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What are class and subtype existentials?

Paul Hudson       @twostraws

You should already know that Swift lets you specify a concrete type as a function parameter, meaning that you can say “this parameter must be a string.” Swift also lets you specify protocols types for function parameters, meaning that you can say “this parameter must conform to Codable.”

Those two are straightforward, but sometimes you need more power. Fortunately, Swift lets us represent existentials of classes and subtypes that conform to protocols, which is dramatic way of saying that you can now refer to types in a more complex way.

As an example, consider this class hierarchy:

class Animal { }

class Dog: Animal, Codable { }
class Cat: Animal, Codable { }

There’s a main Animal class, and two subclasses called Dog and Cat. Both of the subclasses also conform to Codable. Swift lets us combine both Animal (the class) and Codable (the protocol) to write functions that require a parameter to be both at the same time:

func encodeAnimal(with animal: Animal & Codable) {

}

let animal = Dog()
encodeAnimal(with: animal)

Without the Codable part that function would accept instances of the main Animal class even though it wouldn’t work inside, and without the Animal part that function would accept anything that’s Codable – only the combination of the two makes sense.

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