namespaces and expression-qualified names

Jon Zeppieri jaz at bu.edu
Fri Mar 21 00:50:54 PDT 2008


On 3/21/08, Brendan Eich <brendan at mozilla.org> wrote:
>
>  I believe the intention among the core ES4 group over the last couple
>  of years has been to support runtime qualifiers in ES4, as well. JS
>  has computed property names since day 1: o[id] to access o.p. An
>  optional strict mode can't reason about these, but they aren't the
>  only loophole (eval, with, global non-fixtures). Why rule out runtime-
>  computed qualified names (with either part computed at runtime)?

I'm talking about lexical bindings.  I agree with you about object
properties; computed names are already the norm.  For lexical
bindings, the situation (if I understand it correctly) is different.
Because of the way eval is actually implemented in current engines,
it's impractical (or maybe strictly impossible?) to resolve lexical
lookups ahead of time.  With ES4, though, only the operator form of
eval will be able to modify lexical environments, and its use can be
statically detected.  Any environments that can be affected by the
eval operator can be known at compile time, and they can use slow
lookup.  The same holds for with statements.

And, I think, the same would hold for runtime qualified names... but
do you really want to add another way to defeat compile time lookup?
I kind of got the impression that if there were no question of
breaking the web, you would get rid of with (in its current, dynamic
scoping form) and eval's ability to modify local environments.

-Jon



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