Enable more tests that pass

1 files changed, 45 insertions, 0 deletions

diff --git a/tests/run/tcpoly_monads.scala b/tests/run/tcpoly_monads.scala
new file mode 100644
index 000000000..978f88959
--- /dev/null
+++ b/tests/run/tcpoly_monads.scala
@@ -0,0 +1,45 @@
+
+import scala.language.{ higherKinds, implicitConversions }
+
+trait Monads {
+  /**
+   * class Monad m where
+   *   (>>=)  :: m a -> (a -> m b) -> m b
+   *   return :: a -> m a
+   *
+   * MonadTC encodes the above Haskell type class,
+   * an instance of MonadTC corresponds to a method dictionary.
+   * (see http://lampwww.epfl.ch/~odersky/talks/wg2.8-boston06.pdf)
+   *
+   * Note that the identity (`this') of the method dictionary does not really correspond
+   * to the instance of m[x] (`self') that is `wrapped': e.g., unit does not use `self' (which
+   * corresponds to the argument of the implicit conversion that encodes an instance of this type class)
+   */
+  trait MonadTC[m[x], a] {
+    def unit[a](orig: a): m[a]
+
+    // >>='s first argument comes from the implicit definition constructing this "method dictionary"
+    def >>=[b](fun: a => m[b]): m[b]
+  }
+}
+
+/**
+ * instance Monad Maybe where
+ *   (Just x) >>= k = k x
+ *   Nothing  >>= _ = Nothing
+ */
+trait OptionMonad extends Monads {
+  // this implicit method encodes the Monad type class instance for Option
+  implicit def OptionInstOfMonad[a](self: Option[a]): MonadTC[Option, a]
+    = new MonadTC[Option, a] {
+        def unit[a](orig: a) = Some(orig)
+        def >>=[b](fun: a => Option[b]): Option[b] = self match {
+          case Some(x) => fun(x)
+          case None => None
+        }
+      }
+}
+
+object Test extends OptionMonad with App {
+  Console.println((Some("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") >>= (x => Some(x.length))).get)
+}