A State effect can be seen as the combination of both a Reader and a Writer with these operations:

Let’s see an example showing that we can also use tags to track different states at the same time:

  import org.atnos.eff._, all._, syntax.all._

  type S1[A] = State[Int, A]
  type S2[A] = State[String, A]

  type S = Fx.fx2[S1, S2]

  val swapVariables: Eff[S, String] = for {
    v1 <- get[S, Int]
    v2 <- get[S, String]
    _  <- put[S, Int](v2.size)
    _  <- put[S, String](v1.toString)
    w1 <- get[S, Int]
    w2 <- get[S, String]
  } yield "initial: "+(v1, v2).toString+", final: "+(w1, w2).toString


> initial: (10,hello), final: (5,10)

In the example above we have used an eval method to get the A in Eff[R, A] but it is also possible to get both the value and the state with run or only the state with exec.

Instead of tagging state effects it is also possible to transform a State effect acting on a “small” state into a State effect acting on a “bigger” state:

import org.atnos.eff._, all._, syntax.all._

type Count[A] = State[Int, A]
type Sum[A]   = State[Int, A]
type Mean[A]  = State[(Int, Int), A]

type S1 = Fx.fx1[Count]
type S2 = Fx.fx1[Sum]
type S  = Fx.fx1[Mean]

def count(list: List[Int]): Eff[S1, String] = for {
  _ <- put(list.size)
} yield s"there are ${list.size} values"

def sum(list: List[Int]): Eff[S2, String] = {
  val s = if (list.isEmpty) 0 else list.sum
  for {
    _ <- put(s)
  } yield s"the sum is $s"

def mean(list: List[Int]): Eff[S, String] = for {
  m1 <- count(list).lensState((_:(Int, Int))._1, (s: (Int,Int), i: Int) => (i, s._2))
  m2 <- sum(list).lensState((_:(Int, Int))._2, (s: (Int, Int), i: Int) => (s._1, i))
} yield m1+"\n"+m2

mean(List(1, 2, 3)).runState((0, 0)).run
> (there are 3 values
the sum is 6,(3,6))