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

Thorsten Seitz tseitz42 at icloud.com
Thu Jun 9 12:50:25 CDT 2016

> Am 09.06.2016 um 18:49 schrieb Dave Abrahams via swift-evolution <swift-evolution at swift.org>:
> on Wed Jun 08 2016, Jordan Rose <swift-evolution at swift.org> wrote:
>>> On Jun 8, 2016, at 13:16, Dave Abrahams via swift-evolution
>>> <swift-evolution at swift.org> wrote:
>>> on Wed Jun 08 2016, Thorsten Seitz
>>> <swift-evolution at swift.org
>>> <mailto:swift-evolution at swift.org>>
>>> wrote:
>>>> Ah, thanks, I forgot!  I still consider this a bug, though (will have
>>>> to read up again what the reasons are for that behavior).
>>> Yes, but in the case of the issue we're discussing, the choices are:
>>> 1. Omit from the existential's API any protocol requirements that depend
>>>  on Self or associated types, in which case it *can't* conform to
>>>  itself because it doesn't fulfill the requirements.
>>> 2. Erase type relationships and trap at runtime when they don't line up.
>>> Matthew has been arguing against #2, but you can't “fix the bug” without
>>> it.
>> #1 has been my preference for a while as well, at least as a starting
>> point. 
> I should point out that with the resyntaxing of existentials to
> Any<Protocols...>, the idea that Collection's existential doesn't
> conform to Collection becomes far less absurd than it was, so maybe this
> is not so bad.

I think the problem is more that Any<Collection> does not conform to a specific value for a type parameter T: Collection

What I mean by this is that `Collection` denotes a type family, a generic parameter `T: Collection` denotes a specific (though unknown) member of that type family and `Any<Collection>` denotes the type family again, so there is really no point in writing Any<Collection> IMO. 
The type family cannot conform to T because T is just one fixed member of it.
It conforms to itself, though, as I can write
let c1: Any<Collection> = …
let c2: Any<Collection> = c1

That’s why I think that we could just drop Any<Collection> and simply write Collection.


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