I'm looking for a library to be able to express, store and retrieve recurring appointments, like "every Monday at midnight".
I saw Data.Time.Recurrence, which has a nice language, but how to store the appointments? A command like:
now <- getCurrentTime
recur daily `begin` now
produces an infinite list of UTCTime.
You need to define your own serialization for its structures. If "recur" can't be serialized directly, your serialization needs to have a way to represent an invocation of "recur" instead of trying to store its result.
This probably isn't provided because different people/applications have different serialization needs, and it's quite easy to roll the serialization yourself given the library as a starting point.