1 files changed, 69 insertions, 0 deletions
diff --git a/docs/examples/tcpoly/monads/Monads.scala b/docs/examples/tcpoly/monads/Monads.scala
new file mode 100644
index 0000000000..5fcb4cd35f
--- /dev/null
+++ b/docs/examples/tcpoly/monads/Monads.scala
@@ -0,0 +1,69 @@
+package examples.tcpoly.monad;
+
+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)
+   */
+   // Option =:= [x] => Option[x] <: [x] => Any
+//  trait MonadTC[m <: [x] => Any, a] {
+    // MonadTC[m[x], a] x is a type parameter too -- should not write e.g., m[Int] here
+  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 main extends OptionMonad with Application {
+  Console.println(Some("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa") >>= (x => Some(x.length)))
+}
+
+
+/*
+trait MonadTC[m[x], a] requires m[x] {
+  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]
+}
+
+abstract class OptionIsMonad[t[x] <: Option[x], a] implicit extends MonadTC[t, a] {
+  def unit[a](orig: a) = Some(orig) // TODO: problematic.. is a meta-member: not invoked on this
+}
+
+class SomeIsMonad[a] extends OptionIsMonad[Some, a] {
+  def >>=[b](fun: a => Option[b]): Option[b] = fun(x)
+}
+
+class NoneIsMonad[a] extends OptionIsMonad[None, a] {
+  def >>=[b](fun: a => Option[b]): Option[b] = None
+}
+*/
+\ No newline at end of file