varying function by typeclass

I have a function that takes, among other things, a (wrapped) generic type, i.e. "forall a. a". My function can possible produce an error message; is there any way to set up the function so that if the type is an instance of Show include the value along with the error message? I can't think of any, and this is honestly the first time I've been directly frustrated by Haskell's typesystem, so it's bugging me more than it might otherwise. Any suggestions? Abe