Perhaps hack was a strong word.  I'm not referring to type synonyms, but to the fact that Prelude's show class happens to have a special show function for lists, which happens to be handy when writing an instance for Show Char.  I find the coupling here (between Show and a particular instance of Show, namely Show Char) to be disturbing -- Show should not (if you ask me) contain special machinery for specific instances.  Not that I'll lose sleep over it, and of course the benefit (having pretty strings) far outweighs the philosophical cost.