Scala example source code file (callccInterpreter.scala)

This example Scala source code file (callccInterpreter.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, add, app, con, m, m, name, pair, string, term, term, value, value

The Scala callccInterpreter.scala source code

object callccInterpreter {

  def id[a](x: a) = x

  type Answer = Value;

  case class M[A](in: (A => Answer) => Answer) {
    def bind[B](k: A => M[B])          = M[B](c => in (a => k(a) in c));
    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](c => c(a));

  def showM(m: M[Value]): String = (m in id).toString();

  def callCC[A](h: (A => M[A]) => M[A]) = 
    M[A](c => h(a => M[A](d => c(a))) in c);

  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;
  case class Ccc(x: Name, t: 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)) => unitM(Num(m + n))
    case _ => unitM(Wrong)
  }

  def apply(a: Value, b: Value): M[Value] = a match {
    case Fun(k) => k(b)
    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
    case Ccc(x, t) => callCC(k => interp(t, Pair(x, Fun(k)) :: e))
  }

  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));
  val term2 = Add(Con(1), Ccc("k", Add(Con(2), App(Var("k"), Con(4)))));

  def main(args: Array[String]) = {
    System.out.println(test(term0));
    System.out.println(test(term1));
    System.out.println(test(term2));
  }
}