Example of how to use the Cats State datatype to encapsulate state changes in an immutable manner

output.txt

(List(pushed1, pushed1, y, z),pushed1)
Stack is currently List(a, b, c)
(List(pushed2, saw a, a, b, c),pure)
Stack is currently List(b, c)
(List(pushed2_no_a, b, c),pure)

StateExample.scala

import cats._
import cats.data._
import cats.implicits._

// Example of how to use the Cats State datatype to encapsulate state changes in an immutable manner
// Helps us remove error prone boilerplate when chaining state transitions

object StateExample extends App {

  type Stack = List[String]

  // operations, they all take Stack

  val pop = State[Stack, String] {
    case head :: tail => (tail, head)
    case Nil => sys.error("stack is empty")
  }

  def push(a: String) = State[Stack, Unit](stack => (a :: stack, ()))

  // note how we compose a new operation with the for comprehension
  // State methods refer to the previous state
  def stackEdit1(pushed: String): State[Stack, String] = for {
    _ <- pop
    _ <- push(pushed)
    _ <- push(pushed)
    _ <- push(pushed)
    out <- pop
  } yield out

  def stackEdit2(pushed: String): State[Stack, String] = for {
    stack <- State.get[Stack] // we access the current Stack
    _ = println(s"Stack is currently $stack")
    _ <- if (stack.headOption == "a".some) push("saw a") else State.get[Stack]
    _ <- push(pushed)
    _ <- push(pushed)
    _ <- pop
    out <- State.pure("pure")
    // _ <- State.set[Stack](stack) // this would set stack back to how it was in the beginning
  } yield out

  println(stackEdit1("pushed1").run(List("x", "y", "z")).value)

  println(stackEdit2("pushed2").run(List("a", "b", "c")).value)

  println(stackEdit2("pushed2_no_a").run(List("b", "c")).value)
}