Hello, I am using a heterogenous list as in [1] all elements of which are of a given class C. I am awed by the beauty of the code (HList, not mine :-). Here is what I am trying doing: import HListPrelude data T = T Int class C a where foo :: a -> Int instance C T where foo (T i) = i class HList l => CList l instance CList HNil instance (C c, CList cs) => CList (HCons c cs) Since foo maps all class members to Int, hMapOut should be a straight-forward way to produce homogenous Int lists from heterogenous CLists: test :: (CList l) => l -> [Int] test = hMapOut foo That would be too easy, though... ghci (6.4) sez: /home/fis/tmp/Main.hs:16:7: Could not deduce (HMapOut (a -> Int) l Int) from the context (CList l) arising from use of `hMapOut' at /home/fis/tmp/Main.hs:16:7-13 Probable fix: add (HMapOut (a -> Int) l Int) to the type signature(s) for `test' or add an instance declaration for (HMapOut (a -> Int) l Int) In the definition of `test': test = hMapOut foo hugs (version 20050308) sez: ERROR "/home/fis/esim/HList/FakePrelude.hs":113 - Overlapping instances for class "Show" *** This instance : Show (HSucc a) *** Overlaps with : Show (HSucc HZero) *** Common instance : Show (HSucc HZero) (oops? should i upgrade, or is there a switch that i missed?) I am trying to understand the ghci error message, which looks like I might have missed a point. It seems like I should make it more obvious to ghc that CLists actually consist of values of type C. Is that a good guess? But how do I actually do it? I have unsuccessfully to extend the context to (HMapOut (a -> Int) l Int, CList l) or even to (C a, HMapOut (a -> Int) l Int, CList l) and I can't see any reason why the instance declarations of HMap in HListPrelude shouldn't cover my code. I am feeling a little stuck again... thanks, Matthias [1] http://homepages.cwi.nl/~ralf/HList/