src/Pure/Concurrent/future.ML

 fun cancel x = cancel_group (Task_Queue.group_of_task (task_of x));
 
 
 (* future jobs *)
 
-fun assign_result group result res =
-  let
+fun assign_result group result raw_res =
+  let
+    val res =
+      (case raw_res of
+        Exn.Exn exn => Exn.Exn (#2 (Par_Exn.serial exn))
+      | _ => raw_res);
     val _ = Single_Assignment.assign result res
       handle exn as Fail _ =>
         (case Single_Assignment.peek result of
           SOME (Exn.Exn e) => reraise (if Exn.is_interrupt e then e else exn)
         | _ => reraise exn);

 fun get_result x =
   (case peek x of
     NONE => Exn.Exn (Fail "Unfinished future")
   | SOME res =>
       if Exn.is_interrupt_exn res then
-        (case Exn.flatten_list (Task_Queue.group_status (Task_Queue.group_of_task (task_of x))) of
-          [] => res
-        | exns => Exn.Exn (Exn.EXCEPTIONS exns))
+        (case Task_Queue.group_status (Task_Queue.group_of_task (task_of x)) of
+          NONE => res
+        | SOME exn => Exn.Exn exn)
       else res);
 
 fun join_next deps = (*requires SYNCHRONIZED*)
   if null deps then NONE
   else