State examples from herding-cats

Day7.scala

object St0 {
  type Stack = List[Int]

  def pop(s0: Stack): (Stack, Int) =
    s0 match {
      case x :: xs => (xs, x)
      case Nil => sys.error("stack is empty")
    }

  def push(s0: Stack, a: Int): (Stack, Unit) =
    (a :: s0, ())

  def stackManip(s0: Stack): (Stack, Int) = {
    val (s1, _) = push(s0, 3)
    val (s2, a) = pop(s1)
    pop(s2)
  }

  assert(stackManip(List(5, 8, 2, 1)) == (List(8, 2, 1), 5))
}

object St1 extends App {
  import cats._, cats.data.State, cats.instances.all._

  type Stack = List[Int]

  // val pop = State[Stack, Int] {
  //   case x :: xs => (xs, x)
  //   case Nil => sys.error("stack is empty")
  // }
  val pop: State[Stack, Int] = for {
    s <- State.get[Stack]
    (x :: xs) = s
    s <- State.set[Stack](xs)
  } yield x

  // def push(a: Int) = State[Stack, Unit] {
  //   xs => (a :: xs, ())
  // }
  def push(x: Int): State[Stack, Unit] = for {
    xs <- State.get[Stack]
    r <- State.set(x :: xs)
  } yield r

  def stackManip: State[Stack, Int] = for {
    _ <- push(3)
    a <- pop
    b <- pop
  } yield b

  assert { stackManip.run(List(5, 8, 2, 1)).value == (List(8, 2, 1), 5) }

  import cats.syntax.eq._

  def stackyStack: State[Stack, Unit] = for {
    stackNow <- State.get[Stack]
    r <- if (stackNow === List(1, 2, 3)) State.set[Stack](List(8, 3, 1))
         else State.set[Stack](List(9, 2, 1))
  } yield r

  assert { stackyStack.run(List(1, 2, 3)).value == (List(8, 3, 1), ()) }
  assert { stackyStack.run(List(1, 2, 4)).value == (List(9, 2, 1), ()) }
}