> If you made them non-callable by not implementing [[Call]], then typeof would > no longer return "function", which would be ... O_o.
s/Object (implements [[Call]]) | “function” /Object (implements [[Call]] or [[FunctionKind]] is classConstructor) | “function”/g Problem solved! I jest — but making a constructor identifiable as a function while still noting that it’s not “really” callable is certainly possible > On Mar 30, 2015, at 11:27 AM, Domenic Denicola <d...@domenic.me> wrote: > > If you made them non-callable by not implementing [[Call]], then typeof would > no longer return "function", which would be ... O_o. > >> -----Original Message----- >> From: es-discuss [mailto:es-discuss-boun...@mozilla.org] On Behalf Of >> Caitlin Potter >> Sent: Monday, March 30, 2015 11:25 >> To: Yehuda Katz >> Cc: es-discuss@mozilla.org >> Subject: Re: Determine if a value is Callable/Constructible >> >>>> But in either case, these (IsCallable / IsConstructor) are pretty basic >> qualities of objects that a Reflection* api ought to be able to read into, >> imho. >>> >>> What Allen is saying is that the implementation of "throw if >>> constructor" doesn't work by not implementing [[Call]], but rather by >> implementing [[Call]] to throw, so those reflective APIs would say the wrong >> thing, and that this is observable via proxies. >> >> It’s a fair point, constructors do have a [[Call]] internal method — there’s >> no >> reason this needs to be described this way though. Instead of the extra step >> for classConstructors in 9.2.1, class constructors could just as easily not >> have a >> [[Call]] method at all (by default). I guess what I’m getting at is, right >> now >> they aren’t “really” callable, it’s just that the way their >> non-callable-ness is >> expressed makes them appear callable (which probably should not be the >> case). Since they’re intrinsically not callable (currently), they shouldn’t >> be >> treated as callable. >> >> What Domenic is saying later on makes sense, the magic “call-code” method >> (should it ever exist) shouldn’t be a property of the class prototype, so a >> special syntactic form would work better (but also be kind of awful, too). >> >>> >>> Allen, can you say more about why you spec'ed it that way? >>> >>> >>>> >>>> >>>>> On Mar 29, 2015, at 11:51 PM, Caitlin Potter <caitpotte...@gmail.com> >> wrote: >>>>> >>>>> ... >>>>> >>>>> Reflect.isConstructor(fn) -> true if Class constructor, generator, >>>>> or legacy (and non-builtin) function syntactic form >>>>> Reflect.isCallable(fn) -> true for pretty much any function, except >>>>> for class constructors and a few builtins >>>> >>>> I’ve already seen another situation (node’s Buffer) where code could be >> simplified by using a ES6 class definition but where that is prevented >> because >> a class constructor throws when called. >>>> >>>> Just to clarify something. Class constructors actually are “callable”. >>>> You >> can observe this by the fact that Proxy allows you to install an “apply” >> handler >> (the reification of the [[[Call]] internal method) on a class constructor. >> The >> the fact that an object can be [[Call]]’ed is already reflected by the >> typeof >> operator. Class constructors throw when called because at the last minute >> we choose to make their [[Call]] do an explicit throw not because they aren’t >> callable. >>>> >>>> There is no intrinsic reason why we needed to mandate that class >> constructors should throw when called. We even provided a simple and >> straight forward way (new.target===undefined) that a ES constructor body >> can use to determine whether it was called or new’ed. >>>> >>>> I think we should just drop that throws when called feature of class >> constructors.. >>>> >>>> (The restriction was added to future proof for the possibility of >>>> inventing some other way to provide a class with distinct new/call >>>> behavior. I don’t think we need nor can afford to wait for the >>>> invention of a new mechanism which will inevitably be more complex >>>> than new.target, which we already have.) >>>> >>>> Allen >>>> >>>> >>> >>> _______________________________________________ >>> es-discuss mailing list >>> es-discuss@mozilla.org >>> https://mail.mozilla.org/listinfo/es-discuss >> >> _______________________________________________ >> es-discuss mailing list >> es-discuss@mozilla.org >> https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
