change AppUpdate.update function to return a list, so we can return multiple messages from a single event

imperative-shell/update-thread.sml

@@ -1,7 +1,7 @@
 signature UPDATE_THREAD =
 sig
   val run:
-      InputMessage.t Mailbox.mbox
+    InputMessage.t Mailbox.mbox
     * DrawMessage.t Mailbox.mbox
     * FileMessage.t Mailbox.mbox
     * AppType.app_type
@@ -17,13 +17,20 @@ struct
     case updateMsg of
       DRAW drawMsg => Mailbox.send (drawMailbox, drawMsg)
     | FILE fileMsg => Mailbox.send (fileMailbox, fileMsg)
-    | NO_MAILBOX => ()
+
+  fun handleMsgs (drawMailbox, fileMailbox, lst) =
+    case lst of
+      hd :: tl =>
+        let val _ = handleMsg (drawMailbox, fileMailbox, hd)
+        in handleMsgs (drawMailbox, fileMailbox, tl)
+        end
+    | [] => ()
 
   fun loop (inputMailbox, drawMailbox, fileMailbox, model) =
     let
       val inputMsg = Mailbox.recv inputMailbox
-      val (model, updateMsg) = AppUpdate.update (model, inputMsg)
-      val _ = handleMsg (drawMailbox, fileMailbox, updateMsg)
+      val (model, updateMsgs) = AppUpdate.update (model, inputMsg)
+      val _ = handleMsgs (drawMailbox, fileMailbox, updateMsgs)
     in
       loop (inputMailbox, drawMailbox, fileMailbox, model)
     end