Thanks Mark,
so how is anyone supposed to be sure about or check a Brand in ES6 then?
On Fri, Nov 1, 2013 at 1:00 PM, Mark S. Miller <erig...@google.com> wrote:
>
>
>
> On Fri, Nov 1, 2013 at 12:16 PM, Andrea Giammarchi <
> andrea.giammar...@gmail.com> wrote:
>
>> Allen I think we all agree duck typing is more flexible, powerful, easy,
>> etc etc ... but since you mentioned, how would do distinguish between a
>> `Map` and a `WeakMap` without passing through the branding check ?
>>
>> That's a very good example indeed, `'has' in obj` or `'set' in obj` does
>> not grant you any behavior with the signature itself (e.g. `WekMap`
>> does/did not accept primitives as key)
>>
>> AFAIK WebIDL is entirely based on branding, regardless it's a good or a
>> bad thing, that's the way W3C describes "things" since quite ever (and
>> implement them in Java too ... anyway)
>>
>> Why there's no easy way to check for branding if not abusing
>> `Object.prototype.toString` ?
>>
>> If so many libraries are using it because of the cross realm, maybe there
>> is a need for some similar mechanism and implemented in core would make
>> everybody life easier/faster ?
>>
>> The option you give us is: keep abusing `Object.prototype.toString` ...
>> that's OK but it seems to be a dirty solution.
>>
>
> In ES5, Object.prototype.toString was a reliable brand check. It no longer
> is in ES6.
>
>
>
>>
>> Cheers
>>
>>
>>
>>
>> On Fri, Nov 1, 2013 at 11:52 AM, Allen Wirfs-Brock <al...@wirfs-brock.com
>> > wrote:
>>
>>>
>>> On Nov 1, 2013, at 10:40 AM, Anne van Kesteren wrote:
>>>
>>> > On Fri, Nov 1, 2013 at 12:50 AM, Allen Wirfs-Brock
>>> > <al...@wirfs-brock.com> wrote:
>>> >> [isSpecialString]() {return isSpecialString in this};
>>> >
>>> > So this is basically what we have for DOM objects right now. And what
>>> > we're asking for is having this elevated to language-supported
>>> > construct. Be it in the form of a method or something else. It's not
>>> > really clear why sometimes you would use if "brand in obj" and other
>>> > times "Array.isArray(obj)". Seems rather messy.
>>>
>>> Array.isArray is a legacy artifact so I'm not going to try to generalize
>>> from it.
>>>
>>> But, in general, the distinction is between asking "does this object
>>> appear to expose the behavior I need" and "is this object an instance of a
>>> specific known implementation of the behavior I need". Let's call the
>>> first question "duck typing" and the second "branding". Duck typing is a
>>> weaker test but allows for a lot of flexibility and extensibility. Branding
>>> gives you reliable knowledge about certain aspects of a branded object.
>>> But it is less flexible and and systems that extensively using branding
>>> are often harder to extend. IMO branding is over used.
>>>
>>> Array.isArray is a specific brand test based upon how the implementation
>>> of how property definition interacts with the value of the 'length'
>>> property.
>>>
>>> ES currently proves lots of ways to do duck type testing. Built-in
>>> objects (including host objects) can implement branding and brand testing
>>> using various techniques but ES currently (including ES6) is very limited
>>> in its language level sort for reliable branding. The best we have to
>>> offer in ES6 is WeakMaps. In the future, we probably will add mechanism
>>> that make it easier to define branded objects defined via ES code.
>>>
>>> But, before you brand something really think carefully about why you
>>> need such a brand. It may not really be necessary.
>>>
>>> Allen
>>
>>
>>
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>>
>>
>
>
> --
> Cheers,
> --MarkM
>
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