[swift-evolution] Enhanced existential types proposal discussion

[Thorsten Seitz]

> Am 23.05.2016 um 00:18 schrieb Austin Zheng via swift-evolution <swift-evolution at swift.org>:
> 
> I agree; the difference between protocols with and without associated types has been an endless source of confusion for a lot of people.
> 
> Speaking of which, for those who care I rewrote the draft proposal to attempt a much more rigorous treatment of the semantics of the generalized existential, including a discussion about existential type equivalence and subtyping. It would be nice to see people poke holes in my logic so I can patch them up. https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md

I think that *all* methods should be available - at least in principle - with associated types 
- replaced by their upper bounds (i.e. Any if no constraints have been given either by the protocol definition itself or th existential) if in covariant position and 
- replaced by their lower bounds if in contravariant position

As it is not possible to define lower bounds in Swift, the lower bounds are always the bottom type (called `Nothing` in Swift and not be confused with optionals). The bottom type has no members and therefore a method referencing that type cannot be called and is effectively not available.

-Thorsten 

> 
> Austin
> 
>>> On May 22, 2016, at 3:05 PM, Russ Bishop via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 
>>> On May 17, 2016, at 1:55 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> I agree with this. If we're certain we should reskin protocol<> as Any<>, we should frontload that change—in addition to affecting source code, it'd also influence the runtime behavior of type printing/parsing, which can't be statically migrated in the future. I think any discussion of extending existentials has to be considered out of scope for Swift 3, though, so the Any rename deserves its own proposal.
>>> 
>>> -Joe
>> 
>> 
>> Its really unfortunate that the generics work is probably going to be deferred. When you really dive in to protocol-oriented programming and designing frameworks to be native Swift (taking advantage of Swift features) the existential problem comes up a lot and leads to sub-optimal designs, abandonment of type safety, or gobs of boilerplate.  
>> 
>> 
>> Russ
>> 
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