[[Class]] Property of Host Object

Mark S. Miller erights at google.com
Sat Jul 17 12:54:21 PDT 2010


In short, I largely agree with you about the language in the ES5 spec text.
My interpretation of that text derives in large part from my memories of the
conversations that led up to it, and my sense of the intent we were trying
to capture. In the absence of that context, I would probably arrive at the
same reading of ES5 that you have.

At this point, I will wait until others who participated in those
conversations weigh in and state their sense of our agreed intent. Once we
understand what agreed intent there actually was, we can proceed from
there.


On Sat, Jul 17, 2010 at 12:17 PM, Garrett Smith <dhtmlkitchen at gmail.com>wrote:

> On 7/17/10, Mark S. Miller <erights at google.com> wrote:
> > [+es5-discuss as a possible errata issue arises below]
> >
> >
> > On Sat, Jul 17, 2010 at 12:37 AM, Garrett Smith
> > <dhtmlkitchen at gmail.com>wrote:
> >
> >> On 7/16/10, Mark S. Miller <erights at google.com> wrote:
> >> > On Fri, Jul 16, 2010 at 9:54 PM, Garrett Smith
> >> > <dhtmlkitchen at gmail.com>wrote:
> >> >
> >> >> I have a question reqarding [[Class]] property as defined In ES5:
> >> >>
> >> >> | The value of the [[Class]] internal property of a host object
> >> >> | may be any String value except one of "Arguments", "Array",
> >> >> | "Boolean", "Date", "Error", "Function", "JSON", "Math", "Number",
> >> >> | "Object", "RegExp", and "String"
> >> >>
> >> >> May it be something other than a string value? Or must it be a string
> >> >> value?
> >> >>
> >> >
> >> > It must be a string value.
> >> >
> >>
> >> The specification says "may".
> >>
> >
> > Ah. I see the ambiguity. "may" there is modifying "any". It should
> probably
> > have been stated:
> >
> >     ...must be a String value and may be any String value except...
> >
> > In reviewing the document, the possibility that text would allow
> non-String
> > [[Class]]es had not occurred to me. Now that you point it out, and can
> see
> > some reasons why we might want to leave the current text alone and allow
> > non-String [[Class]]es.
> >
>
> I don't.
>
> [[Class]] must be a string value. If the [[Class]] property is absent,
> what happens when the object is supplied to Object.prototype.toString?
> TypeError. Why not avoid that possibility and require host object to
> have [[Class]] be a string value?
>
> >
> >
> >>
> >> >
> >> >>
> >> >> Why must a host object's class be none of the built-in classes
> listed?
> >> >>
> >> >
> >> > So that the [[Class]] property serve as a reliable nominal type check
> >> > for
> >> > the contract that the other internal properties and methods satisfy.
> >> > This
> >> is
> >> > used primarily within the spec itself. Previously, it wasn't clear
> what
> >> was
> >> > meant when the spec said, for example, "if F is a function". Now we
> >> clearly
> >> > say "if the [[Class]] of F is 'Function' " if that's what we mean.
> >> >
> >>
> >> I think I see the problem.
> >>
> >> What you really want to say there is:
> >>
> >> |  The value of the [[Class]] internal property of any non-native host
> >> |  object must be any String value except one of...
> >>
> >
> > Saying "non-native" and "host" together is redundant. Although the
> language
> > of 4.3.6 and 4.3.8 is not as clear as it should be, I read these as
> stating
> > that all EcmaScript objects are either host or native. No object can be
> both
> > host and native. And no object can be neither host nor native.
> >
>
> It is not redundant. Repeating what was written in my last message:
>
> |  While the specification does not preclude the possibility that a host
> |  object may be implemented with native semantics, it nonetheless
> |  defines a host object:
>
>
> I understand that the values for [[Class]] are used internally by the
> specification. One example of that is Array.isArray(x), where the
> [[Class]] property is used internally. Another example is behavior for
> `JSON` reviver.
>
> The specification allows for two types of host objects:
>  * host objects as native objects
>  * host objects as not native objects (does not use native semantics)
>
> From the implementors point of view, what matters the ability derive
> an inference from [[Class]]; if [[Class]] is x then [action]. However,
> from a programmer's perspective, `Object.prototype.toString.call(x)`
> cannot discriminate between objects originating in the host
> environment (what is currently defined as "host object") and native
> objects. Mostly they shouldn't care, but should follow the
> specification approach to derive [[Class]] based inference, however
> they can't follow that because that is not compatible with existing
> implementations (notably IE versions and Opera (which copied IE)), and
> making an "isStrict" supported global flag is not going to provide a
> closely related inference about various types of host objects across a
> wide range of implementations in the wild.
>
> Internet Explorer 9 host objects seem to be absent of problems seen in
> previous versions of IE but there are still host objects that are
> callable, have [[Class]] "Object", and are not implemented with native
> semantics. Two examples of such object are alert and addEventListener.
>
> >
> >
> >>
> >> Because that allows `alert` to be any native ECMAScript object
> >> (Function, Object, etc), while still letting it be defined as a host
> >> object and not violating that spec.
> >
> >
> >
> > "that spec"? What specification demands that alert be a host object? I
> have
>
> I cited 4.3.8; the definition of host object.
>
> > not heard of any. This might be a consequence of the upcoming
> WebIDL-to-ES5
> > language bindings, but I have seen no draft and so have no idea. My own
> > preference would be for these language bindings to result in alert being
> a
> > native Function, but that's an argument for a different standards
> committee
> > ;).
> >
> > If an implementation's alert is a Function, then it is a native object
> and
> > its [[Class]] must be "Function". It can still be an object provided by a
> > host environment whose [[Call]] behavior is written in C++. This simply
> > makes it a host provided native built-in object (4.3.7), not a host
> object.
> >
>
> No, alert is not a defined by ECMAScript specification. It is
> therefore host object, by definition of "host object" in ES5, s 4.3.8.
>
> >
> >
> >
> >> Iff, however, following my
> >> proposed amendment, `alert` had [[Class]] "Object", and it was not a
> >> native ES object (as in IE versions), then it would be a specification
> >> violation.
> >>
> >> > But it is also used from JS. Host objects are exempt from most of the
> >> > specific behaviors specified for specific kinds of native objects.
> Were
> >> > a
> >> > host object to be able to allege to be a kind of native object without
> >> > behaving as that kind of native object behaves, that would be bad.
> >> >
> >>
> >> This is not in the spec:
> >> "without behaving as that kind of native object behaves"
> >>
> >
> > See 4.3.6.
> >
>
> That is where native object is defined.
>
> >
> >>
> >> While the specification does not preclude the possibility that a host
> >> object may be implemented with native semantics, it nonetheless
> >> defines a host object:
> >>
> >> | 4.3.8
> >> |  host object
> >> |  object supplied by the host environment to complete the
> >> |  execution environment of ECMAScript.
> >> |
> >> |  NOTE Any object that is not native is a host object.
> >>
> >> And that means that `alert`, `window`, `document`, XMLHttpRequest, are
> >> all host objects. Whether or not those objects are implemented as
> >> native ECMAScript objects is another matter altogether.
> >>
> >
> > This is the crux. The language there is indeed poorly phrased. But native
> > objects are not host objects.
> >
>
> Where is that stated anywhere in the specification?
>
> > Indeed, it is so poorly phrased that perhaps we should add an errata to
> > clean this up. Sigh. cc'ing es5-discuss.
> >
> >
> >>
> >> It seems the the spec is wrong and that you have misinterpreted it. I
> >> believe that instead it should be written:
> >>
> >>
> > Text missing?
>
> Yes, as I wrote before:
>
> |  The value of the [[Class]] internal property of any non-native host
> |  object must be any String value except one of..
>
> The other possibility is that you have misinterpreted the definition
> of host object to mean something along the lines of "any object that
> is not a native object".
>
> And going by that, referring to the example, `alert` could be a native
> object and would then not be a host object.
>
> [...]
>
> >> > (alert instanceof Function) is not a reliable test in either
> direction.
> >>
> >> No of course not.
> >>
> >> A
> >> > host object as well as a native non-function is perfectly free to
> >> > inherit
> >> > from Function.prototype and thus pass this test. And an actual
> function
> >> may
> >> > be an instance of Function constructor from another frame and so fail
> >> > the
> >> > test. But yes, iff alert is indeed a native function, it's [[Class]]
> >> should
> >> > be "Function".
> >> >
> >>
> >> Therein lies a contradiction: A host object here may be a function.
> >>
> >
> > No it may not. A host object may be callable and it may inherit from
> > Function.prototype, but it may not be a function.
> >
>
> Once again, ES5 does not preclude the possibility that a host object
> may be implemented with native semantics.
>
> >
> >> Yet because it is a host object, that same object's [[Class]] must not
> >> be "Function", and yet again, since it is a function, and any function
> >> must have [[Class]] "Function", then this object's [[Class]] must be
> >> "Function".
> >>
> >> >
> >> >
> >> >>
> >> >> However according to ES5 specs, any host object must not be withing
> >> >
> >> > the set of values that are not allowable and so the assertion could be
> >> >> made that if any object has a [[Class]] that is one of those values,
> >> >> then the object is not a host object.
> >> >
> >> >
> >> > Yes, that is intentional.
> >>
> >> Then it will fail today, as
> >>
> >> javascript: alert(({}).toString.call(alert))
> >>
> >> - will result "[object Object]" or "[object Function]"
> >>
> >
> > 1) No current implementations claim ES5 conformance.
> > 2) On ES5 conformant implementations, alert may well be a function in
> which
> > case the second result will remain correct.
>
> The only way that could be anything other than a contradiction is if
> you have redefined "host object" to be as I suggested above (I'll
> repeat that one more time).
>
> `alert` is a host object by definition of host object in s. 4.3.8. It
> seems as if you have misinterpreted that definition as: "any object
> that is not a native object". Is that how you read it?
>
> That is a significantly different definition than either definition in
> Eds 3 or 5.
>
> Ed 3:
>
> |   A host object is any object supplied by the host
> |  environment to complete the execution environment of
> |  ECMAScript. Any object that is not native is a host object.
>
> Ed 5:
> |  object supplied by the host environment to complete
> |  the execution environment of ECMAScript.
> |
> |  NOTE Any object that is not native is a host object.
>
> Perhaps this is a case of denying the antecedent?
> 1. if (not native) then host object.
> 2. alert is native,
> therefore, alert is not host object. (false conclusion)
>
> <http://en.wikipedia.org/wiki/Denying_the_antecedent>
>
> If the definition of host object is correct then the clause that
> mentions "[[Class]]" property must change as I initially suggested.
> However, if what appears to be your definition of host object was also
> what was intended by the editor, then that definition should be
> rewritten instead.
>
> If a host object is defined as being any object supplied by the host
> environment that is not a native object, then the spec should state
> so. If, however, a host object may be a native object -- and that is
> how it is currently defined, then for any host object that is not a
> native object, that object's [[Class]] property must not be in the set
> of class values for native types.
>
> Garrett
>



-- 
    Cheers,
    --MarkM
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