Hi All I'm having some difficulty with typeclasses. What I'm trying to do should be obvious, but it's still giving me trouble. I want to take a packaged item, and strengthen the constraints on its type. Rather than being just any type that is an instance of A, I want to do a runtime check and do something different if it is a type that is also an instance of B (which implies that it is an instance of A also). I've attached some code that attempts this, but ghci (er, 6.4.2 with -fglasgow-exts) complains: Compiling Main ( Cast.hs, interpreted ) Cast.hs:15:22: Ambiguous type variable `b' in the constraint: `B b' arising from use of `qux' at Cast.hs:15:22-24 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. So, can anyone suggest how I can achieve my goal? thanks, Tom