Scala example source code file (stateInterpreter.scala)

This example Scala source code file (stateInterpreter.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Scala tags/keywords

a, add, app, con, m, m, name, pair, pair, string, term, term, value, value

The Scala stateInterpreter.scala source code

package examples.monads;

object stateInterpreter {

  type State = Int;

  val tickS = new M(s => Pair((), s + 1));

  case class M[A](in: State => Pair[A, State]) {
    def bind[B](k: A => M[B]) = M[B]{ s0 => 
      val Pair(a, s1) = this in s0; k(a) in s1
    }
    def map[B](f: A => B): M[B]        = bind(x => unitM(f(x)));
    def flatMap[B](f: A => M[B]): M[B] = bind(f);
  }

  def unitM[A](a: A) = M[A](s => Pair(a, s));

  def showM(m: M[Value]): String = {
    val Pair(a, s1) = m in 0;
    "Value: " + a + "; Count: " + s1
  }

  type Name = String;

  trait Term;
  case class Var(x: Name) extends Term;
  case class Con(n: Int) extends Term;
  case class Add(l: Term, r: Term) extends Term;
  case class Lam(x: Name, body: Term) extends Term;
  case class App(fun: Term, arg: Term) extends Term;

  trait Value;
  case object Wrong extends Value {
   override def toString() = "wrong"
  }
  case class Num(n: Int) extends Value {
    override def toString() = n.toString();
  }
  case class Fun(f: Value => M[Value]) extends Value {
    override def toString() = "<function>"
  }

  type Environment = List[Pair[Name, Value]];

  def lookup(x: Name, e: Environment): M[Value] = e match {
    case List() => unitM(Wrong)
    case Pair(y, b) :: e1 => if (x == y) unitM(b) else lookup(x, e1)
  }

  def add(a: Value, b: Value): M[Value] = Pair(a, b) match {
    case Pair(Num(m), Num(n)) => for (_ <- tickS) yield Num(m + n)
    case _ => unitM(Wrong)
  }

  def apply(a: Value, b: Value): M[Value] = a match {
    case Fun(k) => for (_ <- tickS; c <- k(b)) yield c
    case _ => unitM(Wrong)
  }

  def interp(t: Term, e: Environment): M[Value] = t match {
    case Var(x) => lookup(x, e)
    case Con(n) => unitM(Num(n))
    case Add(l, r) => for (a <- interp(l, e);
			   b <- interp(r, e);
			   c <- add(a, b))
                      yield c
    case Lam(x, t) => unitM(Fun(a => interp(t, Pair(x, a) :: e)))
    case App(f, t) => for (a <- interp(f, e);
			   b <- interp(t, e);
			   c <- apply(a, b))
		      yield c
  }

  def test(t: Term): String = 
    showM(interp(t, List()));

  val term0 = App(Lam("x", Add(Var("x"), Var("x"))), Add(Con(10), Con(11)));
  val term1 = App(Con(1), Con(2));

  def main(args: Array[String]) = {
    System.out.println(test(term0));
    System.out.println(test(term1));
  }
}