David, thank you very much for the examples. They are very clear. Would it be difficult to post the first one (working in current setup) on Wiki? either https://wiki.haskell.org/GHC/Type_families or https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/type-familie... ? expanding your and Clinton's suggestions, here is one more version. Not sure if this may work however this may be intuitive to C#, Java users: -- Names for the instances data InstanceChoice = ChooseFirst | ChooseSecond | IgnoreBoth -- Determine which instance should be used type family Choose a b where Choose Int y = 'ChooseFirst Choose x Int = 'ChooseSecond Choose x y = 'IgnoreBoth class Closed a b where fun :: a -> b -> Int instance (Choose Int y) => Closed Int y where fun x _ = x instance (Choose x Int) => Closed x Int where fun _ y = y instance (Choose x y) => Closed x y where fun _ _ = 0 the differences: no need to use new keyword "closed" no need for new language pragma non-type family instances may still be defined instances may be defined in other modules (if possible:) multiple type families may be used with the same class. Instance constraint would hint, which (if any) type family to apply in selecting an instance how does this sound?