[swift-evolution] Enhanced existential types proposal discussion

[Douglas Gregor]

> 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 <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…”

> 
> Austin
> 
> On Tue, May 17, 2016 at 9:52 PM, Austin Zheng <austinzheng at gmail.com <mailto:austinzheng at gmail.com>> wrote:
> 
> 
> On Tue, May 17, 2016 at 1:25 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>>  
>> 
>>> 
>>> Within the angle brackets are zero or more 'clauses'. Clauses are separated by semicolons. (This is so commas can be used in where constraints, below. Better ideas are welcome. Maybe it's not necessary; we can use commas exclusively.)
>> 
>> I’m not a fan of the semicolon idea.  I don’t see any reason for this.  The `where` keyword separates the protocol list from the constraints just fine.  The list on either side should be able to use commas with no problem (or line breaks if that proposal goes through).
>> 
>> 
>> I'm leaning towards getting rid of the commas, but would like to write out a few 'dummy' examples to see if there are any readability issues that arise. 
> 
> Replaced with what?  Whitespace separation?  I suppose that might work for the protocol list but it feels inconsistent with the rest of Swift.  Commas plus (hopefully) the alternative of newline seem like the right direction to me.
> 
> Sorry, I completely misspoke (mistyped?). I meant I want to get rid of the semicolons and use commas. I've come to the conclusion that there are no readability issues, protocol<> already uses commas, and semicolons used in this manner don't have a precedent anywhere else in the language.
>  
> 
>>> 
>>> There are five different possible clauses:
>>> 
>>> 'class'. Must be the first clause, if present. Places a constraint on the existential to be any class type. (Implies: Only one can exist. Mutually exclusive with class name clause.)
>>> 
>>> (In the future a follow-up proposal should add in 'struct' or 'value' as a counterpart.)
>> 
>> If we’re going to allow `struct` we should also allow `enum`.  `value` would allow either of those.
>> 
>> 
>> Of course. A future proposal can allow list members to discuss the exact details as to how struct, value, or enum specifiers should work. 
> 
> Yep, agree.  Just mentioning that if we’re going to reference it we should not leave obvious holes in what would be considered.  :)
> 
> Absolutely.
>  
> 
>>> 
>>> Class name. Must be the first clause, if present. (Implies: Only one can exist. Mutually exclusive with 'class'.) Places a constraint on the existential (not really an existential anymore) to be an instance of the class, or one of its subclasses.
>> 
>> It is still be an existential if it includes protocol requirements that the class does not fulfill.  For example, you might have Any<UIView, SomeProtocol> where UIView does not conform to SomeProtocol, but various subclasses do.
>> 
>> 
>> Fair enough. (I don't think the way things work would be affected.)
>>  
>> Your proposal doesn’t discuss composing Any in the way that Adrian’s did like this:
>> 
>> typealias Foo = Any<SomeClass, SomeProtocol, OtherProtocol>
>> Any<AnotherProtocol, Foo>
>> 
>> I didn't think it needed to be discussed. An Any<...> existential type is a type 'expression' just like any other, and should be allowed to participate in other Any<...>s.
>>  
>> 
>> I like the idea of composition as it allows us to factor out constraints.  If we are going to do that we should allow a class to be specified in the composition as long is it is a subclass of all class requirements of Any types it composes.  For example, this should be allowed:
>> 
>> typealias Bar = Any<SubclassOfSomeClass, Foo, AnotherProtocol>
>> 
>> This is still one class requirement for Bar, it just refines the class requirement of Foo to be SubclassOfSomeClass rather than just SomeClass.
>> 
>> This is a good point. There should be clarification as to how special cases of Any<...> used in another Any<...> behave. For example, like you said Any<MyClass, Any<SomeSubclassOfMyClass, Protocol>> should be valid. This will go into any proposal that emerges from the discussion.
> 
> Yes, this is why we need to discuss Any composition.  There are also cases of incompatible associated type constraints which need to be rejected (such as composing two Any’s where one has Element == String and another has Element == Int).
> 
>>  
>> 
>>> Example: Any<UIViewController; UITableViewDataSource; UITableViewDelegate>
>>> "Any UIViewController or subclass which also satisfies the table view data source and delegate protocols"
>>> Dynamic protocol. This is entirely composed of the name of a protocol which has no associated types or Self requirement.
>>> Example: Any<CustomStringConvertible; BooleanType>
>>> "Any type which conforms to both the CustomStringConvertible and BooleanType protocols"
>>> 
>>> I'm going to use 'static protocol' to refer to a protocol with associated types or self requirements. Feel free to propose a more sound name.
>>> 
>>> Self-contained static protocol, simple. This is composed of the name of a static protocol, optionally followed by a 'where' clause in which the associated types can be constrained (with any of the three basic conformance types: subclassing, protocol conformance, or type equality). Associated types are referred to with a leading dot.
>> 
>> Please do not introduce terms “dynamic protocol” and “static protocol”.  We want to support existentials of protocols that have self or associated type requirements.  The dynamic vs static distinction is a limitation of the current implementation of Swift and doesn’t make sense for the long term vision.
>> 
>> I'm not trying to introduce new terms, these are just placeholders. At the same time "protocols with self or associated type requirements" is cumbersome to work with and it would be nice for someone to come up with a descriptive term of art for referring to them.
> 
> I agree that a better term would be useful.  In the meantime, I would prefer something like “trivial” and “nontrivial” protocols.
> 
> I've decided to just use the full name until the community comes up with better names. Clarity is preferable to brevity in this case.
>  
> 
>>  
>> 
>>> 
>>> Example: Any<Collection where .Generator.Element : NSObject, .Generator.Element : SomeProtocol>
>>> "Any type that is a Collection, whose elements are NSObjects or their subclasses conforming to SomeProtocol.”
>> 
>> Swift does not allow disjunction of requirements.  Only conjunctions are supported.  That means the correct reading is:
>> 
>> "Any type that is a Collection, whose elements are NSObjects and their subclasses conforming to SomeProtocol.”
>> 
>> 
>> Yes, that is what I meant. "whose elements are (NSObjects or their subclasses) conforming to SomeProtocol”.
> 
> Ok, good.  Wasn’t quite clear to me.
> 
> Yes, the verbiage will need to be clearer in the future. That sentence could be ambiguously parsed.
>  
> 
>>> 
>>> Bound static protocol. This is the same as a self-contained static protocol, but with a leading "<name> as " which binds the protocol to a generic typealias. The name can be then be used in subsequent clauses to build constraints.
>>> 
>>> Example: Any<T as Collection; IntegerLiteralConvertible where .IntegerLiteralType == T.Element>.
>>> "Any type that is a Collection, and also can be built from an integer literal, in which the collection elements are the same type as the type of the integer used for the integer literal conformance.”
>> 
>> I’m not sure about this, but if we’re going to do it it should be the other way around: `Collection as T` with the alias after the name of the protocol.  
>> 
>> 
>> I like this, it flows better. "Protocol as T where Protocol.Foo == Int, Protocol.Bar : Baz”.
> 
> Why did you introduce an alias here and then not use it?  Did you mean "Protocol as T where T.Foo == Int, T.Bar : Baz"
> 
> Another result of rushing to compose an email. Sorry!
>  
> 
>>  
>> You are also using “dot shorthand” here to refer to an associated type of IntegerLiteralConvertible.  I think “dot shorthand” should be limited to cases where there is only one protocol that is getting constrained.  In other cases, we need to be clear about which protocol we are referring to.
>> 
>> I borrowed dot shorthand from the generics manifesto. But you are right, it should only be allowed if there is one protocol with associated types or self requirements clause in the Any<...> construction.
> 
> I would actually go further and limit it to one protocol period, and possibly even to one protocol and no type names (as types can have nested types and typealiases).  When we allow shorthand it should be immediately unambiguous what the shorthand references with no need to look at type or protocol declarations.
> 
> It might be desirable to propose the proposal with no allowance for shorthand, and have the dot shorthand be a smaller follow-up proposal.
>  
> 
>>  
>> 
>>> 
>>> There will be rules to prevent recursive nesting. For example, if generic typealiases are allowed, they cannot refer to each other in a circular manner (like how structs can't contain themeselves, and you can't create a cyclic graph of enums containing themselves).
>>> 
>>> How an existential can be used depends on what guarantees are provided by the clauses. For example, 'Any<Equatable>' can't be used for much; if there were any methods on Equatable that did not use the associated types at all you'd be able to call them, but that's about it. However, 'Any<Equatable where .Self == String>' would allow for == to be called on instances. (This is a stupid example, since Any<Equatable where .Self == String> is equivalent to 'String', but there are almost certainly useful examples one could come up with.)
>>> 
>>> In order of increasing 'power':
>>> Don't constrain any associated types. You can pass around Any<Equatable>s, but that's about it.
>>> Constrain associated types to conform to protocols.
>>> Fully constrain associated types.
>> 
>> I think we need to spell out pretty clearly what members we expect to be available or not available.  This section probably needs the most design and elaboration.  
>> 
>> For example, we probably can’t access a member who uses an associated type as an input unless it is constrained to a specific type.  On the other hand output types probably don’t need to limit access to a member.  However, if the output type is Self or an associated type the visible signature would have an output type which has the relevant constraints of the existential applied, but no more.  In some cases this means the output type would simply be Any.
>> 
>> Absolutely. This is vaguely what I had in mind but I wanted to get something down first. Thanks for thinking through some of the implications :).
> 
> That’s what I thought.  Just wanted to start the process of elaborating expectations.
> 
>>  
>> 
>> Where this really gets tricky is for compound types like functions, generic types, etc.  Working out the details in these cases is pretty complex.  I will defer to Doug on whether it is best to just defer those cases to the future, leave them up to the implementer, or try to work out all of the relevant details in the proposal (in which case we probably need a type system expert to help!).
>> 
>> Yes, exactly! For example, can Any<...> existentials involving protocols with associated types or self requirements be used within generic function or type definitions? Maybe there's an argument that existential types of this nature are redundant if you have access to generics (e.g. defining a property on a generic type that is a Collection containing Ints; you should be able to do that today). On the other hand, maybe there are use cases I haven't thought of…
> 
> I see no reason they shouldn’t be.  They are not redundant at all.  For example, you may want to store instances in a heterogeneous collection.  You need existentials to do that.
> 
> A simple example of what I was referring to there is something like this:
> 
> protocol P {
>     associatedtype Foo
> 
>     func bar(callback: (Foo) -> ())
> }
> 
> In other words, types in the signature of a protocol member are complex types that reference Self or associated types.  I think you really need a formal understanding of the type system to understand how to expose these members through a constrained existential.  We can probably understand the expected behavior in some of the simpler cases on a case by case basis, but that approach doesn’t scale at all and is arbitrary.  If they’re going to be supported an expert is going to need to be involved in the design.
> 
> Yes. I have some ideas regarding this topic.
>  
> 
>>  
>> 
>> One area you didn’t touch on is “opening” the existential?  Is that out of scope for this proposal?  That would be fine with me as this proposal is already taking on a lot.  But if so, you should mention something about future directions as it is pretty closely related to this proposal.
>> 
>> Yes, existential opening is explicitly separate from this (although I wanted to mention it in the section where I talk about how Any<Equatable> is not very useful). But you are absolutely right, this proposal should discuss how it wants to interact with possible future directions.
>>  
>> 
>> Another area you didn’t touch on is whether Any constructs (and typealiases referring to them) should be usable as generic constraints.  I would expect this to be possible but I think we need to spell it out.
>> 
>> I'm hoping for community input. This is a tricky subject, and at some point we'll bump into implementation limitations.
> 
> I don’t think it’s too tricky.  You can just unpack the constraints of the Any into the list of generic constraints.  Maybe I’m missing something, but I don’t think so.
> 
>>  
>> 
>> -Matthew
> 
> 
> 

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