Hi I came across a problem, which I deeply believe, might be solved in Haskell in much nicer way (than I did it). I have: class (Eq a) => ThatsMyProblem a where fromMyProblem :: a -> Int toMyProblem :: Int -> a data MyType1 = MyType1_1 | MyType1_2 | MyType1_3 Int deriving (Show) instance Eq MyType1 where (==) a b = fromMyProblem a == fromMyProblem b instance ThatsMyProblem MyType1 where [...] data MyType2 = MyType2_1 | MyType2_2 Int deriving (Show) instance Eq MyType2 where (==) a b = fromMyProblem a == fromMyProblem b instance ThatsMyProblem MyType2 where [...] data MyType3 = MyType3_1 | MyType3_2 | MyType3_3 Int deriving (Show) instance Eq MyType3 where (==) a b = fromMyProblem a == fromMyProblem b instance ThatsMyProblem MyType3 where [...] I would very much like to create one single instance like this: instance (FutureVal a) => Eq a where (==) x y = fromFVal x == fromFVal y but that does not seem to work, as I get an error stating “Illegal instance declaration for `Eq a' (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Eq a'” Could you please point me out my mistake and/or direct me to some documentation? -- Mateusz