters" special case.

If duplicate formals are the only such case, then I agree that the
fear of micro-mode is a non-issue. Do we have an accurate record of
the scoping of default value expressions? How about the interaction of
head scope and top body scope? I recall there were problems here, but
I'd need to review our decisions to see if they smell of more


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