package.scala

package object stack {
  trait StackSignature {
    type Stack[T]
    def empty[T]: Stack[T]
    def push[T](s: Stack[T], e: T): Stack[T]
    def peek[T](s: Stack[T]): T
    def pop[T](s: Stack[T]): Stack[T]
    def isEmpty[T](s: Stack[T]): Boolean
  }
  val StackModule: StackSignature = new StackSignature {
    case class Stack[T](content: List[T])
    def empty[T] = Stack[T](Nil)
    def push[T](s: Stack[T], e: T) = s.copy(content = e::s.content)
    def peek[T](s: Stack[T]) = s.content.head
    def pop[T](s: Stack[T]) = s.copy(content = s.content.tail)
    def isEmpty[T](s: Stack[T]) = s.content == Nil
  }
}
// Usage
// import stack._
// val s1 = StackModule.empty[String]
// val s2 = StackModule.push(s1, "FOO")
// println(StackModule.peek(s2)) // FOO
// println(s2.isEmpty) // false
// val s3 = StackModule.pop(s2)
// println(s3.isEmpty) // true