add CHAR_EVENT message type (next is to handle hjkl navigation)

shell/shell.sml

@@ -6,13 +6,12 @@ struct
   fun frameBufferSizeCallback inputMailbox (width, height) =
     Mailbox.send (inputMailbox, RESIZE_EVENT (width, height))
 
-  fun charCallback word =
+  fun charCallback inputMailbox word =
     let
       val word = Word32.toInt word
       val chr = Char.chr word
-      val s = Char.toString chr ^ "\n"
     in
-      print s
+      Mailbox.send (inputMailbox, CHAR_EVENT chr)
     end
 
   fun registerCallbacks (inputMailbox, window) =
@@ -21,6 +20,7 @@ struct
       val () = Input.exportFramebufferSizeCallback resizeCallback
       val () = Input.setFramebufferSizeCallback window
 
+      val charCallback = charCallback inputMailbox
       val () = Input.exportCharCallback charCallback
       val () = Input.setCharCallback window
     in