[swift-evolution] [Manifesto] Completing Generics

Brent Royal-Gordon brent at architechies.com
Thu Mar 3 00:45:44 CST 2016

>> 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 {
			return false
		// If we made it through that `guard`, there is some `Equatable` type `T` which `e1` 
		// and `e2` both contain.
		// (However, I'm not sure why this type couldn't be `Any<Equatable>`.)
		return concreteE1 == concreteE2

Brent Royal-Gordon

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