src/Pure/Tools/jedit.ML

         (case XML.parse txt of
           XML.Elem (("ACTIONS", _), body) => maps (parse_named "ACTION") body
         | _ => [])
     | (_, rc) => error ("Cannot unzip jedit.jar\nreturn code = " ^ string_of_int rc)));
 
-fun is_action a =
-  member (op =) (Lazy.force isabelle_jedit_actions) a orelse
-  member (op =) (Lazy.force isabelle_jedit_dockables) a orelse
-  member (op =) (Lazy.force jedit_actions) a;
+val all_actions =
+  Lazy.lazy (fn () =>
+    Lazy.force isabelle_jedit_actions @
+    Lazy.force isabelle_jedit_dockables @
+    Lazy.force jedit_actions);
 
 in
 
-fun check_action (a, pos) =
-  if is_action a then a
-  else error ("Bad jEdit action " ^ quote a ^ Position.here pos);
+fun check_action (name, pos) =
+  if member (op =) (Lazy.force all_actions) name then name
+  else
+    let
+      val completion =
+        Completion.make (name, pos)
+          (fn completed =>
+            Lazy.force all_actions
+            |> filter completed
+            |> sort_strings
+            |> map (fn a => (a, ("action", a))));
+      val report = Markup.markup_report (Completion.reported_text completion);
+    in error ("Bad jEdit action " ^ quote name ^ Position.here pos ^ report) end
 
 end;
 
 end;