[swift-evolution] Enhanced existential types proposal discussion

[Dave Abrahams]

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