[swift-evolution] namespacing protocols to other types

[Adrian Zubarev]

I disagree with some of your points. Do begin with, I don’t think we should disallow capturing the generic type parameter, because I think there might be a good way to prevent parameterization of nested protocols.

To me this only feels like a natural consequence of nesting protocols anyways. To achieve this we have to provide an explicit associated type which will have a default type that refers to the captured outer generic type parameter. At this point we discover another issue that we cannot disambiguate generic type parameter like associated types yet and would be forced to name the associated type of the protocol differently.

struct Generic<T> {
  protocol P {  
    associatedtype R = T  
    func f() -> R  
  }
}
As you can see I didn’t include the variable in this example, because existential are orthogonal this issue. David Hart and I still want to write a proposal to allow the where clause on typealiases - maybe after the forum officially launches.

Above I said that there is an issue and provided an example that would solve that issue with todays syntax, but I’d rather expand this idea. Consider this syntax of a generic type and a protocol with an associated type.

protocol Proto {
  associatedtype Element
}

Proto.Element // This is an error like this, but it's still allowed in a generic context

func function<P : Proto>(_: P) where P.Element == Int {}

protocol OtherProto : Proto where Element == Int {}

struct Test<Element> {}

extension Test where Element == Int {}

Test.Element // Can/should we allow this?
If we could allow this in Swift then the above example with the nested protocol could be disambiguated nicely.

struct Generic<T> {
  protocol P {  
    associatedtype T = Generic.T  
    func f() -> T  
  }
}
Remember that Generic.T is only the default for P.T if you don’t set it yourself but when you conform or use that that protocol (in a generic context) you can still set it differntly.

This consequence disallows protocol parameterization through nesting in a generic types, but still behaves very similar to nested generic types:

struct Test<T> {
  struct NonGeneric {
    var t: T
  }

  struct Generic<R> {
    var t: T
    var r: R
  }
}

_ = Test<String>.NonGeneric(t: "😎")
_ = Test<String>.Generic<Int>(t: "🤓", r: 42)
——

And for the existential variable inside Genric it really should be something like this (when the where clause is allowed and if we can refer differently to generic type parameters as well):

struct Generic<T> {
    …
    typealias PConstrainedByT = P where T == Self.T
    var object: PConstrainedByT
}



Am 27. Dezember 2017 um 19:53:36, Karl Wagner via swift-evolution (swift-evolution at swift.org) schrieb:

Yeah I wrote that proposal. I eventually stripped it down to just disallow all capturing, but it was still not selected by the core team for review ¯\_(ツ)_/¯

As for capturing semantics, once you start working through use-cases, it’s becomes clear that it's going to require generalised existentials. Otherwise, how would you use a Generic<T>.P?

struct Generic<T> {
    protocol P { func f() -> T }

    var object: P // uh-oh! ‘Generic protocol can only be used as a generic parameter constraint'
}

So, you would need to add a generic parameter to actually use P from within Generic<T>, which of course limits you to a single concrete type of P:

struct Generic<T, TypeOfP> where TypeOfP: Self.P {      // Could this even work? What if P captures TypeOfP?
    protocol P { /* … */ }
    var object: TypeOfP
}

Which is just yucky.

Ideally, the type of ‘object’ should be ‘Any<P where P.T == T>’, to express that it can be any conforming type with the appropriate constraints. You wouldn’t need to write that all out; we could infer that capturing is equivalent to a same-type constraint (or perhaps one of these “generalised supertype constraints” that were pitched recently). But we can’t express those kinds of existentials everywhere in the type-system today, so most examples of capturing fall down pretty quickly.

- Karl

On 25. Dec 2017, at 03:56, Slava Pestov via swift-evolution <swift-evolution at swift.org> wrote:

There was a proposal to allow protocols to be nested inside types at one point but it didn’t move forward.

Basically, if the outer type is a non-generic class, struct or enum, there’s no conceptual difficulty at all.

If the outer type is a generic type or another protocol, you have a problem where the inner protocol can reference generic parameters or associated types of the outer type. This would either have to be banned, or we would need to come up with coherent semantics for it:

struct Generic<T> {
 protocol P {
   func f() -> T
 }
}

struct Conforms : Generic<Int>.P {
 func f() -> Int { … } // Like this?
}

let c = Conforms()
c is Generic<String>.P // is this false? Ie, are Generic<Int>.P and Generic<String>.P different protocols?

Slava

On Dec 24, 2017, at 6:53 PM, Kelvin Ma via swift-evolution <swift-evolution at swift.org> wrote:

is there a reason why it’s not allowed to nest a protocol declaration inside another type?
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