Hello, the following is not Haskell 98: class C a instance C (a,a) Well, GHC allows this with the -fglasgow-exts flag. Surprisingly, I cannot find a section in the GHC User's Guide which states that -fglasgow-exts can be used to allow this kind of instance declarations. Well, in the section on undecidable instances (7.4.4.3), one can read: Note that instance heads may contain repeated type variables. For example, this is OK: instance Stateful (ST s) (MutVar s) where ... But one has to note that in this expample, the different occurences of s aren't in the same type. Furthermore, multiple occurences of the same type variable cannot result in undecidability (as far as I understand). The above-mentioned statement talks about the things that are allowed *without* -fallow-undecidable-instances, and premises that multiple occurences of a type variable are okay. So am I missing something? Best wishes, Wolfgang