preparation for adding new additional threads/mailboxes (namely, the file mailbox)

imperative-shell/update-thread.sml

@@ -1,18 +1,32 @@
-structure UpdateThread =
+signature UPDATE_THREAD =
+sig
+  val run:
+      InputMessage.t Mailbox.mbox
+    * DrawMessage.t Mailbox.mbox
+    * AppType.app_type
+    -> unit
+end
+
+structure UpdateThread :> UPDATE_THREAD =
 struct
   open CML
+  open UpdateMessage
 
-  local
-    fun loop (inputMailbox, drawMailbox, model) =
-      let
-        val inputMsg = Mailbox.recv inputMailbox
-        val (model, drawMsg) = AppUpdate.update (model, inputMsg)
-        val _ = Mailbox.send (drawMailbox, drawMsg)
-      in
-        loop (inputMailbox, drawMailbox, model)
-      end
-  in
-    fun run (inputMailbox, drawMailbox, initial) =
-      loop (inputMailbox, drawMailbox, initial)
-  end
+  fun handleMsg (drawMailbox, updateMsg) =
+    case updateMsg of
+      DRAW drawMsg => Mailbox.send (drawMailbox, drawMsg)
+    | FILE fileMsg => ()
+    | NO_MAILBOX => ()
+
+  fun loop (inputMailbox, drawMailbox, model) =
+    let
+      val inputMsg = Mailbox.recv inputMailbox
+      val (model, updateMsg) = AppUpdate.update (model, inputMsg)
+      val _ = handleMsg (drawMailbox, updateMsg)
+    in
+      loop (inputMailbox, drawMailbox, model)
+    end
+
+  fun run (inputMailbox, drawMailbox, initial) =
+    loop (inputMailbox, drawMailbox, initial)
 end