[Haskell-beginners] special polymorphic default of typeclass depending on other typeclasses?

hi, say i have the following typeclass: class Foo a where bar :: a -> String looks a lot like the Read typeclass, right? (at least i think it should?) well say it's a different meaning (in other terms i can't or do not want to use Read, but i'd like to implement a default version of bar for those instances that also implement Read. is there a way to do so?