Stack Example on iOS, written in Swift..

Stack.swift

import UIKit

public struct Stack<Element> {

    private var storage: [Element] = []

    public init() { }

    public init(_ elements: [Element]) {
        storage = elements
    }

    public mutating func push(_ element: Element) {
        storage.append(element)
    }

    @discardableResult
    public mutating func pop() -> Element? {
        storage.popLast()
    }

    public func peek() -> Element? {
        storage.last
    }

    public var isEmpty: Bool {
        peek() == nil
    }

}

extension Stack: ExpressibleByArrayLiteral {

    public init(arrayLiteral elements: Element...) {
        storage = elements
    }

}

extension Stack: CustomStringConvertible {

    public var description: String {
    """
    \n------- Top (Last pushed) -------
    \(storage.map { "\($0)" }.reversed().joined(separator: "\n"))
    ----- Bottom (First pushed) -----\n
    """
    }

}

// Example: Int
var stack = Stack<Int>()
stack.push(1)
stack.push(2)
stack.push(3)
stack.push(4)
print(stack)

if let poppedElement = stack.pop() {
    print("Popped: \(poppedElement)")
}

// Example: String
let array = ["A", "B", "C", "D"]
var stackString = Stack(array)
stackString.pop()
print(stackString)

// Example: Double
var stackDouble: Stack = [1.0, 2.0, 3.0, 4.0]
stackDouble.pop()
print(stackDouble)