On Sep 16, 2014, at 4:55 PM, Brendan Eich wrote:
> ...
> It would help, I think, if you replied to Kevin's mail dated 12:05pm
> yesterday, which ended:
>
> """
> Well, what I'm trying to point out is that it's always an error to set "this"
> if "new^" is undefined. So it's kind of strange to force the user to
> explicitly write that conditional:
>
> constructor(x, y) {
> if (new^)
> this = new super(x);
> this.y = y;
> }
>
> With the header approach, it's taken care of:
>
> constructor(x, y) : super(x) {
> this.y = y;
> }
>
> (using colon just to avoid bikeshedding)
> """
actually I did respond:
On Sep 15, 2014, at 11:24 AM, Allen Wirfs-Brock wrote:
>
> On Sep 14, 2014, at 8:48 PM, Kevin Smith wrote:
>
>>
>>
>> It seems to me that with the proposed design we're going to have to branch
>> on "new^" continually: if we don't, then the function will always fail if
>> [Call]'ed and it contains a this-setter.
>>
>> With the class create expression approach, that branch is taken care of
>> automatically.
>
> If you want a [[Call]] to the constructor to do an implicit `new`, then you
> need to branch on `new^`:
>
>
> constructor (a,b,c) {
> if (!new^) return new C(a,b,c);
> // continue with instance initialization code
> }
> }
>
> But [[Call]] same as [[Construct]] is not normal default JS behavior, so it
> shouldn't be unexpected that you need to explicitly say something if that is
> what you want. The whole point of new^ is that a constructor function that
> wants differing call vs construct behavior needs to explicitly implement the
> alternatives. Just like today, if you only want your constructors to support
> one or the other of [[Call]]/[[Construct]] you code for what you care about
> and errors will occurs if it is invoke the other way.
>
> Allen
>
or using your above example.
Most people today don't explicitly deal with constructors called as functions.
If you con't care you probably just write
constructor(x, y) {
this = new super(x);
this.y = y;
}
and a runtime error will occur if it is called "as a function".
If you want to also support "called as a function", say to do a `new` like
above you would probably code:
class C extends B {
constructor(x, y) {
if (!new^) return new C(x,y);
this = new super(x);
this.y = y;
}
}
So you only need to branch on `new^` if you want to explicitly support both
constructor and function behavior.
Allen_______________________________________________
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