added file docs/examples/gadts.scala

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@6647 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

docs/examples/gadts.scala

@@ -0,0 +1,22 @@
+package examples
+
+object gadts extends Application {
+
+  abstract class Term[T]
+  case class Lit(x: int) extends Term[int]
+  case class Succ(t: Term[int]) extends Term[int]
+  case class IsZero(t: Term[int]) extends Term[boolean]
+  case class If[T](c: Term[boolean],
+                   t1: Term[T],
+                   t2: Term[T]) extends Term[T]
+
+  def eval[T](t: Term[T]): T = t match {
+    case Lit(n)        => n
+    case Succ(u)       => eval(u) + 1
+    case IsZero(u)     => eval(u) == 0
+    case If(c, u1, u2) => eval(if (eval(c)) u1 else u2)
+  }
+  Console.println(
+    eval(If(IsZero(Lit(1)), Lit(41), Succ(Lit(41)))))
+}
+