add char event which triggers log

shell/update-thread.sml

@@ -2,6 +2,7 @@ structure UpdateThread =
 struct
   open CML
   open MailboxType
+  open InputMsg
 
   fun sendMsg (msg, drawMailbox) =
     case msg of DRAW msg => Mailbox.send (drawMailbox, msg)
@@ -17,6 +18,16 @@ struct
   fun loop (app: AppType.app_type, inputMailbox, drawMailbox) =
     let
       val inputMsg = Mailbox.recv inputMailbox
+      val () =
+        (* if a certain CHAR_EVENT is sent, 
+         * we trigger an exception and log the command history.
+         * This is helpful for manually triggering logs when,
+         * for example, we encounter a bug and would like to see
+         * the history of events that caused it. *)
+        case inputMsg of
+          CHAR_EVENT #"~" => ExceptionLogger.log (Fail "")
+        | _ => ()
+
       val () = ExceptionLogger.addCommand inputMsg
 
       val app = AppUpdate.update (app, inputMsg) handle e => ExceptionLogger.log e