Usually I tend not to give much information to a subroutine intending to restrict its liberty of touching the calling information..... So if I have an instance Inst which should be changed by an outside-Inst computation Sub and I'm calling Sub with Sub(I'),I try to build the story so that Sub can only read Inst or a part of it (I') and return some specially structured information which then I use "inside" Inst to change Inst's state (horribble speech). The feeling I get with Haskell, is that S should have the freedom of changing I and return just I (changed). Is it any right ?? grazie Paolino -- ....lotta dura per la verdura