! Use of undeclared type 'VendingSelection' compiler error

    import Foundation

    // Protocols

    protocol VendingMachineType {
        var selection: [VendingSelection] { get }  // get = it's a read only property
        var inventory: [VendingSelection: ItemType] { get set }
        var amountDeposited: Double { get set }

        init(inventory: [VendingSelection: ItemType])

        func vend(selection: VendingSelection, quantity: Double) throws
        func deposit(amount: Double)
    }

    protocol ItemType {
        var price: Double { get }
        var quantity: Double { get set }   // the quantity can change so it's get set
    }

    // Error Types

    enum InventoryType: ErrorType {
        case InvalidResource
        case ConversionError
        case InvalidKey
    }

    // Helper Classes

    class PlistConverter {
        class func dictionaryFromFile(resource: String, ofType type: String) throws -> [String : AnyObject] {

            guard let path = NSBundle.mainBundle().pathForResource(resource, ofType: type) else {
                throw InventoryType.InvalidResource
            } // .mainBundle() coresponds to app bundle for this app.

            guard let dictionary = NSDictionary(contentsOfFile: path), let castDictionary = dictionary as? [String: AnyObject] else {
                throw InventoryType.ConversionError
            }

            return castDictionary
        }

    class InventoryUnarchiver {
        class func vendingInventoryFromDictionary(dictionary: [String: AnyObject]) throws -> [VendingSelection: ItemType] {

            var inventory: [VendingSelection: ItemType] = [:]

            for (key, value) in dictionary {
                if let itemDict = value as? [String: Double], let price = itemDict["price"], let quantity = itemDict["quantity"] {

                    let item = VendingItem(price: price, quantity: quantity)

                    guard let key = VendingSelection(rawValue: key) else {
                        throw InventoryType.InvalidKey

                    }

                    inventory.updateValue(item, forKey: key)
                }
            }
            return inventory
        }

    }

    // Concrete Types

    enum VendingSelection: String {
        case Soda
        case DietSoda
        case Chips
        case Cookie
        case Sandwich
        case Wrap
        case CandyBar
        case PopTart
        case Water
        case FruitJuice
        case SportsDrink
        case Gum
    }

    struct VendingItem: ItemType {
        let price: Double
        var quantity: Double
    }

    // VendingMachine is better as a class. Classes are good at modeling state
    class VendingMachine: VendingMachineType {
        let selection: [VendingSelection] = [.Soda, .DietSoda, .Chips, .Cookie, .Sandwich, .Wrap, .CandyBar, .PopTart, .Water, .FruitJuice, .SportsDrink, .Gum]
        var inventory: [VendingSelection : ItemType]
        var amountDeposited: Double = 10.0   // setting an initial value to give user money to play with

        // because it's required in the protocol we need the required keyword
        required init(inventory: [VendingSelection : ItemType]) {
            self.inventory = inventory
        }

        func vend(selection: VendingSelection, quantity: Double) throws {
            // add code
        }

        func deposit(amount: Double) {
            // add code
        }
    }