Hello all, I was looking for my master thesis topic and my supervisor suggested an idea of extending class system so it enables refactoring Type Class hierarchy without affecting client source code which is using refactored classes. One example is Functor - Applicative - Monad problem and corresponding proposal [1]. But this proposal instead of allowing the change through extending Type Classes forces client code to prepare for the new class layout and then switch the classes to the new layout. My goal is rather to allow direct changes in class hierarchy without affecting client source code. I have found different proposals approaching this problem on HaskellWiki, some of them are overlapping, some of them refer each other. The most promising to me seems Default superclass instances proposal [2]. This one is somehow implemented in the Strathclyde Haskell Enhancement (SHE) [3] but I haven't found much reference or user experience really. So the reason why I write this email is to ask you for some tips where above mentioned problem occurs in real source code. I would like to investigate some real examples before designing some ad hoc changes to the Type Classes system. Besides that I'd appreciate anyone who has used default superclass instances in SHE to share his experience. And last but not least I am always grateful for any comments and suggestions. Best wishes Frantisek [1] http://www.haskell.org/haskellwiki/Functor_hierarchy_proposal [2] http://hackage.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances [3] https://personal.cis.strath.ac.uk/conor.mcbride/pub/she/