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

[Silent Leaf]

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?
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