Hi, I want to use a rank 2 data type and end up in typ problems that I don't understand. I would really appreciate any hints to the presented cases or pointers to interesting papers that address the background of these problems. I define the following datatype and a simple function that checks whether the list is empty. data Test = Test (forall a . [a -> a]) null1 :: Test -> Bool null1 (Test []) = True null1 (Test _) = False null2 :: Test -> Bool null2 (Test l) = null l null3 :: Test -> Bool null3 (Test l) = case l of [] -> True _ -> False For null1 ghc reports the following typeerror: Couldn't match expected type `forall a. [a -> a]' against inferred type `[a]' In the pattern: [] In the pattern: Test [] In the definition of `null1': null1 (Test []) = True while null2 and null3 don't cause errors. I don't understand why there is a difference between these definitions of null. cp1 :: Test -> Test cp1 (Test l) = case l of [] -> Test [] (x:xs) -> Test (x:xs) cp2 :: Test -> Test cp2 (Test l) = if null l then Test [] else Test (head l:tail l) For cp1 ghc reports the following typeerror: Inferred type is less polymorphic than expected Quantified type variable `a' is mentioned in the environment: xs :: [a -> a] (bound at UpDownTree.hs:36:10) x :: a -> a (bound at UpDownTree.hs:36:8) In the first argument of `Test', namely `(x : xs)' In the expression: Test (x : xs) In a case alternative: (x : xs) -> Test (x : xs) while for cp2 ghc doesn't report an error. I guess there is a similar reason for this error as for the error for null1. I now want to restrict the polymorphic functions in the data type to types for which there is an instance of the type class Eq. data TestEq = TestEq (forall a . Eq a => [a -> a]) null4 :: TestEq -> Bool null4 (TestEq l) = null l For null4 ghc reports the following typeerror: Ambiguous type variable `a' in the constraint: `Eq a' arising from use of `l' at UpDownClass.hs:37:24 Probable fix: add a type signature that fixes these type variable (s) I obviously cannot add a constraint that constraints a type variable that doesn't occur in the type of null4. I will be very pleased for any hints. Regards, Jan