From da7d8783871a7dc654ad8c36b9fbea5028025f47 Mon Sep 17 00:00:00 2001
From: Roland <rk@rkuhn.info>
Date: Wed, 15 Jun 2011 20:39:29 +0200
Subject: [PATCH 1/2] make Channel contravariant

---
 .../scala/akka/actor/actor/ChannelSpec.scala     | 16 ++++++++++++++++
 .../src/main/scala/akka/actor/Channel.scala      |  7 ++-----
 2 files changed, 18 insertions(+), 5 deletions(-)
 create mode 100644 akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala

diff --git a/akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala b/akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala
new file mode 100644
index 0000000000..7300f4a28e
--- /dev/null
+++ b/akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala
@@ -0,0 +1,16 @@
+package akka.actor
+
+import org.scalatest.WordSpec
+import org.scalatest.matchers.MustMatchers
+import akka.dispatch._
+
+class ChannelSpec extends WordSpec with MustMatchers {
+  "A Channel" must {
+    "be contravariant" in {
+      val ap = new ActorPromise(1000)
+      val p: Promise[Any] = ap
+      val c: Channel[Any] = ap
+      val cs: Channel[String] = c
+    }
+  }
+}
diff --git a/akka-actor/src/main/scala/akka/actor/Channel.scala b/akka-actor/src/main/scala/akka/actor/Channel.scala
index 80134f2df8..2896883f2b 100644
--- a/akka-actor/src/main/scala/akka/actor/Channel.scala
+++ b/akka-actor/src/main/scala/akka/actor/Channel.scala
@@ -8,14 +8,11 @@ package akka.actor
  * Abstraction for unification of sender and senderFuture for later reply.
  * Can be stored away and used at a later point in time.
  *
- * Channel cannot be contravariant because of Future providing its value in
- * covariant position.
- *
  * The possible reply channel which can be passed into ! and safe_! is always
  * untyped, as there is no way to utilize its real static type without
  * requiring runtime-costly manifests.
  */
-trait Channel[T] {
+trait Channel[-T] {
 
   /**
    * Scala API. <p/>
@@ -105,7 +102,7 @@ trait Channel[T] {
  * i.e. ! is not guaranteed to fail (e.g. NullChannel would be a
  * counter-example).
  */
-trait AvailableChannel[T] { self: Channel[T] ⇒
+trait AvailableChannel[-T] { self: Channel[T] ⇒
   def safe_!(msg: T)(implicit channel: UntypedChannel = NullChannel): Boolean = {
     if (isUsable) {
       try {

From 6a3049e4c5c1e6af1f6752ff982c665e1031582b Mon Sep 17 00:00:00 2001
From: Roland <rk@rkuhn.info>
Date: Wed, 15 Jun 2011 21:04:33 +0200
Subject: [PATCH 2/2] document Channel contravariance

---
 .../test/scala/akka/actor/actor/ChannelSpec.scala   |  4 ++++
 akka-docs/scala/actors.rst                          | 13 ++++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala b/akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala
index 7300f4a28e..61c898f8be 100644
--- a/akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala
+++ b/akka-actor-tests/src/test/scala/akka/actor/actor/ChannelSpec.scala
@@ -1,3 +1,7 @@
+/**
+ * Copyright (C) 2009-2011 Scalable Solutions AB <http://scalablesolutions.se>
+ */
+
 package akka.actor
 
 import org.scalatest.WordSpec
diff --git a/akka-docs/scala/actors.rst b/akka-docs/scala/actors.rst
index 2d0f105e0e..3019272e7f 100644
--- a/akka-docs/scala/actors.rst
+++ b/akka-docs/scala/actors.rst
@@ -266,7 +266,7 @@ Reply using the channel
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 If you want to have a handle to an object to whom you can reply to the message, you can use the ``Channel`` abstraction.
-Simply call ``self.channel`` and then you can forward that to others, store it away or otherwise until you want to reply, which you do by ``Channel ! response``:
+Simply call ``self.channel`` and then you can forward that to others, store it away or otherwise until you want to reply, which you do by ``channel ! response``:
 
 .. code-block:: scala
 
@@ -274,6 +274,17 @@ Simply call ``self.channel`` and then you can forward that to others, store it a
       val result = process(request)
       self.channel ! result
 
+The :class:`Channel` trait is contravariant in the expected message type. Since
+``self.channel`` is subtype of ``Channel[Any]``, you may specialise your return
+channel to allow the compiler to check your replies::
+
+  class MyActor extends Actor {
+    def doIt(channel: Channel[String], x: Any) = { channel ! x.toString }
+    def receive = {
+      case x => doIt(self.channel, x)
+    }
+  }
+
 .. code-block:: scala
 
   case request =>