As far as I understand it (and the core team as well, judging from their
posts), an existential is just a type defined by an interface of some sort
("there exists a type that is capable of X, Y, and Z"), and you can put any
value of a type that meets that interface in the existential. The associated
types/self types requirements are non-essential add-ons.
I am about to fall asleep, though, so I'd be happy to reread the article
tomorrow and if I'm wrong, make the change.
Austin
> On May 27, 2016, at 1:42 AM, Thorsten Seitz <[email protected]> wrote:
>
>
>> Am 27.05.2016 um 10:30 schrieb Austin Zheng <[email protected]
>> <mailto:[email protected]>>:
>>
>> Thank you for all your great feedback!
>>
>> Let me try rephrasing what I said, because it wasn't very clear. Apologies.
>>
>> NSView is not an existential, like you said.
>>
>> Any<...> syntax is, as far as we've seen, always used for existential types.
>>
>> "Any<NSView>" *looks* like an existential because it has the "Any<...>"
>> syntax, but if it's a synonym for just "NSView" then it actually isn't an
>> existential.
>>
>> So "Any<NSView>" isn't an existential, but it looks like one. This is
>> something I think would be confusing to a lot of people, and also redundant:
>> there is no reason as far as I know to ever write Any<SomeClass> when you
>> could just write SomeClass, so by banning the confusing form we don't lose
>> any expressive power.
>
> Ah, ok! So `Any<NSView>` is just the same as `NSView`. That’s my
> understanding as well :-)
>
> But this does not apply only to classes: `Any<CustomStringConvertible>` is
> just the same as `CustomStringConvertible`, too, because
> `CustomStringConvertible` does not have associated types or self type
> requirements (it is *not* an existential in Wikipedia’s sense).
>
> Banning `Any<Foo>` where `Foo` does not have associated types or self type
> requirements would be the right thing to do, I think, regardless whether
> `Foo` is a class or a protocol. This would make it clear that `Any<Foo>` is a
> *real* existential.
>
> -Thorsten
>
>
>>
>> Hope that helps,
>> Austin
>>
>>> On May 27, 2016, at 1:24 AM, Thorsten Seitz <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>
>>>>
>>>> Am 26.05.2016 um 22:44 schrieb Austin Zheng via swift-evolution
>>>> <[email protected] <mailto:[email protected]>>:
>>>>
>>>> (inline)
>>>>
>>>> On Thu, May 26, 2016 at 12:22 PM, David Hart <[email protected]
>>>> <mailto:[email protected]>> wrote:
>>>> Hi Austin,
>>>>
>>>> I never had te occasion to say thanks for the work you have put in this
>>>> proposal, so thanks! I’m really looking forward to be able to have some
>>>> form of it accepted and implemented in Swift.
>>>>
>>>> Thank you! I just hope a proposal like this one ends up being good enough
>>>> that it means less work for the core team, not more...
>>>>
>>>>
>>>> Here are a few comments:
>>>>
>>>> 1) Why would Any<> and Any<NSView> be illegal? What error messages would
>>>> they generate? Why not make them simply synonymous to Any, and NSView, the
>>>> same way protocol<> currently behaves?
>>>>
>>>> "Any<>" being illegal is a syntactic battle that is being fought over in a
>>>> different thread; I'm not personally invested one way or another. (We
>>>> might not even adopt "Any" syntax specifically; Joe Groff has ideas for a
>>>> different syntax that doesn't use the brackets.)
>>>>
>>>> "Any<NSView>" is an existential, and "NSView" isn't. Existentials'
>>>> metatypes are different from the metatypes of concrete types, and the ways
>>>> they can be used with generics is different as well. My opinion is that
>>>> Any<...> signifies an existential, and allowing the use of
>>>> "Any<SomeClass>" as a concrete type would just confuse people even more.
>>>
>>>
>>> This is something where I still have a problem understanding what an
>>> existential is in the Swift sense. In the "normal“ sense (as defined in
>>> Wikipedia or Haskell) NSView cannot be an existential because it has no
>>> unbound associated types. It cannot be just made an existential either. How
>>> would that work?
>>>
>>> So, what is the meaning of `Any<NSView>` being an existential? How is that
>>> type different from the type `NSView`?
>>>
>>> -Thorsten
>>
>
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