SwiftStack

struct Stack<T> : Sequence {
    
    struct StackNode
    {
        var value: T
        var next: Any? = nil
    }
    
    struct StackGenerator : Generator {
        var top : StackNode? = nil
        
        init(top: StackNode?) {
            self.top = top;
        }
        
        mutating func next() -> T? {
            var value = self.top?.value
            self.top = self.top?.next as? StackNode
            return value ? value! : nil
        }
    }
    
    var top : StackNode? = nil
    
    mutating func push(value: T) {
        self.top = StackNode(value: value, next: self.top as Any?)
    }
    
    mutating func pop() -> T? {
        var value = self.top?.value
        self.top = self.top?.next as? StackNode
        return value ? value! : nil
    }
    
    
    func generate() -> StackGenerator
    {
        return StackGenerator(top: self.top)
    }
}

struct StackNode<T>
{
    typealias Node StackNode<T>
    var value: T
    var next: Node = nil
}

will it work?

DAlOG, no, it is an compiler bug https://twitter.com/jopamer/status/473590251168874496