[swift-evolution] [Manifesto] Completing Generics

[Joe Groff]

> On Mar 2, 2016, at 10:59 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On Mar 2, 2016, at 10:45 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
>> 
>>>> if let storedInE1 = e1 openas T {     // T is a the type of storedInE1, a copy of the value stored in e1
>>>> if let storedInE2 = e2 as? T {      // is e2 also a T?
>>>>   if storedInE1 == storedInE2 { … } // okay: storedInT1 and storedInE2 are both of type T, which we know is Equatable
>>>> }
>>>> }
>>> 
>>> I’m worried that this is not really correct with inheritance. If e1 is an instance of SubClass, and e2 is an instance of SuperClass with SubClass : SuperClass, then your operation is no longer symmetric. Heterogeneous equality just seems like a pain in general.
>> 
>> Following from the generic constants mentioned earlier, perhaps this is expressed with some kind of generic `if let`?
>> 
>> 	func == (e1: Any<Equatable>, e2: Any<Equatable>) -> Bool {
>> 		guard let concreteE1<T: Equatable> = e1 as? T, concreteE2 = e2 as? T else {
> 
> I think here the generic signature should really come first before any patterns, so more like ‘guard let <T : Equatable> …'.
> 
> But this is rather complicated. I fear. Perhaps a well-crafted one-off language feature to handle heterogeneous equality is better.
> 
> Here’s a really not-serious proposal: it seems we could implement GADTs if we allowed enum cases to have generic parameter lists, and extended pattern matching as above.
> 
> enum Foo {
> 	case Something<T>(T)
> 	case Nothing
> }
> 
> let a: [Foo] // could contain many different Something types

And then we introduce higher-order 'protocol' constraints, and implement 'Any' in the library!

enum Any<Constraints: protocol> {
  case Value<T: Constraints>(T)
}

-Joe
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