On Jun 13, 2014, at 7:51 AM, Boris Zbarsky wrote:
> On 6/13/14, 6:33 AM, Tom Van Cutsem wrote:
>> As Allen mentioned, this came up a number of times in TC39 meetings and
>> I believe the fear for exotic objects that require control over
>> [[Construct]] was the biggest show-stopper. If more knowledgeable people
>> like Boris can vouch for the fact that this isn't the case, that would
>> remove the biggest roadblock.
>
> The basic question is how that affects what WebIDL needs here is how it
> should play with subclassing.
>
> If we don't care about subclassing WebIDL objects, there is no issue: WebIDL
> can just override [[Call]] as it does right now and we move on with life.
> Note that some implementations at the moment override [[Construct]], not
> [[Call]] and just throw from [[Call]], so as to preclude web pages from
> relying on the current spec's [[Call]] behavior, since that would presumably
> interact badly with trying to introduce subclassing.
This "we" definitely cares about subclassing WebIDL objects and the ES6 object
creation protocol was design with that in mind.
>
> If we do care about subclassing, then presumably the "construct the right
> sort of object" and "initialize it" actions need to be somewhat decoupled for
> these objects. It seems to me that both Allen's current spec and Jason's
> proposal accomplish that, right?
Right, Jason's proposal introduces an additional degree of variability that my
proposal (assuming that [[Construct goes away) doesn't directly provide. It
kind of comes down to whether that extra flexibility is actually needed.
>
> In Jason's proposal it would look like this, if I understand it right:
>
> subclass[Symbol.new] = function(...args) {
> // Modify args as desired.
> var obj = superclass[Symbol.new](...args);
> // Modify obj as desired.
> return obj;
> }
I don't think so. Unless, I misunderstand I don't think Jason is proposing
eliminating @@create. So, in most cases including, I believe, the WebIDL use
cases a constructor would simply use the default @@new that would be inherited
from Function.prototype. It's definition would be something like:
Function.prototype[Symbol.new] = function (...args) {
let newObj = this[Symbol.create]();
if (! IsObject(newObj) {
//default handling for ill-behaved or missing @@create
}
let ctorResult = this(...args);
if (IsObject(ctorResult) return ctorResult; //ES<6 compatibility
else return newObj;
}
Note that this preserves the separation between object allocation and object
initialization we need to make subclassing work correctly.
> which is pretty similar to how this would be done without classes:
>
> function subclass(...args) {
> // Assume that we're being invoked via new.
> // Modify args as desired.
> var obj = new superclass(...args);
> // Modify obj as desired.
> return obj;
> }
Except that the above doesn't give you true subclassing. The object you return
is an instance of the superclass and not of the subclass. The key thing that
@@create is doing is separating determining the physical characteristics of an
object (making it exotic, branding it, using a custom C struct as its
representation, etc.) from logically initializing it at an appropriate place
within some class hierarchy
>
> except there is nothing that needs to be done if you don't want to munge args
> or obj, since not defining Symbol.new on the subclass will simply find it on
> the superclass.
>
> In Allen's spec, it would look like this:
>
> "subclass constructor": function(...args) {
> // "this" is already set to the right object that got created via
> // looking up Symbol.create and calling it.
> // Modify args as desired.
> superclass.apply(this, ...args);
> // Modify "this" as desired.
> }
>
> Is my understanding of the Allen and Jason's proposals correct? I'd like to
> avoid making any claims of how these interact with WebIDL until I'm sure I'm
> not missing something important.
I think you are missing the key element @@create plays in both proposals. The
only real difference, I think, is whether you have to live with the default
@@create/constructor protocol or whether you are allowed to tweak with that
default.
Allen
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