It's not hard at all and it's actually what the second approach listed by the author shows: class HasRecipient a where get_receiver :: a -> PlayerId

It's not hard at all and it's actually what the second approach listed by the author shows: class HasRecipient a where get_receiver :: a -> PlayerId which adjusted to your example would be class Timestamped a where timestamp :: a -> UTCTime The problem with this approach is that you'll have duplicated data on all instances. In your example, `Quote` has the fields `timestamp` and `sender` in order to satisfy with `Timestamped` and `Msg`. If you had several classes like `Quote` and interfaces like `Timestamped` then you would end up with a lot of duplicated code.