Hi, I have a type class similar to this one. data T class Foo ns a b c | ns -> a, ns -> b, ns -> c where mkFoo :: ns defaultA :: a defaultB :: c -> IO b defaultC :: [T] -> c f :: c -> b -> a -> (b, Int) The idea is, that I define classes of components where the data types of the methods are component-specific. I therefore use a namespace to define the component types (associated types would be nicer, but are not widely supported). Given a few sample instances, I can define a configuration: data X; data XA = XA; data XB = XB; data XC = XC instance Foo X XA XB XC where mkFoo = undefined defaultA = XA defaultB XC = return XB defaultC _ = XC f _ b _ = (b,0) mkX = mkFoo :: X data Y; data YA = YA; data YB = YB; data YC = YC instance Foo Y YA YB YC where mkFoo = undefined defaultA = YA defaultB YC = return YB defaultC _ = YC f _ b _ = (b,1) mkY = mkFoo :: Y config = mkX .*. mkY .*. HNil Using this configuration, I now want to define various functions that work on As, Bs or Cs, respectively. For example, I'd like to have a :: XA :*: XB :*: HNil a = hMap DefaultA config but I fail to figure out how to define DefaultA. I tried with this one: data DefaultA instance Foo ns a b c => Apply DefaultA ns a where apply _ _ = defaultA but I get: Could not deduce (Foo ns1 a b1 c1) from the context (Apply MakeAs ns a, Foo ns a b c) arising from use of `defaultA' at /Users/nominolo/Devel/Haskell/testcase1.hs:126:16-23 Possible fix: add (Foo ns1 a b1 c1) to the class or instance method `apply' In the expression: defaultA In the definition of `apply': apply _ _ = defaultA In the definition for method `apply' I guess I need some type class that also binds b and c. Or maybe I need to do some things completely different at all. Ideally, I'd hope to keep the work for adding new Foo instances as low as possible, though. I attached the relevant parts of the HList implementation and the code. Any suggestions? /Thomas