On Jul 19, 2011, at 6:12 PM, Gavin Barraclough wrote:

> On Jul 19, 2011, at 12:17 PM, Allen Wirfs-Brock wrote:
>> One issue is that <| conceptually modifies the [[Prototype]] of its RHS.  As 
>> an operator (whether special character or keyword based) we can restrict the 
>> RHS to be a literal and define the behavior such that no actual mutation of 
>> [[Prototype]] is necessary.
>> 
>> However, at the value level we have no way to distinguish an object that was 
>> created via a literal and which has no references other than the current 
>> argument value.  So, when we specify Object.make we have to accept an 
>> arbitrary object as the second argument  and specify either [[Prototype]] 
>> mutation or some sort of shallow copy, either of which can be problematic. 
>> 
> 
> On Jul 19, 2011, at 1:27 PM, Brendan Eich wrote:
>> We are *not* gonna standardize mutable [[Prototype]].
> 
> 
> The concept of an operator that logically modifies the RHS operand seems 
> unfortunate,

L <| R does not modify R's [[Prototype]] so much as "preset" it, since R is 
grammatically restricted to be a literal form (object, array, regexp, etc.).


> as does having an operator with an implicit requirement that the RHS be a 
> literal.  The fact that the only reason that setting the prototype is 
> acceptable to us is because the RHS is a literal seems to raise the question 
> whether this really should be the action of an operator?  Do we not just need 
> an extension to the object/array literal syntax to allow the prototype to be 
> specified within the literal?
> 
> Some engines already allow syntax to declare a prototype for an object 
> literal, e.g.:
> 
>       var o = { __proto__: protoObj, a:1, b:2 };
> 
> Does this produce the same results as:
> 
>       var o = protoObj <| { a:1, b:2 };
> 
> ?

Yes.


> I'm sure that we don't want to standardize __proto__, perhaps an option 
> closer to the spec would be:
> 
>       var o = { [[Prototype]]: protoObj, a:1, b:2 };
> 
> Though I'm sure this isn't particularly lovable syntax, particularly if we 
> want to support this for array literals.
> Maybe something based on the idea of the <| operator might work?, say:
> 
>       var o = { |> protoObj, a:1, b:2 };
> 
> Apologies in advance if I'm missing an understanding of an important detail 
> of the <| operator as proposed.

Part of the appeal of <| is that it lets us write class-like expressions:

class D extends B { ... }

vs.

let D = B <| { ... }

but this led down the prototype-first (B and D name prototype objects, not 
constructors) naming path, which I think is too different from current 
constructors-with-prototypes JS pattern. It also led to the class body is 
object literal idea, which I've been arguing against just today.

There are lots of plausible syntactic extensions for presetting [[Prototype]], 
but they all need enforcement that the right-hand side of <| (or the equivalent 
position for alternative syntaxes) is new and not yet accessible to code.

/be
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