The class operator: a bridge between object and function exemplers

Dmitry Soshnikov dmitry.soshnikov at gmail.com
Mon Nov 14 23:50:15 PST 2011


On 15.11.2011 2:01, Brendan Eich wrote:
> On Nov 14, 2011, at 12:16 PM, Allen Wirfs-Brock wrote:
>
>> let Point = {
>>      x:0,
>>      y,0,
>>      constructor(x,y} {
>>          this.x=x;
>>          this.y=y;
>>      }
>> }.constructor;   //<----------- note added property reference
>>
>> let p = new Point(1,2);  //new operator applied to a constructible 
>> function
>
> So can you spec operator new a bit? It tries [[Construct]] and if 
> missing, tries for a .constructor property that it calls on a fresh 
> instance of Object whose [[Prototype]] is the exemplar?
>
> Dave's work on 
> http://wiki.ecmascript.org/doku.php?id=strawman:minimal_classes 
> brought to light some added wiring that comes for free in the 
> prototypal pattern when you write a constructor function as a function 
> declaration: the intrinsic name of the constructor function is the 
> class name.
>
> For exemplars, if constructor is defined using method definition 
> shorthand,
>
> http://wiki.ecmascript.org/doku.php?id=harmony:object_literals#object_literal_property_shorthands
>
> says the intrinsic name will be 'constructor'. This will be a drag in 
> debugging scenarios. You'll want to see Point, Square, Circle, 
> Ellipse, Rectangle, etc. but all you'll get is 'constructor' every time.
>
> The attributes in the 
> http://wiki.ecmascript.org/doku.php?id=harmony:object_literals#object_literal_property_shorthands 
> section are frosty. Is that the right default for methods? Is it the 
> right one for constructor in particular? I'm just asking, this is a 
> separate issue but I spotted it and wanted to get it out before 
> forgetting.
>
>
>> /UnaryExpression/ :
>> *class* /UnaryExpression/
>>            ...
>>
>> The semantics are:
>>  1. if /UnaryExpression/ is undefined or null, return the value of 
>> /UnaryExpression/.
>>  2. Let /obj/ be ToObject(/UnaryExpression/)
>> 3. Return the result of calling the [[Get]] internal method of /obj/ 
>> with argument 'constructor'
>

Sorry, perhaps I'm missing something (correct me if I'm wrong), but 
seems this algorithm just not working.

According to examples, yes, the Point is set to the function previously 
referred by temporary created object's `constructor' property. But 
what's then?

Instances created by such a constructor, will have prototype set to 
`Object.prototype' (since it's the value of `Point.prototype' now). 
Therefore, the do not see neither specified `x' and `y' default (shared, 
static) properties, nor any other method placed on that temp-object:

You may of course try to set Point.prototype = Point after that in a 
hope that it starts to see the properties, but it's obviously useless 
since Point already is set to temp-object's `constructor' property and 
actually is missed by GC.

Dmitry.

> Interesting, so 'constructor' will inherit from Object.prototype if 
> missing from the exemplar. This means
>
> let Point = class {
>      x:0,
>      y,0
> };
>
> let p = new Point(1, 2);
>
> will result in p being constructed via the equivalent of new Number(1).
>
>
>> The class operator  can prefix any UnaryExpression, not just object 
>> literals.  That means that the class operator can be used to classify 
>> objects:
>>
>> if (class p === Point) ...
>
> This is good.
>
> It has the same multiple-global issue that instanceof has.
>
>
>> Note that many object-oriented abstraction designers consider this 
>> for of class testing to be an anti-pattern.
>
> Yup but it has its uses occasionally.
>
>
>> Relationship between the class and instanceof operators
>>
>> There isn't one.  They are different operators.  class is simply a 
>> short hand for accessing an object's constructor property  instanceof 
>> tests a specific inheritance relationship.
>
> Oh, but weren't you going to make instanceof work with an object 
> exemplar on the right?
>
> /be
>
>
>
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