Making "super" work outside a literal?

Axel Rauschmayer axel at rauschma.de
Sat Jun 25 11:13:26 PDT 2011


I don’t know if this has been discussed, but I find the |super| work-around provided by YUI and CoffeeScript (in the generated code) not that bad:

        ThisClass.__super__.foo.call(this, arg1, arg2);

With ThisClass.__super__ pointing to SuperClass.prototype.

As a completely static solution, one could maybe expand a super-call
        super.bar(x, y);
to
        __CurrentObject__.[[Prototype]].bar.call(this, x, y);

On the other hand, I don’t know if a reference to __CurrentObject__ is ever available for some kind of static expansion.

On Jun 25, 2011, at 19:51 , Brendan Eich wrote:

> On Jun 25, 2011, at 10:31 AM, Allen Wirfs-Brock wrote:
> 
>> On Jun 25, 2011, at 6:10 PM, Brendan Eich wrote:
>> 
>>> On Jun 24, 2011, at 1:00 PM, Allen Wirfs-Brock wrote:
>>> 
>>>> If there was a mechanism for lexically addressing this, I would expect |super| to track |this| in parallel.  From a value perspective, |super| is just a synonym for |this|.
>>> 
>>> This is an important point, although what is a non-value perspective in JS?
>> 
>> Perhaps I should have said binding perspective.
> 
> "binding" is a much abused word, but in JS communities I hope we can keep it restricted to names in environments, lexical vs. dynamic vs. argument value to formal parameter binding -- at most!
> 
> But I get what you mean. 'super' by itself is a reference to the same object 'this' denotes, or undefined where 'this' is undefined. 'super' and 'this' are aliases. But 'super'-based property references start from a different prototype chain head.
> 
> 
>>> There are no explicit types. The answer must be an implicit type, the superclass view provided by the [[Prototype]] of the class prototype or ad-hoc containing object in which the method using 'super' was written.
>> 
>> What you are concerned about here may be too subtle for some readers.  Any caching of a |super| based property lookup needs to be keyed by the internal type of the object where the property lookup actually starts rather than the internal type of |this|.
> 
> Yes, I'm concerned about subtlety if 'super' in functions. The tension is between confining 'super' to be valid only in class methods, vs. making it work in any function. But the class-methods-only approach leads to invoke-only methods, to avoid the reparenting or borrowing problem that Object.defineMethod addresses.
> 
> I'm less concerned about 'super' for all functions, plus Object.defineMethod. This is "JavaScript-y" and causes me less concern than a class-based 'super' confinement attempt. But the whole package deal still causes concern, because of the "you forgot to use Object.defineMethod" problem. We're adding another runtime error case, a hazard requiring test coverage.
> 
> Not sure what can be done about this.
> 
> /be

-- 
Dr. Axel Rauschmayer

axel at rauschma.de
twitter.com/rauschma

home: rauschma.de
blog: 2ality.com





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