A way to construct effect environments with private state (potentially impure) and primitive operations.

gistfile1.scala

import scalaz._
import scalaz.Free._
import scalaz.std.function._

// IO-like effect world; like State[S,A] where S is private to the implementation
trait Effects[World] {

  final class Action[+A] private[Effects] (private val t: World => Trampoline[(World, A)]) extends (World => A) {
    def apply(w: World): A = run(w)._2
    def run(w: World): (World, A) = t(w).run
    def map[B](f: A => B): Action[B] = new Action(w => for { (nw, a) <- t(w) } yield (nw, f(a)))
    def flatMap[B](f: A => Action[B]): Action[B] = new Action(w => for { (nw, a) <- t(w); x <- f(a).t(w) } yield x)
  }

  // Actions can only be constructed by subclasses (not happy with the names)
  protected def action[A](f: World => (World, A)): Action[A] = new Action(w => return_(f(w)))
  protected def effect[A](f: World => A): Action[A] = action(w => (w, f(w)))
  protected def unit[A](a: => A): Action[A] = effect(_ => a)

  // Semigroup, Monoid, Monad instances skipped
}

// Console effects for any world
trait ConsoleEffects { this: Effects[_] =>
  def putStrLn(s: String) = unit(println(s))
}

// Effects for a mutable map world
trait MapEffects[K, V] { this: Effects[collection.mutable.Map[K, V]] =>
  def put(k: K, v: V) = effect(_.put(k, v))
  def get(k: K) = effect(_.get(k))
}

object Test extends App {

  // An effect world for a mutable map with console IO
  object MapWorld extends Effects[collection.mutable.Map[String, Int]]
    with MapEffects[String, Int]
    with ConsoleEffects

  import MapWorld._

  // A test action; no way to get a handle to the map
  val action = for {
    a <- get("foo")
    _ <- putStrLn("a is " + a)
    _ <- put("foo", 3)
    b <- get("foo")
    _ <- putStrLn("b is " + b)
    c <- get("bar")
  } yield (a, b, c)
  
  val result = action.run(collection.mutable.HashMap("bar" -> 10)) // UNSAFE
  
  println(result)

  // a is None
  // b is Some(3)
  // (Map(foo -> 3, bar -> 10),(None,Some(3),Some(10)))

}