[Harmony proxies] [[Construct]] semantics breaks the expectation of seeing an object as return value
david.bruant at labri.fr
Fri Sep 23 08:25:39 PDT 2011
Le 23/09/2011 17:17, Tom Van Cutsem a écrit :
> Hi David,
> I believe the relevant use cases here were value types/value proxies,
> which - if added in the future - would want their constructors to
> return non-object values.
Interesting. So maybe it should be kept in mind to relax the current
constraint of [[Construct]] having to return objects (unless types/value
proxies are planned on being objects)
> I searched the archives but can't seem to find the relevant threads,
> still, I'm sure it was discussed here previously.
> 2011/9/23 David Bruant <david.bruant at labri.fr
> <mailto:david.bruant at labri.fr>>
> In ES5.1 - 13.2.2 is defined functions instances [[Construct]]. As
> one can notice, it always returns something of type Object (as per
> ES5.1 - 8.6). Consequently, a call to "new A()" always reliably
> either returns an object or throws an exception.
> Proxies semantics of [[Construct]] states that (step 5) the
> [[Call]] internal method of constructhandler should be called. It
> opens the door to returning things which are not objects (number,
> boolean, undefined, ...).
> I can't think of precedents of this for host objects and I don't
> really see a use case in allowing "new A()" to return something
> else than an object
> When writing functions intended to be used as constructors, people
> may be used of |this| being used as the to-be-returned object and
> expect this behavior even if defining a constructor.
> So i would be in favor of adding a type check between step 5 and 6.
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