Before I go into crazy suggestions, have you looked at the Writer monad?

On Jun 4, 2016 9:25 AM, "martin" <martin.drautzburg@web.de> wrote:
>
> 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.
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