From 83f49b9bebb04a55a45722594e717d95468924ac Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 15 Jul 2004 16:20:37 +0000
Subject: *** empty log message ***

---
 sources/examples/monads/callccInterpreter.scala | 84 +++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 sources/examples/monads/callccInterpreter.scala

(limited to 'sources/examples/monads/callccInterpreter.scala')

diff --git a/sources/examples/monads/callccInterpreter.scala b/sources/examples/monads/callccInterpreter.scala
new file mode 100644
index 0000000000..934fce1a9e
--- /dev/null
+++ b/sources/examples/monads/callccInterpreter.scala
@@ -0,0 +1,84 @@
+object callccInterpreter {
+
+  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 (val a <- interp(l, e);
+			   val b <- interp(r, e);
+			   val c <- add(a, b))
+                      yield c
+    case Lam(x, t) => unitM(Fun(a => interp(t, Pair(x, a) :: e)))
+    case App(f, t) => for (val a <- interp(f, e);
+			   val b <- interp(t, e);
+			   val 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));
+  }
+}
+
-- 
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