ImmutableList.swift

class ImmutableList<T> {
    private let listItems: [T]
    
    init() {
        self.listItems = [T]()
    }
    
    init(listItems: [T]) {
        self.listItems = listItems
    }
    
    func add(item: T) -> ImmutableList<T> {
        
        var listWithAddedItem = self.listItems
        listWithAddedItem.append(item)
 
        return ImmutableList<T>(listItems: listWithAddedItem)
    }
    
    func add(items: [T]) -> ImmutableList<T> {
        
        var listWithAddedItems = self.listItems
        listWithAddedItems.appendContentsOf(items)
        
        return ImmutableList<T>(listItems: listWithAddedItems)
    }
    
    func count() -> Int {
        return self.listItems.count
    }
    
    func items() -> [T] {
        return self.listItems
    }
}

let firstList = ImmutableList<Int>(listItems: [1,2])
firstList.items()                   // [1, 2]

// Adding an item returns a new instance, firstList is untouched
let firstListWithAppended = firstList.add(10)
firstListWithAppended.items()       // [1, 2, 10]
firstList.items()                   // [1, 2]

let firstListWithMultipleAdded = firstList.add(9).add([2,4]).add(11)
firstListWithMultipleAdded.items()  // [1, 2, 9, 2, 4, 11]
firstList.items()                   // [1, 2]