Another Swift box for [Protocol]

map-0-box.swift

// Yet another Swift box....

// I had never run into this explicitly until Andrew Spiess (@ninjazoete) asked about it a couple of days ago:
// https://twitter.com/ninjazoete/status/669530132919525377

// Now it's springing up all over the place in my work.
//
// I'm calling it "map-$0-boxing" for the moment ("identity boxing" is more accurate, but "map-$0" better
// matches the actual code). Anyone have a better or clearer implementation?
//
// I completely get why it's necessary currently (@jckarter confirmed [Type] and [Protocol] are stored
// differently), but it's not really obvious to the reader, so wondering if people should just learn to
// read it, or we should come up with a better way to write it.

protocol ProductType {}
struct Milk: ProductType {}
let milks = [Milk()]

// 
// There's the map-0 box. It "boxes" the conforming type into the protocol
//
let products: [ProductType] = milks.map{$0}

//
//
//

// I've been contemplating something like these, but I don't think there's any way to type-parameterize
// on protocols in Swift, either in functions or in extensions

// func castArrayToProtocol<T, P where T:P>(xs: [T]) -> [P] { return xs.map{$0} }
//
// extension Array where Element: P {
//     func castToProtocol() -> [P] { return map{$0} }
// }


// There's this solution, but I hate it; it's much worse than the map.

extension Array {
    func unsafeCastToArrayOf<T>(type: T.Type) -> [T] {
        return map{$0 as! T}
    }
}

let prods: [ProductType] = milks.unsafeCastToArrayOf(ProductType)

// Anything better?