[swift-evolution] Enhanced existential types proposal discussion

L Mihalkovic laurent.mihalkovic at gmail.com
Thu Jun 9 13:57:47 CDT 2016

decided to play with a different syntax


still have to formalize it, but it does work for many situations, and is consistent with  var  a: P1 & P2

> On Jun 9, 2016, at 6:44 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> on Thu Jun 09 2016, Douglas Gregor <dgregor-AT-apple.com> wrote:
>>> On Jun 8, 2016, at 11:57 AM, Dave Abrahams via swift-evolution
>> <swift-evolution at swift.org> wrote:
>>> on Sun Jun 05 2016, Douglas Gregor <swift-evolution at swift.org
>> <mailto:swift-evolution at swift.org>> wrote:
>>>> Sent from my iPhone
>>>>> On Jun 5, 2016, at 6:41 PM, Matthew Johnson <matthew at anandabits.com> wrote:
>>>>> Sent from my iPad
>>>>>> On Jun 5, 2016, at 6:20 PM, Douglas Gregor <dgregor at apple.com> wrote:
>>>>>>> On May 18, 2016, at 12:35 AM, Austin Zheng <austinzheng at gmail.com> wrote:
>>>>>>> I've put together a considerably more detailed draft proposal,
>>>>>>> taking into account as much of Matthew's feedback as I could. You
>>>>>>> can find it below:
>>>>>>> https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md
>>>>>>> Since there is no chance this will come up for review anytime
>>>>>>> soon, I expect to make significant revisions to it over the next
>>>>>>> month or so. Any feedback would be greatly appreciated.
>>>>>> This is very much Swift 4 territory, but I can’t help myself… so…
>>>>>> The actual feature description is spread out through this very long
>>>>>> document, with user-facing ideas (e.g., using “anonymous associated
>>>>>> types”) intermixed with deeper technical details (existential type
>>>>>> equivalence), so it’s very daunting to read. Please bring the
>>>>>> user-facing features to the front (“Proposed Solution”) with
>>>>>> examples, and save the deeper technical details for “Detailed
>>>>>> Design”. You want more readers to make it through the part that
>>>>>> affects them.
>>>>>> Shortcut 'dot' notation: If there is only one protocol with
>>>>>> associated types specified in the requirements, and there are no
>>>>>> nested Any<...> requirements with where clauses of their own, that
>>>>>> protocol's name can be omitted from the whereclause constraints:
>>>>>> // Okay
>>>>>> // Would otherwise be Any< ~ where Collection.Element == Int>
>>>>>> let a : Any<class, Collection, Any<Streamable, CustomStringConvertible> where .Element == Int>
>>>>>> // NOT ALLOWED
>>>>>> // Both Collection and OptionSetType have associated types.
>>>>>> let b : Any<Collection, OptionSetType where .Element == Int>
>>>>>> FWIW, I think “.Element == Int” should be the only syntax. In
>>>>>> generic signatures, if you have two different protocols with
>>>>>> same-named associated types, and a given type parameter (or
>>>>>> associated type) conforms to both protocols, the associated types
>>>>>> are (implicitly) made equivalent via an inferred same-type
>>>>>> constraint. So there’s no reason to introduce the
>>>>>> “Collection.Element == Int” syntax, because the “Collection” part
>>>>>> is basically irrelevant.
>>>>>> Once existentials have been suitably enhanced, there is a strong
>>>>>> analogy between an existential and a generic signature with a
>>>>>> single type parameter that you can’t name. An existential
>>>>>> Any<Collection where .Element : Equatable> has most of the same
>>>>>> characteristics as a generic something with the signature <T :
>>>>>> Collection where T.Element : Equatable>. Specifically, the sections
>>>>>> on “Existential type equivalence”, “Ordering”, “Real types to
>>>>>> anonymous associated types”, “Anonymous associated types to real
>>>>>> types”. could be reduced to a few small, simple examples and a
>>>>>> mention of the analogous behavior of generics. It will be far
>>>>>> easier to explain this way, and readers don’t need to get immersed
>>>>>> in the details. Where there are differences vs. generics, that’s
>>>>>> important to point out.
>>>>>> “Associated typealias rewriting”: this also falls out of the equivalence with generics + SE-0092.
>>>>>> “Associated types and member exposure”: you don’t make the point
>>>>>> that it only makes sense to refer to the associated types of a let
>>>>>> constant; a var could change its type dynamically, which would
>>>>>> invalidate the typing rules. Did you consider just using
>>>>>> “x.dynamicType” in the type grammar for this? It’s more general, in
>>>>>> that you can refer to associated types but also talk about the
>>>>>> dynamic type of “x” itself, e.g.,
>>>>>> 	let x: Equatable = …
>>>>>> 	let y: Equatable = …
>>>>>> 	if let yAsX = y as? x.dynamicType { … x == yAsX … }
>>>>>> which is (almost?) as powerful as a general “open” expression.
>>>>>> I’m not a fan of the “anonymous associated types” terminology:
>>>>>> these are associated types of a type of some runtime-defined
>>>>>> value. The only thing “anonymous” about them is that it’s harder to
>>>>>> spell the base type; otherwise, they’re just like associated types
>>>>>> of a generic type parameter. Again, the generics analogy is strong
>>>>>> here.
>>>>>> FWIW, I don’t think we’ll ever need “opening existentials” with
>>>>>> what you’ve described here. Also, remember that a method of a
>>>>>> protocol extension essentially opens “Self”, so we already have one
>>>>>> way to open an existential (and that’s probably enough).
>>>>>> I was a little surprised you didn’t point out that AnyObject could become
>>>>>> 	typealias AnyObject = Any<class>
>>>>>> or give the nice “AnyCollection” syntax:
>>>>>> 	typealias AnyCollection<T> = Any<Collection where .Element == T>
>>>>>> the latter of which is fairly important, because it gives nice
>>>>>> syntactic sure to one of the most highly-requested features
>>>>>> [*]. I’d suggest having that example very, very early.
>>>>>> 	- Doug
>>>>>> [*] That generally comes in as “Swift should have parameterized protocols…”
>>>>> Great feedback here Doug.
>>>>> FWIW, we also occasionally get "Swift should have parameterized
>>>>> protocols" in the context of multiple conformances by the same
>>>>> concrete type (as in things like ConvertibleTo<T> protocol).
>>>> I know. From the bugs I've seen it's at least 10x as many requests for
>>>> "any collection of some element type" as for any actually reason why
>>>> one would need parameterize a protocols.
>>> That does, however, speak for the idea that a concise and obvious
>> syntax
>>> should be supported for that use-case.
>>> Personally, it doesn't seem ridiculous to me that some associated
>> types
>>> might usefully be written as type parameters on a protocol.  As
>>> Collection shows, not all associated types are equally important.
>>> Approximately nobody wants the existential “Collection where Index
>> ==
>>> Int.”
>> I’ve toyed with some form of this idea before, and IIRC it was
>> discussed on this list at one point, where some or all associate types
>> move up to the “type parameter” position in the grammar, e.g.,
>> protocol Collection<Element> : Sequence<Element> {
>>  associatedtype Index
>>  associatedtype SubSequence
>> }
>> However, unless that is semantically a type parameter in the sense
>> that one can have a single concrete type conform to Collection<A> and
>> Collection<B> separately, I think it’s misleading to go this route,
>> even if it does give us the clean “Collection<Int>” syntax.
> Excellent point!  I hadn't considered it.
> -- 
> Dave
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