src/Pure/ML/ml_compiler_polyml.ML

     val persistent_reports = reported_tree parse_tree [];
 
     fun output () =
       persistent_reports
       |> map (fn (pos, markup, text) => Position.reported_text pos (markup ()) (text ()))
-      |> implode |> Output.report;
+      |> Output.report;
   in
     if not (null persistent_reports) andalso redirect andalso Multithreading.enabled ()
     then
       Execution.print
         {name = "ML_Compiler.report", pos = Position.thread_data (), pri = 1} output