On Oct 1, 2012, at 12:39 PM, Brendan Eich wrote: > Allen Wirfs-Brock wrote: >> Line 5.c.v is dealing with the ES5 requirement that "host objects" not reuse >> the listed [[Class]] values for anything other than the specified built-ins. >> This version of toStrimg extends the spirit to that restriction to all user >> or implementation defined tag values. It does this by prepending a "~" in >> front of those values if they appear as a tag value. Note that ES5.1 >> includes "Object" in this list so I also included it. But, I actually think >> it probably should be in the censored list. > > This is weird. > > ES5 wanted to help SES and similar languages use some kind of > > original_Object_prototype_toString_call(anyObject).slice(8,-1) > > and not be subject to spoofing. > > Adding @@toStringTag to enable DOM self-hosting runs directly counter to this > goal. > > Prefixing "~" to core-language built-in classes (save "Object"?) does not > resolve the conflict if there are spoofing hazards in other built-ins such as > DOM objects as typically implemented in browsers. > > Mark, can you weigh in? I had not heard of this "~"-prefixing idea.
Let me try explaining this again, if we provide an extension point for parameterizing Object.prototype.toString and it allows arbitrary objects to produce, for example, "[Object Array] " or "[Object Function]" then it invalidates all legacy uses of Object.prototype.toString as nominal type check for those specific built-ins. The "~" prefix is one way to avoid this problem. DOM implementation usage is a separate issue, although arguably a motivating one making Object.prototype.toString extensible. W3C specifications have consistently tried to use [[Class]] as an extension point for parameterizing Object.prototype.toString and implementations have supported them by providing implementation specific mechanisms that allow DOM implementations provide a [[Class]] value to Object.prototype.toString. The proposed ES6 spec. for Object.prototype.toString recognizes the utility of such an extension point. I would certainly be used by a ES-host DOM implementation but it would also be useful for anyone else defining class-like abstractions where they want Object.prototype.toString. The little quicks like the "~" prefix and the exception eating are simply what is necessary to formalize this extensibility model while still preserving the behavior of existing code that depends upon object.prototype.toString as a reliable mechanism to test for instances of the ES<=5.1 built-ins. Allen _______________________________________________ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
