Hi, I wonder if anyone can explain what is going on here and what to do about it. I'm fairly new to Haskell, so apologies in advance if my question seems naive. I've cut my code down to a minimum that reproduces the problem. I receive an error in GHCI in the following code, complaining that it cannot match the rigid type variables for the instance definition for Ainst for the function a3. Can anyone advise about what to do about it? ------------------------------------------------ class A a where a1 :: a a2 :: a -> a a3 :: (B b) => b -> a class B b where b1 :: Int -> b data (B b) => Ainst b = I | J (Ainst b) | K b instance (B b) => A (Ainst b) where a1 = I -- a2 :: (B b, A a) => a -> a a2 = J -- a3 :: (B b, A a) => b -> a a3 = K -- Error! -- a3 = K `asTypeOf` a3 -- Error even with this! data Binst = Val Int instance B Binst where b1 = Val ------------------------------------------------ Test5.hs:17:9: Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the instance declaration at Test5.hs:12:12 `b' is a rigid type variable bound by the type signature for `a3' at Test5.hs:5:13 Expected type: b -> Ainst b1 Inferred type: b -> Ainst b In the expression: K `asTypeOf` a3 In the definition of `a3': a3 = K `asTypeOf` a3 Failed, modules loaded: none.