[swift-evolution] [swift-evolution-announce] [Review] SE-0089: Replace protocol<P1, P2> syntax with Any<P1, P2>

Fri May 27 07:20:00 CDT 2016

From the point of view of the type system `P & Q` is an anonymous supertype of `P` and `Q`.
This would be just the same if the syntax was `Any<P, Q>`. I don’t see a semantic difference here.

Whether that is a "container“ or not seems to be an implementation detail to me but should have nothing to do with the type system.
In what way should it be "lossy“?

Am I missing something?

-Thorsten

> Am 27.05.2016 um 12:30 schrieb L. Mihalkovic <laurent.mihalkovic at gmail.com>:
> 
> It seem to me we are debating the how of a what that has not been defined. The root question behind all these alternatives seems to be to decide if the existential-ness should be carried by a 'container' that is then refined internally, or derived from the presence of the free-floating refinements. This is the question that splits the possible syntaxes into these 2 groups:
> 
> Any<> Any[]
> Type<> Type<> 
> Existential<>
> 
> and the (these are all straw man representations that should not limit the thinking)
> 
> P & Q
> @P and @Q
> is P , Q
> P & Q typed
> 
> If the answer is to use a 'container' then the next question is to see its relationship to the other existing containers: is it the result of a transformation, is it a superset, or a super type; it is obviously lossy, but not entirely if the solution follows in some of Brent's past suggestion to make some existential types instantiate-able (which opens a very similar problem to what java faced for several years when trying to identify a universal collection literal syntax).
> That will narrow down the field of possible matches... until one syntax emerges as conveying the meaning that is reflected by the answers to each question.
> 
> On May 27, 2016, at 10:55 AM, Thorsten Seitz via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> We could just write
>> 
>> let x: P & Q
>> instead of
>> let x: Any<P, Q>
>> 
>> let x: Collection where .Element: P
>> instead of
>> let x: Any<Collection where .Element: P>
>> 
>> let x: P & Q where P.T == Q.T
>> instead of
>> let x: Any<P, Q where P.T == Q.T>
>> 
>> let x: P & Q & R
>> instead of
>> let x: Any<P, Q, R>
>> 
>> let x: Collection
>> instead of
>> let x: Any<Collection>
>> 
>> 
>> This would avoid the confusion of Any<T1, T2> being something completely different than a generic type (i.e. order of T1, T2 does not matter whereas for generic types it is essential).
>> 
>> 
>> -Thorsten
>> 
>> 
>> 
>>> Am 26.05.2016 um 20:11 schrieb Adrian Zubarev via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>> 
>>> Something like type<…> was considered at the very start of the whole discussion (in this thread <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160502/016523.html>), but it does not solve the meaning of an existential type and also might lead to even more confusion. 
>>> 
>>> From my perspective I wouldn’t use parentheses here because it looks more like an init without any label Type.init(…) or Type(…). I could live with Any[…] but this doesn’t look shiny and Swifty to me. Thats only my personal view. ;)
>>> 
>>> 
>>> 
>>> 
>>> -- 
>>> Adrian Zubarev
>>> Sent with Airmail
>>> 
>>> Am 26. Mai 2016 bei 19:48:04, Vladimir.S via swift-evolution (swift-evolution at swift.org <mailto:swift-evolution at swift.org>) schrieb:
>>> 
>>>> Don't think {} is better here, as they also have "established meaning in 
>>>> Swift today".
>>>> 
>>>> How about just Type(P1 & P2 | P3) - as IMO we can think of such 
>>>> construction as "creation" of new type and `P1 & P2 | P3` could be treated 
>>>> as parameters to initializer.
>>>> 
>>>> func f(t: Type(P1 & P2 | P3)) {..}
>>>> 
>>>> 
>>>> On 26.05.2016 20:32, L. Mihalkovic via swift-evolution wrote:
>>>> > How about something like Type{P1 & P2 | P3} the point being that "<...>" has an established meaning in Swift today which is not what is expressed in the "<P1,P2,P3>" contained inside Any<P1, P2,P3>.
>>>> >
>>>> >> On May 26, 2016, at 7:11 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> >>
>>>> >>
>>>> >>> on Thu May 26 2016, Adrian Zubarev <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> >>>
>>>> >>> There is great feedback going on here. I'd like to consider a few things here:
>>>> >>>
>>>> >>> * What if we name the whole thing `Existential<>` to sort out all
>>>> >>> confusion?
>>>> >>
>>>> >> Some of us believe that “existential” is way too theoretical a word to
>>>> >> force into the official lexicon of Swift. I think “Any<...>” is much
>>>> >> more conceptually accessible.
>>>> >>
>>>> >>>
>>>> >>> This would allow `typealias Any = Existential<>`. * Should
>>>> >>> `protocol A: Any<class>` replace `protocol A: class`? Or at least
>>>> >>> deprecate it. * Do we need `typealias AnyClass = Any<class>` or do we
>>>> >>> want to use any class requirement existential directly? If second, we
>>>> >>> will need to allow direct existential usage on protocols (right now we
>>>> >>> only can use typealiases as a worksround).
>>>> >>
>>>> >> --
>>>> >> Dave
>>>> >>
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