Get vs State

gistfile1.scala

// In the following code, Get is a type which has previously been called State - a name which
// the author found to be confusing, since instances aren't actually states!
//
// Instances are in fact functions which obtain (get) a value that depends on some state.
//
// The following code demonstrates using a Get to set as well as get!
//
// claim: even this is less confusing than using the name State!
//
// N.B. Here, 'State' is the type of the er, state.
object app extends App {
  type Get[A, S] = S => (A, S)

  case class State(n: Int)

  def set(n: Int): State = {
    val get = new Get[Unit, State] {
      def apply(s0: State) = ((), s0)
    }
    val (_, s) = get(State(n))
    s
  }

  def get(s0: State): (Int, State) = {
    val get = new Get[Int, State] {
      def apply(s: State) = (s.n, s)
    }
    get(s0)
  }

  // test that get returns the value passed to set
  val s0 = set(1)
  val (n, s1) = get(s0)

  assert(n == 1)
}