1 files changed, 55 insertions, 0 deletions
diff --git a/docs/examples/monads/directInterpreter.scala b/docs/examples/monads/directInterpreter.scala
new file mode 100644
index 0000000000..0e2aae4a7b
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+++ b/docs/examples/monads/directInterpreter.scala
@@ -0,0 +1,55 @@
+object directInterpreter {
+
+  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;
+  case class Num(n: int) extends Value;
+  case class Fun(f: Value => Value)extends Value;
+
+  def showval(v: Value): String = v match {
+    case Wrong => "<wrong>"
+    case Num(n) => n.toString()
+    case Fun(f) => "<function>"
+  }
+
+  type Environment = List[Pair[Name, Value]];
+
+  def lookup(x: Name, e: Environment): Value = e match {
+    case List() => Wrong
+    case Pair(y, b) :: e1 => if (x == y) b else lookup(x, e1)
+  }
+
+  def add(a: Value, b: Value): Value = Pair(a, b) match {
+    case Pair(Num(m), Num(n)) => Num(m + n)
+    case _ => Wrong
+  }
+
+  def apply(a: Value, b: Value) = a match {
+    case Fun(k) => k(b)
+    case _ => Wrong
+  }
+
+  def interp(t: Term, e: Environment): Value = t match {
+    case Var(x) => lookup(x, e)
+    case Con(n) => Num(n)
+    case Add(l, r) => add(interp(l, e), interp(r, e))
+    case Lam(x, t) => Fun(a => interp(t, Pair(x, a) :: e))
+    case App(f, t) => apply(interp(f, e), interp(t, e))
+  }
+
+  def test(t: Term): String =
+    showval(interp(t, List()));
+
+  val term0 = App(Lam("x", Add(Var("x"), Var("x"))), Add(Con(10), Con(11)));
+
+  def main(args: Array[String]) =
+    System.out.println(test(term0));
+}