Typeclasses to the rescue.. ls :: LsOptions -> IO Listing instance ToString Listing where ... instance Default LsOptions where def = LsOptions []

Typeclasses to the rescue.. ls :: LsOptions -> IO Listing instance ToString Listing where ... instance Default LsOptions where def = LsOptions [] Now a single `ls` function can yield regular textual output, or the more specific Listing type can be used if you want to say, map over the files listed. toString <$> ls def map toUpper . filesListed <$> ls def The repl can automate the application of toString of course, so the user can just enter `ls def`. Someone better at this than I could probably find a way to make the def parameter be optional, too.