Testing covariance in Swift

covariance.swift

class Fruit {
    var name = "Fruit"
    var description : String {
        return name
    }
}

class Apple : Fruit {
    init() {
        super.init()
        self.name = "Apple"
    }
}

func printFruit(fruits: Fruit[]) -> String[] {
    return fruits.map() { (fruit : Fruit) -> String in
        return fruit.description
    }
}

let apple1 = Apple()
apple1.description

let apple2 = Apple()
apple2.description

let apples : Apple[] = [apple1, apple2]
// Apple[] is a subtype of Fruit[]
printFruit(apples)



class Banana : Fruit {
    init() {
        super.init()
        self.name = "Banana"
    }
}

func mangleFruit(fruits: Fruit[]) -> String[] {
    fruits[0] = Banana()
    return printFruit(fruits)
}

// Oops, the type system allows this broken code
mangleFruit(apples)

This test is flawed. Arrays have value semantics. When you invoke mangleFruit(apples), the function isn't getting a reference to the existing array; it's getting a NEW array