On Sep 26, 2009, at 3:13 PM, Allen Wirfs-Brock wrote:
From: Maciej Stachowiak [mailto:m...@apple.com]
On Sep 26, 2009, at 8:28 AM, Allen Wirfs-Brock wrote:
...
Essentially,
the semantics of "browser ECMAScript" has been arbitrarily split
into
two independently maintained standards.
Is there any concrete concern on this front other than property
access catchalls?
Every place the WebIDL ECMAScript binding "overrides" an ECMAScript
specification
internal method is a concern as these are special case extensions to
the ECMAScript
semantics. As language designers we need to understand if these
special cases are
exemplars of general deficiencies in the language that should be
addressed.
We have definitely identified catchall property access as such an
area. Are there in fact any others? It's a lot more interesting to
look at specific examples than to expound on the general principles.
See below where I did some study to find other missing capabilities.
In particular now that ES5 is finished, WebIDL has a richer
language to bind to then
it had with ES3. We need a WebIDL binding that maximizes use of ES5
capabilities rather
than inventing non-standard (from an ES perspective) language
extensions.
Updating WebIDL to use ES5 concepts would definitely be worthwhile. At
the time Web IDL was started (early 2007 I think) this wasn't a
practical option, but it is now. In particular, interfaces that don't
have any unusual behavior could be defined as having getters and
setters, and should not need to override internal properties at all.
This would better highlight the capabilities that are needed to
implement the Web platform, but which can't be expressed in the
property descriptor formalism.
I expect there are relatiively few such capabilities, and little
interest in depending on new ones, and therefore we do not really have
a general ongoing problem of language design.
From a quick scan of WebIDL, I see the following:
1) Catchall getters, putters, deleters, definer.
- Variants that can make the catchall check happen either before
or after normal property lookup.
- General string-based name access and index-only versions.
- Note: I think catchall deleters are used only by Web Storage and
not by other new or legacy interfaces.
2) Ability to support being called (via [[Call]]) without being a
Function.
3) Ability to support being invoked a constructor (via [[Construct]])
without being a Function.
4) Ability to support instanceof checking (via [[HasInstance]])
without being a constructor (so myElement instanceof HTMLElement works).
5) Ability to have [[Construct]] do something different than [[Call]]
instead of treating it as a [[Call]] with a freshly allocated Object
passed as "this".
Tentatively, I think all other semantics of Web IDL interfaces can be
implemented in pure ES5.
Regards,
Maciej
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