Sent from my iPad
> On May 26, 2016, at 12:52 PM, Thorsten Seitz via swift-evolution
> <swift-evolution@swift.org> wrote:
>
>
>> Am 26.05.2016 um 07:53 schrieb Austin Zheng via swift-evolution
>> <swift-evolution@swift.org>:
>>
>> The inimitable Joe Groff provided me with an outline as to how the design
>> could be improved. I've taken the liberty of rewriting parts of the proposal
>> to account for his advice.
>>
>> It turns out the runtime type system is considerably more powerful than I
>> expected. The previous concept in which protocols with associated types'
>> APIs were vended out selectively and using existentials has been discarded.
>>
>> Instead, all the associated types that belong to an existential are
>> accessible as 'anonymous' types within the scope of the existential. These
>> anonymous types are not existentials - they are an anonymous representation
>> of whatever concrete type is satisfying the existential's value's underlying
>> type's associated type.
>
> let a : Any<Collection>
> // A variable whose type is the Element associated type of the underlying
> // concrete type of 'a'.
> let theElement : a.Element = ...
>
> In Scala this would be a „path dependent type“ which actually depends on the
> variable `a`.
> What would happen in the following case:
>
> func foo<T: Any<Collection>>(a: T, b: T) {
> // is the type of a.Element equal to the type of b.Element here? (In
> Scala it would not)
> }
As Austin noted, this is unrelated to existentials. The constraint is
identical to 'T: Collection'. Allowing 'Any' in constraints is just a
convenient way to factor them out (by using a typealias).
Because there is only one type parameter there is only one Element. But
because a and b are not existential values you can't access the associated type
of the instance directly (that is something new in Austin's proposal). You
have to use either T or dynamicType.
func foo<T: Collection>(a: T, b: T) {
print(a.dynamicType.Iterator.Element.self ==
b.dynamicType.Iterator.Element.self)
}
// prints true
foo(a: [1, 2, 3], b: [4, 5, 6])
Austin, this brings to mind the question of how you will handle conflicts if
the type defines an instance member with the same name as the asociatedtype.
This should be uncommon due to capitalization conventions but is still a
possibility.
>
> -Thorsten
>
>
>
>> This is an enormous step up in power - for example, an existential can
>> return a value of one of these anonymous associated types from one function
>> and pass it into another function that takes the same type, maintaining
>> perfect type safety but without ever revealing the actual type. There is no
>> need anymore to limit the APIs exposed to the user, although there may still
>> exist APIs that are semantically useless without additional type information.
>>
>> A set of conversions has also been defined. At compile-time 'as' can be used
>> to turn values of these anonymous associated types back into existentials
>> based on the constraints defined earlier. 'as?' can also be used for
>> conditional casting of these anonymously-typed values into potential actual
>> types.
>>
>> As always, the link is here, and feedback would be greatly appreciated:
>> https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md
>>
>> Best,
>> Austin
>>
>>> On Tue, May 24, 2016 at 5:09 AM, Matthew Johnson via swift-evolution
>>> <swift-evolution@swift.org> wrote:
>>>
>>>
>>> Sent from my iPad
>>>
>>> On May 23, 2016, at 9:52 PM, Brent Royal-Gordon via swift-evolution
>>> <swift-evolution@swift.org> wrote:
>>>
>>> >> One initial bit of feedback - I believe if you have existential types,
>>> >> I believe you can define Sequence Element directly, rather than with a
>>> >> type alias. e.g.
>>> >>
>>> >> protocol Sequence {
>>> >> associatedtype Element
>>> >> associatedtype Iterator: any<IteratorProtocol where
>>> >> IteratorProtocol.Element==Element>
>>> >> associatedtype SubSequence: any<Sequence where Sequence.Element ==
>>> >> Element>
>>> >> …
>>> >> }
>>> >
>>> > That's not really the same thing. Any<IteratorProtocol> is an
>>> > existential, not a protocol. It's basically an automatically-generated
>>> > version of our current `AnyIterator<T>` type (though with some additional
>>> > flexibility). It can't appear on the right side of a `:`, any more than
>>> > AnyIterator could.
>>>
>>> After this proposal you should be able to use these existentials anywhere
>>> you can place a constraint, so it would work. You can do this with the
>>> protocol composition operator today and the future existential is just an
>>> extension of that capability.
>>>
>>> >
>>> > What *would* work is allowing `where` clauses on associated types:
>>> >
>>> >> protocol Sequence {
>>> >> associatedtype Element
>>> >> associatedtype Iterator: IteratorProtocol where
>>> >> Iterator.Element==Element
>>> >> associatedtype SubSequence: Sequence where SubSequence.Element ==
>>> >> Element
>>> >> …
>>> >> }
>>> >
>>> > I believe this is part of the generics manifesto.
>>> >
>>> > --
>>> > Brent Royal-Gordon
>>> > Architechies
>>> >
>>> > _______________________________________________
>>> > swift-evolution mailing list
>>> > swift-evolution@swift.org
>>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
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