gistfile1.scala

class Action[S](
                   val when: (S) => Boolean,
                   val then: (S) => Either[String, S],
                   val name: String = "") {

    def apply(state: S): Either[String, S] = {

        if (when(state)) {

            then(state)

        } else {

            Left("The system could not execute the action '" + name + "'.")

        }
    }
}

package com.gmail.drewctaylor.scalaverse.planner

class Goal[S](val actionSet : List[Action[S]], val name : String = "") {
    def action(action: Action[S]) : Goal[S] = {
        new Goal(action :: actionSet, name)
    }

    def apply(state : S) : Either[String, S] = {
        scala.util.Random.shuffle(actionSet).foldLeft(Left("The system could not satisfy the goal '" + name + "' .").asInstanceOf[Either[String, S]])((either : Either[String, S], action : Action[S]) => {
            either match {
                case Left(value) => action(state)
                case Right(value) => Right(value)
            }
        })
    }
}

def a1 = new Action[Int]((i : Int) => i % 2 == 0, (i : Int) => Right(i + 1), "Add one to divisible by 2.");

def a2 = new Action[Int]((i : Int) => i % 3 == 0, (i : Int) => Right(i + 1), "Add one to divisible by 3.");

def a3 = new Action[Int]((i : Int) => i % 4 == 0, (i : Int) => Right(i + 2), "Add two to divisible by 4.");

def g1 = new Goal[Int](List(), "Add");

def g2 = g1.action(a1);

def g3 = g2.action(a2);

def g4 = g3.action(a3);