A strawman for subclassing

Mark S. Miller erights at google.com
Mon Jun 13 22:04:04 PDT 2011

On Mon, Jun 13, 2011 at 5:08 PM, David Bruant <david.bruant at labri.fr> wrote:

> Le 14/06/2011 01:24, Allen Wirfs-Brock a écrit :
> > On Jun 13, 2011, at 3:46 PM, David Bruant wrote:
> >> Hi,
> >>
> >> The subclassing native constructor issue came out several times on the
> >> list and on the web. However, I do not see a strawman for this. There
> >> are strawman:array_create and strawman:array_subtypes, but no mention of
> >> a generic solution.
> > The <| operator is the proposed solution.
> Oh ok, sorry, I hadn't read it yet. Thanks.
> > Using <|  instances with arbitrary [[Prototype]] values can be created
> for any type built-in object type that has a literal representation.  that
> is functions, arrays, regexps, and less interestingly, strings, numbers,
> booleans.  The resulting instance all have the appropriate special internal
> behaviors that are associated with the built-in types.
> It requires the right-hand side to have a specific initialization
> syntax. It currently works for all the "classes" you cited ("functions,
> arrays, regexps, and less interestingly, strings, numbers, booleans"),
> but does not for the current harmony:simple_maps_and_sets for which
> there is currently no specific syntax. Consequently, these could not be
> subclassed (or I am missing something).
> This would be the case for any other native "class" that wouldn't such
> syntax. I'm not sure it's is viable in the long term because it will
> require all subclassable (something people will want to subclass)
> "classes" to have a syntax.
> I like the idea of the <| operator, but a generic non-syntaxy solution
> would be good too in my opinion. It would also allow people to subclass
> host constructors (host objects with [[Construct]]).

We have noticed this issue, and an draft of the classes proposal attempted
to address it. What's left of that, at <

These semantics for constructor chaining preclude defining classes that
inherit from various distinguished built-in constructors, such as Date,
Array, RegExp, Function, Error, etc, whose [[Construct]] ignores the normal
object passed in as the this-binding and instead creates a fresh specialized
object. Similar problems occur for DOM constructors such as HTMLElement.
This strawman can be extended to handle such cases, but probably at the cost
of making classes something more than syntactic sugar for functions. We
leave that to other strawmen to explore.
I continue to believe that it would be valuable to subclass built-ins, that
classes are a fine place to hang this on, and that it should be developed as
a separate strawman proposing to extend classes in this way. The "something
more than syntactic sugar" may be a big impediment to adoption of such a
strawman, and should rule it out for ES-next. Is there some simple
limitation we could impose on classes in the ES-next timeframe to avoid
painting ourselves into a corner, such that we could still consider such a
strawman for ES-after-next?

> David
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