[swift-evolution] [swift-evolution-announce] [Review] SE-0089: Replace protocol<P1, P2> syntax with Any<P1, P2>

[L. Mihalkovic]

On May 25, 2016, at 9:45 AM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:

>> - A few people have also noted the similarity between Any<...> and normal generic types. This is potentially confusing today, but with enough magic language features from the future, Any *could* conceivably be made a library feature in the fullness of time:
>> 
>>    // Let's say 'protocol' constraints indicate second-order constraint variables:
>>    enum Any<Constraints: protocol> {
>>        // And we have GADTs:
>>        case value<T: Constraints>(T)
>>    }
>> 
>>    // And we have user-defined value subtyping:
>>    extension <Constraints: protocol, T: Constraints> T: Any<Constraints> { ... }
> 
> I wanted to reply to this, because this is a beautiful dream, but if we want to treat `Any` like some kind of magical variadic generic type, that has some interesting implications for its design right now.
> 
> 1) In a normal generic type, when you leave out the generic parameters, Swift infers them. (Think of casting an array literal to `Array`, which pins down the collection type but not the elements.) If `Any` were handled similarly, that would mean that `foo as Any` would not necessarily have an empty set of constraints, but would rather have an *inferred* set of constraints. If you wanted to force it to have no constraints, you would need to write `Any<>`. 
> 
> 2) We would not normally expect to be able to cast between a normal generic type and the instance contained in it. Instead, we would expect to use initializers, just as we're now planning to require for Objective-C bridging:
> 
>    let myExistentialInteger = Any<Integer>(myInt)
>    let myInt2 = Int(any: myExistentialInteger)!
> 

How realistic is it to plan something like that considering the details involved in something like (even just to filter it out):

  let myEx = Any<Iterable>([10,"london",20,"tokyo"])

and then would you then propose something like:

let myExistentialInteger = Any<Integer>(myInt) {
  func hashValue -> Int {
    return 42
  }
}



> Note that, if we were passing to an API which specifically expected an `Any<Integer>`, we could just write `Any(myInt)` and type inference would figure out the rest.
> 
> -- 
> Brent Royal-Gordon
> Architechies
> 
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