Hello all, I find myself frequentlly writing types like this data Logger a l = Lgr { runLogger :: a -> Log l -> (Log l, Logger a l) } The purpose is to give a logger a chance to carry an internal state. It could e.g. choose to log only every n invocations. To do this it must keep track of how many times it has been called. I want to leave such things private to the Logger. (1) I know that this is not an unusal thing to do, but it has an OO feel to it. Is there a more functional alternative to it. Should I just not worry? (2) I can write a combinator which creates a Logger from a list of Loggers. Since each Logger potentially returns a new version of itself, I must always re-assemble the combined logger from all the returned new versions. I am worried that this is a costly operation, particularly when most Loggers just return themselves unaltered. I don't have any hard numbers about the performance penalty though. These Loggers are used in a discrete-event-simulation and they will get called many times (once for each event), but only occastionally actually write to the Log.