make transform DSL nicer, see #1966

akka-actor-tests/src/test/scala/akka/actor/FSMActorSpec.scala

@@ -268,7 +268,7 @@ class FSMActorSpec extends AkkaSpec(Map("akka.actor.debug.fsm" -> true)) with Im
         startWith(0, 0)
         when(0)(transform {
           case Event("go", _) ⇒ stay
-        } {
+        } using {
           case x ⇒ goto(1)
         })
         when(1) {

akka-actor/src/main/scala/akka/actor/FSM.scala

@@ -253,8 +253,12 @@ trait FSM[S, D] extends Listeners with ActorLogging {
    */
   protected final def stop(reason: Reason, stateData: D): State = stay using stateData withStopReason (reason)
 
-  protected final def transform(func: StateFunction)(andThen: PartialFunction[State, State]): StateFunction =
+  protected final class TransformHelper(func: StateFunction) {
+    def using(andThen: PartialFunction[State, State]): StateFunction =
       func andThen (andThen orElse { case x ⇒ x })
+  }
+
+  protected final def transform(func: StateFunction): TransformHelper = new TransformHelper(func)
 
   /**
    * Schedule named timer to deliver message after given delay, possibly repeating.

akka-docs/scala/code/akka/docs/actor/FSMDocSpec.scala

@@ -128,7 +128,7 @@ class FSMDocSpec extends AkkaSpec {
         when(SomeState)(transform {
           case Event(bytes: Array[Byte], read) ⇒ stay using (read + bytes.length)
           case Event(bytes: List[Byte], read)  ⇒ stay using (read + bytes.size)
-        } {
+        } using {
           case s @ FSM.State(state, read, timeout, stopReason, replies) if read > 1000 ⇒
             goto(Processing)
         })
@@ -143,7 +143,7 @@ class FSMDocSpec extends AkkaSpec {
         when(SomeState)(transform {
           case Event(bytes: Array[Byte], read) ⇒ stay using (read + bytes.length)
           case Event(bytes: List[Byte], read)  ⇒ stay using (read + bytes.size)
-        }(processingTrigger))
+        } using processingTrigger)
         //#alt-transform-syntax
 
         //#termination-syntax

akka-docs/scala/fsm.rst

@@ -329,8 +329,6 @@ be used several times, e.g. when applying the same transformation to several
 .. includecode:: code/akka/docs/actor/FSMDocSpec.scala
    :include: alt-transform-syntax
 
-The parentheses are required by Scala’s syntax rules.
-
 Timers
 ------