1 files changed, 85 insertions, 0 deletions
diff --git a/sources/examples/monads/errorInterpreter.scala b/sources/examples/monads/errorInterpreter.scala
new file mode 100644
index 0000000000..d797f01a4d
--- /dev/null
+++ b/sources/examples/monads/errorInterpreter.scala
@@ -0,0 +1,85 @@
+object errorInterpreter {
+
+  trait M[A] {
+    def show: String;
+    def bind[B](k: A => M[B]);
+    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]          =  new Suc(a);
+  def errorM[A](msg: String): M[A]  =  new Err(msg);
+
+  def showM(m: M[Value]): String = m.show;
+
+  class Suc[A](x: T) extends M[A] {
+    def bind[A](k: A => M[B]): M[B] = k(x);
+    def show: String = "Success: " + x;
+  }
+  class Err[T](msg: String) extends M[T] {
+    def bind[A](k: A => M[B]): M[B] = new Err(msg);
+    def show: String = "Error: " + msg;
+  }
+
+  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() => errorM("unbound variable: " + x);
+    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 _ => errorM("should be numbers: " + a + "," + b)
+  }
+
+  def apply(a: Value, b: Value): M[Value] = a match {
+    case Fun(k) => k(b)
+    case _ => errorM("should be function: " + a);
+  }
+
+  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
+  }
+
+  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));
+  }
+}